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5 Single fibre electrode measurements - a versatile strategy for assessing the non-uniform kinetics Introduction Redox flow batteries are a good option for large-scale storage of renewable energy and are being deployed at an increasing scale [32, 35]. One of the major advantages of RFBs over other battery technologies is that the electrodes are inert, so have little to no degradation over time or number of charge-discharge cycles under normal operating conditions. Traditionally, RFBs use carbon or graphite felt electrodes as they are stable, conductive and relatively low cost [46, 164, 165]. However the electrochemical performance of these electrodes for the desired reactions can be poor, with current densities typically below 200 mA cm-2, compared to fuel cells which can operate at 1500 mA cm-2 or more [166]. Increased current density would enable cost reductions, as the cell stack in RFBs contributes significantly to the overall capital cost of the battery, where the quantity of expensive ion exchange membranes required to achieve a target power output is proportional to the power density of the cell [80]. Therefore, improving battery performance by increasing the current density will positively influence the economic viability of these batteries. RFB electrode performance depends on many factors: wettability, electrical conductivity, surface area, hydrodynamics, and inherent kinetics of the electrode reactions. It is often hard to decouple these factors in order to understand their role in overall RFBs performance. For example, while the thermal pre-treatment of carbon felts has been suggested to improve the reaction kinetics [167], the improved performance of the electrodes may also be due to an increase in the wettability of the carbon felt or to an increase in surface roughness [90]. For some electrodes, it is relatively easy to determine the kinetics of redox reactions using cyclic voltammetry. For instance, the standard heterogeneous rate constant for a redox reaction at a semi-infinite planar electrode assuming 1D diffusion can be determined from the peak potential separation [168]. This approach has been used to qualitatively compare the reaction kinetics on different carbon felt electrodes [169-171]. Whilst the rate constant will influence the peak potential separation, determining the rate constant from these measurements is very challenging due to the complex mass transport behaviour within these electrodes. Electrode porosity and surface roughness lead to complex diffusion domains which alter measured cyclic 51PDF Image | Electron Transfer Kinetics in Redox Flow Batteries
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