Stability of Highly Soluble Ferrocyanides at Neutral pH for Energy Dense Flow Batteries

Abstract

Ferro- and ferricyanides are among the most employed positive electrolyte materials in aqueous flow battery research but the limited solubility of commonly available sodium and potassium salts is a critical factor limiting application at scale. We systematically study the cation dependent solubility of these materials and, importantly, the cation dependent stability of the anion in aqueous solution. For Li4Fe(CN)6 we report a maximum solubility of 2.3 M and show stable cycling of a 2 M symmetric cell (54 Ah L−1) over 200 days, and also demonstrate solubilities of 1.6 M for ammonium and calcium salts. Importantly, we observe cation dependent electrolyte stability in long-term experiments and investigate the pronounced anion decomposition observed in presence of ammonium cations.[1] While achieving high solubilities leads to increased energy densities, concentrated electrolytes present many challenges such as high osmotic strength and high viscosity. This is particularly pronounced in electrolytes that employ ions with high charge states such as ferrocyanide. Furthermore, the selected cation needs to be compatible with the membrane in the cell.[2] We thus outline several often neglected design aspects that must be considered when developing new active materials for aqueous flow batteries. [1] Reber, D., Thurston, J.R., Becker, M., Marshak, M.P., Stability of highly soluble ferrocyanides at neutral pH for energy dense flow batteries, Cell. Rep. Phys. Sci. 2022, 4, 101215 [2] Waters, S.E., Thurston, J.R., Armstrong, R.W., Robb, B.H., Marshak, M.P., Reber, D., Holistic design principles for flow batteries: Cation dependent membrane resistance and active species solubility J. Power Sources 2022, 520, 230877