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the redox reaction does not have enough time to take place or does not have enough active sites due to low surface active groups. In this case the voltage efficiency trend at 200 mA/cm2 is (%): CF-rGO/Q (64.9) > CF-HT (51.2)> CF- rGO/ROH (38.3). This could be a consequence of the nitrogen and oxygen active functional groups over CF-rGO/Q are larger, as XPS shows (Figure 7.26). Especially larger amount of C-O, O-C=O, graphitic-N and pyrrolic-N are found on CF-rGO/Q surface compared with the other electrodes (Figure 7.25). Moreover, as the 1/Rct is also greater for CF-rGO/Q, when large current densities are applied these active groups facilitate the redox reaction on the electrode surface. Energy efficiency is limited by the Coulombic efficiency when low currents are applied, but by the Voltage efficiency when the current density it is high (Figure 7.30d). b) d) 100 mA/cm2 (cycle 1000) CF-HT CF-rGO/Q CF-rGO/ROH a) 100 100 80 80 60 60 40 40 20 20 00 0 100 200 300 400 500 600 700 800 900 1000 100 95 90 85 80 0 100 200 300 400 500 600 700 800 900 1000 c) 100 80 60 40 20 0 VE / % 69.6 80.3 63.4 EE / % 68.0 78.9 62.1 CF-HT CE VE CF-rGO/Q CE VE CF-rGO/ROH CE VE cycle cycle CE / % 97.7 98.2 97.9 CF-HT CF-rGO/Q CF-rGO/ROH CF-HT CF-rGO/Q CF-rGO/ROH 0 100 200 300 400 500 600 700 800 900 1000 cycle Figure 7.31.- Long term cycling in galvanostatic conditions for 1000 cycles of a 3 electrodes system with CF as counter electrode and Hg/Hg2SO4 as reference in a 0.05M Anthraquinone-2,7-disulfonic acid disodium 3 M MeSO3H solution at a current densities of 100 mA/cm2 displaying a)Coulombic and voltage efficiency (CE and VE). b) Coulombic efficiencies zoomed (CE). c) Energy efficiency (EE). d) Efficiency values after 1000 cycles for the electrodes used at current density of 100 mA/cm2. Finally, the electrodes were tested for 1000 cycles in order to study their long term viability towards the battery application. Those cycles were done at high current density, 100 mA/cm2. As seen in Figure 7.31, Coulombic efficiency for all the electrodes is high, above 95%, and remains constant during cycling. Besides, all samples perform in the range of 97-98% showing the lack of cross-over, OER and HER. Additionally, Voltage efficiency and therefore Energy efficiency are quite stable, as the Voltage efficiency is the limiting factor. The CF-rGO/Q electrode boost the efficiency more than 10% compare with the other electrodes. This improvement 186 Voltage Efficiency / % Energy Efficiency / % Coulombic Efficiency / % Coulombic Efficiency / %PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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