Polymorphism-Driven Coordination Geometry Engineering for Boosting Nitrate Electroreduction in Cu–Pyrazolate Chains

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, β-Cu(Pz)2 and α-Cu(Pz)2 were synthesized, featuring identical chemical compositions and 1-periodic chain structures but distinct local coordination configurations. Remarkably, the α-Cu(Pz)2 exhibits a Faradaic efficiency (FE) of 93.33% for the nitrate reduction reaction (NO3RR), significantly outperforming β-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 α-Cu(Pz)2 enables more delocalized Cu 3d orbitals and stronger Cu-O(NO3-) 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.