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2 Vanadium Redox Flow Battery Review Electron Transfer Kinetics Improving the kinetics of the vanadium redox reactions can improve the voltage efficiency through reduced electrode activation overpotentials. The reduced cell potential therefore can also improve the coulombic efficiency by reducing the cell potential, limiting the amount of charge that goes towards the unwanted side reactions. Combined this can have a dramatic effect on the overall energy efficiency of the cell. The electron transfer kinetics depend on the overpotential applied to the electrodes and the rate constant for the reaction. The rate of the forward reaction is given by: π = π0 ππ₯πβπππΉ(πΈππππβπΈπππ£) (2.8) ππ Where ππ, is the rate of the forward reaction, Ξ±, is the charge transfer coefficient, π, is the number of electrons transferred during the oxidation and reduction reactions, πΉ, is Faradayβs and πΈ β πΈπππ£, is known as the electrode activation overpotential, also denoted as ππππ‘. When considering the reaction in both the forward and reverse directions, the net electron transfer rate at a surface can be calculated using the Butler-Volmer equation: 0 βπΌππΉ(πΈβπΈπππ£ ) (1βπΌ)ππΉ(πΈβπΈπππ£) π=πΉπ΄π (πΆπ πππ π π βπΆππ₯π π π ) (2.9) Where π, is the current, π΄, is the surface area of the electrode, π0, is the standard rate constant, πΆπ ππ, is the concentration of the reduced species at the surface of the electrode, πΆππ₯, is the concentration of the oxidised species at the surface of the electrode, π , is the universal gas constant and π, is the temperature of the system, measured in kelvin. Thus, the current density is proportional to the standard rate constant, but exponentially related to the overpotential. Operation of a cell is deemed to be kinetically limited if the current density is below the mass transfer limit. Improving the electron transfer kinetics leads to a larger π0 value, enabling larger current densities at a given overpotential, or requiring a lower potential to achieve the same current density. 17PDF Image | Electron Transfer Kinetics in Redox Flow Batteries
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