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for 0.5 V (99.1%), fitting with a reversible redox reaction. However, the voltage efficiency reached for this potential limit is remarkable low, 18.2%, which limit its application at such high current density for this system. In order to increase this value it is mandatory to increase the reversibility of the positive redox reaction, as well as reduce the electrode’s polarization. On the bright side, the Coulombic efficiency performed is above 95%, exactly 99.1%. In consequence, there will not have significant electrolyte degradation to stop the process. It can be appreciated in Figure 7.38 showing several charge discharge plots without degradation. However, as it is also shown, the voltage efficiency is the limiting factor of the process. One of the main reasons is the large electrode polarization increasing the ohmic drop, which limits the battery to perform just an 18% of energy efficiency. 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.5 V 0.55 V 0.6 V 1.0 V 99.1 18.2 18.0 91.7 29.6 27.1 66.3 23.7 15.7 47.2 10.6 5.00 25 mA/cm2 25 mA/cm2 GFpristine CE/% VE/% EE/% cut off 0.5 V cut off 0.55 V cut off 0.6 V cut off 1.0 V 0.550 V cut off potential 0.6 0.4 0.2 0.0 -0.2 -0.4 E E E 600 800 cell we ce 0 5 10 15 20 25 30 Capacity / mAh E vs. Ag/AgSO4 / V E/V Figure 7.38.- a) Single cell charge-discharge plots of different cut off potentials (0.5, 0.55, 0.6 and 1 V) vs. capacity at a current density of 25 mA/cm2. b) CE, VE and EE for the different cut off potentials at a current density of 25 mA/cm2. c) Single cell charge-discharge plot of the potential with a cut off 0.55 V vs. time for the working electrode, counter electrode and the difference between these two at a current density of 25 mA/cm2. Additionally, the charge-discharge of the single cell is repeated at lower current density (10 mA/cm2) to verify if it is possible to increase the low voltage efficiency in this regard, as seen in Figure 7.39. Certainly, the voltage efficiency is highly increased, from 18 to 56%, reaching values close to the electrolyte configuration showed before with p-hydroxibenzene as positive active material. Moreover, the capacity is also increased from 3.4 to 15 mAh for the optimum 0.5 V as cut off voltage. However, the coulombic efficiency decreases abruptly when the current density applied is lowered, from 99 to 87%, an indicator of non reversible process increasing during cycling. As previously said, it may be caused by side reactions 0 200 400 time / s 194PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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