Xinpei Gao

Preparation and characterization of nonaqueous proton-conducting membranes with protic ionic liquids.

Hybrid Nafion membranes were successfully fabricated by incorporating with protic imidazolium ionic liquids 1-(2-aminoethyl)-3-methylimidazolium chloride ([MimAE]Cl), 1-(2-hydroxylethyl)-3-methylimidazolium chloride ([MimHE]Cl), and 1-carboxylmethyl-3-methylimidazolium chloride ([MimCM]Cl) for high-temperature fuel cells. The composite membranes were characterized by impedance spectroscopy, small-angle X-ray scattering (SAXS), scanning electronic microscopy (SEM), and thermogravimetric analysis (TGA). The incorporated protic ionic liquids enhance the doping of phosphoric acid (PA) and result in a relatively high ionic conductivity. The Nafion/10 wt % [MimAE]Cl/PA composite membrane exhibits an ionic conductivity of 6.0 mS/cm at 130 °C without humidification. [MimAE]Cl can swell the Nafion matrix more homogeneously than [MimHE]Cl or [MimCM]Cl, which results in a better ionic conductivity. It is notable that the composite Nafion/IL/PA membranes have a better thermal stability than the pristine Nafion membranes.

Nanostructured Proton Conductors Formed via in Situ Polymerization of Ionic Liquid Crystals

Ionic liquid crystals (ILCs) with hexagonal and lamellar phases were successfully fabricated by the self-assembly of a polymerizable amphiphilic zwitterion, which is formed by 3-(1-vinyl-3-imidazolio)propanesulfonate (VIPS) and 4-dodecyl benzenesulfonic acid (DBSA) based on intermolecular electrostatic interactions. The microstructures and phase behaviors of ILCs were studied by polarized microscope (POM) and small-angle X-ray scattering (SAXS). The ILC topological structures can be considered as proton pathways and further fixed by photopolymerization to prepare nanostructured proton-conductive films. The introduction of highly ordered and well-defined ILC structures into these polymeric films radically improves the ionic conductivities.

Nanostructured aqueous lithium-ion conductors formed by the self-assembly of imidazolium-type zwitterions.

In the present study, we have synthesized a room-temperature ionic liquid by mixing imidazolium-type zwitterions with lithium bis(trifluoromethanesulfonyl)imide. We constructed nanostructured aqueous lithium-ion conductors having hexagonal, lamellar, and bicontinuous cubic structures by the self-assembly of this amphiphilic ionic liquid. These nanostructured lithium-ion conductors exhibited an assembled-structure dependent lithium-ion conduction behavior. The introduction of highly ordered and well-defined liquid crystal structures into room-temperature ionic liquid radically changes the conduction mechanism from diffusion to hopping.

Photoresponsive Self-Assembly of Surface Active Ionic Liquid

A novel photoresponsive surface active ionic liquid (SAIL) 1-(4-methyl azobenzene)-3-tetradecylimidazolium bromide ([C14mimAzo]Br) with azobenzene located in the headgroup was designed. Reversible vesicle formation and rupture can be finely controlled by photostimuli without any additives in the aqueous solution of the single-tailed ionic liquid. The photoisomerization of the azobenzene derivative was investigated by (1)H NMR and UV-vis spectroscopy. Density functional theory (DFT) calculations further demonstrate that trans-[C14mimAzo]Br has less negative interaction energy, which is beneficial to aggregate formation in water. The incorporation of trans-azobenzene group increases the hydrophobicity of the headgroup and reduces the electrostatic repulsion by delocalization of charge, which are beneficial to the formation of vesicles. However, the bend of cis-azobenzene makes the cis-isomers have no ability to accumulate tightly, which induces the rupture of vesicles. Our work paves a convenient way to achieve controlled topologies and self-assembly of single SAIL.

Poly(ionic liquid) hydrogels exhibiting superior mechanical and electrochemical properties as flexible electrolytes

Recently, flexible electrolytes have aroused tremendous interest due to their wide applications in flexible electronic devices, such as wearable electronics, roll-up displays, smart mobile devices and implantable biosensors. Herein, novel ionic liquids, whose both cations and anions both can be polymerized, are used to construct flexible poly(ionic liquid) hydrogel electrolytes by one-step synthesis. Originating from the ionic interactions inside the ionic liquids, these hydrogels combine strong and weak crosslinks after polymerization, which makes these hydrogels tough, stretchable, flexible and self-recoverable. Their superior mechanical properties have been certified by mechanical tests. Moreover, these poly(ionic liquid) hydrogels show extremely high ionic conductivities over 1 S m−1 at room temperature and low activation energy. Notably, their electrochemical behaviors can remain stable under different bending angles and successive bending, folding, compressing and twisting. As flexible electrolytes, these poly(ionic liquid) hydrogels give the possibility to be used in fuel cells or supercapacitors, thus, promoting the development of flexible electronic devices.

Temperature-responsive proton-conductive liquid crystals formed by the self-assembly of zwitterionic ionic liquids

In the present study, we synthesized a series of ionic liquids by mixing amphiphilic imidazolium-type zwitterions with sulfonic acids containing different substituent groups. Nanostructured proton conductors having hexagonal and cubic structures were constructed by the self-assembly of these zwitterionic ionic liquids. These nanostructured proton conductors exhibited an assembled-structure dependent proton conduction behavior. The introduction of highly ordered liquid crystal structures efficiently improved ionic conductivity, suggesting the induction of proton conduction through a hopping mechanism. Temperature-responsive ionic conductivity behavior based on phase transition within the self-assembled liquid crystal structures was also observed.

Anion exchange membranes with well-defined ion transporting nanochannels via self-assembly of polymerizable ionic liquids

In this study, we present a facile method to construct highly ordered and well-defined ionic channels in anion-exchange membranes (AEMs) through in-phase photopolymerization of liquid crystals (LCs). Hexagonal and lamellar LC samples were prepared by the self-organization of polymerizable amphiphilic imidazolium-based ionic liquids. The preservation of LC nanostructures in the obtained polymeric membranes was confirmed by the combination of small-angle X-ray scattering (SAXS), polarized optical microscope (POM), and scanning electronic microscopy (SEM) measurements. Phase separations of LCs at the molecular level provided highly ordered and well-defined ionic channels for efficient anion conduction and meanwhile maintained a strong hydrophobic domain to suppress the swelling degree. The membranes synthesized in the present work provide a promising model system for understanding the idea of constructing ionic highways in AEMs through self-organization.

Facile preparation of supramolecular ionogels exhibiting high temperature durability as solid electrolytes

Ionogels, solid-like materials maintaining many of the physicochemical properties of ionic liquids, have aroused much interest as excellent functional materials. Here, a facile strategy of the preparation of supramolecular ionogels is reported. Benzenetricarboxylic acid (H3BTC) and Fe(NO3)3·9H2O are selected as gelators, which form stable supramolecular ionogels within several seconds via metal-coordination interactions in the ionic liquids 1-butyl-3-methylimidazolium chloride (bmimCl) and 1-butyl-3-methylimidazolium benzenesulfonate (bmimBsa). The time for preparing ionogels is reduced dramatically. The ionogel Fe–H3BTC–bmimCl is broken down at 100 °C, but the ionogel Fe–H3BTC–bmimBsa can remain stable at 150 °C, exhibiting high temperature durability. These ionogels are characterized further by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and thermogravimetry (TG) in detail. In addition, oscillatory rheological measurements and electrochemical measurements show that these ionogels display superior mechanical properties and high conductivity due to the high ionic liquid content.

Nanostructured proton-conducting membranes based on polymerizable zwitterionic ionic liquid microemulsions

Ionic liquid (IL)-microemulsions with a polymerizable zwitterionic IL as the polar core were successfully fabricated for the first time using a room temperature IL: 1-(3-sulfopropyl)-3-ethenyl-imidazolium methanesulfonate ([VIPS][MSA]), an anionic surfactant: 4-dodecyl benzenesulfonic acid and styrene. The IL-microemulsion phase diagram was determined. Nanostructured proton-conducting membranes were obtained using an in situ photopolymerization of the IL-microemulsion in different subregions. The Film-Bi obtained using a bicontinuous microemulsion exhibits a higher conductivity and lower activation energy than Film-O/IL or Film-IL/O. The macroscopically interconnected polar zwitterionic IL channels formed in bicontinuous microemulsions can work as proton pathways and supply more hopping sites for proton transportation. This simple and effective method provides a versatile format to prepare IL-based proton conductors.

Formation of supermolecular chiral gels from L-aspartic acid-based perylenebisimides and benzene dicarboxylic acids

Herein, supramolecular gels from L-aspartic acid-based perylenebisimides (APBI) and various isomeric benzene dicarboxylic acids (o-phthalic acid, OPA; isophthalic acid, IPA and terephthalic acid, TPA) were obtained. It has been found that spatial position of carboxyl on the benzene ring plays a key role in the gelation process, and OPA or IPA could lead to gel formation while TPA could not. Intermolecular hydrogen bonding and π–π stacking interactions have been found to guide the aggregation as determined by UV-vis spectra, fluorescence spectra and FT-IR. Moreover the chirality transfer from perylene chromophores to benzene dicarboxylic acids was detected via circular dichroism spectroscopy (CD). The interesting features of the two-component gel system may give a better understanding of supramolecular chirality.