Aqueous redox flow batteries are generally adapted from polymer electrolyte membrane fuel cells that use proton exchange membranes to balance the charge of the cell. Such membranes are well understood and exhibit excellent performance in acidic electrolytes via transport of protons. For neutral pH flow batteries however, the primary cation in solution is not H⁺, but the cation of the active material or supporting electrolyte salt.

The nature of the cation in solution, e.g., hydrated ionic radius and charge density, significantly affects the transport resistance through the membrane and thus the area specific resistance of the cell, a critical parameter that needs to be minimized for high power applications such as flow batteries. Additionally, different cations result in vastly different solubilities of active materials based on redox active anions such as ferrocyanides; the ammonium salt for example is ca. three times more soluble than the sodium salt (1.6 M vs. 0.5 M).^[1] It is therefore critical to not only maximize the concentration of an electrolyte and thus its capacity, but to also choose cations that enable efficient transport across the membrane for efficient flow battery development.

Here, we study the cation dependent resistance of Nafion^TM 212 cation exchange membranes for 12 different alkali and alkylammonium cations and determine the solubilities of 13 ferrocyanides as well as 35 metal-organic chelates based on aminopolycarboxylate ligands. The latter are a particularly promising family of compounds for application in negative electrolytes in near neutral pH aqueous flow batteries with demonstrated discharge voltages as high as 2.1 V and excellent power performance.^[2] We further discuss cation-compatibility for mixed cation electrolytes as well as membrane pretreatments that affect the cells area specific resistance and active species crossover.^[3]

References:

^[1] J. Luo, B. Hu, C. Debruler, Y. Bi, Y. Zhao, B. Yuan, M. Hu, W. Wu, T.L. Liu, Unprecedented capacity and stability of ammonium ferrocyanide catholyte in pH neutral aqueous redox flow batteries, Joule 2019, 3, 149–163.

^[2] B.H. Robb, J.M. Farrell, M.P. Marshak, Chelated chromium electrolyte enabling high-voltage aqueous flow batteries, Joule 2019, 3, 2503–2512.

^[3] S.E. Waters, J.R. Thurston, R.W. Armstrong, B.H. Robb, M.P. Marshak, D. Reber, Holistic design principles for flow batteries: Cation dependent membrane resistance and active species solubility, J. Power Sources 2022, 520, 230877.