The characterization of perfluorsulfonate and sulfonated block copolymer ion exchange membranes is well established in aqueous systems. While many of these membranes have been proved to have favorable conductivity and stability in multiple applications(1,2), similar studies are hardly found for membranes in non-aqueous systems. Although some characterization results on non-aqueous system have been published(3), a complete understanding how sulfonated ion exchange membranes behave in organic solvents is lacking. The largest range of existing methods is mostly dedicated to probing aqueous systems. In light of emerging applications, especially in nonaqueous redox flow batteries (NARFBs), it is certainly desirable to combine various techniques beyond simply conductivity in hope of elucidating some of the membrane properties in non-aqueous system.

A series of protocols is being developed to characterize some fundamental membrane properties. A four-point cell conductivity configuration was adopted to minimize artifacts (4). Swelling behavior was studied to complement conductivity data. In Figure 1, while the 3Mion-825EW membranes in TEA⁺ form overall showed higher conductivity than the Li⁺ form membranes. Acetonitrile soaked-TEA form membranes exhibited a 9-fold boost. This is consistent with relatively high performance of NARFBs using this electrolyte.

To reveal fundamental aspects of this performance, other means of characterization are certainly necessary to look into cation/membrane/solvent matches. Fourier transform infrared spectroscopy (FTIR) was utilized to examine membranes exchanged with different cations (li⁺, Na⁺, TEA⁺ ,etc) so the interaction between the cation and membrane, as well as between the solvent and the membrane can be better understood. Measuring the relaxation time (T₁) of cations and solvents by NMR has been an effective way to unravel the local diffusion kinetics for cation exchange membranes(5). Cation pulsed gradient diffusion experiments by NMR were carried out to further clarify the long-range transport. These data will help us explicate the correlation between the cation behavior/vibrational bonding and macroscopic properties.

On top of indicating good cation/membrane/solvent matches, a more complete and precise understanding of membrane behavior in non-aqueous system is the goal of the study. The behavior of several different membrane types will be discussed.

Acknowledgement

We gratefully acknowledge the support of this work by the U.S. Department of Energy, Office of Electricity Delivery and Energy Reliability (Dr. Imre Gyuk). We also thank 3M for providing membranes.

References

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