Engineered oxidation states in NiCo₂O₄@CeO₂ nanourchin architectures with abundant oxygen vacancies for enhanced oxygen evolution reaction performance

Sluggish oxygen evolution reaction (OER) as the half reaction of water electrolysis hindered the production of clean hydrogen, necessitates a cost-effective electrocatalyst. Spinel nickel cobaltite (NiCo₂O₄), characterized by its lower cost, flexibility in oxidation states of cations, and stable spinel structure, holds promise as an OER electrocatalyst. However, there is still room for improvement in terms of its intrinsic electrochemical activity, conductivity, and active surface area. Inspired by the flexible valence states of Ni and Co, in this study, CeO₂ was introduced into NiCo₂O₄ for further electronic structure adjustment. The result was the synthesis of an urchin-like NiCo₂O₄@CeO₂ (NCOC), and the elucidation of its formation mechanisms, expressed conclusively through reaction formulas. NCOC, characterized by an open architecture and a rough surface, ensures rapid mass transportation and charge diffusion. Within NCOC, numerous oxophilic Ce³⁺ sites with oxygen vacancies construct electron transport pathways, accelerating charge-transfer. This simultaneously induces an increase in the oxidation states and proportions of Ni³⁺ and Co³⁺ in NCOC, reinforcing the covalence of Ni–O and Co–O bonds, expediting the ad/desorption of intermediates during OER. The electronic structure variations of NCOC were further verified through density functional theory (DFT) analyses, revealing alterations in adsorption configurations of intermediates and a decrease in adsorption free energies for intermediates on NCOC during OER. In practical terms, NCOC demonstrated a low overpotential of 228 mV at a current density of 10 mA cm⁻² under 1 M KOH, maintaining long-term electrochemical stability for 100 h. Overall, this study provides a universal approach for designing highly cost-efficient electrocatalysts for OER.