Hybrid Co(III)/Co(IV) Catalytic States Boosting Active Lattice Oxygen in CoNi Hydroxide Nanosheets for Water Electrolysis.

CoNi-based electrocatalysts undergo dynamic reconstruction under anodic potentials, which obscures the active sites and reaction mechanisms in water electrolysis. Herein, in-situ shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) is employed to reveal the evolution of surface structures on CoNi hydroxide nanosheet (NS), compared to the subsurface/bulk structural insights provided by conventional Raman spectroscopy during the oxygen evolution reaction (OER). Key potential-dependent spectral changes in the characteristic A1g and Eg bands of Ni(III)-O, Co(III)-O, and Co(IV)-O indicate surface structures markedly different from the subsurface/bulk structures. Specifically, on Co and CoNi NSs surfaces, both CoOOH and CoO2 coexist, in contrast to only CoO2 phase probed by conventional Raman results. Analysis of A1g/Eg intensity ratios reveals that CoOOH is confined to a near-monolayer at the outermost surface during OER. Ni doping enhances deintercalation of interlayer species (water, anions), accelerating surface CoOOH conversion to CoO2, consistent with the result of electrochemical impedance spectroscopy. Theoretical simulations identify the regulation of the reactivity of lattice oxygen by cation oxidation state, as well as a volcano relationship between OER activity and oxygen vacancy formation energy. CoO2 and Ni-CoOOH, exhibiting the lowest overpotentials, highlight the efficacy of hybrid Co(III)/Co(IV) surface states.