Transport Phenomena in Non-Aqueous Redox Flow Battery Membranes

Grid scale energy storage is becoming increasingly important as the fraction of renewable energy sources in the power grid increases. The energy supply from wind and solar is intermittent and non-dispatchable, and therefore requires energy storage devices to smooth out periods of excess and insufficient power generation. Redox flow batteries (RFBs) have been proposed as a technology to fill this need, but current technologies are aqueous based, which may limit energy density. Switching to non-aqueous solvents could improve energy density, however, the transport properties of the membrane separators needed for efficient and long-lasting operation of these batteries are not well understood in non-aqueous solvents. This dissertation aims to study the relationships between the membrane and battery chemistry and the relevant transport properties for RFBs, ionic conductivity and active material permeability, with the goal of increasing fundamental understanding and the ability to rationally design these membranes for better performance. This begins with the development of a membrane with sufficient stability, conductivity, and permeability to begin testing, but also with the potential to be modified in future work. Charge density of these membranes was varied to find the limits of mechanical stability for the polymer and investigate how the conductivity and permeability properties were affected. Using these membranes, a further study on uncharged material transport was used to develop a model to predict the enthalpy of mixing between the polymer and probe molecules of varied chemical structure. These predicted values correlated well with the measured permeability, and provided insights into the major contributing interactions that affect permeability. Finally, these membranes were crosslinked with a method that allowed controlled solvent uptakes, and were tested in multiple solvents. Conductivity and permeability showed differing dependence on both solvent uptake and the solvent used, which suggested how some solvent specific interactions can affect the transport properties, and how these may be used to improve future membrane selectivity. Altogether, these results identified some new key relationships between the membrane and battery chemistry and the resulting transport properties. These relationships may provide pathways for decoupled control of charged and uncharged material transport, which is highly valuable in designing novel membranes for RFB applications.