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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 that even with the analogous composition, mixing level of constituents could significantly affect the electronic and electrostatic properties. In particular, relatively homogenous intercalation of Bi component might avoid the ohmic resistance across multiple junctions, while increase the point of zero charge for enhanced electro-sorption of Cl−. Furthermore, accelerated life test at 1000 mA cm−2 indicated insignificant dissolution of Bi and Ti during operation for 30 h (data not shown). In order to evaluate the effects of ion migration upon the potential bias (increasing [Cl−] in the anode vicinity), the RCS generation experiments were performed in 50 mM NaCl solutions with or without another electrolyte (50 mM NaClO4) under a galvanostatic condition (Figure S11). The measured values of SRRCS, CERCS, and EERCS in the mixed solutions were 82, 86, and 97% of those in NaCl solutions, respectively. Moderate reductions could be associated with the competing ion migration and lower potential bias in the presence of other electrolyte, which would be a consideration to understand the performance in real (waste)water matrix. On the other hand, one may argue that the elevated CERCS by the heterojunction architecture would be ascribed to the reduced current density, since an increasing current could invigorate OER only when ClER is rate-limited by diffusion of Cl−. However, the operational current density in this study would be lower than the limiting current density for ClER, as corroborated by Figure S12. The observed CERCS values showed negligible correlation with the current density. Therefore, the shift in ClER selectivity should be understood by speciation of surface intermediates with variable reactivity with Cl− (vide infra). 3.5. Formate ion degradation versus RCS generation to assess intermediate speciation In this study, the hydroxyl radical (∙OH) on anode surface could be a nonselective oxidant to be utilized for Cl− oxidation to RCS. Due to the short lifetime (10−6 − 10−3 s), free or bound ∙OH has been quantified using an array of probe compounds such as salicylic acid, benzoic acid, coumarin, benzoquinone, and RNO (p-nitrosodimethylaniline) [26]. Formate ion could be another surface ∙OH prove due to superb reactivity with ∙OH [27], simple quantification method using IC, and relatively low molecular weight to avoid diffusion limitation. In order to further interrogate the speciation between MOx(∙OH) and MOx+1 and their roles on RCS generation, potentiostatic (Ea = 2.0 or 2.5 V NHE) formate ion degradation experiments (initial concentration = 50 mM) were performed using the series of (Bi2O3)x(TiO2)1−x and Bi3Ti7Ox heterojunction anodes (Figure S13). We limited the applied anodic potential up to 2.5 V NHE due to slightly lower electrical conductivity of 50 mM NaCOOH solutions (4.3 mS cm−1) than 50 mM NaCl solutions (5.4 mS cm−1). The differences in ohmic drop between two solutions were estimated to be 0.16 V at maximum. The reaction between HCOO– and > MOx(·OH) could be approximated to pseudo-first-order owing to the facile kinetics (Eq. (10)). >MOx(∙OH) + HCOO– → >MOx + COO–∙ + H2O (10) Fig. 6a illustrates the observed pseudo-first-order rate constants of formate decay (kobsformate) in relation with the pseudo-first order rate constants of RCS generation (kobsClER) that were estimated from the SRRCS (Figs. 3c and 5c) and initial chloride concentration (kobsClER = SRRCS A /V [Cl−]0). The kobsClER was observed to be linearly correlated with kobsformate and, more importantly, the slope of the regressed line was close to the ratio of bimolecular rate constant of ∙OH with Cl− (k·OHchloride = 4.3 × 109 M−1 s−1) to that 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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