Cr-Doped FeC₂O₄ Microrods Formed Directly on AISI 420 Stainless Steel to Enhance Electrochemical NO_3– Reduction to N₂ at Circumneutral pH

We synthesized low-cost cathodes for use in the electrochemical NO 3 – reduction reaction (NO 3 RR) via the simple reconstruction of AISI 420 stainless steel (SS). Thermochemical treatment of the SS in oxalic acid generated iron oxalate (FeC 2 O 4 ) microrods (BL-SS), with further anodization affording Cr-doped Fe 2 O 3 (R-SS) or FeC 2 O 4 (G-SS). G-SS displayed supreme N 2 selectivity during galvanostatic electrolysis at circumneutral pH. Electroanalysis and descriptor/scavenger analysis indicated that Fe sites were the primary active sites of NO 3 – adsorption, with C 2 O 4 2– as the H-binding sites. The C 2 O 4 2– ligands and Cr dopants altered the electronic structures of the Fe sites. A parametric study of the current density, pH, [NO 3 – ] 0, and [Cl – ] 0 indicated an Eley–Rideal N 2 generation mechanism, with NO 2 – as an intermediate. Cl – elevated the N 2 selectivity but reduced the NO 3 RR efficiency. To demonstrate the practical applicability of G-SS with a proposed regeneration strategy, its durability was examined in synthetic and real wastewater matrices. Compared with that in synthetic wastewater, G-SS displayed more stable performance in real wastewater owing to the natural buffering capacity at the cathode, which reduced the corrosion rate. Cr-doped FeC 2 O 4 is viable for use in the low-cost, efficient electrochemical treatment of wastewater containing NO 3 – .