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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 and 69.3°) on surface of these anodes indicated relatively thorough covering even with single spray coating. In comparison, (Bi2O3)3(TiO2)7–L electrodes showed clear patterns of standard α-Bi2O3 (2θ = 27.5, 33.2, and 46.5°), while the repeated coating of the Bi/Ti mixed precursor (-H) alleviated the signal from Bi2O3 buried in TiO2 matrix. The addition of Bi2O3 particles to the precursor gave negligible changes in diffraction peak positions of TiO2, indicating limited thermal diffusion between the separate crystalline lattice structures (TiO2 and Bi2O3) [17]. Therefore, each metal oxide phase was expected to maintain the intrinsic physicochemical properties with minimal doping effects; thus, the outer heterojunction layers could be denoted as (Bi2O3)x(TiO2)1−x. In the presence of Bi2O3 particles, signals from underlying IrO2 (peak near 35°) and Ti substrate (peak near 40°) were invigorated. Considering that the XRD analysis was performed after the electrochemical analyses described hereinafter, these observations suggest the instability of bismuth oxide in anodic environment (vide-infra). 3.2. Voltammetric responses of IrTaOy/(Bi2O3)x(TiO2)1−x heterojunction anodes Areal capacitance from CV within the potential window of water splitting has often been utilized to estimate the ECSA [18], [19]. Fig. 2a (raw CV data in Figure S5) illustrates the areal capacitance measured under anodic potential between 0.2 and 1.0 V NHE in 50 mM NaCl solutions (circum-neutral pH). The highest areal capacitance (22 mF cm−2) of IrTaOy was reduced by about 30% upon addition of surface TiO2 layers. Mixing Bi2O3 particles on surface insignificantly affected or even further decreased the areal capacitance. Within the potential scan range, oxidative transformations of hydrous > TiOH or > BiOH (e.g., to oxyhydroxide or higher oxide) could be limited to especially due to the fully oxidized Ti4+. Nevertheless, considering rather thorough surface passivation by TiO2 layers (at least for IrTaOy/TiO2 anodes as shown in Fig. 1), the electrochemical activity for the heterojunction anodes would be ascribed to a thermal diffusion of Ir components into the upper layer (vide infra) [11]. Linear sweep voltammetry (scan range: 0.8 - 2.0 V NHE, scan rate: 5 mV s−1) in 50 mM NaCl solutions ( Fig. 2b) estimated the anodic potential at 10 mA cm−2 in the order of TiO2-L < IrTaOy < TiO2-H ~ (Bi2O3)1(TiO2)9-L ~ (Bi2O3)3(TiO2)7-L < (Bi2O3)1(TiO2)9-H ~ (Bi2O3)3(TiO2)7-H (Table 1). In these experimental conditions, the current generation would be ascribed to parallel OER and ClER. Electrical conductivity of semiconductor electrocatalysts should be another consideration especially for thick catalyst film. To this end, the outer heterojunction layer mostly lowered the current generation due to the inherently poor electrical conductivity of TiO2 and Bi2O3. More pronounced current decline with included Bi2O3 particles, particularly with higher loading ((Bi2O3)1(TiO2)9-H and (Bi2O3)3(TiO2)7-H), agreed with the concurrent increase in ohmic resistance measured by a current interruption method. The current interruption would collectively evaluate the electrolyte resistance of ion migration, polarization resistance and resistance across the catalytic film, often described as resistances in series in an equivalent circuit [20]. Since the Bi2O3 particles were speculated to be anchored within TiO2 matrix without a direct contact with IrTaOy layer, (Bi2O3)x(TiO2)1−x anodes would suffer from invigorated resistance across the multiple junctions (IrTaOy/TiO2/Bi2O3). Nevertheless, the observed OER and ClER activities were incompletely justified with the areal capacitance and ohmic resistance, particularly for the unexpected elevated current generation from TiO2-L anode. A recent report presented evidences that the OER and ClER overpotential values of IrO2 could be reduced upon a few cycles of atomic layer deposition (ALD) of thin TiO2 layer by shifting surface charge density and overall M-O bond strength [21]. Underlying Ir4+ was speculated to be partly oxidized to result in sub- 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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