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Toray 060 Sigracet 39AA 1186HCBA Sigracet 39AA 6.2. Results and Discussion Pristine Pretreated Figure 6.5: Photographs of the electrode materials used in this work in pristine and pretreated form, with a water droplet applied to each. Pretreatment was performed by baking in air at 400 ◦C for 24 h. The impedance spectra recorded with Sigracet 39AA electrodes are presented in Figure 6.6 (spectra in red colours). A spectrum was also recorded with the cell flooded with 1M KCl adjusted to pH 12 with KOH, as this was later decided to be the supporting electrolyte used for EIS studies of symmetric ferri-/ferrocyanide cells. The pretreatment changed the shape of the impedance both in terms of the high-frequency porous electrode response and the slope of the capacitive tail. These changes would follow from a change in the wetting of the electrodes, a change in the electrode pore geometry, and a change of the double-layer capacitance of the electrode surface. The latter was confirmed by modelling the impedance with the symmetric equivalent circuit presented in Equation 3.57, using the expression for the blocking surface impedance presented in Equation 3.52 and the fixed model parameters presented in Table E.1 in Appendix E. The best fit results are presented in Table E.2 in Appendix E and show a significant change in the pseudo double-layer capacitance. The addition of KOH (∼10 mM) decreased the series resistance by 0.15Ωcm2, likely caused by an increase in ionic conductivity of the electrolyte. The impedance was otherwise practically unchanged, as seen from the similar fit results in Table E.2 in Appendix E. A second set of measurements was carried out on cells assembled with 1186HCBA elec- trodes in pristine form and pretreated for 1 h and 24 h at 400 ◦C. One electrode was placed on each side, gasketed by 0.8 mm Viton, and separated by a Nafion 212 membrane soaked and rinsed in water. The cells were filled with a solution of 1 M KCl adjusted to pH 12 with KOH, which was recirculated at 50 mL min−1 for 5 min to wet the electrodes. The cells were left flooded with the pump off while recording linear potential sweeps at 50 mV s−1 in a two-electrode configuration and impedance spectra from 316 kHz to 1 Hz. The potential sweeps are presented in Figure 6.7. In this case, the current only increased by a factor of 2–3 upon pretreatment for either 1 h or 24 h, and the pretreatment caused a peak to appear at each end of the potential sweeps. Looking at the photographs of pristine and pretreated 1186HCBA in Figure 6.5, a larger increase in surface area could have been expected, as the electrode surface changes from hydrophobic (contact angle >90°) to absorbing the water droplet upon pretreatment. However, as the structure of the cloth is already quite open, the change in hydrophilicity is better explained by a 93PDF Image | Organic Redox Flow Batteries 2023
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