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Energies 2022, 15, 7397 9 of 20 which takes place reversibly even on electrodes without expensive catalysts, with the process of comproportionation in the electrolyte bulk BrO3− + 5Br− + 6H+ → 3Br2 + 3H2O (homogeneous), (5) which proceeds at sufficiently high acidity of the solution. The results of theoretical and experimental studies [145–148] confirmed a number of extremely interesting and unexpected effects: the presence of an anomalous dependence of current density on the intensity of the convective mixing of the solution (which is uncharacteristic of any other known electrochemical mechanisms) combined with an anomalously high cathodic current density of order ~ A cm−2 in bromate solutions of the molar concentration range. Successful confirmation of the developed analytical model predictions with experi- mental rotating disk electrode data for several solutions of bromate anions in sulfuric and phosphoric acids in the molar range of concentrations is presented in the work [149]. Thus, the quantitative agreement of analytical predictions and experimental data allows us to make the following fundamental conclusions on the kinetics of chemical processe: • The high reversibility of the electrochemical step (5) of the EC” mechanism for the bromine/bromide redox couple used in the system, even on carbon electrodes. That is a prerequisite for high current density, sufficient voltage for under load conditions and the specific power of the system; • Irreversibility of the chemical step (5) of the EC” mechanism due to sufficiently high solution acidity (acid volume concentration approximately in the molar range); • The first order of the comproportionation reaction (5) with respect to the anions of bromate and bromide, the second order—with respect to the activity of protons: V = k∗0 [aH]2 BrO3−Br−, k∗0 = k0 fBrO3 fBr, (6) • where aH = 10−pH proton activity (in the form of hydroxonium ions), f BrO3 and f Br are activity coefficients of the bromate and the bromide anions, correspondingly. • The six-electron nature of the electrochemical process (the Br atom changes its ox- idation state from +5, which corresponds to BrO3−, to −1, which corresponds to Br−) during the cycle is a prerequisite for ensuring a high specific energy capacity of the system. Prof. Cho et al. have performed a numerical study of this redox-mediated bromate- based electrochemical energy system focusing on the transient cell behavior analysis and mass transport analysis of the main reagent, which resulted in strengthening the conclu- sions made above about the prospects of such approaches [150]. The numerical model provided by Dr. Chinannai and Dr. Ju has also demonstrated the beneficial features of the H2/Br2/BrO3− system, demonstrating the discharge characteristics of an H2/Br2 cell with BrO3− measured under different C-rates [151]. The bromate system was found to have the innovative features to boost cell performance due to its characteristic autocatalytic reaction to keep regenerating enough reactant for the electrochemical reaction, which may lead to a “no-limiting current condition (i.e., no mass transfer loss)” in well-designed cell and operating conditions. Thorough research by Dr. Modestov’s group showed for 50 cm2 membrane-electrode assembly of hydrogen-bromate flow battery 0.74 W cm−2 power density was reached at the cell voltage 0.7 V while catholyte utilization rate was 0.93 in a single pass through membrane-electrode assembly, see Figures 3 and 4 [144,152]. As a result, based on the high-grade assessments one can conclude that such a system can be of interest to break through the challenging issues in conventional batteriesPDF Image | Halogen Hybrid Flow Batteries
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