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7 Thermal Activation of Carbon Felt for the Positive Vanadium Redox Couple The increase in apparent rate constant for G100 felt thermally treated at 500 oC of 6.3 x, is larger than the recorded increase in capacitance of 2.5 x and physical area of 1.6 x. This indicates that the thermal treatment increased the inherent rate constant of the G100 felt, as well as the electrically active surface area. This result is consistent with the results from the Fe(CN) 3− / Fe(CN) 4− redox couple measurements at G100 in Chapter 6, where the apparent 66 rate constant also increased significantly more than the capacitance. The increased inherent rate constant is likely the result of oxygen containing functional groups forming at the surface during thermal treatment. No apparent rate constant could be measured at thermally treated C100 felt as previously mentioned. However, there appeared to be little change in kinetic performance at 300 oC, at which point the capacitance had already increased by a factor of 9.6 x. BET measurements at C100 treated at 400 oC showed 15.3 x increase in the physical area. This suggest that thermal treatment of C100 felt increases the electrically active surface area, but reduces the inherent rate constant. The same trend was seen in Chapter 6 with the Fe(CN) 3− / Fe(CN) 4− redox 66 couple, where the C100 capacitance increased more significantly than the apparent rate constant. These results show that increases in the electrically active surface area of the carbon felt electrodes are important, as stated by Stimming et al [177]. However, there is sufficient evidence to suggest that the inherent kinetics at the carbon surface do change with thermal treatment. Therefore, it is likely that there are functional groups that can be introduced or removed during thermal treatment, that alter the inherent rate constant for the VO2+ / VO2+ redox couple. These results are supported by observations that oxygen functional groups alter the kinetics during electrochemical treatment, made by Miller et al [158]. 105PDF Image | Electron Transfer Kinetics in Redox Flow Batteries
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