The surface chemistry of Au(111) and Au(100) electrodes under oxygen reduction and evolution potentials

Oxygen electrochemistry plays a central role in various sustainable energy solutions but the factors controlling the electrocatalytic oxygen reduction (ORR) and evolution reactions (OER) are still not fully understood even on model electrodes. As the electrocatalytic (OER and ORR) mechanisms cannot be understood without knowing the electrode state under operation conditions, herein we have employed grand canonical ensemble density functional theory (GCE-DFT) to systematically investigate the pH- and potential-dependent surface coverage of model Au(111) and Au(100) electrodes. Under ORR conditions (0.6–1.1 V vs RHE) both surfaces exhibit a moderate 1/3--2/3 monolayer OH* coverage. No thermodynamically stable OOH* intermediates were found, challenging previous models of Au-catalyzed ORR activity and selectivity. Instead, our results suggest that the pH-, potential-, and coverage-dependent surface charge plays a key role in the ORR activity and selectivity. Close to the OER equilibrium potential both surfaces have high OH or mixed OH/O adsorbate coverage, which transform into surface or bulk oxides above 1.4-1.6 V vs RHE. These oxidation processes coincide with the onset of gold dissolution and indicate that OER active surfaces consist of gold (hydr)oxides rather than metallic gold. Our findings have direct implications in the necessity of explicitly accounting for the effects of surface coverage, applied potential, and pH on electrocatalytic activity and long-term stability of gold electrodes in oxygen electrochemistry.