Cation-regulated nitrate electroreduction to ammonia on Fe-deposited carbon rod electrodes with correction of spontaneous contribution.

Electrochemical nitrate reduction to ammonia (NO3RR) offers a promising route for nitrogen-resource recovery under mild conditions. However, for metal-based nitrate electrolysis systems, the detected ammonia may not originate solely from potential-driven electrocatalysis because the metal electrode can also react with nitrate under open-circuit conditions, leading to overestimation of the apparent ammonia yield and faradaic efficiency (FE). Herein, an Fe-deposited carbon rod electrode (Fe/CR) was used as a model system to examine electrolyte/cation effects and quantify the spontaneous contribution. TEM/EDS, XRD, Fe 2p XPS, ICP-OES and EIS support Fe deposition and improved interfacial charge transfer. At an identical nitrate concentration (0.10 M NO3 -), Fe/CR delivered apparent NH3 yields of 600 ± 3 and 282 ± 2 mg h-1 m-2 with apparent FEs of 96.1 ± 0.3% and 83.2 ± 0.2% in 0.05 M Mg(NO3)2 and 0.10 M NaNO3, respectively. Corresponding open-circuit controls produced 90.0 ± 1.5 and 42.0 ± 1.5 mg h-1 m-2 NH3, accounting for 15.0% and 14.9% of the apparent yields. After correction, the electrochemically attributable NH3 yields remain markedly different at 510 and 240 mg h-1 m-2, with estimated corrected FEs of 81.7% and 70.8% for Mg(NO3)2 and NaNO3, respectively. An ionic-strength-matched Na+ control gave 315 ± 5 mg h-1 m-2, indicating that bulk ionic strength/conductivity differences alone do not explain the full Mg2+-electrolyte enhancement. 15NO3 -/1H NMR supports nitrate as the ammonia nitrogen source. After five cycles, the NH3 yield retention is 98.37%, while post-reaction characterization indicates moderate surface-valence evolution, while ICP-OES of the cycle-5 electrolyte corresponds to trace Fe loss (≤0.23% of the initial loading for that cycle). These results highlight the need to combine electrolyte controls, source verification and cation-specific open-circuit measurements when evaluating metal-based NO3RR.