Strategies for carbon and sulfur tolerant solid oxide fuel cell materials,
incorporating lessons from heterogeneous catalysis
Paul Boldrina*, Enrique Ruiz-Trejoa, Joshua Mermelsteinb, Jose Bermudez
Menendezc, Tomas Ramirez Reinad, Nigel P. Brandona
a: Department of Earth Science & Engineering, Imperial College London
b: The Boeing Company
c: Department of Chemical Engineering, Imperial College London
d: Department of Chemical and Process Engineering, University of Surrey
* Corresponding author: [email protected]
Abstract
Solid oxide fuel cells (SOFCs) are a rapidly emerging energy technology for a low
carbon world, providing high efficiency, potential to use carbonaceous fuels and
compatibility with carbon capture and storage. However, current state-of-the-art materials
have low tolerance to sulfur, a common contaminant of many fuels, and are vulnerable to
deactivation due to carbon deposition when using carbon-containing compounds. In this
review we first study the theoretical basis behind carbon and sulfur poisoning, before
examining the strategies towards carbon and sulfur tolerance used so far in the SOFC
literature. We then study the more extensive relevant heterogeneous catalysis literature for
strategies and materials which could be incorporated into carbon and sulfur tolerant fuel
cells.
Contents
1. Introduction
2. Scope of the review
3. Fundamentals of carbon poisoning
1
3.1 Theoretical studies on carbon deposition in catalysts and fuel cell anodes
4. Fundamentals of sulfur poisoning
4.1 Theoretical studies on sulfur poisoning of catalysts and SOFC anodes
5. Systems approaches to carbon and sulfur tolerance
6. Materials design strategies for carbon tolerance in SOFC anodes
6.1 Ni-YSZ cermets
6.2 Alloying with noble metals
6.3 Alloying or replacement of nickel with base metals
6.4 Replacement of nickel with non-metal electronic conductors
6.5 Increasing alkalinity
6.6 Use of ceria and other oxygen storage materials
6.7 Replacement of cermets with mixed ionic-electronic conductors (MIECs)
6.7.1 Single phase MIECs
6.7.2 Addition of catalytic metal nanoparticles to MIECs
6.8 Regeneration of SOFC anodes deactivated by carbon
7. Materials design strategies for sulfur tolerance in SOFC anodes
7.1 Replacement of YSZ with ceria
7.2 All-ceramic anodes
7.3 Alloying of nickel with other metals
8. Strategies from conventional catalysis
8.1 Carbon tolerance in conventional catalysis
8.1.1 Sulfur passivation
8.1.2 Alloying and bimetallic systems
8.1.3 Promoters
2
8.1.4 Regeneration of Catalysts Deactivated by Carbon Deposition
8.2 Strategies against sulfur poisoning
8.2.1 Noble metal-based catalysts
8.2.2 Alloys, bimetallic and promoters
8.2.3 Support and structural modifications
8.2.4 Regeneration of sulfur poisoned catalysts
9. Conclusions and Perspectives
9.1 Alloying of nickel
9.2 Alkaline promoters and supports
9.3 Ceria, doped ceria and oxygen storage
9.4 Preferential sulfur binding sites
9.5 Non-metal electronic conductors
9.6 Infiltration of nanoparticles
9.7 Regeneration
9.8 Theoretical and computational studies
9.9 Reflections on experimental work
10. Acknowledgements
11. References
1. Introduction to solid oxide fuel cells
Solid oxide fuel cells (SOFCs) are electrochemical devices for the direct conversion of
fuels into electricity. Because they operate by the conduction of oxide ions they are capable
of using a wide variety of fuels including hydrocarbons, syngas, biogas and ammonia, as well
as hydrogen. The oxidation of fuel takes place at the anode, which needs to be active for
electrochemical oxidation of the fuel species and possess both electronic and ionic
3
conductivity. Typically anodes are made either from ceramic-metallic composites (cermets)
where each component provides one aspect of the conductivity, or from a mixed ionic-
electronic conductor (MIEC), a ceramic which provides both ionic and electronic
conductivity.
There are a number of other properties that any materials to be used in SOFC
anodes need to possess, including stability towards high temperatures and highly reducing
conditions, chemical compatibility with other materials such as electrolytes and
interconnect materials, and thermal expansion coefficients matched to the other
components during operation and manufacture. The need for these properties places a
limitation on which materials can be used, for example there are materials with high ionic
conductivity which are not stable in reducing atmospheres, or which have a large thermal
expansion mismatch compared to common electrolyte materials. As well as the direct
electrochemical oxidation of fuel species, other relevant reactions which take place in an
SOFC anode are water-gas shift, steam reforming, dry reforming, Boudouard reaction,
methanation and hydrocarbon decomposition and cracking, among others.
The development of SOFCs has reached an important phase, with rapid technological
advancement over the last decade resulting in multiple programs run by governments
and/or companies testing systems greater than 100 kW, and installed commercial products
in the low kW range combined heat and power market. An initial understanding of the
recent progress of multi-kW-scale SOFC development can be gained by studying the US
Department of Energy’s SOFC program (Solid State Conversion Alliance, SECA) which is
interested in systems of 100 kW and upwards operating on syngas from coal or natural gas.
For the period 2005 – 2007 the SOFC targets were for 1500 hour tests on fuel cell stacks
with performance degradation targets at steady state of <4%/1000 hours, while the latest
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target is for >25 kW stacks with >4 years lifetime and degradation of <0.2%/1000 hours,
with cumulative operation times of between 5,000 and 10,000 hours for this generation of
SOFCs by 2020. The financial year 2015 funding round supported Fuel Cell Energy and Versa
Power to produce a 400 kW system1. Other large projects include Mitsubishi Heavy
Industries demonstrating a 200 kW combined SOFC-gas turbine system operating on syngas
at 900 °C, with a degradation rate of 0.13%/1000 hours2, while Bloom Energy, based in
California, have a commercially-available SOFC capable of generating 100 – 200 kW aimed at
the commercial market, especially data centres, with an installed base of over 30 MW3.
Figure 1 shows a diagram of a combined cycle SOFC with integrated gas turbine.
Figure 1 – Diagram of a combined cycle SOFC system with integrated gas turbine
The focus on degradation rates clearly seen above in the large scale and commercial
programs is a reflection that one of the key issues facing SOFCs is degradation and its effect
on lifetime costs. Performance degradation can be caused by thermal gradients or thermal
cycling, oxidation cycling and long-term incompatibility of components. For SOFCs to
continue to become successful commercially, they will need to operate on carbonaceous
fuels, and be tolerant to common contaminants in those fuels. Two of the most common
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poisons are carbon and sulfur, and current anodes based on composites of nickel and
yttrium-doped zirconia (yttria-stabilised zirconia, YSZ) are not tolerant to them, resulting in
long term degradation and a need for regeneration, which has additional effects on
degradation relating to thermal and/or oxidation cycling. For this reason carbon and sulfur
tolerance is a vital area of research for the next generation of SOFCs to compete with
conventional power plants at the grid scale, and with boilers and combustion engines at
smaller scales.
The application of catalysis in fuels processing has been an important research topic
for several decades. In catalytic processes involving fuels, carbon (from the fuel itself) and
sulfur (present as contaminant) critically affect the performance of the catalyst. Under
certain conditions, this effect can be extremely important and the catalyst is deactivated
quickly, leading to unpractical and/or costly processes. For these reasons, huge research
efforts have focused on the design of catalysts resistant to carbon deposition and sulfur
poisoning. As a result of this, a vast knowledge of possible alternatives to address these
issues has been generated. This literature could provide insights into improving the carbon
and sulfur tolerance in SOFC materials.
2. Scope of the review
This review discusses all aspects of carbon and sulfur tolerance in SOFC anodes, from
mechanistic and theoretical studies to strategies for materials design. In addition, we have
studied the catalysis literature, focussing on fundamental studies and catalysts used in
reactions under conditions similar to those in an SOFC anode (e.g. steam reforming and
partial oxidation). Since, in the end, poisoning by carbon and sulfur may be inevitable, we
have also included sections on regeneration. We have chosen to put these at the end of the
relevant materials design section (e.g. regeneration of SOFCs after carbon deposition is at
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the end of the carbon tolerant SOFC section). In the final section, we have summarised the
various strategies used in catalysts and SOFCs to provide carbon and sulfur tolerance with
lessons learned from each. Certain aspects of this review have been covered in other
reviews in the last decade: Ni-based anodes in hydrocarbon fuels4, sulfur poisoning of Ni-
based anodes and catalysts5-6, anode performance in hydrocarbons7-8, catalyst deactivation
and regeneration9, steam reforming for fuel cells10, internal reforming in fuel cells11 and
sulfur tolerance in hydrogen production catalysts12.
3. Fundamentals of carbon poisoning
Deposition of carbon-containing species on metal catalysts is one of the main causes
of catalyst deactivation and is virtually inevitable in any reaction involving hydrocarbons9, 13-
16. It should be clarified that carbon and coke, although often used interchangeably, refer to
different species. Carbon refers to the product of CO disproportionation whereas coke is
produced by decomposition or condensation of hydrocarbons9, 16-17. However, for the sake of
clarity and readability, only the term carbon will be used in this work.
In reactions involving carbon-containing fuels, the principal reactions leading to
carbon deposition can be summarized as follows13:
2 CO (g) C (s) + CO2 (g)
CnHm (g) n C (s) + m/2 H2 (g)
CO (g) + H2 (g) C (s) + H2O (g)
The first reaction is the disproportionation of carbon monoxide and is commonly
known as the Boudouard reaction, after its discoverer Octave Leopold Boudouard, a French
chemist of the late 19th and early 20th century. It is exothermic at all temperatures but due
to the reduction in entropy becomes more favourable at lower temperatures. The second
reaction is the decomposition of hydrocarbons and conversely is endothermic with an
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increase in entropy, so is favoured at high temperatures. The final reaction is the reverse of
the original “water-gas reaction” used to produce “water gas” (now known as syngas) from
coke using steam. It is distinct from the water-gas shift reaction, which was originally used
to reduce (or shift) the carbon monoxide content of the water gas, so that it could be more
safely used. It has similar thermodynamics to the Boudouard reaction and so is more
favoured at lower temperatures.
Carbon deposition is strongly affected by the presence of sulfur and aromatic
compounds in the fuel13, 18. Sulfur deactivation can either promote or reduce carbon
deposition depending on the conditions19, and the ability of sulfur to potentially improve
carbon tolerance is discussed later on in section 7.1.1. The presence of aromatics in the fuel
tends to increase carbon deposition far more than would be expected from their
concentration in the fuel. This is likely because carbon deposition is thought to proceed
through a mechanism involving the formation of aromatics. Once formed, these aromatics
are less reactive than other compounds in the fuel and serve as nucleation sites for the
formation of polynuclear carbon compounds9, 13. The mechanism of carbon formation varies
with material (e.g., if it is a metal or metal oxide/sulfide)9, 16. This is important because the
effect of the structure and location of carbon on deactivation can be more relevant than the
total quantity of carbon deposited on the catalyst9, 20. In the case of metals, the rate of
carbon deposition is a function of the type of metal, the crystal size, the promoters and the
interaction between the metal and the support9, 21-29.
Formation of solid carbon is favoured thermodynamically in a large proportion of the
potential operating space of SOFCs30. Figure 2 shows the region in which carbon deposition
is favoured at different temperatures, showing that all common carbon containing fuels are
in the carbon deposition region below 1000 °C, including CO and CH3OH. This indicates that
8
oxygen-containing species need to be added to make carbon deposition thermodynamically
unfavourable. Factors which increase the thermodynamic favourability of carbon deposition
include lower temperatures, higher carbon:oxygen ratios and low oxygen fluxes. In addition
to this, carbon deposition is strongly influenced both inside and outside this thermodynamic
window by kinetic factors, especially the relative rates of the forward and reverse
Boudouard and methane decomposition reactions, and the presence of aromatic and
polyaromatic compounds.
Figure 2 – Carbon deposition limit lines in the C-H-O phase diagram. Reproduced by
permission of The Electrochemical Society from J. Electrochem. Soc. 150 (7) A885-A888
(2003). Copyright 2003 The Electrochemical Society
When carbon deposition takes places on metal particles, several situations can lead
to deactivation (Figure 3)9:
Strong chemisorption as a monolayer or physical adsorption in multilayers
blocking access to metal surface sites.
Encapsulation of metal particles, deactivating them completely.
9
Plugging of micro- and mesopores blocking access to the active sites inside
them.
Growth of carbon filaments (whisker carbon) that can stress and fracture the
support or push the metal particles off the support. In the case of SOFC anodes the growth
of this carbon can destroy the structure of the fuel cell.
Dissolution of carbon atoms into the metal, causing a volume expansion. This
is mainly a problem for SOFC anodes, where the metal may have a structural role and
therefore these volume changes can destroy the structure of the anode.
By blocking active sites for catalytic and electrocatalytic reactions, carbon can reduce
the performance of both catalysts and SOFCs. This type of deactivation can occur even at
low levels of carbon deposition, but is generally fully reversible by oxidation of the carbon.
Techniques for achieving this are discussed in sections 5.8 (for SOFC anodes) and 7.1.4 (for
catalysts). Structural deactivation, where carbon deposition causes structural failure, tends
to be the most serious problem caused by carbon poisoning in SOFCs. This mode of
deactivation is caused by longer term running under conditions favourable to carbon
deposition, or when using materials such as nickel which catalyse carbon deposition. In
SOFCs, because the metal component can have some structural role, failure can also occur
by dissolution of carbon into components of the anode, causing a volume expansion which
can result in “dusting”, where the anode becomes pulverised. This tends to occur when
carbon is repeatedly dissolved and removed from the anode materials.
Different types of carbon can be formed in these reactions9, 13. These types of carbon
have different reactivities and morphologies, which affect their potential for deactivation. In
addition, they can react to be transformed in a different type of carbon, thus varying during
the reaction their potential to deactivate the catalyst9, 13, 16.
10
In the case of metal oxides and sulfides, the formation of carbon is the result of
cracking reactions catalysed by acid sites. The rate of carbon deposition depends on the
acidity of the catalyst and its porous structure. In this case deactivation can be caused by
chemical or physical effects. In the case of chemical deactivation, carbon can strongly
adsorb on the acidic sites while physical deactivation is the result of the pore plugging which
blocks access to some catalytic sites.
Figure 3. Different situations in which carbon deposits can lead to deactivation: a)
Carbon layers chemisorbed on metal particles (reprinted from J. Power Sources 2010, 195
(2), 649-661, with permission from Elsevier); b) encapsulation of metal particles by carbon
deposits (Reprinted with permission from J. Am. Chem. Soc. 2006, 128 (35), 11354-11355.
11
Copyright (2006) American Chemical Society); c) growth of carbon nanofilaments that push
metal particles off the support (reprinted from Int. J. Hydrogen Energy 2014, 39 (24), 12586-
12596, with permission from Elsevier); and d) pore blockage by carbon deposits (reprinted
from Chem. Eng. J. 2010, 163 (3), 389-394, with permission from Elsevier).
An ideal carbon tolerant cell would be able to run on hydrocarbons without any
added oxidant and would therefore not require high temperatures, steam generators or
other extra modules which are currently used to mitigate carbon poisoning in SOFC-based
power generators.
Generally, there are two ways for suppressing (or at least minimizing) the rate of
carbon deposition: changing process conditions, such as increasing steam to carbon ratio or
increasing temperature; or developing carbon resistant materials9, 13, 15, 31-32. The rate of
deactivation is related to the balance between the rates of formation and
gasification/oxidation of the carbon, which are strongly influenced by the reaction
conditions and the catalytic activity of the materials towards the different reactions
involved9, 13, 15, 31.
In catalysis, the range of variation of the reaction conditions is often quite limited
since the conditions need to be designed to optimise the yield of the desired product rather
than protect the catalyst. In SOFCs, there is more scope to alter reaction conditions, with
compromises made to cost, power and flexibility. Since Ni is such an effective catalyst for
hydrocarbon decomposition, use of reforming to convert hydrocarbons into syngas can be
effective. This reforming can be done internally or externally. External reforming requires
the extra cost of a separate reforming unit, but has the advantage that the reformer has a
protective effect on the fuel cell. Internal reforming with steam, CO2 or O2 can be effective
due to the high activity of Ni for reforming reactions but certain conditions such as periods
12
at open circuit voltage (OCV) and low oxygen:carbon ratios can result in carbon deposition
33-34. Alternatively, separate reforming layers have been investigated, but these could
complicate fabrication and would need to be composed of a carbon tolerant catalyst 35-38.
Both internal and external reforming have problems with the separation of endothermic
reforming and exothermic oxidation reactions – in external reforming there is a need for
heat exchangers while in internal reforming the proximity of exothermic and endothermic
reactions causes thermal gradients. Both types of reforming reduce the power output of the
cell.
Because of how SOFCs operate, increasing the current density also mitigates against
carbon deposition (at least in sulfur-free fuels), due to the increased flow of oxygen into the
anode side of the cell. This has the advantage of encouraging reforming reactions without
reducing the power output of the cell. Because of the protective effect of oxygen flow
across the electrolyte, SOFCs can be started up under hydrogen with the carbon-containing
fuel being switched on once the cell is already under load, if it is feasible to have a dedicated
hydrogen supply for this purpose.
Running the fuel cell at high temperature can move the conditions outside the
region where carbon deposition is thermodynamically favoured, although this does not
guarantee there will be no carbon deposition. The higher temperatures increase the cost of
components other than the anode, which need to be designed to withstand higher
temperatures, for example above 800 °C, the most suitable alloys for interconnects have
high levels of chromium, which can cause problems with formation of resistive phases39 and
cathode degradation40.The higher temperatures may also reduce the overall lifetime of the
system. Alternatively, with Ni-YSZ anodes, it has been shown that decreasing the
temperature reduces carbon deposition in a cell operating under load in humidified
13
methane as it slows the methane cracking reaction more than the electrocatalytic oxidation,
although high currents and thus high oxygen fluxes into the anode were still required to
eliminate carbon deposition entirely.33
3.1 Theoretical studies on carbon deposition in catalysts and fuel cell anodes
The formation of carbon deposits in catalytic reactions involving hydrocarbons is the
consequence of the dehydrogenation of these hydrocarbons. Methane is the simplest
hydrocarbon and therefore provides the simplest model for understanding the
fundamentals of carbon deposition. The dissociation of CH4 over metal surfaces occurs in
four steps41-42:
CH4(g)a ↔ *CH3a+ *Ha
*CH3a ↔ *CH2a + *Ha
*CH2a ↔ *CHa + *Ha
*CHa ↔ *Ca + *Ha
Considering Ni as the active metal surface, the dissociation of methane can take
place on two different kinds of active sites: those associated with the planar surfaces (or
terraces) and those associated with stepped and defect sites on the metal surfaces41-42.
Considering the planar sites, theoretical studies have shown results that can be surprising at
a first view, as can be seen in Figure 441. The most stable intermediate in planar surfaces is
*CH and the last step of methane dissociation from *CH to produce carbon is an
endothermic process with high activation energy (Table 1)41-43. These data suggest that
carbon deposition should not take place on those Ni surfaces, something that contradicts
what has been widely reported experimentally. However, observing the results from the
stepped sites, the phenomenon of carbon deposition is easily explained. Stepped surfaces
are more reactive than planar, due to electronic and geometrical defects that take place in
14
these low-coordinated surface geometries41-42, 44-46. As a consequence of this, the production
of carbon on stepped surfaces is exothermic and thermodynamically favoured, creating the
driving force for the formation of graphitic carbon deposits42. A similar situation occurs in
the case of other metals and alloys, as can be seen in Table 1. In all the cases, the formation
of carbon is thermodynamically more feasible on stepped than planar surfaces.
Figure 4. Thermodynamic pathway for the dissociation of methane (CH4) on planar
(111) and stepped (211) Ni surfaces. Reprinted from J. Catal. 2007, 247 (1), 20-33., with
permission from Elsevier.
The process starts with the activation of the first C–H bond in methane. As can be
seen in Figure 5, in both cases (planar and stepped surfaces), this takes place over the top of
a surface Ni atom. However, in the case of planar surfaces, the energy barrier is higher than
in the case of stepped (Table 1). This is due to the higher stability of the adsorbed CH3 on
the stepped surface, which gives rise to a stronger bond44. Similarly, the subsequent steps of
the dissociation of methane give rise to species that are more stable on stepped than on
planar surfaces. Finally, whereas CH is the most stable species in planar surfaces, C is the
most stable species on the stepped, favouring its deposition in these sites41.
15
Table 1. Activation barriers (Ea) and reaction energies (∆E) of the different steps in
the dissociation of methane and adsorption energies (Eadsorption) of the different species
involved in the process reported on different metal surfaces. Stepped surfaces are shaded,
exothermic steps are in bold. All values are in kJ/mol.
CH4↔*CH3+*H *CH3↔*CH2+*H *CH2↔*CH+*H *CH↔*C+*H
Ea ∆E Ea ∆E Ea ∆E Ea ∆E
Ni (1 1 1)38 100a 54a 75 a 17 a 29 a -29 a 130 a 63 a
Ni (1 1 1)40 113.9 12.5 74.3 9.6 35.7 -25.1 131.2 53.1
Ni (2 1 1)38 84 a 42 a 88 a 8 a 42 a -33 a 88 a -29 a
Cu (1 1 1)38 188 96 138 92 113 46 205 130
Cu (1 1 1)40 181.4 83.0 141.8 67.5 101.3 30.9 213.2 115.8
Cu (2 1 1)38 138 33 134 79 184 13 176 75
Fe (1 1 1)40 98.4 -60.8 56.0 -30.9 12.5 -66.6 100.3 3.9
Co (1 1 1)40 110.0 4.8 66.6 10.6 30.9 -21.2 120.6 54.0
Cu-Ni (1 1 1)40 105.2 6.8 63.7 4.8 34.7 -34.7 132.2 49.2
Cu-Ni (2 1 1)38 n.a. 29 n.a. 63 n.a. -54 n.a. 21
Fe-Ni (1 1 1)40 120.6 -29.9 68.5 -19.3 30.9 -56.0 111.9 4.8
Co-Ni (1 1 1)40 124.5 4.8 73.3 20.3 32.8 -14.5 121.6 61.8
Rh (1 1 1)44 332 47.3 220 -42.7 26.4 -234 452 224
Rh(1 1 0)44 282 -54.8 127 -46.9 465 -51.0 207 -72.0
Rh(1 0 0)44 261 -53.6 136 -39.7 14.2 -250 284 -93.3
a - Approximate values extracted from Figure 2 in 41
16
(a)
(b)
Figure 5. From left to right, initial, transition and final state for the dissociation of
methane on: a) Ni (1 1 1); and b) Ni(2 1 1). C, H and Ni atoms are represented by dark grey,
black and white colours respectively. Reprinted from Surf. Sci. 2005, 590 (2–3), 127-137,
with permission from Elsevier.
Once carbon has been deposited in the metallic sites, two different processes that
lead to carbon deposits formation can take place. Either C-C bonds can be formed and then
graphitic planes grow parallel to the planar surfaces of the Ni. Graphene is more stable on
planar surfaces because carbon atoms are organized in hexagonal structures that can lie
parallel to the Ni atoms 41-42, 45-48. Alternatively, those isolated atom carbons, once adsorbed,
can dissolve into the bulk Ni forming carbides (Figure 6). As a result of this diffusion, carbon
atoms can reach facets on the support side of the metal particle. These facets are suitable
for the eventual growth of carbon nanotubes42, 47, 49.
(a) (b)
17
Figure 6. (a) Transition and (b) product states of the diffusion of one C atom from an
fcc hollow site to a sublayer octahedral site of the Ni(111) surface. Green (light) spheres
represent Ni atoms and black (dark) spheres represent C atoms. Reprinted with permission
from ACS Catalysis 2011, 1 (6), 574-582. Copyright 2012 American Chemical Society.
However, as stated by Abild-Petersen et al.44, the fact that stepped surfaces are
more active than planar ones does not mean that steps control the activity of the catalyst.
So, if the steps could be blocked, side reactions like carbon deposition would be eliminated,
while only moderately reducing the activity of the catalysts for methane processing. This can
explain the effect that the addition of Au to Ni catalysts has on coke deposition. Au
preferentially binds to low-coordinated Ni sites (like those present on steps). Consequently,
it increases the effective coordination number of adjacent Ni atoms and lowers the Ni
surface energy due to electronic interaction with gold41-42.
Another strategy for decreasing carbon deposition is to increase the reaction rate of
C-O bond formation relative to C-C bond formation. C atoms can be removed from the
surface of the catalyst by oxidation to form CO and CO249-50. Thus, if carbon diffusion and C-C
formation rates are decreased and oxidation rate increased, carbon deposition can be
avoided 49.
Following these ideas, the use of different promoters 41, 51, partial passivation 41, 45 and
alloys 41, 43, 49-50, 52 have been proposed. Table 1 shows that in all cases alloys have higher
thermodynamic barriers to carbon deposition than the metals which make them up,
meaning they are obvious targets. Two clear examples of this can be the effect of alloying Ni
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with Rh or Sn 49-50. The studies by Guo et al.49 and Nikolla et al.50 showed that when Ni is
alloyed with Rh or Sn both carbon and oxygen diffusion in the metal lattice and the C-C and
C-O bond formation are hindered, but to a different extent, as shown in Table 2 and Figure
7. Consequently, the overall carbon deposition rate was diminished. These studies have
supported their theoretical studies with experimental findings that point in the same
direction as the DFT results.
Table 2. Activation barriers (Ea), and reaction energies (∆E) of C-C bond and C-O
formation over different surfaces of Ni(1 1 1) and Ni-Rh (1 1 1). All energies are shown in eV.
C-C formation C-O formation
Ea ∆E Ea ∆E
Ni(1 1 1) -0.90 0.63 -1.75 1.18
Ni2Rh1(1 1 1) -0.69 0.75 -1.66 1.16
Rh(1 1 1) 0.19 1.34 -1.43 1.37
Figure 7. (a) DFT-calculated potential energy surfaces for C-C bond formation on Ni(1
1 1) and Sn/Ni(1 1 1). Inserts show the lowest energy pathways for the attachment of a C
atom to a carbon nucleation center (modelled as a chain of carbon atoms) on the two
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surfaces shown in the insert, (b) C-O bond formation on Ni(1 1 1) and Sn/Ni(1 1 1). Inserts
show the lowest energy pathways for the two surfaces shown in the insert. Ni is depicted as
large blue (light) atom, Sn as a large purple atom, carbon chain as a chain of small black
atoms. Reprinted from Catal. Today 2008, 136 (3–4), 243-248, with permission from
Elsevier.
While the metal particles are regarded as the main sites for carbon deposition, this is
also possible on oxide surfaces, for example both CO and CH4 will form carbon on Y2O3, YSZ
and ZrO2, with the amount of carbon decreasing in that order53-54, so clearly there is a
mechanism for carbon deposition on oxides which is controlled by the surface chemistry.
DFT studies on ceria and doped ceria show that carbon deposition should be
extremely unfavourable on a ceria surface as long as there are oxygen ions available to react
with the carbon atom, which will desorb as CO or CO255. The presence of Ni does not affect
the favourability of this process, indicating that the activity of the ceria in cermets should be
similar to the activity of pure ceria56. DFT studies on Ce2O3, show that surface vacancy
formation is as energetically unfavourable as on YSZ, indicating a low activity towards
oxidation reactions. Combined with experimental measurements showing that the ceria
surface was more active in a more reduced state, this indicates that Ce2O3 is not formed at
the surface57. In fact, DFT modelling shows that it is energetically favourable for CeO2 to
have two oxygen vacancies, providing the explanation for these results and the high
oxidation activity of ceria.
A study on BaCeO3 perovskites found that CeO2-terminated surfaces had much
stronger interactions with CH4 than BaO-terminated surfaces, although they did not link this
to carbon deposition but to methane oxidation58. Unfortunately the thermodynamically-
20
favoured termination under SOFC anodes conditions is BaO, meaning that BaCeO3 should be
inactive for methane oxidation.
4. Fundamentals of sulfur poisoning
In addition to the tolerance to carbon, tolerance to sulfur is required to make an
SOFC or a catalyst flexible to fuels. The interaction of sulfur with anodes in SOFC has been
reviewed in the last few years5, 59-60. Sulfur, contained in all fuels originated from natural
sources (fossil or biogas), can be minimised but will always be present in a wide range of
concentrations, for example from 85 to 5000 ppmw for diesel61. If degradation is
unavoidable, at least a certain degree of regeneration must exist in order to guarantee long
term operation.
Several studies have addressed the influence of sulfur poisoning mainly on Ni/YSZ
anodes operating on H2/H2O 62-68. In recent years, the interest has grown to include carbon
fuels and H2S, again mainly on Ni/YSZ69-75. The reactivity of sulfur is related to the number of
electron pairs available for bonding, therefore, from the chemical point of view, toxicity
decreases in the order H2S, SO2 and SO42- 76. Other compounds of sulfur may exist in the fuels
but it is expected that in the majority of conditions occurring in an SOFC anode all sulfur
compounds are transformed into H2S77. Following the notion of chemical reactivity and
electrons available for bonding, a non-noble metal with electrons available for bonding will
be more affected by sulfur than a ceramic78.
In terms of SOFCs, H2S itself is a fuel that can be oxidised electrochemically and the
obvious choice to oxidise the sulfur is the oxygen ion that is being transported through the
electrolyte. This happens in the same way that hydrogen is oxidised but it should be noted
that three times more electrons are being used per mole in the electrochemical oxidation of
H2S.
21
H2S + 3O2- H2O + SO2 + 6e-
H2 + O2- H2O + 2e-
Examples of the use of SOFCs with H2S as a fuel have been given in the literature but
are rather limited79-86. H2S can and has been used as a fuel and it has been shown that SO2 is
the product of utilisation in a fuel cell with Pt as the catalyst85, 87 or a highly conductive and
catalytically active thiospinel86. It is not clear however, if these thiospinels could operate in
hydrogen rich fuels, are ionically conductive or are resistant to redox cycling. To facilitate
this electrochemical reaction the supply of electrons and oxygen ions must be a continuous
process and therefore, as in the case of hydrogen oxidation in a classic Ni/YSZ anode, the
triple phase boundary (TPB), i.e. the interface between Ni, YSZ and gas phase, is critical to
the performance. Anything that hinders or slows down this supply of oxygen and electrons
to the TPB will have a detrimental effect; examples of hindrance are carbon deposition or
agglomeration of the Ni phase. Similarly, anything that blocks the reaction sites for
hydrogen oxidation or internal reforming will be equally detrimental. In the case of nickel
anodes, sulfur poisoning is one of the reasons for the decreased electrochemical activity. In
what follows, the scope is more concentrated on the presence of H2S in the fuels as
pollutant rather than as fuel.
4.1 Theoretical studies on sulfur poisoning of catalysts and SOFC anodes
Considering H2S as the source of sulfur, the depletion of the anode performance
under H2S containing gas mixtures at elevated temperatures originates from H2S dissociation
leading to the adsorption of atomic sulfur (S*) on the anode surface (i.e. adsorbed on Ni
atoms when a model anode Ni/YSZ is considered)88-89. The strongly adsorbed S* species
block the active sites of the anode surface, decreasing the electrochemical oxidation
performance. Experimental studies have shown that sulfur coverage fits a Temkin isotherm
22
on nickel surfaces in catalysts, where the enthalpy of adsorption of sulfur varies linearly with
coverage90. In solid oxide fuel cell anodes, performance degradation is proportional to sulfur
coverage at constant current density91. Figure 8 shows the coverage of sulfur on Ni and
phase equilibria, highlighting that sulfur coverage is high even at low sulfur concentrations,
with a very strong dependence on temperature. Bulk sulfidation of Ni does not occur until
much higher sulfur concentrations92.
DFT calculations clearly illustrate the situation60. Figure 9 pictures a Ni based anode
built as infinite slabs with an adequate vacuum space (around 15 Å). Under these
circumstances four types of active sites can be imagined including atop, bridge, and three-
fold fcc- and hcp-hollow sites. As schematically illustrated in Figure 9 (c), the mechanism of
S* formation could be described as an interfacial reaction of adsorbed H2S* with the Ni
surface via two elementary steps of S–H bond cleavages (i.e., H2S* /HS* + H* and HS* / H* +
S*).
23
Figure 8 Chemisorption equilibria plotted in the chemical potential diagram for the
Ni–S–H system, log[p(H2S)/p(H2)] vs 1/T plot. Dotted and dashed lines for θs = 0.6 and 0.8,
respectively, are isocoverage lines calculated from the equation given in literature91.
Reproduced by permission of The Electrochemical Society from J. Electrochem. Soc. 157 (6)
B802-B813 (2010). Copyright 2010 The Electrochemical Society
The associated energy barriers for the subsequent steps of dissociation and
adsorption of H2S are presented in Table 3. For sake of comparison, Table 3 includes
analogue calculations for several noble metals. The calculated energies evidence that sulfur
adsorption is clearly a favourable process on Ni surfaces with large exothermic reaction
energies (ΔE) and low activation energies Ea. Furthermore, these computational results
suggest that replacing Ni with noble metals is not a viable solution to mitigate sulfur
poisoning since energy-wise H2S dissociation and S* adsorption also take place on noble
metal surfaces60. The adsorption energies summarized in the table also show that H2S* and
HS* bind to metallic surfaces weaker than S*. Hence in principle greater sulfur resistance
could be achieved by avoiding H2S dissociation on the anode surface, although the latter is
difficult to achieve given that a stronger S* adsorption energy involves a redistribution of
the electronic density that reduces the energy demand for H-S bond breaking.
24
Figure 9 (a) Schematic representation of a slab model with a proper vacuum space
for periodic DFT calculations. (b) Four active sites on a (111) plane. (c) Schematic energy
profile of gas-phase H2S dissociation on Ni (111) forming atomic S* and H*. “*” denotes
surface species. TS1 and TS2 are the transition states. Extracted from Energy and
Environmental Science 2011, 4 (11), 4380-4409 with permission of The Royal Society of
Chemistry
Table 3 Activation barriers (Ea) and reaction energies (ΔE) for the elementary
steps in a H2S dissociative adsorption process and adsorption energies (Eads) of sulfur species
(S*, HS* and H2S*). All values in eV.
metal Ea1a ΔE1
a Ea2b ΔE2
b EadsS* EadsHS* EadsH2S*
Pt(111)93 0.02 -0.90 0.04 -1.19 5.14 3.00 0.90
Pd(111)94 0.37 -1.25 0.04 -0.73 5.15 3.02 0.71
Rh(211) 95 0.01 -1.50 0.32 -1.50 6.0 3.69 1.00
Ni(100)88 0.29 -1.56 0.45 -1.05 5.96 3.72 0.83
Ni(111) 88 0.15 -0.98 0.11 -0.86 5.14 2.95 0.67
a - Ea1 and ΔE1 correspond to H2S* → HS* + H*
b - Ea2 and ΔE2 correspond to HS* → H* + S*
Alternatively to noble metals, alloying Ni with base metals as Cu may result in an
improved sulfur tolerance96. Indeed, DFT calculations evidenced that Cu based anode
materials display better tolerance to carbon deposition and sulfur than Ni based anodes97.
Figure 10 shows the evolution of sulfur adsorption energies with the Ni-Cu alloy
composition. It seems very clear that the alloying approach increases Ni resistance to sulfur
poisoning but the bimetallic system never reaches lower sulfur adsorption energy than
monometallic Cu.
25
A B
Figure 10 a) Supercell models of homogeneous Ni1−xCux as a function of the alloy
composition. Ni and Cu are in grey and in brown, respectively B) Comparison of the
predicted adsorption energies of atomic sulfur on Ni1−xCux(1 1 1) at PAW–GGA–DFT (•) and
GGA–DFT (ο). Adapted from J. Alloys Compd. 2007, 427 (1-2), 25-29 with permission of
Elsevier
The smaller adsorption energy exhibited by the Ni-Cu alloy can be explained in terms
of the density of states (DOS) analysis, as detailed in Norskov’s d-band theory98-99. As shown
in Figure 11, the antibonding orbitals in Ni-S are higher energy than the Cu-S antibonding
orbitals, meaning that it is easier to excite electrons into the Cu-S antibonding orbital to
break the Cu-S bond. This favourable situation has motivated a number of studies focusing
on Cu89, 97, 100,Ni-Cu alloys96, 101 and other alloys such as Ni-Sn102-103 targeting weaker sulfur
interaction with the fuel cell anode. Nevertheless, the main problem of these alternative
materials is their poor catalytic activity for the hydrogen oxidation reaction (HOR).
Simultaneously improving activity for the HOR and reducing poisoning by H2S is difficult as
both are related to the affinity of the material towards hydrogen-containing species104. In
other words, the alloys can effectively enhance the tolerance towards sulfur poisoning but
cell performance is sacrificed in turn.
26
Figure 11 (a) DOS analysis of S* on Ni(111) and Cu(111) in red and blue curves,
respectively. A circle represents the antibonding states around the Fermi level. (b) A scheme
of the energies of bonding and antibonding states corresponded to those of metal d bands.
Adapted from Energy and Environmental Science 2011, 4 (11), 4380-4409 with permission of
The Royal Society of Chemistry
According to thermodynamics, sulfur poisoning of traditional Ni based anodes is
largely unavoidable under a wide range of conditions at very low concentrations of H2S (e.g.
below 0.1 ppm H2S at 800 °C and below 10 ppm at 1000 °C under dry hydrogen)10558.
However, further DFT calculations106 have demonstrated that Ni anodes could be
regenerated through a two-step treatment: (1) addition of H2 to reduce sulfur coverage; (2)
oxidation with oxygen realising S as SO2.
Galea et al. described the sulfur removal pathways via oxidation106. They described a
two-step mechanism. In the first step sulfur concentration is reduced from 0.5 to 0.25
monolayers and in the final stage surface cleaning from 0.25 monolayers of sulfur to
complete sulfur removal is achieved (Figure 12).
27
A B
Figure 12. Energy profiles of the regeneration process via sulfur oxidation. A) Gibbs
free energy (ΔG at 800 ◦C, black line) and enthalpy (ΔH, red line) profile illustrating relative
thermodynamic energy and kinetic pathways of O2 adsorption and SO2 desorption on S–
Ni(111) surface with initial coverage θS = 0.50 ML B) Gibbs free energy (ΔG at 800 ◦C, black
line) and enthalpy (ΔH, blue line) profile illustrating relative thermodynamic energy and
kinetic pathway of O2 adsorption and SO2 desorption on S–Ni(111) surface with initial
coverage θS = 0.25 ML. Adapted from J. Catal. 2009, 263 (2), 380-389 with permission of
Elsevier.
Although this oxidative treatment is effective it has an associated drawback which is
a high likelihood for Ni oxidation. Therefore, ideally this approach could be improved if the
oxygen is supplied by oxygen ion flux through the electrolyte and interacts selectively with
sulfur. In response to this problem, YSZ could be total or partially substituted by other
ceramic phases with higher oxygen conductivity as CGO showing greater sulfur tolerance60.
The presence of a highly oxygen conductive phase in the anode permits a certain degree of
electrochemical oxidation of S* to SO2 facilitating sulfur removal. This strategy of using
mixed oxides with high oxygen mobility seems to mitigate (but not fully eliminate) sulfur
poisoning in both SOFCs and catalytic processes.
5. Systems approaches to carbon and sulfur tolerance
28
As discussed above, carbon and sulfur represent a technical challenge for SOFC
technology. Although this review focuses on anode materials design strategies, currently the
main method for mitigating carbon and sulfur poisoning is processing of the fuel externally
to the SOFC stack, so it is worthwhile briefly reviewing these aspects of SOFC-based power
generation systems which are intended to achieve carbon and sulfur tolerance. Haldor
Topsøe have been involved in gas cleaning for SOFCs for many years, and John Bøgild
Hansen has very helpfully reviewed the company’s experience in this area107.
The main strategy used in relatively clean fuel (e.g. consumer grade natural gas, LPG
etc.) is fuel reforming. This converts most of the hydrocarbons to H2 and CO. The reformer
can be provided with oxidising gas as fresh steam or as recycled anode gas. CO2 can also be
used in so-called dry reforming. The use of reformers has been demonstrated practically in a
number of systems. Reformers can add significantly to the cost of the system, with the cost
reported as being similar to the fuel cell module itself108.
For dirtier fuels, such as gasified biomass or coal, or biogas, fuel processing becomes
more complicated and hence expensive109. The feedstock may contain up to several percent
of sulfur compounds as well as other contaminants such as alkali metals, halides and
phosphorus compounds. For solid fuels, the gasification process which converts the
feedstock into a gaseous form suitable for fuel cells can produce significant amounts of
aromatic compounds, including smaller molecules such as toluene, and larger polyaromatic
compounds which can cause carbon deposition in SOFCs. All of the feedstocks mentioned
above contain methane and/or short chain hydrocarbons, which again can cause carbon
deposition. These feedstocks need several layers of treatment, from desulfurisation to
particulate filtering, although most of this is not exclusive to SOFCs, so may not impact on
the economics of the process compared to competing technologies.
29
For SOFC-based systems using these fuels, the level of desulfurisation required is
crucial to the cost and complexity of the system. For example, to reach levels below 10 ppm,
deep desulfurisation is needed, which is normally carried out at 40 bar of pressure or
higher110-111, necessitating gas compression and increasing safety issues. More recent work
has reduced sulfur below 1 ppm at 10 bar, but this pressure is still too high112.
A final intermediate case is provided by liquid transport fuels, which may be an
important market for SOFCs in future. These have largely been cleaned of contaminants
such as tars and metallic impurities, but may still contain varying levels of sulfur. In general
the level of sulfur in these fuels is being driven downwards due to legislation. Ultra-low
sulfur diesel (ULSD) standards are normally around 10 – 50 ppm, although the actual
content of sulfur may be as low as 2 – 3 ppm. For aviation fuels, the sulfur levels are up to
3000 ppm, with an average of around 600 ppm.
As discussed above, some of these levels of sulfur may be too high for Ni/YSZ
anodes, although there are examples of SOFC stacks being run on reformed ULSD without
desulfurisation. A Topsøe SOFC stack was run on steam reformed ULSD (<10 ppm S) for 1200
hours113. After an initial 150 h period of rapid degradation there was only 0.2%/1000 h
voltage degradation over the rest of the test. Delphi tested a 5-cell stack with simulated
reformate containing 2.5 ppm sulfur, and also found a rapid initial degradation followed by
stable performance114, indicating that SOFCs may be able to operate stably with ULSD
reformate without desulfurisation, albeit with a performance drop caused by sulfur
poisoning. If SOFCs can tolerate ULSD-levels of sulfur, they should be economically attractive
for truck auxiliary power units (APUs)115, and higher sulfur tolerance would allow them to be
used in aircraft APUs).
30
Crucially, both the examples above had no hydrocarbons in the reformate. Even low
levels of compounds such as ethylene are capable of causing carbon deposition on Ni, even
in thermodynamic regimes which do not encourage carbon formation116. In the first example
above, a secondary reformer was used to remove the low levels of hydrocarbons produced
by the first reformer117, while the second example, in common with most studies, used a
simulated reformate without these problematic molecules.
From this summary, several points relating to fuel processing become clear. The first
is that since producing a suitable feed gas for an SOFC from almost any starting material is
technically feasible, then the driver for carbon and sulfur tolerance in the anode itself is
almost entirely economic. The costs of reforming and desulfurisation are each of a similar
order of magnitude to the cost of the fuel cell itself, and become more important for lower
power and/or more portable systems. That being the case, it becomes clear that the key
targets for carbon and sulfur tolerance in SOFC anodes are related to either eliminating or
reducing the specifications for the reforming and desulfurisation units. So for carbon
tolerance, some important targets could be: to be able to operate directly on methane,
propane, ethanol or biogas (methane-carbon dioxide mixtures), preferably without steam
generation or off-gas recycling; or to be able to tolerate low levels of species such as ethene
and tars. Meanwhile, for sulfur tolerance, important targets are tolerance to the low sulfur
levels in ULSD or natural gas (<10 ppm), then for fuels with higher levels of sulfur, tolerance
to the levels of sulfur after hydrodesulfurisation catalysis (i.e. without deep desulfurisation
at high pressure, or ZnO or other sorbents), then finally tolerance to the levels in those fuels
themselves.
6. Materials design strategies for carbon tolerance in SOFC anodes
31
6.1 Ni-YSZ cermets
Cermet-based anodes are the most widely used anodes in SOFCs. Traditionally they
have the advantage that the best oxide ion conductors can be used, while the metal can
provide the catalytic activity and the electronic conductivity. The industry standard material
is yttria-stabilised zirconia (YSZ), so-called because the addition of yttrium ions to the
zirconia stabilises the cubic form of the material under a wide range of temperatures. The
presence of the Y3+ ions also creates oxygen vacancies, which allows oxygen ion transport,
with the maximum conductivity being with 8 mol% of yttria added (Known as 8YSZ,
(ZrO2)0.92(Y2O3)0.08). YSZ is very stable towards high temperatures and a wide range of
oxidising conditions. It is also the most widely used electrolyte, having extremely low
electronic conductivity, meaning that issues of compatibility between the anode and
electrolyte are eliminated by using YSZ in both the anode and electrolyte. The industry
standard metal is nickel. Nickel is relatively cheap and highly active towards various
reactions involving carbon, as well as being active towards electrochemical oxidation. It is
also more stable than other base metals towards high temperatures, and unreactive
towards common electrolytes such as YSZ.
Both components of Ni/YSZ have problems relating to carbon and sulfur tolerance.
Nickel easily dissolves both carbon and sulfur, leading to volume expansions which can
cause structural failure of the anode. Nickel is also an extremely good catalyst for solid
carbon formation, meaning that carbon filaments can be formed, potentially destroying the
structure of the anode and blocking gas diffusion pathways as discussed in section 3. As well
as causing failure of the cell, this propensity towards carbon formation also renders nickel a
poor catalyst for direct oxidation of hydrocarbons, meaning that high quantities of steam
need to be used for cells running on methane or higher hydrocarbons. It also makes nickel
32
susceptible to poisoning by aromatic or polyaromatic compounds which may be present in
gasified coal or biomass118-124.
The problems of YSZ relate to its inertness and consequent inability to mitigate any
of the failings of nickel. It has little activity towards electrochemical oxidation or any of the
other important catalytic reactions and possesses extremely low electronic conductivity,
meaning that once the nickel has deactivated the cell is useless. It also has no oxygen
storage capacity and no ability to absorb sulfur, either of which could help improve carbon
or sulfur tolerance. Strategies to mitigate the issues with Ni and YSZ are described schematically in
figure 13.
Figure 13. Schematics of the most common materials strategies to improve carbon
tolerance. The diagram shows a strategy and does not imply a specific mechanism.
As shown above, the propensity of Ni/YSZ anodes towards carbon deposition is
largely a function of nickel’s ability to catalyse carbon formation. Thus it is natural to look at
partially or entirely replacing the nickel. Since nickel is an exceptional electrocatalyst, many
efforts to replace this have focused on substituting some other potentially active material
for some of the nickel rather than replacing the nickel entirely. The rationale behind this is
two-fold: firstly, heteroatoms could break up large continuous nickel surfaces which are
predisposed towards carbon deposition; and secondly, to enhance the rates of reactions
which compete with carbon deposition, such as carbon oxidation and steam reforming.
33
6.2 Alloying with noble metals
The so-called noble metals (roughly the second and third row transition metals in
groups 8 – 11) may offer enhanced catalysis as well as reducing carbon deposition, and are
known from conventional catalysis to be active in very small quantities. The earliest example
was gold, which causes a reduction in carbon deposition under oxygen-methane mixtures,
at the expense of methane reforming activity. Carbon deposition was reduced by up to 8000
times with one-fifth of the nickel replaced with gold125-126. SOFCs using Au doping have been
tested in dry and humidified methane atmospheres, where they showed no carbon
deposition after 200 hours127. A stabilisation of CHx surface species leading to a reduction in
the rate of graphite formation was found to be responsible128.
Impregnation of Pd into Ni/YSZ anodes showed a marked decrease in polarisation
resistance in hydrogen, methane and ethanol, with suppression but not elimination of
carbon deposition under the carbonaceous fuels129. The same was found for impregnation of
Pd into Ni on Ce0.9Gd0.1O1.95 (CGO)130. Carbon deposition was primarily found to occur in Pd-
poor regions. Likewise impregnation of Ru, also into Ni/CGO anodes, was found to improve
stability under methane, ethane and propane under load and short periods at open circuit
voltage (OCV), with the caveat that a 25 - 40 µm CGO electrolyte was used – CGO possesses
a relatively high electronic conduction under reducing conditions, meaning that there would
be a significant oxygen flux even at OCV131. Carbon deposition was not seen, as measured
from carbon balance analysis. This study also noted one of the problems with the use of Ru,
which is its tendency towards vaporisation during synthesis.
A comparative study looking at Ru, Pt, Pd and Rh on Ni/YSZ found that Ru, Pt and Pd
suppressed carbon deposition under dry methane compared to the unpromoted material,
while Rh actually increased carbon deposition132. In addition, Ru and Pt improved the power
34
density in fuel cell tests. Rh has however been shown in other tests to reduce carbon
deposition on Ni-CGO in microreactors and give more stable performance in button cells in
humidified butane, although the butane used in this paper contained sulfur compounds, so
the improved performance may be due to improved sulfur tolerance133. A further difference
could be due to the high activity of Rh-ceria for water-gas shift compared to Rh on other
supports134.
Silver has been shown to be a good catalyst for CO oxidation, with no propensity
towards carbon deposition135. Co-doping of Ni/YSZ with Ag and Cu was found to reduce
carbon deposition by a factor of three or four relative to samples doped with Cu or Co, with
the carbon deposited being more amorphous136. Silver can also be deposited electrolessly,
and this appears to reduce carbon deposition in dry methane and ethane137. Cells produced
in this way were stable over a period of 100 h in dry methane138.
Noble metals are also used in so-called catalyst or barrier layers in anodes – where a
layer is placed between the active anode and the gas supply. This serves to reduce the
hydrocarbon content in the anode by blocking hydrocarbons from entering or water and
CO2 from leaving. If reforming catalysts are used they can also increase the reforming rate.
This is at the cost of power density, due to the increased resistance to diffusion to the
electrochemically-active layer. In the original work showing this effect, Ru supported on
CeO2 was used in a catalytic reforming layer over a Ni-YSZ anode which showed good
stability in iso-octane-air-CO2 and propane-air mixtures139. Ir-CGO has also been used
successfully140, but more recent work has shown that barrier layers made from materials
which show less reforming activity such as Ni-Cu on Zr-doped ceria141, Ni-doped ceria142,
La0.75Sr0.25Cr0.5Mn0.5O3 (LSCM)-CeO2143, partially-stabilised zirconia and zirconium-doped
ceria144 and even Ni/Al2O3145 can also give low or no carbon deposition in the Ni-based
35
anodes underneath, indicating that the main effect may be the barrier layer effect rather
than the reforming activity. A further drawback to practical use of barrier layers is their non-
conductivity, which may hinder current collection. This could be combatted by incorporation
of reforming catalysts into a mainly metallic composite146. Nevertheless, further work on
barrier layers may be informed by section 7 which discusses sulfur and carbon tolerant
catalyst materials.
6.3 Alloying or replacement of nickel with base metals
A similar rationale is behind the use of top row transition metals Co, Cu and Fe,
which also act to break up the continuous nickel sheets. In comparative studies with Ni/YSZ-
based anodes, all of these elements, while reducing the carbon deposition, also reduce the
electrocatalytic activity compared to pure Ni/YSZ147. Despite this, the benefits of reduced
carbon deposition may outweigh the reduced performance, so these systems have been
extensively studied, including using different fabrication techniques such as impregnation,
microwave irradiation148, and electroless deposition149. Impregnation was used to produce a
series of Ni-Cu alloys on a porous YSZ substrate which was also impregnated with CeO2150.
While no carbon was detected on the pure Cu anode, anodes with a Cu-Ni ratio of 9:1 and
lower displayed significant weight gain due to carbon deposition, although the deposition
seemed to be self-limiting at 4:1 and higher, and the cell structure was not destroyed.
Interestingly, a higher reduction temperature resulted in lower carbon deposition, and it
was suggested that this is caused by copper enrichment at the surface of the alloy. Cell tests
on the 4:1 Cu:Ni anode showed a large increase in performance caused by carbon
deposition improving electronic percolation. A catalytic study of Ni-Cu/YSZ+CeO2 with the
Ni, Cu and CeO2 impregnated into the YSZ also found significant carbon deposition in the
50:50 Ni:Cu sample after exposure to a 2:1 CH4:O2 mixture for 20 h at 800 °C151. The amount
36
of carbon was also not reduced by addition of Pd to the composite. A sample with a 25:75
Ni:Cu mixture in contrast showed no carbon deposition.
Electroless deposition149 produced an inhomogeneous distribution of copper, leading
to carbon deposition in copper-poor areas. When microwave irradiation was used to deposit
copper nanoparticles on a Ni-YSZ anode, the effect was similar to cells produced using
impregnation of a Ni-Cu solution, indicating that alloying during synthesis may not be
necessary to reduce carbon deposition148.
Tests using copper alone have shown very low activities compared to nickel, with
Cu/YSZ anodes showing very low OCV with a dry methane fuel, indicating that it has little
activity towards methane oxidation147, 152-153. This highlights the importance of the ceria used
in a number of the above studies, which will be discussed later.
Iron has also been tested. In a series of studies it was found that iron could reduce
carbon deposition in quantities as low as 10%, in both Ni-Fe/La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM)154
and Ni-Fe/CGO155 anodes. One study compared Ni and Ni0.9Fe0.1 as supports in metal-
supported cells under humidified methane at 650 °C156. They found that while carbon was
deposited in both supports, the carbon in the Ni-Fe support was amorphous, did not retard
the rate of the methane reforming reaction in the support, and prevented carbon
deposition in the Ni-CGO anode layer (from SEM). In contrast, carbon on the Ni support was
highly graphitic, completely deactivated the reforming reaction and led to cell failure due to
carbon deposition in the anode in less than 10 h.
Cobalt, similarly to nickel, has known catalytic activity towards carbon-containing
compounds, so has been investigated in anodes. It seems promising for electro-oxidation of
CO, with alloys with Cu producing higher performance in syngas than an equivalent Ni or Cu
only cell157 and Ni alloys with Co producing higher exchange current densities in syngas than
37
in hydrogen158. Cobalt is expected to have less tendency to carbon deposition than nickel,
but in tests where nickel is entirely replaced with cobalt in a YSZ cell, carbon deposition was
still observed after 15 hours in dry methane. No performance loss was observed however,
indicating that the carbon is not poisoning the activity of the Co/YSZ cell, although it could
still eventually cause structural failure159. Under syngas, cells based on Ni-Co alloys became
completely delaminated in CO:H2 ratios above 60:40, indicating that Ni-Co alloys are still
vulnerable to carbon deposition158.
In a catalytic study, Co-Cu/YSZ+CeO2 with a 50:50 mix of Co and Cu produced by
impregnation into YSZ showed very little carbon deposition after exposure to a 2:1 CH4:O2
mixture for 20 h at 800 °C, much lower than a comparative Ni-Cu sample151. A similar study
conducted with dry butane found that the amount of carbon deposition increased with
increasing metal loading, indicating that the metal is still encouraging the formation of
carbon, despite the lack of nickel160. This carbon was amorphous, and did not cause any
short term degradation of the anode performance, although metal particles were seen
encapsulated in the carbon fibres formed, indicating that the carbon deposition would cause
long term disruption of the anode structure.
Tin is another metal which has been used to reduce the tendency of nickel to form
carbon. Tin has the advantage that it alloys easily with nickel, and the tin segregates to the
surface of the particles, meaning that a large improvement in the stability in dry methane
and isooctane while under load can be achieved with only 1% of tin with respect to nickel161.
The effect of 1% tin in reducing carbon deposition was also seen in ethanol-fuelled SOFCs162.
There has been some debate about the role and effect of tin. One study replicated
some of the testing conditions in reference 161 as well as other conditions with dry and wet
methane at different temperatures, but failed to observe improvements in carbon tolerance
38
under most conditions163. They ascribed this to their use of electrolyte-supported cells
(compared to anode-supported cells in reference 161). Further work by the group suggested
that the tin appears to cause the formation of less stable carbon species, meaning that any
carbon deposited in the electrochemical region is oxidised, but that carbon can still form
outside of this region164. TPO experiments agree that the stability is due to a reduced rate of
graphitic carbon formation rather than total elimination of carbon formation, and that
keeping the cell under load is still necessary165. Another paper suggests that hydroxyl groups
formed at the tin atoms on the surface are responsible for the effect (figure 14)166, while a
study using DFT and microreactor tests showed that the effect is due to an increase in
formation energy of carbon nucleation sites with no increase in energy for CO formation167.
One further study failed to show any improvement when using 1% tin, with increased
carbon deposition on Ni-CGO in microreactor tests on humidified butane, which they
ascribed to a low operating temperature of 600 °C133.
Figure 14 - Polarization curves and power densities of (a) Ni–CGO and (b) Sn/Ni–CGO
anode-supported single cell SOFCs operating at 650 °C with H2 and CH4, and (c) their voltage
variations measured at 650 °C in CH4 as a function of time. Reproduced from Reference 166
with permission of The Royal Society of Chemistry.
6.4 Replacement of nickel with non-metal electronic conductors
39
It is also possible to use electronically-conducting non-metals, and these should have
intrinsically less tendency towards carbon deposition. They can also have the advantage that
nanostructured catalysts, especially precious metal catalysts, can be used without the loss
of activity or function caused by alloying with base metals like copper. The carbon deposited
due to hydrocarbon cracking is conductive, and one study exploited this. Porous YSZ scaffold
was exposed to dry butane, depositing a conductive carbonaceous layer. This was then
impregnated with ceria and/or noble metals to improve the catalysis. Pd showed the best
activity out of Pd, Pt or Rh in these cells 168 (Figure 15). The cells’ performances in butane
showed a much smaller improvement through adding a catalytic metal, which was
suggested to be due to saturation of carbon on the active metal surfaces169.
Figure 15. Potentials (open symbols) and power densities (closed symbols) as a
function of current density at 973 K for H2 (diamonds), n-butane (triangles), and CH4
(circles). In (A), the cell had a C-ceria-YSZ anode; in (B), the anode also contained 1 wt% Pd.
Reproduced by permission of The Electrochemical Society from Electrochem. Solid-State
Lett. 2003, 6 (11), A240-A243. Copyright 2003, The Electrochemical Society.
A longer term test of the Pt/C-CeO2-YSZ cell in dry methane showed a 15% drop in
performance over 100 h. The impedance spectra showed an increase in the Ohmic
resistance, so the loss in performance was attributed to a loss in carbon. An earlier paper by
the same group had shown that for a Cu/C-CeO2-YSZ cell, the OCV in C4H10 settles over time
40
to a value of 0.85 V, implying that an equilibrium is reached between partial oxidation
products170. The authors also observed gradual changes in the performance under load,
implying changes in the carbonaceous layer over time. These results taken together suggest
that the carbonaceous layer will reach an equilibrium over time depending on the fuel,
oxygen flux, presence of catalytic metals and other factors.
A combined thermodynamic and experimental investigation looked at the stability of
a range of electronically-conducting carbides, borides, nitrides and silicides in humidified
hydrogen with a partial pressure of CO of either 10-1 or 10-6 at 950 °C64. Of these, only the
tungsten carbides and molybdenum carbides were stable, and then only at the higher
concentration of CO. Since carbides should have an intrinsic carbon tolerance, as well as
having been investigated in catalysis for various reforming reactions, this marks them out as
potential anode materials. Despite this, tungsten carbide has only recently been
investigated in actual anodes, in a WC-YSZ anode171. The performance with pure WC-YSZ
was poor, but could be improved several times by impregnation of a Ru-CeO2 catalyst. The
cell was stable under dry methane, with low carbon deposition which was not detrimental
to the performance, but careful balancing of the fuel utilisation is required to prevent
oxidation of the WC. A follow-up study tested fuel cells in humidified methane and
methane-hydrogen mixtures, with maximum power densities of ~80 and ~250 mW/cm2
respectively at 900 °C with a 300 µm YSZ electrolyte172. In a further study, the Ru was
replaced with Ni, and this cell showed stable performance over a week under humidified
methane at 850 °C with no evidence of carbon deposition from SEM173. Removal of the ceria
reduced the performance, but the high stability remained, indicated that it is the tungsten
carbide which is preventing carbon deposition on the nickel particles.
41
Molybdenum carbide has been tested in a proton-conducting cell with a
BaCe0.7Zr0.1Y0.2O3 − δ (BCZY) electrolyte operating on ethane174. The carbide was stable and
showed very low levels of carbon deposition both in the cell and when exposed to ethane as
a powder as assessed by thermogravimetry and XPS. Under hydrogen at 0.55 V there was
degradation of 5% over 100 h which was attributed to reduction of the carbide to metallic
molybdenum.
One study looking at molybdenum as a dopant in a Ni-YSZ anode observed extremely
good performance under humidified methane in steam reforming activity, low carbon
deposition and fuel cell tests, especially in materials which were reduced in the humidified
methane rather than in hydrogen. This was suggested to be due to the formation of highly
active molybdenum carbide, but no further investigations were conducted to test this
hypothesis125.
The ability of tungsten and molybdenum to form carbides means they may be able
to Tungsten and molybdenum have also simply been used as promoters in a similar way as
the other base metals described above. In a combined mass spectrometry-
thermogravimetric study, Mo-Cu, W-Cu and Cu doped Ni-YSZ were exposed to dry methane
at 800, 650 and 500 °C. The samples were produced so as to retain Mo and W in their
metallic state. The Mo and W doped samples showed improved tolerance to carbon
deposition, which may have been due to the formation of carbides175.
6.5 Increasing alkalinity
A third strategy involves increasing the basicity of the material, especially by using an
alkaline earth. This strategy is known to reduce carbon deposition in conventional catalysis.
All of the alkaline earths have been tested except Be, which can be highly toxic. They have
strong basicity, and this modifies the electronic state of nearby nickel to make it less active
42
for carbon deposition176. In the particular case of MgO, (Ni,Mg)O solid solutions are formed,
from which the nickel can be reversibly exsolved177. This may help when regeneration is
required.
Microreactor tests using Ni/YSZ cermets doped with small amounts of MgO, CaO and
SrO (0.2, 1 and 2% of anode mass) showed that CaO and SrO suppressed carbon deposition
even at the lowest loadings, while MgO increased the rate of carbon deposition176. A
separate study looking at the same elements plus BaO and La2O3 made similar findings, but
also noted large changes in microstructure and conductivity depending on which promoters
were used, highlighting the need to take into account all the effects of promoters178. They
found that CaO-promoted cells had the highest performance in humidified methane, due to
a combination of good steam reforming activity, high conductivity and low carbon
deposition.
The alkaline earths can also dissociatively uptake water, which is then able to oxidise
nearby carbon. First principles studies indicate that BaO is the best alkaline earth for water
adsorption58 and its effect is shown by a study on the effect of Ba on Ni/YSZ anodes fuelled
with dry propane179. Using thermogravimetic analysis (TGA) and Raman spectroscopy they
observed water incorporation (weight gain and O-H stretching modes) in the anode. This
water uptake may assist the oxidation of carbon on the Ni particle and was further
evidenced using DFT calculations179.
This type of carbon elimination process occurs preferentially at the BaO/Ni
interfaces. The catalyst works synergistically: the water splitting takes place on barium
oxide, the carbon deposition occurs on Ni sites of BaO/Ni and the subsequent steps occur
near the BaO/Ni interfaces. Figure 16 summarizes the proposed mechanism for carbon
mitigation in a BaO/Ni-YSZ composite anode of a fuel cell fed with propane. A combined
43
microreactor and fuel cell study on Ni-Cu/CGO anodes doped or not doped with Ba showed
that the Ba does not reduce the rate of carbon deposition in microreactor tests in dry
methane, but does reduce the rate of carbon deposition in fuel cell tests under load180. It
was suggested that Ba assists in oxygen transfer from the electrolyte to the metal surface,
although other explanations were not ruled out. Impregnating Ni-CGO with BaO also greatly
reduces carbon deposition in humidified CO181.
Because of the mechanisms of carbon suppression of the alkaline earths, the
nanostructure of the anode plays a large role in the results for these elements as the oxide
must be very near to the nickel without completely covering it. Incorporation of CaO by solid
state methods was found to decrease the performance of the cell176, while doping with Ba
by impregnation was found to eliminate carbon deposition while only lowering power
density by around 10%182. Experiments using vapour deposition of Ba on Ni/YSZ showed
remarkable stability in dry C3H8 with a sustained power output of 0.4 W/cm² over 100 hours
compared to complete deactivation after less than 1 hour for a cell without Ba179.
Figure 16 Proposed mechanism for water-mediated carbon removal on the anode
with BaO/Ni interfaces. Large balls in bright blue, green, red, blue-grey and purple are Ni,
Ba, O of BaO or YSZ, Zr and Y, respectively, whereas small balls in red, white and grey are O
44
from H2O, H and C, respectively. D1 is the dissociative adsorption of H2O, whereas D2 is the
dehydrogenation of hydrocarbons or the CO disproportionation reaction. TPB is the triple
phase boundary. Reprinted by permission from Macmillan Publishers Ltd: Nat. Commun. 2,
357, copyright 2011
An expansion of this technique has been to use Ba-containing proton conductors,
which have the dual ability to store water and provide some ionic and electronic
conductivity. Impregnation of yttrium-doped barium zirconate (BYZ)183 reduced carbon
deposition at the same time as improving the performance, but only if the BYZ was present
in the electrochemically active zone. This indicates that the improvement may be due to the
increased electrochemical oxidation activity. A DFT study on Ni on yttrium-doped barium
cerate (BYC) or YSZ indicated that the termination of the surface of the oxide is important –
BaO-terminated surfaces adsorb water much more strongly than ZrO2 or CeO2-terminated
surfaces, and are thus more able to oxidise carbon at the triple phase boundary58.
The water storage capacity of one material – Ni- BaZr0.4Ce0.4Y0.2O3 (BZCY) was actually
measured, and found to be four to five times higher than a selection of other anode and
catalyst materials184. These materials included Ni-BaZrO3, indicating that the water storage
capacity is not solely due to the barium, but may also have some contribution from the
proton or electron conductivity, the other elements present or, as mentioned above,
differences in the preferred surface termination. The Ni-BZCY showed much lower levels of
carbon deposited in microreactor tests at all temperatures and ethanol-steam mixtures, and
anodes based on Ni-BZCY were stable under ethanol-steam for 180 h at 750 °C, in contrast
to Ni-SDC and Ni-YSZ anodes which failed after less than 2 h.
BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) is a MIEC with proton conductivity, but on its own it
has poor performance due to low electronic conductivity185. When impregnated with metals
45
the performance improves markedly, and with Ni-Cu impregnated there is no carbon
deposition as measured by Raman under dry methane at OCV at 750 °C, while Ni-
impregnated cells showed no carbon deposition under humidified methane at OCV at 750
°C. Ni-BZYYb composites have also been used with ethanol as the fuel, in this case there was
significant carbon deposition but it was limited to the outside of the anode and was
amorphous in nature, indicating that these materials may also reduce the amount of
graphitic carbon when carbon deposition does occur186.
Lastly, an in situ Raman study on BaO and barium zirconates showed that, as well as
the water adsorption effect on carbon deposition, there was also a reverse Boudouard
effect in the barium zirconates, where CO2 adsorbed as CO32- ions was able to react directly
with deposited carbon at the triple phase boundary. This effect was not seen in BaO, as the
BaCO3 formed was too stable187.
In theory the alkali metals should also reduce carbon deposition, and this has been
shown for Li in reforming layers in SOFC anodes, where doping with Li or La and Li reduced
carbon deposition in the Ni-Al2O3 layer under an 11.5:1 CH4:O2 mixture. It should be noted
that in this study, Ca was more effective at reducing carbon deposition, but Li (on its own or
combined with La) also showed an improvement in the methane reforming reaction188. The
main concern with use of alkali metals, especially Li, is their volatility. One study used a Li-
ion conducting material, Li0.33La0.56TiO3, as the support rather than Al2O3, and found that this
improved long term stability189. The authors measured the lithium loss, and found that the
lithium content reduced from 4.68 to 4.63 wt% after 100 hours at 800 °C in air, compared to
0.42 to 0.20 wt% for a sample of Li-doped Al2O3. The fact that this study was carried out at
800 °C indicates that the volatility at lower temperatures may be less of a problem.
46
Other than the oxides of alkali metals and alkaline earths, there are a few other basic
oxides which have been tested. Under the conditions of an SOFC anode Mn occurs in the
form of MnO which is a basic oxide. Under wet methane at 800 °C, Ni-YSZ doped with 2 or
5% of the NiO replaced with MnO, the cells lasted less than 1 h, similar to the performance
of an undoped cell. With 10% MnO the cell showed dramatically improved performance,
with no degradation over 40 h190. Microreactor tests showed that the amount of carbon
decreased by over 150 times compared to undoped Ni-YSZ, and this was shown to be due to
a relative decrease in the rate of methane cracking compared to steam reforming.
Conversely, increasing the acidity can worsen carbon deposition. Adding 2.7%
aluminium oxide, an acidic oxide, to a Ni-YSZ anode reduced the amount of carbon
deposition in a simulated CH4-CO2 biogas mixture, due to an improvement in the dry
reforming rate191. However, when the amount of aluminium oxide was increased to 10%, the
carbon deposition was increased due to the increased acidity.
6.6 Use of ceria and other oxygen storage materials
The concept of oxygen storage materials was first used in three-way catalysts, where
a partially reducible oxide is able to supply oxide ions during periods of fuel-rich conditions,
and is reoxidised during fuel-lean conditions192. In SOFCs this may help to prevent carbon
deposition by increasing the rate of supply of oxide ions for oxidising carbon on the surface
of the anode. In reality, for SOFCs the only oxygen storage material of note is ceria and
doped ceria. Although there are potentially other oxygen storage materials relevant to
anode conditions, only one, MnO190, has been used (as discussed above), and the effect of
its oxygen storage capacity was implied rather than confirmed through experiments.
Ceria is particularly attractive due to its high ionic conductivity arising from the
creation of oxygen vacancies on its fluorite lattice when exposed to reductive atmospheres
47
becoming a mixed conductor193-195. These structural defects are known to improve the
oxygen mobility of surface and bulk oxygen of ceria resulting in an enhanced oxygen storage
capacity (OSC) which at the same time benefits the oxidation processes196. These boosted
redox features might be useful to eliminate C and S adsorbed species via oxidation and
release them as CO2 or SO2.
The oxygen vacancies population and consequently the ionic conductivity of ceria
may be enhanced using promoters196-198. In particular, acceptor-dopants (e.g., Sm2O3 or
Gd2O3) are used to substitute some cerium ions in the fluorite structure resulting in the
formation of oxide-ion vacancy site to compensate the charge-balance. Furthermore, the
well-known activity of ceria based catalysts for soot combustion in automobiles makes this
material interesting for SOFC anodes194, 199. For instance, Gd-doped ceria mixed oxide was
employed for methane and hydrogen oxidation exhibiting high current density and good
tolerance towards carbon deposition200.
Initial work used impregnation to introduce ceria into porous Ni-YSZ anodes. It was
reported by some papers to eliminate carbon deposition while using dry methane201, but
others disagree, showing deactivation over only 30 mins.152 Doped cerias can also be
impregnated. Doped cerias are oxide ion conductors, and have some electronic conductivity
under the conditions in a fuel cell anode. This has the effect of extending the triple phase
boundary region, which outweighs the fact that pure ceria is a better catalyst for direct
oxidation than doped ceria153, 202. Impregnation of samarium-doped ceria (CSO)
nanoparticles into a Ni-YSZ electrode produced a cell with stable performance under dry
methane over 1000 h, which was attributed to suppression of nickel sintering and carbon
deposition observed in separate catalytic reactions with methane-air mixtures.203
Impregnation of gadolinium-doped ceria (CGO) into anodes of nickel/scandia-stabilised
48
zirconia (Ni/ScSZ) showed relatively stable performance under humidified methane,
although carbon deposition could still be observed, and had the effect of improving the
performance initially (due to formation of conducting carbon networks), before eventually
degrading.204 It should be noted that Ni/ScSZ cells without CGO also showed relatively stable
but inferior performance, indicating that the main effect of the CGO was to improve
performance rather than reduce carbon deposition.
The ceria-zirconia system is well known in catalysis for its high oxygen storage
capacity as the seven coordinate zirconium ions serve to stabilise Ce3+. Ce0.9Zr0.1O2 was
impregnated into Ni-YSZ anodes and was found to greatly reduce carbon deposition in
methanol at OCV, and almost eliminate it under load, as measured by EDX and TPO205.
Cu/CeO2-YSZ anodes show performance in cell tests under dry methane close to that
of Ni/CeO2-YSZ, but with no carbon deposition152. Importantly, the ceria needs to be
impregnated before the Cu, showing that the catalysed step is the oxidation of
hydrocarbons on ceria using oxide ions from the electrolyte.206 The non-catalytic nature of
the copper was reinforced by a study which showed that the replacement of copper with
gold showed very little change in performance207.
These results indicate that ceria is active towards electrochemical oxidation, while
copper simply acts as a current collector, meaning that the use of ceria allows the complete
replacement of Ni with Cu. The advantage of using copper rather than nickel is that copper
does not catalyse carbon formation, but the low melting point of copper oxide (1326 °C)
means that traditional electrode fabrication methods cannot be used. The above papers use
impregnation of copper nitrate into porous YSZ substrates. To improve the conductivity of
this type of cell, Fe was added to Cu/CeO2-YSZ anodes, causing carbon deposition, which
49
initially improved the performance by improving the conductivity before causing it to
decline slowly208.
Cells based on Cu produced by impregnation may be limited by their electronic
conductivity at low Cu contents. In this case, small (<2 wt%) amounts of carbon deposited
from exposure of the anode to dry butane were found to improve performance, again
because of an increase in electronic conductivity170. The rate of carbon deposition was the
same on YSZ and Cu/YSZ-CGO, implying that Cu and CGO are not catalysing the carbon
deposition. In addition, oxidation and re-reduction returned the cell to its original
performance, suggesting that the carbon deposits caused no permanent changes in the
structure of the anode.
Further improvements to these Cu/CeO2-YSZ anodes can be made by using CSO
rather than pure ceria209. The developed system was suitable for several type of fuels and
conserves high power densities after switching from one fuel to another. Figure 17 presents
the effect of switching fuel type on the cell with the Cu-(doped ceria) composite anode at
973 K. As shown in the plot, 1-butene and ethane leads to the higher power density while
toluene generates a current sensitive current drop. Toluene as an aromatic compound
increases C formation, however they observed that the anode was self-cleaning upon
switching to n-butane. Use of a porous doped ceria interlayer can also reduce carbon
deposition with humidified methane as fuel210.
50
Figure 17. Effect of switching fuel type on the cell with the Cu-(doped ceria)
composite anode at 973 K. The power density is shown as a function of time. The fuels were:
n-butane (C4H10), toluene (C7H8), n-butane, methane (CH4), ethane (C2H6), and 1-butene
(C4H8). Reprinted from Nature 2000, 404 (6775), 265-267, with permission from Nature
Publishing Group.
Since doped cerias are oxide ion conductors in their own right, it is possible to
dispense with the YSZ altogether. Cells produced using Ni-CGO synthesised via a Pechini
method showed no carbon deposition from Raman under humidified methane at 600 °C for
50 hours, although it should be noted that the cells, which used a 20 µm CGO electrolyte,
showed extremely low OCVs, and no attempt was made to find out whether this low OCV
was due to oxygen leaks into the anode or to the non-zero electronic conductivity of CGO211.
High levels of carbon deposition were still observed under humidified propane. A Ni-CSO
anode was operated on dry methane at 600 °C for 72 hours under a current load of 300 mA
with very low levels of carbon detected by FIB-SEM and TPO post-test analysis, although
again thin CSO electrolytes were used and OCVs of ~0.9 V obtained212. Ni on Mo-doped ceria
showed less than 0.04 wt% carbon deposition after exposure to a methane-oxygen mixture
(5:2 molar ratio)213. Cells based on this material using 400 µm LSGM electrolytes showed
51
reasonable stability over 10 h under load and wet methane, although unfortunately the
amount of carbon deposition was not quantified.
There is strong evidence that CSO has high enough electronic conductivity under
hydrogen that the limiting factor is not the triple phase boundary length but the surface
area of the CSO, intimating that an optimal strategy may be to optimise the surface area of
the ceria and improving the catalysis for hydrocarbon oxidation, utilising the minimum
amount of current collecting metal necessary214. This electronic conductivity, along with the
reforming activity of CGO, has been used in the current-collecting layer, where CGO-coated
Ni was used at the top of the anode to reduce exposure of Ni to unreformed methane. The
cells were stable in dry methane at 610 °C over 1000 h, compared to cells without the CGO-
coated Ni layer which failed after <200 h215.
While work on metal-ceria composites has understandably focused on the doped
cerias with the highest ionic conductivities (CSO, CGO etc.) some work has been done on
materials with higher oxygen storage capacities. Ce0.9Zr0.1O2-based (CZO) impregnated
anodes were found by EDX to reduce carbon deposition in humidified methane compared to
CeO2-based anodes216. A larger effect was seen by replacing Ni with Cu, but unlike
replacement of CeO2 with CZO this had a large negative effect on performance for total
replacement. A partial replacement of Ni with Cu on CZO was found to be the best
compromise between carbon tolerance and performance.
A further advantage of ceria and doped ceria is that the methane cracking reaction is
extremely slow. Undoped ceria or ceria doped with varying amounts of Nb or Gd showed
between 0.07 and 0.9 monolayer coverage of carbon after 150 minutes exposure to
methane at 900 °C217, compared to 142 monolayers deposited on Ni/YSZ at the same
temperature218. The electrochemical oxidation of hydrocarbons over doped ceria is still
52
relatively low however219, with higher activities caused by the activity of either Pt current
collectors or Ni towards steam reforming. In theory, cells based on Cu and ceria could be
doped with noble metals to improve their activity, but the noble metals alloy with Cu
forming less active phases169.
6.7 Replacement of cermets with mixed ionic-electronic conductors (MIECs)
6.7.1 Single phase MIECs
At the time of the resurgence in interest in SOFCs in the late 1980s, a concurrent
area of interest was direct hydrocarbon oxidation catalysts, for removal of hydrocarbons
from car and power plant exhausts. It had been established that a number of oxides were
active towards this reaction, and they came to the attention of groups working on SOFCs,
with particular attention paid to the Perovskite family of oxides.
Perovskites are defined as a family of materials, which present the same structure as
the face-centered cubic calcium titanium oxide CaTiO3. The structure of these compounds of
general formula ABO3 may be described as a combination of the oxygen and A-site cations
that form the cubic close-packed (ccp) framework, the oxygen atoms occupy three quarters
of the sites of the cubic close packed layer and the A-site cation, the larger one, the
remaining quarter. The B-site cations occupy one fourth of the octahedral holes of the ccp
arrangement. This structure can be also viewed as the B-site cations occupying the center of
the cubic structure while A and O ions are located at the corners and half edges,
respectively. Perovskites have a high degree of structural and electronic flexibility, with
many different elements and oxidation states able to be incorporated into the structure.
The A-site cation can be a low valence rare earth, alkali or alkaline earth ion, for example La,
Na, Ca, Sr or Ba, while the B-site is a transition metal, such as Ti, Zr, Fe, Co, Ni or Cu. Both
sites are able to accept multiple different ions simultaneously, and this produces
53
possibilities for variable oxidation states220-221. In addition, if there is more than one different
element occupying the B-site, these can become ordered, and Perovskites displaying this
behaviour are known as double Perovskites (materials with only one B-site element or two
or more unordered B-site elements are single Perovskites). Their defect chemistry gives the
potential for them to exhibit MIEC properties under a wide range of partial pressures of O2
at elevated temperatures199.
Early work centred on perovskites of lanthanum with top row transition metals, for
example La0.8Sr0.2FeO3 (LSF), nowadays more familiar as a cathode material, which while it
showed better activity than Pt electrodes and no carbon deposition under dry methane, was
not stable under relevant anode overpotentials (<-0.3 V)222-223. Attention quickly focused on
substituted lanthanum chromites which were already used as interconnects in SOFCs due to
their stability in very low pO2224, despite the fact that the base material exhibited one of the
lowest activities for methane oxidation225.
While lanthanum chromites are not expected to catalyse carbon formation to the
same extent as nickel, carbon deposition has been observed. When exposed to dry methane
in a fixed bed catalytic reactor, at temperatures above 600 °C calcium-doped lanthanum
chromite was observed to catalyse methane decomposition, resulting in an average of half a
monolayer coverage of carbon, compared to 112 for Ni/YSZ under the same conditions226.
While this amount of carbon is small, it was found to have a deleterious effect on the
catalytic reactions. Addition of 3% steam to the 5% methane feed prevented this carbon
build-up. A study of various strontium and manganese-doped lanthanum chromites (LSCM)
containing varying amounts of Cr and Mn found that larger amounts of carbon deposition
for Cr-rich compounds and/or exposure to methane at higher temperatures were linked to
lower selectivities towards the total oxidation of methane relative to partial oxidation227.
54
Due to the low activity of lanthanum chromites, the immediate focus for
improvement was on the activity, rather than on further reducing carbon deposition.
Notwithstanding this, several authors did measure the tendency of doped lanthanum
chromites towards carbon deposition. One such study tested various first row transition
metal dopants to improve the activity as well as alkaline earth dopants to improve the
conductivity. All dopants produced carbon deposition of less than four monolayers at 800
°C. The exception was the Fe-doped material under conditions representing internal
reforming, which produced 69.4 monolayers of carbon228, which is similar to levels which
would be expected from a nickel-based cermet218. Materials which produced no carbon
under any conditions were the Sr and Mg double-doped material and the Co-doped
material. The two Ni-doped materials (singly doped and co-doped with calcium) surprisingly
showed no extra carbon deposition compared to most other dopants, but did produce
considerably higher conversions – between 3 and 5 times higher than any other materials
for the reactions representing partial oxidation and dry reforming, and 1 – 2 orders of
magnitude higher for the reaction representing steam reforming. In fact, although not
known at the time, it is likely that the nickel-doped samples were producing nickel metal
nanoparticles under reducing conditions, which helps explain the vastly improved
catalysis229. This is discussed further below in section 5.7.2.
Composites of LSCM with doped ceria show better activity, as well as increased
carbon deposition. In one study, carbon deposition after exposure to dry methane for 6 h at
750 °C increased from less than 0.1 wt% in pure LSCM to 1.5 wt% in 33 wt% LSCM:67 wt%
lanthanum-doped ceria230. An increase in the amount of the doped ceria improved
performance in fuel cell tests in methane, although above 50 wt% ceria the performance
dropped, probably due to lower electronic conduction. Doping lanthanum chromites with
55
ceria may also help. Iron-doped lanthanum chromite co-doped with 5% Ce showed much
lower carbon deposition at 800 °C in syngas in a microreactor, while symmetrical cells
showed less drop in performance under the same conditions and were able to be
regenerated by 24 h under load in H2231.
Other perovskite-based anode materials have been tested, for example lanthanum
aluminates232, and barium titanate233, but the most studied single perovskite other than
lanthanum chromite is strontium titanate, which is stable and when doped is a MIEC under
reducing conditions. To induce electronic conductivity, the base material can be doped with
La3+ on the A-site (known as LST)234-235 or Nb5+ on the B-site236, with the stoichiometry
controlled to produce either Ti3+, Nb4+ or oxygen deficiency or excess, meaning that this
system is compositionally very flexible. A possible hindrance to using this material is the high
temperature (>1000 °C) reduction needed to induce a suitable degree of electronic
conductivity, and the fact that this conductivity is lost under oxidation. LST does possess
very low propensity towards carbon deposition, with less than 1 wt% of carbon deposited
after 6 h under dry methane at 800 °C in a microreactor237. Composites of LST and CGO in
the same study showed increased carbon deposition, although still less than 2 wt% carbon
with 40 vol% CGO. The increase in carbon deposition was related to the greater degree of
interaction between CGO and methane, but in fuel cells the carbon was not shown to have
any detrimental effect on the performance, with a direct correlation found between
polarisation resistance in the impedance spectrum and propensity towards carbon
deposition in the microreactor tests.
More recently Sr2MMoO6 (where M is a small 2+ cation such as Mg or Ni) double
perovskites (perovskites in which the B-site cations are ordered) have also been used as
SOFC anodes. The ordering occurs as a consequence of the very different charges on the B-
56
site cations, but currently no advantage for carbon or sulfur tolerance of using a double
perovskite rather than a single perovskite related specifically to this ordering has been
suggested. Sr2MgMoO6 showed good activity for CH4 oxidation and stability under short
term testing of 15 hours238. The power density dropped under wet CH4 compared to dry CH4,
indicating that direct oxidation was the main route for CH4 conversion. Materials where the
Mg was partially or fully replaced with Mn performed worse, with a power density of 838
mW/cm² for the pure Mg sample reducing to 650 mW/cm² for the pure Mn sample.
Materials using Co or Ni rather than Mn showed similar performance decreases compared
to the Mg sample239. Co was seen to exsolve from the perovskite as Co metal, although
initial performance was similar. Co and Ni showed different catalytic behaviours – Co acted
mainly through steam reforming, with low performance in dry CH4, while Ni showed no
steam reforming activity. It is important to note that all the above studies were carried out
with Pt current collectors and a doped ceria barrier layer, which later work has suggested
could be responsible for most of the methane oxidation240. A study doping the Mo site with
Nb which did not use a barrier layer or Pt current collector agreed with this poor activity
towards methane oxidation, and suggested that similarly to other MIECs studied, the
catalysed reactions between the methane and the MIEC were likely to be limiting in pure
MIEC-based systems241. The study did find that the amount of carbon deposited was very
low, however this would be expected from a system with poor activity towards methane
conversion.
Due to the structural flexibility of perovskites, they are able to form reduced
compounds while maintaining the perovskite structure, and one promising material which
illustrates this is the A-site layered double perovskite PrBaMn2O5+δ (PBMO)242. This material is
stable across a good pO2 and temperature range, and unlike many of the perovskites
57
described above appears to have some activity towards hydrocarbon oxidation (with silver
used for current collection). Although this material was not tested for carbon deposition or
stability under operation in hydrocarbons, a calcium-doped version, PrBa0.8Ca0.2Mn2O5
(PBCMO) was, and was stable for 50 h in humidified iso-octane followed by 150 h in
humidified propane, with currents of 0.2 – 0.3 A/cm² achieved at 0.6 V at 700 °C243.
6.7.2 Addition of catalytic metal nanoparticles to MIECs
Since MIECs by definition are electronically percolating, a percolating metal phase is
not necessary, but dispersed metals can still be added to promote the catalysis. However,
these metal nanoparticles can also be prone to carbon deposition. Impregnation of metal
salts (typically nitrates) into the anode is a technique borrowed from catalysis, where it is an
extremely widely-used method for producing catalysts. Ni, Pd and Ni-Pd were added to Sr-
doped LaCrO3, with Ni-Pd showing a synergistic effect for methane oxidation in dry
methane, with little or no carbon deposition observed using a carbon balance approach
during testing at 0.5 V and 800 °C244. A short period at OCV was sufficient to completely
deactivate the electrode towards methane decomposition, while returning the cell to 0.5 V
could only recover 60% of the activity. Addition of hydrogen to the cell was necessary to
fully reactivate the cell through methanation of the carbon.
Further work on Ni-Pd and Pd nanoparticles dispersed on an LSCr-CSO anode
suggested that the reaction mechanism for the oxidation is fundamentally different
comparing Ni-Pd alloys with pure Pd particles245. The work suggested that the reaction on Pd
was close to direct electrochemical oxidation, while the reaction on Ni-Pd alloys was likely
through methane cracking followed by electrochemical oxidation of hydrogen, steam
reforming of carbon and electrochemical oxidation of the CO produced. These alloys were
found to be resistant to carbon deposition, and it was proposed that doping of Ce, Sr, La or
58
Sm into the alloy was preventing the formation of carbon fibres, as highlighted by the fact
that the Ni-Pd nanoparticles contained trace amounts of these elements246.
Improvements to the stability of Pd nanoparticles can be achieved by impregnation
of Pd-core/CeO2-shell nanoparticles, which are able to operate on dry methane without
carbon deposition and survive heat treatments in air up to 900 °C with only 9% loss in
performance compared to 40% loss in performance with impregnation of just Pd247.
These results highlight some advantages of using a MIEC combined with dispersed
metal particles compared to cermets:
although the cell can still be deactivated through carbon deposition under
certain conditions, since the metal is not load bearing, complete structural failure does not
occur and the cell can be regenerated.
in cells based on cermets, the only economically feasible method for adding
expensive elements such as Pd is via impregnation into an already formed cermet anode.
This results in segregated Pd and Ni particles, which rules out this synergistic alloying effect
and does not prevent carbon deposition in Pd poor regions130.
An interesting approach towards decoration of MIECs with catalytic nanoparticles is
exsolution, where a reducible metal is incorporated into the oxide structure during
synthesis, and exsolved forming remarkably stable nanoparticles under reducing
conditions248. A feature of these systems is that the nanoparticles can be cyclically
readsorbed and exsolved from the structure. This method has the advantage compared to
traditional impregnation that it produces stronger particle-support interactions and so less
sintering occurs and the particles are more stable. There are two main disadvantages
compared to impregnation: higher reduction temperatures and less control over the
59
composition of the particles – currently there are no reports of alloy nanoparticles
deliberately produced by exsolution.
Exsolution was first (deliberately) tested in SOFC anodes with Ru-doped LSCM249. The
Ru exsolved forming particles up to 5 nm in diameter over 50 hours under hydrogen at 800
°C, doubling the cell performance and reducing the polarisation resistance by a factor of
three. Only 15% of the Ru was found to have exsolved, and the authors suggest that this is
due to a combination of slow diffusion and energetic barriers towards removal of too much
Ru from the perovskite structure. Later studies found that the exsolved Ru particles acted to
hinder carbon deposition on the LSCM250. Aside from improving the performance of cells
running on dry ethanol, it was found in fixed bed tests that carbon deposition was
eliminated compared to around 1 wt% for the Ru-free material250. Co particles can also be
exsolved, and these showed <1% weight gain due to carbon deposition when exposed to dry
methane for four hours, compared to >100% weight gain for Co/CeO2 prepared by
impregnation251.
As mentioned previously, nickel doped into LSCM can exsolve out as nickel
nanoparticles, and depending on the particle size produced these can be resistant to carbon
deposition. Pulse reaction studies on Ni-doped LSCM indicated that essentially all the
methane was converted to carbon dioxide until oxygen stoichiometries below 2.7, where
the methane conversion continues to increase despite CO2 conversion reducing, indicating
that methane decomposition (and consequent carbon deposition) was taking place229. This
was considered to be due to the greater degree of nickel exsolution implied by lower oxygen
stoichiometries and potentially larger particle size. Co particles were also found to exsolve
from Co-doped LSCM, and it was found that exsolved Ni and Co particles have a large effect
on the methane oxidation rate with only a small increase in the rate of carbon deposition
60
compared to LSCM. Exsolved Ni showed far better carbon resistance than impregnated Ni252
(Figure 18).
While exsolved Ni and Co can still show raised levels of carbon deposition, addition
of Cu to form nano-alloys can mitigate this. Ni and Co exsolved from Ce0.8(Co,Ni)0.2VO3
showed significant amounts of carbon deposition on exposure to dry methane at 700 °C,
with 10% weight gain caused by carbon deposition for Co and 27% for Ni, compared to <1%
for the undoped material.253 However, double doping Cu and Co reduced the weight gain to
2%254. Double doping Cu and Ni did not reduce the amount of carbon deposition, probably
because the 50:50 mix of Cu and Ni used is still prone to carbon deposition as discussed in
section 5.3.
Likewise, LSC double-doped with Ni and Fe showed less carbon deposition after
exposure to syngas at 850 °C than singly-doped Ni-LSC. Singly-doped Fe-LSC showed less
carbon deposition than either, but performed worse in fuel cell tests, while Ni-Fe doubly-
doped cells performed best255. XRD and SEM analysis showed that the exsolved Ni and Fe
formed alloy particles of around 25 – 30 nm. The promising symmetrical electrode material
Pr0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ (PSCFN) forms Co-Fe nano-alloys at 900 °C under hydrogen, with
stable performance for 50 h under dry methane and 100 h under dry butane at 800 °C256.
Microreactor tests on reduced PSCFN showed considerable carbon deposition (30 wt%)
under methane at 850 °C and also high activity for methane cracking257. The stable
performance under methane could be explained by the fact that on initial exposure to
methane CO2 (and presumably water) is produced rather than hydrogen, indicating that
there are species active for methane oxidation. In addition, the carbon was able to be
oxidised at 450 °C, implying that it was dispersed and amorphous, and therefore may be
oxidised by oxygen flux under SOFC anode conditions.
61
Figure 18 - Carbon production rates averaged per pulse for LSCM, LSCMCo, LSCMFe,
LSCMNi and LSCM+Ni. Note that the latter is shown on the right axis. Reprinted with
permission from Chem. Mater. 2010, 22 (21), 5856-5865. Copyright 2010 American
Chemical Society.
The exsolution of nanoparticles could be limited by energetic barriers towards
removing B-site cations from a stoichiometric perovskite. This can be combatted by
synthesis of A-site deficient materials, which allow B-site cations to be removed much more
efficiently, allowing even metals such as iron to be exsolved from LST258. Control of the
stoichiometry in this way also allowed Ni metal and CeO2 to be exsolved from lanthanum
cerium titanate259. Exsolution of Ni from Ni and Ce double-doped LST was found to greatly
62
reduce the amount and ease of removal of deposited carbon in microreactor tests in
methane compared to Ni-doped LST260.
SrMoO3 is a MIEC stable under anode conditions, but doping with Ca allowed Mo
nanoparticles to exsolve under reducing conditions. These particles had a small beneficial
effect under hydrogen, but under methane the Ca-doped materials allowed carbon
deposition, in contrast to the undoped material which did not261. While the formation of
carbon implies a greater ability to interact with methane, strangely both the undoped and
doped materials showed very low OCVs which indicates a lack of ability to convert methane.
Nanoparticles of oxides can also be produced through exsolution-type processes.
LSCF impregnated with nickel was used as an anode, where it decomposed into strontium
cobalt iron oxide perovskite with La2NiO4 finely dispersed over the surface262. The La2NiO4
was presumed to be the electrocatalytically-active phase, and the cells exhibited good
performance in dry propane with only a few carbon whiskers observed in the SEM after 100
h of use.
6.8 Regeneration of SOFC anodes deactivated by carbon
As can be seen above, carbon deposition can occur, to varying degrees, on all
materials so far studied. In some circumstances the carbon deposition is not detrimental to
performance, or can even be positive in small amounts as it can improve the electronic
conductivity of the anode. As the amount of carbon increases in an SOFC operating over
potentially tens of thousands of hours deleterious effects such as pore blocking and risk of
structural failure will inevitably increase, so it may be desirable from time to time to remove
this carbon. Clearly it is always possible to remove carbon by heating the cell to high
temperatures in air, but a number of studies have investigated the possibilities for removing
carbon without damaging the cell.
63
Kirtley et al. studied carbon removal from Ni-YSZ using 3% H2O, 10% O2 or 11% CO2 in
nitrogen, and found that the carbon was removed fastest in H2O and slowest in CO2, with
times ranging from 10 – 125 s263. Through examining the OCV and in situ Raman, the authors
were able to identify the stages of carbon removal. First the OCV increased to -0.99
accompanied by the disappearance of carbon peaks in the Raman. This was attributed to the
formation of a CO/CO2 gas mixture. This is followed by the appearance of NiO peaks in the
Raman and an OCV reflecting the thermodynamic equilibrium of the Ni/NiO couple in the
regenerating gas. O2 leads to complete oxidation of Ni to NiO, while H2O and CO2 lead to
partial oxidation. The above study induced carbon deposition from dry methane at OCV, but
carbon induced using diesel reformate under load was able to be removed and the cell fully
regenerated using dry and wet hydrogen, albeit over a time period of 44 h264. Regeneration
via this method was not possible under conditions where the cathode had also degraded,
indicating that the carbon is removed largely by oxygen flux through the electrolyte265.
It is theoretically possible to regain performance without changing the gas mixture
by moving from an operating regime where carbon deposition is favoured to one where it is
not. Ni-YSZ cells were found to completely regain their initial performance after 24 hours
under load at 850 °C in a simulated partial oxidation reformate feed, having previously had
carbon deposited in the same gas mixture at 650 °C under OCV266.
Symmetrical cells (where the electrode material used during fabrication is the same
for anode and cathode) offer interesting theoretical potential for regeneration, given that if
carbon deposition occurs they can simply be reversed, whereupon the deposited carbon will
be exposed to air and thus oxidised. In a recent review on symmetrical electrode materials,
the authors conclude that little work has been done on regeneration of these materials after
carbon deposition267.
64
Table 4 Selected papers reporting improved carbon tolerance in SOFC anodes
through materials strategies.
Noble metals
Metal Fabrication Performance Ref Comments
Au 1.5 mol% Au
deposited onto
Ni/CGO powder,
screen-printed onto
electrolyte
Tested under CH4-H2O,
850 °C, compared to no
Au: 0.15 V higher OCV,
0.2 V higher at 500
mA/cm² (both S/C = 3/2);
no V degradation under
dry methane vs. 0.3 mV/h
degradation under S/C =
1/2 (no Au); less C
deposition (visual)
127 Microreactor and
mechanistic studies
reinforce effect of
gold125-126, 128
Pd 0 – 0.15 mg/cm² Pd
impregnated into
slurry-painted
Ni/CGO electrode
Tested in wet CH4 and
EtOH, 800 °C, OCV,
compared to no Pd: Rp
decreases by 2x in CH4
and 4x in EtOH in
loadings above 0.07
mg/cm²; C deposition still
observed (SEM, EDX)
129 Microreactor tests
suggest carbon
suppression effect132
Ru 0 – 9 wt% RuO2
mixed into Ni/CGO,
Tested in wet CH4, 600 °C,
compared to no Ru:
131 Another paper
agrees that Ru has a
65
formed into a pellet
with ~30 µm CGO
electrolyte
current density increased
by 2x at 0.4 V; also, stable
performance for 20 h, no
C deposition (from
carbon balance) (not
compared to no Ru)
beneficial effect132,
but there are few
papers on Ru due to
problems with its
oxides’ volatility. Ru
doped in ceramic
anodes may be
more feasible249-250
Ag 0.9 – 2.5 wt% Ag
impregnated into
Ni/YSZ anode
supports, 20 µm YSZ
electrolyte
Tested in dry CH4, 750 °C,
0.3 A/cm², compared to
no Ag (at 0.6 A/cm²): 0.9
and 2.5 wt% Ag failed at
12 and 81 h respectively,
1.6% Ag showed no
degradation to 100 h.
Control failed at 5 h. Very
little carbon observed by
EDX
138 A further paper by
the same group
showed similar
results for C2H6137,
while microreactor
results also show C
tolerance136. The
high mobility of Ag
at ~600 °C and
above must be
noted.
Base metals
Metal Fabrication Performance Ref Comments
Cu 0 – 100% Cu
impregnated with Ni
(Cu + Ni = 20 wt%)
Powders and cells tested
in dry CH4: at 700 °C,
powders with Cu:Ni of
150 There are many
papers on Cu with
widespread
66
and CeO2 (10 wt%)
into 400 µm porous
YSZ support with 60
µm YSZ electrolyte
9:1 or 10:0 showed no C
deposition. 4:1 gave <0.1
g C/g. In a cell at 800 °C,
performance of 4:1
improved from ~0.1
A/cm² to >0.6 A/cm² over
500 h due to C
deposition.
agreement that it
reduces C
deposition. Activity
is poor so normally
CeO2 or doped ceria
is used153.
Fe Anode supports
were prepared from
Fe2O3, NiO and CGO,
powders (Fe:Ni
0:100 – 50:50 w/w),
CGO electrolyte
Cells tested in dry CH4,
650 °C, 0.2 A/cm²: Cells
Fe:Ni up to 30:70 gave
similar power densities
(~0.3 A/cm²), 50:50 gave
<0.2 A/cm². All Fe-
containing cells were
stable over 50 h at 0.2
A/cm², Ni only cell
stopped after ~12 h. No
carbon observed on Fe:Ni
10:90 by SEM after test
155 There is some
evidence that Ni:Fe
alloys at 10% Fe are
more active for
methane
oxidation155 and
reforming156. This
level of Fe gives
stable cells with a
variety of oxide ion
conductors154
Co Ni/YSZ and Co/YSZ
were prepared by
coprecipitation then
coated on a 500 µm
Cells tested in dry CH4,
850 °C, OCV: anodic
overpotential remained
stable over 15 h in Co
159 Other papers
confirm that Co is
less vulnerable to C
deposition, but still
67
electrolyte support anode, Ni anode failed. C
deposition still observed
in Co anode by SEM.
vulnerable158, 160.
Activity for CO
oxidation looks
promising157-158
Sn 1% Sn was
impregnated into
Ni/YSZ anode-
supported cells with
a 20 µm YSZ
electrolyte.
Cells tested in dry CH4
and C8H18-air mixtures at
740 °C and 0.6 V and 0.5
V respectively. Stable
performance was
obtained in both fuels
over 6 h (CH4) or 13 h
(C8H18). Ni-only cells
completely deactivated.
161 Despite some
papers showing
little impact of tin133,
163, the bulk of
papers studying
performance and
mechanisms in
SOFCs161-162, 164-167
and catalysts (see
section 8.1.2)
suggest that the
effect of tin is real.
Several papers have
examined 1% and
5% loading, with 1%
being the best.
Non-metal conductors
Phase Fabrication Performance Ref Comments
C Porous YSZ scaffold
impregnated with
Tested in dry CH4 and
C4H10 at 700 °C: Maximum
168 Several papers in
the early 2000s
68
10 wt% CeO2 and
optionally 1 wt% Pt,
Pd or Rh. ~4 wt%
carbon is then
deposited in dry
C4H10 at 700 °C, 100
µm YSZ electrolyte.
power densities of 0.1
W/cm² in C4H10 and 0.02
W/cm² in CH4, similar to
Cu/CeO2-YSZ cells.
Performance in all fuels
was greatly increased by
adding 1% Pd. A 100 h
test of Pt/C-CeO2-YSZ in
CH4 showed large
increase in Ohmic
resistance due to loss of
carbon.
looked promising
for this technique168-
170, however there
have been no
papers since by this
group or others.
WC Porous YSZ scaffold
impregnated with
25 vol% WC, then 5
wt% CeO2 and 5 wt
% Ni
Tested in humidified CH4
at 850 °C at OCV: no
carbon observed visually
after 36 h; at 0.7 V stable
performance of 50
mW/cm² over 24 h with
no carbon observed
visually.
173 This strategy is quite
unexplored, but the
ability of WC to
protect Ni from C
deposition could be
interesting.
Increasing alkalinity
Phase Fabrication Performance Ref Comments
BaO Vapour deposition
of BaO onto an
Tested in dry C3H8 at 750
°C at 0.5 A/cm²: Cell
179 While CaO and SrO
also reduce C
69
anode-supported
NiO/YSZ cell with 15
µm YSZ electrolyte
voltage stable at 0.8 V for
over 100 h compared to
BaO-free cell which failed
after <1 h. Similar results
for wet CO and gasified
carbon. No C deposition
observed by SEM.
deposition176, 178 BaO
appears to be the
best prospect.
Microstructure
appears to be vitally
important.
BZCY Sol-gel synthesis of
NiO/BZCY
composites co-
pressed with CSO to
form an anode-
supported cell with
20 µm electrolyte
Tested in wet C2H5OH at
600 °C at 0.3 A/cm².
Voltage stable at 0.75 V
for 180 h compared to
Ni/YSZ and Ni/CSO which
failed after <2 h due to C
deposition. No carbon
detected or morphology
changes detected by SEM
after testing.
184 Numerous Ba-based
perovskites have
now been tested
including BYZ183,
BYC58 and BZCYYb185.
The efficacy of these
perovskites seems
clear, and
microreactor and
modelling studies
both confirm this
and elucidate the
mechanisms.
Ceria-based oxygen storage materials
Phase Fabrication Performance Ref Comments
CGO Ni-coated CGO by
hydrothermal
Tested in dry CH4 at 610
°C at 1.2 A/cm². Voltage
215 The benefits of ceria
and doped ceria
70
synthesis, 2 µm
layer deposited at
the top of a
conventional
Ni/CGO anode
support, 5 µm CGO
electrolyte
stable at 0.6 V for 1000 h
compared to Ni/CGO cell
without Ni-coated CGO
layer. No carbon
detected by SEM after
test or by Raman after
microreactor tests.
with regards to
carbon tolerance
are so long
established as to be
beyond doubt. Still
work is continuing
with recent studies
including use of high
OSC cerias205, 216,
microstructuring
and the
electrocatalytic
performance of
ceria itself214.
Mixed ionic-electronic conductors
Phase Fabrication Performance Ref Comments
SMMO SMMO powders by
solid state method,
slurry painted onto
250 µm LSGM
electrolyte with CLO
buffer layer. Pt, Au,
Ag and LST current
collectors used.
Cells tested in dry CH4 at
800 °C: OCVs of all cells
were stable over 100 h,
very low OCVs were
observed for Au and Ag
current collectors. At 0.5
V the cell with Pt current
collector was stable for
240 It is becoming clear
that most MIECs
appear to lack
catalytic activity
towards methane
and other
hydrocarbons. In
this paper, the
71
80 h at ~0.3 A/cm² after a
large initial drop.
activity was
dominated by the
current collector.
PBCMO PBCMO prepared by
Pechini method,
screen printed on a
250 µm LSGM
electrolyte with CLO
buffer layer.
Cells tested in wet C8H18
and C3H8 at 700 °C and
0.6 V: stable at 0.2 – 0.3
A/cm² in C8H18 for 50 h
and C3H8 for 150 h. C
deposition was not
measured.
243 This class of reduced
perovskites may
hold some promise
regarding activity
towards
hydrocarbon
oxidation, but much
more work is
needed to confirm
this.
LST+Ni
+ CeO2
LSCNT powders
made by sol-gel,
screen printed onto
a 300 µm YSZ
electrolyte, with Ni
and CeO2 exsolved
in situ
Cells tested in dry CH4 at
900 °C and 0.5 V: The cell
without CeO2 gave an
initial current of 0.4
A/cm² which declined to
0.35 A/cm² over 80 h,
while the cell with
exsolved Ni and CeO2
increased in current from
0.6 – 0.75 A/cm².
Microreactor tests
260 Exsolution of metal
nanoparticles from
perovskites has
been of intense
interest recently,
however long term
studies in operating
fuel cells are lacking.
In this study, the
performance is
increasing after 80
72
showed little carbon
deposition in the material
with Ni and CeO2.
h, perhaps
indicating that Ni is
still exsolving, a
process which is
slow in
stoichiometric
compounds259, 268
7. Materials design strategies for sulfur tolerance in SOFC anodes
There seems to be sufficient consensus in the literature that sulfur will be adsorbed
at the surface of nickel blocking the reaction sites for oxidation or reforming reactions 72, 269;
although initially an unwelcome feature, it can be used as an advantage to minimise carbon
deposition21, 71. It is also accepted that absorption is more dramatic at lower temperatures
and at higher concentration of sulfur62, 66. It is also known that two stages of sulfur poisoning
have been observed, one is the surface absorption of sulfur that blocks the reaction sites
but that can be reversed and a second one related to an in-depth formation of nickel sulfide
that changes the microstructure of nickel and is therefore irreversible71, 270. Figure 19 shows
a possible mechanism for sulfur poisoning in hydrogen and carbon fuel environments62.
There is, however, no consensus in the effect of the current densities on sulfur poisoning.
Some authors reported that increasing current densities leads to a decrease in sulfur
coverage because of its conversion to SO263, 65, 271. On the contrary, other authors have
indicated that sulfur coverage increases with current density60, 272-273. A very good agreement
with the latter view is concluded in the recent modelling work of Riegraf et al274, where the
model involves all gas and solid chemical reactions coupled with electrochemistry. When
73
operating in methane, the adsorbed sulfur supresses the reforming reaction by blocking the
catalytically active sites, and these sites become available if sufficient hydrogen is present to
unblock the sites274.
Again there is agreement that full recovery can be achieved if H2S is removed
completely from the fuel stream but there is a limit beyond which damage is irreversible.
Concentration and temperatures where recovery is possible vary from article to article but
reversibility has been reported independently. This may be related to desorption of sulfur
from the nickel surface and reaction with H2 from the clean stream. Some good examples of
this recovery are the work of Rasmussen and Hagen68, Sasaki62 and Zha63. It is also generally
accepted that the whole surface of nickel is covered and not only the TPB region270.
74
Figure 19. Possible mechanisms of degradation by sulfur poisoning. Taken from J.
Power Sources 2011, 196 (22), 9130-9140. Reprinted with permission from Elsevier.
Whatever change takes place in the anode during poisoning, it must be reversible
and provided that oxygen is migrating to the anode via the electrolyte, it is of paramount
importance that this process is not stopped and that oxidation or removal of adsorbed
sulfur is favourable. To provide sulfur tolerance, the materials and structure of the anode
should therefore be capable of adsorbing sulfur and then react with any of the gaseous
species present H2, H-C or even O2- to form SO2.
From the point of view of the materials modification, the strategies more frequently
used for the development of sulfur tolerant anodes can be summarised as follows:
1) High oxygen transport to increase sulfur oxidation (Figure 20a). In similar
conditions, ScSZ working under H2S/H2 atmospheres shows a higher tolerance to H2S than
YSZ, indicating the importance of a higher oxygen supply through the electrolyte62.
2) Incorporation of additives or partial substitution of nickel (Figure 20b).
Substituting Ni for a more sulfur tolerant metal without compromising H2 activity has been
behind much of the work on alloys89, 96. Some of the earlier work attempted copper97, 275
while the most recent use of additives has been aiming at using these as preferential sites of
sulfur incorporation276, and this is reinforced by a thermodynamics studies showing that
oxides such as BaO and CeO2 reduce the coverage of sulfur on Ni277, which is strongly linked
to performance91. Catalytic activity for hydrogen oxidation reaction and H2S dissociation
seem to follow analogous trends, maintaining the catalytic activity while simultaneously
improving sulfur tolerance difficult via this route104.
3) Use of all-ceramic anodes (Figure 20c). Perovskites are favoured as they can be
tailored on the A and B site to improve ionic conductivity, electronic conductivity, catalytic
75
activity, and resistance (e.g. Mg2+ more resistance to sulfide formation than Cr3+ or Mn2+)278.
Additionally, the reactivity of a ceramic material is expected to be smaller than that of a
metal surface78.
Figure 20. Schematics of the most common materials strategies to improve sulfur
tolerance. The diagram shows a strategy and does not imply a specific mechanism of
desulfurization.
7.1 Replacement of YSZ with ceria
A few papers have compared Ni-YSZ and Ni-CGO electrodes and, for example, Zhang
has shown that degradation in Ni-CGO is lower than in the Ni-YSZ under similar conditions of
operation279. This may not be surprising considering that sulfur can also accumulate in the
surface of the CGO forming Ce0xSy-type phases which can react with O2- to produce SO2.
Recent studies on the adsorption and removal of H2S from fuel streams by rare earth oxides
again suggest that CGO is one of the most promising anodes for operation under H2S
poisoned fuels. Elimination of the adsorbed sulfur can take place in ceria and other rare
earth oxides using a reducing, oxidising, or inert gas or even steam280-281. This may explain
the tolerance to H2S of an anode that has been infiltrated with ceria282 and the minimised
76
potential drop in anodes with lanthanides as additives62. The tolerance of ceria-based
materials to sulfur environments has been known for some years283.
Donor-dopants of ceria have been studied to a lesser extent compared to acceptor
species. Nevertheless some of them are interesting for improving sulfur tolerance, for
example Mo. This dopant is especially desirable for sulfur tolerance goals since it can trap S
forming MoS2. In this sense, Li et al. investigated the electrical properties of the Mo-doped
CeO2 (CMO) as potential anodes for SOFCs. Mo and rare-earth-co-doped Ce0.9-x RExMo0.1O2.1–
0.5x (x=0.2, 0.3) (CRMO) oxides were found to retain their fluorite-type structure under H2 at
elevated temperatures284. The same team demonstrated the remarkable stability of these
Mo-doped CeO2–anodes in wet H2 and wet CH4 mixtures285. As mentioned above Mo is a key
element to incorporate sulfur resilience. In a recent publication, Chen and co-workers
developed a sulfur-resistant SOFC anode by impregnation of Mo0.1Ce0.9O2+δ into a typical Ni-
YSZ material286. Figure 21 shows the successful performance of this material when submitted
to 50 ppm of H2S.
77
Figure 21. Sulfur tolerance test for a CMO-impregnated cell under a current density
of 0.60 A cm−2 at 750 ◦C using H2 and H2 with 50 ppm H2S as the fuel, respectively. Reprinted
from J. Power Sources 2012, 204, 40-45 with permission of Elsevier.
The system allows power densities of 440 mW cm−2 and 420 mW cm−2 using H2 with
50 ppm H2S and methane as fuel, respectively under a current density of 0.60 A cm−2 at 750
⁰C.
7.2 All-ceramic anodes
A number of all-ceramic electrodes have shown promising performance in SOFCs or
SOECs258-259, 287-288. As mentioned before it is expected that an oxide is less prone to adsorb
sulfur than a metal. The classic perovskite SrTiO3 can be doped both in the A and B site or
even have A site deficiency278. Some work has been performed on Y-doped SrTiO3 doped
with Ru and CeO2 showing a limited tolerance to H2S (up to 40 ppm) and especially
reversibility when the H2S stream is removed289. The Sr0.6La0.4TiO3/YSZ (50/50 wt %) anode
showed no degradation in the presence of up to 5000 ppm of H2S in a hydrogen fuel290 and it
has even suggested that the presence of H2S can promote the oxidation of methane291-293. In
general it seems that perovskites are stable towards operation in sulfur, with many
examples being reported, including double perovskites242, lithium-ion conducting
perovskites294.
It is generally recognised that Perovskite-based materials lack the catalytic activity of
nickel. As discussed in section 5.7.2, one method to improve the catalytic activity has been
to dope the perovskite with transition metals which then exsolve out as catalytically-active
nanoparticles on reduction. Little work has been done on the tolerance of these
nanoparticles towards sulfur, but one study showed that Fe nanoparticles ex-solved out of
Sr2Fe1.5Mo0.5O6 forms FeS under 50 ppm H2S in H2, with a decline in activity of around 20%
78
from around 0.1 to around 0.08 W/cm² at 600 °C over a period of 46 hours, followed by
stable operation for a further 200 hours295.
It has also been reported that the presence of H2S improves the performance of the
fuel cell when methane is used as the fuel for La0.4Sr0.6Ba0.1TiO3-d291, 296 but the oxidation of
H2S to SO2 does not seem to be the main reaction as suggested previously269 in SOFCs but
rather a gas reaction with methane and potentially an increase in the conductivity of the
perovskite by some as yet unclear mechanism296. Although doped SrTiO3 has been
independently shown to be stable289, 291 in H2S, the high concentrations used need to be
independently confirmed.
Barium-based perovskites have also shown promise for sulfur tolerance. For
instance, Kan et al. prepared the proton-conducting Ba3CaNb2O9 doped with Mn, Fe and Co
and checked the stability of these materials towards H2S297. They used a 5000 ppm H2S/H2
stream to evaluate whether the investigated samples can be used as electrodes in
contaminated fuels (e.g. natural gas with ppm levels of H2S). Their XRD study indicated that
the samples preserved the double-perovskite structure at 600 ⁰C for 12 h. No secondary
phase was detected due to the formation of sulfides such as MnSx, FeSx, or CoSx. The SEM
study also confirmed that the particle sizes and shapes did not change after H2S treatment.
This result suggests that their materials are physically and chemically stable in the SOFC
working environments. The same group reported enhanced stability of perovskite-type
BaZr0.1Ce0.7Y0.1M0.1O3-δ (M=Fe, Mn and Co) with a substantial chemical stability in 30 ppm H2S/
H2 at elevated temperature during 24 h297.
Another Ba-based Perovskite prepared by Yang et al. seems to be a promising
material276. In a very complete paper they reported outstanding sulfur and coking resistance
of a barium zirconate-cerate co-doped with Y and Yb (BaZr0.1Ce0.7Y0.2-xYbxO3-δ) anode. The
79
terminal voltages of the same cells (with BZCYYb and SDC as electrolyte) at 750°C were
recorded as a function of time when the fuel was contaminated with different
concentrations of H2S. The Ni-BZCYYb anodes for both cells showed no observable change in
power output as the fuel was switched from clean hydrogen to hydrogen contaminated with
10, 20, or 30 ppm H2S. XRD data corroborated the chemical stability of the designed anodes.
A study on Ni-BZCY anodes featured even higher levels of H2S (up to 1000 ppm), used EIS to
show that as well as reducing the anode polarisation losses compared to Ni-CSO, the BZCY-
based anodes showed little increase in Ohmic losses even at 200 ppm H2S, while the Ni-CSO
cell showed severe increases in Ohmic resistance at 100 ppm H2S (figure 22). Post-test EDX
showed large decreases in Ni content in the Ni-CSO anode, which were not seen in the Ni-
BZCY anode298. This indicates that these materials may be hindering restructuring of the Ni
at high sulfur levels.
80
Figure 22. I–V, I–P curves and EIS for the fuel cells with the Ni+SDC (a, c) and Ni+BZCY
(b, d) anodes operating on different fuels at 600 °C. Reproduced from Environ. Sci. Technol.,
2014, 48 (20), pp 12427–12434, copyright 2014 American Chemical Society
The role of barium in the improved tolerance may be related to the reduction of
sulfur chemisorption on nickel. Da Silva and Heck have calculated that the incorporation of
oxides, in particular BaO, reduces the sulfur chemisorption on Ni by minimizing the sulfur
chemical potential and favouring the formation of BaS. This sulfide can be reconverted to
BaO in the presence of water and additional BaO, leading to an in situ regeneration299. It was
also predicted that the addition of BaO enables the anode to tolerate 100 ppm in humidified
H2.
7.3 Alloying of nickel with other metals
The incorporation of additives or secondary phases has been known in metallurgy for
many years. The extraction of nickel (or cobalt) metal from ores involve roasting or
oxidation of the sulfide to the oxide followed by in situ reduction with CO. It should be
noted that all the key elements necessary for oxidation and elimination of sulfur used in
metallurgical processes are present in a fuel cell anode and the analysis of roasting may
provide the key to achieve tolerance to sulfur in SOFC. In roasting, the sulfide minerals are
treated with very hot air and the sulfide is converted to an oxide while sulfur is released as
sulfur dioxide; typical examples being ZnS, FeS2, PbS2 and Cu2S. Roasting is usually carried
out between 500 and 1000 °C300, the same range of operation of SOFC. Improvement of the
roasting process is achieved by adding pyrite (FeS) with the highest rest potential among
sulfide minerals, therefore acting as a cathode which accelerates the oxidation of the other
sulfides. Finally, reduction with CO leads to the formation of the metal although a few
81
metals can be obtained directly by oxidation of their sulfides since their oxide is less stable
than SO2, well known examples being: Cu, Ag and Hg.
Finally, it is worth mentioning the idea of decomposing H2S into hydrogen and sulfur,
both valuable products; several routes have been explored in the past301. The most
straightforward suggestion of relevance to SOFC is that H2S be decomposed thermally
according to:
2H2S → 2H2 + 1/4S8, ΔH = 79.5 kJ/mol
This decomposition has been performed in the presence of MoS2302 between 500-800
°C and more recently, it has been reported H2S can be decomposed in the 700-1000 °C
temperature range using the Perovskite oxide LaSr0.5Mo0.5O3303. The presence of Mo and a
possible decomposition of H2S may be behind the activity and the reported stability of
Sr2Mg1-xMnxMoO6-δ in these complex perovskites anodes238, 304. Mo-containing catalysts are
commonly used in hydrodesulfurisation processes305. The interaction of molybdenum with
sulfur can be modified with the presence of a second metal with direct consequences for
the hydrodesulfurisation properties. In particular the synergistic effects of Ni-Mo bonding
has been proved to be active towards the hydrodesulfurisation306. In contrast, the effects of
Zn, Cu, and Fe on the Mo-S interactions and hydrodesulfurisation activity are less
pronounced. The Ni-Mo and Ni-S-Mo interactions increase the electron density on Mo
making it more chemically active in two key steps for the reactions: the adsorption of S-
containing molecules and the dissociation of H. It is therefore not unthinkable that
molybdenum plays a key role in the sulfur tolerance in anodes containing this metal.
Most studies within the literature regarding the tolerance to sulfur report the effect
of sulfur poisoning on the electrochemical properties as it the most direct way of in-situ
degradation analysis. Therefore, cell voltage changes, power output and area specific
82
resistances are commonly used to describe the changes to the anode upon a modification.
Comparison between the different reports is difficult and therefore we shall provide the
overall change observed in the very same paper when the anodes is modified with the
intention to improve sulfur tolerance. Table 5 presents results from selected papers where
there has been a variation in the anode with the intention to improve the tolerance to
sulfur.
Table 5 Selected papers reporting improved sulfur tolerance in SOFC anodes through
materials strategies.
Cell Modification Sulfur tolerance and figures of merit Reference
Ni-CGO in YSZ
scaffold anode
ScSZ
electrolyte
(L
a0.6Sr0.4)0.99CoO3-
δ cathode
Ferritic (FeCr)
stainless steel
supported
High porosity
(not quantified)
Two stage degradation with area specific
resistance of 0.35 Ω cm2 at 650 °C, 0.25 A
cm-2. Full regeneration possible.
307
Low porosity
(not quantified)
Two stage degradation area specific
resistance of 0.70 Ω cm2 at 650 °C, 0.25 A
cm-2. Full regeneration possible.
Ni/8YSZ and
Ni/CGO
3YSZ
electrolyte
Ni/CGO anode Two stage poisoning. Stack voltage
decrease only to 98.7 % of initial value
upon addition of 2 ppm H2S to fuel at 850
°C, 0.225 A cm-2 after 15 h. Fuel mixture:
308
83
support
LSM cathode
43.8% H2, 6.2% H2O and 50% N2.
Ni/8YSZ anode One stage poisoning. Stack voltage
decrease to 86.5 % of initial value upon
addition of 2 ppm H2S to fuel at 850 °C,
0.319 A cm-2 after 15 h. Fuel mixture:
43.8% H2, 6.2% H2O and 50% N2.
Ni/YSZ or
Ni/CGO anodes
YSZ or CGO
electrolyte
supported
Pt cathode
Ni/CGO anode Polarization resistance of anode is 4.3 Ω
cm2 in 700 ppm H2S in H2, 200 mA/cm2 at
800 °C after 2 h. Regeneration possible.
279
Ni/YSZ anode Polarization resistance of anode is 1.2 Ω
cm2 in 700 ppm H2S in H2, 200 mA/cm2 at
800 °C after 2 h.
Ni-CGO anode
supported
CGO
electrolyte
NdB
a0.75Ca0.25Co2O5+
δ-CGO cathode
Ni/CGO+
BaCe0.9Yb0.1O3−δ
Cell voltage 0.74 V in pure H2 at 650 °C
with 640 mA/cm2 goes immediately to 0.7
V stable over 20 h upon introduction of
500 ppm H2S. Full regeneration possible.
309
Ni/CGO Cell voltage 0.63 V in pure H2 at 650 °C
with 640 mA/cm2 goes immediately to
0.61 V and decreases continuously for 6 h.
Regeneration not possible.
84
Ni1−xCox-YSZ
anodes
YSZ electrolyte
LSM cathode
Ni0.69Co0.31O-YSZ Current exchange density 0.024 A/cm2 in
pure H2 improving to 0.094 A/cm2 in 10%
H2S in CH4 after 15 h at 850 °C.
101
Ni-YSZ Current exchange density 0.018 A/cm2 in
pure H2 improving to 0.032 A/cm2 in 10%
H2S in CH4 after 15 h at 850 °C.
Ni–YSZ or
S
r1−xCexCo0.2Fe0.8
O3−δ anodes
ScSZ
electrolyte
supported
LSM cathode
S
r0.85Ce0.15Co0.2Fe0
.8O3−δ anode
Current density is 0.088 A/cm2 in H2 at 0.9
V.
Lowered to 88 % of initial value upon
addition of 20 ppm H2S at 800 °C after 500
min at constant 0.6 V.
310
Ni-YSZ Current density is 0.080 A/cm2 in H2 at 0.9
V.
Immediate drop in current. After 500 min,
current lowered to 81 % initial normalized
value in 50 ppm H2S at 800 °C at constant
0.6 V.
Ni YSZ anode
YSZ electrolyte
support
SDC/LSCF.
Ni-YSZ +
infiltrated
BaZ
r0.1Ce0.7Y0.1Yb0.1O
3−δ (BZCYYb)
Cell voltage remains above 0.72 V even
upon addition of 30 ppm H2S to pure H2, at
700°C, 0.054 A/cm2. V. slow degradation.
311
Ni-YSZ Cell voltage from 0.72 V in H2 to 0.62 V to
85
20 ppm H2S in H2 within a few minutes at
700°C, 0.054 A/cm2.
Ni-doped
zirconia anode
supported cell
Doped zirconia
LSM cathode
Ni-ScSZ anodes
ScSZ electrolyte
Two stage poisoning mechanisms, one
fast, reversible and one slow and
irreversible. Poisoning of methane
reforming.
Cell voltage 0.7 V in 2 ppm H2S, 13% H2, 58
% H2O, 29% CH4, 850 °C, 1 A/cm2 after 500
h
312
Ni-YSZ
YSZ electrolyte
Cell voltage 0.45 V in 2 ppm H2S, 13% H2,
58 % H2O, 29% CH4, 850 °C, 1 A/cm2 after
500 h.
Two stage poisoning mechanisms.
Poisoning of methane reforming.
Ni-doped
Zirconia anode
YSZ electrolyte
LSM cathode
Ni-ScSZ anode Cell voltage: 0.55 V
at 200 mA/cm2, 800 °C, 100 ppm H2S in H2
after 1000 s with stable performance.
62
Ni-YSZ anode Cell voltage: 0.18 V
200 mA/cm2, 800 °C, 20 ppm H2S in H2
after 1000 s. Null voltage after 1650 s.
Two stage poisoning: initial one is fast and
reversible, second is slow and irreversible
Ni-YSZ anode Ni-YSZ anode Voltage drop 0.12 V, 200 mA/cm2 62
86
Doped zirconia
electrolyte
LSM cathode
ScSZ electrolyte in 5 ppm H2S in H2 at 850 °C.
Ni-YSZ anode
ScSZ electrolyte
Voltage drop 0.52 V, 200 mA/cm2
in 5 ppm H2S in H2 at 850 °C.
Ni-YSZ Anode-
supported
YSZ electrolyte
LSM cathode
Ceria-modified
Ni-YSZ
Cell voltage = 0.6 V at 0.3 A/cm2 in H2 +
200 ppm H2S at 700 °C
313
Ni–YSZ anode Cell voltage = 0.4 V at 0.3 A/cm2, 700 °C in
H2 + 200 ppm H2S
Ni-
BaZ
r0.4Ce0.4Y0.2O3-δ
(BZCY) and Ni-
Sm0.2Ce0.8O1.9
(SDC) anodes
SDC electrolyte
B
a0.5Sr0.5Co0.8Fe0.2
O3-δ (BSCF) and
Sm0.5Sr0.5CoO3-δ
(SSC) cathodes
Ni-BZCY OCV = 1.01 V in pure H2
Stable 148 mW/cm2 in 100 ppm of H2S at
200 mA/cm2, 600 °C for 700 min
298
Ni-SDC OCV = 0.709 V in pure H2
From 137 mW/cm2 to 81 mW/cm2 in 100
ppm of H2S at 200 mA/cm2, 600 °C for 150
min
87
Ni–YSZ anode
supported
YSZ electrolyte
LSCF-GDC
cathode
Bimetallic
coating
Ni-Cu/Co/Fe on
anode
Peak power densities ∼1.4 W/cm2 in H2.
Decreases to 1.0 W/cm2 in 500 ppm H2S-
H2.
Enhanced dry reforming of methane.
141
No extra
coating
Peak power densities ∼1.4 W/cm2 in H2.
NiSn-YSZ
anode
supported
YSZ electrolyte
LSM-YSZ
cathode
Infiltrated NiSn
+ reformer
NiSn/Al2O3
Cell voltage decreases from 0.72 V in pure
H2 to 0.63 V on addition of 500 ppm H2S at
at 850 °C and 1.25 A/cm2. Complete
regeneration.
314
No infiltration
and without
reformer
Cell voltage decreases continuously from
0.5 V to 0.45 V in 48 h at 1.25 A/cm2, 850
°C, 200 ppm in CO2:CH4
8. Strategies from conventional catalysis
The problems faced by fuel cell anodes regarding carbon and sulfur poisoning are
similar in many ways to those faced by conventional catalysts. In fact, in one very important
respect carbon and sulfur tolerance is more challenging in conventional catalysis – there is
no equivalent of the oxygen flux through the electrolyte which occurs in SOFCs, which tends
to reduce the problems with carbon and sulfur. Because of this, it is instructive to look at
solutions for tolerant catalysts. This issue has been studied over a much longer period of
88
time, and more intensively, than for SOFCs, and many of the findings have not yet been
incorporated into SOFC research.
8.1 Carbon tolerance in conventional catalysis
Several strategies have been studied for minimizing the carbon deposition in catalyst
used in reactions involving hydrocarbons, such as steam reforming, dry reforming, partial
oxidation or water gas shift. The use of noble metals, instead of Ni, as the active phase is the
best option in terms of carbon resistance. However, similarly to SOFCs, the high cost and
low availability of noble metals mean that Ni-based catalysts are favoured, and strategies
for minimizing carbon deposition in these catalysts have been developed in the last
decades16, 315.
8.1.1 Sulfur passivation
Sulfur passivation was one of the first strategies developed to diminish carbon
deposition. The first published works, in steam and dry reforming of methane, appeared in
the mid-80s18, 316-317. The approach consists of partially passivating the active centers of Ni
catalysts with sulfur, normally using H2S18, 316-319. Lately the use of alkanethiols for the
passivation has also been proposed with promising results320-321.
Hydrogen sulfide chemisorbs on the nickel surface and blocks access to the catalytic
centres. This blockage decreases the carbon deposition rate more than the methane
reforming rate316. At complete coverage, carbon atoms cannot be dissolved into the nickel
crystal and the whisker growth mechanism is inhibited. However, the complete coverage of
the nickel surface with sulfur results in total deactivation316. Using coverage ratios of around
0.7 it is possible to diminish carbon deposition without compromising reforming activity316-
318. At this coverage ratio it is not possible to inhibit carbon formation. Nevertheless, the
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usual whisker structure is replaced by more amorphous structures, which are less
deactivating316-317, 319. Hence, at this coverage, the reaction still takes place. This is due to the
number of active nickel surface sites needed for each process. Carbon nucleation needs
larger sites, which are almost completely blocked at high coverage rates, whereas methane
reforming reactions can proceed in the smaller sites which are still available18, 316-319.
However, reforming of larger molecules, like toluene and tars, requires large active sites, so
the sulfur passivation can deactivate reforming reactions as well318. This technology, in the
case of the dry reforming of CH4, has been industrially implemented by Haldor Topsøe in the
SPARG process21, 32, 322.
Sulfur passivated catalysts have been also applied to the dehydrogenation of
isobutane, with sulfur passivation improving both the selectivity of the process and
inhibiting carbon formation323.
8.1.2 Alloying and bimetallic systems
The introduction of additional metals that can modify the ability of carbon to
assemble or to dissolve in the bulk metal of the catalyst can drastically reduce the potential
for carbon deposition. A vast number of bimetallic combinations can be found in the
literature. Focusing only on Ni catalysts, bimetallic systems like Ni-Co324-328, Ni-Fe324, Ni-Cu324-
325, 329-332, Ni-Mn324, Ni-Sn167, 325, 333-334 and Ni-NM (being NM a noble metal: Rh, Pt, Pd, Ir, Ru,
Au, Ag)9, 49, 325, 330, 335-341, have been studied, showing in some cases very promising results.
When one or more other metals are introduced into the system, different structures
can be originated depending on the metals’ properties, interactions with the support,
atmosphere, temperature, etc325, 336. A schematic representation of these structures can be
seen in Figure 23. Among these structures, interest in alloys is increasing. The use of alloys
of different metals as the active phase has been deeply developed in the last years. This is
90
due to the superior performance of alloys in terms of conversion and resistance to carbon
deposition341-342.
Figure 23. Possible structures shown by bimetallic nanoparticles: a) core-shell; b)
heterostructure; c) nano-alloy; d) segregation; e) ensembles (adapted from Catal. Today
2012, 197 (1), 190-205, with permission from Elsevier).
The interaction between two or more metals can give rise to geometric and
electronic effects, which could affect carbon deposition325. The geometric effect is the result
of the dilution of the atoms of one metal in the other. Thus, surface ensembles are reduced
in size. This can dramatically affect the catalyst performance, since many reactions depend
on the size of the ensembles, as was explained for the partial sulfur passivation325, 341. The
electronic effect is the result of the difference in electronic affinity between the metals, that
can produce an electronic density increase or a decrease in the main metal depending on
whether the secondary metal has a lower or higher electronic affinity. These modifications
in the electronic density alter phenomena such as adsorption or desorption of species
during the reaction process, affecting activity and selectivity325.
Noble metals are well known to be more resistant to carbon deposition than Ni, as
well as possessing other features such as improved catalytic activity, suppression of Ni
oxidation or sustainability in daily start-stop operations324-325, 340, 343. Among the noble metals,
the most common used in bimetallic systems with Ni is Rh. In this type of catalyst Rh atoms
91
enrich the Ni surface, forming a surface alloy Ni–Rh, rather than dissolving into Ni particles
and forming a bulk alloy. However, the formation of the alloy strongly depends on the
support used and its interaction with the metallic particles324, 340. In addition, preparation
conditions can also affect the carbon resistance of the bimetallic system. Thus, if the
catalysts are calcined in oxygen at high temperatures, metal segregation can occur, giving
rise to lower carbon resistance339. The presence of Rh increases the energy barriers of C
diffusion and C–C bond formation whereas the O diffusion and C–O bond formation are not
significantly affected. As a consequence, the global rate of carbon deposition is decreased49,
325. Similar behaviour has been found in the case of Ni-Pt systems325, 336. The presence of Pt
has been found to promote the formation of small NiO crystals, which facilitates the
reduction to Ni0 and improves Ni dispersion340. The versatility of Ni-Pt system allows the
creation of different surface structures (core shell, monolayers, alloys) that need to be
controlled to minimize carbon deposition336, 340.
Although less studied, Au and Ag have given rise to interesting results in terms of
carbon resistance5, 330. Particularly in the case of the Ni–Au alloys, it has been found that the
presence of a small amount of gold on a supported nickel catalyst can induce a significant
effect on the carbon formation process during the steam reforming of methane5. Au makes
the diffusion of the CHx species (intermediates in carbon growth) significantly difficult,
preventing carbon nucleation330. In the dry reforming of methane, the presence of gold
promotes the formation of carbonaceous species which have high reactivity with CO2, thus
facilitating gasification340.
However, the elevated cost of noble metals makes it more practical from the
industrial point of view to develop noble metal-free catalysts49, 324-332, 344. Ni-Co might be a
more affordable option. Ni-Co bimetallic catalysts show a synergetic effect that makes the
92
catalyst more active and resistant to carbon deposition than Ni and Co monometallic
catalysts324, 326-327. Ni and Co benefit from the electronic effects that appear in bimetallic
systems. They present different oxidation states depending whether they are used in
monometallic or in bimetallic catalysts, indicating an electronic transfer between Ni and Co
in the bimetallic catalyst324. This protects metal from oxidation during the reaction and
confirms the near-distance interaction between the two metal atoms, making it easier to
form Ni–Co alloy on the catalyst surface324. In addition to the synergetic effect, the
formation of various spinel-type solid solutions with the supports improves the metal-
support interaction and therefore the carbon resistance324, 327. Cu-Ni system stability has
been found to be dependent on temperature and Cu/Ni ratio 329, 331. Copper seems to
stabilize the structure of the active site on Ni surface, thus preventing sintering or loss of
nickel crystallites. Adding Cu into Ni catalyst system can fine-tune the catalytic activity, so
that carbon formation and removal can be balanced, preventing deactivation by carbon
accumulation 329-330, 332. However, an excessive load of Cu could give rise to a Cu-rich alloy
that can increase carbon deposition 331.
Sn/Ni alloys seem to be the most promising alternative, not only for their high
resistance to carbon deposition but also for the low price of Sn compared to noble metals167,
325, 333-334. Sn/Ni alloys have shown up a huge potential for improving the carbon resistance in
steam reforming processes by modifying the relative kinetics of C-O and C-C bond
formation. Once again, in this case the formation of the surface alloy is favoured over the
bulk alloy, especially at low Sn loadings167, 333. DFT studies showed that the presence of Sn,
which is mainly located at the surface of the alloy particles, imposes an important barrier to
carbon diffusion in Ni crystallites, thus hindering carbides formation and the subsequent
93
nucleation of carbon 167, 334. These theoretical results were confirmed in steam reforming of
various hydrocarbons 167, 334.
8.1.3 Promoters
The addition of different promoters can affect the interaction between the metal
and the support or the acid-basic nature of the support, which modifies its tendency to give
rise to carbon deposition 27, 29. The use of several promoters can be found in the literature,
including Li 22, Na 345-346, K 22, 29, 346-350, Mg 15, 17, 22, 346, 351-359, Ca 346, 351, 360-361, La 22, 26-27, Zr 25, 326, 335, 337,
362-369, Mn 29, 348, Ce 29, 348, 370.
Alkali (Li, Na, K) 22, 29, 345-350 or alkali earths (Mg, Ca) 17, 22, 346, 351, 360-361, are usually
introduced in catalyst formulations with the aim of accelerating carbon removal from the
catalyst surface due to their basic nature371. K2O can reduce the carbon deposition rate in
reforming processes, but occasionally it also compromises the catalytic activity 22, 29, 347-350.
This reduction of the carbon deposition is a consequence of an improvement of the
gasification rate of the carbon deposits, thus improving the stability of the catalyst.
However, the interaction of K with Ni gives rise to large NiO crystalline particles. This species
has high mobility, thus promoting aggregation of particles and decreasing the activity, while
larger Ni particles also promote carbon deposition21-24, 31. K2O has also been used as a
promoter in Ni catalysts for the water–gas shift reaction345 and in bimetallic Ni-Mo catalysts
for the dry reforming of propane 372, presenting in these cases improvements both in carbon
resistance and catalytic activity. Similar behaviour to K, although less pronounced has been
observed in the case of Na2O and CaO 346, 351, 360-361, 373-375. In the case of CaO some researchers
have suggested that while the amount of carbon deposits increases, the reactivity of the
deposits also increases, leading to a higher stability of the catalyst 351, 360, 374.
94
The use of Mg has also been shown as an interesting option for minimizing carbon
deposition, although in this case the preparation of the catalyst and the interaction of Mg
with the support play a critical role 17, 22, 345, 352, 354-356, 358-359. If Mg is used as a promoter of
Ni/Al2O3, it interacts with Ni, leading to a NiO-MgO solid solution, but when Mg is used as a
dopant of the Al2O3 support, it can react to produce MgAl2O4 spinel. In both cases, Ni
sintering is prevented and the carbon deposition is reduced whereas the catalytic activity
increases, but the effect achieved by the spinel has been shown to be quantitatively better
than that from the solid solution.
Mn is used as a promoter, especially in dry reforming of methane, to reduce carbon
deposition, both in Ni and Co based catalysts 364-365, 375-378. MnOx forms patches that partially
cover the Ni surface giving a similar effect to that from sulfur passivation 375-377. Moreover, Ni
dispersion is improved 376, 378 and the moderate basicity of the MnOx improves CO2
adsorption and increases carbon gasification rate by forming reactive carbonate species 376-
377.
Lanthanide oxides have also been thoroughly studied, with La and Ce oxides the
most promising promoters 379-381. La2O3 has been found to affect positively both activity and
resistance to carbon deposition22, 26-27. Two different effects are promoted by the presence of
La2O3. On one hand, its basicity promotes the absorption of CO2, giving rise to lanthanum
oxycarbonates 22, 26-27, 382-383. These species play a role in conserving the stability of the
catalyst, since they promote the CO2 decomposition to CO and O, which can increase the
carbon gasification rate 22, 27. On the other hand, the interaction between La and Ni forms a
mixed oxide (NiLa2O4) in the same way as happens with Al2O3. This phase prevents the
sintering of Ni particles, thus reducing carbon deposition 27, 382. It has also been found that
the performance can be improved by the addition of alkaline oxides 384 or other lanthanides
95
like Ce or Pr 383, 385. In the case of the combination of La-Ce, it has been found that after
reduction of the catalysts, particles of a mixed oxide appear on top of nickel particles. This
decoration of Ni particles reduces the ensemble of Ni similarly as in the case of sulfur
passivation, thus lowering the probability of the nucleation of carbon precursors 383.
CeO2 constitutes another interesting promoter for minimizing and even suppressing
carbon deposition 29, 348. It has been used as support as well, but the conversions were lower
probably due to the strong metal-support interaction. However the results showed that its
use as promoter is much better, giving rise to high conversions and resistance to carbon
deposition 370, 386. It should be noted that the amount of CeO2 used as a promoter should not
exceed certain value to avoid compromising the catalytic activity338, 387. The high resistance
to carbon deposition comes from the oxygen storage capacity and oxygen mobility that
ceria presents 192, 388. CeO2 can store and release reversibly a large amount of oxygen, thus
increasing its availability for gasifying the carbon deposits29, 348, 353. As discussed in section 5.6
CeO2 exhibits excellent redox properties with a Ce3+-Ce4+ equilibrium and the coexistence of
CeO2 and Ce3O4 192, 353, 388. This behaviour can influence the oxidation state of atoms on the
surface of the active metal particles (for example Rh0/Rhδ+) favouring the activation of
reacting molecules 338, 386, 389. Other features that can improve catalyst performance are that
CeO2 gives rise to a better dispersion of the active phase, enhancing the catalyst
performance and inhibiting the transition of the γ-Al2O3 used as support to the low-surface-
area α-Al2O3 at high temperatures 387. When CeO2 is used as dopant of the Al2O3, CeAlO3
species are formed. This species completely inhibits the growth of filamentous carbon,
although amorphous carbon is still deposited (Figure 24) 380, 383, 387. It has been suggested that
these species suppress the growth of this filamentous carbon by chemical blocking rather
than by gasifying them after they have been deposited in the catalyst 387.
96
Figure 24. Carbon deposition models in the steam reforming of hydrocarbons over
Ni/Al2O3 and Ni-Ce/Al2O3 catalyst: a) not doped; and b) doped with Ce (adapted from J.
Catal. 2005, 234 (2), 496-508, with permission from Elsevier).
Finally, ZrO2 is also able to enhance carbon deposition resistance 25, 335, 362, 364-366, 368.
This promoter enhances the dissociation of CO2, forming oxygen intermediates near the
contact between ZrO2 and Ni. These intermediates increase the rate of gasification of the
carbon deposits 362. In addition, ZrO2 has both basic and weak acidic sites, which improves its
resistance to carbon deposition 326, 367. However, the main interest in ZrO2 seems to be its
use in combination with CeO2, since ZrO2 enhances the oxygen storage capacity and oxygen
ion mobility of CeO2. This enhancement of CeO2 properties results in an improvement of the
resistance against carbon deposition 335, 337, 363-364, 367, 369, 390.
8.1.4 Regeneration of Catalysts Deactivated by Carbon Deposition
Under certain conditions catalyst deactivation due to carbon deposition can be
inevitable even in the most resistant catalysts. For this reason the regeneration of the
catalysts is extremely important to maximize the benefit obtained from them9, 13. Catalysts
deactivated by carbon deposition can be regenerated using different gasifying agents (in
97
order of gasification rate): oxygen, steam, carbon dioxide or hydrogen. The reactions
involved in the regeneration processes with these gasifying agents are shown below 9, 13, 391-
393.
C + O2 CO2
C + H2O H2 + CO
C + CO2 2CO
C + 2H2 CH4
As discussed above, different types of carbon can deposit on the catalyst surface13.
Thus, they will behave differently during the regeneration. The carbons formed on Ni
catalysts involved in reactions with hydrocarbons can be monoatomic carbon, polymeric
amorphous films, vermicular fibres or whiskers, nickel carbide and graphitic films9, 13. Both,
preparation of the catalyst (metal loading, calcination temperature, particle size, use of
promoters) and reaction conditions (temperature, H/C ratio, presence of carbon precursors)
can affect the type and amount of carbon deposited.
Due to the differences in reactivity between the different types of carbon deposits,
different conditions should be applied. Thus, less ordered and more reactive carbons
(monoatomic carbon or amorphous) need lower temperatures and weak gasifying agents
(about 400 °C in H2 or H2O) whereas graphitic carbon needs higher temperatures and strong
gasifying agents (700-900 °C in air)9, 392. However, as O2 is the strongest and cheapest
gasifying agent, in industry catalysts are usually regenerated in air at about 600 °C9.
Although the catalytic activity can be recovered almost completely under certain
conditions, catalysts lose activity after each recovery cycle due to different reasons13, 394. For
example, regeneration in air is a very exothermic process that can lead to hot spots. These
hot spots can lead to metal reorganization or sintering, thus deactivating the catalysts in the
98
attempt of recovering the catalytic activity lost due to carbon deposition9, 13, 392. The main
reasons for losing catalytic activity during regeneration are:
Loss of metal particles that were pulled out from the support due to the
formation of carbon filaments9
Oxidation of the metals9, 13, 26, 390, 395. Although it can be reversed by
subsequent reduction of the catalyst, it sometimes gives rise to the irreversible formation of
different structures that can be inactive.13, 396-397 398
Sintering9, 26, 393, 398-399
Damage of the support328, 400.
However, in the same way that the addition of promoters or the formation of alloys
can enhance catalytic activity and resistance against carbon deposition, the regeneration
can be positively affected by the presence of these promoters27, 361 and alloys325-326, 328, 395.
Thus, the presence of small amounts of noble metals can improve reducibility of the main
metal324-325, 340. Moreover, in some cases, after a few regeneration cycles the performance of
the catalyst can be enhanced325. These strategies for improving carbon resistance can help
to facilitate carbon removal during regeneration of the spent catalyst.
Table 6. Strategies to minimize carbon deposition in catalysts
Strategy Catalysts Process Ref.
Sulfur
passivation Ni(S) Steam reforming
of CH4
320-321
Ni(S)/Al2O3 CO2 reforming of
CH4
316
Ni(S)/MgAl2O4 CO2 reforming of 316
99
CH4
Dehydrogenation
of isobutane
323
Bimetallic
catalysts
Ni-Au/MgAl2O4 Steam reforming
of n-butane
342
Ni-Co/Al2O3 Steam reforming
of glycerol
328, 393
Ni-Co/CeO2-Al2O3 CO2 reforming of
CH4
379
Ni-Co/MgAl2O4 CO2 reforming of
CH4
324, 327
Ni-Co/MgO-ZrO2 CO2 reforming of
CH4
326
Ni-Cu/Al2O3 CO2 reforming of
CH4
331
Ni-Cu/SiO2 CO2 reforming of
CH4
329
Ni-Cu/MgO-SiO2 Steam reforming
of ethanol
358
Ni-Cu/CaO-SiO2 Steam reforming
of ethanol
358
Ni-Cu/ZnO-Al2O3 Steam reforming 332
100
of methanol
Steam reforming
of ethanol
332
Ni-Mn/MgAl2O4 CO2 reforming of
CH4
324
Ni-Mo/Al2O3 Steam reforming
of gasoline
344
Ni-Mo-K2O/Al2O3 CO2 reforming of
propane
372
Ni-Mo/Al2O3 Steam reforming
of gasoline
344
Ni-Pt/Al2O3 CO2 reforming of
CH4
336
Ni-Re/Al2O3 Steam reforming
of gasoline
344
Ni-Rh/SiO2 CO2 reforming of
CH4
339
Ni-Rh/CeO2-Al2O3 CO2 reforming of
CH4
49, 338
Ni-Rh/CeO2-ZrO2 Steam reforming
of ethanol
335
Methanation of
carbon dioxide
337
Ni-Sn/YSZ Steam reforming 167, 333-334
101
of methane
Steam reforming
of propane
167, 333-334
Steam reforming
of isooctane
167, 333-334
Co-Rh/CeO2-ZrO2 Steam reforming
of ethanol
335
Co–Rh/SiO2 Oxidative steam-
reforming of
ethanol
395
Co–Ru/SiO2 Oxidative steam-
reforming of
ethanol
395
Promoters
Ni-K2O Water gas shift 345
Ni-CaO/Al2O3 CO2 reforming of
CH4
346, 351, 360, 374
Ni/CeO2-Al2O3 CO2 reforming of
CH4
348, 353, 370, 387, 389
Steam
gasification of
polypropylene
357
Steam reforming
of propane
380
102
Oxidative
reforming of
hexadecane
383
Ni-K2O/Al2O3 CO2 reforming of
CH4
22, 346-350
Ni/La2O3-Al2O3 CO2 reforming of
CH4
22, 27
Steam reforming
of ethanol
401
Steam reforming
of propane
380
Ni/CeO2-ZrO2-Al2O3 Steam reforming
of CH4
369, 397
Ni/CeO2-La2O3-Al2O3 Oxidative
reforming of
hexadecane
383
Ni-Li2O/Al2O3 CO2 reforming of
CH4
22
Ni-MgO/Al2O3 CO2 reforming of
CH4
22, 346
351-355
Steam reforming
of ethanol
356
Steam
gasification of
357
103
polypropylene
Ni-MnO/Al2O3 CO2 reforming of
CH4
348, 365, 376-377
Ni-Na2O/Al2O3 CO2 reforming of
CH4
346
Ni/ZrO2-Al2O3 CO2 reforming of
CH4
362
Ni/ CeO2-ZrO2 Methanation of
carbon dioxide
337, 363
Autothermal
reforming of
isooctane
390
Ni-CaO/La2O3 CO2 reforming of
CH4
384
Ni-SrO/La2O3 CO2 reforming of
CH4
384
Partial oxidation
of CH4
367
Ni-MgO/SiO2 CO2 reforming of
CH4
359
Ni-MnO/SiO2 CO2 reforming of
CH4
378
Ni-ZrO2/SiO2 CO2 reforming of
CH4
378
104
Ni-K-Ca/ NaZSM-5 CO2 reforming of
CH4
361
Ni-MgO/Zeolite HY CO2 reforming of
CH4
375
Ni-MnO/Zeolite HY CO2 reforming of
CH4
375
Ni-Co/CeO2-Al2O3 CO2 reforming of
CH4
379
Ni-Co/MgAl2O4 CO2 reforming of
CH4
324, 327
Ni-Co/MgO-ZrO2 CO2 reforming of
CH4
326
Ni-Cu-MgO/SiO2 Steam reforming
of ethanol
358
Ni-Cu-CaO/SiO2 Steam reforming
of ethanol
358
Ni-K/CeO2-Al2O3 CO2 reforming of
CH4
29
Ni-Mo-K2O/Al2O3 CO2 reforming of
propane
372
Ni-Rh/CeO2-Al2O3 CO2 reforming of
CH4
49, 338
Ni-Rh/CeO2-ZrO2 Methanation of
carbon dioxide
337
105
Steam reforming
of ethanol
335
Co/CeO2-ZrO2 CO2 reforming of
CH4
364
Co-Rh/CeO2-ZrO2 Steam reforming
of ethanol
335
Pt/CeO2-Al2O3 Oxidative
reforming of
hexadecane
383
Pt/ZrO2-Al2O3 CO2 reforming of
CH4
368
Partial oxidation
of CH4
368
Pt/CeO2-ZrO2 Partial oxidation
of n-tetradecane
366
Pt/CeO2-La2O3-Al2O3 Oxidative
reforming of
hexadecane
383
Rh/CeO2-ZrO2 Partial oxidation
of n-tetradecane
366
8.2 Strategies against sulfur poisoning
Sulfur containing molecules are frequent impurities in fuels and oil-derived
feedstock. These impurities, even at very low concentrations, are responsible for
106
heterogeneous catalyst deactivation. Millions of dollars are lost in chemical and oil
industries as a result of sulfur poisoning 402 403.
Generally speaking, two different approaches have been intensively studied to face
this problem. The first consists of the sulfur removal from the fuel via hydrodesulfurization
and involves a thoughtful catalyst design to achieve high efficiencies (see section 5). In
industrial reactors, sulfur is removed to levels below 0.1 ppm by a multiple step process,
finishing with adsorbents normally based on ZnO404-405. However, a balance has to be struck
between cost, convenience and effectiveness, and significant savings can be made if higher
levels of sulfur can be tolerated. In this sense, the second strategy is to develop sulfur-
tolerant catalysts able to operate in sulfur-rich reaction mixtures78. This second approach is
in line with the aim of this review, which is to provide an overview of the current status in
carbon and sulfur tolerant systems. At this point, a brief reiteration of the fundamental basis
of sulfur poisoning given in detail in section 4 may help to understand the developed
strategies.
Sulfur poisoning takes place due to sulfidation of the active catalytic species, namely
metallic particles and/or metal oxides13. In the case of a metallic particle (Me) and
considering H2S as the source of sulfur the process could be simplified as follows:
Me0 + H2S ----- MeS + H2
At high T its effect should decrease, because sulfidation is thermodynamically
unfavored. However, its kinetics is favoured and the result can be different to the expected,
depending on the metal used (for example, with Ni ΔG is not positive even at 1000 °C).
Sulfur as a poison causes a multifold effect in the catalytic activity. Firstly, sulfur adsorption
physically blocks the catalyst active sites limiting accessibility for the reactants and reducing
the probability of reactant molecules encountering each other. Secondly, by virtue of its
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strong chemical bond it electronically modifies the neighbour metal atoms thus modulating
their ability to adsorb and/or dissociate reactant molecules9. In addition, the catalyst surface
could be reconstructed due to the strong chemical adsorption. Finally, the presence of
strongly bonded sulfur species on the surface of the catalyst hinders the diffusion of both
product and reactant species. Figure 25 schematizes the multiple effects caused by sulfur in
a metal supported catalyst.
A
BA B
CAS S S S
MMM MM M
support
Electrons withdrawal
Site blockage
Hindered reactants encounter limited products
diffusion
Figure 25 Simplified representation of the multifold poisoning effect due to sulfur
chemisorption (M represents an active metal. A and B represent reactant molecules and C
the reaction product)
In this scenario, catalyst deactivation must be overcome and/or the poisoned
catalysts must be regenerated. It must be always kept in mind that the degree of poisoning
depends on the studied reaction, process conditions and the involved catalysts, among
other factors. Consequently, a specific catalyst and/or strategy is required for each process.
In response to these needs, intensive research has been carried out in the field of
heterogeneous catalysis in the last decades generating a wide variety of multicomponent
catalysts with different natures and different features aimed at sulfur poison mitigation.
Herein, a summary of the most conventional approaches and proposed materials are
discussed.
8.2.1 Noble metal-based catalysts
108
Nickel-based catalysts are still the most preferred materials for reforming reactions
due to their good performance, low cost, relatively simple preparation and wide availability
406-408. However, apart from the well-known Ni deactivation due to sintering and carbon
deposition, this metal is one of the most sensitive active phases towards sulfur poisoning 409-
410. The chemical equilibrium of sulfidation at 900 °C for Ni is much more favourable
compared to the values obtained for Ru, Pt, Rh or Co underlining that Ni is the most sulfur
sensitive metal among the conventional reforming active phases13.
In this sense, the use of noble metals based catalysts is a good choice from an
activity and sulfur tolerance point of view, although the cost must be considered411-412. Mono
and bimetallic Pt-based catalysts developed by Farrauto et al. were stable under continuous
operation when exposed to sulfur-containing streams in reforming reactions411. Pt/CGO was
successfully employed in the steam reforming of isooctane to produce hydrogen
demonstrating complete sulfur tolerance413. In this study the authors compared the
performance of this material with a similar Ni/CGO and a conventional Pt/Al2O3. Only Pt
supported on ceria resisted the effect of sulfur. The latter indicates that not only the active
phase matters but also the support plays a crucial role in sulfur resistance. Apparently, the
Pt atoms in the Pt/CGO are more electron-deficient than Pt atoms in Pt/Al2O3 limiting the
interaction with S species413. However, Pt tolerance towards S poisoning also depends on
the considered reaction. For example, in the WGS reaction many high performance Pt based
catalysts suffer from severe deactivation when exposed to sulfur9, 414-419. Furthermore, this
adverse effect seems to be proportional to the amount of sulfur. For example for a Pt/ZrO2
catalyst, Xue and co-workers reported that the conversion went from 44% in the absence of
H2S to 25% (50 ppm H2S) to 14 % (200 ppm H2S) and finally 12% when 1000 ppm H2S were
introduced into the reactant mixture414.
109
Not only the considered reaction, but also the nature of the noble metal influences
the sulfur tolerance capacity of the catalysts. In other words, not all the noble metals exhibit
the same sulfur resistance. For example, McCoy et al. and Azad et al. demonstrated in
different papers that Rh is remarkably less sensitive than Pd towards sulfur poisoning420-422 .
The combination of metals (Rh-Pd) enhanced the tolerance, conserving high and stable
conversion during 12 h of reaction (50 ppm of H2S were used as a sulfur source). The
unsuitability of Pd for sulfur tolerance was also shown in the work of Goud et al.25 Their
results show the deactivation of a Pd/ZrO2 catalyst on the reforming of hexadecane after a
few hours of operation. The inefficiency of Pd was also evidenced in the WGS this time using
ceria as a support and SO2 as a source of S423.
As mentioned above sulfur poisoning can be envisaged as a steric and an electronic
effect. From the electronic point of view, sulfur ligands withdraw electron density from the
metals. For instance, the differences among Rh, Pt and Pd can be explained in terms of
electronic effects. Theoretical calculations for model clusters S/M12 (M = Rh, Pt and Pd)
indicate that the tendency of a metal to lose d electrons increases in the following order: Rh
< Pt < Pd9, 103, 424. This agrees well with the relative occupancy of the d shell in the isolated
elements: Rh: d8s1< Pt d9s1<Pd d10s0. This tendency correlates with the decrease of density
of states around the Fermi level for these elements (25 % reduction for Rh, 50 % for Pt and
approximately 55 % for Pd). According to the latter, and strictly considering electronic
effects, Pd is the most vulnerable to sulfur poisoning among the three mentioned noble
species, in good agreement with the observed behaviour in reforming reactions425.
In summary, noble metals may constitute an alternative to mitigate the sulfur
poisoning effects in heterogeneous catalysts. Nevertheless, there is no guarantee that these
precious metals will completely tolerate sulfur and indeed they frequently fail depending on
110
the reaction conditions and the sulfur concentration. In addition, the nature of the noble
metal is a factor to take into account. In this sense, Rh seems to be one of the most
promising.
8.2.2 Alloys, bimetallic and promoters
Many efforts have been made aiming to improve the sulfur tolerance capacity of the
traditional Ni based catalysts for reforming reactions12. The use of promoters and bimetallic
combinations (whether alloys or not) have been a frequent strategy in recent years. For
example, Xie et al. investigated the behaviour of Ni, Rh, and Ni-Rh supported on CeO2-Al2O3
catalysts in the steam reforming of hydrocarbons, introducing sulfur into the reactant
mixture 426-427 None of the Ni-containing catalysts was stable to sulfur-laden mixtures,
although the Ni-Rh catalyst requires more time before deactivation; over 60 h on-stream.
Moreover enhanced carbon deposition due to sulfur was observed, especially for Ni-based
materials, but also noble metal combinations, for example in a commercial Pt-Rh/ZrO2
catalyst for the steam reforming of ethanol/gasolines428. A small amount of sulfur (5 ppm)
was enough to deactivate this catalyst after 22 h on stream.
Other Ni based bimetallic combinations have been tried. For example, Wang et al.
carried out screening of catalysts for liquid hydrocarbon reforming using Ni-Mo, Ni-Co and
Ni-Re supported on Al2O3 and introducing 20 ppm of sulfur in the reactant mixture429. All the
bimetallic samples exhibit superior performance to the primary monometallic Ni with Ni-
Re/Al2O3 being the most active sample. Indeed, this Ni-Re/Al2O3 sample showed an
outstanding performance maintaining hydrocarbon conversions around 90% during a 300 h
test in a sulfur-containing stream and at relatively low reforming temperatures (580 °C). A
similar positive effect due to the addition of Re in a Ni/Zeolite ZSM5 system was reported
elsewhere highlighting the ability of Re to mitigate sulfur poisoning430. In addition, Re can be
111
employed not only as a part of bimetallic systems but also as a promoter of an already
active catalyst. For example, Murata et al. developed a very active Ni/Sr/ZrO2 catalyst but
with poor sulfur tolerance431. In order to improve sulfur resistance a series of dopants were
added including Re, La, Nd, Sm, Ce, Yb, Eu and Mo. Among all the dopants only La and
especially Re enhanced sulfur tolerance. Actually the best sample in this study was
Ni-Sr/ZrO2 with 5 wt% Re, which was able to remain stable during 30 h processing a
commercial premium gasoline. It can be argued that rhenium seems to be the most
promising metal to diminish Ni sulfur poisoning with the extra benefit of enhanced catalytic
activity, although the exact mechanism ascribed (sulfur tolerant alloy formation or sacrificial
phase) varies between different studies.
Some other traditional bimetallic systems are Ni-Mo and Ni-W. As indicated in the
paper of González et al., the addition of Mo and W to Ni-based catalysts reduces
deactivation in steam reforming432. The idea is to use Mo as a sacrificial agent given its
facility to be sulfidized. In the presence of any sulfur species Mo would tend to form MoS2,
Ni atoms would not be affected and so in principle the active sites should be available.
Indeed, the electronic interaction between Ni and Mo in the Ni-Mo ensemble increases Mo
electron density easing its interaction with electronegative ligands such as S78. In other
words, Ni promotes the formation of MoS2 and in some particular applications, for example
hydrodesulfurization reactions, Ni is considered a promoter while Mo is the metal that
carries out the sulfur removal. In reforming reactions the classic paper of Bartholomew
proved that a Ni-catalyst doped with Mo was more sulfur resistant than the Ni catalyst
alone in a feed containing 10 ppm sulfur 433.
The combination of an active metal for reforming reactions such as Ni or Pt with Sn is
another widely explored alternative16, 434-437. Dumesic et al. obtained very promising results
112
in hydrogen production from biomass reforming using Ni-Sn catalysts434. In principle,
bimetallic Ni-Sn phases were designed to avoid Ni deactivation due to C deposition. As
proposed by Trimm the similar electronic structure of carbon and elements of groups IV and
V of the periodic system may favor the interaction of these metals with Ni 3d electrons
thereby reducing the chance of nickel interactions to carbon16. Further, as explained by
Rodriguez and Hrbek, the addition of tin to platinum is a good strategy to prevent sulfur
poisoning78. Tin may act as a site blocker to platinum avoiding the noble metal interaction
with sulfur and improving the stability of the reforming catalysts78. Tin and platinum form
well defined alloys that are very stable103. When compared to pure Sn and Pt, these alloys
exhibit a lower chemical reactivity towards sulfur-containing species such as SO2, H2S, S2 and
thiophene438-439. Figure 26 adapted from Rodriguez´s paper underlines the superiority of the
Pt-Sn alloy in terms of sulfur uptake compared to the monometallic systems.
Among the typical site blockers (Cu, Au, Ag, Zn, Sn) tin is the best choice to promote
sulfur tolerance of Pt based catalysts78. The electronic perturbations arising from the Pt-Sn
bond produce a system which has remarkably low reactivity towards sulfur poisoning78.
Other types of bimetallic systems and alloys involving noble metals have been
proposed aiming to gain sulfur resistance440-442. Bimetallic Pt-Pd and Pt-Ni catalysts were
significantly higher sulfur tolerant compared to the monometallic Pt based catalysts during
50 h of a stability test440. A commercial catalyst from BASF based on Pt-Rh was also tested
for the ATR of JP8442. The addition of 125 ppm of sulfur in the stream slightly deactivated the
catalysts on the first 250 h of operation. A more demanding stability test based on start-
up/shutdown operations strongly affected the catalysts’ performance with these series of
start/stop cycles the main reason for the catalysts’ deactivation.
113
0 1 2 3 4 5 6 7 8 9 100,00
0,05
0,10
0,15
0,20
0,25
0,30
0,35
0,40
Sulphur uptake 300 - 310 K
Sn/Pt (111)
Pt (111)
Polycrystalline SnTo
tal s
ulph
ur c
over
age
(ML)
SO2 exposure (L)
Figure 26. Total sulfur uptake for the adsorption of SO2 on polycrystalline Sn, Pt(111),
and a Sn/Pt(111) alloy. Adapted from Acc. Chem. Res. 1999, 32 (9), 719-728. Copyright 1999
American Chemical Society.
As mentioned in the previous section, among the noble metals Pd seems to be the
least sulfur tolerant. Nevertheless, bimetallic combinations also open up a route to improve
Pd-based catalysts sulfur resistance78. Metal-metal interactions reduce the electron donor
capacity of Pd limiting its tendency to form strong bonds with sulfur-like ligands443. In
particular, Pd-Rh, Pd-Ni and Pd-Mn may present a good catalytic behaviour and be notably
less sensitive to the presence of sulfur-containing molecules in the reactant mixtures than
pure Pd78.
Briefly, it can be concluded that most of the bimetallic systems proposed in the
literature exhibit superior performance (higher catalytic activity and enhanced carbon and
sulfur resistance) compared to their individual counterparts. Several reasons account for
the positive results obtained with the bimetallic materials: (i) a change in the number of
active sites (cooperative effects); (ii) the sacrificial role played by one of the species forming
114
the bimetallic system leaving free and available the second metal; (iii) an electronic effect
coming from the metal-metal interactions resulting in less sensitive materials towards sulfur
poisoning (the bimetallic bonding modifies the chemical reactivity of the metal towards
sulfur-containing molecules, "ligand effect")
The addition of promoters is an alternative to the bimetallic systems. Special
attention has been devoted to alkali metals in this regard. Due to their electropositive
behaviour they can easily donate electrons to sulfur ligands thus shielding the interaction
between sulfur species and the actual active phase of the catalyst. Apart from the electronic
effect, these types of promoters may act as a site blocker species, physically hindering the
arrival of sulfur to the catalytic active centre. Ferrandon and co-workers demonstrated that
the addition of potassium to a Rh/Al2O3 catalyst in gasoline steam reforming appreciably
increased sulfur tolerance444. They pointed out that sulfur adsorption on the Rh/Al2O3 was
limited due to site blockage attributed to K. In turn, they found a drawback: alkali inclusion
increased the temperature in the catalyst bed by inhibition of the endothermic steam
reforming reaction more than the partial oxidation processes. At the same time, this effect
enhanced the sulfur tolerance beyond the initial expectations when K was intended to be a
mere sorbent since the stability of sulfide species decreases with temperature.
8.2.3 Support and structural modifications
So far all the discussed approaches are focused on the metallic active phase of the
catalysts. However, similarly to SOFC anodes, conventional catalysts are composed of
metal/oxide mixtures and therefore the role of the support and its behaviour towards sulfur
poisoning should not be disregarded. Indeed, on the surface of a metal oxide sulfur can
interact with the metal oxygen sites, producing species that have different electronic
properties (i.e. sulfides and sulfates) and maybe responsible for catalyst deactivation.
115
In this sense, one of the most widely used strategies to alleviate sulfur poisoning is to
select supports with high oxygen mobility13. It is well established that oxygen mobility
mitigates the carbon deposition which can accompany sulfur poisoning445-450 and presumably
helps avoid the formation of inactive metal sulfides. As mentioned in previous sections,
ceria is one of the most desirable supports when oxygen mobility is required451-452. In this
way, a rather sulfur tolerant catalyst was developed by Xue et al. using Pt supported on
alumina impregnated with ceria and gadolinia450. In this report the Pt/CGO-alumina catalysts
were compared vs. a conventional Pt/Al2O3 sample. Only the ceria based materials resulted
in immunity to sulfur attack, with significant differences depending on the order of addition
of ceria and gadolinia. Interestingly, the sample where the ceria was impregnated first was
the most stable, which the authors ascribe to an improved Pt-CeO2 interaction. This catalyst
presented good activity in commercial-gasoline reforming with relatively high sulfur
concentration (100-500 ppm provided by thiophene). The authors argue that Pt possesses
different electronic properties when supported on bare alumina compared to the ceria-
alumina based support. Pt metallic sites in alumina are unable to resist sulfur poisoning. A
valuable point of this paper is the redox mechanism that the authors proposed for sulfur
elimination. Under steam reforming conditions, thiophene was transformed to H2S which is
released and eliminated from the cycle via reduction and re-oxidation of the ceria-doped
support450. Azad and Duran obtained also some interesting results using Rh/CeO2 based
materials420. In this work, the presence of 50 ppm of H2S “activates” the catalysts increasing
H2 yields in the steam reforming of toluene. They suggested that such positive effect could
be due to the formation of Ce2O2S which presumably promotes the activity of the supported
Rh. Actually, in this particular situation ceria is acting as a sulfur sorbent and the redox
properties of ceria are useful since the reduced oxide (Ce2O3) is more prone to trap sulfur.
116
It is well known that the redox properties of ceria can be boosted by the use of
promoters resulting in materials with enhanced oxygen storage capacity196, 198, 453.
Laosiripojana et al. investigated the steam reforming of biomass tar using Ni-Fe supported
on MgO-Al2O3, coated with CGO454. The results indicated that the formation of various Ce-O-
S phases influences the catalytic activity with the sulfates having a positive effect in the
oxygen mobility and therefore increasing the activity and the sulfides producing an activity
drop. Some other examples using Pd/CeO2 samples and CuO and Y2O3 as metal oxide
additives benefit the reforming performance. These dopants increase H2 yield due to an
increase in metal surface area available for reaction. In addition, CuO increased the stability
against sulfur poisoning due to the oxide acting as a sacrificial sulfidation site, taking the
sulfur species away from the active metal and/or the ceria support421.
Some groups proposed other types of support modifications in order to improve
sulfur tolerance. For instance, incorporation of the active metal into the crystal structure of
the oxide phase, followed by exsolution of metal particles on reduction with the aim of
stabilizing the particles and at the same time increasing metal dispersion. Smaller, more
stable particles should improve sulfur tolerance since the sintering of Ni particles leads to
larger crystallites that are more easily poisoned76, 455. For example, Ni particles can be
stabilized on hexaaluminate structures 456-458. Smith et al. prepared nickel hexaaluminate
dispersed on zirconia doped ceria catalysts obtaining rather good sulfur tolerance in the
partial oxidation of methylnaphthalene458.
Pyrochlore-like structures are also interesting to avoid sulfur poisoning. Pyrochlores
are a class of ternary metal oxides based on the fluorite structure with a cubic unit cell with
a general formula of A2B2O7. An important property of these materials is that catalytically
117
active noble metals can be substituted isomorphically on the B site to form a
crystalline catalyst457. In particular, metals like Ru, Rh and Pt can be introduced into the B
site of the pyrochlore structure because they meet ionic radius constraints and have the
required oxidation state. In this situation, the metal is included in the solid network and
somehow protected towards sulfur species. The group of Spivey has done intensive research
on this type of materials457, 459-462. For example, they found that a La/Sr/Zr/Ni-pyrochlore
loses some activity with 50 ppm of dibenzothiophene at the initial stages of the reforming
reactions. However, the deactivation was not continuous with time on stream. This suggests
that the poisoning species are adsorbed on the catalyst surface in the initial steps but are
not accumulated on the surface itself. In addition, almost complete activity was recuperated
when sulfur was removed from the stream. Similar results were obtained when Rh instead
of Ni was introduced in the pyrochlore lattice462. The pyrochlore structure, although it
experienced some deactivation, was more tolerant to sulfur compared to a reference
Rh/Al2O3 catalyst.
In a similar way to the carbon tolerance section, Table 7 summarizes the developed
catalytic formulations following a given strategy to mitigate sulfur poisoning.
Table 7. Strategies to minimize sulfur poisoning in catalysts
118
119
Strategy Catalysts Process Ref.
Bimetallic catalysts
Ni-Co/MgO-Al2O3
Partial oxidation
reforming of
isooctane
409
Ni-Co/ Al2O3
Steam reforming of
liquid
methylcyclohexane
429, 432
Ni-Mo/ Al2O3
Steam reforming of
liquid
methylcyclohexane
429
Ni-Re/ Al2O3
Steam reforming of
liquid
methylcyclohexane
429
Steam reforming of
gasoline
430
Ni-Fe/MgO-Al2O3
Partial oxidation
reforming of
isooctane
409
Ni-Rh/ CeO2-Al2O3
Steam reforming of
liquid hydrocarbons
426
Ni-Sn/MgO-Al2O3
Steam reforming of
glycerol
407
Ni-Sn/CeO2-MgO-Al2O3
Steam reforming of
glycerol
406
Rh-Pd/ Gd2O3-CeO2
Steam reforming of
toluene
422
Rh-Pd/ ZrO2-CeO2
Steam reforming of
toluene
422
8.2.4 Regeneration of sulfur poisoned catalysts
Since most of catalysts are expensive and industrial productivity in many cases
depends on the catalysts’ performance, there is a need to reactivate or regenerate them.
Although the regeneration method is rather catalyst-specific, generally, they involve thermal
treatments in oxygen, hydrogen or steam atmospheres.
As indicated by Bartholomew the toxicity of the sulfur species depends on how many
electron pairs are available for the interaction with the metals9 . In general toxicity
decreases as follows: H2S > SO2 >SO42-
etc. in the order of increased shielding by oxygen.
Therefore oxidation treatment to eliminate sulfides is an alternative to recover activity.
Ideally, the main goal of oxygen treatments is to remove all the sulfur species as SO2 (Ssolid +
O2 gas → SO2 gas) at high temperature. For example, Choudhary et al. managed complete
activity recovery of a Ni-ceria based catalyst after being exposed to 7400 ppm of thiophene
by thermal treatment at 800 °C in an O2/N2 50:50 mixture463. An inherent drawback of this
procedure is the oxidation of the active phase (Ni) during the recovery thus making
necessary a reduction step before re-running the reaction.
Apart from the active phase oxidation, this type of oxidative treatment involves
other disadvantages that limit its application and therefore it cannot be considered as a
general regeneration procedure. More precisely, the exothermicity associated with this
process may produce irreversible catalyst deactivation by thermal degradation and/or phase
transformation of the active components13. For instance, some authors reported irreversible
formation of the inactive NiAl2O4 spinel when they tried to re-activate a Ni/Al2O3 reforming
catalyst using diluted oxygen at high temperatures396. In this scenario, oxidative treatment is
only useful in some specific cases when the oxidation at high temperatures would not risk
modifying the catalyst structure.
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Alternatively to oxygen, thermal treatments in steam can be applied to re-activate
sulfur contaminated catalysts. One of the seminal works in this area was carried out by
Rostrup-Nielsen dealing with Ni-based catalysts deactivated upon H2S exposure464. This
indicated that steam can remove sulfur as hydrogen sulfide via:
Ni-S + H2O → NiO + H2S
Although H2O may produce also some oxidation of reduced Ni:
Ni + H2O → Ni-O + H2
At temperatures between 800 and 900 °C up to 90% of the sulfur can be removed
from the catalyst surface. In the same paper, the positive role of alkali promoters such as Ca
and Mg in the steam regeneration was discovered. The catalysts doped with small amounts
of calcium and magnesium were easier to re-activate. In turn, some other dopants like K or
Na did not improve the regeneration process, most likely because sulfur is converted into a
form that is retained in the catalysts in the presence of K and Na.
Complete recuperation of reforming activity for bulk Ni catalysts was found by
Hassini et al. using Ar/steam mixtures465. Regenerated catalysts were characterized by
means of XPS indicating complete sulfur removal from the catalyst surface after the steam
treatment. Nevertheless, as indicated above, Ni oxidation occurred and some oxidised Ni
species were identified by infrared spectroscopy underlining again the risk of altering the
catalysts structure when an oxidative treatment is applied.
Reducing atmospheres do not present the catalyst oxidation drawback observed
when the spent samples are treated with steam or oxygen. In this sense, this alternative is
currently viewed as the most desirable way to remove sulfur from catalysts. Typically, sulfur
is released as H2S by the direct reaction of adsorbed sulfur species and H2 (Ssolid + H2 gas → H2S
121
gas). According to the thermodynamics of sulfide formation, this process is essentially
reversing the metal sulfide equilibrium formation13.
Cheekatamarla et al. observed complete regeneration of a molybdenum carbide
catalyst deactivated upon exposure to 500 ppm of benzothiophene using a sequential
thermal treatment: first one hour in He and later one hour in hydrogen at 900 °C for both
processes466. The authors claimed that the heating step in He may remove weakly adsorbed
sulfur species while for the chemisorbed species (likely forming metal sulfides) heating in
hydrogen was required.
The effectiveness of the hydrogen thermal treatment depends as expected on the
sulfur concentration used in the catalytic test. For example Hepola et al.318, 467 demonstrated
through temperature programmed hydrogenation (TPH) that complete sulfur removal from
a commercial Ni based catalysts was achieved when 500 ppm of H2S was used. On the
contrary, when the H2S concentration was increased up to 2000 ppm the hydrogen
treatment was not sufficient to eliminate all the chemisorbed sulfur. Figure 27 represents
the TPH profiles discussed in ref318
. It is clear that 2000 ppm provoked a strong adsorption of sulfur on the catalyst’s
surface making complete sulfide removal almost impossible.
122
2000 ppm H2S500 ppm H2S
300 400 500 600 700 800 900 1000Temperature (oC)
H2S
con
cent
ratio
n (a
.u.)
Figure 27. TPH profiles (70% Ar/30% H2) of Ni based catalysts after exposure to 500
and 2000 ppm of H2S in N2 at 2 MPa, 900 C and 4-6 h. (reprinted from Appl. Catal. B 1997, 14
(3-4), 305-321, with permission from Elsevier)
Other reducing mixtures have been successfully employed to regenerate sulfur
poisoned catalysts. For example Arosio et al. demonstrated that CH4-reductive pulses can
partially recuperate a sulfur-contaminated Pd/Al2O3 catalyst spent in methane
combustion468. A small increase of the temperature up to 600 °C using short time pulses (2
min.) gave almost complete catalyst regeneration. Such a treatment combines extensive
sulfate decomposition with a PdO reduction/oxidation cycle. The authors claimed that the
reductive regeneration of sulfur-poisoned catalysts with CH4-containing atmospheres could
be more effective than the analogous treatment in H2, possibly due to the milder reducing
action resulting in minor formation of sulfide species on the catalyst surface468.
In summary, there are routes to regenerate sulfur-poisoned catalysts via thermal
treatments in different atmospheres. However, the success of this process depends on
several factors such as sulfur concentration, strength of sulfur interaction with the catalyst
surface, catalyst composition and its susceptibility to be affected by the recovery treatment,
123
etc. This complex situation makes necessary a careful choice of the thermal treatment and
may not ensure complete activity recuperation.
9. Conclusions and Perspectives
Solid oxide fuel cells (SOFCs) generate electricity and heat electrochemically from
hydrogen and/or carbon-based fuels. The electrodes in SOFCs need to exhibit electronic
conductivity, oxygen ion conductivity and catalytic activity. The fuel oxidation takes place at
the anode, where the deactivation by carbon and/or sulfur is one of the key challenges in
SOFC technology.
We have reviewed the approaches used in catalysis to prevent or minimise the
effects of carbon or sulfur on catalysts. Carbon and sulfur poisoning are much more
challenging in conventional catalysis since, as opposed to SOFCs, normally there is no
oxygen flux that could help to minimise their deleterious effect. Some strategies have been
shown to work in both catalysis and SOFCs, we can have confidence that the effect is real
and the basic knowledge is in place to expand or refine those strategies.
It is clear that the search for carbon and sulfur tolerance in catalysts and solid oxide
fuel cells is exemplified by the properties of one element, nickel. Its unrivalled propensity to
catalyse carbon-carbon bond formation is matched by superiority to other base metals in a
variety of other useful reactions. It is also extremely vulnerable to the electron withdrawing
effects of sulfur. The search, then, has focused on two different goals – first to mitigate
carbon deposition and sulfur poisoning in nickel-based catalysts, and second to find
catalysts which approach the activity (and cost) of nickel without vulnerability towards
sulfur poisoning or catalytic activity towards carbon formation. Several strategies to achieve
these goals have emerged in the SOFC and catalysis literature.
9.1 Alloying of nickel
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Alloying of nickel is a strategy that can have an effect on both carbon and sulfur
tolerance. Alloying can improve carbon tolerance by reducing the rate of carbon-carbon
bond formation, reducing the amount of the most destructive and deactivating graphitic
carbon, and/or increasing the rate of competing reactions, such as carbon oxidation. For
sulfur tolerance, nickel is the element most vulnerable to sulfidation, so alloying with almost
anything improves sulfur tolerance. Conversely, since nickel is an excellent catalyst for many
of the reactions in an SOFC anode, alloying may reduce the activity for these reactions.
This strategy has been implemented in a number of different ways in both catalysis
and SOFC studies. The classic example in catalysis is addition of noble metals such as Rh and
Au, and these have been used in SOFCs with some success. The use of noble metals in SOFCs
is complicated by the larger total amount of metal, meaning that proportionally more of the
expensive noble metals need to be used. For this group of elements, the developments in
MIECs and non-metal electronic conductors for SOFC anodes may allow more realistic
amounts of these metals to be used, and nanoalloys of Ni with Au, Rh or Re may be
promising for carbon and sulfur tolerance.
In both fields the issue of cost has driven a search for cheaper alternatives. For
obvious reasons the top row transition metals from Fe through to Cu have been explored
extensively. These seem to be effective in reducing the overall carbon deposition, and
decreasing the amount of graphitic carbon. In the case of these promoters, research could
switch to other issues affecting SOFCs, for example tolerance to redox cycling or
compatibility with electrolytes and other components, as their ability to mitigate carbon
deposition seems largely agreed upon.
Outside of the top row transition metals, there are some other candidates for carbon
and sulfur tolerance, the most promising of which is tin. Tin has been trialled in both
125
catalysis and SOFC anodes and appears to confer both carbon and sulfur tolerance. A
further advantage of tin is that the mechanism by which it works is reasonably well known,
meaning this is a good target for further testing in terms of long term stability and
compatibility. Another promising element is molybdenum, which is widely used in catalysis.
It has complicated chemistry, with different carbide, sulfide and oxide phases being stable
under possible regimes in an SOFC anode, meaning that further work to clarify its behaviour
is needed. However, it is potentially a promising electrocatalyst in its own right, so further
investigation may be fruitful.
9.2 Alkaline promoters and supports
It is well known in catalysis that basic oxides reduce carbon deposition by increasing
the carbon oxidation rate. This is thought to work as the basic sites act as stores for highly
reactive hydroxyl radicals. The strategy has found use in the SOFC literature, with elements
such as Ba and La looking the most promising for further investigations. The use of alkali
metals is underexplored compared to catalyst science, due to the higher mobility of these
elements, and also their potential for poisoning the catalytic reactions. The vapour
pressures of their oxides approaches that of Ni at 1000 °C (~10-10 bar) at temperatures
ranging from ~800 °C for Li2O down to ~500 °C for K2O, while the melting points of Na2O
(1132 °C) and K2O (740 °C) are also a concern. The move to intermediate temperature fuel
cells may bring at least Na and Li into play.
One interesting strategy which currently appears to be unique to the SOFC literature
is the use of basic cationic conductors such as Li+ and H+ conductors for carbon tolerance
(although the latter have recently begun to be used as catalysts for the reverse water-gas
shift reaction, the link to carbon tolerance has not been made469-470). These maintain the
basicity of the materials promoted solely with simple non-conducting alkali and alkaline
126
earth oxides but add in some extra conductivity to improve both the carbon tolerance and
electrochemical performance. In the case of Li+ conductors the Li is also stabilised so less
volatile.
One aspect which needs exploration regarding these basic promoters is their sulfur
tolerance, especially regarding their ability to mitigate carbon deposition in a sulfur-
containing gas feed.
9.3 Ceria, doped ceria and oxygen storage
It is fair to say that the discovery of the redox properties of ceria and doped cerias
has revolutionised both catalysis and SOFC science, especially given the oxide ion and
electronic conductivity of doped cerias. These materials work both by acting as a store for
oxygen which is then able to react with carbon species, and by their ability to trap sulfur
species. It has also been shown conclusively that doped cerias are both electrocatalysts and
catalysts in their own right for important reactions such as electrooxidation of hydrogen and
reforming of hydrocarbons. It is clear that doped cerias will continue to be incorporated into
the current and next generations of SOFC anodes.
With such a useful and varied class of materials there are obviously many fruitful
avenues for research. One of the most obvious is the use of the extremely high oxygen
storage capacity materials found in three-way catalysts and other catalytic systems. The
earliest of these, the Ce-Zr system, has been somewhat investigated, but it does not appear
that other ceria-based systems have been used at all in SOFCs. It is also worth noting that
although ceria-based oxygen storage materials are favoured because of their relative
structural stability on redox cycling, the use of impregnation and MIECs may allow the use of
less stable oxygen storage materials.
127
Other possible routes for further investigation include trying to improve the catalytic
activity of doped cerias. Current work in SOFCs has focused on the doped cerias with the
highest ionic conductivity, but a focus on the activity towards electrooxidation and
reforming of hydrocarbons may be useful, especially where ceria is not the only ionic
conducting species. Work to improve the sulfur storage capacity could also be important.
9.4 Preferential sulfur binding sites
Phases which preferentially bind sulfur, and thereby lower the sulfur coverage on Ni
or other active metals have been used in both SOFCs and catalysis. In catalysis species such
as Cu, Zn and Mo are known to act as sulfur sorbents in preference to Ni, while in SOFCs this
effect has been noted in ceria and Ba-containing compounds. In addition, there is literature
on sulfur sorbents for gas cleaning which may be useful405. The deposit of a barrier layer (i.e.
the first point of contact with the fuel) that protects the most electrochemically active area
of the anode (i.e. close to the electrolyte layer) is known to protect against carbon
deposition in SOFCs, but has not yet been investigated for sulfur poisoning.
9.5 Non-metal electronic conductors
Removal of the metal electronic-conducting phase solves many of worst effects of
carbon and sulfur poisoning, and there are two possible solutions for this. Non-metal
conductors in a cermet such as carbides or carbon retain the benefits of cermets, such as
the ability to independently optimise the electronic and oxide conducting phases, as well as
the disadvantages, such as having to match thermal expansion coefficients and more
complicated microstructure optimisation. MIECs lose both the advantages and
disadvantages of cermets. Both non-metal conductors and MIECs as potential solutions have
deficiencies in different areas. Non-metal conductors are generally under-researched and in
particular more work on stability is needed. MIECs are lacking in either electronic or ionic
128
conductivity, and some of the more widely used materials, such as the strontium titanates,
require high processing temperatures and are difficult to fabricate into anodes. Both
solutions are lacking in catalytic activity and will likely require a further catalytic phase.
9.6 Infiltration of nanoparticles
In catalysis, infiltration of porous structures with metal nanoparticles is a common
practice to maximise the active surface while simultaneously hindering carbon deposition by
decreasing the area of graphitic growth. In SOFCs, this approach was first used because the
low melting point of copper oxide meant that Cu-YSZ anodes could not be produced by the
conventional solid state route. Since then it has been used to add a variety of metals
(including nickel) and now has been proved that it can improve important parameters such
as the triple phase boundary length.
There are many issues to be resolved with infiltration, especially relating to long
term stability and feasibility of scaling up the process to industrial-sized anodes, but the
reason it is interesting for carbon and sulfur tolerance is that it allows much greater control
over the chemistry and structure of the electrode. The exploration of the possibilities in
SOFC anodes is only just beginning but already we have seen that the infiltration of barium
or ceria allows fine dispersion of the promoter over the surface of the material, enhancing
carbon tolerance by ensuring that any given nickel particle is close to a particle of the
promoter.
In the future we could see more complex oxygen storage materials or more
advanced catalysts incorporated into the anode structure by this method. The advances in
MIECs and possibilities of non-metal conducting phases such as carbon and carbides should
allow designed catalyst nanoparticles (whether containing nickel or not) to be added
without their effect being destroyed by alloying into the percolating metal phase.
129
9.7 Regeneration
Catalysts deactivated by carbon deposition are commonly regenerated by stopping
the process and then passing a stream of cleaning gas (hydrogen, steam, carbon dioxide or
oxygen). All of these gases are eventually present in a SOFC anode: as fuel, as a product of
oxidation, as a permeant gas, etc. The literature on regeneration of SOFC anodes is very
sparse, and modifications of the anode to aid regeneration are non-existent. Nevertheless, it
has already been shown that it is possible to remove carbon deposits from Ni-YSZ anodes by
a variety of gases, and also by oxygen flux through the electrolyte.
Ideally anodes would be designed so that they can be regenerated without use of
alternative feedstocks or extensive downtime, but failing this they need to be designed to
be regenerated at the minimum temperature for as short a time as possible, and be able to
withstand any changes which take place during regeneration. From the catalysis literature it
is probable that many promoters which prevent carbon deposition in the first place are also
effective in aiding regeneration, whereas for sulfur tolerance where sacrificial phases are
used, these might bind more strongly to sulfur, requiring harsher conditions for
regeneration.
Exsolution of nanoparticles from MIECs and symmetrical SOFCs also provide
interesting alternatives to conventional cermet anodes in terms of regeneration. While this
potential benefit has been noted in the literature on these materials there is little published
experimental work proving it.
9.8 Theoretical and computational studies
It has become clear in the last five years that theoretical and computational
chemistry is finally becoming able to provide accurate insights into the chemical behaviour
of materials and even interfaces471. Very recently accurate predictions have even been made
130
as to the structure and properties of previously unknown materials relevant to SOFCs472. A
historical problem in SOFC research (and scientific research in general) is the lack of
coordination between groups working in different fields. Groups working in fields such as
materials chemistry and detailed in situ and ex situ characterisation would surely benefit
from incorporating insights from theoretical chemistry in the future, and making sure their
work is relevant to the challenges in SOFCs.
9.9 Reflections on experimental work
Many in the literature have claimed experimental results of tolerance against carbon
and sulfur. However, caution needs to be taken in the techniques used to analyze these
results, and during the research and writing of this review we have noted some points
relating to this:
Claiming carbon tolerance by lack of performance degradation. It is certainly
true that an anode is carbon tolerant if it maintains performance over a long period of time
regardless of whether or not carbon is actually present. However, it cannot be claimed that
a lack of degradation means that there is no carbon, as it has been shown many times that
cells can operate without performance degradation for significant periods despite carbon
being deposited. Certainly a testing period of a day or a week as used in many papers is not
long enough to claim carbon tolerance in the absence of other data proving that either
there is no carbon or that the carbon has reached some kind of steady state.
Claiming carbon tolerance by lack of carbon in SEM. While SEM is clearly a
useful technique for assessing microstructure, the lack of carbon whiskers in an SEM image
is not proof of a lack of carbon. Even EDX needs careful sample preparation for accurate
quantitative analysis, for example polishing.
131
Low measured OCVs. This applies to two different systems – thin doped ceria
electrolytes and air leakage into the anode chamber. It has been shown that leaks causing
an increase in measured OCV of less than 0.02 V from 1.223 V to 1.239 V in dry methane
results in a decrease in implied water content (as calculated from the H2/O2 equilibrium) of
30% from 0.24% to 0.17%473. This is enough to result in a dramatic decrease by over half in
the amount of carbon deposition over a period of one hour. Likewise, many groups using 10
– 50 µm CGO and CSO electrolytes report OCVs below 0.9 V, which implies a substantial
oxygen flux at OCV through the electrolyte caused by the electronic conductivity of doped
ceria (or through a slightly permeable electrolyte). This oxygen flux or leakage should be
extremely effective at preventing carbon deposition and improving sulfur tolerance, but if
the aim is to study the carbon and sulfur tolerance of the electrode materials then care
should be taken to account for this.
Current collectors. The paper referred to above also showed that the
coverage of silver current collector paste can have a large effect on carbon deposition, even
completely preventing it if the electrode is completely covered473. Presumably this works in
a similar way to the barrier layer concept discussed in section 5.2. There has also been
considerable controversy over the use of platinum current collectors, with claimed high
power densities in dry methane for some MIECs being shown to be almost entirely due to
the use of platinum current collectors and doped ceria electrolytes240.
Humidification. As shown above, the exact level of humidification can have a
profound effect on carbon and sulfur tolerance. Many studies report 3% humidification
levels, which if using a bubbler, implies a water temperature of just under 25 °C (25 °C
would actually be 3.1%). This seems quite warm for a lab temperature, although the authors
of this review are based in Britain so maybe used to cooler temperatures than many. The
132
humidity level at 20 °C is 2.3%, and at 16 °C is 1.8%, care should be taken that the correct
humidity levels are being reported. It should also be borne in mind that these are 100%
relative humidity levels, whereas it is known that bubblers are not necessarily effective in
reaching 100% relative humidity474.
Interaction between carbon and sulfur. Many papers claim carbon and sulfur
tolerance, with separate experiments done to prove each using model gas feeds (e.g. one
experiment with dry methane and another with H2S in H2). However, it is well known from
both catalysis and SOFC literature that there is strong interaction between carbon and
sulfur, with each having the possibility of hindering or promoting the other under different
operating conditions. While there are undoubtedly advantages in simplifying the system by
separately studying carbon and sulfur tolerance, it does not necessarily follow that a system
which is separately carbon and sulfur tolerant will be simultaneously carbon and sulfur
tolerant. This could especially be the case where sulfur may react with promoters which are
present to reduce carbon deposition, for example with BaO. More work needs to be done
on sulfur-containing carbonaceous fuel in SOFC anodes.
In this review, we have seen the different materials solutions to carbon and sulfur
tolerance in catalysis and solid oxide fuel cells, but there is also an aspect of different
experimental techniques in the two fields. The foundational techniques of catalysis are well
established over decades, with a focus on gas phase techniques such as chemisorption
measurements and temperature-programmed reactions. Some of these have started to be
incorporated into SOFC studies, for example temperature-programmed oxidation. In SOFCs,
electrochemical impedance spectroscopy (EIS) has long been a key feature of the
investigations, and although it has not been used in catalysis, with advances in impedance
133
analysers, analysis techniques such as distribution of relaxation times (DRT) and modelling,
it is possible that EIS could start to be used in model catalysis studies.
Because of the more widespread nature of catalysis science, newer techniques have
generally been adopted in catalysis first, and later in solid oxide fuel cells. An example of this
is in situ Raman, which has been known in catalysis since the early 1990s, but is now being
used to investigate SOFCs. Other techniques such as high-resolution TEM, XPS and XRD are
following a similar path. A notable counter-example is FIB-SEM and tomography in general,
which appears to have been far more enthusiastically adopted in the SOFC community than
in catalysis.
As can be seen, the experience gained in the field of catalysis has had an increasing
influence in the research paths in SOFC and hopefully in the near future, this inspiration may
be reciprocated as catalysis can profit from the experience of SOFCs. Although electro-
catalysis at low temperatures is common, the high temperature regime is still an area of
opportunity for catalysis as it features the unique capability of supplying/extracting O2- or H+
to reactant species under an electric bias. A few examples are the synthesis of ammonia at
atmospheric pressure475, the non-Faradaic electrochemical modification of catalytic activity87
and the electrochemical reduction of CO2 and H2O476-477.
It is clear that the SOFC community has made great strides towards carbon and
sulfur tolerance over the last decade. Going forward, the strategies already implemented at
lab scale need to be incorporated into more commercially-focused devices, while at lab
scale the learnings from catalysis should be used to develop materials which are carbon and
sulfur tolerant, especially at lower temperatures. We hope that this review is able to help
with both of these.
10. Acknowledgements
134
Funding for this effort has been provided by Boeing Research & Technology
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Biographies
Paul Boldrin received a PhD in materials science from Queen Mary University of London
working on continuous hydrothermal synthesis of nanomaterial for catalysis. This was
followed by postdoctoral work in chemistry at the University of Liverpool working initially on
high throughput discovery of catalysts as a research associate and later as research
coordinator. Currently he is a postdoctoral research associate at Imperial College London
working on solid oxide fuel cells. His research interests include characterisation of the
catalytic and electrocatalytic processes occurring in solid oxide cells and membrane
reactors, and the use of nanomaterials in those devices.
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Enrique Ruiz-Trejo obtained his PhD. in Materials from Imperial College and immediately
after was appointed lecturer at Universidad Nacional Autónoma de México. He was then
awarded a Humboldt scholarship at the Max Planck Institute for Solid State Research. In
2009 he moved to Denmark as Senior Scientist at Risoe National Laboratories for
Sustainable Energy followed by a position as Research Fellow at the University of St
Andrews. Since 2012 he is Research Associate in Fuel Cells and Materials Processing at
Imperial College. His areas of interest include materials for energy applications and gas
separation membranes, the development of electrodes for fuel cells and the manufacture of
metal-ceramic composites.
Joshua Mermelstein is a fuel cell systems engineer at the Boeing Company in Huntington
Beach, CA with an expertise in solid oxide fuel cell (SOFC) and proton exchange membrane
(PEM) fuel cell systems. He is currently the lead scientist for fuel cell system development
within Boeing’s Electronic and Information Solutions Advanced Technology Programs (ATP).
Joshua is currently leading efforts as the chief engineer for Boeing’s development of a 50 kW
reversible solid oxide fuel cell (RSOFC) system used for microgrid energy storage. Joshua
also provides technical support for the development of other SOFC and PEM based fuel cell
systems throughout Boeing.
Joshua earned his Bachelor’s degree in Chemical Engineering from the University of Arizona
in 1999, Masters from the University of Southern California in 2000, and Ph.D. from Imperial
College of London in 2010 with the Department of Chemical Engineering and Fuel Cell
Research Group of the Energy Futures Lab, after working in industry as a chemical engineer
for 7 years. His research at Imperial College focused on the impact and mitigation of carbon
formation on SOFC anodes arising from biomass gasification tars through steam reforming,
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partial oxidation, and dry reforming technologies. Joshua has published over 10 publications
related to his work in this field. His career background spans 10+ years of industry
experience in chemical/process engineering, cryogenic and compressed gases, hydrogen
and fuel cell technology, fuel cell electric vehicles, plug-in hybrids and BEVs, alternative and
renewable energy for stationary power, hydrogen production, and combined heat and
power (CHP) for energy efficiency.
Jose M. Bermúdez graduated in Chemical Engineering (2008) and got a MSc in Process and
Environmental Engineering (2010) from the University of Oviedo, Spain. He obtained his PhD
in Chemical Engineering (2013) from the same university. His PhD Thesis deals with the CO2
reforming of coke oven gases to produce syngas for methanol synthesis and was developed
in the National Institute of Coal-CSIC (Spain). He worked in this research centre for more
than 5 years, where he was involved in the development of microwave-assisted processes in
the field of energy, mainly focusing on pyrolysis, gasification and catalytic heterogeneous
reactions. He gained a postdoctoral position in Imperial College London in 2014, where he is
working on the thermochemical stability of mixed ionic-electronic conductors for oxygen
transport membranes. He is also involved in the development of thermochemical processes
like supercritical water upgrading or catalytic hydrocracking of heavy oils and biomass. He
has co-authored more than 30 peer-reviewed papers and 2 patents on these topics and has
been finalist of the Best Young Researcher Award 2015 of the Spanish Group of Coal.
Tomas Ramirez Reina received his PhD in Chemistry from the University of Seville (Spain) in
2014 under the supervision of Prof. Odriozola and Dr. Ivanova. For his PhD work, he was
awarded “best PhD thesis 2014” by the Spanish Society of Catalysis (SECAT). He worked as
visiting researcher in 2011 in the Brookhaven National Laboratory (NY, USA) and in 2012 in
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the Institute of Chemical Engineering ICE-HT (Patras, Greece). In 2014 he moved to UK as
Research Associate in the Chemical Engineering Department at Imperial College London.
Currently, Dr. Reina is a lecturer in the Department of Chemical and Process Engineering in
the University of Surrey. His research interests include the development of advanced
heterogeneous catalysts for energy and sustainability. In particular, his work is focused on
clean hydrogen production, selective oxidation and hydrocarbon upgrading.
Nigel Brandon's research is focused on electrochemical devices for energy applications, with
a particular focus on fuel cells, electrolysers, and batteries. He is Director of the UK Research
Council Energy programme funded Hydrogen and Fuel Cells SUPERGEN Hub, and Co-
Director of the SUPERGEN Energy Storage Hub. He was the founding Director of the Energy
Futures Lab at Imperial College, and a founder of Ceres Power, an AIM listed fuel cell
company spun out from Imperial College in 2000. In 2014 he was appointed to the BG Chair
in Sustainable Gas and as founder Director of the Sustainable Gas Institute at Imperial
College.
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