Gradient Spin-echo NMR Technique Research Articles

Mass Transfer Parameters and Ionic Association of Dual-Cation Electrolyte for Hybrid Capacitor System

[Introduction] Hybrid capacitors, which combine an activated carbon (AC) electrode with a large-capacity faradic (psuedocapacitive or battery) electrode, are the focus on an increase in energy density of electric double layer capacitors (EDLCs). One representative faradic electrode material for such hybrid systems is lithium titanate (Li4Ti5O12, LTO), which was used as a negative electrode in one of the first hybrid systems1) and has been extensively investigated since then 2). In this connection, we have reported a dual-cation electrolyte system composed of lithium tetrafluoroborate (LiBF4) and spiro-(1,1’)-bipyrrolidium tetrafluoroborate (SBPBF4) dissolved in propylene carbonate (PC), where the latter cation gives high ionic conductivity in the PC-based electrolytes3). The dual-cation electrolyte system raised rate capability of LTO/AC hybrid capacitors to a level comparable to EDLCs, thanks to the enhancement of LTO reaction kinetic. Still, the detailed mechanism of improved rate capability by dual-cation system has not been completely understood. The difficulty of the analysis lies in the electrolyte containing multiple ion spices, whose contribution to the rate capability cannot be simply divided into individuals.The purpose of this study was to evaluate the contribution of individual ionic species by using pulsed gradient spin-echo (PGSE) NMR technique. Specifically, by determining the self-diffusion coefficient of dual cation electrolyte system, we attempted to break down the ionic conductivity of the whole electrolyte system into individual ionic species and to calculate a carrier number of ions. [Experimental]Electrolytes used in this report were prepared by dissolving 1 M LiBF4 and x M SBPBF4 in PC (0<x<3). All the electrolyte was prepared by mixing components with magnetic stirrer overnight in the Ar-filled glove box. PGSE-NMR was conducted to determine self-diffusion coefficients of Li+(7Li), SBP+(1H) and BF4 -(19F) in electrolytes with maximum gradient strength of 13.5 T/m at different durations. The interval between pulse gradients was set as 50 ms. Ionic conductivity of prepared electrolytes was measured by electrochemical impedance spectroscopy (EIS). [Results and discussion]Self-diffusion coefficients of different ions obtained from PGSE-NMR were plotted in Figure 1, with different concentration of SBPBF4 (0, 1, 2, and 3 M) in the 1 M LiBF4/PC electrolyte. Diffusion coefficients for all three ions decreased with an increment of added SBPBF4 concentration, while the value of SBP+ is higher compared to other two ions, especially Li+, in all cases. Based on obtained self-diffusion coefficients, we calculated the transport number t x of ions (x = Li+, SBP+, or BF4 -) according to the following equation(1) t x = N x D x /(N[Li+]D[Li+] + N[BF4 -] D[BF4 -] + N[SBP+] D[SBP+]) (1)where N x and D x is a concentration and self-diffusion coefficient of corresponding ions, respectively. Using this t x, we further calculated ionic conductivity of each ion by simply multiplying electrolyte ionic conductivity by t x. The evaluated values are shown in Figure 2. Like diffusion coefficients shown in Figure 1, Li+ ionic conductivity decreased with an increment of SBPBF4 concentration. Interestingly, cations ionic conductivity, which is sum of Li+ and SBP+, kept increased with an addition of SBPBF4. In order to discuss cations dissociation in electrolytes with presentence of SBPBF4, we evaluated number of carrier for Li+ based on the peak shifts observed in 7Li NMR spectra and theory of Haven ratio where degree of dissociation is considered as λEIS/λNMR. Here again, with a SBPBF4 concentration increase, number of Li+ carrier decreased, while number of SBP+ carrier increased. In conclusion, a dual-cation electrolyte (Li+ and SBP+) decreases Li+ ionic conductivity and its number of carrier, however, increases the two parameters for the SBP+. The present result suggests that the presence of highly conductive and dissociated SBP+ accelerates charge compensation of LTO reaction, and thus enhanced its rate capability. [References]1) G.G. Amatucci et al., J. Electrochem. Soc., 148 (8) A930 (2001).2) K. Naoi et al., Acc. Chem. Res., 46 (5), 1075 (2013).3) T. Ueda et al., 5th International Conference on Advanced Capacitors (ICAC 2016), 2O-03, Otsu, Japan (2016). Figure 1

Microemulsions with the ionic liquid ethylammonium nitrate: phase behavior, composition, and microstructure.

In this study, we investigate properties of microemulsions which consist of the ionic liquid (IL) ethylammonium nitrate (EAN), the nonionic surfactant C12E3 and an n-alkane, namely n-dodecane or n-octane. The compositions of the coexisting phases are calculated from the densities and volumes of the respective phases. Since the interfacial tension between the water-rich and the oil-rich phase in traditional microemulsions (containing water and oil) relates to the microstructure, spinning drop tensiometry is used to measure the interfacial tension σab and to estimate the domain sizes. Finally, measuring the self-diffusion coefficients of all components via the Fourier Transform Pulsed Gradient Spin Echo (FTPGSE) NMR technique allowed distinguishing between continuous and discrete structures. Our results indicate that the general principles underlying water-n-alkane-CiEj microemulsions can indeed be transferred to oil-in-EAN droplet and the respective bicontinuous microemulsions, while differences are observed for EAN-in-oil droplet microemulsions.

Influence of lithium salt addition on ionic conductivity and transporting properties of lithium bis(trifluoromethanesulfonyl)imide-doped glycine-based ionic liquid electrolyte

The effects of the incorporation of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) on the ionic transporting and physicochemical properties of a glycine-based ionic liquid (IL) are studied. The ionization conditions of each ionic species in LiTFSI-doped glycine-based ionic liquid electrolytes ([MGlyA][TFSI]), are characterized by diffusion coefficients of the species measured using the pulsed gradient spin-echo (PGSE) NMR technique. Temperature-dependent viscosity, ionic conductivity, molar conductivity, and self diffusion coefficient in neat [MGlyA][TFSI] and LiTFSI-doped [MGlyA][TFSI] followed the Vogel–Tamman–Fulcher equation at all concentrations. The experimental molar conductivity (Λ) of neat [MGlyA][TFSI] and LiTFSI-doped [MGlyA][TFSI] is lower than that of the calculated molar conductivity (ΛNMR) from PGSE NMR technique over the entire temperature range, demonstrating that not all the diffusive species contribute to the ionic conduction, that is, free-ions, ionic pairs, and/or clusters coexist in ILs.

Effects of cations and anions on transport properties in tetrafluoroborate-based ionic liquids

The self-diffusion coefficient, diffusion radius, dissociation and association degree, and transport numbers of various cations (methylpyrrolidinium, 1-methyl-2-oxopyrrolidinium, methylpiperidinium, ethylmorpholinium, and methylmorpholinium) and tetrafluoroborate anion in ionic liquids are measured by observing 1H and 19F nuclei using the pulsed-field gradient spin-echo (PGSE) NMR technique at various temperatures. The self-diffusion coefficients determined for the cations and anions of ionic liquids by PGSE NMR method exhibit tetrafluoroborate anion have higher value than cations over a wide temperature range. The temperature dependence of the self-diffusion coefficient obeys the Vogel–Tamman–Fulcher (VTF) equation, and the VTF parameters were presented. The self-diffusion coefficient is analyzed in terms of the Stokes–Einstein equation and the Nernst–Einstein equation, the hydrodynamic radius of ILs is calculated from Stokes–Einstein equation and the molar conductivity (Λ NMR) calculated from the Nernst–Einstein equation is derived without considering ionic association. The dissociation degree of cations and anions range from 72 to 86% for [PipMe][BF 4], [PyrMe][BF 4], [MorMe][BF 4], and [MorEt][BF 4], whereas the ratio is 32% for [PyrOMe][BF 4] at 303 K, this difference can be explained by taking the ionic association into consideration for [PyrOMe][BF 4].

Electrochemical studies and self diffusion coefficients in cyclic ammonium based ionic liquids with allyl substituents

Several cyclic ammonium-based ionic liquids (ILs) with allyl substituent are synthesized, these allyl substituent ILs have high ionic conductivity (up to 4.72 mS cm −1 at 30 °C) and wide electrochemical window of 5 V. The electrochemical behaviors of two organometallic redox couples Fc/Fc + (ferrocene/ferrocenium) and Cc/Cc + (cobaltocene/cobaltocenium) have been studied in these ILs, the calculated Stokes–Einstein product ( Dη T −1) of ferrocene in ILs is larger than that of cobaltocenium in ILs. The self-diffusion coefficients of cation and anion in these ILs are studied using pulsed gradient spin-echo NMR technique. There are very few reports where electrochemically derived diffusion coefficients and self diffusion coefficients are available for comparison, so a new key concept in electrochemistry could be developed in this paper.

Transport properties in two pyrrolidinium-based protic ionic liquids as determined by conductivity, viscosity and NMR self-diffusion measurements

Conductivity and viscosity measurements of pyrrolidinium hydrogen sulfate, [Pyrr][HSO 4], and pyrrolidinium trifluoroacetate [Pyrr][CF 3COO] were performed at various temperatures over a wide temperature range (i.e., from T = 273 K to 353 K). The results were utilized in the Stokes–Einstein equation to investigate the proton conductivity in both PILs. The self-diffusion coefficients ( D) of the cation and anion species in both studied PILs were independently determined in the same temperature range by observing 1H and 19F nuclei with the pulsed-field gradient spin-echo NMR technique. With regard to the mechanism of self-diffusion, based on the values of the coefficients, a relatively large difference was observed between the two ionic liquids (ILs). Independently of the temperature, the D values indicated that the diffusion of both ions was similar, signifying that they were tightly bound together as ion pairs. Nevertheless, mobile protons attached to nitrogen atoms exhibited D values five times higher than those of the pyrrolidinium cation or hydrogenosulfate anion in [Pyrr][HSO 4], and twofold those in the case of [Pyrr][CF 3CO 2]. In order to comment d.c. conductivities results, the self-diffusion coefficients determined by PGSE NMR were converted into charge diffusivity D by means of the Nernst–Einstein equation. In a similar way, a viscosity-related diffusivity D was calculated with the aid of the Stokes–Einstein equation. The temperature-independent cation transference number and the effective hydrodynamic radius were also deduced from these equations. Such parameters play an important role in charge and mass transports in ILs. Moreover, proton conduction follows a combination of Grotthuss- and vehicle-type mechanisms, which confirms that Brønsted acid–base ionic liquid systems are good candidates as proton conductors in fuel cells or supercapacitor electrolyte devices operating under anhydrous conditions at elevated temperatures.

Ion–ion interactions of LiPF 6 and LiBF 4 in propylene carbonate solutions

Self-diffusion coefficients of Li + D Li + , PF 6 − D PF 6 − and solvent propylene carbonate (PC) D PC in LiPF 6−PC solutions were determined at 298 K by the pulse gradient spin echo (PGSE) NMR technique over the salt concentration range of 0.1–3.0 M (M = mol dm – 3 ). The order of the diffusion coefficients was found to be D Li + < D PF 6 − < D PC over the concentration range examined, and they were monotonically decreased with increasing the salt concentration. Haven ratio Λ/ Λ NMR, where Λ and Λ NMR represent the ionic conductivity measured electrochemically and that estimated via the Nernst-Einstein equation using the diffusion coefficient, respectively, was evaluated as the measure of the ion–ion interaction in the LiPF 6–PC solutions. Though Λ/ Λ NMR values for LiPF 6-solutions decrease with increasing the salt concentration, they were greater than those for LiBF 4–PC solutions over the whole concentration range examined, which indicates that the ion pair formation ability of PF 6 – ion is weaker than that of the BF 4 – ion. The smaller value of the ionic conductivity for the highly concentrated LiPF 6–PC solution (above 2.0 M) than that of the LiBF 4-solutions can be attributed to the more rapidly increased viscosity relative to the LiBF 4-solution. Classic molecular dynamics (MD) simulations for the respective LiPF 6 and LiBF 4-solution of 0.5 and 1.0 M were also carried out based on the effective pair potentials. Diffusion coefficients, ionic conductivity and Haven ratio for these solutions were calculated from MD trajectories, and they qualitatively agree with those evaluated by experiments. Pair correlation functions g LiO( r) (for Li +–O (PC) pair) and g LiPF 6 ( r) (for Li +–PF 6 – pair) or g LiBF 4 ( r) (for Li +–BF 4 – pair) revealed that the lithium ion weakly forms the contact ion pairs with PF 6 –, whilst strongly with BF 4 –, which supports the present experimental results. Moreover, the simulation results show that both anions in the contact ion pairs predominantly take the monodentate form, which is in contrast to the multidentate coordination predicted by ab initio calculation in gas phase.

Neutral and cationic main group element cages of germanium(ii) with pyrazolyl ligands: solid state structures, DFT calculations and advanced solution NMR investigations

Two equivalents of 3,5-bis(trifluoromethyl)pyrazole or 5-methyl-3-(trifluoromethyl)pyrazole react with [Ge{N(SiMe(3))(2)}(2)] to give dimeric homoleptic germanium(ii) compounds [{Ge(3,5-RR'pz)(2)}(2)] (R = R' = CF(3) (1); R = Me, R' = CF(3) (2)), which provide cationic triply pz-bridged cages of the general formula [Ge(mu(2)-3,5-RR'pz)(3)Ge](+)[OTf](-) (R = R' = CF(3) (3); R = Me, R' = CF(3) (4)) upon treatment with one equivalent of triflic acid (HOTf, HSO(3)CF(3)). The compounds were studied using the pulsed field gradient spin-echo (PGSE) NMR technique and (19)F,(1)H-HOESY NMR methods. Single crystal X-ray structure determinations of all compounds are reported. The results obtained are verified by density functional theory calculations.

Water and proton transport properties of hexafluorinated sulfonated poly(arylenethioethersulfone) copolymers for applications to proton exchange membrane fuel cells

In the present study, we examine the water and proton transport properties of hexafluorinated sulfonated poly(arylenethioethersulfone) (6F-SPTES) copolymer membranes for applications to proton exchange membrane fuel cells (PEMFCs). The 6F-SPTES copolymer membranes build upon the structures of previously studied sulfonated poly(arylenethioethersulfone) (SPTES) copolymer membranes to include CF 3 functional groups in efforts to strengthen water retention and extend membrane performance at elevated temperatures (above 120 °C). The 6F-SPTES copolymer membranes sustain higher water self-diffusion and greater proton conductivities than the commercial Nafion ® membrane. Water diffusion studies of the 6F-SPTES copolymer membranes using the pulsed-field gradient spin-echo NMR technique reveal, however, the fluorinated membranes to be somewhat unfavorable over their non-fluorinated counterparts as high temperature membranes. In addition, proton conductivity measurements of the fluorinated membranes up to 85 °C show comparable results with the non-fluorinated SPTES membranes.

Toolbox for 3D imaging and modeling of porous media: Relationship with transport properties

Porous media can be considered as interfacial systems where an internal surface partitions and fills the space in a complex way. Meaningful structural features appear on a length-scale where physical chemistry plays a central role either to impose a specific organisation on the material or to strongly modify the dynamics and the thermodynamics of the embedded fluids. A key issue is to understand how the geometrical and interfacial confinement affects numerous phenomena such as molecular diffusion, excitation relaxation, reaction kinetics, phase transitions, adsorption and capillary condensation. We will first review some experimental techniques able to image the 3D structure of disordered porous media. In the second part, we will analyse the geometrical and particularly some topological properties of a disordered porous material. We will discuss the interest and the limits of several strategies for obtaining 3D representations of various pore networks starting from an incomplete set of morphological characterisations. Finally, connection between geometry and diffusive transport will be presented, with emphasis on the application of pulsed gradient spin echo NMR technique as a tool for a multiscale analysis of transport in a confining geometry.

A New Tool in Peptide Engineering: A Photoswitchable Stilbene‐type β‐Hairpin Mimetic

Peptide secondary structure mimetics are important tools in medicinal chemistry, as they provide analogues of endogenous peptides with new physicochemical and pharmacological properties. The development, synthesis, photochemical investigation, and conformational analysis of a stilbene-type beta-hairpin mimetic capable of light-triggered conformational changes have been achieved. In addition to standard spectroscopic techniques (nuclear Overhauser effects, amide temperature coefficients, circular dichroism spectroscopy), the applicability of self-diffusion measurements (longitudinal eddy current delay pulsed-field gradient spin echo (LED-PGSE) NMR technique) in conformational studies of oligopeptides is demonstrated. The title compound shows photoisomerization of the stilbene chromophore, resulting in a change in solution conformation between an unfolded structure and a folded beta-hairpin.

Effects of alkyl chain on transport properties in 1-alkyl-3-methylimidazolium hexafluorophosphates

We studied the transport properties in a series of 1-alkyl-3-methylimidazolium hexafluorophosphates with the alkyl side chains being butyl, hexyl, and octyl, which are expressed by [BMIM][PF 6], [HMIM][PF 6], and [OMIM][PF 6], respectively. The self-diffusion coefficients of the cation ( D cation) and anion ( D anion) species in the ionic liquids were independently determined at a fixed temperature of 323.2±0.1 K by observing 1H and 19F nuclei with the pulsed-field gradient spin-echo NMR technique together with the electric conductivities. Based on these experimental results, the effects of alkyl side chain on transport properties in 1-alkyl-3-methylimidazolium hexafluorophosphates were discussed in terms of interionic interactions.

Self-diffusion coefficients of 1-butyl-3-methylimidazolium hexafluorophosphate with pulsed-field gradient spin-echo NMR technique

The self-diffusion coefficients of the cation and anion species in 1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF6], were independently determined over a wide temperature range by observing 1H and 19F nuclei with the pulsed-field gradient spin-echo NMR technique. The self-diffusion measurements were carried out for two samples of [BMIM][PF6], of which one was synthesized in our laboratory and the other was purchased commercially (97%). There has been a relatively large difference observed in the self-diffusion coefficients between the two samples. It has become apparent that a small amount of impurity significantly affects the transport phenomena in the ionic liquid. On the basis of the hydrodynamic model, moreover, the self-diffusion coefficients of [BMIM][PF6] were discussed in terms of intermolecular interactions.

Nanometric sponges made of water-soluble hydrophobic dendrimers.

The behavior of a fifth generation phosphorus dendrimer having a hydrophobic interior and hydrophilic end groups is studied in solution in water using the pulsed-field gradient spin-echo NMR technique. Addition of THF to this solution induces dramatic changes concerning the size of the dendrimer (+150% in volume), T1 relaxation of THF, and viscosity. This dendrimer behaves like a highly singular nanotool, able both to modify its environment and to be modified by it.

Diffusion NMR spectroscopy for the characterization of the size and interactions of colloidal matter: the case of vesicles and nanoparticles.

We report the application of the pulse gradient spin-echo (PGSE) NMR technique (PGSE NMR) to the analysis of large colloidal materials, specifically vesicles formed from macromolecular amphiphiles and nanoparticles. Measurements of size and size distribution were demonstrated to be comparable to those obtained through dynamic light scattering or hydrodynamic chromatography. In comparison to these more common analytical methods, the use of PGSE NMR is particularly advantageous in that, as a spectroscopic technique, it adds chemical selectivity to the study of physical dimensions. In this way, chemically different species contemporarily present in a sample may be individually studied. In addition, we demonstrate the use of PGSE NMR to probe the existence of equilibria between macroamphiphiles present in solution and those present in vesicles or on the surface of nanoparticles. This feature in particular opens exciting possibilities for the characterization of the phase behavior and of the surface adsorption phenomena of colloids.

Ionic conduction mechanisms of lithium gel polymer electrolytes investigated by the conductivity and diffusion coefficient

Conduction properties of the polymer gel electrolytes of lithium were investigated based on the electrical conductivity and diffusion coefficient using the pulsed gradient spin echo NMR technique. The gel electrolytes prepared by the phase inversion method showed characteristic features in diffusive behavior depending on the kind of immiscible solvent used during the preparation process of the porous polymer membranes. The diffusion coefficients of several kinds of gel were composed of two components that were attributed to the different phases, the trapped solution in the cavities in the membrane and the swollen polymer chains due to the immersed solution. The swollen polymer dominates the high conductivity of this type of gel electrolytes due to the homogenous network appropriate for carrier migration through the chains.

Diffusion Coefficients in Cold Sulfuric Acid Solution

The diffusion coefficients of dimethyl sulfoxide, dimethyl sulfone, 15N-labeled DNO3, and 17O-labeled D2O in solutions of 0.6−0.8 weight fraction D2SO4 in D2O were studied at temperatures from 248 to 298 K by the pulsed gradient spin-echo NMR technique. Diffusion coefficients for these compounds are reported, and an empirical technique for extrapolating the results to sulfuric acid of stratospheric compositions and temperatures is discussed. The viscosity of 0.7 weight fraction D2SO4 in D2O at temperatures from 248 to 298 K is also reported.

Expression of aquaporins in Xenopus laevis oocytes and glial cells as detected by diffusion-weighted 1H NMR spectroscopy and photometric swelling assay

Expression of aquaporins (AQP) and water permeability were studied in Xenopus laevis oocytes and immobilized glial cells by a pulsed-field gradient spin echo NMR technique and a photometric swelling assay. Oocytes injected with poly(A) RNA from C6-BU-1 cells showed increased swelling behavior under hypoosmotic stress due to expressed water channels as compared to control oocytes. The swelling could be reversibly inhibited by HgCl 2. Furthermore, the intracellular relaxation time and the apparent intracellular diffusion coefficient of water in oocytes were determined by diffusion-weighted 1H NMR experiments to be T 2=36 ms and D app,intra=0.18×10 −3 mm 2/s. In immobilized C6 and F98 cells the mean exchange time of intracellular water was found to be 51 ms which increased to 75 ms upon chronic treatment (4 days) in hypertonic medium. Additional hybrid depletion experiments with antisense oligonucleotides directed against AQP1 were performed on oocytes and C6 cells. Moreover, different water channel subtypes of glial cells were assessed by a reverse transcriptase polymerase chain reaction assay. With this, the mRNA encoding AQP1 could be detected in primary cultures and glial cell lines, whereas AQP4 mRNA was found in astroglia-rich primary cultures, but not in F98 and C6 cells. Our results show that water permeability in glial cells is mainly mediated by water channels which play an important role in the regulation of water flow in brain under normal and pathological conditions.

Self-diffusion of water in thermoreversible gels near volume transition: Model development and PFG NMR investigation

Pulsed filed gradient spin echo (PFG-SE) NMR technique has been used to measure the self-diffusion coefficient ( D self) of water in a thermoreversible hydrogel, Poly(N-isopropylacrylamide). D self, was measured as a function of temperature (−5–50°C) and hydration level. A theoretical model to predict D self was built by using a combination of lattice fluid hydrogen bond theory and free volume theory. The fully swollen gel did not show any discontinuous change in D self with temperature, Whereas, a partially swollen gel, showing the existence of two kinds of water having diffusivities differing by order of magnitude, exhibits a discontinuous change in D self. These results were explained within the theoretical framework developed by us. The present work thus enables an elucidation of diffusion phenomenon at the volume transition point for the first time.

Dynamic properties of aqueous solutions of ethylene glycol oligomers as measured by the pulsed gradient spin-echo NMR technique at 25[deg

Self-diffusion coefficients for binary systems containing water and an oligomer of ethylene glycol [HO–(CH2CH2–O)nH, n=1–5], have been measured at 25°C in the whole concentration range by the pulsed gradient spin-echo NMR technique. Upon adding oligomer to pure water, both the oligomer and water self-diffusion coefficients decrease until a certain composition is reached, after which they remain approximately constant. This evidence is interpreted in terms of molecular interactions, especially hydrogen-bond formation. A relation between mutual and self-diffusion coefficients for these systems is found and commented upon.