Long-term stability of ferri-/ferrocyanide as an electroactive component for redox flow battery applications: On the origin of apparent capacity fade

We assess the suitability of potassium ferri-/ferrocyanide as an electroactive species for long-term utilization in aqueous organic redox flow batteries. A series of electrochemical and chemical characterization experiments was performed to distinguish between structural decomposition and apparent capacity fade of ferri-/ferrocyanide solutions used in the capacity-limiting side of a flow battery. Our results indicate that, in contrast with previous reports, no structural decomposition of ferri-/ferrocyanide occurs at tested pH values as high as 14 in the dark or in diffuse indoor light. Instead, an apparent capacity fade takes place due to an electroless reduction of ferricyanide to ferrocyanide, via electroless oxygen evolution reaction. We find that this parasitic process can be further exacerbated by carbon electrodes, with apparent capacity fade rates at pH 14 increasing with an increased ratio of carbon electrode surface area to total amount of ferricyanide in solution. Based on these results, we report a set of operating conditions that enables the cycling of alkaline ferri-/ferrocyanide electrolytes, and demonstrate how apparent capacity fade rates can be engineered by the initial cell setup. If protected from direct exposure to light, the chemical stability of ferri-/ferrocyanide anions allows for their practical deployment as electroactive species in long duration energy storage applications.