Acid-Base Chemistry of a Model IrO2 Catalytic Interface

Heterogeneous electrocatalysts for the oxygen evolution reaction (OER) operate via inner-sphere processes that are highly sensitive to the composition and structure of the interface. Iridium oxide (IrO2) is one of the most efficient catalytic materials for the OER, yet the atomic scale structure of its aqueous interface is largely unknown. Herein, the hydration structure, proton transfer mechanisms and acid-base properties of the rutile IrO2(110)-water interface are investigated using ab-initio based deep neural-network potentials and enhanced sampling simulations combined with a recently proposed collective variable that allows a unique identification of the protonation states of the reactive species. The intrinsic proton affinities of the different surface sites are characterized by calculating their surface acid dissociation constants, which yield a point of zero-charge in good agreement with experiments. A large fraction (≈ 80%) of adsorbed water dissociation is observed, together with a short lifetime (≈ 0.5 ns) of the resulting terminal hydroxyls, due to rapid proton exchanges between adsorbed H2O and OH species at adjacent surface Ir sites. This rapid surface proton transfer supports the suggestion that the rate-determining step in the OER may not involve proton transfer across the double layer into solution, but rather depend on the concentration of oxidized sites formed by the deprotonation of terminal and bridging hydroxyls, as indicated by recent experiments.