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electrodes at a current applied of 100 mA/cm2 has a Coulombic efficiency values that follows: CF-HT (96.6)> CF-rGO/Q (96.2)> CF-N (93.7). Although the three samples present closer values and above 95%, the voltage efficiency decreases when large current densities are handled. It is due to the redox reaction does not have enough time to take place. Energy efficiencies are limited by the voltage efficiency, and therefore has the same tendency of the values obtained (Figure 7.19d). The energy efficiency values for a current applied of 50 mA/cm2 have values around a 60%, following the same trend as for Coulombic and voltage efficiencies: CF-HT (59.1)> CF-rGO/Q (49.6)> CF-N (45.8). 100 80 60 40 20 100 80 60 40 20 100 mA/cm2 (cycle 1000) CF-HT CE / % 96.2 VE / % 43.2 EE / % 41.6 CF-HT CE VE EE 00 0 200 400 600 800 1000 cycle Figure 7.20.- Long term cycling in galvanostatic conditions for 1000 cycles of a 3 electrodes system with GF as counter electrode and Hg/Hg2SO4 as reference in a 0.05M Tiron 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 CF-HT electrode was tested for 1000 cycles in order to study its long term viability towards the battery application, as it is the one performing better towards the Tiron redox reaction. Those cycles were done at high current density (100 mA/cm2). As seen in Figure 7.20, the coulombic efficiency performed for the GF-HT electrode is high enough, around 96%, and remains constant during cycling. Moreover, Voltage efficiency and therefore Energy efficiency are quite stable, as the Voltage efficiency is the limiting factor. The GF-HT electrode boost the efficiency more than 10% compare with the other electrodes for similar currents, as seen in Figure 7.19. This improvement can be maintained along the life cycling with an energy efficiency of 41.6%. In brief, the treatments done on the cathode electrodes for aqueous organic redox flow batteries has the objective to increase the surface active area by electrode functionalization with nitrogen and oxygen groups, especially active are the oxygenated-N and C=O groups. These are more abundant on the CF-HT surface. Therefore the electrodes become more electrochemically active, as CV showed by an increase in the reversibility and mass transfer towards the Tiron redox reaction on CF-HT comparatively to the other two electrodes (CF-N and CF-rGO/Q). Moreover, the charge transfer resistance is reduced due to that functionalization. 174 Voltage Efficiency / % Coulombic Efficiency / %PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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