In situ fabrication of a Ni-Fe-S hollow hierarchical sphere: an efficient (pre)catalyst for OER and HER

Designing low-cost, highly active and stable electrocatalysts is very important for various renewable energy storage and conversion devices. We fabricated a hierarchically porous Ni-Fe-S hollow hierarchical sphere through a solvothermal reaction of Ni2+, Fe3+ with a mixed solution of glycerol and isopropanol and a S2- ion-exchange process. The incorporation of hybrid metal atoms significantly improved the oxygen evolution reaction (OER) activity of the hollow hierarchical sphere. Typically, the Ni-Fe-S hollow hierarchical sphere exhibits superior OER and HER activities to the single-phase Ni-S and Fe-S hollow hierarchical spheres, as well as most of the bifunctional electrocatalysts when catalyzing full water splitting. To drive a current density of 10 mA cm(-2), the Ni-Fe-S hollow hierarchical sphere requires an overpotential of only 223 and 115 mV for the OER and HER, respectively. Furthermore, the Ni-Fe-S hollow hierarchical sphere catalyst shows excellent durability after a long-term test. The superior activity and long-term stability demonstrate that the Ni-Fe-S hollow hierarchical sphere catalyst has promising potential for application in large-scale water splitting.