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Following the 24h period with the temperature controlled to 30◦C, the temperature controller was switched off and the setup was left to cool to room temperature. A new spectrum was recorded from 100 kHz to 100 mHz at an electrolyte flow rate of 50 mL min−1 to check if the decrease in series resistance with elevated temperature was reversible. As shown in Figure 6.11, the series resistance increased back to ∼1.18 Ω cm2 after the system returned to room temperature. Figure 6.11: Nyquist plots recorded on a one-container symmetric cell containing 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. The impedance was recorded from 100kHz to 100mHz at a flow rate of 50mLmin−1 before (black), during (red), and after (blue) controlling the temperature to 30 ◦C. To quantitatively assess how the impedance changes with temperature, spectra were re- corded with the temperature controlled to 31–35◦C in increments of 1◦C. The system was left to stabilise for 30 min after each increase in temperature before recording the im- pedance from 100 kHz to 100 mHz with the electrolyte recirculated at 50 mL min−1. The five resulting spectra are shown in Figure 6.12 (a). A consistent decrease in series resis- tance with each increase in temperature is observed. Furthermore, the total impedance also decreased with temperature, which can be seen on Figure 6.12 (b), where the spectra were shifting along the real axis until the high-frequency parts overlapped. The decrease is related to the physicochemical processes occurring at a faster rate when temperature is increased. The impedance spectra shown in Figure 6.12 (a) were modelled using the symmetric cell model presented in Equation 3.57 and the fixed model parameters listed in Table E.1 in Appendix E. The resulting best fit parameters are listed in Table E.3 in Appendix E. The spectrum recorded at 31 ◦C is presented separately in Figure 6.13 together with the results from the CNLS fit. The remaining four impedance spectra and their corresponding fits are presented in Figure E.1 in Appendix E. The model was able to simulate the experimental impedance well, which is visible from both the Nyquist plot (Figure 6.13 (a)), the semi- logarithmic Bode plot (Figure 6.13 (b)), and the relative fit residuals that are closely spaced around 0 % (Figure 6.13 (c)). Furthermore, the Kramers-Kronig test resulted in residuals closely spaced around 0 % (Figure 6.13 (d)), which suggests the recorded data is of good quality. 6.2. Results and Discussion 99PDF Image | Organic Redox Flow Batteries 2023
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