As a typical single ion conductor, the perfluorsulfonate (PFSA) ion exchange membrane performance predominantly relies on the interactions amongst cation, sulfonate group and solvent. Such solvation information is crucial to the performance of electrochemical devices. The well-studied interaction between water and proton exchange membranes paved the way for the development of fuel cells and redox flow batteries (1–3). The emerging non-aqueous redox flow battery (NARFB) features alkyl or organic cations. Given the overall inferior performance of NARFBs to aqueous systems, it is important to try to find paths to improve the conductance of the membrane. Despite this and its importance in determining conductivity, the understanding of PFSA cation solvation in non-aqueous solvent is limited so far. We previously reported the surprisingly high ionic conductivity of bulky tetraalkylammonium cations in organic solvents compared to conventional alkyl cations(4). To further investigate the fundamental aspects of cation solvation, we applied different characterization techniques to build connections between ionic conductivity and cation-solvent interaction.

Ion mobility data will be discussed in light of solvent uptake. As an example, Figure. 1 shows the relationship between the degree of solvation and relative quantity of deuterated acetonitrile solvating tetrabutylammonium (TBA) form 825EW 3M PFSA. The degree of solvation was derived from solvation signal picked up from carbon-13 NMR on cyanide carbon of deuterated acetonitrile. The degree of solvation increased accordingly with the amount of solvent until it reaches saturation when excess solvent molecules would not participate in the solvation. The respective diffusion coefficients from NMR pulsed gradient diffusion experiments on cations were also presented. A clear signal corresponding to solvent molecules involved in solvation is captured by FTIR and this will also be analyzed as a function of composition and ion type.

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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3. Zawodzinski TA, Derouin C, Radzinski S, Sherman RJ, Smith VT, Springer TE, et al. Water Uptake by and Transport Through Nafion® 117 Membranes. J Electrochem Soc. 1993;140(4):1041–7.
4. Lou K, Peng J, Tang Z, Fujimoto C, Zawodzinski TA. Investigation of Ionic Conductivity, Uptake and Cation Diffusion of Perfluorsulfonate and Sulfonated Block Copolymer Ion Exchange Membrane in Non-Aqueous Solvents. Meet Abstr. 2017;MA2017-01(2):166–166.

Figure 1. Degree of solvated CN vs relative acetonitrile quantity of TBA⁺ form 825EW 3M membrane in deuterated Acetonitrile.