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Although the effect of pretreatment is markedly different for the two materials, it is clear from the photographs in Figure 6.5 that pretreatment is required for both materials to make them hydrophilic and thereby increase the wetting with aqueous electrolytes. The impedance spectra recorded on cells assembled with 1186HCBA cloths are presen- ted in Figure 6.6 (spectra in blue colours). Pretreating the cloths for 1h decreased the series resistance by 0.12 Ω cm2, whereas baking for 24 h increased the series resistance by 1.25 Ω cm2 compared to pristine cloths. The reason for this behaviour is still unclear, but one possible explanation for the large increase in series resistance could be contact issues between the electrodes and the membrane. The shape of the impedance response was otherwise largely unchanged between pristine and pretreated electrodes. When comparing the spectra of cells assembled with carbon papers and cloths, two notable differences are observed. The porous electrode response is longer for the 1186HCBA cloths, which is caused by their higher thickness (1.321 mm compared to 0.560 mm for 2×Sigracet 39AA papers). The most striking difference is the series resistance between the two sets of spectra. Except for the chosen electrode material, the only significant difference between the experiments with carbon papers and cloths was the applied membrane pretreatment. Nafion 212 was initially pretreated by soaking in 1 M KOH, as this had been reported in the literature for alkaline systems [24, 53, 64]. However, since the series resistance was markedly higher than the theoretical 0.368Ωcm2 expected for a Nafion 212 membrane exchanged with K+ ions [88] (based on numbers for Nafion 117), it was surmised that the KOH pretreatment was detrimental to the membrane performance. The 1186HCBA cells were therefore assembled with a Nafion 212 membrane soaked only in water, and the effect of membrane pretreatment was studied in a new set of measurements. 6.2.1.2 Membranes The unusually high series resistances observed for cells assembled with Nafion 212 pre- treated with 1 M KOH led to the suspicion that the pretreatment protocol was responsible. This was supported by previous studies that showed a considerable change in series resis- tance depending on the pretreatment protocol [59, 60]. A set of experiments with various Nafion 212 pretreatment protocols was therefore carried out to find a suitable way to pre- treat the membranes used for impedance studies. The pretreatment protocols consisted of soaking a piece of Nafion 212 in various solutions for various amounts of time and then rinsing it with water. The membranes were then used to assemble cells with 2×Sigracet 39AA papers on each side of the membrane, gasketed by 0.42 mm Viton. The cells were in all cases flooded with 1M KCl adjusted to pH 12 with KOH, which was recirculated at 50 mL min−1 for 5 min to wet the electrodes. While recording the impedance, the cells were left flooded with the pump off. The spectra recorded from 316 kHz to 1 Hz with a Gamry Reference 3000 potentiostat are presented in Figure 6.8. The starting point is the membrane pretreated only with water, which resulted in a series resistance of 1.05Ωcm2. Exposing Nafion 212 to solutions at pH 12 or above increased the series resistance in all three cases (spectra in green and red colours in Figure 6.8). Interestingly, the membrane soaked in 1 M KOH for 28 h resulted in a lower series resistance than the one soaked in 1 M KCl adjusted to pH 12 for 25 h. A protocol found in the literature was also tested [29], consisting of heating the membrane in 80◦C deionised water for 20min and soaking in 5% H2O2 for 35min. This protocol 6.2. Results and Discussion 95PDF Image | Organic Redox Flow Batteries 2023
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