Pulsed Field Gradient NMR Technique Research Articles

Application of pfg NMR diffusometry to study diffusion and ionization phenomena in plasticized polymeric membranes doped with ionic liquids

Ionic liquids (ILs) are increasingly used as promising organic solvents for a variety of applications, including electrochemical processes, novel separation techniques and innovative methods of synthesis, due to their low volatility, excellent solvation properties, high ionic conductivity and electrochemical stability. Moreover, the tunable properties of ILs determined by their composition make them favorable candidates as dopants for composite and/or functional polymeric materials, e.g. in the membranes of chemical sensors. To effectively use ILs in polymeric materials and control the characteristics of the latter, one should develop a complete understanding of the properties of ILs in the polymeric phase, such as their diffusion and ionic behavior.In our previous work, we applied pulsed field gradient (PFG) NMR technique to verify the results of a chronopotentiometric method conventionally used to determine the diffusion coefficients and electrolytic properties of electrolytes in plasticized poly(vinyl chloride) (PVC) membranes of chemical sensors. The results obtained with these two independent methods were in good agreement, confirming that PEG NMR diffusometry is a viable technique for assessing the diffusion properties of species even in highly viscous polymeric media of low polarity.Here we report a comprehensive study of the transport and ionic behavior of IL 1-hexyl-3-methyl-1H-imidazolium bis[(trifluoromethyl)sulfonyl]amide ([C6Meim][NTf2]) in a poly(vinyl chloride) matrix, performed in situ by means of PFG NMR diffusometry. A number of sets of spectra for the 1H and 19F nuclei of the [C6Meim][NTf2] ionic liquid in PVC membranes doped with the plasticizer bis(2-ethylhexyl)sebacate were registered at different amplitudes of the magnetic field gradient. The mobility of both individual ions of the IL and the formed ion associates was investigated over a broad range of IL concentrations in the PVC matrix; in addition, the ionization degree and dissociation constant of [C6Meim][NTf2] were determined and their dependence on the IL content was traced.

Elastin-like Peptide as a Model for Disordered Proteins: Diffusion Behaviour in Self-Crowding Conditions.

Despite the current high interest, there is limited information on diffusion data for intrinsically disordered proteins (IDPs). This study investigates the effect of crowding on the diffusion behaviour of an elastin-like peptide (ELP), by combined pulse field gradient (PFG) and static field gradient (SFG) NMR techniques. We interpret our findings in terms of highly dynamic chain assemblies with weak interactions, resulting in ELP diffusion that is primarily governed by the viscous flow of the solvent. The diffusion behaviour of the peptide appears to resemble that of globular proteins rather than flexible linear polymers over a wide concentration range.

Nanocomposite Anion Exchange Membranes for Aems Electrolyzers: Ion Transport and Carbonation Studies

Anion-exchange membranes (AEMs) have received increased interest in recent years as the electrolyte separator in different electrochemical energy conversion and storage (EECS) devices. [1] AEM fuel cells and electrolyzers devices can reach the performance level required by applications with electrocatalysts that do not require a high loading of platinum-group metals (PGMs) due to the alkaline environment at the electrodes. However, the AEM based EECS development and implementation is significantly hindered by the anion exchange membrane (AEM) stability during cell operation, their low OH- conductivity and the low kinetics of the electrocatalysts. In fact, the electrolytic anion-exchange membrane (AEM) is a crucial component which should allow good charge and water transport, i.e. high ionic conductivity, but should also guarantee good chemical, thermal and mechanical stability and high durability. [2]In this work, nanocomposite AEMs based on Polysulfone ionomer (PSU) and two different nanoadditives, Layered Double Hydroxide (LDH) and Nanoscale Ionic Materials (NIM), have been developed and studied. [3] LDHs are mineral anionic clays with elevated ion exchange capacity (IEC). They consist of the positively charged metal hydroxide layers with anions located in the interlayer space. This allows strong hydration of the material together with a large number of hydroxyls on the host layers forming a dense network of hydrogen bonds along the two-dimensional surface, therefore facilitating OH- ion conduction by diffusion mechanism. NIMs have a nanostructure consisting of a spherical silica core functionalized with an ionic oligomeric corona. Due to the extremely high number of functional groups present, these materials are able to significantly increase the ionic conduction capacity of the resulting electrolyte. [4]PSU is a thermoplastic polymer with high thermal stability, good chemical resistance and mechanical properties. However, since it has two activated positions per repetition unit for electrophilic aromatic substitution, it can undergo high degrees of functionalisation, which reduce the mechanical properties. We developed a two steps procedure, with allow a functionalization degree between 60 and 80%, preserving its stability.Nanocomposites AEMs were investigated in both OH- and HCO3 - forms, comparing swelling capacity, ionic conductivity and water diffusion. The latter was studied by NMR spectroscopy, measuring the self-diffusion coefficient by the Pulse Field Gradient (PFG) NMR techniques. The nanocomposite membranes are able to maintain good hydration at high temperatures, and to create an adequate nanostructure with the polymer chains, which favour the Grotthuss diffusion mechanism for the OH- ions. Such feature is reflected in the ionic conductivity and in the alkali stability, where they demonstrated the highest conductivity and a reduced membrane degradation rate. Finally, electrolysis cell tests were conducted on MEAs based on these hybrid membranes, and preliminary results showed very promising performance.

Mobility of Li+, Na+, and Cs+ cations in Nafion membrane, as studied by NMR techniques

The local mobility and diffusion of Li+, Na+, and Cs+ cations in Nafion 117 membrane were explored by 7Li, 23Na, and 133Cs spin relaxation and pulsed field gradient NMR techniques. It was shown that the macroscopic mass transfer of cations is controlled by ion motion near sulfonate groups. Lithium and sodium cations, whose hydrated energy is higher than the water hydrogen bond energy, are moving together with water molecules, but cesium cations possessing a low hydrated energy are jumping directly between the neighboring sulfonate groups.

(Invited) Ions Transport Properties and Carbonation Process Investigation of Nanocomposite Anion Exchange Membranes Containing Layered Double Hydroxide

Electrochemical energy conversion and storage (EECS) technologies such as fuel cells and electrolyzers are expected to play a pivotal role owing to their compatibility with the environment and high energy conversion efficiency. Anion-exchange membranes (AEMs) have received increased interest in recent years as the electrolyte separator in different EECS devices.[1] They have numerous advantages respect to the proton-exchange membranes, such as operation at high pH conditions, significantly lower cost (based on low-cost raw materials), less corrosive environment, and significantly lower fuel crossover (one order of magnitude lower than the acidic counterparts). AEM fuel cells and electrolyzers devices can reach the performance level required by applications with electrocatalysts that do not require a high loading of platinum-group metals (PGMs) due to the alkaline environment at the electrodes.However, the AEM based EECS development and implementation is significantly hindered by the anion exchange membrane (AEM) stability during cell operation, their low OH- conductivity and the low kinetics of the electrocatalysts. In fact, the electrolytic anion-exchange membrane (AEM) is a crucial component which should allow good charge and water transport, i.e. high ionic conductivity, but should also guarantee good chemical, thermal and mechanical stability and high durability. [2]We report on an extensive study on nanocomposite AEMs based on tetramethylammonium Polysulfone ionomer (PSU) and Layered Double Hydroxide (LDH) as nanofiller. [3] PSU is a thermoplastic polymer with high thermal stability, good chemical resistance and mechanical properties. However, since it has two activated positions per repetition unit for electrophilic aromatic substitution, it can undergo high degrees of functionalisation, which reduce the mechanical properties. We developed a two steps procedure, with allow a functionalization degree of about 80%, preserving its stability. Concerning the LDHs, these mineral anionic clays are excellent inorganic anionic conductors with elevated ion exchange capacity (IEC). They consist of the positively charged metal hydroxide layers with anions located in the interlayer space. This allows strong hydration of the material together with a large number of hydroxyls on the host layers forming a dense network of hydrogen bonds along the two-dimensional surface, therefore facilitating OH- ion conduction by diffusion mechanism.PSU/LDH AEMs were investigated in both OH- and forms, comparing swelling capacity, ionic conductivity and water diffusion. The latter was studied by NMR spectroscopy, measuring the self-diffusion coefficient by the Pulse Field Gradient (PFG) NMR techniques. The nanocomposite membranes are able to maintain good hydration at high temperatures, and to create an adequate nanostructure with the polymer chains, which favour the Grotthuss diffusion mechanism for the OH- ions. Such feature is reflected in the ionic conductivity and in the alkali stability, where they demonstrated the highest conductivity and a reduced membrane degradation rate.In addition, the 13C-NMR technique was used to investigate carbonation processes in the presence of CO2, showing that the presence of the LDH platelets into the AEM remarkably reduces the conversion rate of hydroxyl groups, as much as slow down the diffusion of carbonate ions.Finally, electrolysis cell tests were conducted on MEAs based on these hybrid membranes, and preliminary results showed very promising performance. Acknowledgments This work has been supported by the Italian Ministry for University and Research (MUR) for funding through the FISR 2019 project AMPERE (FISR2019_01294). References Vincent, I. and Bessarabov, D. (2018) Renewable and Sustainable Energy Reviews 81, 1690–704G. Arges, L. Zhang, ACS Appl. Energy Mater. 1 (2018) 2991–3012,Simari, C.,...and Nicotera, I. (2022) Electrochimica Acta, 403, p. 139713.

NMR spectroscopy of lipidic cubic phases.

Lipidic cubic phase (LCP) structures have been used for stabilisation and crystallisation of membrane proteins and show promising properties as drug carriers. In this mini-review, we present how NMR spectroscopy has played a major role in understanding the physico-chemical properties of LCPs and how recent advances in pulsed field gradient NMR techniques open new perspectives in characterising encapsulated molecules.

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.

Molecular and Ionic Diffusion in Ion Exchange Membranes and Biological Systems (Cells and Proteins) Studied by NMR.

The results of NMR, and especially pulsed field gradient NMR (PFG NMR) investigations, are summarized. Pulsed field gradient NMR technique makes it possible to investigate directly the partial self-diffusion processes in spatial scales from tenth micron to millimeters. Modern NMR spectrometer diffusive units enable to measure self-diffusion coefficients from 10−13 m2/s to 10−8 m2/s in different materials on 1 H, 2 H, 7 Li, 13 C, 19 F, 23 Na, 31 P, 133 Cs nuclei. PFG NMR became the method of choice for reveals of transport mechanism in polymeric electrolytes for lithium batteries and fuel cells. Second wide field of application this technique is the exchange processes and lateral diffusion in biological cells as well as molecular association of proteins. In this case a permeability, cell size, and associate lifetime could be estimated. The authors have presented the review of their research carried out in Karpov Institute of Physical Chemistry, Moscow, Russia; Institute of Problems of Chemical Physics RAS, Chernogolovka, Russia; Kazan Federal University, Kazan, Russia; Korea University, Seoul, South Korea; Yokohama National University, Yokohama, Japan. The results of water molecule and Li+, Na+, Cs+ cation self-diffusion in Nafion membranes and membranes based on sulfonated polystyrene, water (and water soluble) fullerene derivative permeability in RBC, casein molecule association have being discussed.

Hydration, self-diffusion and ionic conductivity of Li+, Na+ and Cs+ cations in Nafion membrane studied by NMR

Hydration of Nafion 117 perfluorinated sulfonic cation-exchange membrane in alkaline ion forms was investigated by high resolution 1H NMR. Hydration numbers of Li+, Na+ and Cs+ cations were 5 ± 1, 6 ± 1 and 1 ± 0.2, correspondingly for membrane equilibrated with water vapor at 98% RH. As opposed to Li+ and Na+, which form separate ion pair, Cs+ cation directly contacts with membrane sulfonate group. Cation self-diffusion coefficients were measured by pulsed field gradient NMR technique on 7Li, 23Na and 133Cs nuclei for the first time. Self-diffusion coefficients are changed in the next rows Li+ ≤ Na+ > Cs+. Self-diffusion activation energies of Li+ and Na+ cations are about 20 kJ/mol which is close to water self-diffusion activation energy in these membranes, but Cs+ self-diffusion activation energy is distinctly more (25 kJ/mol). Ionic conductivities calculated on the basis of Nernst–Einstein equation from cation self-diffusion coefficients 1.6∙10−2, 2∙10−2, 6∙10−3 S/cm for Li+, Na+, Cs+ cations, correspondingly, are closely approximating to conductivities measured by impedance spectroscopy: 1.3∙10−2, 1.1∙10−2, 2.3∙10−3 S/cm for Li+, Na+, Cs+ cations, correspondingly, but calculated values are appreciably more compared with experimental meanings.

Transport properties in molten carbonates: self-diffusion and conductivity measurements at high temperature

A better understanding and use of molten carbonate fuel cells (MCFCs) requires more detailed consideration on transport properties in melt. The combination of different methodological developments can be one solution to improve our comprehension. Here we present 23Na and 7Li self-diffusion coefficients measured by pulsed field gradient (PFG) NMR technique combined with electrical conductivity obtained by a 4-electrode set up, in the eutectic mixture Li2CO3–Na2CO3 (52:48 %mol) at high temperature (up to 1050 K), and under pure CO2 atmosphere. The results were compared with known experimental data from literature obtained by radiotracers techniques and 2-electrode set up and also with some calculations of the transport properties.

Achieving Stable Cycling of Li-Metal Anodes in Li-O2 Batteries: Optimizing Solvation Environment in Ionic Liquid/Solvent Blend Formulations

The realization of a truly rechargeable, high capacity lithium-oxygen (Li-O2) battery faces serious technological barriers relating to material/intermediate stability, electrochemical reversibility of the cathode discharge processes, and gas supply challenges.1 Furthermore, to maximize energy densities of Li-O2 cells, the utilization of Li-metal anodes is highly desired, thus introducing additional and serious interfacial challenges in the form of electrolyte degradation and susceptibility to dendrite formation on cycling. These factors ultimately create serious safety issues and further contribute to the reduced cyclability of Li-O2 cells that is already limited by the poor reversibility of Li2O2 formation at the cathode. Without introducing artificial anode protection or isolation of Li from a cathoylte compartment, the electrolyte for a high-performance Li-O2 device must exhibit complementary stability at both reactive interfacial regions of the two different electrodes. In many cases, the solvents that mediate good performance at the Li-O2 cathode exhibit poor stability at Li-metal and, consequently, new electrolyte formulations geared towards good stability and performance at both the cathode and the anode are required.In this work, we report on the development of blend electrolytes of ionic liquids (ILs) and molecular solvents for improving cell performance in Li-O2 batteries with considered focus on enhancing stability at the Li-metal anode. Through combined experimental and theoretical investigations, we present how the electrochemical stability of Li-O2-cathode relevant molecular solvents at the Li-metal anode can be regulated by tuning formulation properties. Wide formulation ranges, including dilute to highly concentrated Li-salt mixtures are explored within the ternary formulations of IL, molecular solvent and Li-salt. Good performance in symmetrical Li|electrolyte|Li cells is achieved through improving the Coulombic efficiency of the electroplating/stripping processes, as quantified in asymmetric Li|Cu cells. In lieu of strong SEI forming additives, these observed stability enhancements are found to be highly reliant on Li+-solvent solvation interactions within the liquids. Shifts in vibrational modes associated with changes in coordination, as measured by Raman and IR spectroscopy, provide insights into the nature of these important solvent and salt interactions. These experimental insights are supported by computational modelling of local solvation environments in select exemplary mixtures. Pulsed-field gradient NMR techniques are also utilized to understand what information the diffusional motion of differing species within the IL/solvent blends further reveals about the nature of these stabilizing coordination interactions. The implications of complete formulation design towards achieving mutual stability at anode and cathode interfaces is demonstrated within full cell (Li-O2) testing using composite air-cathodes. 1. D. Aurbach, B. D. McCloskey, L. F. Nazar, and P. G. Bruce, Nat. Energy, 1 16128 (2016).

Cavity-controlled diffusion in 8-membered ring molecular sieve catalysts for shape selective strategy

Shape selectivity is the most important feature and advantage of molecular sieve catalysis. Diffusion of reactant and product molecules inside the confined environments with channels and cavities is the essential aspect of shape selective catalysis of molecular sieve catalysts. Elucidating the diffusion mechanism of molecules in the confined space of catalysts is of great importance for the efficient utilization of the porous materials in the catalytic reaction as well as the strategy proposal of reaction control in shape-selective catalysis. However, it is still a great challenge to understand and quantify the diffusion behavior at the molecular level for the establishment of the diffusion mechanism within the crystal of microporous materials as so far. In this work, with advanced pulsed field gradient (PFG) NMR technique, self-diffusion coefficients of alkanes (methane, ethane and propane) in three cavity-type molecular sieves (LEV, CHA and RHO) with very close eight-membered ring (8-MR) windows were determined. Furthermore, molecular dynamics (MD) simulations predicted the energy barrier for crossing 8-MR to be overcome. Based on MD simulations and a continuous-time random-walk (CTRW) method, not only the diffusion trajectory of the molecule can be visually displayed, but also the diffusion behavior is quantitatively described as the inter-cavity hopping process with the success extraction of the jump frequency (f) and jump length (L). The substantial role played by cavity structure and dimension in diffusion within cavity-type molecular sieves with the close 8-MR windows were revealed in depth, which would help to reveal the mechanism of cavity-controlled diffusion and propose the shape-selective strategy in the methanol-to-olefins process.

Effect of Temperature and Water Content on the Properties of Solid Protic Electrolyte of Tetra Calixarene Sulfonic Acid: NMR Study

The methods of 1H NMR and pulsed field gradient NMR are used to study the protic solid electrolyte of calix[4]arene-para-sulfonic acid in the temperature range of –15 to 24°C and water content λ of 1.6 to 5.7 H2O molecules per SO3H. Analysis of the 1H signal intensity in NMR spectra showed that no ice phase is formed in the whole studied temperature and water content range. The hydration numbers of H+(H2O)h complexes are calculated on the basis of the temperature dependences of chemical shift values. The pulsed field gradient NMR technique is used to determine self-diffusion coefficients. Activation energies Eact are calculated on the basis of the temperature dependences of self-diffusion coefficients. Good agreement of the data on diffusion Eact and protic conductivity are shown. A corollary of the Nernst–Einstein equation is used to calculate the values of protic conductivity on the basis of self-diffusion data. The calculated values of protic conductivity in the water content λ range of 1.6 to 5.7 agree with the data obtained experimentally.

NMR of cellulose nanocrystals, mesoporous media, and liquid crystal assemblies

ABSTRACT This article gives an overview of cellulose nanocrystals (CNCs), liquid crystal templating based on suspensions of CNC rods, and their corresponding templated mesoporous materials. NMR cryoporometry was used to give pore size distributions in chiral nematic mesoporous silica (CNMS) and organosilica (CNMO) films, while NMR pulsed-field gradient technique was used to study diffusion anisotropy of water absorbed in these films. 14N-NMR was used to probe the orientation of a calamite liquid crystal (LC) confined in the pores of CNMO films. Effects due to confining hydrogen-bond assemblies (HBAs) in CNMS and CNMO films were investigated by means of 2H-NMR and 19F-NMR. These results were compared with the ordering in the corresponding neat HBA. The ability to tune/switch the film’s optical responses via orientation ordering of LCs in the pores of CNMO films was demonstrated by NMR.

Emulsion Breakage Mechanism Using Pressurized Carbon Dioxide

The production of water during crude oil extraction may result in the formation of stable water-in-oil emulsions. Such emulsions are problematic for a variety of reasons; for example, they increase the fluid viscosity and hence the pumping costs. Previously, Ling; [NMR Studies of the Effect of CO2 on Oilfield Emulsion Stability. Energy Fuels 2016, 307, 5555–5562] have shown that treating these water-in-crude oil emulsions with subcritical CO2 at 50 bar can lead to their breakage. These measurements utilized benchtop NMR pulsed field gradient (PFG) techniques to monitor the evolution in the emulsion droplet size distribution, which is a precursor to emulsion breakage. Experimental limitations meant, however, that the measurements were performed only following depressurization of the applied CO2 and as such were unable to directly distinguish between two potential mechanisms for emulsion breakage as proposed in the literature: (i) CO2 bubble formation within the water droplets upon depressurization or (ii) ...

Translational diffusion of unfolded and intrinsically disordered proteins.

Translational (or self-diffusion) coefficient in dilute solution is inversely proportional to the size of a diffusing molecule, and hence self-diffusion coefficient measurements have been applied to determine the effective hydrodynamic radii for a range of native and nonnative protein conformations. In particular, translational diffusion coefficient measurements are useful to estimate the hydrodynamic radius of natively (or intrinsically) disordered proteins in solution, and, thereby, probe the compactness of a protein as well as its change when environmental parameters such as temperature, solution pH, or protein concentration are varied. The situation becomes more complicated in concentrated solutions. In this review, we discuss the translational diffusion of disordered proteins in dilute and crowded solutions, focusing primarily on the information provided by pulsed-field gradient NMR technique, and draw analogies to well-structured globular proteins and synthetic polymers.

Improved lithium ion dynamics in crosslinked PMMA gel polymer electrolyte.

Since PMMA-based gel polymer electrolytes could substitute PVDF-HFP based gels currently used in Li-ion batteries at lower financial and environmental costs, we investigate here the solvation and transport properties of the lithium ions in a crosslinked PMMA-based gel polymer electrolyte by a combination of thermal and electrochemical methods, Raman spectroscopy, pulse field gradient (PFG) and electrophoretic NMR (eNMR) techniques, as well as ab initio calculations. The conductivity of the gel containing 10 wt% polymer is only reduced by 14% relative to the liquid electrolyte. In addition, the co-solvation by polymer functional groups, a priori expected to slow lithium transport relatively to the anion, has instead a positive effect on lithium transport. Indeed, the ester groups not only participate in lithium solvation and increase ionic dissociation, but since this interaction is rather weak, rather than lowering the lithium diffusion relatively to other species, it mainly decorrelates lithium transport from anionic mobility. Compared to its liquid fraction, the gels show, at the same time, better dissociation and a higher lithium transference number, which results in a higher cationic conductivity, despite the overall conductivity loss.

Self-diffusion of water-soluble fullerene derivatives in mouse erythrocytes

Self-diffusion of water-soluble fullerene derivative (WSFD) C60[S(CH2)3SO3Na]5H in mouse red blood cells (RBC) was characterized by 1H pulsed field gradient NMR technique. It was found that a fraction of fullerene molecules (~13% of the fullerene derivative added in aqueous RBC suspension) shows a self-diffusion coefficient of (5.5 ± 0.8)·10−12 m2/s, which is matching the coefficient of the lateral diffusion of lipids in the erythrocyte membrane (DL = (5.4 ± 0.8)·10−12 m2/s). This experimental finding evidences the absorption of the fullerene derivative by RBC. Fullerene derivative molecules are also absorbed by RBC ghosts and phosphatidylcholine liposomes as manifested in self-diffusion coefficients of (7.9 ± 1.2)·10−12 m2/s and (7.7 ± 1.2)·10−12 m2/s, which are also close to the lateral diffusion coefficients of (6.5 ± 1.0)·10−12 m2/s and (8.5 ± 1.3)·10−12 m2/s, respectively. The obtained results suggest that fullerene derivative molecules are, probably, fixed on the RBC surface. The average residence time of the fullerene derivative molecule on RBC was estimated as 440 ± 70 ms. Thus, the pulsed field gradient NMR was shown to be a versatile technique for investigation of the interactions of the fullerene derivatives with blood cells providing essential information, which can be projected on their behavior in-vivo after intravenous administration while screening as potential drug candidates.

The application of pulse field gradient (PFG) NMR methods to characterize the efficiency of separation of water-in-crude oil emulsions

Demulsification of water-in-crude oil emulsions is an essential and sometimes challenging procedure for crude oil processing facilities. Pulse field gradient (PFG) NMR techniques are known to monitor the dynamics of emulsion separation. This method has limitations that restrict its application to some crude oils. A comprehensive methodology applicable to all types of crude oil regardless of its viscosity, without assumptions, and providing a large number of data with fast measurements, is proposed in this paper. The coalescence and sedimentation of unstable emulsions was observed through simultaneous measurements of the evolution of the brine profile and droplet size distribution (DSD). Measurements of emulsions after stabilization, with and without the contribution of the free water layer, revealed the residual emulsified water quantity and location in the sample. A new, faster approach to separate the oil and water overlapping T2 relaxation signals was demonstrated on real water-in-crude oil emulsions, using the root mean square displacement (RMSD) measured with the spoiler recovery and a loop of 13-interval pulsed field gradient stimulated echo (PFGSTE) oneshot sequences. The residual water within the crude oils after separation was determined and used to quantify the efficiency of the demulsifier used.

Multi-Temperature in Situ Magnetic Resonance Imaging of Polarization and Salt Precipitation in Li-Ion Battery Electrolytes

The parameterization and validation of robust electrochemical models for lithium-ion batteries in automotive applications, which are intended to address both the state of charge (SOC) and state of health (SOH) estimation problems, is an important milestone in their widespread deployment. Much effort has been focused on the mass transport characteristics of the electrode domains, but a significant fraction of overall polarization stems from concentration gradients that form in the electrolyte domain and their attendant effect on the mass transport parameters, which have recently been shown by in situ MRI to be quite significant, even within modest operational regimes.1 The effect of temperature on these concentration gradients remains to be explored by in situ MRI. The mass transport in the electrolyte domain is typically modelled as being governed by material balance of a binary salt in a concentrated solution, which can be realized by modifying the Nernst-Planck equation to have non-constant, concentration-dependent coefficients (Equation 1).2 This system has a steady-state concentration gradient (Equation 2) when migration flux driven by the potential and diffusion flux against the concentration gradient are in equilibrium (see Figure 1A). Using in situ MRI, the evolution of this gradient toward the steady-state upon constant-current charging can be monitored (Figure 1B). A surprising outcome of these studies was that at 10°C, an equal parts (v/v) mixture of EC/DEC with 1.00 M LiPF6 exhibited anomalous concentration gradient formation (Figure 2) with a current density corresponding to 6 A·m-2 (or ~C/4), whereas the same mixture and cell construction could be polarized without difficulty at room temperature. On comparing raw MR images from before the polarization and after the relaxation of the disrupted concentration gradient, it is clear that the overall signal intensity is lower whilst the shape of the profile is retained, which is a direct indicator that salt precipitation has occurred. There are two routes by which this salt precipitation phenomenon can be avoided: lowering the salt concentration, so that the absolute concentration at the upper portion of the gradient is decreased proportionately for an equivalent current density, and the selection of an electrolyte with better low-temperature solvation characteristics. Successful polarization and visualization of the electrolyte domain in situ was achieved with a 0.85 M variant of the equal-parts EC/DEC mixture utilized above, at a series of temperatures. This constitutes a means of avoiding the critical concentration at the low temperature where the 1.00 M variant exhibited precipitation, but it would come with a trade-off of reduced ionic conductivity. More significantly, a ‘full-strength’ 1.00 M LiPF6 in EC/PC/DMC, 5:2:3 (v/v) mixture was also successfully polarized at low, room, and elevated temperatures, with the same 6 A·m-2 applied current density. Previous ex situ experiments using the pulsed-field gradient NMR technique, in conjunction with conductivity measurements, had revealed that ion pairing at low temperature was significantly reduced in electrolyte mixtures where the fraction of high-dielectric solvent was maximized.3 A quantitative comparison of the steady-state concentration gradients at a series of inter-electrode distances and temperatures was also conducted with this electrolyte formulation, to assess how the magnitudes of the gradients varied with temperature, and to test the hypothesis inherent in Equation 2 that the inter-electrode spacing does not play a role in determining the magnitude of the steady-state gradient. Within 3σ confidence intervals, the inter-electrode spacing was not found to affect the inter-electrode spacing, which directly confirms that the in situ MRI technique can generate steady-state mass transport parameters which are relevant to real battery modelling efforts. A significant effect of the temperature on the magnitude of the steady-state concentration gradient is also illustrated in these multi-temperature imaging experiments, demonstrating the importance of characterizing the spatial variation and temperature-dependence of electrolyte mass transport characteristics for electrochemical model parameterization and validation. References (1) Krachkovskiy, S. A.; Bazak, J. D.; Werhun, P.; Balcom, B. J.; Halalay, I. C.; Goward, G. R. Visualization of Steady-State Ionic Concentration Profiles Formed in Electrolytes during Li-Ion Battery Operation and Determination of Mass-Transport Properties by in Situ Magnetic Resonance Imaging. J. Am. Chem. Soc. 2016, 138 (25), 7992–7999. (2) Sethurajan, A. K.; Krachkovskiy, S. A.; Halalay, I. C.; Goward, G. R.; Protas, B. Accurate Characterization of Ion Transport Properties in Binary Symmetric Electrolytes Using In Situ NMR Imaging and Inverse Modeling. J. Phys. Chem. B 2015, 119 (37), 12238–12248. (3) Krachkovskiy, S. A.; Bazak, J. D.; Fraser, S.; Halalay, I. C.; Goward, G. R. Determination of Mass Transfer Parameters and Ionic Association of LiPF 6 : Organic Carbonates Solutions. J. Electrochem. Soc. 2017, 164 (4), A912–A916. Figure 1