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Study of redox flow battery systems for residential applications Table 3.1 - Side-reactions, for each half-cell, of vanadium active species with mobile vanadium species. Adapted from [43, 53, 54]. VO2+ + 2V2+ + 4H+ VO2+ + V3+ 2VO2+ + V2+ + 2H+ 3V3+ + 2H2O 2VO2+ 3VO2+ + H2O Reaction location Side reaction Negative half-cell Negative half-cell Negative half-cell Positive half-cell Positive half-cell VO2+ + V2+ + 2H+ 2V3+ + H2O VO2+ + V3+ 2VO2+ Another issue related to the IEMs is the water osmosis. Water transfer is typically observed in all type of membranes but with different behaviour. On a CEM, the net volumetric transfer is towards the positive tank, whereas with an AEM the net volumetric transfer is towards the negative tank. This leads to electrolyte dilution on one half-cell tank and increase of concentration on the other. The same can cause vanadium precipitation thus decreasing the performance and capacity of the battery [29, 38]. 3.1.2 Hydrogen and oxygen evolution Other side-reactions that may occur are the hydrogen and oxygen evolutions at the negative and positive half-cells, respectively: 2H+ + 2e- H2 2H2O O2 + 4H+ + 4e- Since the negative half-cell reaction has a lower reduction potential than the hydrogen reduction reaction, hydrogen evolution and V3+ reduction occur at the same time [55]. On the other hand, oxygen evolution only happens when the battery is overcharged, since it requires a lower potential than VO2+ oxidation potential [22, 54]. Hydrogen evolution not only changes the pH, but may also interrupt electrolyte flow, increase the cell resistance [22] and decrease the coulombic efficiency by consuming part of the charging current and by covering some active areas for the redox reactions [54]. This side- reaction also causes ionic imbalance which cannot be recovered by remixing electrolyte. The oxygen evolution also causes performance decrease on the battery since it oxidizes the electrode [22, 54]. Both reactions also lead to solution instability followed by precipitation of the positive electrolyte since the solution becomes more concentrated, contributing to capacity fade [29]. Chapter 3: Capacity fade and state of charge in VRFB 15PDF Image | Tubular Vanadium Air Redox‐flow battery
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