Design and synthesis of noble-metal-free bifunctional catalysts for efficient and robust electrochemical water splitting are of significant importance in developing clean and renewable energy sources for sus-tainable energy consumption.Herein,a simple three-step strategy is reported to construct cobalt-iron nitride/alloy nanosheets on nickel foam (CoFe-NA/NF) as a bifunctional catalyst for both hydrogen evo-lution reaction (HER) and oxygen evolution reaction (OER).The electrocatalyst with optimized composi-tion (CoFe-NA2/NF) can achieve ultralow overpotentials of 73 mV and 250 mV for HER and OER,respectively,at a current density of 10 mAcm-2 in 1 M KOH.Notably,the electrolyzer based on this elec-trocatalyst is able to boost the overall water splitting with a cell voltage of 1.564 V to deliver 10 mAcm-2 for at least 50 h without obvious performance decay.Furthermore,our experiment and theoretical calcu-lation demonstrate that the combination of cobalt-iron nitride and alloy can have low hydrogen adsorp-tion energy and facilitate water dissociation during HER.In addition,the surface reconstruction introduces metal oxyhydroxides to optimize the OER process.Our work may pave a new pathway to design bifunctional catalysts for overall water splitting.