Three-Dimensional Hourglass-Type Polyoxometalate-Based Metal–Organic Frameworks for Electrocatalytic Nitrate Reduction to Ammonia

Electrocatalytic nitrate reduction reaction (e-NO3RR) provides a promising route for ambient NH3 production and nitrate valorization, but synchronizing multielectron transfer with stepwise hydrogenation remains challenging. Herein, two isostructural three-dimensional polyoxometalate-based metal–organic frameworks, [M(H2O)2HNCP]2(H4M2P4Mo6O33)[M0.5(H2O)3]·4H2O (M-P4Mo6-3D, M = Ni, Co), were hydrothermally synthesized from reduced hourglass {P4Mo6} clusters, Ni2+/Co2+ nodes, and HNCP linkers. Single-crystal X-ray diffraction confirms triclinic P1̅ structures with identical frameworks. The metal centers connect discrete {P4Mo6} units into hourglass M{P4Mo6}2 secondary building units that extend into three-dimensional coordination networks. Topological analysis using ToposPro identifies the framework as a uninodal six-connected pcu net with a point symbol of {412,63}. Ni–P4Mo6-3D exhibits superior e-NO3RR performance, achieving 98.6% NH3 Faradaic efficiency and an NH3 yield rate of 14.63 mg h–1 mgcat.–1 at −0.9 VRHE in neutral electrolyte. Electrochemical measurements indicate enhanced intrinsic activity and charge-transfer kinetics. Potential-dependent in situ FTIR and Raman spectroscopy reveal oxygenated and hydrogenated nitrogen intermediates, supporting a stepwise NO3– → *NO3 → *NO2 → *NO → *NOH → *NHOH → *NH2OH → *NH2 → NH3 reduction pathway.