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Highlights Bimetallic nickel cobalt sulfide (Ni,Co)S2 nanosheet arrays were demonstrated as a multifunctional catalyst for OER, HER, and ORR.First principle calculations were performed to probe the rate-limiting step, which involves the formation of *OOH from HO− on the (Ni,Co)S2 surface.A water-splitting system was designed with the (Ni,Co)S2 serving as both cathode and anode, and a Zn–air battery cathode electrocatalyst. Electronic supplementary material The online version of this article (10.1007/s40820-018-0232-2) contains supplementary material, which is available to authorized users., The development of efficient earth-abundant electrocatalysts for oxygen reduction, oxygen evolution, and hydrogen evolution reactions (ORR, OER, and HER) is important for future energy conversion and energy storage devices, for which both rechargeable Zn–air batteries and water splitting have raised great expectations. Herein, we report a single-phase bimetallic nickel cobalt sulfide ((Ni,Co)S2) as an efficient electrocatalyst for both OER and ORR. Owing to the synergistic combination of Ni and Co, the (Ni,Co)S2 exhibits superior electrocatalytic performance for ORR, OER, and HER in an alkaline electrolyte, and the first principle calculation results indicate that the reaction of an adsorbed O atom with a H2O molecule to form a *OOH is the potential limiting step in the OER. Importantly, it could be utilized as an advanced air electrode material in Zn–air batteries, which shows an enhanced charge–discharge performance (charging voltage of 1.71 V and discharge voltage of 1.26 V at 2 mA cm−2), large specific capacity (842 mAh gZn−1 at 5 mA cm−2), and excellent cycling stability (480 h). Interestingly, the (Ni,Co)S2-based Zn–air battery can efficiently power an electrochemical water-splitting unit with (Ni,Co)S2 serving as both the electrodes. This reveals that the prepared (Ni,Co)S2 has promising applications in future energy conversion and energy storage devices. Electronic supplementary material The online version of this article (10.1007/s40820-018-0232-2) contains supplementary material, which is available to authorized users.