Characterization Techniques and Cation Exchange Membrane for Non-aqueous Redox Flow Battery

The motivation of this work comes from one of the major problems of emerging non-aqueous flow battery (NAFB) that a separator or membrane which facilitates conductivity and blocks redox species crossover does not exist. Although many aspects of principles can be mirrored from mature fuel cell and aqueous flow battery, it is found that some well-defined membrane properties in aqueous systems such as swelling, transport and interactions are different in non-aqueous solvents to some extent. However, the approach of this work does follow the way perfluorosulfonate ion exchange membrane (PFSA) facilitated development of fuel cell and aqueous flow battery in the past. The aim of the work is not to identify or suggest one or one type of membrane that fits the non-aqueous flow battery. This is quite impossible based on the extremely diverse scope of published non-aqueous redox species. Thus, effort to understand and establish fundamentals of transport and interactions within membrane is the goal of the work to bridge the polymer design/synthesis and practical flow battery development is the goal of this work.

The highlight of this work includes observation of high ionic conductivity of tetraalkylammonium form PFSA of high organic solvent swelling as well as, solvation and interactions among the cation, organic solvent and membrane, low swelling but high ionic conductivity of asymmetric organic cation as a charge carrier for non-aqueous flow battery from an interaction perspective.

In terms of techniques, advanced Fourier-transform infrared spectroscopy (FTIR) and Nuclear Magnetic Resonance (NMR) methods beyond simple material characterization are designed in order to probe interactions and solvation. Although the methodologies involved here have long been used to study aqueous or non-aqueous electrolytes dating back to the 50s and 60s, this work aims to push these methods to another level inside of the membrane, a non-homogeneous and non-aqueous environment. Local information is separated from bulk property to understand transport dynamics and reveal membrane properties.