Redox flow batteries are emerging energy storage devices that are being explored for grid scale storage of electrical energy. A key component of a flow battery is the separator membrane, which separates the two compartments of the battery. The two compartments each contain a different redox active molecule that can store charge. The major source of degradation in these batteries is associated with the "crossover" of electrolyte species across the separator membrane - i.e. the failure of the separator membrane. In this project we will explore this crossover effect and quantify the importance of the different structural factors on crossover. The primary measurement tool will be solution state NMR spectroscopy which can quantify crossover directly, and in a time-resolved fashion. We will also carry out solid state NMR experiments to understand how the electrolyte species adsorb and diffuse inside the membrane. By carrying out these experiments on a range of separators and electrolyte materials, we will begin to understand how materials structure influences crossover. This project will lead to new fundamental insights on degradation processes in redox flow batteries and will inform the synthesis of improved separators materials.