Electrocatalytic Hydrogen Production and Synergistic Formaldehyde Degradation

This study reports a simple molten salt method for the large-scale synthesis of NiS/NiS2 heterostructures. The NiS/NiS2 sample obtained at 350 °C exhibits a high hydrogen evolution reaction (HER) activity. At 10 mA cm–2, low HER overpotentials are required for NiS/NiS2 in both acidic (196 mV) and alkaline electrolytes (161 mV). Furthermore, density functional theory (DFT) calculations demonstrate that NiS/NiS2, with an optimized electron structure, presents a significantly lower Gibbs free energy for hydrogen adsorption (0.201 eV), compared with single NiS (0.413 eV) and single NiS2 (0.953 eV). This suggests that the NiS/NiS2 heterostructure enables more favorable thermodynamics for hydrogen adsorption/desorption. In addition, an electrolyzer was assembled using NiS/NiS2 as the electrodes and formaldehyde as an electrolyte solution. It was observed that in a neutral electrolyte, the required cell voltage of a formaldehyde-based electrolyzer (2.52 V) at 10 mA cm–2 was 0.33 V lower than that of a conventional water-based one (2.85 V). The cell voltage has decreased by 11.6%, demonstrating that the electrolyzer is highly energy-efficient. Also, the formaldehyde-based electrolyzer has a higher Faradaic efficiency (FE) of 80.17%, compared to the water-based electrolyzer (73.58%). Moreover, 90.4% of formaldehyde is degraded after running for 2 h at a cell voltage of 3 V. This study confirms that this innovative electrolyzer can not only efficiently degrade formaldehyde but also simultaneously produce hydrogen with a lower energy consumption. This work presents an innovative strategy for both wastewater treating and green hydrogen production.