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Study of redox flow battery systems for residential applications during charging step and the reverse during discharging step or water and vanadium ions crossing from the positive tank to the negative tank during the charging step and from the negative to the positive tank during the discharging step. Figure D.10, in Appendix D.4, shows that during cycle 21 the volumetric crossover direction during the charging step was from the positive tank to the negative tank and during discharging it was from the negative tank to the positive tank. This volume changes were also reported by other authors [43, 76, 79, 85]. Thus, the discharging step was identified as the responsible for the net volumetric crossover. The volume change observed on each tank can be attributed mainly to water transfer since the mass transfer is from positive tank to negative tank during the charging step and the vanadium concentration increases on the positive tank. The water transfer can occur through osmosis, electro-osmosis and by water molecules being dragged by protons and vanadium ions across the membrane as hydration shell [78, 90-92]. The membrane water crossover as hydration shell of H+ takes the same direction of the electrons, which can explain why there is a volume change in different directions during the charging and discharging steps. Though, since the current density is the same for both steps and its durations are similar, electrolyte volume in each tank should not suffer significant change at the end of any cycle [43, 92]. On the other hand, since the vanadium concentration is higher on the positive side and osmosis occurs from the diluted to concentrated tank [81], it plays a key role in net volumetric crossover [92]. Agar et al. [53] reported that for a Nafion like membrane in a electrochemical cell operating at same charge and discharge current density, the osmotic crossover is always towards the positive tank regardless the cycle step. Thus, osmosis is identified as the main reason for net volumetric crossover to occur in this battery. The vanadium concentration increase on positive tank is related to the fact that vanadium ions are transported by water through osmosis [87]. Also, the presence of ion concentration gradient results in V2+ and V3+ ions crossing to the positive tank and VO2+ and VO2+ ions crossing to the negative tank. Since the diffusion rate on a cation exchange membrane is different for each ion (Figure 5.9), V2+ >> VO2+ > VO2+ >> V3+, ion crossover from the negative tank to the positive tank will be greater than the reverse direction [43, 87, 92]. The V2+ ions then react with VO2+ ions to form VO2+ ions and discharge the positive tank and since VO2+ diffusion rate is lower than V2+, an accumulation of vanadium ions on the positive tank is observed. Additionally, since VO2+ diffusion rate is higher than V3+, there will be a net crossover of VO2+ ions to the negative tank during pre-charging step which lead to concentration increase as it was observed. Convection and migration mechanisms were excluded as the main reason for net ion crossover as the pressure was always greater on the positive tank (Appendix D.4, Figure D.9) and operation current density was equal for charging and discharging steps [53]. Chapter 5: Results and discussion 36PDF Image | Tubular Vanadium Air Redox‐flow battery
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