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Membranes 2022, 12, 1228 3 of 16 These fundamental studies paved the way towards power sources where bromate is used as an oxidant [21,22]. Very high values of the current and power densities (well over 1 A/cm2 and 1 W/cm2, respectively), as well as of the faradaic efficiency (over 93% in the course of a single catholyte passage through the cell), were attained for hydrogen-bromate MEA in the course of their discharges. They also revealed an excellent ability to increase the current and the power of such devices proportionally to the membrane surface area, from 0.5 cm2 to 50 cm2. Even though the cathodic section of the H2-BrO3− MEA was constructed and func- tioning in a close analogy to those for RFBs of other types the former was used as a primary power source based on the energy of the overall hydrogen-bromate reaction: 3 H2 + BrO3− = Br− + 3 H2O, i.e., only within the discharge regime. Until this study no at- tempts to perform this global process in the opposite direction, i.e., to regenerate both bromate and hydrogen have been made. At the same time, a hypothetic possibility for the H2-BrO3− MEA to function in a cyclic way, i.e., as a set of discharge and charge stages, would strongly enhance the above merits of this device. The goal of this study was to shed light on this important aspect of bromate-based power sources. The process of the oxidative transformation of a bromide solution, in particular up to the bromate one, has been extensively explored in the literature. Its first stage gives molecular bromine where the tribromide ion, Br3−, is present as an intermediate, in confor- mity with the thermodynamic analysis and experimental studies of the solution compo- sition [52–55]. Even though thermodynamic analysis predicts a direct transition of Br2 to bromate in the course of a further oxidation process, its multi-electron character excludes it as a direct electrode reaction. Therefore, the subsequent formation of the oxygen-containing compounds of Br corresponding to its positive oxidation degrees proceeds via a set of chemical and electrochemical steps due to the generation of numerous intermediates. As a result, the rate and current efficiency of the further electrolysis may depend strongly on pH and on the electrochemical conditions. Several publications have addressed the problem of the electrolytic production of bromates from bromides. The electrolysis of a bromide-containing disinfectant liquid (pH = 6–9) was studied to optimize its current output [56]. It was shown that the use of a RuO2/TiO2/Ti anode allows one to achieve a 67% current efficiency during the electrolysis of bromides at pH > 8.5 [57]. In addition, the electrolysis of bromides salts at the pH of the electrolyte (slightly alkaline) was carried out on a PbO2 anode with a current efficiency of 90% and a current density of 0.2 A/cm2 [58]. A current efficiency of 98–99% was achieved at pH 8.5–9.5 for the electrolysis of bromides on the RuO2/TiO2/Ti anode with the addition of Na2Cr2O7 [59]. Finally, a 100% current efficiency was reported for the electrolysis of bromide-containing solutions at a high pH on boron-doped diamond [60,61] It should be noted that the main goal of these studies [56–61] was to optimize the current efficiency in order to minimize the effect of side reactions, rather than to minimize the applied voltage, while the latter is of primary importance for the energy-storing cycle. Besides this, the electrolysis was carried out an under alkaline pH, which is not compatible with the conditions of our study where the oxidative electrolysis has to be carried out for a strongly acidized bromide solution produced at the stage of bromate reduction inside the same discharge device. The goal of this work has been to perform the transformation between bromate and bromide inside the hydrogen-bromate MEA in a cyclic way, i.e., to carry out the stages of charge and discharge repeatedly for the same overall reaction: 3 H2 + BrO3− = Br− + 3 H2O in both directions in order to assess the degree of the reagent and energy recovery as well as the prospects for its use for energy storage. This study has been carried out for a model electrolyte (mixed sulfuric and hy- drobromic acid solution) supplied into the electrolytic cell of the positive electrode of the hydrogen-bromate MEA: Freidenberg H23C8 Pt-C/GP-IEM 103/Sigracet 39AA, HBr + H2SO4 with a surface area of 4 cm2. The completeness of the bromide to bromate conversion andPDF Image | Hydrogen-Bromate Flow Battery
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