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9 Conclusions and Recommendations Thermal treatment of four different carbon fibers was carried out to further understand the kinetics of Fe(CN) 3− / Fe(CN) 4− at the carbon fiber surface. The apparent rate constant 66 was calculated for thermally treated and untreated carbon fiber samples, using the previously validated methods for the simulation of cyclic voltammetry and electrochemical impedance spectroscopy. The capacitance of the carbon fibers was calculated from cyclic voltammetry and can be considered a proxy for the electrochemically active surface area. The apparent rate constant was found to increase more significantly than the capacitance at three of the four fibers tested, whilst the final fiber had only a modest increase in the apparent rate constant. This result strongly suggests that the kinetics for the reaction depend on different functional groups at the surface, as well as the electrochemically active surface area. Whilst thermal treatment improved the apparent rate constant for the Fe(CN) 3− / Fe(CN) 4− 66 redox couple, the redox couples of most interest form a commercial perspective are the V2+ / V3+ and VO2+ / VO2+. Cyclic voltammetry of the vanadium redox couples at single fiber electrodes is difficult, as the water splitting and hydrogen evolution side reactions will complicate the measurement of the voltammograms. However, due to the small voids within carbon felt electrodes, the positive redox couple can be analysed through cyclic voltammetry, as the reactant concentration in the small voids within the felt depletes quickly, reducing the peak potential separation relative to single fiber electrodes. Using the previously calculated void size distribution, an apparent rate constant was calculated for felt samples of three different carbon felts, thermally treated for 1 hour at 100 oC increments from 200 oC to 700 oC. Thermal treatment primarily improved the apparent rate constant at the different carbon felts. 124PDF Image | Electron Transfer Kinetics in Redox Flow Batteries
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