Solid Oxide Fuel Cells (SOFC) - ENDURANCE -...

Solid Oxide Fuel Cells (SOFC)

Dr Waldemar BujalskiSenior Research Fellow

School of Chemical EngineeringCollege of Engineering and Physical Sciences

University of Birmingham

Outline• PEMFC – Proton Exchange Membrane Fuel Cell• SOFC - How it works?• A bit of theory• Standard SOFC design• SOFC components and materials• Cell and stack design• Planar cell and stacks fabrication methods• Cell and stack testing• Tubular SOFC designs (large and micro-tubular)• Work on SOFC at Chemical Engineering

– FP 6 European Project– Rolls Royce IP SOFC concept and testing facilities at

Birmingham (some thermal testing results)

PEMFC (Proton Exchange Membrane Fuel Cell)

Anode reaction: Cathode reaction:

−+ +⇒ eHH 222 OHeOH 22 2212 ⇒++ −+

Nernst equation for EMF(electromotive force or reversible open circuit voltage)

Overall electrochemical reaction for SOFC:

leads (for pressure given in bar) to:

where:E - the electromotive force (or reversible open circuit voltage), VEº - the EMF at standard pressure, VR - the universal gas constant, 8.314 JK-1mol-1F - Faraday constant (the charge of one mole of electrons), 96485 Coulombspi - partial pressure (i - hydrogen, oxygen and steam), bar

12 2 2 ( )2 steamH O H O+ →

12

2 2

2

0 ln2

H O

H O

p pRTE EF p

⎛ ⎞⎜ ⎟= +⎜ ⎟⎝ ⎠

PEM Stack

MicroCHP

Micro CHP in House

MicroCHP system at Lye

Microcab car

Hybrid – Electric/Fuel Cells (1.2kW Ballard)

• 10 large cylinders, each containing 30 kg of metal hydride power.

• Gives about 5 kg of hydrogen.• Operating pressure is < 10 bar

PEM Fuel Cell Batteries & Motor

Successful ProjectsHydrogen Fuel Cell Narrow Boat

The Protium Project

How it works?• The SOFC is a complete

solid-state device that uses an oxide-conducting ceramics material as electrolyte:– Only two phases involved

i.e. gas and solid;– Both hydrogen and carbon

monoxide can act as fuels;– A negatively charged ion

(O=) is transferred from the cathode through the electrolyte to the anode with water produced at the anode.

Schematic showing reactions and charge transfer in a

typical solid oxide fuel cell fuelled with Hydrogen

Reaction at the cathode;

O2 + 4e- → 2O2 -

Porous anode

Electrolyte

Porous Cathode

Oxygen

Hydrogen

External Load

e-

e-e-

e-

O2-

H2

O2

Reaction at the anode;

2H2 + 2O2- → 2H2O + 4e-

Development can be traced back to Nernst (1899) who was the first to describe zirconia (ZrO2) as an oxygen ion conductor.

Nernst equation for EMF(electromotive force or reversible open circuit voltage)

Overall electrochemical reaction for SOFC:

leads (for pressure given in bar) to:

where:E - the electromotive force (or reversible open circuit voltage), VEº - the EMF at standard pressure, VR - the universal gas constant, 8.314 JK-1mol-1F - Faraday constant (the charge of one mole of electrons), 96485 Coulombspi - partial pressure (i - hydrogen, oxygen and steam), bar

12 2 2 ( )2 steamH O H O+ →

12

2 2

2

0 ln2

H O

H O

p pRTE EF p

⎛ ⎞⎜ ⎟= +⎜ ⎟⎝ ⎠

Standard SOFC• Based on an electrolyte of zirconia stabilised with the addition of a small

percentage of yttria (Y203). Above temperature of 800ºC zirconia becomes a conductor of oxygen ions (O=) and typically the state of the art zirconia based SOFC operates between 800 and 1100ºC (this presents both challenges for the construction and durability but also opportunities e.g. for combined cycles application such as CHP, etc.).

• The anode is usually a zirconia cermet (an intimate mixture of ceramic and metal). The metallic component is usually Ni (nickel chosen amongst other things because of its high electronic conductivity and stability under chemically reducing and part reducing conditions; it can also be used as catalyst for direct internal reforming on the anode!);

• Cathodes proved to be difficult from the point of view of choosing the right material (initially nobel metals were used but proved too expensive for mass production!). At present, most cathodes are made from electronically conducting oxides or mixed electronically conducting and ion-conducting ceramics. The most common cathode material of the latter type is strontium-doped-lanthanum manganite (SLM).

SOFC components• Electrolytes:

– Zirconia doped with 8 to 10 mole % yttria (yttria-stabilised zirconia (YSZ)) is still the most effective electrolyte for the high temperature SOFC (but others such as Bi2O3, CeO2 and Ta2O5have been also investigated with mixed success). Advantages:

• Zirconia is highly stable in both the reducing and oxidising environments that are experienced at the anode and cathode, respectively;

• The ability to conduct O= ions is brought about by th fluorite crystal structure of zirconia in which some of the Zr4+ ions are replaced by Y3+ ions. When ion exchange occurs, a number of oxide-ion sites become vacant because of three O= ions replacing four O= ions. Oxide-ion transport occurs between vacancies located at tetrahedral sites in the perovskite lattice.

• The ionic conductivity of YSZ (0.02 Scm-1 at 800ºC and 0.1 Scm-1 at 1000ºC) is comparable with that of liquid electrolytes and it can be made very thin (25-50 μm) ensuring that the ohmic loss in the SOFC is comparable with other fuel cell types.

SOFC components• Electrolytes:

– Most recently other materials have been produced with enhanced oxide-ion conductivity at temperatures lower than that required by zirconia. For example, LaSrGaMgO (LSGM) proved to be a superior oxide-ion electrolyte that provides performance at 800ºC comparable to YSZ at 1000ºC.

Specific conductivity versus reciprocal temperature for selected solid-oxide electrolytes

Typical single cell performance of LSGM electrolyte (500 μm thick)

SOFC components• Anode:

– The anode of state of the art SOFCs is a cermet made of metallic nickel and a YSZ skeleton. Advantages:

• The zirconia serves to inhibit sintering of the metal particles and provides a thermal expansion coefficient comparable to that of the electrolyte;

• The anode has a porosity (20-40%) so that mass transport of reactants and product gases is not inhibited;

• However, there is some ohmic polarisation loss at the interface between the anode and the electrolyte and bi-layer anodes have been introduced in order to overcome this problem (often a small amount of ceria is added to the anode cermet; this improves the tolerance of the anodes to temperature and redox cycling );

• Control of the particle size of the YSZ can also improve the stability of the anode under redox conditions.

SOFC components• Cathode:

– The cathode is also a porous structure which must allow rapid mass transport of reactant and product gases. Strontium-doped lanthanum manganite (La0.84Sr0.26)MnO3, a p-type semiconductor, is the most commonly used material;

– Other materials may be also used:• Particularly attractive is p-type conducting perovskite structure that

exhibits mixed ionic and electronic conductivity. This is especially important for lower-temperature operation since polarisation of the cathode increases significantly at those conditions. The advantages are particularly apparent in cells operating at around 650ºC and as well as the perovskites, lanthanum strontium ferrite, lanthanum strontium cobalite and n-type semiconductors are better electrocatalysts than the state of the art LSM because they are mixed conductors.

SOFC components• Interconnect material (in general):

– The interconnect is the means by which connection is achieved between neighbouring fuel cells. In planar fuel cell technology this it the bipolar plate (however, it’s different for tubular geometry).

• Metals can be used as interconnects but they are expensive “inconel” type stainless steels (particularly for stacks that need to operate above 800ºC);

• Conventional steels also have a mismatch in thermal expansion coefficient with the YSZ electrolyte;

• Development of new alloys (e.g. Cr-5Fe-1Y2O3 Siemens/Plansee alloy) lead to poisoning the cathode with chromium leading to its deposition at the three phase (LSM/YSZ/gas) boundary and causing rapid de-activation of the cathode;

• An advantage for the low temperature SOFC is that cheaper materials may be used (such as austenitic steels, which do not contain chromium);

• Metal interconnects also tend to form oxide coatings which can limit their electrical conductivity and acts as a barrier to mass transport.

SOFC components• Interconnect material (tubular design):

– The use of a ceramic material for the interconnect is favoured for the tubular design and lanthanum chromite being the preferred choice (its conductivity is further enhanced when some of the lanthanum is substituted by magnesium or other alkaline earth elements).

– However, the material needs to be sintered to quite high temperature (1625ºC) to produce a dense phase and this makes fabrication very difficult:

• All the cell components need to be compatible with respect to chemical stability and mechanical compliance (i.e. similar thermal expansion coefficients);

• The various layers need to be deposited in such a way that good adherence is achieved without degrading the material due to the use of too high sintering temperature;

• Many of the methods of fabrication are proprietary and lots of research is being devoted in this area.

SOFC components• Sealing materials:

– Major issue with SOFCs is the method of sealing the ceramic components to obtain gas tightness, particularly with planar design. The wide temperature ranges pose significant difficulties.

– Usual approach is to use glasses that have transition temperature close to the operating temperature of the cell (these materials soften when the cells are heated up and form a seal all around the cell and this is particularly important for the planar design in which a number of cell can be assembled in one layer).

– A particular problem is the migration of silica from such glasses (especially onto the anodes) causing degradation in cell performance.

– For all-ceramic stacks, glass ceramics have been used, but migration of the silica component can still be a problem on both the anode and cathode sides.

Cell and stack designs

• Numerous designs od SOFCs emerged since 1930s but since 1960s most development was focused on:– Planar SOFC

• Short stack planar (Julich)• Integrated Planar (Rolls Royce)

– Tubular SOFC• Siemens Westinghouse (relatively large diameter

>15 mm)• Microtubular SOFC (Adelan) (diameter < 5 mm)

Cell and stack designs

• Planar SOFC design– Cell componets are configured as flat plates

which are connected in electrical series:

Cell and stack designs• Planar SOFC design – key requirements:

– Electrical performance:• Minimise ohmic losses thus the current path in the components (especially those having

low electrical conductivity) must be designed to be as short as possible;• There must be good electrical contact and sufficient contact area between the

components;• The current collector must also be designed to facilitate current distribution and flow in

the stack.– Electrochemical performance:

• Design must provide for full open circuit voltage and minimal polarisation losses thus significant gass leakage or cross-leakage and electrical short must be avoided;

• Fuel and oxidant must be distributed uniformly not only across the area of each cell but also to each cell of the stack;

• The gases must be able to quickly reach the reaction sites to reduce potential for mass transport limitations.

– Thermal management:• Means for stack cooling and more uniform temperature distribution during operation.

The design must permit the highest possible temperature gradient across the stack.– Mechanical/structural integrity:

• Adequate mechanical strength for assembly and handling. Mechanical and thermal stresses must be kept to minimum to prevent cracking, delamination or detachment of the components under the variety of operating conditions the stack can experience.

Cell and stack designsThe most important design feature of the Planar SOFC relates to gas flow configuration and gas manifolding which can be arranged in several ways

Fuel and oxidants flows can be arranged as cross-flows, co-flows or counter-flow with potentially significant effects on temperature and current distribution within the stack depending on the stack precise configuration.

Cell and stack designs• Flow channels are used to:

– Increase uniformity of gas distribution and to promote heat and mass transfer in each cell;

– Often, the design takes care of sufficient pressure drop through the cell to promote cell-to-cell flow uniformity within the stack;

– The flow channels electrically connect the interconnect and the electrodes, thus the contact area must be considered in the design to minimise contact resistance losses.

Cell and stack designs• Gas manifolds (external

and internal):– Any stack design must

include gas manifold for routing gas from a common supply point to each cell and removing; they often require sealing to prevent gas leakage or crossover;

– Manifolds should be design to have a low pressure drop in order to provide uniform flow distribution to the stack.

Planar Cell designs• These can be broadly classified into two

categories:– Self supporting where one of the cell components

(often the thickest layer) acts as the cell structural support (thus the single cell can be design as electrolyte, anode or cathode supported);

– In the external supporting configuration, the single cell is configured as thin layers on the interconnect or a porous substrate;

– Again, the most common cell materials are:• Electrolyte: yttria-stabilised zirconia (YSZ);• Anode: lanthanum strontium manganite (LSM);• Cathode: nickel/zirconia cermet (Ni/YSZ).

Planar Cell designs

Micrograph of a planar cell on porous metal structure

Features of planar single cellCell configuration Advantage Disadvantage

Self-supportingElectrolyte supported Relatively strong structural support

from dense electrolyteLess susceptible to failure due to anode re-oxidation

Higher resistance due to lower electrolyte conductivityHigher operating temperatures required to minimise ohmic loses

Anode supported Highly conductive anodeLower operating temperature via use of thin electrolyte

Potential anode re-oxidationMass transport limitations due to thick anodes

Cathode supported No oxidation issuesLower operating temperature via use of thin electrolyte

Lower conductivityMass transport limitation due to thick cathodes

External supportingInterconnect supported Thin cell components for lower

operating temperatureStronger structures from metallic interconnects

Interconnect oxidationFlowfield design limitations due to cell support requirement

Porous substrate Thin cell components for lower operating temperaturePotential for use of non-cell material for support to improve properties

Increased complexity due to addition of new materialsPotential electrical shorts with porous metallic substrate due to uneven surface

Cell fabrication– Based on deposition approach wide range of techniques have

been used focusing on the aim of making thin (5-20μm) YSZ electrolytes:

• Sputtering - involving electrical discharge in argon/nitrogen mixtures to deposit YSZ;

• Dip coating - porous substrates are immersed in YSZ slurries of colloidal size particles. Deposited films are then dried and fired;

• Spin coating - YSZ films are produced on a dense or porous substrate by spin coating a sol-gel precursor followed by heat treatment at relatively low temperatures;

• Spray pyrolysis - a solution consisting of powder precursor and /or particles of the final composition is sprayed onto a hot substrate followed by a sintering step to densify the deposited layer;

• Other methods - electrophoretic deposition, slip casting, plasma spraying, electrostatic assisted vapour deposition, vacuum evaporation, lase spraying, transfer printing, sedimentation method, and plasm metal organic chemical vapour deposition.

Stack testing

Design will be shown and discussed at the laboratory session in the afternoon.

Stack testing

Tubular SOFC design

• Two general types are being pursued:– Large diameter (> 15

mm) (Siemens Westinghouse Power Corporation;

– Very small diameter (< 5 mm) (Adelan, UK; Adaptive Materials Inc., USA).

This design will be discussed and its performance investigate/tested further at the laboratory session this afternoon.

microtubular

Large tubular SOFC design

• The cell components are deposited in the form of thin layers on a cylindrical tube.

– In the earlier designs the tube was made of calcia-stabilised zirconia; this porous support tube (PST) acted both as a structural member onto which the active cell components were fabricated and as a functional member to allow the passage of air to the cathode during cell operation;

– This porous support tube was fabricated by extrusion followed by sintering at an elevated temperature (problems with air flow);

– The porous support tube was eliminated and replaced by a doped LaMnO3 tube (air electrode-supported cell, AES) leading to significant improvements in performance (see next figure)

Eliminating the need for gas-tight seals

Dr W. BujalskiSenior Research Fellow

Chemical Engineering• Group leader: Professor Kevin Kendall, FRS

“Development of solid oxide fuel cells (SOFC) using a range of existing hydrocarbon fuels”

– The main work on fuel cells is emphasised on SOFC’s with projects on:

• Materials specification for electrolyte, electrode and interconnect;• Operating conditions of the SOFC, especially with direct injection of various

fuels including methane, propane, butane, iso-octane, methanol, ethanol, ethers, biodiesel and biogas;

• The potential of use of supercritical fluids for fuel processing to convert biomass into syngas;

• Fuel cells operating on hydrogen from waste sugar (and also biogas –mainly CH4) looking at flow and catalysis issues in collaboration with other groups;

• There are 10 PhD students in the group at different stages of their research advancement.


Chemical Engineering

• Since February 2004 we have been involved in EU Project:

Sixth Framework Programme (Priority 6):SUSTAINABLE ENERGY SYSTEMS

“Realising reliable, durable energy efficient and cost effective SOFC systems”


Aims of the ProjectReal-SOFC is an Integrated Project aimed at:

1. Solving the persisting generic problems of ageing with planar Solid Oxide Fuel Cells (SOFC) in a concerted action of the European fuel cell industry and research institutions. This includes:

• gaining full understanding of degradation processes;• finding solutions to reduce ageing;• producing improved materials that then will be tested in stacks.

2. In this process further consideration will be given to:• the design of cost effective materials;• low cost components;• optimised manufacturing process.


Structure of the Project1. Overall, the project has been split into seven, so called, Working Packages (WP) in

order to cater for the strongly interdisciplinary character of SOFC technology. There are 26 partners involved in the project from 13 European countries with four from United Kingdom and Department of Chemical Engineering at The University of Birmingham being one of them.

2. We are involved in WP 1 i.e. “Understanding the ageing of SOFC in industrial applications”, which focuses on the identification of the mechanisms responsible for the degradation of SOFC stacks (two different designs to be provided for testing by Rolls Royce (UK) and Forschungszentrum Jülich GmbH (Germany) i.e. industrial partners, respectively) and the determination of the sensitivity of these mechanisms for the operational parameters relevant to those realistic systems.

3. This WP 1 is led by Energy Research Centre of the Netherlands and consists of seven sub-tasks with The University of Birmingham responsible for Task 4 (WT 1.4) i.e. “Cycled stack operation for 50 to 100 cycles at defined conditions”. This involves both experimental testing and computer modelling for predicting reliability and durability of cells and stacks.

.

Framework 6 “Sustainable Energy Systems” Integrated Project Framework 6 “Sustainable Energy Systems” Integrated Project

Work Task 1.4

• Leader for Work Package 1 “Understanding of aging of SOFC for industrial applications”

• Responsible partner for Work Task 1.4“Cycled stack operation for 50 to 100 cycles at defined conditions”


University of Birmingham

Professor Kevin Kendall, FRSDr Waldemar Bujalski

Department of Chemical EngineeringUniversity of Birmingham

EdgbastonBirmingham B15 2TT, UK

[email protected]; [email protected]

mailto:[email protected]

mailto:[email protected]


Activities at UBHAM

The “state of art” SOFC Test Station from Advanced Measurements Inc. has been fully operational now with all the auxiliary equipment in place and all the “Health and Safety” issues related to working with hydrogen sorted out!


Activities at Birmingham

• Experimental testing of R-R IP-SOFC tubes has been carried out at different levels of “sophistication” as the test facilities developed:– Initially, temperature tests of the R-R tubes for a range of

controlled temperature ramps (from 1 deg C/min to 10 deg C/min) and gas flow rates (from 0 to 10 l/min) with and without air preheated in the process were carried out.

• General outcome:– No signs of any damage to the cells was noticed. Even the

highest temperature ramps rates used did not cause any obvious damage to the integrity of the tube!

“multi-cell membrane electrode assembly (multi-cell MEA) module”


Work with hydrogen at Birmingham

• Experimental runs were carried out using hydrogen attempting achieving of 50 current load cycles from 0 to the iDP=2.7 A (i.e. current Design Point value) and back to OCV.– Procedure provided by R-R for the run was

adhered to which conformed to general testing procedures used in the programme;

– This procedure was unified/synchronised across the manufactures/partners in order to arrive at comparable results for different makes of the fuel cells tested in the programme;

– There were 16 thermocouples placed in the box for monitoring the temperature in vicinity of the tube.


Conclusions

• The experimental rig enabled fully programmable/automated operation of the test thus providing a regime for reproducible testing conditions in cycling tests;

• The conditions for the test itself were in accordance with the stack providers requirements/limits and were specified for the programme of “standardisation” agreed by the Real-SOFC participants;

• Temperature (16 thermocouples monitoring within the test box as well as air preheater and furnace temperature) and electronic load control allowed detailed monitoring of cyclic behaviour;

• First run consisted of 50 full cycles (up and down i.e. from open circuit to design value of electronic load of 2.7 A). The nominal temperature for the cycling work was set at 900ºC but the actual one experienced by the tube in the box was about 920ºC;

• The module has shown some increase in resistance at the end of the cycling process. Initial visual inspection of the module in the test box identified a possible defect in one of the cells, however, when the module was removed from the box after completion of additional cycling tests leading to 93 cycles in total no physical damage to its structure was identified;

• Overall, the tube performed very well. Even after reaching the limit for the allowed minimum tube voltage of 9 V under full load (i.e. 2.7 A) the cut off point was reached at the next load level of 2.6 A for all the remaining test points.


Thermal Cycling with Second Generation

Cycling results for 102 thermal cyclesat 10 C/min for Rolls Royce IP-SOFC

(blue arrows indicate re-starts)

Number of thermal cycles, [-]

0 10 20 30 40 50 60 70 80 90 100 110

OC

V or

U, [

V]

7

8

9

10

11

Open Circuit Voltage [V] Voltage at IDP of 2.5 A i.e. 0.278 A/cm2 [V]

IV curves as a function of temperature cyclesfor IP SOFC - RR_134W-40 10 cell tube (10 C/min)

Specific current load, i, [A/cm2]

0.00 0.05 0.10 0.15 0.20 0.25 0.30U

, [V

]7

8

9

10

11

Initial IV curve (1 C/min heating/cooling rate)After 1 cycle (10 C/min up; 5 C/min down to 500 C & 2 C/min to 200 C)After 10 cyclesAfter 20 cyclesAfter 50 cyclesAfter 102 cycles

IDP=2.5 A


Further work within the Real SOFC

• Further experimental work:

– To continue work with R-R tube to be followed by a bundle;– Get a full “post mortem” info on the damaged tubes in order to

draw some sound conclusions on possible modes of failure;– Also, to try to analyse the already acquired data fully (there is

lots of important, unravelled information there!);– Carry out the thermal cycling (already done) and redox cycling

experiments with those tubes.

• Test with first generation of Jülich module using thermal cycling, soon.


Future work at Birmingham

• Substantial additional capital funds have been recently secured from AWM (Advantage West Midlands i.e. the regional development agency) for expanding our activities in the following areas:

– CHP (testing production unit under the laboratory conditions and in the field i.e. in a house in the Black Country housing estate);

– Further development of our testing facilities for variety of fuel cells and their stacks;

– Fuel reforming and biofuels work in collaboration with Biosciences;

– Modelling work in collaboration with Maths department.

References

• “High temperature Solid Oxide Fuel Cells. Fundamentals, design and applications”, (Edited by: S.C. Singhal and K. Kendall), Elsevier Ltd., Oxford, UK (2003)

• “Fuel cell systems explained”, J. Larminie and A. Dicks, John Wiley and Sons, Ltd., England (Second Edition, 2003)

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