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Membranes 2022, 12, 1228 12 of 16 • • Energy efficiency (ηE) of a hydrogen-bromate flow battery charge—discharge tests— the multiplication product of Coulombic and voltaic efficiencies; Capacity utilization (Qdischarge /Qtot )—the ratio of the charge received while dis- charging the battery to the theoretical charge in percent (6 × F × c × V, where F is Faraday’s constant, c is the concentration of electroactive compounds, V is the electrolyte volume). No. Qcharge, ×103 Kð 1 3.1 2 2.3 3 1.1 4 0.5 Table 1. Characteristics of the charge—discharge cycle of a hydrogen-bromate flow battery according to Figure 6. Qdischarge, ×103 C Ucharge, V Udischarge, V ηQ, % ηU, % ηE, % Qdischarge/Qtot, % 1.9 1.6 1.2 61 75 46 90 1.6 1.6 1.1 70 69 48 70 0.9 1.6 1 82 62 51 40 0.4 1.5 1 80 67 54 20 The results of capacity utilization (Qcharge/Qtot) for cycles 1 and 2 are overestimated because of the contribution of the side reaction of water electrolysis. The battery charac- teristics are at a level high enough for practical use. The values of coulombic, voltaic and energy efficiencies are high, but the charge and cycle duration have decreased significantly. The decrease in capacity utilization should be considered as the most noticeable negative effect. It occurs as a result of a significant diminution of the charge which passes in the course of the charging stage of the battery. The causes of the diminution are various, but primarily it is due to a decrease in the total amount of Br atoms in the system. Therefore, we should turn to the analysis of the Br atoms’ amount in the catholyte and in the cell as a whole before and after the measurements (Table 2). Table 2. Estimation of losses of Br-containing compounds after charge-discharge tests. Catholyte Escaping hydrogen trap Anodic half-cell Total amount of bromine atoms Amount of Bromine Atoms Before Cycling, mmol 3.98 0 0 3.98 Amount of Bromine Atoms After Cycling (Figure 5), mmol 3.07 0.06 0.06 3.19 Relative Change in Bromine Content, % 77 1.6 1.6 80.2 The results of the analysis of the electrode potential variation during cycling are demonstrated in Figure 6a, curve 1. The charging segments of this dependence indicate that the overvoltage of the catholyte oxidation decreases from cycle to cycle. This decrease is caused by the growth of the surface of the positive electrode. This is clearly evidenced by an increase in its capacitive current of more than 20 times after cycling, compared to that before cycling, according to the CV measurements in Figure 8a. Subsequently, the harging current density or the charge transfer voltage is reduced. It should be emphasized that the increasing capacitance/development of the surface are caused by the destructive effect of high potentials and strong oxidizers on carbon material. However, this destructive effect did not cause any noticeable increase in cell resistance during the four cycles performed (Figure 8b): the increase in its impedance during measurements at a high frequency did not exceed 16%. On the other hand, the total resistance of the cell when current was passed (determined by the ratio of the power surge during the transition from charge to discharge to the current surge) grew noticeably— almost three times. Considering the previously noted decrease in the cathode polarization and the relative constancy of the MEA ohmic resistance we assumed that the reason for the increase in total resistance was anode polarization, which increases from cycle to cycle. Similar problems were noted in [16,18] and are apparently caused by the catalytic surfacePDF Image | Hydrogen-Bromate Flow Battery
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