The electrochemical reduction of CO2 to chemical fuels and value-added chemicals is a viable pathway to store renewably generated electrical energy and to mitigate the negative impact of anthropogenic CO2 production. Herein, we study how the local reaction environment dictates the mechanism and kinetics of CO2 reduction to CO at an Ag electrode. The local reaction environment is determined using a hierarchical model that accounts for multistep reaction kinetics, specific surface charging state at a given electrode potential, and mass transport phenomena. The model reveals vital mechanistic insights into the reaction behavior. The increasing Tafel slope with overpotential is seen to be influenced by the surface charging relation and mass transport effects. In addition, the decrease of the CO current density at high overpotentials is found to be caused not only by the decrease in CO2 concentration due to mass transport, surface charge effects, and pH increase but also by lateral interactions between HCOOad, COOHad, and Had. Moreover, we explore how the electrolyte properties, including bicarbonate concentration, solvated cation size, and CO2 partial pressure, tune the local reaction environment.

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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 HCOO m is the main active intermediate with HCOO − as the main precursor. CO ad 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 HCOO m 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.