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Chapter 6. Electrochemical Impedance of Single Flow Cells Figure 6.23: Total, series, and resolved porous electrode resistances against flow rate obtained through CNLS fitting of the 0.05 M DHAQ+0.05 M DHAHQ impedance presen- ted in Figure 6.21. The total resistances are shown with circular markers, whereas the contributions are shown with triangular markers. The "Low-frequency" resistances were determined as the low-frequency intercept of the Nyquist plots in Figure 6.21 with the real axis. Notice the different scaling of the y-axes. by a decrease in the effective electrolyte flow rate, as it was observed that electrolyte accumulated in the pulsation dampener bottle at the highest flow rates. Furthermore, the porous electrode impedance is higher than that observed for the DHAQ system (see Figure 6.21), which is presumably caused by the lower diffusion coefficient of DBEAQ compared to DHAQ (1.58 × 10−6 cm2 s−1 versus 4.8 × 10−6 cm2 s−1). The impedance of DBEAQ was modelled using the symmetric cell model presented in Equation 3.57. An RQ element was not added in this case, as the high-frequency process observed for the DHAQ system was not observed in this case. The CNLS fits were carried out with the fixed model parameters of Sigracet 39AA papers and DBEAQ listed in Table E.1 in Appendix E, and the resulting best fit parameters are listed in Table E.6 in Ap- pendix E. The results from the fit of the impedance recorded at 50 mL min−1 are presented in Figure 6.25, whereas the remaining impedance spectra with their corresponding best fit models are presented in Figure E.4 in Appendix E. Even without the extra RQ element, the model captured the shape of the impedance well, although with increasing residuals in both the Kramers-Kronig and relative residuals plots above 1 Hz (Figure 6.25 (c) and (d)) caused by the high-frequency noise. The porous electrode impedance was resolved into the distributed ohmic, charge transfer, and finite diffusion resistances using Equations 3.54, 3.55, and 3.56. These are shown together with the modelled series resistance and the total resistances in Figure 6.26. The two total resistances are almost identical, with a difference of ∼0.9 % based on the lowest resistance. The total resistance decreased between 10 mL min−1 and 50 mL min−1 and then appeared to stabilise. As for the DHAQ experiment, which was conducted in the same cell 110PDF Image | Organic Redox Flow Batteries 2023
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