(452h) Developing Ion Exchange Membranes with Ultrahigh Charge Densities

Ion exchange membranes (IEMs) are dense polymer films with covalently attached ionizable moieties. IEMs are used extensively in electrochemical water and energy applications such as electrodialysis, fuel cells, flow batteries, and electrolysis. These technologies broadly require exclusive and expedient transport of designated ions through IEMs, quantified as membrane selectivity and throughput, respectively. Unfortunately, a trade-off between these properties impedes membrane performance, wherein highly selective membranes typically have low throughput and vice versa. As a result, either the membrane selectivity or throughput commonly limit the energy efficiency or intensity of electrochemical technologies. Our prior research identified that the volumetric charge density of IEMs essentially defines the upper bound of the performance trade-off. So, to improve IEMs universally, we sought to design materials with significantly higher charge densities than those of currently available membranes.

It is synthetically challenging to prepare membranes with high charge densities because hydrophilic polymers used to form IEMs expand as they swell with water. To minimize swelling without sacrificing the charge density of the membrane, we synthesized a series of cross-linkable charged monomers. We polymerized the monomers using synthetic strategies capable of finely tuning and maximizing the charge density of the resultant IEM. The performance of these membranes was probed by measuring the NaCl ionic conductivity and salt permeability, which allowed calculation of the counter-ion throughput and selectivity. To add context, we characterized the performance of 40 commercially available IEMs and surveyed structural properties from approximately 1,000 reported IEMs.

Many IEMs prepared from these specialized monomers achieved ultrahigh charge densities (~5 M), greatly exceeding every charge density surveyed in the literature (<4 M). As a result, select membranes simultaneously achieved substantially higher selectivities and throughputs than all 40 commercial IEM controls. With such favorable performance, this class of ultrahigh charge density membranes promises significantly improved performance of IEM-based technologies.