Low-Molecular-Weight Supramolecular Ionogel Based on Host-Guest Interaction

Recent Advancements in Gel Polymer Electrolytes for Flexible Energy Storage Applications

Since the last decade, the need for deformable electronics exponentially increased, requiring adaptive energy storage systems, especially batteries and supercapacitors. Thus, the conception and elaboration of new deformable electrolytes becomes more crucial than ever. Among diverse materials, gel polymer electrolytes (hydrogels, organogels, and ionogels) remain the most studied thanks to the ability to tune the physicochemical and mechanical properties by changing the nature of the precursors, the type of interactions, and the formulation. Nevertheless, the exploitation of this category of electrolyte as a possible commercial product is still restrained, due to different issues related to the nature of the gels (ionic conductivity, evaporation of filling solvent, toxicity, etc.). Therefore, this review aims to resume different strategies to tailor the properties of the gel polymer electrolytes as well as to provide recent advancements in the field toward the elaboration of deformable batteries and supercapacitors.

Mechanisms of ion transport in lithium salt-doped zwitterionic polymer-supported ionic liquid electrolytes

Recent experimental results have demonstrated that zwitterionic ionogel comprised of polyzwitterion (polyZI)-supported lithium salt-doped ionic liquid exhibits improved conductivities and lithium transference numbers than the salt-doped base ionic liquid electrolyte (ILE). However, the underlying mechanisms of such observations remain unresolved. In this work, we pursued a systematic investigation to understand the impact of the polyZI content and salt concentration on the structural and dynamic properties of the poly(MPC) ionogel of our model polyZI ionogel, poly(2-methacryloyloxyethyl phosphorylcholine) [poly(MPC)] supported LiTFSI/N-butyl-N-methylpyrrolidinium TFSI base ionic liquid electrolyte. Our structural analyses show strong lithium-ZI interaction consistent with the physical network characteristic observed in the experiments. An increase in polyZI content leads to an increased fraction of Li+ ions coordinated with the polyZI. In contrast, an increase in salt concentration leads to a decreased fraction of Li+ ions coordinated with the polyZI. The diffusivities of the mobile ions in the poly(MPC) ionogel were found to be lower than the base ILE in agreement with experiments at T > 300 K. Analysis of ion transport mechanisms shows that lithium ions within the poly(MPC) ionogel travel via a combination of structural, vehicular diffusion, as well as hopping mechanism. Finally, the conductivity trend crossover between the poly(MPC) ionogel and the base ILE was rationalized via a temperature study that showed that the base ILE ions are influenced more by the variation of temperature when compared to the poly(MPC) ions.

Amidoamine Oxide Surfactants as Low-Molecular-Weight Hydrogelators: Effect of Methylene Chain Length on Aggregate Structure and Rheological Behavior

Rheology control is an important issue in many industrial products such as cosmetics and paints. Recently, low-molecular-weight compounds have attracted considerable attention as thickeners/gelators for various solvents; however, there is still a significant need for molecular design guidelines for industrial applications. Amidoamine oxides (AAOs), which are long-chain alkylamine oxides with three amide groups, are surfactants that act as hydrogelators. Here, we show the relationship between the length of methylene chains at four different locations of AAOs, the aggregate structure, the gelation temperature T_gel, and the viscoelasticity of the formed hydrogels. As seen from the results of electron microscopic observations, the aggregate structure (ribbon-like or rod-like) can be controlled by changing the length of methylene chain in the hydrophobic part, the length of methylene chain between the amide and amine oxide groups, and the lengths of methylene chains between amide groups. Furthermore, hydrogels consisting of rod-like aggregates showed significantly higher viscoelasticity than those consisting of ribbon-like aggregates. In other words, it was shown that the gel viscoelasticity could be controlled by changing the methylene chain lengths at four different locations of the AAO.

Ionogels: recent advances in design, material properties and emerging biomedical applications

Ionic liquid (IL)-based gels (ionogels) have received considerable attention due to their unique advantages in ionic conductivity and their biphasic liquid-solid phase property. In ionogels, the negligibly volatile ionic liquid is retained in the interconnected 3D pore structure. On the basis of these physical features as well as the chemical properties of well-chosen ILs, there is emerging interest in the anti-bacterial and biocompatibility aspects. In this review, the recent achievements of ionogels for biomedical applications are summarized and discussed. Following a brief introduction of the various types of ILs and their key physicochemical and biological properties, the design strategies and fabrication methods of ionogels are presented by means of different confining networks. These sophisticated ionogels with diverse functions, aimed at biomedical applications, are further classified into several active domains, including wearable strain sensors, therapeutic delivery systems, wound healing and biochemical detections. Finally, the challenges and possible strategies for the design of future ionogels by integrating materials science with a biological interface are proposed.

Soret Effect of Ionic Liquid Gels for Thermoelectric Conversion

Cations and anions can accumulate at the two ends of an ionic conductor under temperature gradient, which is the so-called Soret effect. This can generate a voltage between the two electrodes, and the thermopower can be higher than that of the electronic conductors because of the Seebeck effect by 1-2 orders in magnitude. The thermoelectric properties of ionic conductors depend on the ionic thermopower, ionic conductivity, and thermal conductivity. Compared with other ionic conductors, like liquid electrolytes and hydrogels, ionogels made of an ionic liquid and a gelator can have the advantages of high thermopower and high stability. Great progress was recently made to improve the ionic conductivity and/or ionic thermopower of ionogels. They can be used in ionic thermoelectric capacitors (ITECs) to harvest heat. In addition, they can be integrated with electronic thermoelectric materials to harvest heat from both temperature gradient and temperature fluctuation, which can be caused by waste heat.

A pH-Sensitive, Stretchable, Antibacterial Artificial Tongue Based on MXene Cross-Linked Ionogel

Dehydration has always been a confusing problem for a hydrogel exposed to air, which restricts its application in practical detection. While an ionogel with unique properties can lock water molecules efficiently due to its low vapor pressure, the design and development of an ionogel with excellent water-locking properties and stability to achieve satisfactory detection are thus highly desirable. In this work, a pH-sensitive, stretchable, antibacterial, and stable ionogel artificial tongue was fabricated through dual cross-linking. The artificial tongue consisted of MXene as a cross-linking agent, phenol red as an indicator, gelatin, N-(2-hydroxyethyl) acrylamide, and 1-butyl-3-methylimidazolium chloride as the three-dimensional polymer network. A small quantity of MXene was used as a cross-linker the first time to promote the initiator in the system to generate free radicals, accelerating the reaction process and the multilevel linkage among the polymers. Meanwhile, the indicator phenol red was introduced into the ionogel successfully for the first time, showing great stability in 5 weeks. After evaluation by the Brand-Altman analysis, it was found that the method of pH measurement using ionogels had good consistency with the pH meter measurement method. Additionally, the prepared ionogel presented excellent water retention, mechanical stretchability, antibacterial property, and stability, as well as good test results in the test of artificial simulated saliva, demonstrating great potential in clinical applications.

Emerging iongel materials towards applications in energy and bioelectronics

In the past two decades, ionic liquids (ILs) have blossomed as versatile task-specific materials with a unique combination of properties, which can be beneficial for a plethora of different applications. The additional need of incorporating ILs into solid devices led to the development of a new class of ionic soft-solid materials, named here iongels. Nowadays, iongels cover a wide range of materials mostly composed of an IL component immobilized within different matrices such as polymers, inorganic networks, biopolymers or inorganic nanoparticles. This review aims at presenting an integrated perspective on the recent progress and advances in this emerging type of material. We provide an analysis of the main families of iongels and highlight the emerging types of these ionic soft materials offering additional properties, such as thermoresponsiveness, self-healing, mixed ionic/electronic properties, and (photo)luminescence, among others. Next, recent trends in additive manufacturing (3D printing) of iongels are presented. Finally, their new applications in the areas of energy, gas separation and (bio)electronics are detailed and discussed in terms of performance, underpinning it to the structural features and processing of iongel materials.

Nanosponges based on self-assembled starfish-shaped cucurbit[6]urils functionalized with imidazolium arms

A new porous material based on the first supramolecular cucurbituril-based nanosponge was synthesized by the functionalization of cucurbit[6]uril with twelve 1-(2-bromoethyl)-3-methyl-1H-imidazol-3-ium arms. The porous structure and the high adsorption capacity were demonstrated through surface area measurements and carbon dioxide adsorption. The new supramolecular sponge showed attractive properties such as (i) a highly porous structure that allowed CO₂ capture, (ii) the possibility to reuse the adsorbed CO₂ for organic synthesis, and (iii) an exciting thermal stability up to around 800 °C, with the potential use of this material in high temperature reactions. Finally, the reuse of CO₂ was successfully investigated in the carboxylation reaction of phenylacetylene.

Elastic and Thermoreversible Iongels by Supramolecular PVA/Phenol Interactions

Iongels have attracted much attention over the years as ion-conducting soft materials for applications in several technologies including stimuli-responsive drug release and flexible (bio)electronics. Nowadays, iongels with additional functionalities such as electronic conductivity, self-healing, thermo-responsiveness, or biocompatibility are actively being searched for high demanding applications. In this work, a simple and rapid synthetic pathway to prepare elastic and thermoreversible iongels is presented. These iongels are prepared by supramolecular crosslinking between polyphenols biomolecules with a hydroxyl-rich biocompatible polymer such as poly(vinyl alcohol) (PVA) in the presence of ionic liquids. Using this strategy, a variety of iongels are obtained by combining different plant-derived polyphenol compounds (PhC) such as gallic acid, pyrogallol, and tannic acid with imidazolium-based ionic liquids, namely 1-ethyl-3-methylimidazolium dicyanamide and 1-ethyl-3-methylimidazolium bromide. A suite of characterization tools is used to study the structural, morphological, mechanical, rheological, and thermal properties of the supramolecular iongels. These iongels can withstand large deformations (40% under compression) with full recovery, revealing reversible transitions from solid to liquid state between 87 and 125 °C. Finally, the polyphenol-based thermoreversible iongels show appropriated properties for their potential application as printable electrolytes for bioelectronics.

Supramolecular self-healing materials from non-covalent cross-linking host-guest interactions

The introduction of non-covalent bonds is effective for achieving self-healing properties because they can be controlled reversibly. One approach to introduce these bonds into supramolecular materials is use of host-guest interactions. This feature article summarizes the development of supramolecular materials constructed by non-covalent cross-linking through several approaches, such as host-guest interactions between host polymers and guest polymers, 1 : 2-type host-guest interactions, and host-guest interactions from the polymerization of host-guest inclusion complexes. Host-guest interactions show self-healing functions while also enabling stimuli-responsiveness (redox, pH, and temperature). The self-healing function of supramolecular materials is achieved by stress dispersion arising from host-guest interactions when stress is applied. Reversible bonds based on host-guest interactions have tremendous potential to expand the variety of functional materials.

Pillararene-Containing Polymers with Tunable Conductivity Based on Host-Guest Complexations

Regulating the conductivity of conducting polymers has spurred increasing studies, aiming at meeting different demands in various fields, including chemosensors, photovoltaic cells, and so on. Herein, linear pillar[5]arene-containing conjugated polymers were designed and synthesized via metathesis cyclopolymerization of pillar[5]arene-functionalized 1,6-heptadiyne. Upon addition of an ionic guest, such polymers could form inclusion complexes, of which the glass transition temperature decreased dramatically. With the aid of ionic guest and host-guest complexations between the pendant pillararenes and guest, these supramolecular materials exhibited tunable conductivity from 10^-12 to 10^-3 S·cm^-1 at 30 °C. In addition, compared with the polymers without pendant pillar[5]arenes, such polymers showed better compatibility with the ionic guest, which could prevent the leakage of the latter one and was good for the conductivity of the material.

Effect of β-Cyclodextrin on Physicochemical Properties of an Ionic Liquid Electrolyte Composed of N-Methyl-N-Propylpyrrolidinium bis(trifluoromethylsulfonyl)amide

Ionic liquids (ILs) are promising electrolyte materials for developing next-generation rechargeable batteries. In order to improve their properties, several kinds of additives have been investigated. In this study, β-cyclodextrin (β-CD) was chosen as a new additive in IL electrolytes because it can form an inclusion complex with bis(trifluoromethylsulfonyl)amide (TFSA) anions. We prepared the composites by mixing N-methyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)amide/LiTFSA and a given amount of triacetyl-β-cyclodextrin (Acβ-CD). The thermal behaviors and electrochemical properties of the composites were analyzed by several techniques. In addition, pulse field gradient NMR measurements were conducted to determine the self-diffusion coefficients of the component ions. The addition of Acβ-CD to the IL electrolytes results in the decrease in the conductivity value and the increase in the viscosity value. In contrast, the addition of Acβ-CD to the IL electrolytes induced an improvement in the anodic stability because of the formation of an inclusion complex between the Acβ-CD and TFSA anions. CDs are potential candidates as additives in IL electrolytes for electrochemical applications.

Supramolecular Thermotropic Ionic Liquid Crystals Formed via Self-Assembled Zwitterionic Ionic Liquids

Supramolecular thermotropic ionic liquid crystals (ILCs) with hexagonal and lamellar phases were fabricated by the self-assembly of zwitterionic ionic liquids, which were formed by 3-(1-alkyl-3-imidazolio) propanesulfonate with different alkyl chain lengths C _nIPS ( n = 12, 14, 16) and 3,4,5-tris(dodecyloxy)benzoic acid (TDBA) based on intermolecular electrostatic interactions. The phase behaviors of ILCs were investigated by differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), and polarized optical microscopy (POM). The highly ordered and well-defined microstructure of ILCs can be considered to be proton pathways and to radically improve the ionic conductivity, suggesting the induction of proton conduction through a hopping mechanism.

Mechanical and self-recovery properties of supramolecular ionic liquid elastomers based on host–guest interactions and correlation with ionic liquid content

Supramolecular materials have received considerable attention due to their higher fracture energy and self-recovery capability compared to conventional chemically cross-linked materials. Herein, we focus on the mechanical properties and self-recovery behaviours of supramolecular polymeric elastomers swollen with ionic liquid. We also gained insight into the correlation between ionic liquid content and mechanical properties. These supramolecular polymers with ionic liquid can be easily prepared from bulk copolymerization of the host–guest complex (peracetylated cyclodextrin and adamantane derivatives) and alkyl acrylates and subsequent immersion in ionic liquid such as 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. The supramolecular polymeric elastomers showed a self-recovery ability, which the conventional chemically cross-linked elastomers with ionic liquid cannot achieve.

Supramolecular ionic liquid elastomers showed higher fracture energy than chemically cross-linked ionic liquid elastomers and also self-recovery ability.