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.

Enhanced chlorine evolution from dimensionally stable anode by heterojunction with Ti and Bi based mixed metal oxide layers prepared from nanoparticle slurry.

This study reports enhanced current (CERCS) and energy efficiency (EERCS) of reactive chlorine species (RCS) generation on Ir7Ta3Oy anode by Ti/Bi mixed metal oxide heterojunction layers despite reductions in pseudo-capacitance and film conductivity. In potentiostatic electrolysis of 50 mM NaCl solutions, dramatic improvement (0.61 mmol cm-2 hr-1 at 2.5 V NHE) was noted by simple coating of thin (~2 µm) TiO2 layer from ball-milled TiO2 nanoparticle (80-100 nm) suspension, even with moderate elevation in voltammetric wave. Decoration of Bi2O3 particles (1 - 2 µm) showed limited or adverse effects for RCS generation and stability. However, Bi-doped TiO2 layers prepared from polyol-mediated or co-precipitation methods marked the highest CERCS (~100%) and EERCS (8.16 mmol Wh-1 at 2.5 V NHE) by increased mixing level and effective shift in surface charge. Surface ·OH exclusively mediated the RCS generation whose further transformation to higher oxide could be restrained by the heterojunction layer.