Polyoxometalate as Hydrogen–Electron Hub in Metal–Organic Complex for Electrocatalytic Nitrate Reduction to Ammonia

Electrocatalytic nitrate reduction to ammonia (ENRA) is a sustainable strategy for decentralized ammonia synthesis and wastewater purification, but its efficiency in neutral media is limited by slow water dissociation and insufficient active hydrogen (*H) supply. Herein, we address this challenge through the rational design of chain-like polyoxometalate-based metal-organic complexes that feature dinuclear ([M(H₂O)₂C₂₀O₂N₄H₁₁]₄[SiW^VI ₁₂O₄₀], M-SiW₁₂-1D) and trinuclear ([M_1.5(H₂O)₂C₂₀O₃N₄H₁₁]₄[PW^VI ₇W^V ₅O₄₀], M-PW₁₂-1D) architectures (M = Ni, Co, Fe) that create continuous electron channels. Among them, trinuclear Ni-PW₁₂-1D exhibits exceptional ENRA performance in neutral electrolytes, delivering a very high Faradaic efficiency of 95.0% and a remarkable ammonia yield rate of 16.9 mg h^-1 mg_cat. ^-1 at -1.2 V vs. RHE. In situ Fourier-transform infrared spectroscopy, differential electrochemical mass spectrometry, and density functional theory calculations are used to elucidate the synergistic mechanism operating in this system: polyoxometalate (POM) moieties promote water dissociation to generate *H that migrates to metal-organic reaction sites. Meanwhile, electron transfer from the POM to the metal cluster enhances the adsorption of the nitrate intermediate and lowers the energy barrier of the rate-determining step (NO → *NOH). This synergy balances hydrogenation and suppresses the competing hydrogen evolution reaction, providing insights into multinuclear cluster structure-activity relationships for sustainable nitrogen cycling.