The Duality of Ion Effects: How Cation Precipitation Sites and Anion Interactions Shape Electrochemical Nitrate Reduction

Electrochemical nitrate reduction to ammonia (ENRA) offers a sustainable alternative to the energy-intensive Haber-Bosch process, while efficiently removing nitrate (NO₃^-) from impaired water. However, the influence of coexisting substances on ENRA performance remains underexplored. This study investigates the effects of various ionic species and dissolved organic matter on the performance of a Cu@Ni electrode, known for its high selectivity toward ammonia. In the presence of HCO₃^- and Ca²⁺, the NO₃^- removal efficiency remained stable at 83.2% over six cycles. This stability is attributed to the preferential formation of calcium carbonate scales at the electrode edges, driven by electrostatic repulsion between the negatively charged cathode and (bi)carbonate ions, thereby preserving active sites. Conversely, Mg²⁺ ions precipitated directly on the active sites due to favorable precipitation conditions induced by the elevated local pH, which impaired the catalytic performance. In contrast, Ca²⁺ exhibited a positive effect by forming ion pairs and condensing electric double layers. Among anions, Cl^- significantly enhanced NO₃^- removal via electrochlorination, whereas S^2- caused electrode poisoning, severely reducing the efficiency. In real NO₃^--polluted groundwater, the Cu@Ni electrode demonstrated a negligible ENRA performance. However, acid or base pretreatment significantly improved ENRA efficiency from almost 0% to over 50% of NO₃^- removal. These findings underscore the critical role of ionic composition in ENRA optimization and highlight pretreatment as a viable strategy for enhancing performance in complex water matrices.