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in the electrolyte composition or cycling protocol between each. In any case, based on the redox potentials of DHAQ and K4[Fe(CN)6] at pH 14 determined in this work (−0.670V and 0.519 V), a theoretical cell voltage of 1.189 V was expected. Experiment 1. The cell was assembled with 7×Toray 060 carbon papers on each side (baked at 400◦C for 30h), gasketed by 1mm PTFE, and separated by a Nafion 117 membrane soaked in water, with a resulting electrode compression of 24.8 %. The negolyte consisted of 15mL 0.267M DHAQ dissolved in 1.534M KOH (= 1.0M OH–) and the posolyte of 15 mL 0.4 M K4[Fe(CN)6] dissolved in 1.0 M KOH, giving a theoretical capacity of 160.8 mA h. An excess of DHAQ was used at this point because it was assumed that 25 % of the quinone would dimerise in solution [63]; this assumption was later abandoned. The electrolytes were purged with nitrogen before starting the measurements, and an inert atmosphere was kept inside the containers during measurements. The setup as a whole was however not protected from oxygen. Figure 5.5 shows the data recorded during galvanostatic cycling at ±100mAcm−2, voltage limits of 0.6V and 1.7V, a flow rate of 100mLmin−1, and room temperature, using an MTI 8 Channel Battery Analyzer. The data was recorded in three sets of 101 cycles, with the system left at OCV in the discharged state (DHAQ in oxidised state) for 30 h after the first 101 cycles, and left at OCV in the charged state (DHAQ in reduced state) for 24 h after the next 101 cycles. This was done to assess any difference in stability between the oxidised and reduced state of DHAQ. The total duration of the experiment including the OCV periods was 194 h. The charge-discharge curves presented in Figure 5.5 (a) show a clear decrease in recovered capacity on successive cycles. They furthermore show that the initial charge and discharge potentials increased and decreased, respectively, on successive cycles, which is likely the result of an increasing membrane resistance. The capacity data in Figure 5.5 (b) shows that the capacity decreased by 15.4mA, or 10.1% of the achieved capacity, in a non- linear manner over the first 101 cycles. This does not fit the observations made in previous cycling studies of DHAQ in both symmetric and full cell configurations, where the capacity fade was approximately linear from the beginning [24, 53]. However, DHAQ was capacity- limiting from the beginning in these studies, which was not the case in the experiment presented here, so the first part of Experiment 1 is not representative for the capacity fade of DHAQ. The total capacity recovered slightly between cycle 101 and 102 where DHAQ was left in the oxidised state (note that the extra 30 h at OCV between cycle 101 and 102 were left out in Figure 5.5 (b) for clarity). After this resting period, the capacity decreased linearly with time for the next 101 cycles, with a rate of 17.8mAhd−1 or 11.6%d−1 based on the charge capacity. The linear decrease suggests that DHAQ at this point had degraded sufficiently to become the CLS. The fade rate is around 2.4 times higher than reported in the literature [24, 53], which is likely caused by the system not being fully protected from oxygen. From cycle 202 to 203, where DHAQ was left in the reduced state, a large decrease in capacity is observed, which fits well with the earlier linear decrease with time. This suggests that the reduced state of DHAQ is the least stable and that degradation is time-dependent. This is accordance with the observations made by Goulet et al., who attributed the capacity loss to the formation of DHA and further into irreversible degradation products [53] (see also Figure 2.5). Over the last 101 cycles, the fade rate increased to 19.4 mA h d−1 or 12.7 % d−1 based on the charge capacity. 5.2. Results and Discussion 77PDF Image | Organic Redox Flow Batteries 2023
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