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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 with HCOO– (k·OHformate = 3.2 × 109 M−1 s−1). These findings strongly evinced that MOx(∙OH) would be the exclusive intermediate for the RCS generation (Eq. (5)), while MOx+1 driven ClER (Eq. (6)) would play a minor role for our heterojunction anodes. In spite of previous reports [28] on HCOOH degradation by IrO2/IrO3 couple in acid (pH 0), oxygen atom transfer from MOx+1 to HCOO– could be excluded in the current experimental conditions. Ozone, as a presumed analog of the higher oxide, is known to undergo sluggish kinetics both with Cl− (<3 × 10−3 M−1 s−1) and HCOO– (1 × 102 M−1 s−1) [29], [30]. Provided that free and bound ∙OH react with HCOO– at analogous bimolecular rate constants, the steady- state concentration of surface bound ∙OH ([>MOx(∙OH)]SS) could be estimated from kobsformate using eq. (11). [>MOx(∙OH)]SS = kobsformate (s−1) V / (A k·OHformate) (11) As shown in Figure S14a, the [>MOx(∙OH)]SS estimates were in similar order with our previous study [11]. Eq. (12) given by a pseudo-steady-state approximation further estimated the rate constant for Eq. (2) (k2), assuming that >MOx(∙OH) and >MOx+1 are exclusively responsible to the formate ion oxidation and OER, respectively. k2 = CEOER I × 1 (12) 2F A[>MOx( OH)]SS where CEOER is the CE of OER calculated from the charge balance (1 – CE of formate ion degradation). Figure S14b substantiated that the outer heterojunction layers significantly lowered k2 values of IrTaOy. Under further assumption that transition from >MOx(∙OH) to >MOx+1 (Eq. (2)) is limiting the OER rate, CERCS would be determined by the ratio of k2 to kobschloride. Despite the apparent inverse relation between k2 and CERCS noted in Fig. 6b, relatively weak correlation suggested the liberation of O2 from >MOx+1 (Eq. (4)) could also determine the OER rate depending on the outer layer composition. For example, a strong binding of O on Bi might allow the ClER to overwhelm OER on the Bi3Ti7Ox-2 anode with relatively high k2 value. 4. Conclusions This study prepared Ir7Ta3Oy DSAs with outer heterojunction layers based on mixed Bi and Ti oxide to compare the RCS generation efficiency during competitive OER and ClER in dilute (50 mM) NaCl solutions. Crack free, anatase dominant TiO2 layers prepared from ball-milled P60 nanoparticles brought about significantly enhanced SRRCS, CERCS, and EERCS, while TiO2-L even elevated the anodic current wave possibly owing to a tuned overall M-O bond strength. However, decoration of Bi2O3 particles on the outer TiO2 films ((Bi2O3)x(TiO2)1−x) gave limited or adverse effects on voltammetric response, RCS generation, and stability presumably because of multiple junction formation and specially separated Bi2O3 phase. To this end, Bi3Ti7Ox heterojunction layers with elevated mixing level of Bi marked CERCS values near unity, by an effective increase in the point of zero charge. Consequently, the highest SRRCS and EERCS were noted by Ir7Ta3Oy/TiO2-L and Ir7Ta3Oy/Bi3Ti7Ox, respectively. Parallel formate ion degradation experiments revealed that the RCS generation would be exclusively mediated by >MO (·OH) 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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