Ferrocyanide Electrolytes Unsuitable for Symmetric Flow Batteries Due to Decomposition

Ferrocyanide in Flow Batteries

The ferrocyanide/ferricyanide redox couple ([Fe(CN)6]4−, Fe(CN)6]3−) is frequently employed in flow batteries. Its popularity stems from high redox stability, favorable kinetics, significant solubility (0.7-1.2M), and a redox potential of +0.22V (vs saturated Ag/AgCl), with stability under high pH conditions.

Absence in Symmetric Systems

Despite its advantages, ferrocyanide salts are not used in symmetric flow battery systems. These systems start with the same electrolyte in both the catholyte and anolyte, with redox reactions occurring from this mixed state. Common examples include ZnBr2, ZnI2, Vanadium, and Fe systems using FeCl2 or FeSO4 salts.

Initial Challenges: Insoluble Precipitates

One initial hurdle is that ferrocyanide forms insoluble substances, such as Prussian blue, with most heavy metal cations. This would prevent metal reduction in the anolyte, as seen with Zn2+ to Zn metal or Fe2+ to Fe metal, due to solid precipitation. While this issue can be mitigated using pyrophosphates or strong chelating agents, it does not resolve the core problem.

Decomposition at Low Potentials

The primary reason ferrocyanide is unsuitable for symmetric systems is its decomposition at the anode under low reducing potentials. This process forms some Fe metal and releases free cyanide. Studies of ferrocyanide solutions under reducing potentials show that ferrocyanide is not stable below approximately -0.5V, with clear cathodic peaks indicating decomposition after repeated cycling to negative potentials.