110th Anniversary: The Dehydration and Loss of Ionic Conductivity in Anion Exchange Membranes Due to FeCl₄– Ion Exchange and the Role of Membrane Microstructure
McGrath, Michael J. | Patterson, Nicholas | Manubay, Bryce C. | Hardy, Samantha H. | Malecha, John J. | Shi, Zhangxing | Yue, Xiujun | Xing, Xing | Funke, Hans H. | Gin, Douglas L. | Liu, Ping | Noble, Richard D.
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 H₂O/mol ion exchange site) upon exposure to ferric chloride/hydrochloric acid solutions were studied. Raman spectroscopy and iron sorption measurements show that FeCl₄– 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 FeCl₄– is the dominant mechanism for membrane dehydration. The ionic resistivity of the AEMs containing FeCl₄– increased by as much as 4 orders of magnitude due to dehydration and, at high FeCl₃ 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 FeCl₄– sorption, and a correspondingly greater dehydration and resistivity increase in dilute FeCl₃ solutions.Show more [+] Less [-]