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

Abstract

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 Ni0.33Fe0.67O y /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 y /TiO2 at 30 mA cm–2 in 50 mM NaCl solutions with near-neutral pH. Correlations of ClER CE of variable Ni x Fe1–x O y /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 Ni0.33Fe0.67O y , characterized as biphasic NiFe2O4 and NiO, effectively lowered the O binding energy of TiO2 by electronic interaction across the junction. The OER activity of Ni0.33Fe0.67O y superior to the other Fe-doped Ni oxides suggested that the conductive OER intermediates generated on Ni0.33Fe0.67O y could also facilitate the ClER as an ohmic contact. Stability tests and NH4 + degradation in synthetic and real wastewater confirmed the feasibility of Ni0.33Fe0.67O y /TiO2 heterojunction anode for mediated water treatments in mild pH.

Controlling the Selectivity of Chlorine Evolution Reaction by IrTaOx/TiO2 Heterojunction Anodes: Mechanism and Real Wastewater Treatment

“…5,36 Likewise, a prior work on NiFeO x / TiO 2 demonstrated that an elevated E FB corresponds to a greater distance of the p-band center from the Fermi level and weaker M−O binding. 13 In this study, CE RCS correlated moderately with E ZC (Figure 4a) and more strongly with E FB (Figure 4b). Overall, weaker M−O bonds (as described by the escalated E ZC and E FB ) with the increase in the outer TiO 2 loading could rationalize more facile reactivity of surface •OH with Cl − for ClER.…”

“…20,21 The relative atomic proportions of Ir and Ta estimated by EDS decreased with an increase in TiO 2 loading (Table S3). Although interstitial substitution could occur for Ir and Ta upon repetitive TiO 2 loading and annealing, 13 the major cause of this reduction in the atomic percentage of Ir and Ta is likely to be the limited depths of X-ray penetration in EDS. 22 XRD patterns (Figure 1d) represent crystalline lattice structures of the heterojunction anodes (T1, T3, and T5), referenced with the reported XRD patterns for IrO 2 , 23 TiO 2 anatase, 24 and Ti metal.…”

“…5 In contrast, apparent modulation of the surface Ti valency was previously noted for NiFeO x /TiO 2 due to stronger electronic interactions. 13 The O 1s spectra (Figure 1f) were deconvoluted into three peaks (binding energies of 529.9, 530.9, and 531.9 eV) indicative of lattice oxygens bound to saturated Ti atoms (O L ), surface hydroxyl group (O H ), and oxygen vacancy (O v ), respectively (Figure S10 and Table S4). 16 Coordinately unsaturated sites (CUS) including an O vacancy (26.1% O v ) and OH moieties (31.1% O H ) were observed prevalently on IrTaO x , presumably due to defects caused by the Ta component mixed in.…”

“…Imperative functions of the IrTaO x layer as an electron sink were corroborated by the impaired performance of the Ti/ TiO 2 anode. The CE RCS and SR RCS in 50 mM NaCl (Figure S22) were less than 20% and 0.2 mmol cm −2 h −1 , respectively, most likely due to oxidation of the Ti substrate underneath at drastically elevated cell voltages of >15 V. 13 Figure S23 represents the effects of IrTaO x loadings (3−9 coating sequences) at constant potential regimes (2.5 and 3.0 V NHE). Comparing the CE RCS of T6 (6 coats for each layer) at 2.5 V NHE with Figure 3 ruled out the influence of the bias mode (constant potential versus constant current) on CE RCS at comparable j values near 30 mA cm −2 .…”

Electrochemical Chlorine Evolution by Bi–Ti Oxide with a Heterojunction on Ir–Ta Oxide: Insights from the Effects of Layer Configurations

“…In spite of the outstanding ClER performance of the optimized anodes in 50 mM NaCl (in accordance with the prior reports), , various ionic matrix in wastewater can potentially interfere with the ClER in real application scenarios. In particular, PO 4 3– (ranging up to ∼20 mM in wastewater) , would make a strong complexation with the Ti-enriched outer layer to hinder chloride adsorption as reported for metal oxide electrocatalysts. − Therefore, further study is necessary to tackle the practical applicability of the heterojunction anodes for mediated treatment of a wide array of wastewater. Lastly, an optimization of the loading of Ir 7 Ta 3 O z (as ohmic contact) is warranted to deal with the recent price fluctuation of precious Ir.…”