Microemulsion Electrolytes for Redox Flow Batteries

The work presented covers exploration of microemulsions as electrolytes for redox flow batteries (RFBs). Microemulsions are structured fluids comprised of oil, water, and emulsifiers that allow for the unique combination of nonaqueous redox species with aqueous supporting electrolyte.

We demonstrated the first microemulsion RFB using ferrocene (posolyte) and menadione (negolyte) in sodium dodecyl sulfate (SDS) and polysorbate 20 microemulsions. Ferrocene exhibited a quasi-reversible electron transfer reaction regardless of microemulsion composition. Electrochemical reversibility of menadione was, however, only possible in SDS microemulsions. Polarization curve analysis and electrochemical impedance spectroscopy revealed significant redox species and ion transport limitations through the porous electrodes and the membrane. This work highlighted the importance of understanding interactions between microemulsions and RFB cell components (porous electrodes, membrane), for improving performance for the next generation technology.

We further explored the role of surfactant in modulating kinetics and transport in porous electrodes using vanadium cations as redox species. Surfactant decreased vanadium volumetric transport rates by increasing the hydrophilicity of the electrode, and therefore, increasing the electrode-electrolyte interactions. Surfactant had a more complex eect on kinetics. Vanadium (IV/V) half-reaction kinetics decreased with addition of surfactant. Vanadium (II/III) half-reaction kinetics demonstrated a surprising dependence on surfactant chemistry and electrode type. These findings suggest surfactants could be used as an electrolyte additive to modify electrode properties.

A fundamental understanding of the electron transfer mechanism is needed to utilize microemulsions as energy storage electrolytes. To probe the mechanism, a model system that allows for isolation of the redox molecule is necessary. Rubrene is a hydrophobic redox compound that is insoluble in water in neutral and radical cation forms. Electrochemical oxidation of rubrene is possible, however, in microemulsions when sodium hydroxide is the only supporting electrolyte. Contrary to observations made in emulsion systems, rubrene oxidation in microemulsions seemingly violates phase electroneutrality. Our results demonstrate that electron transfer reactions within nanometer-sized domains are not subject to restrictions which govern larger systems.

The work presented here includes the first microemulsion electrolyte RFB, as well as studies aimed at deeper understanding of kinetics and transport in porous electrodes and mechanisms of electron transfer for microemulsion systems.