AbstractDevelopment of hydrogen as clean and efficient energy carrier for future is imperative. Water electrolysis, is considered as one of the most promising ways to realize large-scaled hydrogen production. However, a big obstacle of it is to reduce the electric energy consumption for water oxidation in the anode. Engineering of hierarchical architectures on the electrocatalysts could provide abundant active sites and thus boost the sluggish reaction kinetics of water oxidation. Herein, a sequential synthesis method is developed for in-situ growth of ultrathin Co₉S₈ nanosheets vertically aligned on N and S co-doped reduced graphene oxide (Co₉S₈/N,S-rGO) as novel and efficient electrocatalysts for water splitting. This architecture with vertically aligned ultrathin Co₉S₈ nanosheets on N,S/rGO is adopted to facilitate the electron transport and exposure of active sites. Benefiting from the synergetic catalysis between Co₉S₈ nanosheets and N,S/rGO, Co₉S₈/N,S-rGO presents remarkable electrocatalytic activity towards oxygen evolution with a low overpotential (266 mV to achieve current density of 10 mA cm⁻²), small Tafel slope of 75.5 mV dec⁻¹, and good durability in alkaline medium. This remarkable OER electrocatalytic activity is outperforms most of the known noble-metal-free electrocatalysts.