# Ex-Situ Evaluation of Commercial Polymer Membranes for Vanadium Redox Flow Batteries (VRFBs)
## 4 Conclusions  
A membrane in a VRFB battery cell acts as a separator between the anode and cathode compartment to separate the active species, an electronic insulator, and an ionic conductor facilitating the transport of ions such as protons, or sulfate ions to maintain charge balance within the cell. For VRFB applications, the ideal membrane should have high ionic conductivity, low vanadium ion permeability, superior ion selectivity, good chemical stability, high mechanical stability and optimal water transportation properties as well as low cost. In this report, five membrane properties, including chemical stability, mechanical property, ion conductivity/ area resistance and vanadium ion crossover (ion selectivity), as well as membrane swelling property, were identified as critical factors to determine the selection of membranes for VRFB. Chemical stability for the selected membranes was evaluated in fully charged vanadium electrolyte (V5+ solution) and the membrane degradation rate was detected by the amount of VO2+ ions generated in V5+ solution and the changes in tensile strength before and after the chemical stability test. Of the membranes tested in this work, based on VO2+ concentration generated in V5+ solution, the CMV showed the lowest chemical stability, while FAP-450 and PFSA based membranes (DF, FS-930 and N212) showed excellent stability. However, according to the post-analysis of the tensile strength results, FAP-450 exhibited inferior stability. AHA and FAP-450 presented superior intrinsic tensile strength. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Membranes**} & **Dimensional Changes** & **Mass Changes (\%)** \\ \cline{2-3}  & **(Volume Changes) (\%)** & **Mass Changes (\%)** \\ \hline N212 & \(8\pm 4\) & \(6\pm 0.2\) \\ \hline FS-930 & \(3\pm 1\) & \(3\pm 1\) \\ \hline DF & \(6\pm 3\) & \(4\pm 2\) \\ \hline AMV & \(2\pm 2\) & \(9\pm 1\) \\ \hline CMV & \(0\pm 1\) & \(1\pm 1\) \\ \hline AHA & \(7\pm 1\) & \(8\pm 2\) \\ \hline FAP-450 & \(42\pm 8\) & \(57\pm 3\) \\ \hline VAN & \(2\pm 1\) & \(101\pm 6\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Dimensional changes and mass changes of membranes measured at 21 \({}^{\circ}\)C and RH 40% in 1.6 M VOSO4 solution (with 2 M H2 SO4). Amongst the identified five membrane properties for membrane screening, ion selectivity is the most important factor on VRFB performance. The outstanding ion selectivity implies the best balance between ion conductivity and vanadium ion permeability, possibly leading to the greatest VRFB performance. AMV, CMV, AHA, FAP-450, and DF showed higher ion selectivity. For the dimensional changes, all of the membranes are comparable, except FAP-450. To compare these eight (8) commercial membranes for VRFB application, each membrane candidate was ranked according to their ex-situ characterization results and is shown in Table 6. The best membrane for a certain property gets a rank of #1, counting one (1) point, and the worst gets a rank of #8, counting eight (8) points, which means that a higher point indicates that a membrane is less suitable for a VRFB. The overall property score for each membrane is the sum of the points of each property as an overall performance indicator to screen the membranes for VRFB applications. Due to the negligible difference in dimensional changes among the membranes (less than 10%), the point for all membranes in this property is treated the same as one (1) point, except FAP-450. Judging from the overall property score in Table 6, the fluorinated PFSA membranes (Nafion N212, Fumapem FS-930, and Dongyue DF) and the hydrocarbon AEMs (Selemion AMV, Neosepta AHA, and Fumapem FAP-450) are recommended as membrane candidates for further in-situ VRFB evaluation. On the other hand, VANADion(tm-20 VAN and Selemion CMV membranes are not recommended for VRFB applications based on the ex-situ characterization results in this report.