Controllable synthesis of nickel sulfide nanocatalysts and their phase-dependent performance for overall water splitting.

The exploitation of economical and highly efficient bifunctional electrocatalysts to promote oxygen evolution and hydrogen evolution reactions (OER and HER) for water splitting devices is urgently needed. Herein, a series of NiSx (i.e., NiS, Ni3S2, NiS2) nanocrystals with controllable phase and composition have been synthesized via a facile polyol solution process and the corresponding electrocatalytic properties towards OER and HER have been systematically investigated. Electrochemical results reveal that Ni3S2 exhibits superior OER and HER performance to NiS and NiS2, achieving 1.63 V to reach a current density of 10 mA cm-2 in the overall water splitting device, which is comparable to that of noble metal catalysts. Experimental and theoretical calculation investigations demonstrate that the remarkable catalytic properties of Ni3S2 could be attributed to the intrinsic metallic conductivity, abundant active sites and optimal Gibbs free-energy for catalyst-H* for HER. Moreover, a thicker layer of catalytically active species of NiOOH was generated on the surface of Ni3S2 due to the higher proportion of Ni, leading to a better OER performance. These results should shed light on the design and development of low cost and efficient transition metal chalcogenide electrocatalysts through phase and composition regulation for advanced electrochemical energy conversion devices.