# Ex-Situ Evaluation of Commercial Polymer Membranes for Vanadium Redox Flow Batteries (VRFBs)
###### Abstract  
Polymer membranes play a vital role in vanadium redox flow batteries (VRFBs), acting as a separator between the two compartments, an electronic insulator for maintaining electrical neutrality of the cell, and an ionic conductor for allowing the transport of ionic charge carriers. It is a major influencer of VRFB performance, but also identified as one of the major factors limiting the large-scale implementation of VRFB technology in energy storage applications due to its cost and durability. In this work, five (5) high-priority characteristics of membranes related to VRFB performance were selected as major considerable factors for membrane screening before in-situ testing. Eight (8) state-of-the-art of commercially available ion exchange membranes (IEMs) were specifically selected, evaluated and compared by a set of ex-situ assessment approaches to determine the possibility of the membranes applied for VRFB. The results recommend perfluorosulfonic acid (PFSA) membranes and hydrocarbon anion exchange membranes (AEMs) as the candidates for further in-situ testing, while one hydrocarbon cation exchange membrane (CEM) is not recommended for VRFB application due to its relatively high VO2+ ion crossover and low mechanical stability during/after the chemical stability test. This work could provide VRFB researchers and industry a valuable reference for selecting the polymer membrane materials before VRFB in-situ testing. m +
Footnote †: journal:  
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a premiere commercial perfluorosulfonic acid (PFSA) membrane used in VRFBs, which is a cation exchange membrane (CEM) discovered in the late 1960s by [PERSON] of DuPont and currently is a trademark of Chemours [18]. A large number of research works have been published recently to compare the performance of these traditional Nafion membranes in VRFB application [19; 20; 21; 22]. [PERSON] et al. [20] installed Nafion(r) N115, Nafion(r) NR-212, and Nafion(r) NR-211 into 3-cell 1 kW stacks to evaluate their VRFB performances. The thinnest membrane, NR-212, is advisable for use at high current densities to serve the needs for a VRFB such as reduced cost, ease of handling, and high performance. A series of Nafion membranes, N112, N1135, N115, and N117, were also evaluated and compared by [PERSON] et al. [21]. They reported that N115 was the most suitable for VRFB applications due to its favourable trade-off between vanadium ion crossover and membrane resistance. The VANADion membrane is a brand-new composite PFSA membrane developed by Union Chemical Industrial Co., Ltd., for VRFB application. Compared to N115, [PERSON] et al. claimed that the VANADion membrane is a promising membrane candidate for VRFB due to its better energy efficiency (EE), higher electrolyte utilization and similar capacity retention [23]. However, all the PFSA membranes exhibit vanadium crossover and high cost issues regardless of their thickness, ion-exchange capacity (IEC) and reinforcement. Therefore, considerable efforts have been devoted to investigating hydrocarbon-based materials as an alternative solution to PFSA-based membranes driven by cost-effective mass production in the marketplace [9; 10; 11; 12; 13; 14; 15; 16; 17]. It is thought that hydrocarbon-based anion exchange membranes (AEMs) could suppress vanadium permeability due to the Donnan exclusion effect [12]; however, their chemical and mechanical stability in VRFB operational conditions and lackluster ionic conductivity make their viability questionable [24; 25]. Hydrocarbon-based membranes were primarily developed for fuel cell applications; as such, there are limited publications about the use of commercial AEMs in VRFBs. [PERSON]'s research group has extensively studied the chemical stability of the commercially available AEMs including New Selemion (type 2), Selemion AMV, New Selemion Type 3H and a sample from Tokuyama [26; 27].