In situ formation of surface-induced oxygen vacancies in Co₉S₈/CoO/NC as a bifunctional electrocatalyst for improved oxygen and hydrogen evolution reactions

Creating oxygen vacancies and introducing heterostructures are two widely used strategies in Co-based oxides for their efficient electrocatalytic performance, yet both strategies have rarely been used together to design a bifunctional electrocatalyst for an efficient overall water splitting. Herein, we propose a facile strategy to synthesize oxygen-defect-rich Co₉S₈/CoO hetero-nanoparticles with a nitrogen-doped carbon shell (ODR-Co₉S₈/CoO/NC) through the in situ conversion of heterojunction along with surface-induced oxygen vacancies, simply via annealing the precursor Co₃S₄/Co(OH)₂/ZIF-67. The as-prepared ODR-Co₉S₈/CoO/NC shows excellent bifunctional catalytic activities, featuring a low overpotential of 217 mV at 10 mA cm⁻² in the oxygen evolution reaction (OER) and 160 mV at 10 mA cm−2 in the hydrogen evolution reaction (HER). This performance excellency is attributed to unique heterostructure and oxygen defects in Co₉S₈/CoO nanoparticles, the current work is expected to offer new insights to the design of cost-effective, noble-metal-free electrocatalysts.