Membrane Screening for Iron–Chrome Redox Flow Batteries

Since the electrolyte in an iron chrome redox flow battery (ICRFB) is inexpensive, the cost of the separator can contribute up to 38% of the CapEx cost of an ICRFB. Since the membrane also influences the RFB performance, it is the aim of this study to screen various commercial ion‐exchange membranes (IEMs) and a microporous separator (MPS) in an ICRFB to identify possible alternative membranes to the currently used Nafion. The suitability of six cation (CEMs) and two anion exchange membranes (AEMs), as well as one MPS, is investigated. No discharge curves are attained with either of the AEMs, which probably result from the formation of an anionic FeCl4− species at the elevated operating temperatures (65 °C) used, confirming literature on the unsuitability of AEMs for ICRFBs. Similarly, although the MPS is stable in the ICRFB electrolyte, it yields a high capacity decay ascribed to excessive crossover. Whereas all six CEMs yield similar CE, VE, and EE values, the fumatech FS‐950 yields a comparable capacity decay but higher EE and capacity discharge than the currently used Nafion 117 counterpart. Due to the significant cost reduction, modified or customized MPS should be further evaluated. Alternative membranes for iron chrome redox flow batteries are investigated including cation and anion exchange membranes (AEMs) and microporous separators (MPS). AEMs are unsuitable due to the formation of an anionic FeCl4− species at 65 °C. FS‐950 shows comparable capacity decay but higher energy efficiency and discharge capacity than Nafion 117. Modified MPSs should be further investigated.