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; however, challenges persist in direct electrocatalytic nitrate reduction (e-NO₃RR) under neutral conditions, including competing hydrogen evolution and sluggish reaction kinetics. Herein, a series of transition-metal–porphyrin metal-organic frameworks (MOFs), NU-902(M) (M = Fe, Co, Ni, Cu), are engineered via coordination between metalated tetrakis(4-carboxyphenyl)porphyrin (M-TCPP) and Zr₆O₄(OH)₄(CO₂)₁₂ clusters, constructing a hierarchical mesoporous architecture. Notably, NU-902(Cu) achieves a Faradaic efficiency (FE) of 71.65% with an NH₃ yield rate of 13.76 mg·h⁻¹·mg_cat.⁻¹ at −0.9 V vs. reversible hydrogen electrode (RHE), outperforming analogous frameworks based on Fe (14.01 mg·h⁻¹·mg_cat.⁻¹, 63.83%), Co (9.51 mg·h⁻¹·mg_cat.⁻¹, 71.51%), and Ni (2.40 mg·h⁻¹·mg_cat.⁻¹, 67.99%). Moreover, the FE of NU-902(Cu) remains above 65% after 500 cycles, and powder X-ray diffraction characterization confirms no significant collapse of the framework structure, indicating excellent structural stability. This work establishes porphyrinic MOFs as efficient platforms for sustainable nitrogen valorization and provides experimental guidance for modulating metal-specific activity in electrocatalysis.