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.