Understanding the in-situ structure of iridium oxide single crystal surfaces during oxygen evolution reaction

Iridium oxides have been considered as a benchmark catalyst exhibiting high activity, albeit a modest stability, for the oxygen evolution reaction (OER) in acidic media. Their structure, however, is debated, with phase transformations possible at the surface or in the bulk under oxidizing potentials. Probing these phenomena at the molecular level could, in turn, provide insights into the reasons for observed stability losses. In this study, we computationally probe the structural evolution of a single crystal (110) surface of IrO₂ using a combination of thermodynamic phase diagrams and density functional theory (DFT) calculations. We analyze co-adsorption effects at high and mixed OER intermediate coverages using a customized graph theory algorithm, while solvation corrections for hydrophilic OH* and H₂O* intermediates are treated in the context of water bilayer models. Our results indicate that the top iridium sites on the (110) surface are completely covered with water molecules in aqueous environments. To initiate the reaction, this water undergoes further dissociation, forming a monolayer of OH* on the top sites, and protonating lattice oxygen atoms at bridging iridium atoms to form OH^# species. Our phase diagram analysis further indicates that that the bridge OH^# groups oxidize to give back bridge lattice oxygen atoms at oxidizing potentials of 1.40 V. The top OH* groups, on the other hand, oxidize to O* groups as the potential is raised to 1.90 V. These computationally predicted phase transitions correctly predict trends measured by Suntivich et al. However, the computational results overpredict the experimental voltages, and we discuss their sensitivity in the context of DFT functionals, changes in oxidation state evaluated with a Bader charge analysis, and DFT+U corrections. Further, work is being done in explicitly determining coverage dependent solvation corrections using an in-house formalism based on ab initio molecular dynamics, inspired from simulated annealing.