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8 Half β cell performance of Carbon Felt Samples Half-Cell Measurements Half-cell measurements enable the contribution to the overpotential from the positive and negative electrodes to be calculated independently of each other. This is important for identifying whether electrode modifications improve the kinetics for both redox couples. Half- cell measurements have been recorded using a number of different cell set-ups, Schweiss et al. used a double-half cell design, whereby the same electrolyte was used at 50% SOC for both the positive and negative electrode. For the V2+ / V3+ couple, this means V2+ is oxidised at the positive electrode and V3+ is oxidised at the negative electrode. By drawing the electrolytes from the same tank the SOC should be maintained at 50%, provided there is not a large imbalance in the charge transferred to side reactions between the negative and positive electrode [229]. Fetyan et al. used a full cell, but with reference electrodes in each half cell to measure the potential of each half-cell reaction [230]. If a half-cell is used for polarization measurements, the apparent rate constant can be calculated from the current density, overpotential and geometric surface area of the electrode by rearranging Eq. 2.9: π0 = π (2.9) βπΌππΉ(πΈβπΈπππ£ ) (1βπΌ)ππΉ(πΈβπΈπππ£) πΉπ΄(πΆπ πππ π π βπΆππ₯π π π ) Where π, is the current, π΄, is the surface area of the electrode, π0, is the standard rate constant, CRed, 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, π, is the temperature of the system in K. This Equation is only valid if the concentration of the oxidised and reduced species at the surface do not change. This is conditional upon having sufficient electrolyte flow to limit concentration polarization at the electrode surface and therefore calculated apparent rate constant values should be considered estimates. 114PDF Image | Electron Transfer Kinetics in Redox Flow Batteries
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