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Chapter 6. Electrochemical Impedance of Single Flow Cells resulted in the lowest observed series resistance, specifically 0.46 Ω cm2. However, this type of pretreatment was previously observed to increase the crossover of VO2+ through Nafion, which was explained by swelling of the membrane’s micro micelle structure [60]. Figure 6.8: Nyquist plots recorded on cells assembled with Sigracet 39AA papers, Nafion 212 membranes subjected to various pretreatments (as stated in the legends), and flooded with 1 M KCl adjusted to pH 12 with KOH. The impedance was recorded from 316 kHz to 1 Hz at room temperature. Based on these results, it was decided to pretreat Nafion 212 membranes used for the following EIS experiments by simply rinsing and soaking in water before use. It was later found that the pH of electrolytes initially at pH 12 could decrease substantially in a cell with such a membrane, presumably due to protons from the membrane leaking into the electrolyte. It would therefore be of interest to continue this study at a later point to find an appropriate pretreatment protocol for Nafion, and in general for other membrane materials suited for the use in alkaline RFBs. 6.2.1.3 Pulsation Dampening Pulsation dampeners were introduced to the symmetric cell configuration to assess whether or not they would improve on the impedance data quality. It is especially the low- frequency part that is affected by pulsations from the peristaltic pump, as an uneven flow of electrolyte through the cell causes a low-frequency fluctuation in the diffusion layer [72]. The effect was tested on a one-container symmetric ferri-/ferrocyanide cell, assembled with 2×pretreated Sigracet 39AA papers on each side, gasketed by 0.42mm Viton, and separated by a Nafion 212 membrane soaked in water. The electrolyte con- sisted of a 50 mL 0.1 M/0.1 M mixture of K3[Fe(CN)6] and K4[Fe(CN)6] dissolved in 1 M KCl adjusted to pH 12 with KOH, which was recirculated at 50mLmin−1. The pulsa- tion dampener consisted of a 100mL glass bottle fitted with a 4-port HPLC cap placed between the pump outlets and cell inlets. Figure 6.9 (a) shows spectra recorded with a Gamry Reference 3000 potentiostat from 100kHz to 10mHz, before and after the pulsation dampener was implemented. The 96PDF Image | Organic Redox Flow Batteries 2023
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