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 Ni 0.33 Fe 0.67 O y /TiO 2 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 Ir 7 Ta 3 O y /TiO 2 at 30 mA cm –2 in 50 mM NaCl solutions with near-neutral pH. Correlations of ClER CE of variable Ni x Fe 1– x O y /TiO 2 ( 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 TiO 2 to be the active ClER center. The underlying Ni 0.33 Fe 0.67 O y, characterized as biphasic NiFe 2 O 4 and NiO, effectively lowered the O binding energy of TiO 2 by electronic interaction across the junction. The OER activity of Ni 0.33 Fe 0.67 O y superior to the other Fe-doped Ni oxides suggested that the conductive OER intermediates generated on Ni 0.33 Fe 0.67 O y could also facilitate the ClER as an ohmic contact. Stability tests and NH 4 + degradation in synthetic and real wastewater confirmed the feasibility of Ni 0.33 Fe 0.67 O y /TiO 2 heterojunction anode for mediated water treatments in mild pH.