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good electrocatalytic properties for the vanadium redox couple reactions, while concentration overvoltage can be minimized by employing mass transfer enhanced electrodes, high flow rates and ensuring good electrolyte flow distribution. In the following sections is studied the cell resistance while modifying the base electrode, graphite or carbon felt, with materials such as TiO2, TiO2:H and TiO2:N synthesized coatings, as well as Atomic layer deposited (ALD) CeO2. The results indicated that the voltage drop varied with respect to the materials properties. Materials with a larger electrochemical active area, high conductivity, and a hydrophilic nature showed a higher specific capacitance and energy storage efficiency with lower internal resistance. Besides, the material/component selections for cell preparation as well as cell design are very important for improving the performance of the VRFB. In addition to the voltage efficiency, Coulombic efficiency will also impact on cell performance. Coulombic efficiency is affected by the selectivity of the membrane that determines the rate diffusion of the vanadium ions from one half-cell to the other, leading to self-discharge. Coulombic efficiency is also affected by side reactions such as hydrogen and/or oxygen evolution during charging and air oxidation of the V2+ ions in the negative half-cell electrolyte. Gassing side reactions are a property of the electrode material as well as the electrolyte flow rate. Gassing an air oxidation not only reduce Coulombic Efficiency but can also decrease capacity over extended operation, requiring electrolyte rebalancing in order to restore capacity as discussed by Corcuera and Skyllas-Kazacos144. Coulombic efficiency is therefore dramatically affected by the properties of the membrane as well as battery design and operation. 2.7.3 Cell Components and Design Requirements The exploded view of a single cell of the VRFB is shown in Figure 2.8. The basic VRFB cell is constructed from two half-cells, each containing an end plate, an insulating frame, current collectors (usually copper or brass), a graphite plate with flow channels for the electrolytes (anolyte and catholyte) flow, carbon or graphite felt (GF) of the desired thickness, and a gasket. The half-cells are separated by an ion exchange membrane that prevents mixing of the two half-cell electrolytes while allowing a transfer of hydrogen ions to complete the circuit. 38PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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