Energy-level engineered hollow N-doped NiS_1.03 for Zn–Air batteries

Transition metal sulfides are attracting great attention as promising electrocatalysts for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). However, their sluggish reaction kinetics and poor stability hinder their commercial use. Here, we report the facile design and synthesis of N-doped NiS_1.03 hollow spheres (N-NiS_1.03 HS) for use as efficient bifunctional OER/ORR electrocatalysts. By simultaneous energy-level engineering via N doping and structural modification via hollow nanosphere designing, synthetic N-NiS_1.03 HS can be produced; it exhibits a low overpotential of 250 mV (1.48 V vs. reversible hydrogen electrode (RHE)) at a current density of 10 mA cm^–2 (E_{j = 10}) for the OER and a high half-wave potential (E_1/2) of 0.72 V for the ORR; further, its bifunctional OER/ORR performance is comparable to that of noble metal electrocatalysts. First-principle calculation results reveal that N dopants can simultaneously downshift the d-band center and reduce the overpotential of the rate-determining step of NiS_1.03, thus assisting in the adsorption/desorption of oxygen intermediates on active sites and accelerating reaction kinetics in electrocatalytic processes. Furthermore, N-NiS_1.03 HS-based Zn–air batteries exhibit a small charge–discharge gap (0.64 V), high open-circuit potential (1.41 V), large specific capacity (821 mA h g^–1 _Zn at 10 mA cm^–2), and long-term cycling stability (5 mA cm^–2 for 60 h), thus showing great potential as future energy-efficient metal–air batteries.