Tuning eg orbital occupancy of Ni sites in Cr-doped Ni3S2 to steer intermediate adsorption for efficient water splitting.

Electrocatalytic water splitting enables sustainable green hydrogen production, yet sluggish oxygen/hydrogen evolution reaction (OER/HER) kinetics hinder its practical commercialization. Trinickel disulfide (Ni3S2), with abundant active sites and favorable intrinsic conductivity, is a competitive bifunctional electrocatalyst. However, the pristine Ni3S2 features overfilled eg orbital states at Ni sites accompanied by an up-shifted d-band center, causing suboptimal intermediate adsorption and then yielding large overpotentials. Herein, Cr heteroatom doping is employed to reconstruct the d-orbital electronic configuration at Ni sites and achieve differentiated orbital modulation toward catalytic reactions. The formed Cr 3d-S 2p-Ni 3d orbital hybridization in the developed NiS-Cr-0.5 optimizes eg orbital filling state, balances the adsorption/desorption of oxygen-containing intermediate (*OH, *O, *OOH), and lowers the rate-determining step (RDS) energy barrier for OER. Meanwhile, the modulated d-band center tunes the adsorption strength of *H intermediates and accelerates water dissociation kinetics, contributing to the enhanced HER performance. Accordingly, the orbital engineering endows NiS-Cr-0.5 with excellent bifunctional catalytic activity in 1.0 M KOH electrolyte. It only requires overpotentials of 175 mV and 134 mV to drive 10 mA cm-2 for OER and HER, respectively. Moreover, the assembled water-splitting system of NiS-Cr-0.5//NiS-Cr-0.5 achieves a cell voltage of 1.565 V at 10 mA cm-2, comparable to or even superior to most reported nickel sulfide-based electrocatalysts. This work establishes an orbital descriptor-activity correlation, providing a feasible orbital engineering strategy for the design of advanced electrocatalysts.