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2 Vanadium Redox Flow Battery Review The performance of a VRFB cell with a serpentine flow field was compared to a flow through design. The serpentine flow field improved the distribution uniformity of the electrolyte and reduced the overpotential in the electrode. The cell with the serpentine flow field had an energy efficiency of 80.7% at 40 mA cm-2 and 2.5 mL s-1, outperforming the cell with flow through design which returned a maximum energy efficiency of 75.9% at 40 mA cm-2 and 1.2 mL s-1. This higher energy efficiency of the serpentine flow field is significant enough to overcome a 1% higher pumping cost, relative to the flow through design [76]. VRFB cells with interdigitated flow fields have been demonstrated to outperform cells with flow-through carbon electrodes. Thicker electrodes are required in flow through configurations to reduce the pressure drop, the thicker electrodes increase the ohmic losses due to contact resistances, lowering the performance of the cell. When scaled up to larger active area electrodes, the pressure drop increases less for interdigitated flow field configurations and thus they are more suited for use in large cells with higher power output [77]. However, analysis of a range of different interdigitated variants showed that they were susceptible to poor distribution of electrolyte within the cell and therefore the performance is typically below that achievable using serpentine flow channels in large cells [78]. New methods to improve electrolyte distribution through electrodes include the use of static mixers and toroidal cell designs [79]. Toroidal cell designs using wedge-shaped cells enables varied felt compression, with higher electrode compression at the outlet, optimising the material properties through the cell. Wedge cells tested without mixers had smaller overpotentials than traditional parallel plate designs and resulted in a 15% increase in energy efficiency. This in turn increased the energy capacity, due to larger depth of charge and discharge between voltage cut-offs. The addition of static mixers can further improve mass transport, with minimal increased pumping cost by reducing the concentration gradients across the cell. It is expected that with the addition of static mixers, energy efficiency improvements of over 20% could be achieved. 22PDF Image | Electron Transfer Kinetics in Redox Flow Batteries
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