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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 stoichiometric TiO2−x outer layer, significantly elevating the electrical conductivity. Even for much thicker TiO2 coating (greater than1 μm) in this study, Ir ions might be diffused into the TiO2 matrix during the thermal annealing, leading to a vertical concentration gradient of Ir (in mixture with Ti). Effective ionic radius of Ir4+ (62.5 pm) is more similar with Ti4+ (60.5 pm) than Bi3+ (103 pm), supporting the observed enhancement only with the TiO2–L layer. For the TiO2–H heterojunction anode, more pronounced increase in ohmic resistance led to net reduction in anodic wave, as in the case of far thicker (~30 μm) TiO2 layers on IrTaOy in our previous report [11]. 3.3. Reactive chlorine generation by IrTaOy/(Bi2O3)x(TiO2)1−x heterojunction anodes Reactive chlorine generation was comparatively evaluated in 50 mM NaCl solutions (Fig. 3) at variable applied anodic potentials (2.0, 2.5, and 3.0 V NHE). Throughout this study, the corresponding cell voltage and ohmic resistance (measured by current interruption at 200 mA) ranged 4–6 V and 3.3–5.6 Ω, respectively. The current density in potentiostatic condition (Fig. 3a) was in general agreement with the LSV shown in Fig. 2b. EE always decreased with the greater applied potential (cell voltage) due to the increment in ohmic drop, whereas CE varied insignificantly. Despite moderate variations in ohmic drop and iR-compensated anodic potential among anodes, the insignificant dependency of CE on the applied potential would allow a qualitative comparison. The CE and EE values of the control IrTaOy electrode were averaged to 52% and 4.23 mmol Wh−1 at 2.5 V NHE, respectively, while the heterojunction layers substantially augmented the CE and EE for RCS generation (in Fig. 3b and d). Accordingly, despite the general reductions in the operational current density, heterojunction anodes marked greater specific RCS generation rates (Fig. 3c) than the control. The most remarkable enhancement was noted for TiO2-L with more than 70% increase in SRRCS, due to the elevated voltammetric response. Utilizing nanoparticle slurry precursor with relatively simple preparation procedure, these results outperformed our previous report on IrTaOy/BixTi1−xOz heterojunction anodes by casting aqueous Ti-glycolate solutions prepared by a peroxo- route [11]. It would be a rational postulation that the RCS generation through charge transfer between the physi-sorbed ∙OH and Cl− (Eq. (5)) would be far more facile than oxygen atom transfer from higher oxide (Eq. (6)) [5]. In comparison, the >MOx+1 form would more preferentially mediate OER (Eq. (4)) than (>MOx (∙OH), Eq. (3)) [10]. Therefore, the speciation of hydrous metal oxide under an anodic bias (MOx(∙OH) versus MOx+1) is expected to determine the selectivity between OER and ClER [22], [23]. More predominant steady-state surface concentration of >MOx(∙OH) would account for the overall enhancements of CE and EE for RCS4+ generation on heterojunction architectures, as suggested previously [11], [13]. In order words, primary Ti species on surface of heterojunction anodes would prevent transformation into the higher oxide, whereas the control IrTaOy would favor >MOx+1 formation by Ir4+/Ir6+ transition. Compared to IrTaOy/TiO2 anodes, on the other hand, Bi2O3 particles on surface generally lessened the CERCS and EERCS, except for (Bi2O3)3(TiO2)7-H. Our previous report [11] proposed competitive roles of Bi mixing for ClER on the solution casted IrTaOy/BixTi1−xOz heterojunction anodes; accelerating interaction with Cl− (via a positive shift in surface charge) but hampering ClER kinetics through the higher oxide formation (via Bi3+/Bi5+ redox transition). Accordingly, CE values of ClER were noted to be maximized at Bi fraction of 0.1–0.3 [11]. In this study, the former effect was insignificant, most presumably because of spatially separated TiO2 and Bi2O3 phase. Nevertheless, (Bi2O3)3(TiO2)7-H with the highest surface loading of Bi showed comparable CE and EE values with IrTaOy/TiO2 anodes. 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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