Manipulation Of Proton-Conducting Membrane Nanostructures Based On Ionic Liquids And Their Self-Assemblies

As a new kind of "green solvents", ionic liquids (ILs) have attracted increasing attentions due to their extraordinary properties. ILs are now not only considered as important alternative solvents, but also as materials with unique and tuneable properties which can be easily adjusted by suitable selection of cations and anions for a specific function. Moreover, we can also realize the functionalization of traditional ordered self-assembled structures by the incorporation of functional ILs. Such investigation has deep significance to further extend the application of both ILs and ordered self-assembled structures.During the last ten years, the self-assembled structures based on ILs have been applied in a lot of fields, especially in the proton exchange membrane fuel cells (PEMFC). As the key factor of PEMFC, the properties of proton-conducting membranes have direct influence on the practical performance of fuel cells. It is noteworthy that the better conductive properties are resulted from the more ordered nanostructures in proton-conducting membranes. Taking all these factors into account, it is a new challenge to manipulate the proton-conducting membrane nanostructures based on ILs and their self-assembled structures. We expect to shed light on the potential of nanostructured proton conductors in practical applications.In this dissertation, ILs with different structures and special functional groups were designed and synthesized, and the self-assembled aggregates based on these ILs were studied systematically. Further, the manipulating effect of self-assembled aggregates on the properties of proton-conducting membranes was also investigated. Our aim is to establish the dependence of conductive properties on the ordered self-assembled structures. Our research is helpful to explore a good solution to enhance the conductive properties of proton-conducting membranes. The outline and contents of this dissertation are as follows:1. The aggregation behaviors of a series of long chain ILs alkyltriphenylphonium bromides (CnTPB, n=12,14) were investigated in two different room-temperature ILs, the protic ethylammonium nitrate (EAN) and the aprotic 1-butyl-3-metylimidazolium tetrafluoroborate ([bmim][BF4]). Additionally, the effects of ILs with different anions, 1-butyl-3-methylimidazolium acetate ([bmim][OAc]) and 1-butyl-3-methylimidazolium benzoate ([bmim][PhCOO]) on the aggregation behaviors of dodecyltriphenylphosphonium bromide (C12TPB) in aqueous solution were also investigated. Following results were obtained.① CnTPB can form micelles in both EAN and [bmim][BF4]. The obviously smaller size of micelle in EAN indicates more compact packing of CnTPB molecules, which is caused by the stronger solvophobic interactions in protic EAN. Based on the surface tension measurement and thermodynamic analysis, the micellization of CnTPB in EAN is enthalpy-driven throughout the whole temperature range, similar to the micellization of ionic surfactant in aqueous solution. While the thermodynamic parameters of CnTPB in [bmim][BF4] vary in different tendency. Based on the enthalpy-entropy compensation, the micelles of CnTPB in aprotic [bmim][BF4] are more stable than that in protic EAN. The analysis of 1H NMR reveals that EAN and [bmim][BF4] can not only act as solvent, but also participate in the micelle formation.② Both [Bmim][PhCOO] and [Bmim][OAc] can facilitate the micelle formation of C12TPB, and a better surface activity of aqueous C12TPB solution can be obtained with the addition of [Bmim][PhCOO]. DLS data demonstrate that [Bmim][PhCOO] makes C12TPB to pack more densely. Compared with [Bmim][OAc], the introduction of benzene rings in the anions of [Bmim][PhCOO] can promote the micellization of C12TPB more efficiently due to the enhanced hydrophobic effect and π-π stacking interaction. When [Bmim][OAc] or [Bmim][PhCOO] was added in the C12TPB solution, the imidazolium cations of the both ILs may participate in the micelle formation with the butyl chains of imidazolium cations immersing in a hydrophobic core. While the anions of the two ILs affect in different ways. Both [OAc]- and [PhCOO]- can be adsorbed at the micellar surfaces due to the electrostatic interaction. While only [PhCOO]- can aligned into the C12TPB micelles with the enhanced hydrophobic effect and π-π stacking interaction, resulting in a smaller CMC values and micellar growth.2. In this chapter, a polymerizable amphiphilic zwitterion was prepared in a facile method by 3-(1-vinyl-3-imidazolio)propanesulfonate (VIPS) and 4-dodecyl benzenesulfonic acid (DBSA) based on intermolecular electrostatic interactions. According to the hard-soft acid-base theory, the preferential interactions between 4-dodecyl benzenesulfonate anions and the imidazolium cationic part of the zwitterion encourage the formation of zwitterionic amphiphile with polymerizable moiety. The VIPS-DBSA aqueous mixtures can construct hexagonal and lamellar phases with the increasing component contents. SAXS results indicate that the topological structure in liquid crystals (LCs) is sensitive to both temperature and water contents. Hence, it is pertinent to fix the LC nanostructure by in situ photopolymerization. SAXS and POM analysis demonstrate that the proton-conductive polymeric films with hexagonal or lamellar nanostructures were successfully derived. The water channels in LCs can be considered as proton pathways. The more ordered nanostructures in proton-conducting membranes can provide more ionic conduction pathways for proton hopping, resulting in a higher conductivity and lower activation energy.3. In this chapter, a novel IL microemulsion was prepared by a polymerizable zwitterionic room-temperature ionic liquid 1-(3-sulfonyl)propyl-3-vinyl-imidazolium methanesulfonate ([VIPS][MSA]),4-dodecyl benzenesulfonic acid (DBSA) and styrene (St). Electrical conductivity measurement was used to identify the microstructure of IL microemulsion. Then the microstructure of IL microemulsion was preserved in a resultant polymeric matrix through in situ photopolymerization to obtain three kinds of proton-conducting membranes, Film-O/IL, Film-Bi and Film-IL/O. Especially, the conductivity of Film-Bi is one order of magnitude high than that of Film-O/IL and Film-IL/O. Compared with the IL content, the nanostructures in proton-conducting membranes have more remarkable influence on the conductive properties. For the bicontinuous microemulsion, the continuous ionic liquid channel can be considered as hydrophilic pathway while the continuous styrene phase can be consider as hydrophobic channel. The intersection and overlap of these two continuous pathways enhance the hydrophilic-hydrophobic phase separation. A more distinct inter-phase separation in Film-Bi results in a higher conductivity and lower activation energy.4. In this chapter, hybrid membranes based on Nafion and protic imidazolium ILs, i.e. 1-(2-aminoethyl)-3-methylimidazolium chloride ([MimAE]Cl), 1-(2-hydroxylethyl)-3-methylimidazolium chloride ([MimHE]Cl) and 1-carboxylmethyl-3-methylimidazolium chloride ([MimCM]Cl) were fabricated by solution-casting method. Then the Nafion/IL membranes were doped by phosphoric acid (PA) to prepare the Nafion/IL/PA composite membranes. Due to the interactions between protic groups in ILs and PA, the incorporated protic ILs enhance the doping degree of PA. In the Nafion/IL/PA composite membranes, PA can act as "proton donor" while the protic groups in ILs can act as "site" for proton hopping. Hence, the Nafion/IL/PA composite membranes can exhibit outstanding conductive properties at high temperature. The ionic conductivity of Nafion/10wt%[MimAE]Cl/PA composite membrane is 6.0 mS/cm at 130℃ without humidification. SAXS results show that [MimAE]Cl can swell the Nafion matrix more homogeneously than [MimHE]Cl and [MimCM]Cl, which results in a better ionic conductivity. According to the TG results, the water contents of the Nafion/IL/PA composite membranes are reduced due to the ionic domains are filled with ILs, and the temperatures of thermal degradation are relatively raised. The incorporated ILs and PA can act as plasticizers and increase the thermal stability of Nafion matrix.5. Chemical modified Nafion composite membranes were successfully fabricated by five kinds of protic ionic liquids (PILs) with different cations,1-butylammonium methanesulfonate (BA-MS), tributylammonium methanesulfonate (TBA-MS), 2,4,6-trimethylphenylammounium methanesulfonate (TMA-MS), butane-1,4-diammonium methanesulfonate (BDA-MS), and N-(2-aminoethyl)ethane-1,2-diammonium methanesulfonate (DETA-MS). The interactions between Nafion ionomer and different geometric cations of PILs were discussed by the comparison of nanostructures, dynamic-mechanical properties and thermal stabilities of the Nafion/PIL composite membranes. With the incorporation of PILs, a more distinct inter cluster separation can be organized within the composite membranes and results in a significant increase in ionic conductivity than pristine Nafion under anhydrous conditions. SAXS results revealed the mean distance between ionic clusters consists with the molecular dimension of PIL cations. The insertion of PILs, especially the PILs with bulky cations, enhances the phase separation of Nafion. The interactions between ionomer and cations can also be reflected via DMA and TGA. The PILs with linear cations, including BA-MS, BDA-MS and DETA-MS can form smaller nanoclusters and have stronger interactions with the ionic side of Nafion, which results in a more flexible composite membrane. Nevertheless, TBA-MS and TMA-MS containing relatively bulky cations can enhance the thermal stability of sulfonated groups of Nafion due to the larger charged clusters.