Valence oscillation and dynamic active sites in monolayer NiCo hydroxides for water oxidation

Abstract

Monolayer materials are endowed with an additional degree of freedom to modulate electronic structures and catalytic performances. Here, we report a direct synthesis of monolayer Ni(OH)2 on electrodes by in situ electrochemical conversion and a fundamental investigation of their catalytic activity. The monolayer structure greatly promotes hydrogen and oxygen release processes to produce dynamic active sites for the oxygen evolution reaction (OER) at a lower potential. Lattice doping with cobalt further tunes the electronic structure to reduce the overpotential. In situ experiments revealed Ni and Co valence state oscillation in NiCo hydroxides, which has been attributed to sequential dehydrogenation and deoxygenation processes, and fundamentally contributes to the dynamic generation of OER active sites. This study defines an in situ conversion process to yield monolayer layered double hydroxides (LDHs) and establishes a critical fundamental understanding of the origin of the active sites in monolayer LDHs for the OER. Layered double hydroxides of transition metals are known to be highly active for water oxidation, but the nature of their active sites and reaction mechanism are still elusive. Now, a monolayer NiCo hydroxide catalyst, in situ prepared on the working electrode, is reported to exhibit valence oscillation and dynamic generation of active sites during water oxidation.