Oxygen Evolution and Reduction on Fe-doped NiOOH: Influence of Solvent, Dopant Position and Reaction Mechanism

Abstract: The oxygen evolution reaction (OER) is the limiting factor in an electrolyzer and the oxygen reduction reaction (ORR) the limiting factor in a fuel cell. In OER, water is converted to O₂ and H⁺/e⁻ pairs, while in ORR the reverse process happens to form water. Both reactions and their efficiency are important enablers of a hydrogen economy where hydrogen will act as a fuel or energy storage medium. OER and ORR can both be described assuming a four-step electrochemical mechanism with coupled H⁺/e⁻ transfers between four intermediates (M-*, M-OH, M = O, M-OOH, M = active metal site). Previously, it was shown for mixed metal-oxyhydroxides that an unstable M-OOH species can equilibrate to an M-OO species and a hydrogenated acceptor site (M-OOH/eq), enabling a bifunctional mechanism. Within OER, the presence of Fe within a nickel-oxyhydroxide (NiOOH) acceptor site was found to be beneficial to lower the required overpotential (Vandichel et al. in Chemcatchem 12(5):1436–1442, 2020). In this work, we present the first proof-of-concept study of various possible mechanisms (standard and bifunctional ones) for OER and ORR, i.e. we include now the active edge sites and hydrogen acceptor sites in the same model system. Furthermore, we consider water as solvent to describe the equilibration of the M-OOH species to M-OOH/eq, a crucial step that enables a bifunctional route to be operative. Additionally, different single Fe-dopant positions in an exfoliated NiOOH model are considered and four different reaction schemes are studied for OER and the reverse ORR process. The results are relevant in alkaline conditions, where the studied model systems are stable. Certain Fe-dopant positions result in active Ni-edge sites with very low overpotentials provided water is present within the model system. Graphic Abstract: [Figure not available: see fulltext.]