NiMoO 4 containing O-vacancy cooperated with bimetallic sulfides as efficient bifunctional electrocatalyst for overall water splitting

MoS 2 is a potential transition metal sulfide (TMS) with excellent performance for hydrogen evolution reaction (HER) in acidic medium. However, MoS 2 has poor stability owing to its metastable properties and therefore is easily oxidized to MoO 3 which can hinder the HER process. In this work, MoO 3 on the surface of the TMS is transformed to NiMoO 4 with plentiful oxygen vacancies (Ov) favorable for HER and oxygen evolution reaction (OER) by an acid treatment to form an Ov-containing transition bimetallic sulfide/oxide catalyst (OvNiMoO 4 @Ni 3 S 2-MoS 2-rGO-NF, denoted as NiMoO 4 @NiMoS-rN). The electrocatalyst shows exceptional performance for HER and OER in terms of increased intrinsic activity, electron transfer efficiency, reaction kinetics and durability on account of the Ov, synergistic effect among different phases and bimetallic active sites. The overpotentials of NiMoO 4 @NiMoS-rN electrode when achieving 10 mA cm-2 are 45 mV and 195 mV towards HER and OER in alkaline, respectively, and the performance holds nearly unchanged during a 50-h stability test. When assembled to a two-electrode system, the current density of 10 mA cm- 2 can keep steady for 50 h without degradation. The strategy combined by phase transition, interface and defect engineering via a facile method provides a practical idea for design and modulation of bifunctional electrocatalysts.