Oxygen Evolution Activity on NiOOH Catalysts: Four-Coordinated Ni Cation as the Active Site and the Hydroperoxide Mechanism

The NiOOH catalyst as obtained dynamically from electrodeposition of Ni2+(aq) in the borate-containing electrolyte was observed to exhibit much higher oxygen evolution activity at a near-neutral pH range (7-9) compared to other NiOx-based materials. Here, we demonstrate that this intriguing high activity is owing to the high concentration of Ni cationic vacancy on the nascent ultra-small NiOOH particles (<3 nm). By using first-principles calculations, we compute the thermodynamics of Ni dissolution and clarify the mechanism of oxygen evolution reaction (OER) on the gamma-NiOOH surface. We show that (i) similar to 4% Ni cations on the surface of gamma-NiOOH dissolve at pH = 7 and 1.73 V versus reversible hydrogen electrode; (ii) on the pristine gamma-NiOOH surface, OER proceeds via the lattice peroxide mechanism (*H2O -> *OH -> *O-(OH)-H-latt* -> O-O-latt -> O-2) with an overpotential of 0.70 V; (iii) in the presence of Ni cationic vacancies, OER proceeds via the hydroperoxide mechanism (*OH + *H2O -> *2OH -> *OOH -> O-2) with an overpotential of 0.40 V. Our electronic structure and geometrical structure analyses demonstrate that the structural flexibility at the four-coordinated Ni site nearby Ni vacancy, featuring the ability to bind two terminal oxo species, is key to boost the activity. Considering the presence of the active OOH intermediate, our theory thus implies that the ultra-small oxide nanoclusters with ample cation vacancies could be a paradigm in catalyst design for oxidation reactions.