Synergistic Effect of MoO<sub>2</sub>–Ni<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> Heterostructures In Situ Grown\non Nickel Foam Enhances the Efficiency of Hydrogen Evolution Reaction\nin Simulated Seawa

Hydrogen\n(H<sub>2</sub>) with a high gravimetric energy density\n(142 MJ/kg) and zero carbon emissions is a green energy source. The\nelectrocatalytic hydrogen evolution reaction (HER) is a prominent\nstrategy for hydrogen production, and the essential technology of\nelectrocatalysts focuses on creating catalysts that are highly efficient,\ncost-effective, and excellently stable. Herein, a heterostructure\nelectrode/catalyst consisting of MoO<sub>2</sub>–Ni<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>/NF (where NF = nickel foam) was fabricated\nusing (NH<sub>4</sub>)<sub>6</sub>[NiMo<sub>9</sub>O<sub>32</sub>]·6H<sub>2</sub>O as precursor via a two-step method utilizing hydrothermal\nsynthesis and chemical vapor deposition (CVD). Thanks to the remarkable\nsynergistic effect occurring at the interfaces of the heterostructure,\nthe catalytic efficiency of MoO<sub>2</sub>–Ni<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>/NF can outperform that of other catalyst materials.\nIn particular, the MoO<sub>2</sub>–Ni<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>/NF electrode exhibits overpotentials of 66 and 258\nmV at 10 mA cm<sup>–2</sup>, along with low Tafel slopes of\n56.03 and 85.32 mV/dec in 1 mol/L KOH and simulated seawater electrolyte,\nrespectively. Density functional theory calculations (DFT) validate\nthat the Gibbs free energy (Δ<i>G</i><sub>H*</sub>) values for hydrogen adsorption of MoO<sub>2</sub> (110)/Ni<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> (−222) with 0.033 eV are\nmuch closer to zero, similar to Pt/C. In situ FTIR spectra indicate\nthat the synergistic effect of MoO<sub>2</sub> (110)/Ni<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> (−222) can further create more\ncatalytic active sites and modulate intermediate H* adsorption to\npromote the HER process. Overall, this study highlights the potential\nof nanostructured MoO<sub>2</sub>–Ni<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> heterostructures for application in efficient hydrogen\nproduction under seawater conditions.