Alkaline flow batteries have attracted much attention as an alternative to vanadium electrolyte. All alkaline chemistries reported so far are based on the use of potassium ferrocyanide (K4Fe(CN)6) as active species in the positive compartment in combination with electroactive organic compounds in the negative one. Since potassium ferrocyanide is the only material found to be active for the positive compartment, the intrinsic electrochemical properties of this species has become of high scientific interest.
Cycling stability is one of the most important parameters of the active species for redox flow batteries. Recently, the cycling stability of the ferro-ferricyanide couple has been studied. The authors concluded that ferrocyanide was not stable at strong alkaline media releasing free cyanide and causing a rapid capacity fading in the battery. These results have generated an intensive debate in the community since they challenge all the existing alkaline redox flow batteries formulations.
Herein*, the stability of ferrocyanide in alkaline media is critically re-evaluated and conclude that ferrocyanide in strong alkaline media is not unstable as previously proposed. Actually, the capacity losses that were interpreted as instability of the ferro-ferricyanide couples were caused by an unbalance in the state of charge of the positive and the negative electrolyte due to the evolution of oxygen in the positive side during battery charging.
(*) Páez, T., Martínez-Cuezva, A., Palma, J., Ventosa, E.
Revisiting the cycling stability of ferrocyanide in alkaline media for redox flow batteries
(2020) Journal of Power Sources, 471, art. no. 228453, DOI: 10.1016/j.jpowsour.2020.228453
More information: Jesús Palma firstname.lastname@example.org, Head of the Electrochemical Processes Unit.