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leading to a limited specific capacity and a modest electrolyte-utilization ratio (SoC). Consequently, the partially reduced sample (CF@TiO2 N500) favours the V3+/V2+ due to a reduced charge transfer resistance as an increased number of oxygen active sites (-OH and -O), especially hydroxyl groups, are present on the surface (Figure 6.25), and therefore, more vanadium ions are capable of reaction simultaneously. Finally, for the sample where TiN is partially formed (CF@TiO2 N900) there are two conditions that favour the V3+ reduction229: i) Increase of –N groups on the carbon felt and over the TiO2 surface forming TiN, due to the nitride process that promotes the number of active sites, enhancing the electron charge transfer at the interface electrode/vanadium. ii) Increase of oxygen groups, especially hydroxyl electroactive sites (-OH) over the electrode surface, for which electron charge transfer is facilitated by the nitrogen groups, most favoured by pyrrolic-N230,231. 6.1.4 Cerium oxide (CeO2) In this section is studied the demonstration of ALD Ceria deposit over commercial Graphite Felt and its function as catalyst for the positive VRFB redox reaction (VO2+/VO2+) by itself (GF@CeO2) , as well as, after heat treatment under reductive atmosphere (GF@CeO2-x) for high performance all-vanadium redox flow batteries (VRFB) as a simple and eco-friendly strategy, well-suited for large-scale applications (Figure 6.37). Analogously to the previous section, where Hydrogen- treated rutile TiO2 shell in a graphite felt core (GF@TiO2:H) is used as negative enhanced electrode89. It is done to further improve the device, similarly to the negative reaction enhancement by this method, showen in previous section, the positive reaction is improved using a better electrode as the positive compartment would be now the limiting factor in the overall cell performance. Moreover, significant improvements in charge and electron transfer processes towards VO2+/VO2+ redox reaction is achieved using GF@CeO2-x electrodes due to the enhancement of CeO2 charge and electron transfer after hydrogen annealing. It is a consequence of the formation of oxygen vacancies in the lattice structure, similarly as the case of hydrogen treated TiO2. We have achieved a good quality performance for VRFB not only in terms of high capability rates (250 mAcm-2) but also the capacity retention achieved (60% at 200 mAcm-2). Additionally, high coulombic efficiency (CE, 98%) and attonishing stability, over 100 cycles at 200 mA/cm2, demonstrating the feasibility of achieving an outstanding long-term stability, using 1.5 M Vanadium ions in 3 M H2SO4. 131PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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