Flow Battery Molecular Reactant Stability Determined by Symmetric Cell Cycling Methods

Abstract

We present an unbalanced compositionally-symmetric flow cell method for revealing and quantifying different mechanisms for capacity fade in redox flow batteries that are based on molecular energy storage. We utilize it, accompanied in some cases by a corresponding static-cell cycling method, to study capacity fade in cells comprising anthraquinone di-sulfonate, di-hydroxy anthraquinone, iron hexacyanide, methyl viologen, and bis-trimethylammoniopropyl viologen. In all cases the cycling capacity decay is reasonably consistent with exponential in time and is independent of the number of charge-discharge cycles imposed. By introducing pauses at various states of charge of the capacity-limiting side during cycling, we show that in some cases the temporal fade time constant is dependent on the state of charge. These observations suggest that molecular lifetime is dominated by chemical rather than electrochemical mechanisms. These mechanisms include irrecoverable chemical decomposition and recoverable interactions with cell materials. We conclude with recommendations for cell cycling protocols for evaluating stability of single electrolytes. Growing interest in redox flow batteries (RFBs) for grid-scale en-ergy storage has prompted development of new redox-active organic and organometallic molecules synthesized from commonly available raw materials containing earth-abundant elements such as carbon, nitrogen and oxygen. Flow batteries based upon these synthetic com-pounds might achieve lower electrolyte capital costs than current vanadium-based technologies without the scarcity risks associated with finite resources. 1 To become a cost-effective solution for match-ing of intermittent renewable energy supply to demand, these flow batteries should retain most of their capacity over decadal time scales. As opposed to the elements of the periodic table, e.g. transition metals, traditionally used for the redox-active components of flow batteries, these new organic and organometallic compounds may be susceptible to molecular decomposition, leading to an additional mechanism for flow battery charge storage capacity fade.