Cobalt and nickel selenide nanowalls anchored on graphene as bifunctional electrocatalysts for overall water splitting

The development of efficient and low-cost solar-driven overall water splitting systems for sustainable hydrogen generation is still a challenge. Herein, cobalt and nickel (Co and Ni) selenide nanowalls have been prepared on a conductive graphene coated Ni mesh substrate and used as electrocatalysts for hydrogen generation and oxygen evolution. The bifunctional CoSe nanowalls showed a high electrochemically active surface area, which manifested a low onset potential and high structural stability for overall water splitting. It afforded a 10 mA cm(-2) current density at -78 mV vs. RHE for the hydrogen evolution reaction and 150 mA cm(-2) at 1.74 V vs. RHE for the oxygen evolution reaction. No significant degradation was observed after a 24 h chronoamperometric test. A solar water electrolysis system was constructed by connecting the CoSe (+) and CoSe (-) pair with a photovoltaic cell in tandem, which showed a highest solar-to-hydrogen conversion efficiency of 6.65%. Its superior performance over industrial water electrolyzers provided a new possibility towards clean energy storage and utilization.