First-principles description of electrocatalytic characteristics of graphene-like materials

Graphene-like materials (GLMs) have received much attention as a potential alternative to precious metal-based electrocatalysts. However, the description of their electrocatalytic characteristics may still need to be improved, especially under constant chemical potential. Unlike the case of conventional metal electrodes, the potential drop across the electrical double layer (ϕ_D) at the electrode-electrolyte interface can deviate substantially from the applied voltage (ϕ_app) due to a shift of the Dirac point (eϕ_G) with charging. This may in turn significantly alter the interfacial capacitance (C_T) and the relationship between ϕ_app and free-energy change (ΔF). Hence, accurate evaluation of the electrode contribution is necessary to better understand and optimize the electrocatalytic properties of GLMs. In this work, we revisit and compare first-principles methods available to describe the ϕ_app-∆F relation. Grand-canonical density functional theory is used to determine ΔF as a function of ϕ_app or electrode potential (ϕ_q), from which the relative contribution of eϕ_G is estimated. In parallel, eϕ_G is directly extracted from a density functional theory analysis of the electronic structure of uncharged GLMs. The results of both methods are found to be in close agreement for pristine graphene, but their predictions deviate noticeably in the presence of adsorbates; the origin of the discrepancy is analyzed and explained. We then evaluate the application of the first-principle methods to prediction of the electrocatalytic processes, taking the reduction (hydrogenation) and oxidation (hydroxylation) reactions on pristine graphene as examples. Our work highlights the vital role of the modification of the electrode electronic structure in determining the electrocatalytic performance of GLMs.