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.