Article pubs.acs.org/JPCB
Cite This: J. Phys. Chem. B XXXX, XXX, XXX−XXX
Fully-Zwitterionic Polymer-Supported Ionogel Electrolytes Featuring a Hydrophobic Ionic Liquid Morgan E. Taylor and Matthew J. Panzer* Department of Chemical & Biological Engineering, Tufts University, 4 Colby Street, Medford, Massachusetts 02155, United States
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ABSTRACT: In this report, fully-zwitterionic (ZI) copolymer scaffolds for ionogel electrolytes have been synthesized via in situ photopolymerization using various molar ratios of 2-methacryloyloxyethyl phosphorylcholine (MPC) and sulfobetaine vinylimidazole (SBVI) within the hydrophobic ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI TFSI). Depending on the chemical composition of the ZI scaffold, ionogel room temperature ionic conductivities are found to vary between 2.5 and 6.7 mS cm−1 at a fixed 20 mol % total polymer content. Compressive elastic moduli also exhibit a strong dependence on the co-monomer ratio, with values between 23 kPa and 11 MPa observed because of different degrees of ZI physical cross-linking. These results, together with NMR chemical shift analysis, suggest that the phosphorylcholine ZI group of MPC interacts more strongly with EMI TFSI, while SBVI prefers to self-aggregate and form dipole−dipole cross-links in the ionic liquid (IL). Self-diffusivity measurements of the EMI+ cations and TFSI− anions in both ionogel and ZI solution samples confirm that slower ion diffusion in MPC-containing systems is due to attractive zwitterion/IL interactions, and not merely reduced mobility in the presence of a polymeric scaffold. This work highlights the importance of relative zwitterion/IL and ZI dipole−dipole interactions on the properties of a novel class of fully-ZI polymersupported ionogel electrolytes containing a hydrophobic IL suitable for future electrical energy storage applications.
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be enhanced.9−13 Beyond choosing the components of the scaffold, there are also different methods for synthesizing polymer-supported ionogels, including swelling of polymer membranes with IL,14 cosolvent evaporation,15 or in situ photopolymerization.16 Of these options, in situ photopolymerization is often the preferred choice because it provides the best control of the final material composition and is a rapid synthesis performed at ambient temperatures.17 Another benefit to the in situ approach is that ILs have been shown to increase polymerization rates and polymer molecular weights compared to conventional organic solvents.18,19 While immobilizing the IL in a polymeric scaffold can be advantageous for numerous reasons, it comes at the cost of decreasing the IL mass fraction in the gel composite, which in turn reduces ionic conductivity. Therefore, there is an interest in designing scaffold materials that can interact favorably with the IL to improve ion dissociation and/or transport, and consequently, gel ionic conductivity. Zwitterions contain an equal number of positively and negatively charged groups at different locations within the same molecule; they are overall charge-neutral and can exhibit high dipole moments. Interactions between the charged groups
INTRODUCTION Ionic liquids (ILs) are molten salts at or near ambient temperature that consist of weakly coordinated cations and anions. ILs have received considerable attention in recent years because of their many advantageous properties, including ultralow volatility, high thermal stability, and moderate ionic conductivity (∼0.1−20 mS cm−1).1,2 Possessing both a high ion density and a wide electrochemical stability window (∼3− 5 V), certain ILs may be well-suited to replace existing electrolytes for the development of safer electrical energy storage technologies, including batteries and supercapacitors.3,4 While ILs offer greater safety than conventional organic electrolytes because of their ultralow volatility and nonflammable nature, there still remains the undesirable possibility of leakage when using a liquid phase IL electrolyte. Supporting an IL using a solid framework is thus a convenient strategy for further improving safety, preventing accidental electrolyte leakage, while also allowing for more flexible or robust composite systems. Common methods to achieve this include immobilizing the IL using a solid matrix, such as a polymeric or inorganic nanoparticle-based scaffold, to create what is referred to as an ionogel (or ion gel),5−7 as well as polymerizing one of the IL ions to create a poly(ionic liquid).8 Polymeric scaffolds are a popular choice for ionogels because through careful selection of the polymer chemistry, desired gel properties can be finely tuned and gel stability can © XXXX American Chemical Society
Received: June 22, 2018 Revised: August 10, 2018 Published: August 15, 2018 A
DOI: 10.1021/acs.jpcb.8b05985 J. Phys. Chem. B XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry B
field gradient spin-echo (PGSE) NMR spectroscopy. The results of this study reveal that the mechanical and electrical properties of fully-ZI polymer-supported ionogels are highly dependent on both ZI dipole−dipole and zwitterion/IL ion interactions, with a clear difference in behavior observed for two ZI monomers having distinct chemical functionalities.
of zwitterionic (ZI) additives and lithium salts within an iondense IL electrolyte have been shown to enhance the conductivity of such systems by improving ion dissociation.20,21 Zwitterions have also gained attention for polymersupported gels because ZI functional groups can form dipole− dipole cross-links that boost the mechanical stability of the composite.22,23 When covalently incorporated into a polymeric scaffold, ZI cross-links can significantly improve mechanical properties, leading to more robust and stiff gels, as demonstrated in various studies of hydrogel materials.24,25 Our group recently demonstrated the effectiveness of increasing ZI cross-link density within a non-ZI/ZI copolymer scaffold in order to tune the mechanical properties of a series of nonaqueous, hydrophobic ionogels containing 80 mol % 1ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI TFSI).26 To our knowledge, this was the first report to describe the incorporation of ZI groups into a polymeric ionogel scaffold. Another recent study demonstrated enhanced mechanical properties using a ZI homopolymer scaffold within a hydrophilic, water-miscible IL.27 However, there is still a need to further understand the implications of zwitterion selection for hydrophobic IL-based ionogels that offer higher electrochemical stability for energy storage applications28 and to determine how relative zwitterion/IL and ZI dipole−dipole interactions can influence ionogel properties. In this study, we examined the properties of a suite of fullyZI polymer-supported ionogels for potential solid-state electrolyte applications. The ionogels were synthesized via in situ free radical polymerization of ZI (co)monomers, sulfobetaine vinylimidazole (SBVI) and/or 2-methacryloyloxyethyl phosphorylcholine (MPC), within a well-studied hydrophobic ionic liquid, EMI TFSI. To our knowledge, this represents the first time that an ionogel electrolyte containing a fully-ZI copolymer scaffold (i.e., synthesized using only ZI monomers) in a hydrophobic ionic liquid has been demonstrated. The molecular structures of the ionogel components are shown in Figure 1. Ionogels were fabricated
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EXPERIMENTAL METHODS Materials. The ionic liquid, EMI TFSI, was purchased from EMD Performance Materials and stored in a nitrogen-filled glovebox (
