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7 Thermal Activation of Carbon Felt for the Positive Vanadium Redox Couple Introduction Thermal treatment has been shown in numerous studies to improve the round trip efficiency of VRFB cells [167, 218-220]. Explanations given for the improved efficiency include an increase in oxygen functional groups at the surface, increased electrode area from pitting and improved wettability. However, as discussed in Chapter 4, accurate quantification of the carbon felt kinetics is challenging. Accurate quantification of the inherent kinetics will enable identification of the most effective physical and chemical changes possible. It has been shown that cyclic voltammetry and EIS of single carbon fibers electrodes can be used to calculate the apparent rate constant distribution of a carbon felt sample, Chapter 4. This data can then be used to accurately model the behaviour of a carbon felt sample, if the void size distribution is known. However, for the VO2+ to VO2+ oxidation reaction and subsequent reduction reaction, the standard reduction potential, 𝐸0 = 1.00 𝑉 𝑣𝑠 𝑆. 𝐻. 𝐸 is close to the standard reduction potential of the water splitting reaction, 𝐸0 = 1.23 𝑉 𝑣𝑠 𝑆. 𝐻. 𝐸. As the kinetics for the VO2+ to VO2+ oxidation reaction is considerably lower than for the Fe(CN) 3− / Fe(CN) 4− redox couple, it is not possible to record a vanadium oxidation peak 66 at a single carbon fiber electrode. The applied overpotential required to achieve vanadium redox peaks leads to large current contributions from water splitting, preventing accurate analysis of cyclic voltammetry measurements. However, as discussed in Chapter 2 due to the relatively small voids within carbon felt, the peak separation during cyclic voltammetry at carbon felt electrodes is significantly reduced, relative to a single fiber electrode. If the sweep rate is low enough for the rate constant of the chosen electrode, it is possible to record clear vanadium oxidation peaks. Whilst it has been shown that EIS is a more reliable method for calculating the rate constant of single carbon fibers, this is not a practical option for measuring the kinetics at a felt sample, as there is no validated model to account for the void size distribution within the felt. 92PDF Image | Electron Transfer Kinetics in Redox Flow Batteries
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