Long-Term Stability of Ferri/Ferrocyanide As an Electroactive Component for Redox Flow Battery Applications: On the Origin of Apparent Capacity Fade

Abstract

The attraction of aqueous organic redox flow batteries (AORFBs) lies in the potential for low mass-production cost and long lifetime of the organic molecules. To reach cell potentials >1.0 V, several AORFBs have employed the ferri/ferrocyanide redox couple as posolyte in alkaline conditions. Recent works have reported significant amounts of capacity fade of this redox couple at high pH, attributed either to chemical decomposition associated with cyanide ligand dissociation and irreversible hydroxylation of the iron complex [1,2], or due to cell unbalancing associated with electrochemical oxygen evolution reaction (OER) [3].We assess the chemical and electrochemical stability of ferri/ferrocyanide utilizing a volumetrically unbalanced, compositionally symmetric cell method [4]. A series of electrochemical and chemical characterization experiments was performed to distinguish between “real” capacity fade (redox-active is structurally damaged) and “apparent” capacity fade (redox-active remains structurally intact), when ferri/ferrocyanide electrolytes are used in the capacity-limiting side of a flow battery. Our results indicate that, in contrast with previous reports [1,2], no chemical 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 OER. We find that this parasitic process can be further enhanced 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 further 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 at alkaline pH values up to at least 14.