Triple microenvironment synergy engineering by grain boundaries of hydrophilic Cu electrode for boosting low-concentration nitrate electroreduction

Electrochemical reduction of NO 3 – (ERN) is a promising route for sustainable water treatment and nitrogen recovery. However, its practical implementation is hindered by the low Faradaic efficiency (FE) and NH 4 + yield rate of conventional electrodes under low NO 3 – concentrations (≤50 mg-N L -1 ). These limitations primarily arise from insufficient reactant accessibility and the undesired hydrogen evolution reaction (HER). Here, we developed a copper electrode modified with polyvinylpyrrolidone (PVP) and grain boundary (GB) defects as an ERN catalyst (denoted as PVP-Cu). The PVP-Cu electrode achieved a FE of 81.82 % and an NH 4 + yield rate of 0.44 mg-N h −1 cm −2 , approximately 3-fold higher than its counterparts in a 30 mg-N L -1 NO 3 – solution. Characterization results reveal that both PVP modification and GB defects synergistically enhance the triple microenvironment. Specifically, (i) the hydrophilic PVP layer enhances mass transfer, concentrating NO 3 – near active sites, (ii) GB defects facilitate water dissociation, and (iii) these features produce active hydrogen species while suppressing their recombination into undesired H 2 . This integrated “enrichment-activation-suppression” mechanism significantly improves the ERN performance. Additionally, the PVP-Cu electrode exhibited robust resistance to environmental interferences, a widely pH range, minimal Cu leaching, and excellent operational stability. This work establishes a general microenvironment engineering strategy to enhance ERN performance, advancing technologies suitable for decentralized water purification and nutrient recovery.