Effect of Anionic, Cationic, and Non-Ionic Surfactants on the Structural Stability and Morphology of Perfluorinated Sulfonic Acid Membranes

Abstract

Redox flow batteries (RFB) are a critical technology for stationary grid-scale energy storage, with potential usage with intermittent power sources, such as wind and solar. RFBs are attractive for long-duration energy storage because the anolyte and catholyte are stored separately external to the electrochemical reactor, which allows the RFBs to be modular and stationary. Separating the electrolytes and the electrochemical cell decouples energy and power densities, unlike batteries that store charge in the solid-state, such as Li-ion, Pb-acid, or Ni-Cd. A standard RFB chemistry is the all-vanadium redox flow battery (VRFB), which utilizes the four oxidation states of vanadium, with V2+, V3+, V4+, and V5+. Recently, new electrolytes are being studied in energy storage devices. Such as the first redox flow battery to utilize microemulsions as the electrolyte.Microemulsions are a breakthrough electrolyte for energy storage (BEES) that is emerging as an alternative to traditional RFB electrolytes because they produce thermodynamically stable, homogeneous solutions of polar and non-polar molecules. The stabilization of oil and water solutions allows microemulsion electrolytes to accommodate organic redox species with fast electron transfer rates that are not soluble in the aqueous phase while still offering the high conductivity of an aqueous salt electrolyte. Microemulsions consist of two immiscible phases stabilized by a surfactant, a non-polar "oil" phase, solubilizing the redox material, and the polar phase, which typically consists of an aqueous brine that provides charge balance and conductivity. Additionally, if the redox molecule was a non-polar liquid, it could serve as the entire oil phase, maximizing the energy density. An attractive property of a microemulsion electrolyte is that each component can be environmentally and biologically safe. For example, the aqueous phase only contains salt and water, and surfactants are found in many household, personal, and food products. Furthermore, some redox materials investigated in our lab are vitamins such as menadione (vitamin K3) and tocopherol (vitamin E).This talk describes the structural, mechanical, and morphological studies of surfactants' effects on perfluorinated sulfonic acid membranes. Small-angle X-ray scattering monitored the morphological changes as a function of surfactant concentration and soaking time, revealing how the surfactant penetrated and altered the polymer membrane. FTIR and NMR spectroscopy are also employed, providing insight into any polymer membrane degradation or dissolution. We demonstrate that surfactant choice is not only crucial to the stability of the microemulsion used as an electrolyte but can ultimately destroy the separator in a redox flow battery.This work was supported as part of the Breakthrough Electrolytes for Energy Storage and Systems (BEES2), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0019409.