110th Anniversary: The Dehydration and Loss of Ionic Conductivity in Anion Exchange Membranes Due to FeCl 4 – Ion Exchange and the Role of Membrane Microstructure
Anion-exchange membranes (AEMs) often dehydrate and lose ionic conductivity in ferric chloride solutions used in all-iron or iron–chromium redox flow batteries (RFB). In this work, the change in material and ionic transport properties of three AEMs with similar concentrations of ion exchange sites and hydration numbers (mol H 2 O/mol ion exchange site) upon exposure to ferric chloride/hydrochloric acid solutions were studied. Raman spectroscopy and iron sorption measurements show that FeCl 4 – occupies a fraction of the AEM ion exchange sites that depends on the ferric chloride concentration in the external solution. The AEM hydration number is linearly proportional to the amount of iron sorbed in the AEMs, suggesting that the displacement of the original hydrated Cl – counterions for unhydrated FeCl 4 – is the dominant mechanism for membrane dehydration. The ionic resistivity of the AEMs containing FeCl 4 – increased by as much as 4 orders of magnitude due to dehydration and, at high FeC l3 solution concentrations (≥1.4M), also due to nonideal solution effects. Although the three AEMs have similar ion exchange site concentrations, the AEM with the closer local spacing of ion exchange sites exhibits higher FeCl 4 – sorption, and a correspondingly greater dehydration and resistivity increase in dilute FeCl 3 solutions.