(621ah) Trends in Formic Acid Electro-Oxidation on Model Transition Metal Surfaces: A Density Functional Theory Study
Formic Acid (FA) is a renewable fuel that can be produced either from biomass or directly from carbon dioxide CO2. It can be used as a hydrogen storage material for in situ production upon its decomposition [1,2]. Moreover, being relatively non-toxic  (unlike methanol) and a liquid at room temperature (unlike hydrogen), FA can be conveniently used in Direct Formic Acid Fuel Cells (DFAFCs), enabling a safe renewable fuel for transportation and portable applications. DFAFCs offer low temperature operation and mild crossover coupled with high power densities owing to relatively facile kinetics and favorable thermodynamics [4,5]. Consequently, FA electro-oxidation has attracted considerable attention both in theoretical and experimental literature. Unlike FA vapor-phase decomposition [6-8], trends across the periodic table for FA electro-oxidation are still unclear. To this end, we present a first-principles, self-consistent periodic density functional theory (PW91-GGA) study of FA electro-oxidation on model (111) and (100) facets of eight fcc metals (Au, Ag, Cu, Pt, Pd, Ni, Ir, and Rh) and (0001) facets of four hcp metals (Co, Os, Ru, and Re). Using the calculated binding energies of key adsorbates including formate (HCOO), carboxyl (COOH), carbon monoxide (CO), and hydroxyl (OH), together with a simple computational electrochemical model , we develop thermochemical free energy diagrams, calculate onset potentials, and identify potential-determining steps for all surfaces. Finally, we use these energetics to develop volcano plots to be used for the design of improved FA electro-oxidation catalysts.
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