Cr-Doped FeC2O4 Microrods Formed Directly on AISI 420 Stainless Steel to Enhance Electrochemical NO3- Reduction to N2 at Circumneutral pH.

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 C2O42- as the H-binding sites. The C2O42- 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-.

Ni-Fe Oxides/TiO2 Heterojunction Anodes for Reactive Chlorine Generation and Mediated Water Treatment.

Reactive chlorine-mediated electrochemical water treatment necessitates selective chlorine evolution reaction (ClER) versus parallel oxygen evolution reaction (OER) in mild pH (7-10), with minimal deployments of precious electrocatalysts. This study reports Ni0.33Fe0.67Oy/TiO2 heterojunction anode prepared by a straightforward sol-gel coating with thermal decomposition at 425 °C. The ClER current efficiency (CE, 70%) and energy efficiency (2.3 mmol W h-1) were comparable to benchmarking Ir7Ta3Oy/TiO2 at 30 mA cm-2 in 50 mM NaCl solutions with near-neutral pH. Correlations of ClER CE of variable NixFe1-xOy/TiO2 (x: 0.33, 0.8-1) with the flat-band potential and p-band center, as experimental descriptors for surface charge density, nominated the outer TiO2 to be the active ClER center. The underlying Ni0.33Fe0.67Oy, characterized as biphasic NiFe2O4 and NiO, effectively lowered the O binding energy of TiO2 by electronic interaction across the junction. The OER activity of Ni0.33Fe0.67Oy superior to the other Fe-doped Ni oxides suggested that the conductive OER intermediates generated on Ni0.33Fe0.67Oy could also facilitate the ClER as an ohmic contact. Stability tests and NH4+ degradation in synthetic and real wastewater confirmed the feasibility of Ni0.33Fe0.67Oy/TiO2 heterojunction anode for mediated water treatments in mild pH.