# Ex-Situ Evaluation of Commercial Polymer Membranes for Vanadium Redox Flow Batteries (VRFBs)
## 3 Results and Discussion  
### Membrane Chemical Stability  
An accelerated chemical stability test was carried out to screen the selected membranes by soaking the membranes in a solution containing 1.6 M V\({}^{5+}\) and a total of 4.0 M SO\({}_{4}\)\({}^{2-}\) at 40 \({}^{\circ}\)C. During the 10 days of immersion, the soaking solutions were monitored by both visual observation of the colour change in V\({}^{5+}\) solution and UV-Vis spectroscopy. Figure 1 shows the photograph of the V\({}^{5+}\) soaking solutions with membranes as well as blank solution (1.6 M pure V\({}^{5+}\) fully understood solution without membrane) at 0 day, 1 day and 10 days. After membrane immersion for 1 day, the solutions with N212, AHA, AMV, CMV and VAN membranes began to turn dark, indicating the release of VO\({}^{2+}\) ions in the soaking solutions, while the solutions with DF, FS-930 and FAP-450 membranes retained their original colour. Generally, the colour of pure the VO\({}^{2+}\) ion and pure V\({}^{5+}\) ion electrolyte is yellow and blue, respectively. When the membrane is oxidized by V\({}^{5+}\) ions in the soaking solution, some V\({}^{5+}\) ions with a yellow colour are reduced to VO\({}^{2+}\) ions with a blue colour [27; 39]. The observed colour changes in some V\({}^{5+}\) solutions are possibly due to the generated blue colour from VO\({}^{2+}\) ions, suggesting that N212, AHA, AMV, CMV and VAN membranes started degradation in the first day. However, there was no obvious colour change in the solutions containing DF, FS-930 and FAP-450 membranes, indicating that these three membranes were more stable than the other selected membranes in the first 24 h. After 10 days, all the solutions went to further darken (mixture of yellow and blue) except DF and FAP-450 membranes, implying that more V\({}^{5+}\) ions were reduced upon reaction with the investigated membranes. In contrast, the colour of the soaking solutions of the vials containing DF and FAP-450 did not change with time, which was similar to that observed in the blank solution. This indicates that VO\({}^{2+}\) ions in the immersion solution with DF and FAP-450 membranes were not visible and the membrane degradation rate was lower than others or no oxidation reaction occurred between membranes and V\({}^{5+}\) electrolyte in these 10 days. Figure 2 shows a photograph of each membrane before and after the chemical stability test, proving that all of the investigated membranes survived without any cracking after 10 days of soaking in V\({}^{5+}\) solution at 40 \({}^{\circ}\)C. A colour change from light to dark for the AMV, AHA, FAP-450 and VAN membranes was noticed due to their absorbance of the electrolyte, while the colour changed from black to yellow for CMV, possibly ascribable to the oxidation reaction of V\({}^{5+}\) ions with CMV membrane. Contrastingly, N212, DF and FS-930 kept their original respective colours. comparison, the VO\({}^{2+}\) ion concentration in the solution containing the CMV membrane was two orders of magnitude greater than that with baseline membrane (N212) on the 10 th day of soaking. It revealed that the CMV membrane generated the largest amount of VO\({}^{2+}\) ions and showed the worst stability among the selected membranes. DF and FAP-450 membranes display the best stability followed by FS-930 and then N212 (baseline). It is worth mentioning that the change in VO\({}^{2+}\) concentration detected by UV-Vis for DF and FAP-450 membranes was insignificant, suggesting that there was very minor degradation for these two membranes in 10 days and being reflected by the lack of any colour change of the membrane in soaking solution, as shown in Figure 2. Moreover, VAN, AHA and AMV membranes were less stable than the baseline membrane. The chemical stability for the membranes is in the order of DF \(\approx\) FAP-450 \(>\) FS-930 \(>\) N212 \(>\) AMV \(>\) AHA \(>\) VAN \(>\) CMV.