Structure Sensitivity of Formic Acid Electrooxidation on Transition Metal Surfaces: A First-Principles Study

Abstract

Formic acid presents several advantages for use as a fuel in fuel cells. We present a first-principles based analysis of electrooxidation trends for formic acid on the close-packed facets of eight fcc metals: Au, Ag, Cu, Pt, Pd, Ni, Ir, and Rh, and four hcp metals: Co, Os, Ru, and Re. To explore the structure sensitivity of this reaction on the fcc metals, we also studied the open (100) facet of these eight metals. We find that the open facets of Au, Ag, Cu, Pt, and Pd are more energy-efficient (i.e. require less overpotential) for formic acid electrooxidation when compared to their close-packed facets. The opposite is true for the stronger-binding metals: Ni, Ir, and Rh. Using the free energy of adsorbed CO * and OH * as reactivity descriptors, we cast the thermochemistry of the reaction network into phase diagrams showing regions of rate-determining steps, together with their calculated free energies. This allows the identification of bimetallic alloys, potentially possessing improved electrocatalysis for formic acid electrooxidation, compared to Pt or Pd. We discuss the need for anode catalysts for direct formic acid fuel cells (DFAFCs) to diminish CO * poisoning, by promoting the formation of formate instead of carboxyl intermediate.

Electrocatalytic Activities towards the Electrochemical Oxidation of Formic Acid and Oxygen Reduction Reactions over Bimetallic, Trimetallic and Core–Shell-Structured Pd-Based Materials

“…CO formation by dehydration of formic acid proceeded via the carboxyl pathway (i.e., COOH*). Mavrikakis et al, calculated free-energy diagrams of formic acid oxidation on Pt and Pd electrocatalysts, including the carboxyl pathway, formate pathway (i.e., HCOO*) and indirect carboxyl-mediated mechanism [40]. According to their calculations, the reaction mechanisms via formate and carboxyl have very similar onset potentials to Pd (111), in agreement with other reported results [41].…”

Recent advances in formic acid electro-oxidation: from the fundamental mechanism to electrocatalysts

“…62 And therefore, catalysts with high activity for CO electro-oxidation can efficiently remove the surface adsorbed CO and thus significantly promote the FAEO. 43 Just like the identification of reactive intermediates, bridge-bonded formate (HCOO b *), dimeric formic acid, carboxylate (*COOH), CO 2 and other species were once thought to be the precursors to CO. 57,[63][64][65] As for the formation origination of CO ads , Cuesta et al proposed that the presence of adjacent platinum atoms would lead to the formation of CO ads . This view has been widely accepted, and many efforts have been made to construct discontinuous platinum sites to mitigate the poisoning of platinum.…”

MEA Preparation for Direct Formate/Formic Acid Fuel Cell—Comparison of Palladium Black and Palladium Supported on Activated Carbon Performance on Power Generation in Passive Fuel Cell

“… A general formic acid oxidation mechanism on Pd and Pt anodes of direct formic acid fuel cells (DFAFCs) by Elnabawy et al Paths: blue, direct via formate; black, direct via carboxyl; and red, indirect via carboxyl. Reproduced from [ 12 ]. …”