Understanding the Chemical Shifts of Aqueous Electrolyte Species Adsorbed in Carbon Nanopores

Interfaces between aqueous electrolytes and nanoporous carbons are involved in a number of technological applications such as energy storage and capacitive deionization. Nuclear magnetic spectroscopy is a very useful tool to characterize ion adsorption in such systems thanks to its nuclei specificity and the ability to distinguish between ions in the bulk and in pores. We use complementary methods (density functional theory, molecular dynamics simulations, and a mesoscopic model) to investigate the relative importance of various effects on the chemical shifts of adsorbed species: ring currents, ion organization in pores of various sizes, specific ion–carbon interactions, and hydration. We show that ring currents and ion organization are predominant for the determination of chemical shifts in the case of Li+ ions and hydrogen atoms of water. For the large Rb+ and Cs+ ions, the additional effect of the hydration shell should be considered to predict chemical shifts in agreement with experiments

Understanding the Chemical Shifts of Aqueous Electrolyte Species Adsorbed in Carbon Nanopores

Interfaces between aqueous electrolytes and nanoporous carbons are involved in a number of technological applications such as energy storage and capacitive deionization. Nuclear magnetic spectroscopy is a very useful tool to characterize ion adsorption in such systems thanks to its nuclei specificity and the ability to distinguish between ions in the bulk and in pores. We use complementary methods (density functional theory, molecular dynamics simulations, and a mesoscopic model) to investigate the relative importance of various effects on the chemical shifts of adsorbed species: ring currents, ion organization in pores of various sizes, specific ion–carbon interactions, and hydration. We show that ring currents and ion organization are predominant for the determination of chemical shifts in the case of Li+ ions and hydrogen atoms of water. For the large Rb+ and Cs+ ions, the additional effect of the hydration shell should be considered to predict chemical shifts in agreement with experiments

Lithium Conductivity and Ions Dynamics in LiBH₄/SiO₂ Solid Electrolytes Studied by Solid-State NMR and Quasi-Elastic Neutron Scattering and Applied in Lithium–Sulfur Batteries

Composite solid-state electrolytes based on ball-milled LiBH₄/SiO₂ aerogel exhibit high lithium conductivities, and we have found an optimal weight ratio of 30/70 wt % LiBH₄/SiO₂ with a conductivity of 0.1 mS cm^–1 at room temperature. We have studied the Li⁺ and BH₄^– dynamics using quasi-elastic neutron scattering and solid-state nuclear magnetic resonance and found that only a small fraction (∼10%) of the ions have high mobilities, whereas most of the LiBH₄ shows behavior similar to macrocrystalline material. The modified LiBH₄ is formed from interaction with the SiO₂ surface and most probably from reaction with the surface silanol groups. We successfully applied these composite electrolytes in lithium–sulfur solid-state batteries. The batteries show reasonable capacity retention (794 mAh g^–1 sulfur after 10 discharge–charge cycles, Coulombic efficiency of 88.8 ± 2.7%, and average capacity loss of 7.2% during the first 10 cycles)

Applications of NMR Crystallography to Problems in Biomineralization: Refinement of the Crystal Structure and ³¹P Solid-State NMR Spectral Assignment of Octacalcium Phosphate

By combining X-ray crystallography, first-principles density functional theory calculations, and solid-state nuclear magnetic resonance spectroscopy, we have refined the crystal structure of octacalcium phosphate (OCP), reassigned its ³¹P NMR spectrum, and identified an extended hydrogen-bonding network that we propose is critical to the structural stability of OCP. Analogous water networks may be related to the critical role of the hydration state in determining the mechanical properties of bone, as OCP has long been proposed as a precursor phase in bone mineral formation. The approach that we have taken in this paper is broadly applicable to the characterization of crystalline materials in general, but particularly to those incorporating hydrogen that cannot be fully characterized using diffraction techniques

High-Rate Intercalation without Nanostructuring in Metastable Nb₂O₅ Bronze Phases

Nanostructuring and nanosizing have been widely employed to increase the rate capability in a variety of energy storage materials. While nanoprocessing is required for many materials, we show here that both the capacity and rate performance of low-temperature bronze-phase TT- and T-polymorphs of Nb₂O₅ are inherent properties of the bulk crystal structure. Their unique “room-and-pillar” NbO₆/NbO₇ framework structure provides a stable host for lithium intercalation; bond valence sum mapping exposes the degenerate diffusion pathways in the sites (rooms) surrounding the oxygen pillars of this complex structure. Electrochemical analysis of thick films of micrometer-sized, insulating niobia particles indicates that the capacity of the T-phase, measured over a fixed potential window, is limited only by the Ohmic drop up to at least 60C (12.1 A·g^–1), while the higher temperature (Wadsley–Roth, crystallographic shear structure) H-phase shows high intercalation capacity (>200 mA·h·g^–1) but only at moderate rates. High-resolution ^6/7Li solid-state nuclear magnetic resonance (NMR) spectroscopy of T-Nb₂O₅ revealed two distinct spin reservoirs, a small initial rigid population and a majority-component mobile distribution of lithium. Variable-temperature NMR showed lithium dynamics for the majority lithium characterized by very low activation energies of 58(2)–98(1) meV. The fast rate, high density, good gravimetric capacity, excellent capacity retention, and safety features of bulk, insulating Nb₂O₅ synthesized in a single step at relatively low temperatures suggest that this material not only is structurally and electronically exceptional but merits consideration for a range of further applications. In addition, the realization of high rate performance without nanostructuring in a complex insulating oxide expands the field for battery material exploration beyond conventional strategies and structural motifs

Ion Dynamics in Li₂CO₃ Studied by Solid-State NMR and First-Principles Calculations

Novel lithium-based materials for carbon capture and storage (CCS) applications have emerged as a promising class of materials for use in CO₂ looping, where the material reacts reversibly with CO₂ to form Li₂CO₃, among other phases depending on the parent phase. Much work has been done to try and understand the origin of the continued reactivity of the process even after a layer of Li₂CO₃ has covered the sorbent particles. In this work, we have studied the lithium and oxygen ion dynamics in Li₂CO₃ over the temperature range of 293–973 K in order to elucidate the link between dynamics and reactivity in this system. We have used a combination of powder X-ray diffraction, solid-state NMR spectroscopy, and theoretical calculations to chart the temperature dependence of both structural changes and ion dynamics in the sample. These methods together allowed us to determine the activation energy for both lithium ion hopping processes and carbonate ion rotations in Li₂CO₃. Importantly, we have shown that these processes may be coupled in this material, with the initial carbonate ion rotations aiding the subsequent hopping of lithium ions within the structure. Additionally, this study shows that it is possible to measure dynamic processes in powder or crystalline materials indirectly through a combination of NMR spectroscopy and theoretical calculations

Ring Current Effects: Factors Affecting the NMR Chemical Shift of Molecules Adsorbed on Porous Carbons

Nuclear magnetic resonance (NMR) spectroscopy is increasingly being used to study the adsorption of molecules in porous carbons, a process which underpins applications ranging from electrochemical energy storage to water purification. Here we present density functional theory (DFT) calculations of the nucleus-independent chemical shift (NICS) near various sp²-hybridized carbon fragments to explore the structural factors that may affect the resonance frequencies observed for adsorbed species. The domain size of the delocalized electron system affects the calculated NICSs, with larger domains giving rise to larger chemical shieldings. In slit pores, overlap of the ring current effects from the pore walls is shown to increase the chemical shielding. Finally, curvature in the carbon sheets is shown to have a significant effect on the NICS. The trends observed are consistent with existing NMR results as well as new spectra presented for an electrolyte adsorbed on carbide-derived carbons prepared at different temperatures

Revealing Local Dynamics of the Protonic Conductor CsH(PO₃H) by Solid-State NMR Spectroscopy and First-Principles Calculations

A joint study incorporating multinuclear solid-state NMR spectroscopy and first-principles calculations has been performed to investigate the local structure and dynamics of the protonic conductor CsH­(PO₃H) in the paraelectric phase. The existence of the superprotonic phase (>137 °C) is clearly confirmed by NMR, in good agreement with the literature. The variable-temperature ¹H, ²H, and ³¹P NMR data further reveal a distribution of motional correlation times, with isotropic rotation of the phosphite ion being observed below the superprotonic phase transition for a small but gradually increasing subset of anions. This isotropic rotation is associated with fast local protonic motion, with the distribution of correlation times being tentatively assigned to internal defects or surface adsorbed H₂O. The phosphite ion dynamics of the majority slower subset of phosphite ions is quantified through analysis of variable-temperature ¹⁷O spectra recorded from 34 to 150 °C, by considering a model for the pseudo <i>C</i>₃ rotation of the phosphite ion around the P–H bond axis below the phase transformation. An extracted activation energy of 0.24 ± 0.08 eV (23 ± 8 kJ mol^–1) for this model was obtained, much lower than that reported from proton conductivity measurements, implying that no strong correlation exists between long-range protonic motion and <i>C</i>₃ rotations of the phosphite. We conclude that proton conduction in CsH­(PO₃H) in the paraelectric phase is governed by the activation energy for exchange between donor and acceptor oxygen sites, rotation of the phosphite units, and the lack of isotropic rotation of the phosphite ion. Surprisingly, coalescence of ¹⁷O NMR resonances, as would be expected for rapid isotropic reorientations of all phosphite groups, is not observed above the transition. Potential reasons for this are discussed

A Multinuclear Solid-State NMR Study of Templated and Calcined Chabazite-Type GaPO-34

The open-framework gallophosphate GaPO-34 is prepared with either 1-methylimidazole or pyridine as the structure-directing agent. ¹³C and ¹H NMR spectra for these two variants of the as-made GaPO-34 are fully assigned, confirming the presence of the protonated amine and water within the pores of both materials. ³¹P MAS NMR confirms the presence of three crystallographic P sites, while ⁷¹Ga MAS and MQMAS NMR spectra reveal three crystallographic Ga sites: two tetrahedral and one six-coordinate. Simulations of ⁶⁹Ga MAS NMR spectra from these results are in good agreement with spectra acquired at <i>B</i>₀ = 20.0 T, and assignments are supported by first-principles calculations. ¹⁹F MAS NMR proves the presence of Ga-bridging fluoride within the as-made materials, leading to the six-coordinate gallium. Calcination removes the organic species and fluoride, yielding a microporous chabazite-type GaPO₄, containing one tetrahedral Ga site. Exposure to moist air yields calcined, rehydrated GaPO-34 containing four-, five-, and six-coordinate gallium. Upon heating this material, loss of crystallinity is observed by powder X-ray diffraction and NMR, with the latter revealing a range of P and Ga environments. The thermal instability of calcined, rehydrated GaPO-34 contrasts with the isomorphous aluminophosphate, showing that apparently analogous materials may have important differences in reactivity

In Situ NMR Spectroscopy of Supercapacitors: Insight into the Charge Storage Mechanism

Electrochemical capacitors, commonly known as supercapacitors, are important energy storage devices with high power capabilities and long cycle lives. Here we report the development and application of in situ nuclear magnetic resonance (NMR) methodologies to study changes at the electrode–electrolyte interface in working devices as they charge and discharge. For a supercapacitor comprising activated carbon electrodes and an organic electrolyte, NMR experiments carried out at different charge states allow quantification of the number of charge storing species and show that there are at least two distinct charge storage regimes. At cell voltages below 0.75 V, electrolyte anions are increasingly desorbed from the carbon micropores at the negative electrode, while at the positive electrode there is little change in the number of anions that are adsorbed as the voltage is increased. However, above a cell voltage of 0.75 V, dramatic increases in the amount of adsorbed anions in the positive electrode are observed while anions continue to be desorbed at the negative electrode. NMR experiments with simultaneous cyclic voltammetry show that supercapacitor charging causes marked changes to the local environments of charge storing species, with periodic changes of their chemical shift observed. NMR calculations on a model carbon fragment show that the addition and removal of electrons from a delocalized system should lead to considerable increases in the nucleus-independent chemical shift of nearby species, in agreement with our experimental observations