Our research
From Grey Lab Page
MATERIALS CHEMISTRY: STRUCTURE AND FUNCTION
We use a wide range of techniques, including solid state NMR and diffraction, to investigate local structure and the role that this plays in controlling the physical properties of a wide range of technologically-important, but disordered, materials. Conventional structural techniques, such as powder and single-crystal X-ray and neutron diffraction, characterize the "long-range" order, giving an average view of a structure; as a system becomes more disordered, these methods become progressively less useful. Even the most disordered system will, however, contain some local order. Solid-state nuclear magnetic resonance (NMR) probes the local environment of a particular nucleus, and is ideally suited to study such materials. Systems currently under investigation include lithium-ion batteries, fuel cell materials, catalysts and molecular sieves. By using a combination of short range (NMR) and long range (XRD) structural techniques, we can build up a detailed structure of the disordered compound - this helps determine how the particular material functions and provides insight as to how it can be improved. Molecular dynamics simulations provide complementary information concerning the structure and dynamics of, for example, ions in superionic conductors or molecules sorbed on surfaces.
Anionic conductors: We use 17O and 19F magic angle spinning NMR to study oxide and fluoride conduction. By identifying individual crystallographic or interstitial sites in often highly disordered materials, we can determine which anion sites are responsible for ionic conduction and obtain a much deeper understanding of how these materials function as "superionic" conductors. The ionic materials under investigation find potential uses as membranes in solid oxide fuel cells and as oxygen sensors.
Batteries: In a related project, we use lithium NMR to investigate cathodes for rechargeable lithium-ion batteries. Here, the mechanism of lithium intercalation and deintercalation are probed by using 6Li/^7Li NMR and the effect of this on local structure and electronic and magnetic properties are investigated. Even the structures of the cathode materials used in "ordinary" commercial alkaline and lithium batteries (primary, i.e., non-rechargeable batteries- used routinely in, for example, flashlights and cameras) are poorly understood, in part because they contain considerable disorder. The number and location of protons in manganese dioxide are key in determining the power that can be obtained from these systems, yet standard structural techniques such as X-ray diffraction are not sensitive to the locations of these protons. We use deuterium NMR to locate the deuterons (in the deuterium-exchanged materials) and to follow their motion as a function of temperature.
In-situ NMR of Lithium ion batteries: To monitor dynamic structural changes occurring in the active materials used in a battery subjected to electrochemical cycling, we have developed a wide variety of in- and ex-situ NMR techniques, the former being applied synchronously with the cycler. The ex-situ studies of active materials involve arresting the electrochemical cycling at various stages, and subsequent extraction of the active materials which are studied by NMR under fast MAS. Ex-situ, although rich in information content, may fail to capture short-lived metastable phases appearing at various stages of cycling. To capture such processes, we use a flexible plastic bag battery placed inside the NMR spectrometer and NMR spectra are recorded synchronously at uniform interval, while the battery undergoes electrochemical cycling. We demonstrated the immense scope of in-situ technique, in a Si-based Li battery, where appearance of short-lived metastable phases (the peak at -10 ppm) of lithium silicides are accurately captured. (Featured in Chemical and Engineering News, American Chemical Society, NMR method reveals hidden battery chemistry).
Environmental Chemistry: We are members of the NSF Center for Environmental Molecular Sciences (CEMS), which involves research groups from geosciences, materials science, marine sciences and physics at Stony Brook, Brookhaven National Laboratory, Temple University and Penn State. Our current interests involve the use of NMR to characterize (i) new layered materials designed to sorb high concentrations of pollutants and (ii) natural systems such as the hydrated iron and manganese oxides often found in soil samples; these oxides represent important sorbates for toxic ions such as Pb2+ or H2AsO4-.
Gas Sorption and Catalysis: Projects in this area include the study of gas adsorption and reactivity on catalysts and zeolites. In recent work, we showed that new double resonance experiments can be used to determine key internuclear distances, in order to determine the structures of molecules bound inside zeolites. We are currently using 17O NMR to study the oxygen framework sites in zeolites. Our aim is to use this NMR method to follow cation exchange processes and gas binding. We have built equipment to allow structural changes to be monitored while performing catalytic reactions, by using in situ synchrotron X-ray diffraction (XRD) methods. The in situ X-ray diffraction methods are used in conjunction with NMR experiments to determine catalyst structure, and how this changes during the catalytic reaction (e.g., the dismutation reaction of hydrochlorofluorocarbon (HCFC)-22 (HCF2Cl) over gamma-alumina). A combination of short range (NMR) and long range (XRD) structural techniques are required to determine a detail picture of catalyst structure.
