Decoupling charge transport from the structural dynamics in room temperature ionic liquids

Understanding the Positive Role of Ionic Liquids in CO2 Capture by Poly(ethylenimine)

CO2 capture technology is one of the most important technical methods for significantly mitigating CO2 emissions in a low-carbon context. The recent invention of mixed absorbents comprising poly(ethylenimine) (PEI) and ionic liquids (ILs) provides a novel strategy for efficiently capturing CO2, and this has garnered widespread attention. However, the intermolecular interactions between the IL and other constituents during the CO2 absorption process remain unclear. In this present work, a series of density functional theory (DFT) calculations and molecular dynamics simulations were conducted to investigate the positive role of IL in CO2 capture by PEI. The results showed that the formation of hydrogen bonds between the IL anion and the amino groups of PEI primarily drives the addition of IL to PEI. During the CO2 absorption process, the IL anion not only can absorb CO2 but also exerts a dehydrogenation effect on the amino group of PEI, facilitating enhanced interaction between PEI and CO2. Additionally, the IL substantially reduces the viscosity of PEI, promoting the diffusion of CO2 within the system and enhancing the absorption rate. Based on the information on interaction energy and viscosity, we can easily make theoretical predictions for the optimal proportion of IL to be added. The above results provide fundamental insights to promote the industrial application of the PEI/IL system for CO2 capture.

Preserving fast ion dynamics while introducing mechanical rigidity in gelatin-based ionogels

Ionogels are gels containing ions, often an ionic liquid (IL), and a gelling agent. They are promising candidates for applications including batteries, photovoltaics or fuel cells due to their chemical stability and high ionic conductivity. In this work we report on a thermo-irreversible ionic gel prepared from a mixture of the ionic liquid 1-butyl-3-methylimidazolium ([BMIM]) dicyanamide ([DCA]), water and gelatin, which combines the advantages of an ionic liquid with the low cost of gelatin. We use (i) dielectric spectroscopy to monitor the ion transport, (ii) dynamic light scattering techniques to access the reorientational motions of the ions, as well as fluctuations of the gel matrix, and (iii) rheology to determine the shear response from above room temperature down to the glass transition. In this way, we are able to connect the microscopic ion dynamics with the meso- and macroscopic behavior of the gelatin matrix. We show, by comparing our results to those for a IL-water mixture from a previous study, that although some weak additional slow relaxation modes are present in the gel, the overall ion dynamics is hardly changed by the presence of gelatin. The macroscopic mechanical response, as probed by rheology, is however dominated by the gel matrix. This behaviour can be highly useful e.g. in battery gel electrolytes which prevent electrolyte leakage and combine mechanical rigidity and flexibility.

A fractal structural feature related to dynamic crossover in metallic glass-forming liquids

The dynamic crossover in supercooled liquids initially predicted by model coupling theory has been widely accepted, but its underlying structural origin is still an open issue for glass-forming liquids. By molecular dynamics simulations of binary CuZr liquids, the present work verifies that high pressure could enhance this crossover, facilitating the studies on the structural features at the crossover temperature T_c. We discover that the topological connectivity of icosahedral clusters is responsible for this dynamic crossover, rather than all clusters. T_c is the temperature at which the connectivity degree between these clusters reaches a maximum and the dynamic heterogeneity begins to keep stable. Below T_c, the fractal topological structures appear in the medium-range order scale. The icosahedral clusters with a certain connectivity pattern can be regarded as a fractal structural unit. By employing the established fractal analysis method, the fractal dimension D of the icosahedral network is calculated. Our results indicate that the D value increases monotonically with increasing pressure and the fractal behavior of the icosahedral network is an inherent feature of metallic glasses. We also find similar fractal behavior in clusters with high local five-fold symmetry. Our findings shed light on the origin of a dynamic crossover in the deep supercooled region of metallic glasses and also demonstrate the important role of icosahedral clusters in uncovering the fractal behavior of metallic glass.

Dielectric Study of Tetraalkylammonium and Tetraalkylphosphonium Levulinate Ionic Liquids

Broadband dielectric spectroscopy in a broad temperature range was employed to study ionic conductivity and dynamics in tetraalkylammonium- and tetraalkylphosphonium-based ionic liquids (ILs) having levulinate as a common anion. Combining data for ionic conductivity with data obtained for viscosity in a Walden plot, we show that ionic conductivity is controlled by viscosity while a strong association of ions takes place. Higher values for ionic conductivities in a broad temperature range were found for the tetraalkylphosphonium-based IL compared to its ammonium homolog in accordance with its lower viscosity. Levulinate used in the present study as anion was found to interact and associate stronger with the cations forming ion-pairs or other complexes compared to the NTf₂ anion studied in literature. In order to analyze dielectric data, different fitting approaches were employed. The original random barrier model cannot well describe the conductivity especially at the higher frequencies region. In electric modulus representation, two overlapping mechanisms contribute to the broad low frequencies peak. The slower process is related to the conduction mechanism and the faster to the main polarization process of the complex dielectric permittivity representation. The correlation of the characteristic time scales of the previous relaxation processes was discussed in terms of ionic interactions.

Low-frequency dynamics in ionic liquids: Comparison of experiments and the random barrier model

By examining the fine features of dielectric spectra of ionic liquids, we show that the derivative of real permittivity progressively broadens at low frequencies when the glass transition is approached...

On the temperature and pressure dependence of dielectric relaxation processes in ionic liquids

Molecular dynamics of ionic liquids in an electric field can be decomposed into contributions from translational motions of ions, rotational motions of permanent dipoles and - in the case of ions equipped with long alkyl-chains - motions of ionic aggregates. The discrimination of these contributions in the dielectric spectrum is quite involved, resulting in numerous controversies in the literature. Here, we use dielectric spectroscopy at ambient and elevated pressures of up to 550 MPa to monitor the changes of the observed processes in five supercooled ionic liquids with octyl-chains independent of pressure and temperature. In most of the ionic liquids under investigation two dynamical processes are observed, one of them is identified as the ion hopping process, which we describe by the MIGRATION model. It turns out that this process is closely connected to the glass transition step as measured by differential scanning calorimetry. Concerning the second process, we rule out motions of aggregated ions to be its origin by comparison of our results with X-ray scattering literature data at elevated pressure. Instead, we tentatively ascribe it to dipolar reorientations and show that the dielectric strength of this slow process decreases as a function of increasing relaxation time, i.e. for decreasing temperatures and increasing pressures. We compare this behavior with literature data of other ion conducting systems and discuss its microscopic origin.

Modulation of Cation Diffusion by Reversible Supramolecular Assemblies in Ionic Liquid-Based Nanocomposites

Ionic liquid (IL) properties, such as high ionic conductivity under ambient conditions combined with nontoxicity and nonflammability, make them important materials for future technologies. Despite high ion conductivity desired for battery applications, cation transport numbers in ILs are not sufficient enough to attain high power density batteries. Thus, developing novel approaches directed toward improvement of cation transport properties is required for the application of ILs in energy-storing devices. In this effort, we used various experimental techniques to demonstrate that the strategy of mixing ILs with ultrasmall (1.8 nm) nanoparticles (NPs) resulted in melt-processable composites with improved transport numbers for cations at room temperature. This significant enhancement in the transport number was attributed to the specific chemistry of NPs exhibiting a weaker cation and stronger anion coordination at ambient temperature. At high temperature, significantly weakened NP-anion associations promoted a liquid-like behavior of composites, highlighting the melt-processability of these composites. These results show that designing a reversible dynamic noncovalent NP-anion association controlled by the temperature may constitute an effective strategy to control ion diffusion. Our studies provide fundamental insights into mechanisms driving the charge transport and offer practical guidance for the design of melt-processable composites with an improved cation transport number under ambient conditions.

Influence of the Internal Structure and Intermolecular Interactions on the Correlation between Structural (α) and Secondary (β-JG) Relaxation below the Glass Transition Temperature in Neat Probucol and Its Binary Mixtures with Modified Saccharides

Broadband dielectric spectroscopy (BDS) has been used to study the molecular dynamics and aging process in neat probucol (PRO) as well as its binary mixtures with selected acetylated saccharides. In particular, we applied the Casalini and Roland approach to determine structural relaxation times in the glassy state of the examined systems (so-called isostructural times, τ_iso). Next, using the calculated τ_iso, primitive relaxation times of the coupling model were obtained and compared to the experimental secondary β (Johari-Goldstein (JG) type) relaxation times. Interestingly, it turned out that there is a correlation between the β-JG and the structural (α)-relaxation processes below the glass transition temperature (T < T_g) in each investigated sample. This is a new observation compared to previous studies demonstrating that such a relationship exists only in the supercooled liquid state of neat PRO. Moreover, it was revealed that the stretching parameters obtained from the aging procedure are very close to the ones determined by fitting the dielectric data above the T_g with the use of the Kohlrausch-Williams-Watts function, indicating that the aging process is governed by the α-relaxation. Complementary Fourier transform infrared and X-ray diffraction measurements allowed us to find a possible reason for these findings. It was demonstrated that although there are very weak intermolecular interactions between PRO and modified saccharides, the intra- and intermolecular structure of PRO is practically unaffected by the presence of modified saccharides.

Ultraslow Relaxation in Aprotic Double Salt Ionic Liquids

A mixture of two pure ionic liquids (ILs) or double salt ILs (DSILs) can push the limits of ILs in terms of unraveling their unique physicochemical properties and potential in clean technology. While the correlated ion dynamics and heterogeneity in the bulk of pure ILs have been reported, such a phenomenon at longer timescales in DSILs has never been elucidated. Here, a combination of temperature-dependent polarized dynamic light scattering and rheological measurements has been employed to reveal the presence of structural and ultraslow relaxation in three DSILs, each containing a 1-ethyl-3-methylimidazolium cation and two different anions. The slow relaxation caused by Brownian diffusion of cluster-like arrangements occurs at a timescale of a few to several hundred milliseconds; both the relaxation processes, nevertheless, are Arrhenius in nature. Notably, slow relaxation in the DSILs is much different compared to that in the pure ILs. The decay of intensity correlation functions (ICFs) and average hydrodynamic correlation length of the clusters and their response to temperature markedly vary with the nature of the two anions present in the DSILs. Stretched exponential analyses of the ICFs disclose the cluster-to-cluster transfer of ionic species as well as percolation dynamics among clusters. The identity of anions also governs whether the DSILs follow or violate the Stokes-Einstein relationship or not.

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.

Ionic conductivity of deep eutectic solvents: the role of orientational dynamics and glassy freezing

Dielectric spectroscopy reveals that the ionic conductivity of deep eutectic solvents is closely coupled to their reorientational dipolar relaxation dynamics.

Molecular dynamics of 1-ethyl-3-methylimidazolium triflate ionic liquid studied by 1H and 19F nuclear magnetic resonances

The molecular dynamics of an ionic liquid (IL) composed of a 1-ethyl-3-methylimidazolium cation and a triflate (trifluoromethanesulfonate) anion, abbreviated as [Emim][TfO], were studied by NMR spectroscopy. By measuring the temperature-dependent high-field 1H and ¹⁹F spin-lattice relaxation (SLR) rates, the frequency-dependent ¹H and ¹⁹F SLR dispersion curves using fast-field-cycling relaxometry, and the temperature-dependent 1H and ¹⁹F diffusion constants, and by utilizing the fact that the primary NMR-active nucleus on the Emim cation is ¹H, whereas on the TfO anion it is ¹⁹F, the cationic and anionic dynamics were studied separately. A single theoretical relaxation model successfully reproduced all the experimental data of both types of resonant nuclei by fitting all the data simultaneously with the same set of fit parameters. Upon cooling, [Emim][TfO] exhibited a supercooled liquid phase between T_SL = 256 K and the crystallization temperature T_Cr ≈ 227-222 K, as confirmed by differential scanning calorimetry (DSC) experiments. Theoretical analysis revealed that within the liquid and the supercooled liquid states of [Emim][TfO], the ¹H and ¹⁹F relaxation rates are affected by both the rotational and translational diffusional processes with no discontinuous change at T_SL. While the rotational diffusion is well described as an Arrhenius thermally activated process, the translational diffusion undergoes strong freezing dynamics that are well described by the Vogel-Fulcher model assuming a freezing temperature of T₀ = 157 K. The existence of the supercooled liquid region in the [Emim][TfO] IL should be taken into account when using this IL for a specific application.

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.

Mechanism of Conductivity Relaxation in Liquid and Polymeric Electrolytes: Direct Link between Conductivity and Diffusivity

Combining broadband impedance spectroscopy, differential scanning calorimetry, and nuclear magnetic resonance we analyzed charge and mass transport in two polymerized ionic liquids and one of their monomeric precursors. In order to establish a general procedure for extracting single-particle diffusivity from their conductivity spectra, we critically assessed several approaches previously employed to describe the onset of diffusive charge dynamics and of the electrode polarization in ion conducting materials. Based on the analysis of the permittivity spectra, we demonstrate that the conductivity relaxation process provides information on ion diffusion and the magnitude of cross-correlation effects between ionic motions. A new approach is introduced which is able to estimate ionic diffusivities from the characteristic times of conductivity relaxation and ion concentration without any adjustable parameters. This opens the venue for a deeper understanding of charge transport in concentrated and diluted electrolyte solutions.

Self-assembly and structural relaxation in a model ionomer melt

Molecular dynamics simulations are used to understand the self-assembly and structural relaxation in ionomer melts containing less than 10% degree of ionization on the backbone. The self-assembly of charged sites and counterions shows structural ordering and agglomeration with a range of structures that can be achieved by changing the dielectric constant of the medium. The intermediate scattering function shows a decoupling of charge and counterion relaxation at longer length scales for only high dielectric constant and at shorter length scales for all dielectric constants. Overall, the slow structural decay of counterions in the strongly correlated ionomer system closely resembles transport properties of semi-flexible polymers.

Recent progress on dielectric properties of protic ionic liquids

Protic ionic liquids (PILs) are key materials for a wide range of emerging technologies. In particular, these systems have long been envisioned as promising candidates for fuel cells. Therefore, in recent years special attention has been devoted to thorough studies of these compounds. Amongst others, dielectric properties of PILs at ambient and elevated pressure have become the subject of intense research. The reason for this lies in the role of broadband dielectric spectroscopy in recognizing the conductivity mechanism in protic ionic systems. In this paper, we summarize the dielectric results of various PILs reflecting recent advances in this field.

Dynamics of the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulphonyl)imide studied by nuclear magnetic resonance dispersion and diffusion

NMR was used to study the dynamics of the ionic liquid Bmim-Tf2N in the supercooled regime. Rotation and translation molecular dynamics exhibit a transition at T_c∼1.26 T_g.

Study on the temperature-dependent coupling among viscosity, conductivity and structural relaxation of ionic liquids

The frequency-dependent viscosity and conductivity of three imidazolium-based ionic liquids were measured at several temperatures in the MHz region, and the results are compared with the intermediate scattering functions determined by neutron spin echo spectroscopy.

Observation of highly decoupled conductivity in protic ionic conductors

Using dielectric spectroscopy, we report the observation of highly decoupled conductivity in a newly synthesized protic ionic conductor, lidocaine di-(dihydrogen phosphate).

Translation-rotation decoupling and nonexponentiality in room temperature ionic liquids

Using a combination of light scattering techniques and broadband dielectric spectroscopy, we have measured the temperature dependence of structural relaxation time and self diffusion in three imidazolium-based room temperature ionic liquids: [bmim][NTf(2)], [bmim][PF(6)], and [bmim][TFA]. A detailed analysis of the results demonstrates that self diffusion decouples from structural relaxation in these systems as the temperature is decreased toward T(g). The degree to which the dynamics are decoupled, however, is shown to be surprisingly weak when compared to other supercooled liquids of similar fragility. In addition to the weak decoupling, we demonstrate that the temperature dependence of the structural relaxation time in all three liquids can be well described by a single Vogel-Fulcher-Tamann function over 13 decades in time from 10(-11) s up to 10(2) s. Furthermore, the stretching of the structural relaxation is shown to be temperature independent over the same range of time scales, i.e., time temperature superposition is valid for these ionic liquids from far above the melting point down to the glass transition temperature. We suggest that these phenomena are interconnected and all result from the same underlying mechanism--strong and directional intermolecular interactions.