Rational design of hexanuclear zirconium-cluster-based porphyrinic metal-organic frameworks for high-efficiency electrocatalytic nitrate reduction to ammonia

Green ammonia (NH₃) synthesis is pivotal for sustainable agriculture and hydrogen energy carriers, yet challenges persist in direct electrocatalytic nitrate reduction (e-NO₃RR) under neutral conditions, including competing hydrogen evolution and sluggish reaction kinetics. Herein, we engineer a series of transition metal-porphyrin MOFs (NU-902 (M), M = Fe, Co, Ni, Cu) via coordination between metalated tetrakis(4-carboxyphenyl)porphyrin (M-TCPP) and Zr₆O₄(OH)₄(CO₂)₁₂ clusters, constructing a hierarchical mesoporous architecture. Crucially, NU-902 (Cu) achieves a record Faradaic efficiency (FE) of 71.65% with an NH₃ yield rate of 13.76 mg h^-1 mg^-1_cat.at −0.9 V vs. RHE, outperforming similar frameworks based on Fe (14.01 mg h^-1 mg^-1_cat.63.83%), Co (9.51 mg h^-1 mg^-1_cat.71.51%), and Ni (2.40 mg h^-1 mg^-1_cat.67.99%). Moreover, the FE of NU-902 (Cu) remains above 65% after 500 cycles, and PXRD characterization confirms no significant collapse of the framework structure, which indicates its excellent stability. This work establishes porphyrinic MOFs as efficient platforms for sustainable nitrogen valorization and provides experimental guidelines for metal-specific activity modulation in electrocatalysis.