Nanosheets Assembled Nickel Sulfide Nanosphere with Enriched Ni3+ Active Sites for Efficient Water-Splitting and Zinc-Air Batteries

Exploiting a low cost and highly efficient multi-functional electrocatalyst for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is indispensable for promoting the development of overall water-splitting and Zn-air battery technology. In this work, we develop a one-step facile hydrothermal approach to successfully synthesize nickel sulfide nanospheres from nanosheets. The micro-nanostructure and surface metal valence state can be engineered by tuning the phase and composition of the nickel sulfides. Electrochemical results show that a pyrite-type NiS2 catalyst with surface enriched Ni3+ sites exhibits enhanced catalytic activities towards tri-functional electrocatalysis: overpotentials of 241 mV and 147 mV to achieve a current density of 10 mA cm(-2) in 1.0 M KOH for OER and HER, respectively; and a half-wave potential of 0.80 V for ORR. As for overall water-splitting, a low voltage of 1.66 V is required at 10 mA cm(-2), which is even lower than that required by RuO2 + Pt/C (1.69 V). The catalyst also exhibits robust durability: the overpotential reduces by only 4.8% over 37 h of chronopotentiometry and maintains almost 100% faradaic efficiency. The hierarchical NiS2 nanospheres also promote performance in a Zn-air battery with lower discharging-recharging overpotentials (0.8 V) and a longer cycle life (>120 cycles at 10 mA cm(-2)) than Pt/C. Furthermore, theoretical analysis demonstrates that the pyrite-type NiS2 with octahedrally coordinated Ni3+ active sites tailors the adsorption free energies of H* and OH* intermediates to optimal values, contributing to the outstanding electrochemical properties. This work sheds light on the rational design of highly efficient transition metal chalcogenide nanocatalysts through composition and surface chemistry engineering for next-generation energy conversion technologies.