The stoichiometry, efficiency, and product distribution for ethanol oxidation in fuel cell hardware has been determined at 80 • C for commercial Pt/C, PtRu/C and PtSn/C anode catalysts. The amounts of ethanol consumed and acetic acid and acetaldehyde produced were determined by proton NMR spectroscopy while CO 2 was measured with a non-dispersive infrared CO 2 monitor. The Pt/C catalyst was most selective for the complete oxidation of ethanol to CO 2 at all potentials and therefore produced the highest number of electrons per ethanol molecule (stoichiometry). Consequently, it would provide the highest efficiency for a fuel cell, and for an electrolysis cell at high current densities. However, PtRu/C provided much higher currents at low overpotentials and therefore better electrolysis efficiency than Pt/C at low current densities. The main product at the PtRu/C catalyst was acetic acid, with ≥ 86% conversion at potentials ≥ 0.35 V vs. a dynamic hydrogen electrode. The PtSn/C catalyst also provided high yields of acetic acid (65-75%), with substantial production of CO 2 (26-27%) at high potentials. The electrochemical oxidation of ethanol in cells with proton exchange membrane (PEM) electrolytes is of central importance to the development of energy technologies based on bio-ethanol. PEM-based direct ethanol fuel cell (DEFC) 1,2 power systems are potentially one of the best alternatives to internal combustion engines, 3 although their efficiencies will need to be increased significantly. Alternatively, ethanol electrolysis cells (EEC) can be used to produce hydrogen. [4][5][6] In an EEC, ethanol is oxidized at the anode and protons are reduced to hydrogen at the cathode. In a DEFC, ethanol is oxidized at the anode and oxygen is reduced to water at the cathode. The attraction of these technologies arises from their high theoretical efficiencies and the perception that emissions will be low. However, the incomplete oxidation of ethanol at all known catalysts currently results in low efficiency cells and large amounts of byproducts.2 To achieve high energy efficiencies in EECs and DEFCs the faradaic efficiency (ε F ) for the electrochemical oxidation of ethanol to carbon dioxide (Eq. 1) must be high.2,7 However, in practice low yields of CO 2 have generally been reported, with the major products being acetaldehyde (Eq. 2) and acetic acid (Eq. 3).The faradaic efficiency is the ratio of the average number of electrons transferred per molecule of ethanol (n av ) to the maximum of 12 for the complete oxidation to CO 2 (ε F = n av /12). It is determined by the product distribution according to Eq. 4,where n i is the number of electrons transferred to form product i and f i is the fraction of ethanol converted to product i. The importance of n av in determining the efficiency of ethanol oxidation technologies makes product analysis of central importance in the development of better anode catalysts. 9Although there have been many advances in the electrochemical performances (voltage efficiency) of ethanol oxidation catalysts...

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Polarization curves, product distributions, and reaction stoichiometries have been measured for the oxidation of ethanol at anodes consisting of Pt and PtRu bilayers and a homogeneous mixture of the two catalysts. These anode structures all show synergies between the two catalysts that can be attributed to the oxidation of acetaldehyde produced at the PtRu catalyst by the Pt catalyst. The use of a PtRu layer over a Pt layer produces the strongest effect, with higher currents than a Pt on PtRu bilayer, mixed layer, or either catalyst alone, except for Pt at high potentials. Reaction stoichiometries (average number of electrons transferred per ethanol molecule) were closer to the values for Pt alone for both of the bilayer configurations but much lower for PtRu and mixed anodes. Although Pt alone would provide the highest overall fuel cell efficiency at low power densities, the PtRu on Pt bilayer would provide higher power densities without a significant loss of efficiency. The origin of the synergy between the Pt and PtRu catalysts was elucidated by separation of the total current into the individual components for generation of carbon dioxide and the acetaldehyde and acetic acid byproducts.

Pt nanoparticle catalysts supported on carbon black coated with mixtures of Ru and Sn oxides (Pt/Ru-Sn oxide/C) have been screened for their activity and stoichiometry for ethanol oxidation. The compositions, loadings, and properties of the mixed Ru + Sn oxide support layers were varied by changing the concentration of base used during their deposition. Cyclic voltammetry at ambient temperature showed that Ru oxide alone provided a significant increase in activity at low potentials over Pt/C, while a small amount of Sn provided a large additional benefit. The most active catalyst had Ru and Sn loadings of 17% and 1.2% by mass, respectively. This was also the most active catalyst for ethanol oxidation at 80 • C in proton exchange membrane electrolysis cells, where it provided four times more current than a commercial Pt/C catalyst at 0.25 V vs. a hydrogen evolving cathode. This is comparable to the performance of commercial PtRu/C, and the Pt/Ru-Sn oxide/C catalyst was more selective for the complete oxidation of ethanol to CO 2 . Thus, in direct ethanol fuel cells, Pt/Ru-Sn oxide/C catalysts would provide greatly increased current and power densities over Pt/C, and improved efficiencies over PtRu/C.

Direct ethanol fuel cells (DEFCs) offer one of the attractive alternatives to conventional combustion technologies as low carbon power sources for vehicles and could become important power sources for consumer electronics and distributed power systems. They provide very high theoretical efficiency (97%), and ethanol is a safe, plentiful, and renewable resource that can be simply stored and handled through the current infrastructure. However, the development and commercialization of DEFCs are hampered by low practical efficiencies and the production of acetaldehyde and acetic acid byproducts as well as carbon monoxide. New anode catalysts are needed to solve these problems, and these need to be evaluated in terms of their electrochemical performance, efficiency, and emissions. In combination with computational studies, various experimental techniques including DEMS, in situ FTIR, and NMR could be employed to provide insights regarding the mechanisms of the ethanol electrooxidation reaction occurring at the newly designed anode catalysts. This insight is necessary to provide the understanding required to allow highly active catalytic materials to be developed and thus enhance the performance and efficiency of DEFCs.

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Mixed Ru–Sn oxides have been deposited onto a high surface area carbon support by thermal decomposition of Ru and Sn acetylacetonate (acac) complexes. Adsorption of preformed Pt nanoparticles produced catalysts with enhanced low potential activity for the oxidation of ethanol in aqueous sulfuric acid at ambient temperature and in a proton exchange membrane (PEM) cell at 80 °C. Varying the oxide composition between Ru0.38Sn0.62O2 and Ru0.67Sn0.33O2 did not influence the catalyst’s activity greatly but did increase stability in the sulfuric acid solution. Higher stability was observed in the PEM cell, where a Pt/Ru0.55Sn0.45O2/C anode provided much higher currents than a commercial Pt/C catalyst for ethanol oxidation at low potentials. Anodes for direct ethanol fuel cells can be fabricated by coating a carbon fibre paper backing layer consecutively with carbon black, Ru(acac)3 + Sn(acac)2, and Pt nanoparticles, with appropriate thermal processing.