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 (CoSx) hollow nanospheres (HNSs) and their catalytic efficiencies for hydrogen and oxygen evolution reactions in alkaline media. Three CoSx 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 CoSx series, which would provide insight into the rational design of transition metal chalcogenides for highly efficient hydrogen and oxygen-involved electrocatalysis.