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7 Thermal Activation of Carbon Felt for the Positive Vanadium Redox Couple Physical area measurements using BET As previously mentioned in Chapter 6, the increase in capacitance can be caused by a combination of three factors. Thermal treatment can oxidise the surface, forming pores and increasing the roughness of the electrode. This increases the physical area of the electrode and can be measured using BET isotherms. The ratio of basal to edge plane sites can also alter the capacitance significantly, as the specific capacitance of the basal plane is 2 μF cm2, increasing to 60 − 70 μF cm-2 for edge plane sites. Finally, the wettability of the felt also influences the capacitance, if only a fraction of the physical area of the felt is wetted the measured capacitance will be reduced. BET measurements were conducted on the untreated and thermally treated felt samples, with GFE felt thermally treated at 600 oC, G100 at 500 oC and C100 at 400 oC. The expected specific surface area for each carbon felt was calculated assuming perfectly smooth cylindrical fibers, with the average fiber diameters for each felt reported in Chapter 6 and assuming a density of 2 g cm-3 for the carbon fibers, (Table 7.2). Table 7.2: Specific surface area of the different carbon felts based on fiber diameter and BET measurements. Felt Type C100 G100 GFE Expected (m2 g-1) 0.24 0.22 0.2 Untreated (m2 g-1) 0.39 0.37 0.36 Roughness Factor 1.6 x 1.7 x 1.8 x Thermally Treated (m2 g-1) 6.00 0.61 6.92 Increase Factor 15.3 x 1.6 x 19.1 x The roughness factors are modest and feasible and suggest that there is a consistently high level of wetting. 97PDF Image | Electron Transfer Kinetics in Redox Flow Batteries
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