Shifting surface oxidation cycle to high-valence subsurface boosts water oxidation

Abstract

Molecular reactants adsorption and their subsequent redox on surface are two major processes in every electrochemical reaction. Thus, 3d-transition-metal (M) oxide has attracted significant attention for its proper binding with the reactants and its facile metal oxidation cycle (Mn+M(n+1)+) to facilitate the reactants’ redox such as oxygen evolution reaction (OER) (*OH  *O  *OOH  O2). However, the metal oxidation cycle on the surface has still generated thermodynamic energy cost and surface degradation, so limited efficient electrocatalytic processes. Here, we eliminate the metal cycle from the surface and hand it over to a non-catalytic subsurface. In a multivalent metal oxide CoWO4-δ as a first showcase, we reveal a dramatic shift of the metal oxidation cycle from the Co (Co2+ Co3+) surface to an embedded but easily oxidizable W (W5+W6+) subsurface. By moving the cycle to the stable subsurface, we relieve the repetitive surface metal oxidation during OER and exploit the strong oxidation capability of the high-valence elements (W5+) protected in subsurface. Disentangling the adsorption site and the oxidation cycle site is a powerful emerging strategy in electrocatalysis. This concept aligns with the paradigm of leveraging subsurface sites to overcome traditional scaling relations. Here, we provide a clear demonstration of this principle in a multivalent CoWO4-δ system, offering unprecedented synergistic effects such as lowered overpotential and alleviated surface degradation to be generally applied to various electrochemical processes.