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Microstructure determines water and salt permeation in commercial ion exchange membranes

preprint
revised on 15.10.2018 and posted on 15.10.2018 by Ryan Kingsbury, Shan Zhu, Sophie Flotron, Orlando Coronell
Ion exchange membrane (IEM) performance in electrochemical processes such as fuel cells, redox flow batteries, or reverse electrodialysis (RED) is typically quantified through membrane selectivity and conductivity, which together determine the energy efficiency. However, water and co-ion transport (i.e., osmosis and salt diffusion / fuel crossover) also impact energy efficiency by allowing uncontrolled mixing of the electrolyte solutions to occur. For example, in RED with hypersaline water sources, uncontrolled mixing consumes 20-50% of the available mixing energy. Thus, in addition to high selectivity and high conductivity, it is desirable for IEMs to have low permeability to water and salt in order to minimize energy losses. Unfortunately, there is very little quantitative water and salt permeability information available for commercial IEMs, making it difficult to select the best membrane for a particular application. Accordingly, we measured the water and salt transport properties of 20 commercial IEMs and analyzed the relationships between permeability, diffusion and partitioning according to the solution-diffusion model. We found that water and salt permeance vary over several orders of magnitude among commercial IEMs, making some membranes better-suited than others to electrochemical processes that involve high salt concentrations and/or concentration gradients. Water and salt diffusion coefficients were found to be the principal factors contributing to the differences in permeance among commercial IEMs. We also observed that water and salt permeability were highly correlated to one another for all IEMs studied, regardless of polymer type or reinforcement. This finding suggests that transport of mobile salt in IEMs is governed by the microstructure of the membrane, and provides clear evidence that mobile salt does not interact strongly with polymer chains in highly-swollen IEMs.

Funding

National Science Foundation DGE-1144081

History

Email Address of Submitting Author

ryanskingsbury@alumni.unc.edu

Institution

University of North Carolina at Chapel Hill

Country

United States

ORCID For Submitting Author

0000-0002-7168-3967

Declaration of Conflict of Interest

No conflict of interest.

Version Notes

v2 - Correct contact info for coauthor Shan Zhu, remove typo and uppercase words in title. Fix a text flow issue in the Description / Abstract. v3 - fix a typo in the abstract. Fix pagination issue in the SI. v4 - correct Equation 11

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