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The Voltage efficiency, which is the ratio of charge and discharge voltage, follows the same trend as the Energy efficiency, a consequence of being the limiting factor of the battery performance. The Coulombic efficiency, the difference between charge and discharge capacity, shows similar values for all samples (Figure 6.35), above 95% for all current densities. The exception being for carbon felt above current density 100 mA cm-2, due to an enormous overpotential that causes a Coulombic efficiency drop to approximately a 20%. As HER increase, the overpotential does increase as well, which is confirmed in Figure 6.34d, causing critical imbalances between charge and discharge process that ultimately makes the system to fail when CF is used as negative electrode. Moreover, Figure 6.34d shows the Voltage profiles between 0.8 and 1.8 V for carbon felt, CF@TiO2 nitride at 500oC and 900oC, applying increasing current densities. It is obvious the larger ohmic drop for CF than nitrided electrodes, especially low ohmic drop is observed for CF@TiO2 N900 capable of a lower polarization when high currents are applied. Considering the different charge-discharge profiles, it is clear the logarithmic tendency of the overpotential when increasing the current density, seen in Figure 6.34d. a) 1.8 1.6 1.4 1.2 1.0 0.8 c) b) 1.8 1.6 1.4 1.2 1.0 0.8 CF@TiO2 N500 1.8 1.6 1.4 1.2 1.0 0.8 CF@TiO2 N900 25 mA/cm2 37.5 mA/cm2 50 mA/cm2 100 mA/cm2 125 mA/cm2 150 mA/cm2 0 2 4 6 8 10 12 14 Specific Capacity / AhL-1 d) 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0 CF 0 2 4 6 8 10 12 14 Specific Capacity / A h L-1 0 2 4 6 8 10 12 14 Specific Capacity / A h L-1 CF CF@TiO2 N500 CF@TiO2 N900 20 40 60 80 100 120 140 160 j / mA cm-2 25 mA/cm2 37.5 mA/cm2 2 50 mA/cm 100 mA/cm2 25 mA/cm2 37.5 mA/cm2 50 mA/cm2 100 mA/cm2 125 mA/cm2 150 mA/cm2 E/V E/V Overpotential / V E / V Figure 6.34 – Charge-discharge voltage profiles between 1.8 and 0.8 V at different current densities (25 to 150 mA cm-2) for a) CF, b) CF@TiO2 N500 and c) CF@TiO2 N900. d) Overvoltage during charge period comparatively to Eocv. 126PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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