Electron Transfer-Induced Metal Spin-Crossover at NiCo₂S₄/ReS₂ 2D–2D Interfaces for Promoting pH-universal Hydrogen Evolution Reaction

The development of an efficient pH-universal hydrogen evolution reaction (HER) electrocatalyst is essential for practical hydrogen production. Here, an efficient and stable pH-universal HER electrocatalyst composed of the strongly coupled 2D NiCo₂S₄ and 2D ReS₂ nanosheets (NiCo₂S₄/ReS₂) is demonstrated. The NiCo₂S₄/ReS₂ 2D–2D nanocomposite is directly grown on the surface of the carbon cloth substrate, which exhibits excellent HER performance with overpotentials of 85 and 126 mV at a current density of 10 mA cm⁻² and Tafel slopes of 78.3 and 67.8 mV dec⁻¹ under both alkaline and acidic conditions, respectively. Theoretical and experimental characterizations reveal that the chemical coupling between NiCo₂S₄ and ReS₂ layers induces electron transfer from Ni and Co to interfacial Re-neighbored S atoms, enabling beneficial H atom adsorption and desorption for both acidic and alkaline HER. Simultaneously, an electron transfer-induced spin-crossover generates high-spin interfacial Ni and Co atoms that promote water dissociation kinetics at the NiCo₂S₄/ReS₂ interface, which is the origin of the superior alkaline HER activity. NiCo₂S₄/ReS₂ also shows decent catalytic activity and long-term durability for oxygen evolution reaction, and finally bifunctionality for overall water splitting. This study suggests a rational strategy to enhance water dissociation kinetics by inducing spin-crossover via electron transfer.