Effect of bromine complexing agents on the performance of cation exchange membranes in second-generation vanadium bromide battery

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 (50μm) membrane shows a higher energy efficiency at a current density of 20mA-cm^-2. A preliminary test of ChiNaf at two different concentration ratios of MEM and MEP evaluated at 4mA-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 (25μm) membrane at 4mA-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 20mA-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. Battery performance: Complexing agents affect the performance of both ChiNaf and VF11 membranes used in vanadium bromide redox flow cells owing to the formation of an organic layer on the membrane surface, which acts as an insulation barrier, and thus, increases membrane resistance. The VF11 membrane performs better than ChiNaf, with a higher coulombic efficiency and energy efficiency (see figure).