Multicapacitor Approach to Interfacial Proton-Coupled Electron Transfer Thermodynamics at Constant Potential

Abstract

Theoretical calculations of interfacial thermodynamics at constant potential enhance understanding of heterogeneous electrocatalytic reactions. Herein, a strategy is devised for computing reaction thermodynamics for electrochemical proton-coupled electron transfer (PCET), a key elementary step in a wide range of electrocatalytic processes. In this approach, Gibbs free energies obtained from constant charge periodic density functional theory calculations are transformed to grand potentials by using a grid-based mapping procedure in conjunction with a multicapacitor model that accounts for constantly charged surface adsorbates. The energetic contribution of the adsorbate capacitor to the grand potential is found to be essential for computing proton-coupled redox potentials at constant potential. This strategy is applied to graphite-conjugated catalysts, wherein organic acids are attached to carbon surfaces through a conductive phenazine bridge. In these systems, PCET occurs at both the phenazine bridges and the organic acids, which can be negatively or positively charged. For a given graphite-conjugated catalyst active site, the charge of the organic acid adsorbate remains virtually constant for the relevant range of electrode potentials. Moreover, the potential-dependent graphite surface charge density, which excludes this adsorbate charge, is consistent across all systems studied. Within this framework, the potential-dependent PCET reaction free energies are independent of cell size, thereby avoiding the need for computationally expensive cell size extrapolation techniques. The proton-coupled redox potentials computed with this strategy are in agreement with experimental data. This computational strategy, as well as the conceptual insights about the impact of charged adsorbates on electrochemical interfaces, is applicable to other materials and processes.