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follow its influence on the impedance response and cell resistances in more detail, and to examine e.g. the surface chemistry of the electrodes before and after being exposed to the electrolytes. 6.2.2 Influence of Flow Rate on the Electrochemical Impedance of Symmetric Cells The studies of the experimental conditions presented in the previous subsections were carried out in preparation for studying the influence of electrolyte flow rate on the im- pedance. An increasing flow rate is expected to decrease the impedance by decreasing the finite diffusion resistance. By recording the impedance at different flow rates and by modelling the data, this relation can be quantified. These results can be used to make decisions on which flow rate is the optimum for a given system, in terms of decreasing the finite diffusion resistance without increasing the pumping losses unnecessarily. 6.2.2.1 Ferri-/Ferrocyanide The ferri-/ferrocyanide couple was used as the model system for these studies, using the one-container symmetric cell configuration with all modifications mentioned in Chapter 6.1.1 (pulsation dampening, temperature control, and UV light and oxygen protection). The cell was assembled with 2×pretreated Sigracet 39AA papers on each side, gasketed by 0.42 mm Viton, and separated by a Nafion 212 membrane soaked in water. An electrolyte consisting of 50 mL 0.1 M K3[Fe(CN)6]+0.1 M K4[Fe(CN)6] dissolved in 1 M KCl adjusted to pH 12 with KOH was used. The cell was heated to 35◦C and left to stabilise with the electrolyte flow rate set to 50 mL min−1 for approximately 4 d to allow the membrane to equilibrate with the electrolyte (this experiment was performed in continuation of the experiment presented in Section 6.2.1.5). The influence of electrolyte flow rate on the impedance was investigated by recording impedance spectra from 100kHz to 30mHz at flow rates ranging from 10–90mLmin−1 in increments of 10 mL min−1. These measurements were conducted with a BioLogic SP- 300 potentiostat and the resulting spectra are shown in Figure 6.16. A clear decrease in impedance is observed with increasing flow rate, which levels off at the higher flow rates. The charge transfer (high-frequency arc) and finite diffusion (low-frequency arc) processes are only distinguishable at the lowest flow rate of 10 mL min−1. This stands in contrast to the impedance of ferri-/ferrocyanide presented in Figure 6.12, where the transition from charge transfer to finite diffusion was clearly visible at a flow rate of 50mLmin−1. The unclear resolution is likely due to the depression of the impedance response presumably caused by the build-up of a contact resistance (see Section 6.2.1.5). As previously, the impedance spectra were fitted using the symmetric cell model presented in Equation 3.57. Due to the increased depression of the impedance, the basic model did not produce satisfactory fits and was in some cases not able to resolve the impedance into the charge transfer and finite diffusion processes. It was therefore decided to add an RQ element to the model to represent the presumed contact resistance. The CNLS fits were carried out with the fixed model parameters listed in Table E.1 in Appendix E, and the resulting best fit parameters are listed in Table E.4 in Appendix E. Figure 6.17 presents 6.2. Results and Discussion 103PDF Image | Organic Redox Flow Batteries 2023
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