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Enhanced chlorine evolution from dimensionally stable anode by heterojunction with Ti and Bi based mixed metal oxide layers prepared from nanoparticle slurry (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. 1. Introduction Chlorine (Cl2) is a widely utilized industrial reagent for polymer synthesis globally. In chemical industry, chlor-alkali process would be one of the most commercialized methods to electrolyze concentrated NaCl solutions for generation of Cl2, H2, and caustic soda (NaOH). Dimensionally stable anodes (DSAs), based on precious metal oxides such as RuO2 and IrO2, have been most often used for these purposes owing to the supreme activity and stability [1], [2], [3]. Variable secondary components have been explored primarily for stability enhancement, to come up with the mixed metal oxide anodes, Ir7Ta3Ox and Ru3Ti7Ox as the optimal DSAs compositions for many decades [1], [3], [4]. Aqueous Cl2 (commonly noted as free chlorine) is also a famous oxidant for drinking water disinfection and, recently, electrolytic reactive chlorine species (RCS) has been utilized for on-site treatment of saline wastewater as well [5]. Owing to the value consuming nature of environmental technologies, however, the scarcity of Ir and Ru components have substantially constrained a large scale application of the electrochemical chlorination for water treatment. In addition, the DSAs active for chlorine evolution reaction (ClER, E°(Cl2/Cl−) = 1.36 V NHE) also effectively catalyze oxygen evolution reaction (OER, E° (O2/H2O) = 1.23 V NHE) [6], a parasitic side reaction with respect to the water treatment. A sufficient anodic potential bias on hydrous metal oxide (>MOx) would initiate the formation of surface hydroxyl radical (>MOx(∙OH), Eq. (1)) whose further oxidation could lead to higher oxide (>MOx+1, via Eq. (2)) formation. It has been widely accepted that the overall adsorption energy of oxygen to metal (M−O bond strength) is known to be the principal factor of OER activity mostly via Eq. (4) [7], [8], [9], [10]. > MOx + H2O → > MOx(∙OH) + H+ + e− (1) > MOx(∙OH) → > MOx+1 + H+ + e− (2) > MOx(∙OH) → > MOx + 1/2 O2 + H+ + e− (3) > MOx+1 → > MOx + 1/2 O2 (4) In a presence of chloride ions, parallel ClER mechanism on metal oxide electrocatalysts can be described by Eqs. (5), (6), where the ClER could share active sites with the competing OER [6], [8]; i.e., MOx(∙OH) and MOx+1 mediate the ClER as well. > MOx(∙OH) + Cl− → > MOx + 1/2 Cl2 + OH − (5) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7539370/?report=printable[10/12/2020 8:49:16 AM]PDF Image | Enhanced chlorine evolution
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