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

Abstract

When coated on top of Ir-based dimensionally stable anodes, binary Bi−Ti oxides could enhance the chlorine evolution reaction (ClER) efficiency for chlor-alkali and mediated water treatment. However, the underlying mechanism and the effects of Bi necessitated further clarification. This study prepared Bi x Ti 10-x O y heterojunction layers on an existing Ir 7 Ta 3 O z anode by thermal decomposition (425 °C) of TiOSO 4 and Bi(NO 3 ) 3 under varied Ti:Bi ratios and precursor matrix (inorganic versus organic solvents). Plain material characterizations (scanning electron microscopy and X-ray diffraction) were performed together with voltammetry and potentiostatic electrolysis (in 50 mM NaCl, 50 mM HCOONa, and 0.1 M NaClO 4 ). The top Bi x Ti 10−x O y was nominated as the Cl − binding site with Ir 7 Ta 3 O z as the ohmic contact. Bi 12 TiO 20 and β-Bi 2 O 3 (formed at x > 5) on the outer layer could elevate the reactivity of surface bound hydroxyl radicals and the ClER selectivity. However, relatively loosely packed and thick overlayers (due to glycolate and isopropanol in precursor) aggravated the current and energy efficiency of ClER as well as the anodic stability.

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

“…These observations imply that TiO 2 could provide the active site for Cl – chemisorption. In particular, chemisorption of Cl – (Volmer step) would occur on the surface-bound •OH (the principal mediator of ClER on TiO 2 ) to form •HOCl, as described by the following equations. > MO x + H 2 normalO → prefix> MO x false( • OH false) + H + + e − > MO x false( • OH false) + Cl − → > MO x false( • HOCl false) + e − …”

“…These observations imply that TiO 2 could provide the active site for Cl – chemisorption. In particular, chemisorption of Cl – (Volmer step) would occur on the surface-bound •OH (the principal mediator of ClER on TiO 2 ) to form •HOCl, as described by the following equations. …”

“…In other words, the exchange current density (inversely correlated with the charge transfer resistance) decreased with the TiO 2 heterojunction, presumably due to the pore and/or ohmic resistance through the TiO 2 film added to the solution resistance. However, facilitated chemisorption of Cl – on TiO 2 (Volmer step) could increase the charge transfer coefficient, effectively elevating the intrinsic ClER activity of the heterojunction anodes …”

“…Indeed, LSVs for Ti/TiO 2 samples (either 1 or 6 coats) showed insignificant anodic waves (Figure S17), while Nyquist plots (Figures S18 and S19) indicated behaviors just as a resistor and capacitor in series. However, a possibility that the reactants (e.g., H 2 O and Cl – ) could migrate through the pores of the TiO 2 film to react on IrTaO x cannot be excluded completely . In addition, the variations in ECSA, solution resistance, and intrinsic activity were incompletely correlated with the TiO 2 layer thickness due to the uncertainty related to surface crack and pinholes.…”

“…In addition, we discovered that applying solution-casted TiO 2 layers onto the Ir-based MMO significantly enhanced the selectivity and energy efficiencies of ClER . This finding was attributed to an elevated reactivity of surface-bound hydroxyl radicals (•OH) with Cl – . The BiO x component mixed within the TiO 2 matrix could also influence the ClER selectivity but jeopardize the long-term stability of the top coat.…”