Cation–Carbonate Ion Pairing Suppresses Electrochemical Nitrate Reduction

Nitrate reduction (NO3RR) has emerged as a promising strategy for sustainable remediation of nitrate-contaminated water. However, advances in nitrate conversion systems have mainly focused on achieving high performance under idealized electrolytes, while the mechanisms governing stability in realistic water matrices remain poorly understood. Groundwater typically contains multivalent cations and buffering anions that can fundamentally alter the interfacial reaction environment, yet their coupled influence on NO3RR performance has not been systematically investigated. Here, we investigate NO3RR performance in a complex electrolyte representative of groundwater, containing Mg2+, Ca2+, and HCO3-, using a benchmark Cu catalyst and a flow-by electrochemical reactor. Through 100-h experiments combined with in situ Raman spectroscopy, ex situ surface characterization, and multiphysics modeling, we show that precipitation of Mg2+ and Ca2+ on the cathode surface alone cannot fully account for observed performance losses. Our results suggest that elevated concentrations of cation-carbonate ion pairs near the electrode-electrolyte interface likely limit nitrate transport to Cu active sites, suppressing nitrate conversion and shifting selectivity. These findings are consistent with performance losses in NO3RR systems originating from an interfacial aqueous-phase phenomenon driven by cation-carbonate ion pairing, providing new mechanistic insight for the design of electrochemical water treatment systems capable of operating in chemically complex electrolytes.