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Chapter 5. Cell Cycling 5.2.2.2 PPAQ(Na) against Ferrocyanide Although a reasonable estimate of the PPAQ(Na) diffusion coefficient was not determined, due to the presumed low purity, it displayed interesting molecular electrochemistry. The redox potential of −0.537V results in a theoretical cell voltage of 1.056V against ferri- /ferrocyanide, so it was decided to investigate PPAQ(Na) further with a full cell study. The cell was assembled with 7×Toray 060 carbon papers baked at 400 ◦C for 30 h, a Nafion 117 membrane soaked in water, and 1 mm PTFE gaskets. The negolyte consisted of 10 mL 0.2 M PPAQ(Na) dissolved in 1.0 M KOH and the posolyte of 15 mL 0.4 M K4[Fe(CN)6] dissolved in 1.0 M KOH. The electrolytes were purged with nitrogen beforehand and an inert atmosphere was kept inside their containers during measurements, but the setup was not otherwise protected from oxygen. A galvanostatic cycling protocol was conducted at a current density of ±25mAcm−2, voltage limits of 0.75V and 1.2V, a flow rate of 100mLmin−1, room temperature, and using a MTI 8 Channel Battery Analyzer. The results are presented in Figure 5.9. The charge-discharge curves in Figure 5.9 (a) show cycling with low overvoltages between charge and discharge. Although the system was cycled at a low current density, only about 11.8 mA h of the theoretical 107.2 mA h could be accessed, as shown on Figure 5.9 (b). This observation is consistent with the small normalised peak currents observed with CV, and therefore supports the hypothesis that the purity of the used PPAQ salt is lower than expected. The loss of capacity is additionally fairly high, with a fade rate of 0.3 mA h d−1 based on the charge capacity, which equals 2.5 % d−1 of the available capacity. As an extra note, the experiment was originally planned for 100 cycles, but during the charge of the 78th cycle, the voltage hit the programmed safety limit of 2.0 V, leading to the interruption of the experiment. The used instrument was overall not very precise at switching at the defined voltage limits, showcasing how important good instrumentation is for long-term cycling experiments. 5.2.2.3 DBEAQ against Ferrocyanide The full cell cycling of DBEAQ against K4[Fe(CN)6] was the last cycling experiment to be carried out during this work. DBEAQ was chosen as a negolyte to once again reproduce results from the literature [24], but also to record impedance spectra of the system at different conditions (these results are presented in Chapter 6). The cell was assembled with 3×Sigracet 39AA carbon papers on each side, gasketed by 0.42mm Viton and separated by a Nafion 212 membrane soaked in water. In this case, the electrode compression was determined by measuring the thickness of the cell assembly with and without the two sets of gaskets and electrodes. A difference of 0.3 mm was measured, meaning the gaskets were compressed by 28.6 %. The resulting electrode compression was 37.5 %. The negolyte consisted of 10 mL 0.1 M DBEAQ dissolved in 0.21 M KOH to achieve a pH value of 12 (this was checked with a pH meter), whereas the posolyte consisted of 60mL 0.15M K4[Fe(CN)6]+0.05M K3[Fe(CN)6] dissolved in 1.0M KCl adjusted to pH 12 with KOH. This gives a theoretical capacity of 53.6mAh. A KNF FPD 06 flow pulsation dampener was placed between each of the cell inlets and the pump head to dampen the pulsations from the peristaltic pump. The cell was heated to 35 ◦C with a heating cartridge inserted into each end plate, which were controlled by a 82PDF Image | Organic Redox Flow Batteries 2023
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