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The need for large scale energy storage has become a priority to integrate renewable energy sources into the electricity grid. Redox flow batteries are considered the best option to store electricity from medium to large scale applications. However, the current high cost of redox flow batteries impedes the wide spread adoption of this technology. The membrane is a critical component of redox flow batteries as it determines the performance as well as the economic viability of the batteries. The membrane acts as a separator to prevent cross-mixing of the positive and negative electrolytes, while still allowing the transport of ions to complete the circuit during the passage of current. An ideal membrane should have high ionic conductivity, low water intake and excellent chemical and thermal stability as well as good ionic exchange capacity. Developing a low cost, chemically stable membrane for redox flow cell batteries has been a major focus for many groups around the world in recent years. This paper reviews the research work on membranes for redox flow batteries, in particular for the all-vanadium redox flow battery which has received the most attention.

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The effect of bromine complexing agents, N‐ethyl‐N‐methyl‐morpholinium bromide (MEM) and N‐ethyl‐N‐methyl‐pyrrolidinium bromide (MEP) on the performance of different perfluorinated cation exchange membranes, ChiNaf and VF11, is assessed in the G2 vanadium bromide battery (V/Br). It is noted that in the absence of MEM and MEP, the thicker ChiNaf (50 μm) membrane shows a higher energy efficiency at a current density of 20 mA cm−2. A preliminary test of ChiNaf at two different concentration ratios of MEM and MEP evaluated at 4 mA cm−2 shows that a mixture of 0.19 M MEM and 0.56 M MEP provides a slightly higher cell efficiency than the other tested compositions. However, better cell performance is obtained with the VF11 (25 μm) membrane at 4 mA cm−2 using 0.19 M MEM and 0.56 M MEP. Further studies using VF11 with a mixture of 0.19 M MEM and 0.56 M MEP at 20 mA cm−2 provide direct evidence that the addition of MEM+MEP lowers the cell performance through a reduction in voltage efficiency. The increased membrane resistance after the introduction of MEM and MEP marks the formation of an organic layer after bromine reacts with MEM and MEP and is deposited onto the membrane.

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