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12.6 (94) at 150, 200, 300 and 25 mA/cm2. The efficiencies values remain constant, achieving CE~96% for charge/discharge rates applied and EE values attain up to 67.5% at 200 mA/cm2. It is clearly observed an efficiency decrease related to the concentration as it is expected due to an increasing polarization of the ionic species solvated.3 Finally, based on all the presented results, the mechanistic interpretation is proposed for all electrodes used (Figure 6.21). Firstly, V3+ ions interact with the hydroxyl groups on the graphite felt surface, generating O-V bonds, which extract a proton to the solution. Consequently an electron is donated to the V3+ reducing it to V2+ and breaking the previously formed bond (O-V) and taking the proton again from the solution. In this case, this redox reaction competes with the hydrogen evolution reaction as the protons are easily adsorbed to the C surface to form an H-H bond and lastly be desorbed. The last reaction is kinetically more favourable causing imbalance and low performance, for which we look for a catalyst to inhibit HER and enhance the V3+/V2+ redox reaction. Secondly, after the coverage with TiO2 there are two conditions that favour the V3+ reduction: i) Increase of –OH groups due to the presence of titanium dioxide itself that promote the number of active sites for the reaction to happen, enhancing the electron charge transfer at the interface electrode/vanadium. ii) Highly stable proton absorption over the Ti-O bond inhibiting side reaction as HER. However, large electrode polarization is dominant at high rate capability leading to a limited specific capacity and a modest electrolyte-utilization ratio. Thirdly, the partially reduced sample (TiO2:H) favours the V3+/V2+ due to an increment in electron mobility as an increasing hydroxyl active sites are present on the surface, and therefore, more vanadium ions are capable of react simultaneously. 104PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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