Abstract
Tailoring the coordination geometry of metal centers through polymorphism offers a powerful approach to isolating the structural origin of catalytic activity in metal-organic frameworks (MOFs). Herein, two copper-pyrazolate (Pz) polymorphs, α-Cu(Pz)(2) and β-Cu(Pz)(2) were synthesized, featuring identical chemical compositions and 1-periodic chain structures but distinct local coordination configurations. Remarkably, the β-Cu(Pz)(2) exhibits a faradaic efficiency (FE) of 93.33% for the nitrate reduction reaction (NO(3)RR), significantly outperforming α-Cu(Pz)(2) (53.10%). Comprehensive structural analyses, in situ spectroscopy, and density functional theory (DFT) calculations revealed that the coordination geometry governs the electronic structure of the Cu active centers. Specifically, the cis-configured β-Cu(Pz)(2) enables more delocalized Cu 3d orbitals and stronger Cu-O (NO(3) (-)) electronic coupling, thereby promoting nitrate adsorption and activation. This work demonstrates that MOF polymorphism allows precise tuning of electronic structures, offering a fundamental design principle for the development of advanced electrocatalysts toward sustainable nitrogen-cycle chemistry.