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Study of redox flow battery systems for residential applications VO2+ of the positive tank and, consequently, the respective state of charge increase which further limits the amount of energy that can be stored in the negative tank as it was observed (Appendix D.3.1, Figure D.1). As vanadium concentration did not change drastically, the volumetric crossover can be justified by the hydration shell of VO2+ and VO2+ ions as [VO(H2O)5]2+ and [VO2(H2O)3]+, respectively, where water molecules are carried by vanadium ions (Figure 5.4) [43, 80-82] and so, despite the volume change, the vanadium concentration remained nearly constant. Figure 5.4 – Vanadium ion diffusivities (red>orange>yellow>blue), net crossover of vanadium ions and water for VisBlue 6 (FAP450 ion exchange membrane) battery after 35 cycles. Adapted from [77, 79, 83] Volterion GmbH reportedly used expanded graphite as bipolar plates for VisBlue 6 battery which may be permeable to electrolyte causing mass transport from one tank to the other. It was observed a decrease of potential efficiency, from 70 % to 68 %, which is in agreement with the fact that there is swelling of the bipolar plate, increasing ohmic resistance. The electrolyte crossover may be further enhanced by such swelling which causes pores to open, increasing permeability to electrolyte. Sulfate ions, transported through an anionic membrane by Grotthuss mechanism with the opposite direction to current, are used as conduction ions [79, 83] and water osmosis may occur due to sulfuric acid concentration gradient [82], though sulfate ions concentration was not measured and it may not be excluded as a cause for volumetric crossover. This should not be verified since charging steps were longer than discharging steps, causing more SO42- ions to be transferred from the negative to positive tank [78, 79, 84]. Chapter 5: Results and discussion 29PDF Image | Tubular Vanadium Air Redox‐flow battery
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