(759f) Fundamental Aspects of Formic Acid Electro-Oxidation on Bimetallic Catalysts
Direct formic acid fuel cells (DFAFCs) have emerged as a viable power source as demands for alternative energy sources for transportation and portable energy storage have increased1. Formic acid (FA) is a carbon-neutral fuel that can be produced from biomass2 or via the reduction of CO23. DFAFCs have numerous advantages compared to other similar fuel cells4 because they have an equilibrium cell voltage (1.40 V) that is higher than that shown by hydrogen (1.23 V) and methanol (1.21 V) fuel cells among others5. Additionally, FA is liquid at room temperature (unlike hydrogen), is less toxic than methanol5,and shows less crossover through Nafion® membranes in fuel cells6. Despite these key advantages of DFAFCs, the best monometallic catalysts, platinum and palladium, are unstable in acidic media, are poisoned by CO during FA electro-oxidation, and are costly4. Alloying Pt and Pd with less elxpensive metals reduces the amount of Pt or Pd in the catalyst (and lowers the cost) and can improve both the stability and activity of the catalyst7,14. Here, we present a density functional theory (PW91-GGA) study of FA electro-oxidation on the (111) facet of bimetallic Pt or Pd catalysts alloyed with Au, Ag, Cu, Pt, Pd, Ir, Rh, Ru, and Re to better understand the activity of these catalysts. For each catalyst, we calculate free energy diagrams to investigate the interconversion between CO, OH, carboxyl (COOH), and formate (HCOO) on each surface. This allows us to calculate the onset potential of three key reaction mechanisms: direct oxidation of FA via COOH, direct oxidation of FA via HCOO, and the indirect oxidation of FA that forms CO for each surface. We then compare the onset potential of each mechanism to determine preferred reaction pathways and the propensity of each catalyst to be poisoned by CO.
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