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Membranes 2022, 12, 1228 current direction was reversed, thereby switching to the discharge half-cycle. Figure 4a demonstrates the resulting plot for the cell voltage vs. the number of equiv- alent charges passed during the process described above, i.e., the passed charge divided by the charge equivalent, Q/Qequiv. The latter is defined as the amount of charge required to change the oxidation degree of all Br atoms in the electrolyte by one unit which is equal to the product: Qequiv = C × V × F where C and V are the initial concentration of bromide anions in the electrolyte and its volume, respectively, and F is the Faraday constant. 8 of 16 2.0 1.8 1.6 1.4 1.2 1.0 2.0 1.5 1.0 0.5 0.0 (a) A, a.u. U, V 02468 Number of charge equivalents, Q/Qequiv (b) 0,1 eq 0,4 eq 0,9 eq 1,1 eq 1,2 eq 1,7 eq 2,0 eq 6,2 eq 8,2 eq 550 600 1 cycle charge 300 350 400 450 500 l, nm Figure 4. Charging curve of H2-BrO3− flow battery (a) and the evolution of the corresponding Figure 4. Charging curve of H2-BrO3− flow battery (a) and the evolution of the corresponding spec- spectrum of the catholyte (b) in the course of galvanostatic polarization by 0.075 A/cm2, starting com- trum of the catholyte (b) in the course of galvanostatic polarization by 0.075 A/cm2, starting compo- position of the positive-electrode solution: 0.3 M HBr + 3 M H2SO4, solution flow rate: 30 mL/min, sition of the positive-electrode solution: 0.3 M HBr + 3 M H2SO4, solution flow rate: 30 mL/min, exit exit of the negative electrode is open to the atmosphere. of the negative electrode is open to the atmosphere. At the initial moments of charging with the transfer of 1 charge equivalent (1/6 from At the initial moments of charging with the transfer of 1 charge equivalent (1/6 from the theoretical charge for 13 mL 0.3 M HBr + 3 M H2SO4), part of the formed Br2 due the theoretical charge for 13 mL 0.3 M HBr + 3 M H2SO4), part of the formed Br2 due to to presence of Br− transforms into Br3− whose contribution dominates in the spectra presence of Br− transforms into Br3− whose contribution dominates in the spectra of the of the solution due to its very large extinction. In this case, the side reaction of water solution due to its very large extinction. In this case, the side reaction of water oxidation oxidation proceeds simultaneously with the main oxidation reaction of bromide described in Equation (3), but to a lesser extent. The concentration of bromide anion in the solution decreases after 1 charge equivalent flowing. Consequently, the quantity of tribromide anions drops down and a pure bromine peak appears on the UV-Vis spectra of the solution (Figure 4b). Then the Br2 concentration increases, confirmed by the spectral peak upsurge. Later, Br2 becomes oxidized and produces intermediate products with a positive oxidation state (+1), the HBrO signal appears on the Figure 4b, but its absorption peak goes beyond the measurement range of the UV—Vis method. The charging curve shows a strong voltage increase while passing 8 charge equivalents (Figure 4a). This indicates the exhaustion of electroactive Br-containing compounds in an oxidation state below (+5). Finally, we observed only the side reaction of water electrolysis with the O2 release after 8.2 charge equivalents. Before the discharge, the spectra indicate the absence of both Br2 and HBrO, which have absorption bands at 396 nm and 266 nm, respectively [65–67]. Therefore, the electrolyte contains only the bromate anion. The beginning of the discharge curve is demonstratedPDF Image | Hydrogen-Bromate Flow Battery
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