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_0.33Fe_0.67O/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 Ir₇Ta₃O/TiO₂ at 30 mA cm^-2 in 50 mM NaCl solutions with near-neutral pH. Correlations of ClER CE of variable NiFe_1-O/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 Ni_0.33Fe_0.67O, characterized as biphasic NiFe₂O₄ and NiO, effectively lowered the O binding energy of TiO₂ by electronic interaction across the junction. The OER activity of Ni_0.33Fe_0.67O superior to the other Fe-doped Ni oxides suggested that the conductive OER intermediates generated on Ni_0.33Fe_0.67O could also facilitate the ClER as an ohmic contact. Stability tests and NH₄⁺ degradation in synthetic and real wastewater confirmed the feasibility of Ni_0.33Fe_0.67O/TiO₂ heterojunction anode for mediated water treatments in mild pH.