Probing Acetonitrile Solvation in Perfluorsulfonate Ion Exchange Membrane By in-Situ FTIR

Abstract

In-situ FTIR is a successful tool to study water absorption and interactions within the Perfluorsulfonate Ion Exchange Membrane (PFSA)(1–4). The versatility of the technique can reveal in-depth information related to proton transport in PFSA, including proton dissociation and solvation. Another desirable feature is that experiments under controlled water content(λ) is also possible. Studies of PFSA membranes equilibrated with different λ was fundamental to understand membrane behavior(5). The volatile nature of acetonitrile and many organic solvents adds to the inaccuracy of many classic membrane characterization methods. To overcome this, techniques tailored for membrane incorporated with organic solvents are necessary. In this study, acetonitrile and lithiated PFSA were chosen to revisit the in-situ IR technique. The goal of this work is to examine acetonitrile’s role in membrane swelling and to create a ‘slow-motion picture’ of how acetonitrile molecules behave upon being imbibed into the membrane, especially at low λ. The vibrational signal of cyanide group (~2250 cm-1)does not overlap with other signals from the membrane and is known to respond to dipole interactions by shifting to higher wavenumber(6). This feature makes an acetonitrile-swollen system an even more favorable candidate for in-situ IR study compared to its aqueous counterpart. For studying proton transport in membrane, initial water buildup in membrane yields a weak and broad signal, making the already obscure -OH stretching region even more complicated to analyze. Without interference from hydrogen bonding, lithium ion solvation with cyanide can be clearly observed. Coupled with monitoring -SO3- area, it is totally feasible to simultaneously see both cation dissociation from membrane (Fig.1A) and cation solvation in solvent (Fig.1B) in a real-time manner to compare with fully swollen state. 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 Elabd YA, Baschetti MG, Barbari TA. Time-resolved Fourier transform infrared/attenuated total reflection spectroscopy for the measurement of molecular diffusion in polymers. J Polym Sci Part B Polym Phys. 2003;41(22):2794–807.Hallinan DT, Elabd YA. Diffusion of Water in Nafion Using Time-Resolved Fourier Transform Infrared−Attenuated Total Reflectance Spectroscopy. J Phys Chem B. 2009 ;113(13):4257–66.Hallinan DT, Elabd YA. Diffusion and Sorption of Methanol and Water in Nafion Using Time-Resolved Fourier Transform Infrared−Attenuated Total Reflectance Spectroscopy. J Phys Chem B. 2007 ;111(46):13221–30.Kunimatsu K, Bae B, Miyatake K, Uchida H, Watanabe M. ATR-FTIR Study of Water in Nafion Membrane Combined with Proton Conductivity Measurements during Hydration/Dehydration Cycle. J Phys Chem B. 2011 21;115(15):4315–21.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.Barthel J, Buchner R, Wismeth E. FTIR Spectroscopy of Ion Solvation of LiClO4 and LiSCN in Acetonitrile, Benzonitrile, and Propylene Carbonate. J Solut Chem. 2000 ;29(10):937–54. Figure.1A: SO stretching shift due to solvation; Figure.1B: Normalized CN stretching area upon acetonitrile buildup in the membrane. Blue dashline indicates corresponding fully swollen state. Figure 1