# Ex-Situ Evaluation of Commercial Polymer Membranes for Vanadium Redox Flow Batteries (VRFBs)
## 3 Results and Discussion
### Membrane Mechanical Property  
Sufficient mechanical strength is one of the essential requirements for VRFB membranes, because these membranes suffer different stresses, from battery assembly to VRFB operation, including the high cell assembly pressure, various mechanical shocks, and dimensional changes from dehydration (dry) to hydration (wet) conditions. Most notably, membranes are required to maintain their mechanical strength in a long-term VRFB application to tolerate the harsh chemical environments (high concentration of supporting electrolyte (H\({}_{2}\)SO\({}_{4}\)) and highly oxidative reactivity of the electrolyte species).  
The intrinsic tensile strengths of the investigated membranes were measured before the chemical stability test at room temperature (RT, 21 \({}^{\circ}\)C) and 40% relative humidity (RH) and are summarized in Table 2. The PFSA membranes (N212, DF, FS-930), AMV and CMV displayed similar peak stress (\(\sim\)30 MPa), indicating that the intrinsic mechanical properties of these membranes are close. In contrast, VAN as a PFSA-based composite membrane exhibited a lower tensile strength than other PFSA membranes due to its unique dual-layered structure [23]. A thin, dense Nafion layer (\(\sim\)20 \(\upmu\)m) coated on a thick microporous layer (\(\sim\)210 \(\upmu\)m) allowed VAN to withstand a superior peak load, but relatively low peak stress. In addition, two hydrocarbon membranes, AHA and FAP-450 showed slightly higher peak stress (\(\sim\)40-50 MPa). Before the chemical stability test, the order of strength of the membranes is as follows: AHA \(\geq\) FAP-450 > N212 \(\approx\) DF\(\approx\) FS-930 \(\approx\) CMV \(\approx\) AMV > VAN.  
Figure 3: Membrane chemical stability carried 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 for 10 days. **(a)** Concentration of V\({}^{4+}\) ions in the electrolyte solutions with time for all the investigated membranes; **(b)** Zoomed-in curves of **(a)** without CMV and VAN; **(c)** Concentration of V\({}^{4+}\) ions in the electrolyte solutions normalized by the mass of the membrane against time; **(d)** Zoomed-in curves of **(c)** without CMV and VAN.  
After 10 days' chemical stability test, all the tested membranes were visibly in good shape, which makes it difficult to identify the degradation done to the membranes. Therefore, the mechanical strength tests for the investigated membranes were conducted after the accelerated chemical stability test to evaluate the chemical/mechanical stabilities of the membranes. The ultimate peak stress losses of the selected membranes after chemical stability testing are presented in Table 2. It revealed that the FAP-450 membrane showed the highest tensile strength loss (40%) after chemical stability testing, although the changes of VO\({}^{2+}\) concentration in the soaking solution for the FAP-450 membrane were imperceptible. The AHA membrane displays the lowest mechanical property loss (4%) followed by VAN and AMV. All the PFSA dense membranes had -20% tensile strength losses after soaking in V\({}^{5+}\) solution for 10 days, which presented a lower mechanical stability than that of hydrocarbon membranes. The thickness of the membrane samples is a factor possibly affecting the mechanical stability of the membranes due to the extended time that the V\({}^{5+}\) ions attack the backbone of the inner polymer in thicker membranes. As one of the indicators for membrane chemical stability, the mechanical property loss of the investigated membranes after soaking in V\({}^{5+}\) solution for 10 days is in the order of FAP-450 \(>\) CMV \(>\) DF \(>\) N212 \(>\) FS-930 \(>\) AMV \(>\) VAN \(>\) AHA.