Pulsed Field Gradient NMR Research Articles

Diffusion spreadability as a probe of the microstructure of complex media across length scales.

Understanding time-dependent diffusion processes in multiphase media is of great importance in physics, chemistry, materials science, petroleum engineering, and biology. Consider the time-dependent problem of mass transfer of a solute between two phases and assume that the solute is initially distributed in one phase (phase 2) and absent from the other (phase 1). We desire the fraction of total solute present in phase 1 as a function of time, S(t), which we call the spreadability, since it is a measure of the spreadability of diffusion information as a function of time. We derive exact direct-space formulas for S(t) in any Euclidean space dimension d in terms of the autocovariance function as well as corresponding Fourier representations of S(t) in terms of the spectral density, which are especially useful when scattering information is available experimentally or theoretically. These are singular results because they are rare examples of mass transport problems where exact solutions are possible. We derive closed-form general formulas for the short- and long-time behaviors of the spreadability in terms of crucial small- and large-scale microstructural information, respectively. The long-time behavior of S(t) enables one to distinguish the entire spectrum of microstructures that span from hyperuniform to nonhyperuniform media. For hyperuniform media, disordered or not, we show that the "excess" spreadability, S(∞)-S(t), decays to its long-time behavior exponentially faster than that of any nonhyperuniform two-phase medium, the "slowest" being antihyperuniform media. The stealthy hyperuniform class is characterized by an excess spreadability with the fastest decay rate among all translationally invariant microstructures. We obtain exact results for S(t) for a variety of specific ordered and disordered model microstructures across dimensions that span from hyperuniform to antihyperuniform media. Moreover, we establish a remarkable connection between the spreadability and an outstanding problem in discrete geometry, namely, microstructures with "fast" spreadabilities are also those that can be derived from efficient "coverings" of space. We also identify heretofore unnoticed, to our best knowledge, remarkable links between the spreadability S(t) and NMR pulsed field gradient spin-echo amplitude as well as diffusion MRI measurements. This investigation reveals that the time-dependent spreadability is a powerful, dynamic-based figure of merit to probe and classify the spectrum of possible microstructures of two-phase media across length scales.

Relationship between whey protein nitrogen index of skim milk powder and the heat stability of recombined filled evaporated milk

In this study, it was evaluated whether the whey protein nitrogen index (WPNI) of a range of skim milk powder (SMP) samples was correlated with the heat stability of the corresponding recombined filled evaporated milk (RFEM), as well as with the extent of lactosylation and changes in whey proteins (WPs). Pulsed field gradient NMR results showed that the diffusion coefficients of lactose in all samples varied only to a small extent, while the diffusion coefficients of the WP fraction ranged more widely. A lower heat stability of RFEM produced from SMP with a lower WPNI was found as compared with those with a higher WPNI: principal component analysis (PCA) and correlation analysis revealed that the decreased heat stability could be at least partly explained by the increased calcium solubilization induced by the lower pH of dispersions of SMP with a lower WPNI. Although it was claimed over the years that high-heat SMP (with a low WPNI) is beneficial to ensure a better heat stability of RFEM, it is clear from our results that a low WPNI of SMP alone was not sufficient to guarantee a better RFEM-heat stabilizing functionality.

Dynamics of ethylammonium nitrate near PTFE surface

Self-diffusion of ions in the protic ionic liquid ethylammonium nitrate (EAN) was studied by 1H NMR pulsed field gradient techniques between 294 and 393 K in the presence of a PTFE insert in a 5-mm NMR tube. At all temperatures, the bulk diffusion of ions (measured by 1H and 15N NMR) can be described by a unique diffusion coefficient. The presence of solid hydrophobic surfaces of PTFE induces regions of EAN in their vicinity, where diffusion of ions, both cations and anions, is reduced compared to the bulk values. An additional line-shape analysis in 1H NMR spectra showed that local mobility of ethylammonium cations in the surface layers near PTFE is also reduced.

Long‐term efficiency for reducing entanglements of nascent polyethylene by a polystyrene‐modified Ziegler‐Natta catalyst

AbstractThe weakly entangled ultrahigh molecular weight polyethylene (UHMWPE) was synthesized by a Ziegler‐Natta catalyst, where the titanium tetrachloride was anchored on the polystyrene (PS)‐modified silica. The PS chains were successfully incorporated into silica hierarchical pores even with the size less than 10 nm through the in situ free‐radical polymerization of styrene. The self‐diffusion coefficient and crystallization of probing molecules inside the pores were investigated by the pulsed field gradient NMR and thermoporosimetry to address the swollen behavior of incorporated PS blocks. This PS blocks compartmentalized the polyethylene chains, where the less entangled UHMWPE was synthesized with an exceptional activity at 70°C. The ubiquitous PS isolators effectively hindered the formation of chains overlaps during the polymerization, showing a long‐term efficiency to reduce the entanglements of nascent UHMWPE even at 4 h of polymerization The toughness/stiffness/strength balance of weakly entangled UHMWPE was significantly improved.

Structure and Dynamics in Self-Healing Polymer Electrolytes Based on PVDF-HFP for Battery Applications

Lithium dendrite formation is a key limitation in rechargeable Li metal batteries. Here we report on a novel polymer electrolyte with self-healing capabilities, consisting of PVdF-HFP membranes imbibed with an ionic liquid electrolyte EMIM-TFSI + LiTFSI and mechanically reinforced by alumina nanowires. Similar membranes were reported to have self-healing capability via resistance to dendrite formation at the electrode-electrolyte interface.1 We have investigated the structure and dynamics in these membranes by nuclear magnetic resonance (NMR). Double resonance and 2D NMR methods show that the EMIM cation interacts with CF3 group present in HFP, forming an ion-dipole association which may be related to the desired self-healing properties. Moreover, addition of alumina nanowires has been reported2 to increase the ionic conductivity in the membrane in addition to imparting mechanical strength. We have used pulsed field gradient (PFG) NMR to investigate how alumina nanowire doping into the PVDF-HFP-IL matrix improves the Li+ diffusivity. Additional characterizations such as electron dispersion X-ray (EDX) spectroscopy to study nanowire distribution in the polymer and ionic conductivity measurements, will also be discussed. Keywords: Ionic Liquids, Self-healing, Dendrite formation, PVDF-HFP membranes, NMR spectroscopy, EDX References Chen, T. et al. Ionic liquid-immobilized polymer gel electrolyte with self-healing capability, high ionic conductivity and heat resistance for dendrite-free lithium metal batteries. Nano Energy 54, 17–25 (2018).Kwon, S. J. et al. Influence of Al2O3 Nanowires on Ion Transport in Nanocomposite Solid Polymer Electrolytes. Macromolecules 51, 10194–10201 (2018).

Water Domain Enabled Transport and Enhanced Stability in Aqueous Solid Polymer-in-Salt Electrolytes for Lithium-Ion Batteries

As lithium-ion batteries (LIBs) continue to grow in use for energy storage applications, concerns over their safety have influenced research for alternative materials, particularly solid-state electrolytes to replace the highly flammable and electrochemically unstable liquid organic electrolytes used in commercial LIBs. Aqueous-based solid polymer electrolytes (SPEs) offer a safer approach by using water as an abundant, cheap, and non-toxic solvent in an inherently nonflammable polymer matrix. This class of electrolyte also offers decreased cost and weight, ease of processibility, and increased long-term chemical and mechanical stability. However, widespread commercial adoption of SPEs for lithium-ion batteries has been hindered by subpar transport properties, namely, ionic conductivities <1 mS/cm at room temperature and slower Li+ transport compared to anion transport due to slow polymer chain mobility. The use of water as a solvent also lends to restrictive electrochemical stability windows (ESWs) due to its nonideal redox properties that lead to minimal anodic and cathodic limits, dominated by hydrogen evolution at potentials beyond common anode materials (>2 V vs. Li/Li+).To combat these challenges, our work builds upon the concept of super-concentrated salt systems. In the solid-state regime, highly concentrated “polymer-in-salt” systems can exhibit an ion cluster effect allowing for fast cationic transport decoupled from polymer chain segmental motion. In the aqueous regime, highly concentrated “water-in-salt” systems exhibit the formation of a passivating solid electrolyte interphase (SEI) through reduction of TFSI- to form LiF on the anode surface. The presence of water in the “water-in-salt” system also gives rise to a disproportionation of Li+ solvation environments, namely TFSI-rich and water-rich domains, allowing for fast cationic transport through the water domains. By including a highly concentrated “water-in-salt” system into a polymer matrix, the unique aqueous solid polymer-in-salt electrolyte (ASPE) demonstrates preferential Li+ transport compared to anion transport, high ionic conductivity, and extended ESW. This system also exhibits unique stability in air that eliminates the need for meticulously dry environments and solution processing, which is desirable to manufacturers for substantial savings in productions costs.We demonstrate that the ASPE comprised of poly(ethylene oxide) (PEO), water, and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) salt can achieve a high room temperature ionic conductivity of >1.75 mS/cm. Pulsed field gradient NMR shows a lithium transference number of 0.66, which was corroborated with electrochemical impedance spectroscopy. Molecular dynamic simulations reveal that the exceptional transport properties of the ASPE are likely due to decoupling of ion conduction from the polymer-assisted transport seen in typical SPEs, instead being dominated by water-assisted vehicular transport. The system exhibits stability to ~1.5 V vs. Li/Li+, enabling access to anode materials like LTO and LVPF previously incompatible with aqueous electrolytes. Cycling performance was assessed using LTO/LMO, a ~2.8 V output, to demonstrate the capability of the ASPE to promote intercalation and deintercalation in these electrodes.Further work demonstrates improvement to the ESW of the ASPE by changing the composition of the “water-in-salt” electrolyte to a highly concentrated hybrid aqueous/ionic liquid electrolyte and changing the polymer matrix. The inclusion of ionic liquid as a second, non-aqueous solvating molecule reduces the amount of water needed in the system and provides an alternative pathway for quality SEI formation. High molecular weight polymer offers a solvating matrix to reduce the overall concentration of water while simultaneously reducing water activity through changes in the hydrogen bonding structure. Additionally, the new polymer matrix exhibits superior thermodynamic stability at high voltages and aids in decoupling ionic mobility from polymer mobility, making it a strong choice for highly concentrated systems without sacrificing transport properties. This new class of ASPE demonstrates a remarkable extended cathodic limit <1 V vs. Li/Li+ and overall ESW >4 V with minimal electrochemical activity up to 5.5 V vs. Li/Li+. Cyclic voltammetry demonstrates compatibility with commercial LTO and TNO anodes, which when paired with appropriate high voltage cathode material can enable fast-charging systems with increased energy density.

The charge transport mechanism in Brønsted-acidic protic ionic liquid/water systems – An NMR and QENS study

In this study, a protic ionic liquid (PIL), 2-Sulfoethylmethylammonium triflate [2-Sema][TfO] is considered as a potential new proton conducting electrolyte for future polymer membrane fuel cells capable of ambient air operation above 100 °C. The proton dynamics of the PIL with residual water are examined as a function of the hydration level on different time scales using pulsed field gradient nuclear magnetic resonance (PFG-NMR) and quasi-elastic neutron scattering (QENS). The separation of the different contributing relaxation processes enables a quantification of the proton fractions for the underlying hopping or vehicular motions. The hopping motion of the water in the time scale of picosecond and the vehicular motion in the time scale of nanosecond are detected by means of QENS. Such dynamic processes can be well described by the Chudley-Elliot jump model. This emphasised the presence of fixed jump lenghts. In the timescale of millisecond, the cooperative transport of the active protons of the acidic SO3H group and of the H2O molecules, as well as the vehicular transport of the PIL cations are detected by NMR. The different diffusion coefficients obtained by the NMR and QENS techniques are discussed in detail.

Experimental analysis on dynamics of liquid molecules adjacent to particles in nanofluids

Quasi-elastic neutron scattering (QENS) and pulsed-field-gradient nuclear magnetic resonance (PFG-NMR) analyses of a nanofluid composed of silicon dioxide (SiO2) nanoparticles (sized 100, 300, or 500 nm) and a base fluid of ethylene glycol aqueous solution were performed. The aim was to elucidate the mechanism increase in the thermal conductivity of the nanofluid above its theoretical value. Typically, the self-diffusion coefficient of liquid molecules in nanofluids can indicate the effect of the nanoparticles on thermal transport in nanofluids as well as the molecular dynamics of the liquid surrounding the nanoparticles. At 298 and 333 K, the obtained experimental results indicate that SiO2 particles may decrease the self-diffusion coefficient of the liquid molecules in the ethylene glycol aqueous solution because of their highly restricted motion around these nanoparticles. The self-diffusion coefficient of the liquid molecules comprising 300 nm-sized particles is lower than that of the nanofluids comprising 100 and 500 nm-sized particles. At a constant temperature, the thermal conductivity increases as the self-diffusion coefficient of the liquid molecules decreases in the SiO2 nanofluids.

Molecules, the Ultimate Nanomotor: Linking Chemical Reaction Intermediates to their Molecular Diffusivity.

The intellectual community focused on nanomotors has recently become interested in extending these concepts to individual molecules. Here, we study a chemical reaction according to whose mechanism some intermediate species should speed up while others slow down in predictable ways, if the nanomotor hypothesis of boosted diffusion holds. Accordingly, we scrutinize the absolute diffusion coefficient (D) during intermediate steps of the catalytic cycle for the CuAAC reaction (copper-catalyzed azide-alkyne cycloaddition click reaction), using proton pulsed field-gradient nuclear magnetic resonance to discriminate between the diffusion of various reaction intermediates. We observe time-dependent diffusion that is enhanced for some intermediate molecular species and depressed for those whose size increases owing to complex formation. These findings point to the failure of the conventional Stokes-Einstein equation to fully explain diffusivity during chemical reaction. Without attempting a firm explanation, this paper highlights aspects of the physics of chemical reactions that are imperfectly understood and presents systematic data that can be used to assess hypotheses.

Quantifying diffusion of organic liquids in a MOF component of MOF/Polymer mixed-matrix membranes by high field NMR

Confinement of crystals of metal–organic frameworks (MOFs) in polymers to form MOF/polymer mixed-matrix membranes (MMMs) can lead to changes in intra-MOF diffusion. Such changes in intra-MOF diffusion were previously demonstrated for light gases, and attributed to a reduction in MOF framework flexibility. However, to our knowledge, no direct measurements of intra-MOF diffusion in MMMs and of the related MOF confinement effect on diffusion have been reported for organic liquids. In this work, 13C pulsed field gradient (PFG) NMR was used to quantify self-diffusion of methanol, ethanol, p-xylene, and o-xylene inside MOF crystals of the type ZIF-71, which were dispersed in a Torlon polymer to form MMMs. The intra-ZIF self-diffusivities in the MMMs were compared with the corresponding self-diffusivities measured in beds of ZIF-71 crystals. The observed self-diffusivity dependencies on diffusion time were explained by crystal boundary effects. The corresponding values of intra-ZIF self-diffusivities not perturbed by such effects were found to be the same, within uncertainty, in the MMMs and ZIF-71 beds. The observed lack of an influence of the ZIF-71 confinement in Torlon on diffusion is explained, and an option to increase diffusion selectivity is discussed.

Diffusion and Viscosity of Unentangled Polyelectrolytes

We report chain self-diffusion and viscosity data for sodium polystyrene sulfonate (NaPSS) in semidilute salt-free aqueous solutions measured by pulsed-field gradient NMR and rotational rheometry, respectively. The molecular weight of NaPSS is characterized using five techniques. Relationships between Mw and the intrinsic viscosity and diffusion coefficient in excess salt are established. These are helpful for the accurate determination of the molar mass of NaPSS. The observed concentration dependence of η and D are consistent with the Rouse–Zimm scaling model if the monomeric friction coefficient (ζ) is concentration-dependent. The concentration dependence of ζ exceeds that expected from free-volume models of diffusion, and its origin remains unclear, possibly being related to electrostatic effects. Correlation blobs and dilute chains with equivalent end-to-end distances exhibit nearly equal friction coefficients, in agreement with scaling. Viscosity and diffusion data are combined using the Rouse model to calculate the single-chain dimensions of NaPSS in salt-free solution, and the results overpredict direct SANS measurements of the radii of gyration by a factor of ≃1.4.

Li+ and Oxidant Addition To Control Ionic and Electronic Conduction in Ionic Liquid-Functionalized Conjugated Polymers

Conduction of ions and charge (electrons) often follow distinct materials design rules, presenting a significant challenge for the development of homogeneous materials that are good at both. The fundamental interactions that dictate ionic and electronic conduction in mixed conductors are still unclear. Here, we characterize the ionic and electronic conduction of a class of mixed polymeric conductors in which ionic liquid groups are tethered to an electron-conducting conjugated polymer backbone. A model conjugated polymeric ionic liquid, poly{3-[6′-(N-methylimidazolium)hexyl]thiophene}BF4– (P3HT-IM), is synthesized and shown to have significant long-range ordering. Chemical oxidation of the polymer results in a room-temperature electronic conductivity of 10–2 S cm–1. The polymer is also capable of dissolving Li+ salt up to a concentration of rsalt = 1 [moles of salt]/[moles of monomer]. The polymer displays a monotonic increase in ionic conductivity with salt concentration, reaching a maximum room-temperature ionic conductivity of 10–5 S cm–1 at the highest concentration of rsalt = 1. Notably, this is among the first studies to characterize both ionic conductivity and electronic conductivity of an ionic liquid-functionalized conjugated polymer upon the addition of an oxidant and salt. All-atom molecular dynamics (MD) simulations indicate that the imidazolium side chains promote the formation of a percolated network of solvation sites at high salt concentrations, which facilitates ion transport. Pulsed-field gradient nuclear magnetic resonance diffusivity measurements and MD indicate a lithium transference number around 0.5, suggesting that the percolated solvation network promotes lithium transport in a way that is unique from many ion-conducting systems. These results suggest that the addition of diffuse, ionic liquid-like groups to a conjugated polymer backbone serves as an effective design approach to facilitate simultaneous lithium-ion conduction and electronic conduction in the absence of a solvent.

Binding of Dipeptides to Fatty Acid Membranes Explains Their Colocalization in Protocells but Does Not Select for Them Relative to Unjoined Amino Acids.

Dipeptides, which consist of two amino acids joined by a peptide bond, have been shown to have catalytic functions. This observation leads to fundamental questions relevant to the origin of life. How could peptides have become colocalized with the first protocells? Which structural features would have determined the association of amino acids and peptides with membranes? Could the association of dipeptides with protocell membranes have driven molecular evolution, favoring dipeptides over individual amino acids? Using pulsed-field gradient nuclear magnetic resonance, we find that several prebiotic amino acids and dipeptides bind to prebiotic membranes. For amino acids, the side chains and carboxylate contribute to the interaction. For dipeptides, the extent of binding is generally less than that of the constituent amino acids, implying that other mechanisms would be necessary to drive molecular evolution. Nevertheless, our results are consistent with a scheme in which the building blocks of the biological polymers colocalized with protocells prior to the emergence of RNA and proteins.

Unusual ion transport behaviour of ethylammonium nitrate mixed with lithium nitrate

The diffusivity of ions and ionic conductivity of ethylammonium nitrate (EAN) mixed with lithium nitrate (LiNO3) has been carried out as a function of Li-salt concentration and temperature. An unusual behavior of ion diffusivities and ionic conductivities of the mixtures are observed over a range of Li-salt concentration and temperature. The diffusivities of EA+ and Li+, as measured by pulsed-field gradient (PFG) nuclear magnetic resonance (NMR) diffusometry, are found to be comparable in the lower temperature range. An overall decrease in the ion diffusivity is observed with an increase in the concentration of LiNO3. A lower degree of dissociation of ionic complexes in the presence lower concentration of the Li-salt (less than 6 mol. %) resulted in lower ionic conductivity. In the higher concentration range of Li-salt the Li+ diffusivity is monotonously decreased with an increase in the concentration. In the lower concentration range, the Li+ diffusivity exceeded the diffusivity of EA+ cation demonstrating the release of Li+ from the associates. Being enclosed between glass plates, the diffusivities of EA+ and Li+ showed peculiarities similar to the earlier observed results for neat nitrate ILs: accelerated diffusivity of cations and reversible alteration of diffusivities under the influence of strong static magnetic field.

Structure and dynamics of large ring polymers

We report a comprehensive study on the molecular conformation and dynamics of very large poly(ethylene oxide) rings in the melt: (i) for all rings, independent of the ring size, by small angle neutron scattering we observe a crossover from a strong Q-dependence at intermediate Q to a Q−2 dependence at higher Q. Constructing a generic model including a crossover from Gaussian statistics at short distances to more compact structures at larger distances, we find the crossover at a distance along the ring of Ne,0=45±2.5 monomers close to the entanglement distance in the linear counterpart. This finding is clear evidence for the predicted elementary loops building the ring conformation. (ii) The radius of gyration Rg(N) follows quantitatively the result of numerous simulations. However, other than claimed, the crossover to mass fractal statistics does occur around N≅10Ne,0, but up to N≅44Ne,0, the relation Rg(N)∼N0.39 holds. The self-similar ring dynamics was accessed by pulsed field gradient-NMR and neutron spin echo spectroscopy: we find three dynamic regimes for center of mass diffusion starting (i) with a strong subdiffusive domain ⟨rcom2(t)⟩∼tα(0.4≤α≤0.65), (ii) a second subdiffusive region ⟨rcom2(t)⟩∼t0.75 that (iii) finally, crosses over to Fickian diffusion. The internal dynamics at scales below the elementary loop size is well described by the ring Rouse motion. At larger scales, the dynamics is self-similar and follows very well the predictions of scaling models with a preference for the fractal loopy globule model. Finally, we note that the key results were previously published in the form of two letters [Kruteva et al., ACS Macro Lett. 9, 507–511 (2020) and Kruteva et al., Phys. Rev. Lett. 125, 238004 (2020)].

23Na NMR T1 relaxation measurements as a probe for diffusion and dynamics of sodium ions in salt-glycerol mixtures.

Mixtures of sodium salts with oxygen-containing molecules are useful from the perspective of applications such as sodium ion batteries because they fill the gap between deep eutectic solvents and molten salt hydrates. In a previous work, the physical properties (such as diffusion coefficients, conductivity, viscosity, and glass transition temperature) of four salts, namely, Na2B4O7 · 10H2O, NaOAc · 3H2O, NaBr, and NaOAc, were measured with glycerol. Pulsed-field gradient (PFG) nuclear magnetic resonance (NMR) was also used to measure self-diffusion coefficients of 1H-bearing species. However, the technique was not able to measure diffusion of sodium ions due to the very fast NMR relaxation rate of such species, resulting in loss of the PFG NMR signal. In the current work, this study is expanded using 23Na T1 relaxation measurements which, under certain assumptions, can be translated into diffusion coefficients. Analysis of the physical properties is then correlated with self-diffusion coefficient measurements to elucidate information about structure and ionic mobility. It is shown that NaOAc · 3H2O, NaBr, and NaOAc fit models for ionic conductivity and diffusion, which are consistent with ionic liquids where charge transport is limited by ionic mobility rather than the number of charge carriers. The waters of hydration of NaOAc · 3H2O do not appear to form a separate phase but are instead strongly coordinated to the cation. In contrast, Na2B4O7 · 10H2O appears to form a water-rich phase with enhanced sodium mobility.

The Role of Rotational Motion in Diffusion NMR Experiments on Supramolecular Assemblies: Application to Sup35NM Fibrils.

Pulsed-field gradient (PFG) NMR is an important tool for characterization of biomolecules and supramolecular assemblies. However, for micrometer-sized objects, such as amyloid fibrils, these experiments become difficult to interpret because in addition to translational diffusion they are also sensitive to rotational diffusion. We have constructed a mathematical theory describing the outcome of PFG NMR experiments on rod-like fibrils. To test its validity, we have studied the fibrils formed by Sup35NM segment of the prion protein Sup35. The interpretation of the PFG NMR data in this system is fully consistent with the evidence from electron microscopy. Contrary to some previously expressed views, the signals originating from disordered regions in the fibrils can be readily differentiated from the similar signals representing small soluble species (e.g. proteolytic fragments). This paves the way for diffusion-sorted NMR experiments on complex amyloidogenic samples.

Ion Transport and Electrochemical Properties of Fluorine-Free Lithium-Ion Battery Electrolytes Derived from Biomass

Unlike conventional electrolytes, ionic liquid (IL)-based electrolytes offer higher thermal stability, acceptable ionic conductivity, and a higher electrochemical stability window (ESW), which are indispensable for the proper functioning of Li-ion batteries. In this study, fluorine-free electrolytes are prepared by mixing the lithium furan-2-carboxylate [Li(FuA)] salt with the tetra(n-butyl)phosphonium furan-2-carboxylate [(P4444)(FuA)] IL in different molar ratios. The anion of these electrolytes is produced from biomass and agricultural waste on a large scale and, therefore, this study is a step ahead toward the development of renewable electrolytes for batteries. The electrolytes are found to have Tonset higher than 568 K and acceptable ionic conductivities in a wide temperature range. The pulsed field gradient nuclear magnetic resonance (PFG-NMR) analysis has confirmed that the (FuA)− anion diffuses faster than the (P4444)+ cation in the neat (P4444)(FuA) IL; however, the anion diffusion becomes slower than cation diffusion by doping Li salt. The Li+ ion interacts strongly with the carboxylate functionality in the (FuA)− anion and diffuses slower than other ions over the whole studied temperature range. The interaction of the Li+ ion with the carboxylate group is also confirmed by 7Li NMR and Fourier transform infrared (FTIR) spectroscopy. The transference number of the Li+ ion is increased with increasing Li salt concentration. Linear sweep voltammetry (LSV) suggests lithium underpotential deposition and bulk reduction at temperatures above 313 K.

A Study of Ferrocene Diffusion in Toluene/Tween 20/1-Butanol/Water Microemulsions for Redox Flow Battery Applications

Redox flow batteries (RFBs) possess multiple advantages as a flexible energy storage solution. However, RFB researchers are still facing many challenges in finding an appropriate electrolyte. Microemulsions have recently been proposed as a promising alternative RFB electrolyte because of their ability to accommodate organic redox species with fast electron transfer rates that are not soluble in aqueous phase, while still offering the high conductivity of an aqueous salt electrolyte. In this work, we focused on understanding the transport of ferrocene (Fc) in a toluene/Tween 20/1-butanol/water model microemulsion and studied the compositional influence on Fc diffusion. The results show that Fc redistributes among the oil, surfactant, and water microenvironments, and the corresponding diffusion and partition coefficients are quantified. Thus, a tortuous path diffusion model is proposed to describe the mass transport of Fc to an electrode surface. Diffusion coefficients are also obtained by pulsed-field gradient nuclear magnetic resonance (PFG NMR), while the values for Fc diffusion are substantially higher than those from electrochemical measurements, suggesting that they measure samples in different ways. The current contributions from each microenvironment indicate that the Fc permeability is much higher in the oil, even though the electron transfer reaction is likely occurring in the surfactant.

Concentration- and Temperature-Driven Evolution of V5+ Solution Structure in Redox-Flow Battery Electrolytes Studied By Multinuclear NMR

The ongoing implementation of renewable energy generation systems into worldwide electrical grids increasingly demands additional energy storage platforms to maintain stability through their associated intermittency, with vanadium redox-flow batteries being a forefront solution owing to the decoupling of energy- and power-density that they afford.1 However, a lingering performance issue associated with this chemistry in its conventional format, with the vanadium ions dissolved in concentrated aqueous sulfuric acid, is the poor thermal stability of the V5+ cation in solution. This leads to precipitation as the hydrated penta-coordinated vanadate complex, stable at room temperature, is deprotonated and converted to insoluble V2O5 by a condensation reaction induced at elevated temperature.2 This process ultimately limits the concentration of the electrolyte – adversely impacting the available energy density – and its operation at temperatures above ~310 K. While alternatives to this relatively simple electrolyte composition have shown great promise in rectifying this thermal- and concentration-induced instability of the solvated V5+, via the introduction of mixed acid solvent systems3 or stabilizing additives,4 a complete understanding in the original formulation at a range of length- and time-scales and across a multitude of conditions will aid the further development of alternative systems, and provide a means to benchmark their performance at a very fundamental level.This study therefore aims to greatly expand on previous multinuclear NMR work2,5 on the elucidation of this precipitation mechanism by combining a thorough exploration of concentration effects on V5+ solvation structure and dynamics in conjunction with an investigation of their thermal behavior. Ten samples spanning the concentration range of 0 M to 1.7 M V5+ (all with 5 M H2SO4) were examined over temperatures from 298 K to 383 K ascending, and subsequently likewise descending. Combining 1H, 17O, 33S, and 51V linewidth and shift measurements, the evolving solution structure with increasing temperature – and the associated hysteresis on again decreasing it – is mapped as a function of the V5+ concentration. In particular, the extent of the thermal hysteresis previously established with 17O and 51V linewidth and shift measurements at high V5+ concentration2 is quantitatively examined as the concentration is lowered. This spectroscopic analysis is complemented by a similar variable-concentration and variable-temperature study of the 1H self-diffusivity via pulsed-field gradient (PFG) NMR and by 17O longitudinal (T1) relaxation measurements, to determine both the effect of this evolving solution structure on the dynamics and transport, and how they are impacted by increasing V5+ concentration via the associated activation energy trends. The 1H PFG-NMR measurements, coupled with high-accuracy dynamic viscosity measurements via the falling-ball method, are also used to examine the evolution of the effective proton hydrodynamic radius as the V5+ aggregation reaction occurs. The totality of the data, collected across such a wide range of concentration and temperature conditions, yields an unprecedented map of the V5+ solvation phenomena and solution dynamics which spans molecular to bulk length- and time-scales, and which will serve as a useful platform of comparison for analyzing the effectiveness of alternative solvent systems and stabilizing agents presently undergoing development. References M. Skyllas-Kazacos, M. H. Chakrabarti, S. A. Hajimolana, F. S. Mjalli, and M. Saleem, J. Electrochem. Soc., 158, R55 (2011) https://iopscience.iop.org/article/10.1149/1.3599565.M. Vijayakumar et al., J. Power Sources, 196, 3669–3672 (2011) http://dx.doi.org/10.1016/j.jpowsour.2010.11.126.L. Li et al., Adv. Energy Mater., 1, 394–400 (2011) http://doi.wiley.com/10.1002/aenm.201100008.N. Kausar, A. Mousa, and M. Skyllas-Kazacos, ChemElectroChem, 3, 276–282 (2016) http://doi.wiley.com/10.1002/celc.201500453.M. Vijayakumar et al., Chempluschem, 80, 428–437 (2015) http://doi.wiley.com/10.1002/cplu.201402139.