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Chapter 6. Electrochemical Impedance of Single Flow Cells Although clear trends were observed in the resistance with an increase in stacked Sigracet 39AA papers and increased compression rate of the 1186HCBA cloths, the absolute values may not be completely correct. The measured resistance also depends on how well the cell was assembled, as e.g. a misalignment of the electrodes in the gaskets will increase the resistance. It would therefore be necessary to repeat the measurements several times to obtain more reliable absolute values. Cells were subsequently assembled with a Nafion 212 membrane and flooded with 1M KCl to study the influence of electrode pretreatment on a blank system. The first set of measurements was carried out on cells assembled with Sigracet 39AA electrodes, one with pristine and one with pretreated papers (baked at 400◦C for 24h). Two papers were stacked on each side and separated by a Nafion 212 membrane soaked in 1 M KOH and rinsed in water. Both cells were filled with 1M KCl, which was recirculated at a flow rate of 50 mL min−1 for 5 min to wet the electrodes. The cells were left flooded with the pump turned off while recording linear potential sweeps at 50 mV s−1 in a two- electrode configuration and impedance spectra from 316 kHz to 1 Hz, both with a Gamry Reference 3000 potentiostat. The potential sweeps are presented in Figure 6.4 and show that pretreatment increased the background current by a factor of approximately 500. A simple test of the hydrophilicity of pristine and pretreated Sigracet 39AA was later performed by applying a water droplet to their surface. As shown in Figure 6.5, water has a contact angle above 90° on pristine Sigracet 39AA, whereas the pretreated piece absorbed the water. The effect of pretreatment was also studied by capturing scanning electrode microscopy (SEM) images, which showed an increase in pore sizes upon pretreatment (see Figure D.1 in Appendix D). The increase in background current is thus likely caused by an increase in internal surface area in contact with the electrolyte, which substantiates the necessity of pretreatment of this type of carbon material. Figure 6.4: (a) Linear potential sweeps recorded at 50 mV s−1 in a two-electrode configu- ration on cells assembled with either pristine or pretreated Sigracet 39AA carbon papers. The cells were flooded with 1 M KCl. (b) Zoom around the voltammogram recorded with pristine carbon papers. 92PDF Image | Organic Redox Flow Batteries 2023
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