Predicting the Solubility of Organic Redox Molecules in Non-Aqueous Flow Battery Electrolytes

Abstract

Large scale energy storage systems are essential to sustain an electrical supply grid which are powered by multiple energy conversion technologies including intermittent sources such as solar and wind energies. Redox Flow Batteries (RFB) is emerging as potential candidate for such a grid scale energy storage systems. Typically, the efficiency and energy density of the RFB depends on the operating voltage and solubility of redox active species in electrolyte system. The restriction in operating voltage associated with aqueous electrolytes fueled research towards non-aqueous organic electrolyte system which have wider electrochemical window. However, the solubility of redox active species in non-aqueous electrolyte system still remains as a major hurdle in realizing the commercial non-aqueous RFB technology. Apparently, the search for optimal electrolyte system and redox active species is actively pursued. In this regard, our research group developed a novel ferrocenium based ionic liquid as redox active species. The solubility of these ferrocenium species in traditional carbonate and/or ether based solvents depend on the solvation structure and dynamics of electrolyte system. To unravel the solvation structure and possible dissolution mechanism we carried out density functional theory (DFT) based calculations and verified the results with X-ray and Nuclear Magnetic Resonance (NMR) spectroscopic measurements. This powerful combination of theory and experimental measurements helped us to derive the solubility mechanism of redox active species in non-aqueous electrolyte system. Our efforts are focused towards developing a predictive level model which can aid in rational designing of optimal redox molecule and solvent systems for non-aqueous RFB technology.