FeO₄-Type Active Sites Grown on Fe-Doped Ni Core Surfaces during the Initial Oxygen Evolution Reactions: Fe-Doping Effect?

Clean hydrogen production by water splitting is hampered by sluggish oxygen evolution reactions (OER). The most promising high-activity, high-stability, and low-cost OER catalyst working in alkaline conditions is a core-shell nanostructure whose Fe-doped Ni (NiFe) core particles are encapsulated by N-doped graphitic (NC) shells. However, hidden beneath the irregular NC shells, the active site in the core as well as the origin of the favorable Fe-doping effect has not been well identified. We thus herein use density functional theory calculations and derive an active site model on the core by mimicking the oxidation of the NiFe metal surface during the initial OER cycles required for catalyst activation. During these cycles at pH 14, an oxophilic Fe atom protrudes out of the O-covered surface and sits on Ni oxide layers grown on top of the core surface, forming an FeO₄-type structure that requires very low overpotentials for subsequent OER cycles. An equivalent NiO₄-type structure, if it forms, may require even lower overpotentials, but the protrusion of less oxophilic Ni into surrounding O atoms needs to overcome a much higher free energy barrier. Indeed, the FeO₄-type structure has been suggested as an active site whose facile dissolution as Fe(OH)₄^- explains for the activity loss of catalysts directly exposed to the electrolyte.