Efficient bifunctional water splitting catalysts enabled by crystalline–amorphous NixSy@NiFe LDH heterojunctions

Abstract

Developing crystalline–amorphous heterojunctions presents a promising pathway to enhance the electrocatalytic performance of both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). This study reports the exploration of novel crystalline–amorphous Ni_xS_y@NiFe LDH heterostructures for efficient HER and OER, which are synthesized via in situ growth of NiFe-LDH on nickel sulfide (Ni_xS_y) nanowires. The resultant three-dimensional core–shell structures remarkably increase the active sites, enhance the charge transfer, and facilitate the gas release during the catalytic process. In situ Raman spectroscopy and density functional theory (DFT) calculations verify that the introduced Fe could boost the OER activity by promoting the structural reconstruction to form the disorder of NiOOH@NiFeOOH species, and reducing the energy barrier for conversion of oxygen-containing intermediates. The heterojunction interface in Ni_xS_y@NiFe LDH modifies the electron distribution, thus significantly lowering the Gibbs free energy of hydrogen adsorption (ΔG_H* = 0.1 eV) compared to that of Ni_xS_y. Correspondingly, the Ni_xS_y@NiFe LDH exhibits superior bifunctional performance for the HER and OER in alkaline solution, delivering high current densities of −100 and 200 mA cm⁻² at low overpotentials of 159 and 250 mV for the HER and OER, respectively, as well as an excellent stability against operation over 250 h at 200 mA cm⁻², implying its promise toward commercial applications.