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3 Electrode modifications Carbon fiber microelectrodes were used to simplify the complexities involved with kinetic analysis of carbon felt materials. Linear sweep voltammetry and electrochemical impedance spectroscopy analysis methods were used to calculate the rate constant for the V2+ / V3+ and VO2+ / VO2+ redox couples, as well as the effect on kinetics of electrochemical oxidation and reduction. It was found that electrochemical reduction inhibits the kinetics of V2+ / V3+ redox couple, but enhances the kinetics of VO2+ / VO2+. These changes were assigned to inherent kinetics of different functional groups, as the capacitance was consistent during testing after a pre-treatment procedure was carried out. XPS analysis demonstrated that electrochemical oxidation increased the oxygen content on the surface. Interestingly, there was a clear difference in treatment potential between activation and de-activation. For instance, electrochemical reduction below -0.3 V, relative to a mercury sulfate reference electrode, was required to increase the rate constant for the VO2+ / VO2+ reaction. However, the increased rate constant remained until the potential increased to over 0.9 V. The same phenomenon was observed for the V2+ /V3+ reaction, with application of electrode potentials over 0.1 V increasing the rate constant, but a reduction potential of – 1.3 V was required to reverse this improvement. Similar results were observed at a range of different carbon surfaces in other work by the same authors [124, 125]. It is possible to combine different surface treatment methods, for instance graphite felt was soaked in 98% sulphuric acid for 5 hours before thermal treatment at 450 oC for 2 hours. An increase in COOH groups was observed, with kinetic performance improvements determined from a reduction in peak separation during cyclic voltammetry, with only a minor increase in physical surface area, measured by Brunauer–Emmett–Teller theory of gas adsorption (BET) [126]. 35PDF Image | Electron Transfer Kinetics in Redox Flow Batteries
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