Anodic Catalyst Design for the Ethanol Oxidation Fuel Cell Reactions
Xiaowei Teng Department of Chemical Engineering, University of New Hampshire, Durham, New Hampshire 03824, United States
Rising demands for energy, coupled with concerns over environmental pollution and growing fossil fuels cost, pose great needs for clean and efficient power sources. Since the energy conversion efficiency of heat engines is confined by Carnot cycle limitation (typically < 35%), the electrochemical oxidation of small organic molecules (SOMs) becomes attractive due to its high thermodynamic efficiency (higher than 70%) and low emission of CO2 footprint via low temperature direct fuel cell reactions. Compared to hydrogen and methanol, ethanol is a promising fuel in the so-called the direct ethanol fuel cell (DEFC) reaction for several reasons. First, ethanol is less toxic; Second, ethanol is easy to store and transport due to its relatively higher boiling point; Third, ethanol has higher energy density due to the nature of twelve–electron transfer upon complete oxidation; Fourth, ethanol has been qualified as a substantial energy source since it can be produced in large quantities from sugar– and cellulose–containing raw materials. However, the implementation of the DEFC technology has been hindered by the sluggish ethanol oxidation reaction (EOR) at the anode, due to the lack of an active anode catalyst.
Platinum (Pt) is the most common catalyst in both anode and cathode fuel cell reactions because of its excellent properties in the adsorption and dissociation of SOMs. However, the expense of Pt catalysts is a major impediment in the commercialization of fuel cell technology, since it alone account for approximately 54% of the total fuel cell stack cost. Many studies have reported the enhancement of the electrocatalytic performance of Pt by adding additional elements to form Pt/M (M: Ru, Sn, Ir, Bi, Pd, Ru, Rh, Mo etc.) binary or ternary catalysts, or by developing non-Pt alternative electrocatalysts with high activity to EOR and low–cost in order to implement the DEFC technology.
Due to the importance of this issue, this paper will report fundamental understanding of the EOR on the catalyst surface and discuss the current anodic catalysts for the ethanol oxidation fuel cell reactions. Several binary and ternary catalysts systems with different crystalline structures, i.e. alloyed and non-alloyed catalysts, will be discussed. Moreover, current advances on several Pt-free anodic catalysts, such as iridium (Ir)-based catalysts for the EOR will be discussed.
Fig. 1 Production and distribution network of ethanol in U.S.
Demand for energy, coupled with concerns over environmental pollution and growing fossil fuel costs has created a great need for clean and efficient power sources for the nations. The United States is responsible for a quarter of the world's total carbon emissions, and Americans' per capita emissions are five times the world average. Since a major source of carbon emissions in the United States is the transportation system, a practical path to achieve significant emissions reductions is to find alternative fuels to gasoline, as well as alternative power devices to the internal combustion engine from which thermodynamic efficiency is subject to Carnot cycle limitation (typically < 35%).1 Over the last forty years, a great deal of attention has gone into developing low–temperature fuel cells, devices that can
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directly electro-oxidize Small Organic Molecules (SOMs) for electricity with a high thermodynamic efficiency (up to 97%), and provide an alternative path to power.2, 3 Table 1 Comparison of different fuels in low temperature direct fuel cell reaction.
Hydrogen Formic acid Methanol Ethanol
Energy Density (kWh/kg) 32.8[a] 0.42 [b]
1.7 6.1 8.0
Boiling Point (ºC) -253 101 65 78
Toxicity & Safety Flammable toxic Very toxic Non-toxic
biomass Infrastructure Not ready Not Ready Not ready Ready
Efficiency at 25 ºC[c] 83% ~ 100% 97% 97%
[a] Theoretical energy density of hydrogen gas; [b] Energy density of hydrogen gas compressed at a pressure of 5000 pounds per square inch (psi) or 340 atm; [c] Thermodynamic efficiency is equal to the changes of Gibbs free energy (ΔG) per changes of enthalpy (ΔH) upon the complete combustion.
As shown in Table 1, ethanol (CH3CH2OH) has many advantages over hydrogen (H2), methanol (CH3OH) and formic acid (HCOOH) as a fuel in fuel cells, though the latter three are by far the most studied fuels in fuel cells research.4, 5 Ethanol has a lower toxicity, a higher thermodynamic efficiency, and contains a higher energy density (twelve–electron transfer in complete oxidation). Furthermore, it has a higher boiling point for safer storage in transportation applications. More importantly, ethanol can be produced in large quantities from biomass through a fermentation process from sugar–containing resources like from sugar cane, wheat, corn, or cellulose–containing raw materials such as grass and straw. Bio-generated ethanol (or bio-ethanol) is thus attractive since growing crops for biofuels absorbs much of the carbon dioxide emitted into the atmosphere from fuel used to produce the biofuels, and from burning the biofuels themselves.6, 7 This is in sharp contrast to the use of fossil fuels for harvesting energy. Thus ethanol has been recognized as a substantial energy source in the future of ‘green’ technology, mainly in the Midwest of U.S. Worldwide, bioethanol production exceeded 17 billion US gallons in 2008. Blends of gasoline containing 85% of denatured ethanol (E85) have recently appeared at fuelling stations in the U.S. with a well-established production and distribution infrastructure across the country as Figure 1 shows.
1.2 Reaction kinetics of the EOR
Fig. 2 Scheme of the DEFCs in Proton Exchange Membrane Fuel Cells (PEMFCs) and Alkaline Fuel Cells (AFCs).
Although electrooxidation of alcohol fuels, especially ethanol, has been a subject of increasing interest, the kinetics of alcohol oxidation are much slower than for hydrogen oxidation reaction.2, 8 Among the reactions shown in Figure 2, oxygen reduction reaction (ORR) at cathode is basically diffusion-limited, while electrooxidation of liquid fuel at anode is primarily catalytically driven. Therefore, identifying effective anode electro-catalysts becomes imperative to improve
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the overall reaction rate of the DEFCs. In addition, ethanol contains two carbons (C2). During EOR various one carbon (C1) intermediates such as carbon monoxide (CO) and carbohydrate (CHx), as well as two carbon (C2) intermediates such as acetaldehyde (CH3CH=O), acetic acid (CH3COOH) and CH3CO are produced. Strongly adsorbed species such as CO and CHx could poison the Pt surface and reduce the charge transfer rate of the EOR considerably. Therefore, designing catalysts that can mitigate the poisoning of adsorbed reaction intermediates is vital to improve the kinetics of EOR and advance the development of DEFC technology. A significant enhancement of the electrocatalytic activity has been reported by modifying Pt with secondary species to form Pt/M (M: Ru, Sn, Mo etc).2, 9-11 In general, M atom is oxophilic, which acts as an oxygen supplier at low potentials and mitigates the poisoning effect by oxidizing the meta-stable intermediates with adsorbed water fragments. Complete oxidation of ethanol into CO2 via C–C bond cleavage is mechanistically difficult. In-situ differential electrochemical mass spectrometry (DEMS) and Fourier Transform Infrared Spectroscopy (FTIRS) have been used to study the electro-oxidation of ethanol by identifying the adsorbed intermediates at different potentials, mainly on the surface of Pt-based catalysts.12-17 Results from electrochemical analysis on particle catalyst and single crystal model catalyst prepared in ultrahigh vacuum (UHV), as well as density functional theory (DFT) calculation suggest that most of the current from the EOR is produced from partial oxidation of ethanol to acetaldehyde or acetic acid, involving the activation of both the C–H and O–H bonds of ethanol instead of the C–C bond as presented in Figure 3. For example, DEMS studies showed that only 1 % of ethanol was completely oxidized into CO2 on the surface of Pt3Sn/C alloy at room temperature, with the major products of acetaldehyde and acetic acid.16
Fig. 3 The reaction paths involved in the electrooxidation of ethanol to acetaldehyde, acetic acid and CO2. Through the primary paths of EOR (black arrows in Figure 3), acetaldehyde forms on Pt surface in lower potentials. Since acetaldehyde is weakly bound to Pt surface, it can readily desorb on the surface of catalysts or it can be oxidized further to form acetate or acetic acid as primary products in EOR, which are very inert in electrocatalysis. Formations of acetaldehyde or acetic acid deliver only 2 or 4 electrons, compared to the 12 electrons upon the formation of CO2, thus, efficiency of incomplete EOR is low. Meanwhile, acetaldehyde can possibly be further reacted to form C1 intermediates (CO, CHx) via C–C bond cleavage, and eventually oxidized to CO2. Several DFT calculations and experiments suggest that CO2 formation via paths of acetaldehyde, as of following equation, whereas the pathways are marked by solid green arrows in Figure 3: → → + → 2 is more favourable than direct formation of CO2 as of the pathways marked by dash green arrow: → + → 2
Lai et al reported a systematic study of the oxidation of ethanol and acetaldehyde on well-defined Pt single-crystal surfaces,18 showing a strong structural sensitivity of the EOR. They found that the poisoning of defect sites (step sites) is slow for EOR at lower potentials, leading to a positive effect of the step density on the conversion of ethanol to acetaldehyde, and enhancing the rupture of the C–C and total oxidation to CO2. At higher potentials, however, step sites could be covered by strongly adsorbed C1 and C2 intermediates and play a less important role in the activation of C–C bond. The structure sensitive nature of EOR at low potential was also confirmed by DFT calculation conducted by Neurock,19 whereas EOR over Pt (111) surface and Pt (211) surface were compared to elucidate the effort of step sites on the C–C splitting paths. He found a clear reduction in the overall endothermicity for nearly all of the paths of EOR in moving from Pt (111) to (211) surface, indicating the step sites are more active than the terrace site for C–C cleavage. Extrapolation of the reaction mechanism from a single crystal surface under UHV remains elusive to rational catalyst design for practical application. However, promotional effect of high density of stepped atoms on EOR has been demonstrated in electrochemically synthesized tetrahexahedral Pt nanoparticles with high-index facets such as {730}, {210} and {520}.20 These tetrahexahedral Pt nanoparticles showed greatly enhanced current density for the EOR as compared to equivalent Pt surface areas of nanoparticles with different shapes (e.g. nanospheres that have low-index facets), which has been attributed to the increased number of step sites and dangling bonds at high-index facets.
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The effect of particle size on the electrocatalytic activity has been of interest to both the heterogeneous catalysis and electrocatalysis. A computer simulation was used to calculate the total number of atoms on various positions of platinum nanoparticles at the given sizes (diameters for spherical nanoparticles). To simplify the treatment, perfect cubo-octahedron was used to model sphere-like particles and assumed the most active sites locate at given positions, such as edges and corners. The formulas used are listed in Table 2, where m is the number of atoms located on one edge for a given particle. The total number of active sites (e.g. atoms at edge and corners) per surface atoms with a given size is obtained as shown in Figure 4, which showed that there is an exponential increase in the low coordination sites as particle size increses in the range from 10 nm to 1 nm. Although the low coordinated sites (e.g. edge and corner) could be catalytically active, it is not trivail to synthesize nanocatalysts with small sizes (less than a few nm) and narrow size distributions. According to the Gibbs-Thomas equation:21
SRT
Vr m
ln
2* γ=
where r* is the critical radius at which the particles showing a zero growth rate, γ is the specific surface energy of particles, Vm is molar volume of the particles, R is the gas constant, T is the reaction temperature and S is the supersaturation of the monomer), the synthesis of small particles requires a higher reaction temperature and supersaturation level (thermodynamic control). On the other hand, a higher temperature will result in a larger reaction rate constant for particle growth, and therefore a larger particle size (kinetic control). Therefore, optimal experimental parameters (e.g. precursor concentration, reaction temperature and time) are imperative to make small, uniformly distributed nanocatalysts. Table 2. Number of available sites at the given positions on the surfaces of Pt spherical Nanoparticles
(100) surface (111) surface all surface edge and corner
sphere* 12m2-36m+30 4m2-20m+24 16m2-32m+18 24m-36
*: Modeled after cubo-octahedron.
1.3.2 Heterogeneous Structure of the Electrocatalysts
The electronic structure of materials is associated with their heterogeneous structural diversity strongly, such as phase segregation, intermetallic alloy, random alloy, and near surface alloy (NSA) (Figure 5). Understanding heterogeneous structure of multi-component catalysts is a key factor to advance an efficient EOR catalyst. The superior performance of multi-component electrocatalysts for the EOR compared to its monometallic counterparts has been well demonstrated in various Pt-based binary catalysts (Pt/TM, TM: Ru, Sn, Pd, Co, Fe, Ni and Pb etc.). The synergistic effects found in multi-component electrocatalysts, resulting from carefully tailoring the heterogeneous structure (spatial distribution of different atoms inside the particle), may offer the opportunity to optimize the EOR with improved kinetics and CO2 formation. At present, two types of heterogeneous electrocatalysts have been intensively studied and hold great promise in catalysis in particular:
Fig. 4 The ratios of the edge and corner atoms on the surface of cubo-octahedron nanoparticles as function of size.
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Fig. 5 Schematics of four different heterogeneous nanostructures: (a) Phase segregation (surface of metal M1 is modified by irreversibly adsorbed M2); (b) Intermetallic alloy (atoms M1 and M2 are mixed with definite lattice proportions); (3) Random alloy (homogeneous alloy, atoms M1 and M2 are mixed statistically in accordance with the overall concentration). (4) NSA alloy (atoms M1 are core–rich and M2 are shell–rich) Homogeneous AB alloys (where metals A and B are mixed within the catalyst statistically in accordance with the overall composition): Previous results have shown that different types of atoms are in close proximity in the homogenous alloy that may optimize the bifunctional and electronic effects between each component, delivering a better electrochemical performance in EOR with higher current densities and lower peak potentials compared to pure Pt. ACBS NSA alloys (where metal B atoms overlay metal A substrate, showing B shell-rich and A core-rich structure): Experimental and theoretical efforts mainly by Adzic, Mavrikakis and Nørskov showed that Pt- and Au-skinned structures were able to weaken the bonding between reaction intermediates and catalysts due to the electronic effect, leading to potentially higher catalytic activities in the ORR compared to pure Pt.22-30 Although few skinned alloys have been developed for electrooxidation of alcohol, especially ethanol, similar electronic effects could mitigate the poisoning of reaction intermediates by weakening their bonding to catalysts’ surface, and eventually improve the reaction kinetics. Four methods have been employed to make ACBS structures in literature: (i) deposition of a monolayer of metal B on metal A single crystal under UHV, followed by high temperature annealing; (ii) controlled dissolution of a less noble element (A) from noble metal alloys (AB) on the surface of electrode in an electrochemical cell; (iii) deposition of noble metal (B) through a spontaneous irreversible redox process on a less noble metal (A) on the surface of electrode in an electrochemical cell; and (iv) atomic layer deposition (ALD) using organometallic precursors (B) on the thin film of another metal (A). These methods can yield fine skinned structures via precise control of synthetic conditions. However, their intrinsic complexity in preparation demands a simple solution phase synthesis.
1.3.3 Structural Characterizations
Fig 6. (a, c) High Angle Annular Dark Field (HAADF) images and (b, d) normalized EELS line scans of PtBi and Ir-Sn/C catalysts obtained using aberration-corrected STEM. EELS line scans indicated (c) a PtBi homogenous alloy structure and (d) an Ir-core-rich and Sn-shell-rich skinned structure.
Since the size and heterogeneous structure and composition all play important roles in the functionality of the catalyst, to provide new fundamental insights into the structure/property relationships of the various nanoarchitectured materials, various techniques has be used to analyze the size, crystalline and electronic structures of the proposed materials. In addition to conventional techniques such as Scanning Electronic Microscopy (SEM) and TEM (to measure geometric features of the catalysts); Energy-Dispersive X-ray Spectroscopy (EDS) (to analyze the chemical composition of the materials); X-Ray Diffraction (XRD) will be conducted to determine the degree of alloy formation of the catalysts, two state–of–the–art techniques have been intensively studied in recent years to decipher the “real” heterogeneous structure of the catalysts.
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Aberration-corrected Scanning Transmission Electronic Microscopy (STEM): Microscopy was revolutionised in the 1930s by using electrons to illuminate specimens, creating images at much higher magnifications than optical microscopes allowed. But electron lenses are inherently poor compared to optical lenses and soon the design of electron microscopes had improved to the state where the performance was mainly limited by spherical aberration, a feature of all round lenses that causes image distortion and limits the resolution. Although attempts to correct spherical aberration were made as early as 1940’s, the realisation of spherical aberration correction wasn’t achieved until 1997 by Ondrej Krivanek, Nicklas Dellby and Mick Brown. At present, the aberration-corrected STEM has been dramatically improving spatial resolution and beam current, exhibiting a strong ability to analyze materials with single atom resolution and providing great opportunities to interpret the real structure of individual nanomaterials with equipped electron energy loss spectroscopy (EELS) and/or energy-dispersive X-ray spectroscopy (EDS). Figure 6 shows aberration-corrected STEM measurements of Pt/Bi nanowire and carbon-supported Ir/Sn nanoparticles conducted at the Centre for Functional Nanomaterials at the Brookhaven National Laboratory.31, 32 EELS line scans equipped with STEM have demonstrated the ability to generate high resolution signals, so that the distribution of heterogeneous atoms within a single particle can be determined with a resolution of ~ 1 Å. EELS line scans indicated a PtBi homogenous alloy structure with uniformly mixed Pt and Bi spectra cross the nanowire. In contrast, for the Ir/Sn catalyst, EELS line scan demonstrated that a higher Sn signal at the edges compared to the centre of the particle, showing Ir-core-rich and Sn-shell-rich skinned structure.
Fig 7. Schematics of X-ray absorption spectroscopy (XAS) presented in energy space (a) and R space (b).
Extended X-ray absorption fine structure (EXAFS): X-ray absorption spectroscopy (XAS) is a well-established technique which provides element specific information on the electronic and structural properties for metallic nanostructures through its two modifications, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). EXAFS is a well-established tool for investigating the element-resolved structure of the nanomaterials, since the local environment and electronic properties of atoms of each resonant element can be studied separately. The EXAFS region, defined as a spectrum region between several hundreds of eV to ~1000 eV above the absorption edge, contains the structural information for coordination environment around the absorbing element (Figure 7). Such parameters including ligand types, coordination numbers, bond distance and disorders can be determined by EXAFS analysis. Through EXAFS, energy edges of metal component may be analysed concurrently. Quantitative analysis will be done by computing theoretical phase and amplitude functions for both single scattering and multiple scattering paths and by performing a nonlinear least-square fit to the data.33 Parameters describing electronic properties (e.g. correction to the photoelectron energy origin) and local structural environment (e.g. coordination number and bond length) around absorbing atoms will be varied in the fit. During the fitting, multiple constraints obtained from other analyses such as TEM, XRD and EDS will be applied. Finally, coordination numbers (N) will be obtained from the best fit for all pairs (A-A, A-B, and B-B) in binary AB catalysts to identify the heterogeneous structure of the newly synthesized nanocatalysts (Table 3).34-36
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Table 3. Relationship between coordination number and heterogeneous structure in EXAFS analysis.
Coordination Number (N) Heterogeneous Structure
NA-B = NB-A = 0 [a] Metal A and metal B in separated phases (e.g. core/shell particle)
NA-Metal > NB-Metal > 0 [b] Skinned alloy : Metal A core-rich, metal B shell-rich
NA-Metal = NB-Metal > 0 homogeneous alloy: A and B mixed statistically within the particle
NB-Metal > NA-Metal > 0 Non-random (skinned) alloy: B is core-rich and A is shell-rich
[a]. NA-B: average No. of B atoms surrounding A atom NB-A: average No. of A atoms surrounding B atom
[b]. NA-Metal: average No. of A and B atoms surrounding A (NA-Metal = NA-A + NA-B) NB-Metal: average No. of A and B atoms surrounding B (NB-Metal = NB-B + NB-A)
2. Binary and Ternary Pt-based Electrocatalysts for the EOR
2.1 Pt-based binary catalysts
Pt-based bimetallic materials (e.g., Pt/Sn, Pt/Ru) are the most commonly used catalysts in EOR.11 The increased activity over these multicomponent catalysts compared to their monometallic Pt counterpart has predominantly been attributed to bifunctional effect and ligand or electronic effect. In bifunctional effect, the transition metal (Sn, Ru) has a strong interaction with water (oxophilic) to form an oxygenated species (e.g. OHads), while Pt actively adsorbs and dissociates the ethanol; in the electronic effect, the electronic back-donation from the transition metal changes the d-band structure of Pt, and therefore weakens the bonding between the adsorbents and Pt. However, the enhanced current in Pt binary alloys results more from the formation of acetaldehyde and/or acetic acid than from the splitting of C–C bond. Since Pt is the active metal for C–C activation, these alloyed materials with decreased surface Pt actually lower the total conversion to CO2. In a noble metal-transition metal binary system, higher transition metal (TM) content will generate more TM-OHads complexes, resulting from the dissociative adsorption of water, which facilitate the oxidation of adsorbed SOMs. However, higher TM content will also decrease the occupancy of active noble metal atoms on the surface, and consequently impair the overall performance of dissociation of adsorbed SOMs. Therefore, an optimal composition usually exists as a result of such rival effects. PtSn: Many studies have indicated that PtSn/C is the best catalyst for the improving the reaction kinetics of the EOR in comparison with other Pt-based binary materials. Density function theory (DFT) calculations showed that the energy barriers for CO oxidation on Pt3Sn/Pt (111), Pt3Sn (111), and Pt (111) surfaces are 0.64 eV, 0.68 eV, and 0.82 eV, respectively, and water dissociation energy is lower on Sn (0.44 eV) than that on Pt (0.67 eV).37 Therefore, Sn component can enhance CO oxidation on Pt by promoting H2O dissociation on Sn to form Sn–OH (bifunctional mechanism), and weaken the Pt-CO interaction by altering electronic properties of Pt through Pt-Sn bonding (ligand effect). Although Pt/Sn has been considered as the most effective catalyst for EOR, detailed rules of Sn on the Pt for the EOR remain unclear and even controversial. Discrepancies exist primarily on the effect of alloyed and non-alloyed Sn on the EOR kinetics. For example, Antolini and Jiang both reported non-alloyed PtSnO2/C showed faster EOR kinetics than alloyed catalysts.38-40 However, De Souza and Godoi (Journal of Power Sources 195, 3394, 2010) showed that alloyed PtSn has better EOR kinetics.15, 41 PtRu: The optimal ratio of Ru in Pt-Ru binary system for the electrooxidation of SOMs has been intensively studied with various controversial results, especially for the methanol oxidation reaction (MOR). Watanabe and Loffler reported Pt50Ru50 alloy had the best performance in MOR.42, 43 While Gasteiger et al found that for the MOR, the optimal Ru content on the surface of polycrystalline Pt-Ru bulk alloys had strong temperature-dependency: ~10 % and ~30 % surface Ru were the optimal ratios at 25°C and 60°C, respectively.44, 45 They also proposed that the surface enrichment of Pt could be responsible for controversial results on the optimal Ru ratios in Pt-Ru systems, which was largely ignored by previous studies. Similarly, Iwasta and Abruna found a broad maximum in activity for MOR between 10 to 40 % of surface Ru for Pt-Ru alloy at room temperature.46, 47 Furthermore, Cuesta studied the cyanide-modified Pt (111) electrodes for MOR, and found out that ensemble of three contiguous of Pt atoms was indispensable for CO formation through dehydrogenation of methanol, strongly indicating Pt3Ru1 might be the perfect surface composition to mitigate the CO poisoning, which is consistent with above reported results for MOR.48
2.2 Pt-based ternary catalyst
In addition to bimetallic catalysts, ternary catalysts such as PtRuSn, PtRhSn, PtIrMo, PtRuMo and PtSnMo, become a subject of increasing interest for the EOR.49-51 The addition of a third component further modifies the structure and electronic states of the alloys, with a hope of leading to a better catalyst for the EOR. Although many ternary alloys demonstrate an enhancement of current density of the EOR, their ability to cleave C–C bond of ethanol to form CO2 is
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controversial. For example, addition of Mo to PtRu catalyst increased the current density of the EOR, accompanied with high yields of C2 products instead of improving CO2 formation. On the other hand, Kowal et al showed that PtRh/SnO2 electrocatalysts made by the under potential deposition (UPD) technique on the surface of the working electrode yielded not only a higher current density over EOR, but also a high level of CO2 formation compared to Pt.52 Although mechanistic details remain obscure, the prevailing view indicates that Rh increases the yield of CO2 by helping break C–C bond, while Sn supplies the oxygenated species needed to oxidize the blocking intermediates. Thus, formation of an alloyed structure where Pt, Rh and Sn atoms are in close proximity could be favorable because the cooperative effect from Rh and Sn in the vicinity of Pt sites could provide high activity toward ethanol oxidation and better tolerance for poisoning species. In particular, DFT calculations indicated that interactions between the PtRh alloy and SnO2 play a crucial role in C–C bond breaking, whereas Rh with highly-lying d band accelerated the C–C activation of Pt and SnO2 provided OHads species that oxidized the C1 intermediate formed at Rh and Pt sites. The results from these ternary systems are encouraging. However, most of the studies were unable to synthesize and/or verify the ternary alloyed structure. A PtRu, PtRh and PtIr binary alloy mixed with SnO2 or MoO2 were often observed. Therefore, the reported ternary systems might not be as optimized as homogeneous alloys, in which synergic effect between Pt, Rh/Ir and Sn/Mo could be further reinforced. In this regard, advanced synthesis and characterization procedures which can produce and validate the ternary homogeneously alloyed catalysts will be highly plausible for optimizing the reaction kinetics and CO2 formation for the EOR.
Table 4. EXAFS analysis results for Pt52Sn(36-x)Rh12–SnxO2x /C, Pt36Rh10–Sn54O108/C and Pt30Rh30–Sn40O80/C
NSn–M 6.4 + 2.4 0 1.0 + 0.9 In a recent effort, EXAFS has been used to verify, for the first time, the ternary alloyed structure of Pt/Rh/Sn systems.53 Table 4 shows the best fit values of coordination numbers of various Pt/Rh/Sn nanoparticles, indicating clearly that heterogeneous bonds were formed between Rh–Sn, Pt–Sn as well as Pt–Rh. In a typical Pt52Rh12Sn36/C catalyst, the coordination numbers of Rh–Rh (NRh-Rh) and Sn–Sn (NSn-Sn) were very small, even though distinct Sn–Pt coordination was observed (NSn-Pt = 5.4 + 2.3). This might be due to the low concentration of metallic Rh and Sn, evident also by the less distinct Sn–Rh coordination number (NSn-Rh = 1.0 + 0.6). On the other hand, the strong Sn–O coordination (NSn–O = 2.9 + 0.7) unambiguously indicated the formation of tin oxide clusters. Meanwhile, whether Rh is also present as oxide is unclear due to the high uncertainty of the Rh–O coordination (NRh–O = 2.5 + 2.8). By analyzing the metal–metal total coordination number, the metal placement pattern within Pt52Rh12Sn36/C catalyst is examined. Since Rh and Sn can be present in both oxides and metallic (alloy) phases, the values of Rh–metal coordination number (NRh–M = 6.6 + 1.3), and Sn–metal coordination number (NSn–M = 6.4 + 2.4) have meanings of average results over the alloy and oxide phases. If these values are compared with the Pt–M coordination numbers directly, such comparison will result in underestimating the distribution of Rh–M and Sn–M in the alloy phase. Had EXAFS been able to extract these numbers for the metal alloy phase separately, they would have been higher and thus closer to the Pt–metal
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coordination number (NPt–M = 8.5 + 0.4), indicating a more homogeneous alloy than the average coordination numbers indicate. Therefore, it strongly suggests that the coexistence of bi–phase throughout the Pt52Rh12Sn36/C catalyst: a homogeneous alloy containing Pt, Sn and Rh, and tetragonal SnO2 clusters in segregated phase. Combined with overall chemical composition obtained from EDS, the stoichiometry of the bi–phase can be expressed as Pt52Rh12Sn36/C.
3. Non-Pt based Alternative Electrocatalysts for the EOR
In addition to Pt based materials, theoretical and experimental efforts have studied other metals catalysts, seeking an alternative electrocatalyst with even higher activity and selectivity toward EOR compared to Pt. Ferrin et al recently studied the decomposition of ethanol on the close-packed facets of ten metals (Cu, Pt, Pd, Ni, Ir, Rh, Co, Os, Ru, and Re). They calculated the C–C and C–O bond-breaking Turnover Frequencies (TOF) on various metals using DFT method and found that Ir, Rh and Ru metals could decompose ethanol with higher kinetics (quickly removing CO intermediates via C–O cleavage) and selectivity to CO2 formation (via C–C cleavage) compared to Pt. These results indicate strongly that Ir, Rh and Ru have great potential as new class of electrocatalysts for the EOR with a faster reaction kinetics (breaking the C–O bond easily) and a better tendency to form CO2 (breaking C–C bond easily). Catalysts containing Pt, Rh and Ru have been well demonstrated for the EOR. A few Ir-M (M: Se, V, Co, Sn) catalysts have also been tested for ORR and electrooxidation of various SOMs. In recent efforts, Du et al have synthesized and tested several carbon supported Pt-based (e.g. Pt-Bi, Pt-Sn, PtRhSn/C) and Ir-based (Ir-Sn/C and Ir-Ru) alloyed nanocatalysts for the EOR.31, 32, 53, 54 Results shown in Figure 8 indicate that the Ir and Ir-based alloys exhibit an unusually high electrocatalytic response compared to commercial Pt/C (ETEK) and PtRu/C (Johnson Matthey, JM). In particular, IrSn/C showed much lower onset potential in polarization curves and compared to Pt/C (Figure 9a). The chronoamperometric (CA) analysis (Figure 9b) conducted to analyze the chemical stability of the anode electrocatalyst for one hour shows that Ir-Sn catalysts have at least 15 times higher current density after one hour measurement compared to pure Ir, Pt and PtRu electrodes. In addition, (111) surfaces of Ir, Pd, Pt and an Ir-M (M: Sn, Ru, Rh) clusters were modelled for the reaction:
CH2COads CH2-ads + COads
which has been considered as a key step toward C–C bond scission. It was found that the reaction energy for CH2CO is exothermic over all considered surfaces, but the Ir and Ir alloy surfaces have more exothermic reaction energies than the Pt surface. These calculations were conducted in the gas phase on model metal surfaces, and ignored the effect of aqueous environment, or dielectric background. Although this is different from the realistic reaction environment, this simplification is often used as a starting point to understand the real catalytic reactions.
Fig 8. (a) Polarization curves and (b) CA curves (at 0.2 V) of Ir, Ir-Sn, Ir-Ru, Pt-Sn, PtRhSn, and commercial Pt (ETEK) and PtRu (Johnson Matthey, JM). The measurements were conducted in 0.5 M H2SO4 and 0.5 M ethanol at room temperature. (c) Reaction energies for breaking of CH2CO over the different surfaces. Lower energy values indicate more likely reaction pathways.
4. EOR Catalysts for Alkaline Ethanol Fuel cell
In alkaline fuel cells (AFCs), where a high pH medium functions as an electrolyte, the kinetics of the EOR can be greatly accelerated. This might be due to the excellent interaction between the catalysts and the hydroxyl group in an alkaline medium, which facilitates the dehydrogenation of ethanol as indicated by experimental results and Density Functional Theory (DFT) calculations. As one of the platinum group metals, Pd has similar catalytic properties to Pt, but is much lower in materials cost (Table 5). In particular, the abundance of Pd on the Earth’s crust is 200 times higher than Pt (0.6 part per billion [ppb] vs. 0.003 ppb), making it very attractive for long-term industrial applications.
Materials and processes for energy: communicating current research and technological developments (A. Méndez-Vilas, Ed.)____________________________________________________________________________________________________
Fig 9. Electrochemical measurements of Pd/SnO2/C and Pd/C (ETEK) catalysts for the EOR. (a) CVs from –6 V~ -0.3 V (forward scans). (b) IT measurements at -0.1 V. (c) Tafel plot for the EOR at a scan rate of 1 mV s-1. Despite its good activity, complete oxidation of ethanol in an alkaline medium on Pd surface remains difficult. In-situ DEMS and FTIR results indicated the ability of Pd for breaking the C–C bond of ethanol was slightly better than that of Pt under the same conditions. However, the overall selectivity for ethanol oxidation to CO2 species in alkaline media was still low (around 2.5%). Acetic acid is still the major product, and the rate-determining step is the removal of the adsorbed ethoxy (CH3COads) by adsorbed hydroxyl (OHads). To study and develop AFCs technology using ethanol as a fuel and Pd-based materials as anode catalysts, a better understanding of the structure-electroactivity relationship in the EOR is needed. While several Pd-based binary catalysts (Pd-M, M: Ir, Fe, Bi, Co, Cu, Ni, Au, Ag) have been studied as alternatives to Pt in the oxygen reduction reaction (cathode reaction) and the oxidation of SOMs (anode reaction) in AFCs, there are a large number of unanswered questions regarding the role of atomic structure in determining electroactivities. In a recent effort, Du et al have investigated the electroactivity of Pd/SnO2/C catalysts in a half-cell in 0.5 M KOH and 0.5 M ethanol electrolyte (Figure 9).54 The CV curves (after 100 cycles) demonstrated that Pd/SnO2/C had 4.2 times higher current density than commercial Pd/C (ETEK). Pd/SnO2/C also showed superior long-term activity from IT measurements compared to Pd/C (Figure 9b). Moreover, a linear region of the Tafel plots shows that Pd/SnO2/C had a slope of 131 mV/dec, indicating the EOR in the range from -0.5 V to -0.2 V (vs. Hg/HgO) was dominated by the adsorption of hydroxyl on the Pd-SnO2 surface (Figure 9c). Table 5. Price of platinum group metals as of Feb, 2013
Metal Rhodium Platinum Iridium Palladium Osmium Ruthenium
Price ($/oz) 2000 1740 1050 785 380 180
5. Summary
The catalytic response depends on the structure of the material which, in turn, is a function of the material’s morphology, composition and atom distribution that are all very intricately affected by the synthesis and processing methods. It is therefore critical that the complicated EOR processes should be understood at the nanoscale level using new capabilities in materials synthesis, structure characterization and functionality measurement. Armed with this knowledge, it will be possible to synthesize a new genre of materials with controlled structures and desired electro-catalytic properties, namely, a class of electrocatalysts with high efficiency to the complete oxidation of the ethanol. Several critical needs that could require new design rules for EOR catalysts are enumerated:
Novel Nanostructured Anodic Electrocatalysts that can minimize accumulation of strongly-adsorbed reaction intermediates to increase EOR kinetics (higher current density), meanwhile activating C–C bond of ethanol to increase EOR efficiency (higher current density from CO2 formation).
Rigorous Synthetic Approach that can synthesize desired materials with precisely controlled composition, size/size distribution, morphology and heterogeneous nanostructure.
Advanced Characterizations both in situ and ex situ that can comprehensively describe the structures of catalyst and the generation of reaction intermediates during EOR using photon- (e.g., X-ray), spectroscopic-, and imaging- techniques.
Novel Computer Modelling that can investigate the structure and dynamics of the catalysts surface to provide an understanding of factors contributing to electrocatalytic reactions, validated by experiments on model systems to ensure reliability of computational simulations, especially in collaboration with characterizations using X-ray, electron, electrochemical and spectroscopic techniques.
Materials and processes for energy: communicating current research and technological developments (A. Méndez-Vilas, Ed.)____________________________________________________________________________________________________
Acknowledgements This work is support by the University of New Hampshire and National Science Foundation (CBET-1159662, CHE-1152771).
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