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the surface of the carbon felt electrode. The composite electrode composed of graphene and catalysts was developed and evaluated in the VRFB. In parallel, the feasibility of metal-decorated CNT, N-doped CNT, and carbon nano-walls were also examined with a view to their potential as electrodes for the VRFB. 2013 and 2014 were the most promising years in terms of the electrode research, with approximately 54 articles published. Graphene or graphene oxide (GO) were mainly investigated as an electrode or catalyst for the VRFB, and the graphite felt electrode decorated with various metal oxides (e.g. Nb2O5, WO3, CeO2, and PbO2) showed attractive electrochemical performance. Research on the electrochemical performance of the CNT/CNF grown carbon felt electrode was also reported in this period. Moreover, it was highlighted that Bi decoration could notably improve the reactivity of the carbon felt electrode towards redox reactions of V ions. On the other hand, biomaterials were proposed as a new type of catalyst, which can be easily adopted as the electrode for the VRFB. Accordingly, since pristine carbon materials offer a poor kinetic reversibility, they require functionalization treatments in order to guarantee highly efficient VRFB systems to operate at a large (charge/discharge) current density (> 25 mA/cm2).Although the various explained approaches that modifies the battery’s electrodes have provided some enhancement, the poor performance of the electrochemical reaction that takes place in the negative side of the battery remains a challenge, reducing efficiency and stability in VRFB systems166,167. (6.1) Furthermore, taking into account the reduction standard potential values for VRFB negative reaction (2.17) compare to hydrogen-evolution reaction (HER) (6.1), makes the last one thermodynamically favourable, as it has been demonstrated in section 4.1, Figure 4.5c. However, individual studies of electrochemical processes on the negative electrodes demonstrated that HER and VRFBs negative reaction occur simultaneously168. As a consequence, HER directly competes with V3+/V2+ reaction in VRFB, causing imbalances, an increment in the overall cell polarization and performance losses over cycling. Additionally, oxygen evolution reaction (OER) also could take place as a side reaction in all-vanadium redox flow batteries. However, due to the highly acidic working conditions it is not thermodynamically favoured, as well as following Le Chatelier's principles. Besides, the OER (6.2) potential is positively shifted related to the positive reaction in VRFBs (2.18). Carbon based electrodes and also polar plates are subjected to corrosion within an oxygen evolution reaction, in order to avoid it experimentally the cut off potential should be set below 1.9 V. (6.2) 80PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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