Modeling Electrocatalytic Oxidation of Formic Acid at Platinum

Abstract

The formic acid oxidation reaction (FAOR) is a model electrocatalytic reaction involving multiple proton-coupled electron transfer steps. Despite of repeated research extending over more than fifty years, the FAOR is still an active research field in which several important questions including reaction mechanisms, the activity dependence on the electrode structure, the hysteresis between positive- and negative-going scan in cyclic voltammetry (CV), and, especially, the pH effect, remain elusive yet. To shed some light on these puzzles, we herein develop a microkinetic model for the FAOR at Pt(111) which uses a reaction mechanism supported by microscopic and mechanistic information from density functional theory calculations and spectroscopic characterizations, formulates the mechanism using fully microkinetic modeling without designating a rate-determining step, and incorporates double-layer effects by means of a mean-field description for the electrode-electrolyte interphase. Moreover, chemisorbed intermediates play multifaceted roles in this formulism: they are reactants with lateral interactions, site-blockers, as well as modifiers of the double-layer structure and properties. The model is parameterized using CV data of Pt(111) in perchlorate electrolyte with different pHs, revealing that HCOOm is the main active intermediate with HCOO− as the main precursor. COad on defect sites induces the voltage hysteresis through modifying surface charging relation (the main effect) and blocking adsorption sites (the minor effect). It is also found that the higher current density as the pH increases from 0.11 to 1.42 is the result of two opposing factors: higher concentration of HCOO− in bulk solution and stronger double layer effects that suppress HCOOm formation. The presented work demonstrates that consideration of double-layer effects and an integrated view of multifaceted roles of reaction intermediates are a sheer necessity for FAOR.