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Chapter 6. Electrochemical Impedance of Single Flow Cells 6.2 Results and Discussion 6.2.1 Influence of Experimental Conditions The influence of various experimental conditions on the impedance of symmetric cells was investigated by recording the impedance after each systematic change of a condition. When necessary, the results were quantified by modelling the impedance with a suitable equivalent circuit model. 6.2.1.1 Cell Hardware and Electrodes The cell hardware and electrodes contribute to the ohmic resistance through their intrinsic electrical resistivities and contact resistances between the individual parts. To quantify this resistance, the impedance of cells assembled with only gaskets and electrodes was recorded. The cells were assembled with a varying number of either Sigracet 39AA carbon papers or 1186HCBA carbon cloths, which were in either pristine or pretreated form, and compressed by applying varying torques to the bolts. The Sigracet 39AA papers were pretreated by baking in air at 400◦C for 24h. The 1186HCBA cloths were pretreated at the same temperature for either 1 h or 24 h. A 0.42 mm Viton gasket was used for every two pieces of Sigracet 39AA, whereas a 0.8mm Viton gasket was used for every piece of 1186HCBA. While assembling the cells, their thickness was measured to obtain a more reliable estimate of the electrode compression. The thickness was determined as an average of eight points measured around the cell frame with a caliper. These values and the calculated compression rates are reported in Table A.4 in Appendix A. The spectra recorded across all configurations of Sigracet 39AA papers using a Gamry Reference 3000 potentiostat are presented in Figure 6.2 (a). The resistances were deter- mined at the low-frequency intercept with the real axis and are shown in Figure 6.2 (b) against the number of stacked papers. The resistance increased with the number of stacked carbon papers, but not in a completely linear fashion. This was presumably caused by the electrode compression changing with the number of stacked papers, although the cells were tightened to the same torque. This is evident from the cells assembled with 2× and 4× papers tightened to 6.78 N m, which resulted in compression rates of 44.6 % and 33.9%, respectively (see Table A.4 in Appendix A). The resistance also increased upon pretreatment of the carbon papers, which could be an effect of new surface groups chan- ging the electrical conductivity of the material. On the other hand, increasing the torque from 3.39 N m to 6.78 N m did not change the resistance significantly, which suggests the papers were at their compressibility limit. Interestingly, the extrapolated resistance at 0 stacked carbon papers (shown as the dashed lines in Figure 6.2 (b)) does not match between the cells with pristine and pretreated electrodes. The spectra recorded across all configurations with 1186HCBA cloths are presented in Figure 6.3 (a) and the corresponding resistances are shown in Figure 6.3 (b). In this case, increasing the compression rate (by increasing the applied torque) decreased the resistance. Increasing the baking time decreased the resistance, which could again be the result of changing surface groups. The resistances measured for the 2×1186HCBA cloths are lower than for 4× and 6×Sigracet 39AA papers, even though the total thickness of the 1186HCBA stack was higher than for the Sigracet 39AA stacks. This is likely owed 90PDF Image | Organic Redox Flow Batteries 2023
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