Journal
ANNUAL REVIEW OF MATERIALS RESEARCH, VOL 51, 2021
Volume 51, Issue -, Pages 21-46Publisher
ANNUAL REVIEWS
DOI: 10.1146/annurev-matsci-080619-010139
Keywords
ionophore; ion-conducting channel; ion exchange membrane; selectivity; ion transport; vanadium-redox-flow battery
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Funding
- Bundesministerium fur Forschung und Technologie (BMBF) [03EK3045A]
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This review highlights the importance of properly balanced ion-coordinating interactions over steric effects for selective ion transport in biological systems. It also emphasizes the relevance of steric effects for the selectivity of ionic transport through ion exchange membranes (IEMs). Reduced hydration leads to higher selectivity but lower transport rates, with near-perfect selectivity achieved through ion-conducting channels and sterically constrained transport in IEMs.
wThis review discusses selective and fast transport of ionic species (ions and their associates) through systems as diverse as ion-conducting transmembrane proteins and ion exchange membranes (IEMs) in aqueous environments, with special emphasis on the role of electrostatics, specific chemical interactions, and morphology (steric effects). Contrary to the current doctrine, we suggest that properly balanced ion-coordinating interactions are more important than steric effects for selective ion transport in biological systems. Steric effects are more relevant to the selectivity of ionic transport through IEMs. As a general rule, decreased hydration leads to higher selectivity but also to lower transport rate. Near-perfect selectivity is achieved by ion-conducting channels in which unhydrated ions transfer through extremely short hydrophobic passages separating aqueous environments. In IEMs, ionic species practically keep their hydration shell and their transport is sterically constrained by the width of aqueous pathways. We discuss the trade-off between selectivity and transport rates and make suggestions for choosing, optimizing, or developing membranes for technological applications such as vanadium-redox-flow batteries.
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