Conductivity Mechanism in Polymerized Imidazolium-Based Protic Ionic Liquid [HSO3–BVIm][OTf]: Dielectric Relaxation Studies

Crystallization Kinetics of Phosphonium Ionic Liquids: Effect of Cation Alkyl Chain Length and Thermal History

The effects of alkyl chain length on the crystallization kinetics and ion mobility of tetraalkylphosphonium, [P666,n][TFSI], (n = 2, 6, 8, and 12) ionic liquids were studied by differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS) over a wide temperature range. The liquid-glass transition temperature (Tg) and ion dynamics examined over a broad T range were almost insensitive to structural modifications of the phosphonium cation. In contrast, the crystallization kinetics were strongly affected by the length of the fourth alkyl chain. Furthermore, the thermal history of the sample (cold vs melt crystallization) significantly impacted the crystallization rate. It has been found that the nature of crystallization phenomena is the same across the homologous series, while the kinetic aspect differs. Finally, electric conductivity in supercooled liquid and crystalline solid phases was measured for all samples, revealing significant ionic conductivity, largely independent of the cation structure.

Inflection Point in Pressure Dependence of Ionic Conductivity as a Fingerprint of Local Structure Formation

In this study, we employed dielectric spectroscopy to investigate the effect of temperature and pressure on the ion dynamics of phosphonium ionic liquids (ILs) differing by the length of an alkyl chain, [P666,n][TFSI] (n = 2, 6, 8, 12). We found that both temperature and pressure dependence of dc-conductivity (σdc) determined for all examined ILs herein exhibit unique characteristics, unusual for aprotic ILs. Two regions differing by ion self-organization have been identified from the derivative analysis of σdc(T-1) data. On the other hand, isothermal measurements performed at elevated pressure revealed a unique concave-convex character of σdc(P) dependences, resulting in a clear minimum in the pressure behavior of activation volume. Such an inflection point characterizing the pressure dependence of σdc in [P666,n][TFSI] ILs can be considered an inherent feature of ion dynamics governed by structural self-assembly. Our results offer a unique perspective to link the ion mobility at various T-P conditions to the nanostructural organization of ionic systems.

Effect of bulky anions on the liquid-liquid phase transition in phosphonium ionic liquids: Ambient and high-pressure dielectric studies

Although the first-order liquid-liquid phase transition (LLT) has been reported to exist in various systems (i.e., phosphorus, silicon, water, triphenyl phosphite, etc.), it is still one of the most challenging problems in the field of physical science. Recently, we found that this phenomenon occurs in the family of trihexyl(tetradecyl)phosphonium [P_666,14]⁺ based ionic liquids (ILs) with different anions (Wojnarowska et al in Nat Commun 13:1342, 2022). To understand the molecular structure-property relationships governing LLT, herein, we examine ion dynamics of two other quaternary phosphonium ILs containing long alkyl chains in cation and anion. We found that IL with the anion containing branched -O-(CH₂)₅-CH₃ side chains does not reveal any signs of LLT, while IL with shorter alkyl chains in the anion brings a hidden LLT, i.e., it overlaps with the liquid-glass transition. Ambient pressure dielectric and viscosity measurements revealed a peculiar behavior of ion dynamics near T_g for IL with hidden LLT. Moreover, high-pressure studies have shown that IL with hidden LLT has relatively strong pressure sensitivity compared to the one without first-order phase transition. At the same time, the former exposes the inflection point indicating the concave-convex character of logτ_σ(P) dependences.

Antifreezing Proton Zwitterionic Hydrogel Electrolyte via Ionic Hopping and Grotthuss Transport Mechanism toward Solid Supercapacitor Working at -50 °C

Hydrogel electrolyte is widely used in solid energy storage devices because of its high ionic conductivity, environmental friendliness, and non-leakage property. However, hydrogel electrolyte is not resistant to freezing. Here, a high proton conductive zwitterionic hydrogel electrolyte with super conductivity of 1.51 mS cm^-1 at -50 °C is fabricated by random copolymerization of acrylamide and zwitterionic monomer in the presence of 1 m H₂ SO₄ and ethylene glycol (EG). The antifreezing performance and low temperature conductivity are ascribed to hydrogen bonds and ionic bonds between the components and water molecules in the system and can be tuned by changing the monomer ratio and EG contents. The proton hopping migration on the ionic group of the polymer chains and Grotthuss proton transport mechanism are responsible for the high proton conductivity while Grotthuss transport is dominated at the glassy state of the polymer chains. The electrolyte-assembled supercapacitor (SC) offers high specific capacitance of 93.5 F g^-1 at 25 °C and 62.0 F g^-1 at -50 °C with a capacitance retention of 91.1% and 81.5% after 10 000 cycles, respectively. The SC can even work at -70 °C. The electrolyte outperforms most reported antifreezing hydrogel electrolytes and has high potential in low-temperature devices.

Exploration of Ion Transport in Blends of an Ionic Liquid and a Polymerized Ionic Liquid Graft Copolymer

In this study, we prepared a composite membrane consisting of a poly(1-butyl-3-vinylimidazolium-tetrafluoroborate) (poly([BVIM]-[BF₄])) polymerized ionic liquid graft copolymer (PILGC) and a blend of PILGC and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]-[BF₄]) ionic liquid (IL) to explore techniques for improving the conductivity of PILGCs, which is normally three orders of magnitude lower than that of ILs. PILGCs, which are environmentally friendly, have attracted much interest. To gain a better understanding of ion transport in composites, the mechanisms of ion transport in composite components should be explored. We investigated anion transport in ILs and PILGCs and were able to obtain the correct ion transport mechanisms in IL-PILGC blends based on a previous work. We performed molecular dynamics (MD) simulations, which are commonly used to investigate molecular mechanisms. According to the MD simulation results, in most IL-PILGC blends of various compositions, the contributions of cations are greater than those of anions. This is one reason that blends have higher conductivities than their component PILGCs. To the best of our knowledge, we are the first to identify ion transport mechanisms in PILGCs and their blends with ILs by exploring subdiffusive ion motion regimes. The ratio of the number of cages with more than three cationic branch chains in the blend with 50 wt % PILGC, the blend with 80 wt % PILGC, and the PILGC was 0.26:0.39:0.65. Therefore, the ratio of firm cages gets a promotion as the PILGC content increases. Because the ratio of fast ions decreases as the ratio of firm cages increases, the blend with 80 wt % PILGC has lower anion diffusivities than the blend with 50 wt % PILGC. It was inappropriate to probe ion transport in PILGCs (or IL-PILGC blends) solely via analyzing ion association interactions. Analysis of only ion association interactions led to the incorrect conclusion that the time scales of ion transport in PILGCs are given by the continuous ion association time, which is the time when the ion association remains paired rather than the time when an ion is caught inside a cage. Proper methods should be used to obtain more accurate theories.

Free-radical polymerization of 2-hydroxyethyl methacrylate (HEMA) supported by the high electric field

In macromolecular science, tunning basic polymer parameters, like molecular weight (Mn) or molecular weight distribution (dispersity, Đ), is an active research topic. Many prominent synthetic protocols concerning chemical modification of...

Ion and Proton Transport In Aqueous/Nonaqueous Acidic Ionic Liquids for Fuel-Cell Applications-Insight from High-Pressure Dielectric Studies

The use of acidic ionic liquids and solids as electrolytes in fuel cells is an emerging field due to their efficient proton conductivity and good thermal stability. Despite multiple reports describing conducting properties of acidic ILs, little is known on the charge-transport mechanism in the vicinity of liquid-glass transition and the structural factors governing the proton hopping. To address these issues, we studied two acidic imidazolium-based ILs with the same cation, however, different anions-bulk tosylate vs small methanesulfonate. High-pressure dielectric studies of anhydrous and water-saturated materials performed in the close vicinity of T_g have revealed significant differences in the charge-transport mechanism in these two systems being undetectable at ambient conditions. Thereby, we demonstrated the effect of molecular architecture on proton hopping, being crucial in the potential electrochemical applications of acidic ILs.

Enhanced Performance of Polymer Electrolyte Membranes via Modification with Ionic Liquids for Fuel Cell Applications

The polymer electrolyte membrane (PEM) is a key component in the PEM fuel cell (PEMFC) system. This study highlights the latest development of PEM technology by combining Nafion® and ionic liquids, namely 2-Hydroxyethylammonium Formate (2-HEAF) and Propylammonium Nitrate (PAN). Test membranes were prepared using the casting technique. The impact of functional groups in grafting, morphology, thermal stability, ion exchange capacity, water absorption, swelling and proton conductivity for the prepared membranes is discussed. Both hybrid membranes showed higher values in ion exchange capacity, water uptake and swelling rate as compared to the recast pure Nafion® membrane. The results also show that the proton conductivity of Nafion®/2-HEAF and Nafion®/PAN membranes increased with increasing ionic liquid concentrations. The maximum values of proton conductivity for Nafion®/2-HEAF and Nafion®/PAN membranes were 2.87 and 4.55 mScm-1, respectively, equivalent to 2.2 and 3.5 times that of the pure recast Nafion® membrane.

Light-mediated controlled and classical polymerizations of less-activated monomers under high-pressure conditions

An entirely new robust setup combining light and high pressure was applied to polymerize less-activated monomers via three different “green” polymerization protocols.

Impact of Imidazolium-Based Ionic Liquids on the Curing Kinetics and Physicochemical Properties of Nascent Epoxy Resins

We investigated the influence of anion type (salicylate, [(MOB)MIm][Sal], vs chloride, [(MOB)MIm][Cl]) of imidazolium-based ionic liquid (IL) and its content on the curing kinetics of bisphenol A diglicydyl ether (DGEBA of molecular weight M n = 340 g/mol). Further physicochemical properties (i.e., glass transition temperature, T g, and conductivity, σdc) of produced polymers were investigated. The polymerization of the studied systems was examined at various molar ratios (1:1, 10:1, and 20:1) at different reaction temperatures (T reaction = 353-383 K) by using differential scanning calorimetry (DSC). Interestingly, both DGEBA/IL compositions studied herein revealed significantly different reaction kinetics and yielded materials of completely distinct physical properties. Surprisingly, in contrast to [(MOB)MIm][Cl], for the low concentration of [(MOB)MIm][Sal] in the reaction mixture, an additional step in the kinetic curves, likely due to the combined enhanced initiation activity of anion (salicylate)-cation (imidazolium-based), was noted. To thoroughly analyze the kinetics of all studied systems, including the two-step kinetics of DGEBA/[(MOB)MIm][Sal], we applied a new approach that relies on the combination of the two phenomenological Avrami equations. Analysis of the determined constant rates revealed that the reaction occurring in the presence of the salicylate anion is characterized by higher activation energy with respect to those with the chloride. Moreover, DGEBA/[(MOB)MIm][Sal] cured materials have higher T g in comparison to DGEBA polymerized with [(MOB)MIm][Cl] independent of the IL concentration. This fact might indicate that, most likely, the products of hardening are highly cross-linked (high T g) or oligomeric linear polymers (low T g) in the former and latter cases, respectively. Such a change in the chemical structure of the polymer is also reflected in the dc conductivity measured at the glass transition temperature, which is much higher for DGEBA cured with [(MOB)MIm][Cl]. Herein, we have clearly demonstrated that the type of anion has a crucial impact on the polymerization mechanism, kinetics, and properties of produced materials.

Hydrogen bonding and charge transport in a protic polymerized ionic liquid

Hydrogen bonding and charge transport in the protic polymerized ionic liquid poly[tris(2-(2-methoxyethoxy)ethyl)ammoniumacryloxypropyl sulfonate] (PAAPS) are studied by combining Fourier transform infrared (FTIR) and broadband dielectric spectroscopy (BDS) in a wide temperature range from 170 to 300 K. While the former enables to determine precisely the formation of hydrogen bonds and other moiety-specific quantized vibrational states, the latter allows for recording the complex conductivity in a spectral range from 10-2 to 10+9  Hz. A pronounced thermal hysteresis is observed for the H-bond network formation in distinct contrast to the reversibility of the effective conductivity measured by BDS. On the basis of this finding and the fact that the conductivity changes with temperature by orders of magnitude, whereas the integrated absorbance of the N-H stretching vibration (being proportional to the number density of protons in the hydrogen bond network) changes only by a factor of 4, it is concluded that charge transport takes place predominantly due to hopping conduction assisted by glassy dynamics (dynamic glass transition assisted hopping) and is not significantly affected by the establishment of H-bonds.

Comparing ion transport in ionic liquids and polymerized ionic liquids

Polymerized ionic liquids (polyILs) combine the unique properties of ionic liquids (ILs) with macromolecular polymers. But anion diffusivities in polyILs can be three orders of magnitude lower than that in ILs. Endeavors to improve ion transport in polyILs urgently need in-depth insights of ion transport in polyILs. As such in the work we compared ion transport in poly (1-butyl-3-vinylimidazolium-tetrafluoroborate) (poly ([BVIM]-[BF4])) polyIL and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]-[BF4]) IL. The diffusivities of ions in the polyIL and IL were measured and computed. According to the results of the molecular dynamics simulations performed, in the IL the coupling motion between an anion and the ions around determines the ion diffusivities, and the ion association lifetime gives the time scale of ion transport. But in the polyIL, the hopping of an anion among cages composed of cationic branch chains determines the diffusivity, and the associated anion transport time scale is the trap time, which is the time when an anion is caught inside a cage, not the ion association lifetime, as Mogurampelly et al. regarded. The calculation results of average displacements (ADs) of the polyIL chains show that, besides free volume fraction, average amplitudes of the oscillation of chains and chain translation speed lead to various diffusivities at various temperatures.

Access to Thermodynamic and Viscoelastic Properties of Poly(ionic liquid)s Using High-Pressure Conductivity Measurements

In this paper, we examine the transport properties of a 1,2,3-triazolium-based poly(ionic liquid) (PIL) at ambient and elevated pressure up to 475 MPa. We show that the isothermal and isobaric conductivity measurements analyzed in the 3D plane give a unique possibility to estimate the thermodynamic (isothermal compressibility and thermal expansion coefficient) properties for PILs having a charge transport fully controlled by viscosity. This result, providing a direct connection between thermodynamic and dynamic properties of PILs, is of significant importance for both material scientists and practical applications.

Charge transport and glassy dynamics in polymeric ionic liquids as reflected by their inter- and intramolecular interactions

Polymeric ionic liquids (PILs) form a novel class of materials in which the extraordinary properties of ionic liquids (ILs) are combined with the mechanical stability of polymeric systems qualifying them for multifold applications. In the present study broadband dielectric spectroscopy (BDS), Fourier transform infrared spectroscopy (FTIR), AC-chip calorimetry (ACC) and differential scanning calorimetry (DSC) are combined in order to unravel the interplay between charge transport and glassy dynamics. Three low molecular weight ILs and their polymeric correspondents are studied with systematic variations of anions and cations. For all examined samples charge transport takes place by glassy dynamics assisted hopping conduction. In contrast to low molecular weight ILs the thermal activation of DC conductivity for the polymeric systems changes from a Vogel-Fulcher-Tammann- to an Arrhenius-dependence at a (sample specific) temperature Tσ0. This temperature has been widely discussed to coincide with the glass transition temperature Tg, a refined analysis, instead, reveals Tσ0 of all PILs under study at up to 80 K higher values. In effect, below the Tσ0 charge transport in PILs becomes more efficient - albeit on a much lower level compared to the low molecular weight pendants - indicating conduction paths along the polymer chain. This is corroborated by analysing the temperature dependence of specific IR-active vibrations showing at Tσ0 distinct changes in the spectral position and the oscillator strength, whereas other molecular units are not affected. This leads to the identification of charge transport responsive (CTR) as well as charge transport irresponsive (CTI) moieties and paves the way to a refined molecular understanding of electrical conduction in PILs.

Influence of molecular weight on ion-transport properties of polymeric ionic liquids

We report the results of atomistic molecular dynamics simulations on polymerized 1-butyl-3-vinylimidazolium-hexafluorophosphate ionic liquids, studying the influence of the polymer molecular weight on the ion mobilities and the mechanisms underlying ion transport, including ion-association dynamics, ion hopping, and ion-polymer coordinations. With an increase in polymer molecular weight, the diffusivity of the hexafluorophosphate (PF₆^-) counterion decreases and plateaus above seven repeat units. The diffusivity is seen to correlate well with the ion-association structural relaxation time for pure ionic liquids, but becomes more correlated with ion-association lifetimes for larger molecular weight polymers. By analyzing the diffusivity of ions based on coordination structure, we unearth a transport mechanism in which the PF₆^- moves by "climbing the ladder" while associated with four polymeric cations from two different polymers.

How is charge transport different in ionic liquids? The effect of high pressure

Modern ionic liquids (ILs) are considered green solvents for the future applications due to their inherited advantages and remarkable transport properties. One of the ubiquitous properties of ILs is their intrinsic ionic conductivity. However, understanding of the super-Arrhenius behavior of the ionic conductivity process at elevated pressure still remains elusive and crucial in glass science. In this work, we investigate the ion transport properties of 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide: [C₄mim][NTf₂], 1-butylimidazolium bis[(trifluoromethyl)-sulfonyl]imide: [C₄Him][NTf₂] and 1-butylimidazolium hydrogen sulfate: [C₄Him][HSO₄] ILs in the supercooled liquid state using dielectric spectroscopy at ambient and high pressure. We present the experimental data in the dynamic window of the conductivity formalism to examine the charge transport properties. The frequency-dependent ionic conductivity data have been analyzed using the time-temperature superposition principle. In the Arrhenius diagram, the thermal evolution of the dc-conductivity reveals similar temperature dependence for both protic and aprotic ILs thus making it difficult to distinguish the ion dynamics. However, our results demonstrate the key role of high pressure that unambiguously separates the charge transport properties of protic ILs from aprotic ones through the apparent activation volume parameter. We also highlight that the activation volume can be employed to assess the information connecting the ability of ionic systems to form H-bond networks and the impact of proton transfer involved in the conduction process.

A facile route to well-defined imidazolium-based poly(ionic liquid)s of enhanced conductivity via RAFT

Examining the relationship between the glass transition temperature, conductivity and molecular weight of tailored imidazolium-based PILs synthesized via RAFT.

Constructing Straight Polyionic Liquid Microchannels for Fast Anhydrous Proton Transport

Polymeric ionic liquids (PILs) have triggered great interest as all solid-state flexible electrolytes because of safety and superior thermal, chemical, and electrochemical stability. It is of great importance to fabricate highly conductive electrolyte membranes capable to operate above 120 °C under anhydrous conditions and in the absence of mineral acids, without sacrificing the mechanical behavior. Herein, the diminished dimensional and mechanical stability of poly[1-(3H-imidazolium)ethylene]bis(trifluoromethanesulfonyl)imide has been improved thanks to its infiltration on a polybenzimidale (PBI) support with specific pore architecture. Our innovative solution is based on the synergic combination of an emerging class of materials and sustainable large-scale manufacturing techniques (UV polymerization and replication by microtransfer-molding). Following this approach, the PIL plays the proton conduction role, and the PBI microsieve (SPBI) mainly provides the mechanical reinforcement. Among the resulting electrolyte membranes, conductivity values above 50 mS·cm^-1 at 200 °C and 10.0 MPa as tensile stress are shown by straight microchannels of poly[1-(3H-imidazolium)ethylene]bis(trifluoromethanesulfonyl)imide cross-linked with 1% of dyvinylbenzene embedded in a PBI microsieve with well-defined porosity (36%) and pore diameter (17 μm).