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7 Thermal Activation of Carbon Felt for the Positive Vanadium Redox Couple at single fiber electrodes, due to depletion of the reactants in the felt voids, causing the concentration at the electrode surface to decline faster. The peak potential separation during cyclic voltammetry is an indicator of kinetics, with closer peaks indicating faster kinetics for a given void distribution. There is evidence of unwanted side reactions at the thermally treated GFE electrode. This is likely the oxygen evolution reaction, which has a standard reduction potential of 1.03 V vs Ag|AgCl. The thermal treatment of the GFE electrode therefore improved the apparent rate constant for the VO2+ / VO2+ redox couple as well as the water splitting reaction. The peak heights and distance from the standard reduction potential differ for a number of the electrodes, indicating that the charge transfer coefficient, 𝛼 is not always close to 0.5, as was observed for the Fe(CN) 3− / Fe(CN) 4− redox couple, investigated in 66 Chapters 4-6. Of the untreated felts, C100 clearly had the smallest peak separation at 339 mV, compared to 452 mV for GFE and 818 mV for G100. Thermally treating the felts in air for 1 hour was shown to have a positive effect on the reaction kinetics, with large reductions in peak potential separation observed at GFE and G100 felts. Cyclic voltammetry could not be carried out for G100 at 700 °C, as the felt had oxidised and lost mechanical integrity, with the same phenomena occurring for C100 at 600 °C and GFE at 800 oC. Measurement of the peak potential separation during cyclic voltammetry using the C100 felt was not possible after treatment at 400 °C and above, due to the large increase in capacitance demonstrated in Figure 7.2. From the peak separation the mean apparent rate constant of the carbon felts can be found, if the void size distribution is known. In this work we were restricted to only having micro-CT data for an uncompressed GFE felt sample. Identification of the mean rate constant in this cell set up is further complicated by the compression exerted around the outer rim of the felt sample, likely causing a reduction in the average felt thickness, changing the pore size distribution slightly. 101PDF Image | Electron Transfer Kinetics in Redox Flow Batteries
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