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• CoWO 4-x @C exhibits comparable HER activity to Pt/C in acidic and alkaline mediums. • CoWO 4-x @C requires only 295 mV overpotential to deliver 10 mA cm−2 in the OER test. • 1.57 V is required for CoWO 4-x @C to attain 10 mA cm−2 in alkaline water splitting. Development of earth-abundant, efficient and stable electrocatalysts for water splitting is of crucial importance for environmentally friendly energy conversion and storage. Here, we report an oxygen deficient cobalt tungstate nanoparticles with diameter of 8 nm (CoWO 4-x @C) as efficient bifunctional electrocatalyst for hydrogen evolution reaction (HER) and oxygen reduction reaction (OER), which exhibits comparable HER activity to commercial Pt/C with overpotentials of 32.5 mV and 46.8 mV vs. RHE to deliver current density of 10 mA cm−2 in acidic and alkaline mediums, respectively, ascribed to the rich oxygen vacancies facilitating the hydrogen adsorption and its relative recombination, respectively. Additionally, undetectable degradation is observed for CoWO 4-x @C after 10,000 potential cycles indicating high durability. Meanwhile, CoWO 4-x @C requires only 295 mV overpotential to deliver 10 mA cm−2 in the OER test, which is better than the benchmarking IrO 2 (313 mV). 1.57 V comparably lower than Pt/C-IrO 2 (1.59 V) is required for achieving the water splitting current density of 10 mA cm−2 without any degradation for 12 h. [ABSTRACT FROM AUTHOR]