Se-NiSe₂ hybrid nanosheet arrays with self-regulated elemental Se for efficient alkaline water splitting

Understanding the catalytic mechanism of non-noble transition metal electrocatalysts is crucial to designing high-efficiency, low-cost, and durable alternative electrocatalysts for water splitting which comprises the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this work, Se-NiSe₂ hybrid nanosheets with a self-regulated ratio of ionic Se (I-Se) to elemental Se (E-Se) are designed on carbon cloth by solution synthesis and hydrothermal processing. The effects of the I-Se/E-Se ratios on the electrocatalytic characteristics in HER and OER are investigated systematically both experimentally and theoretically. The optimized bifunctional electrocatalyst needs overpotentials of only 133 mV to deliver an HER current density of 10 mA cm⁻² and 350 mV to generate an OER current density of 100 mA cm⁻² in 1.0 mol L⁻¹ KOH. Based on the density-functional theory calculation, surface-adsorbed E-Se is beneficial to optimizing the electron environment and the adsorption/desorption free energy of hydrogen/water of the hybrid catalyst, consequently facilitating the electrocatalytic water splitting process. There is a proper I-Se/E-Se ratio to improve the catalytic activity and kinetics of the reaction and the optimized E-Se adsorption amount can balance the interactions between I-Se and E-Se, so that the catalyst can achieve appropriate Se-H binding and active site exposure for the excellent electrocatalytic activity. To demonstrate the practicality, the assembled symmetrical device can be powered by an AA battery to produce hydrogen and oxygen synchronously. Our results provide a deeper understanding of the catalytic mechanism of transition metal selenides in water splitting and insights into the design of high-efficiency and low-cost electrocatalysts in energy-related applications.