Phase and composition controlled synthesis of cobalt sulfide hollow nanospheres for electrocatalytic water splitting

Developing cheap, highly efficient and stable electrocatalysts for both oxygen and hydrogen evolution reactions (OER and HER) is extremely meaningful to realize large-scale implementation of water splitting technology. Herein, we report the phase and composition controlled synthesis of cobalt sulfide (CoS_x) hollow nanospheres (HNSs) and their catalytic efficiencies for hydrogen and oxygen evolution reactions in alkaline media. Three CoS_x compounds, i.e., Co₉S₈, Co₃S₄, and CoS₂ HNSs, were precisely synthesized by simply adjusting the molar ratio of carbon disulfide to cobalt acetate using a facile solution-based strategy. Electrochemical results reveal that the as-prepared CoS₂ HNSs exhibit superior OER and HER catalytic performance to Co₉S₈ and Co₃S₄ HNSs in 1.0 M KOH, with overpotentials of 290 mV for the OER and 193 mV for the HER at 10 mA cm^-2, and the corresponding Tafel slopes of 57 and 100 mV dec^-1, respectively. In addition, the CoS₂ HNSs exhibit remarkable long-term catalytic durability, which is even superior to precious metal catalysts of RuO₂ and Pt/C. Moreover, an alkaline electrolyzer assembled using CoS₂ HNSs as both anode and cathode materials can achieve 10 mA cm^-2 at a low cell voltage of 1.54 V at 60 °C with a faradaic efficiency of 100% for overall water splitting. Further analysis demonstrates that the surface morphology, crystallographic structure and coordination environment of Coⁿ⁺ active sites in combination determine the HER/OER activities in the synthesized binary CoS_x series, which would provide insight into the rational design of transition metal chalcogenides for highly efficient hydrogen and oxygen-involved electrocatalysis.