Abstract
Ferrocyanides 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. Here, 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, corresponding to electrolyte capacities of 54 Ah L−1, over 200 days. We also demonstrate solubilities of 1.6 M for ammonium and calcium salts and investigate the pronounced anion decomposition observed in the presence of ammonium cations. Our observations lead to a discussion of challenges associated with the osmotic strength of concentrated electrolytes that employ ions with high charge states, such as ferrocyanide.
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