Nowadays, the charge-discharge test progress. But it showed that the discharge capacity is reduced rapidly as the cycle progresses in vanadium redox flow battery. So i don`t know how to do. Thank you.
If you are running the tests under the same conditions (temperature and discharge current) then it seems like the battery is deteriorating; its internal resistance increases and as a result the losses are high and efficiency is low.
There isn't a lot of information here to tell you exactly what's going on. Is it simply the charge capacity that is fading? Or is it the cell current/voltage?
A likely situation is if your electrolyte was balanced initially, you could be unbalancing it by running some H2 evolution on your negative side. What that would do, is that with every cycle your negative side will be incompletely charged, and upon discharge, your positive side will retain some charge. On the next cycle, you'll have an incomplete charge of the 2/3 again, and after discharge your 4/5 side will become more charged. At least that's what you'd get if you use voltage as the cutoff point. If you charge your battery close to 100% SOC, these kinds of issues becomes very likely. An easy way to tell if this is the case is to look at the SOC of each electrolyte with your battery fully charged or fully discharged.
Its primarily because of the permeability of positively charged vanadium ions through negatively charged membranes (Here Nafion e.g.). This permeation of Vanadium ions can not be eliminated, however, greatly suppressed by using anion exchange membranes.
other factors such as membrane degradation, unbalancing of electrolyte by H2 evolution (occurs due to the difference in the vanadium ions diffusion rate) also influence the capacity fading.
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