(324b) Ion-Exchange Membranes with Ultrahigh Charge Densities

Ion-exchange membranes (IEMs) are used in water purification (e.g., electrodialysis, membrane capacitive deionization, etc.) and energy generation/storage (e.g., reverse electrodialysis, fuel cells, flow batteries, etc.) technologies. The role of IEMs is to efficiently permeate counter-ions (ions with opposite charge to that of the fixed ions) while preventing the transport of co-ions (ions with similar charge to that of the fixed ions) and solvent. IEMs with higher counter-ion throughput and selectivity are needed to increase the efficiency of existing technologies and enable their use in emerging electrochemical technologies for water and energy applications. One of the most important membrane properties that affects counter-ion throughput and selectivity in IEMs is the charge density of the hydrated membrane. Higher charge densities should enhance both counter-ion throughput and selectivity, but the charge densities of nearly all reported IEMs are bound within a relatively narrow range owing to synthetic limitations. Attempts to increase the charge density of IEMs generally focus on introducing additional ionizable functional groups on the polymer backbone. However, the additional charged groups render the polymer more hydrophilic, which increases the water content of the membrane. Higher water content is beneficial for ion throughput, but excessive swelling ultimately dilutes the charge density of the hydrated membrane and deteriorates the membrane selectivity. As a result of the coupling between charge density and water content, IEMs are bound by a permeability-selectivity tradeoff – highly permeable membranes exhibit low selectivity and vice versa. In this presentation, I will discuss strategies for synthesizing novel IEMs with ultrahigh charge densities that exhibit combinations of remarkably high counter-ion throughput and selectivity.