Cr-Doped FeC₂O₄ Microrods Formed Directly on AISI 420 Stainless Steel to Enhance Electrochemical NO_3- Reduction to N₂...

We synthesized low-cost cathodes for use in the electrochemical NO₃^- reduction reaction (NO₃RR) via the simple reconstruction of AISI 420 stainless steel (SS). Thermochemical treatment of the SS in oxalic acid generated iron oxalate (FeC₂O₄) microrods (BL-SS), with further anodization affording Cr-doped Fe₂O₃ (R-SS) or FeC₂O₄ (G-SS). G-SS displayed supreme N₂ selectivity during galvanostatic electrolysis at circumneutral pH. Electroanalysis and descriptor/scavenger analysis indicated that Fe sites were the primary active sites of NO₃^- adsorption, with C₂O₄^2- as the H-binding sites. The C₂O₄^2- ligands and Cr dopants altered the electronic structures of the Fe sites. A parametric study of the current density, pH, [NO₃^-]₀, and [Cl^-]₀ indicated an Eley-Rideal N₂ generation mechanism, with NO₂^- as an intermediate. Cl^- elevated the N₂ selectivity but reduced the NO₃RR 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 FeC₂O₄ is viable for use in the low-cost, efficient electrochemical treatment of wastewater containing NO₃^-.