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Section 2.10 Concentration overpotential Voltage in V Voltage in V SoC in % Figure 2-16: Sample cycle between cell voltage limits of 1.1 V and 1.7 V for the illustration of the difference between the electrode area and the electrode surface regarding the computation of the concentration overpotential If the current density iDL is calculated from the cell current, IC, using the electrode area, AE, the concentration overpotential contributes significantly to the rising cell voltage towards the end of the charging process and the dropping voltage towards the end of the discharging process, as shown in Figure 2-15. If the total electrode surface is used to compute the current density in the diffusion layer, the concentration overpotential is negligible. In the cycle bounded by cell voltage limits, the usage of the electrode area leads to a 35 % shorter cycle time, as shown in Figure 2-16. Again, if the total electrode surface is deployed, the concentration overpotential can be neglected. There are two main reasons, why neither the electrode area nor the electrode surface is the correct area for calculating the current density. The first reason is that the exact determination of the electrochemically active surface of a graphite felt is difficult [62]. There might be structures in the electrode, which are blocked at the end. Hence, they contribute to the total electrode surface, but are not available to the electrochemical reactions. In addition, the wettability of the surface as well as the surface structure can affect the effective electrochemical active surface. The second reason is that the electrochemical activity is not uniformly distributed in a flow cell [51]. Hence, the current density will vary over the electrochemically active surface. 47PDF Image | Model-based Design Vanadium Redox Flow Batteries
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