(429d) Poly Ionic Liquid Membranes for Fast and Selective Ion Transport

Ion exchange membranes (IEMs)—comprising water and ion-conducting polymers—are critical components in diverse electrochemical technologies, including fuel cells, redox flow batteries, and electrodialysis. These membranes must be both highly conductive and highly selective so specific ions. However, IEMs are constrained by a fundamental trade-off between these two properties: increasing water content enhances ionic conductivity but lowers charge density, which diminishes selectivity. Conversely, increasing charge content improves selectivity but compromises mechanical stability due to electrostatic repulsion and excessive swelling.

To address this challenge, we have developed a new class of anion-exchange membranes (AEMs) based on poly(ionic liquid) architectures. These membranes feature a microphase-separated structure consisting of a highly charged, ion-conductive domains for selective transport, and a mechanically robust support domain for structural integrity. A key innovation is an alkyl chain interlocking mechanism, which reinforces structural integrity while enabling the formation of a continuous ion transport pathway, combined with closely spaced fixed charge groups that facilitate ion transport. These membranes exhibit over an order-of-magnitude enhancement in counter-/co-ion selectivity compared to commercial analogs, as well as exceptional H⁺ blocking ability—a particularly difficult property to achieve in conventional AEMs due to the unique transport mechanism of H⁺ ions. The distinct microphase separation was investigated by comprehensive x-ray characterization, and ion transport behavior was quantitatively described using Transition State Theory. These IEMs were successfully implemented as proton-blocking membranes in an acid recovery process, outperforming state-of-the-art commercial membranes. This work illustrates how rational design can overcome entrenched transport limitations and unlock new performance regimes for IEMs.