Conductivity, permeability, and stability properties of chemically tailored poly(phenylene oxide) membranes for Li+ conductive non-aqueous redox flow battery separators

Abstract

Non-aqueous redox flow batteries can operate at a higher voltage and energy density than aqueous systems, but developing these batteries will require new membrane separators engineered specifically for non-aqueous applications. Herein, we report the preparation and characterization of a series of membranes engineered specifically for a non-aqueous redox flow battery by functionalizing a poly(phenylene oxide) (PPO) backbone with increasing amounts of a highly sulfonated side chain, phenoxyaniline trisulfonate (POATS). These POATS-PPO membranes appear to be dimensionally stable over a period of at least four months in the non-aqueous electrolyte, and they exhibit lithium ion conductivities greater than that of previously reported control membranes. Ionic conductivity values, measured in non-aqueous and aqueous electrolytes, reveal solvent-specific ionic conductivity properties that differ from expected scaling relationships based on the ionic conductivity of the bulk electrolyte solution. The permeability of the membranes to ferrocene, a representative redox active molecule, does not change significantly with the degree of functionalization of the membrane. As a result, selectivity increases due to the increase in ionic conductivity as the degree of functionalization increases. Overall, the characterization of flow battery-relevant electrochemical properties suggests that these POATS-PPO membranes are promising materials for non-aqueous flow battery applications.