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.