Dual-Functional Medium-Entropy Quinary Sulfides for Photocatalytic Hydrogen Evolution and Ethylene Glycol Oxidation

The use of solar energy for hydrogen (H₂) production via water splitting is rapidly emerging as a promising clean energy source. Progress in this field depends on developing high-performance, stable catalysts that efficiently drive the hydrogen evolution reaction (HER). In this study, a simple hydrothermal method is employed to synthesize a medium-entropy quinary sulfide photocatalyst (Cd_{1-<i>x</i>-<i>y</i>}Ni_<i>x</i>Mo_<i>y</i>Zn_0.45S) capable of generating H₂ directly from water without the use of additional cocatalysts. The quinary photocatalyst retains a distinct hexagonal lattice despite the partial substitution of Cd with earth-abundant elements Ni, Mo, and Zn, significantly reducing the Cd content without disrupting the crystalline phase. The optimal composition, Cd_0.39Ni_0.09Mo_0.07Zn_0.45S (CNMZS-3), exhibits a 6-fold enhancement in the H₂ evolution rate of 2437.87 μmol g^-1 h^-1 compared to pristine CdS (419.75 μmol g^-1 h^-1), aided by engineered sulfur vacancies. CNMZS-3 also demonstrates excellent stability, maintaining its structural integrity and catalytic performance for 72 h with minimal degradation. Density functional theory (DFT) analyses reveal that the Mo sites serve as the most active centers for H adsorption, while Ni improves photoabsorption, creating a synergistic effect that boosts HER activity. Replacing the conventional oxygen evolution reaction (OER) with ethylene glycol (EG) oxidation further increases H₂ production to 3746.74 μmol g^-1 h^-1 over 4 h, accompanied by the formate formation. Remarkably, CNMZS-3 also performs effectively in artificial seawater, achieving H₂ evolution rate of 1786.79 μmol g^-1 h^-1. These findings highlight medium-entropy quinary sulfides as versatile bifunctional photocatalysts for H₂ production from both freshwater and seawater, as well as for value-added chemical generation from EG.