Symmetric Cell Study of Ferri/Ferrocyanide Stability As a Flow Battery Active Material

The attraction of aqueous organic redox flow batteries (AORFBs) lies in the potential for low mass-production cost and long lifetime of the organic reactants. To reach a cell potential greater than 1.0 V, several AORFBs have employed the ferri/ferrocyanide redox couple as a positive electrolyte (posolyte) in an alkaline full cell. Recent works [1,2] have reported significant amounts of capacity fade of this redox couple at high pH, attributed to an apparent chemical decomposition associated with cyanide ligand dissociation and subsequent irreversible hydroxylation of the iron complex. An in-depth analysis of the chemical and electrochemical stability of ferri/ferrocyanide is performed utilizing a volumetrically unbalanced, compositionally-symmetric cell method [3]. The dependence on pH of the apparent interaction of carbonaceous electrodes with active species will be reported, and keys to suppressing electrolyte degradation will be discussed. The accuracy of electrochemical techniques (e.g. high-precision coulometry) and chemical characterization (e.g. NMR, UV-vis) to understand ferri/ferrocyanide degradation will also be compared. Collectively, these results will highlight opportunities for improving energy storage systems. [1] J. Luo, A. Sam, B. Hu, C. DeBruler, X. Wei, W. Wang, and T.L. Liu, “Unraveling pH dependent cycling stability of ferricyanide/ferrocyanide in redox flow batteries”, Nano Energy 42, 215 (2017). [2] J. Luo, B. Hu, C. Debruler, Y. Bi, Y. Zhao, B. Yuan, M. Hu, W. Wu, and T.L. Liu, “Unprecedented Capacity and Stability of Ammonium Ferrocyanide Catholyte in pH Neutral Aqueous Redox Flow Batteries”, Joule, in press (2018). [3] M-A. Goulet, M.J. Aziz, “Flow Battery Molecular Reactant Stability Determined by Symmetric Cell Cycling Methods”, Journal of the Electrochemical Society 165, A1466 (2018).