Smart electrically responsive hybrid ion-selective membranes for selective gated transport of ionic species

© 2018 The Royal Society of Chemistry. Ion-exchange membrane materials have been developed to allow for selective transport based on the absolute charge and valence of the species. The transport of ions across the dense, charge neutral membranes is dominated by a hopping mechanism between ionic sites. The ionic transport rate is therefore limited by both process conditions, including primarily the concentration of ionic species in solution and the current passing across the system, and also material properties including the overall resistance and ionic site density of the membrane. A route to tackle this challenge is to develop gated ionic transport materials from electrically responsive hybrid ion-exchange membranes able to alter their ionic conductance under external stimuli. Here, hybrid ion-exchange membranes were synthesized by incorporating selective anion or cation exchange resins across porous electrically conductive reinforcement materials for application in electro-dialysis. The electrically conductive nature of the reinforcement allowed for the super-imposition of a secondary electrical field acting as a gate keeper for the direct polarisation of the hybrid membranes upon electro-dialysis operation. The secondary electrical field generated across pairs of hybrid ion-exchange membranes was shown to provide an additional driving force either blocking the transport or promoting the migration of ionic species by up to 6 fold. Ionic transport mechanisms across the selective gated ion-exchange membranes are also discussed considering the response of the system under specific current/voltage stimuli. This novel approach offers unprecedented control over the ionic transport mechanisms with potential applications in microfluidics, resource recovery, desalination and chemical synthesis, offering a cost-effective solution to selective ionic transport.