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8 Conclusions 1. A reference electrode based on silver/silver sulfate to be inserted in filter press cells of VRFB has been proposed. This easy-to-make reference electrode provides valuable information for “in-operando” conditions. For ammoxidized graphite felts, the hydrogen evolution at the negative electrode during the charge decreases the coulombic efficiency whereas the slower kinetics of the positive electrode contributes more to the decrease in voltage efficiency. The use of this easy-to-make reference electrode also opens new perspectives for the experimental routes allowing a fast, reliable and feasible way for proceeding in the study, analysis and control of VRFBs. 2. In order to overcome the drawbacks of VFRB related to thermal stability at 60oC, several charge and discharge experiments at 20 mAcm-2 were carried out to demonstrate the outstanding performance of the dPEI-containing positive electrolyte. Stable performance with an energy efficiency of 88% was observed during a course of 40 cycles. This is probably due to the stabilization of the V (V) ions carried out by the dPEI- containing electrolyte. The energy density at 60 o C was 19.5 W h L-1. Note this value is higher than that obtained at 25o C (ca.18 WhL-1). This is probably due to the water evaporation in each compartment leading to an increment of proton concentration. Therefore, a cost-effective formulation of dPEI-based positive electrolyte for VFRB was successfully demonstrated in term of the thermal stability and energy density. The highly improved energy capacity (ca. 18 WhL-1) and the excellent electrochemical performance (ca. 85 % of energy efficiency) over 60oC of operation temperature could boost the marketability of VRFB technology. 3. Along the experimental work done within the group, it has been accomplished much in terms of electrode modifications, obtaining the following conclusion points that summarize them: a) Titanium dioxide especially in its rutile (110) phase acts as a powerful electrocatalyst towards the VRFBs negative half-cell reaction. Increasing the active groups on the electrode surface and decreasing the side reactions taking place (HER). However, when the TiO2 thickness increases above certain limit the semiconductor properties enlarge the charge transfer resistance towards the vanadium negative redox reaction due to an ohmic drop. In spite that fact, kinetically the reaction towards V3+/V2+ is faster when the graphite felt is completely covered by the nanorods (GF-TiO2(MNRs)). In order to improve the electrodes performance, it would be relevant to improve the conductivity when the fibers are completely covered by titanium dioxide nanorods in its rutile phase. 208PDF Image | Redox Flow Batteries Vanadium to Earth Quinones
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