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Chapter 5. Cell Cycling The electrolyte reservoirs consisted of two glass vials or bottles with holes drilled into the lid to accommodate the inlet and outlet tubes. Most of the tubing consisted of 1.6mm inner diameter (ID) and 3.2 mm outer diameter (OD) perfluoroalkoxy alkane (PFA) tubing from Swagelok or PTFE tubing of the same dimensions from Buch & Holm. Through the peristaltic pump heads, either Norprene tubing with 3.1mm ID and 6.4mm OD from Sigma-Aldrich or Masterflex Chem-Durance tubing with 3.1 mm ID and 6.3 mm OD was used. The pump tubing was connected to the normal tubing using 6.4mm to 3.2mm PFA reducing unions from Swagelok. The electrolyte solutions were circulated using a Shenchen LabV1 pump fitted with either a Masterflex L/S Easy-Load II pump head with four rollers or two Shenchen YZ1515x pump heads with three rollers each. Full cell cycling experiments were carried out galvanostatically, either with or without a potential hold at the end of each half-cycle or after a certain number of half-cycles. Sym- metric cell cycling was performed potentiostatically. Polarisation curves were recorded at different SOC values by establishing the accessible capacity with a full galvanostatic charge and then discharging the system in increments by drawing a galvanostatic current for a predefined time period. The used currents, potentials, limits, cell cycling protocol, and instrument will be mentioned specifically for each experiment. 5.2 Results and Discussion The results are divided into subsections of symmetric and full cell cycling, and the latter further into subsections corresponding to the investigated quinone negolyte. The order matches the order in which the quinones were introduced in Chapter 4. 5.2.1 Symmetric Cells The [Fe(CN)6]–3/[Fe(CN)6]–4 couple was studied in a symmetric configuration to confirm that it is reasonably stable over longer periods of cycling. Two separate experiments were carried out, each employing a different cell cycling protocol, as presented in the following. Experiment 1. The cell assembly consisted of 3×Toray 060 carbon papers on each side of a Nafion 212 membrane and gasketed by 0.42 mm Viton, resulting in an electrode com- pression of 26.3 %. The negolyte consisted of 10 mL 0.2 M K4[Fe(CN)6] and the posolyte of 10 mL 0.1 M K3[Fe(CN)6], both dissolved in 1 M Na2SO4. This gives a theoretical capacity of 53.6mAh. The electrolytes were purged with nitrogen before starting the measurements and an inert atmosphere was subsequently kept around the containers, but the setup as a whole was not otherwise protected from oxygen. The cell was cycled galvanostatically at ±40mAcm−2 to voltage limits of ±0.4V using a BioLogic SP-300 potentiostat, at an electrolyte flow rate of 60mLmin−1 and room temperature. A total of 34 cycles were recorded over 18 h. The results are presented in Figure 5.1. The system exhibited high coulombic efficiencies of approximately 99.95 % but a significant capacity fade, which was observed over two distinct regions. The capacity fade was initially 0.69 mA h d−1 based on the charge capacity, which equals 1.38 % d−1 of the total achieved charge capacity (49.87mAh). From the eighth cycle, the fade rate decreased to 0.08mAhd−1, or 0.16%d−1, which is closer to the 0.6%d−1 previously reported in 72PDF Image | Organic Redox Flow Batteries 2023
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