Selective Mass Accumulation at the Metal–Polymer Bridging Interface for Efficient Nitrate Electroreduction to Ammonia and Zn-Nitrate Batteries

The electrochemical conversion of nitrate (NO₃^-), a common nitrogen source in industrial wastewater and contaminated groundwater, into ammonia (NH₃), signifies an approach to wastewater treatment and NH₃ production. Nevertheless, its selectivity and activity at low NO₃^- concentrations and industrial current densities are constrained by limited mass transfer around the electrode. Here, we report a metal-polymer bridging interface constructed by anchoring Cu/Cu₂O nanoparticles onto a two-dimensional (2D) Cu-based benzene dicarboxylate (CuBDC) coordination polymer via in situ electroreduction (denoted as E-CuBDC). This interface weakens the electrostatic repulsion and regulates the distribution/migration of NO₃^- and H₂O, creating a Janus NO₃^--rich and H₂O-poor domain near the catalyst surface. Operando characterizations and theoretical simulations indicate that the metal-polymer bridging interface selectively accumulates NO₃^- and reduces the energy barrier toward the reduction of *NH₂OH to *NH₂, overcoming the mass transfer limitations at a low NO₃^- concentration. E-CuBDC exhibits a high Faradaic efficiency (FE) of over 90% across wide NO₃^- concentrations (7.1-100 mM NO₃^-) and high applied voltages. Additionally, it achieved stable NH₃ production over 100 h at ampere-level current densities. When applied in a Zn-NO₃^- system, this newly developed E-CuBDC catalyst demonstrates an outstanding power density and FE for NH₃ production, showcasing its great potential for large-scale electrochemical conversion and storage systems. This study presents a generalizable strategy for constructing metal-polymer interfaces to regulate interfacial mass transport.