Matthew T. Dunstan

Origin of additional capacities in metal oxide lithium-ion battery electrodes

Metal fluorides/oxides (MF(x)/M(x)O(y)) are promising electrodes for lithium-ion batteries that operate through conversion reactions. These reactions are associated with much higher energy densities than intercalation reactions. The fluorides/oxides also exhibit additional reversible capacity beyond their theoretical capacity through mechanisms that are still poorly understood, in part owing to the difficulty in characterizing structure at the nanoscale, particularly at buried interfaces. This study employs high-resolution multinuclear/multidimensional solid-state NMR techniques, with in situ synchrotron-based techniques, to study the prototype conversion material RuO2. The experiments, together with theoretical calculations, show that a major contribution to the extra capacity in this system is due to the generation of LiOH and its subsequent reversible reaction with Li to form Li2O and LiH. The research demonstrates a protocol for studying the structure and spatial proximities of nanostructures formed in this system, including the amorphous solid electrolyte interphase that grows on battery electrodes.

Inhibiting the interaction between FeO and Al 2 O 3 during chemical looping production of hydrogen

Hydrogen of high purity can be produced by chemical looping using iron oxide as an oxygen carrier and making use of the reaction between steam and either iron or FeO. However, this process is viable only if the iron oxide can be cycled between the fully-oxidised and fully-reduced states many times. This can be achieved if the iron oxide is supported on refractory oxides such as alumina. Unfortunately, the interaction between alumina and oxides of iron to form FeAl2O4 hinders the kinetics of the reactions essential to the production of hydrogen, viz. the reduction of Fe(II) to metallic iron by a mixture of CO and CO2 prior to the oxidation by steam. Here, oxygen carriers containing Fe2O3 and Al2O3 were doped with Na2O and, or, MgO, in order to inhibit the formation of FeAl2O4 by forming NaAlO2 or MgAl2O4, respectively. The performance of the modified oxygen carriers for producing hydrogen, i.e. cyclic transitions between Fe2O3 (or Fe3O4) and metallic Fe at 1123 K were investigated. It was found that the interaction between FeO and Al2O3 was successfully mitigated in an oxygen carrier containing Mg, with an Al: Mg ratio of 2, resulting in consistently stable and high capacity for producing hydrogen by chemical looping, whether or not the material was oxidised fully in air in each cycle. However, the oxygen carrier without Mg only remained active when a step to oxidise the sample in air was included in each cycle. Otherwise it progressively deactivated with cycling, showing substantial interaction between Al2O3 and oxides of Fe.

Probing Oxide-Ion Mobility in the Mixed Ionic–Electronic Conductor La2NiO4+δ by Solid-State 17O MAS NMR Spectroscopy

While solid-state NMR spectroscopic techniques have helped clarify the local structure and dynamics of ionic conductors, similar studies of mixed ionic–electronic conductors (MIECs) have been hampered by the paramagnetic behavior of these systems. Here we report high-resolution 17O (I = 5/2) solid-state NMR spectra of the mixed-conducting solid oxide fuel cell (SOFC) cathode material La2NiO4+δ, a paramagnetic transition-metal oxide. Three distinct oxygen environments (equatorial, axial, and interstitial) can be assigned on the basis of hyperfine (Fermi contact) shifts and quadrupolar nutation behavior, aided by results from periodic DFT calculations. Distinct structural distortions among the axial sites, arising from the nonstoichiometric incorporation of interstitial oxygen, can be resolved by advanced magic angle turning and phase-adjusted sideband separation (MATPASS) NMR experiments. Finally, variable-temperature spectra reveal the onset of rapid interstitial oxide motion and exchange with axial sites at ∼130 °C, associated with the reported orthorhombic-to-tetragonal phase transition of La2NiO4+δ. From the variable-temperature spectra, we develop a model of oxide-ion dynamics on the spectral time scale that accounts for motional differences of all distinct oxygen sites. Though we treat La2NiO4+δ as a model system for a combined paramagnetic 17O NMR and DFT methodology, the approach presented herein should prove applicable to MIECs and other functionally important paramagnetic oxides.

Large scale in silico screening of materials for carbon capture through chemical looping

Chemical looping combustion (CLC) has been proposed as an efficient carbon capture process for power generation. Oxygen stored within a solid metal oxide is used to combust the fuel, either by releasing the oxygen into the gas phase, or by direct contact with the fuel; this oxyfuel combustion produces flue gases which are not diluted by N2. These materials can also be used to perform air-separation to produce a stream of oxygen mixed with CO2, which can subsequently be used in the conventional oxyfuel combustion process to produce sequesterable CO2. The temperature and oxygen partial pressures under which various oxide materials will react in this way are controlled by their thermodynamic equilibria with respect to reduction and oxidation. While many materials have been proposed for use in chemical looping, many suffer from poor kinetics or irreversible capacity loss due to carbonation, and therefore applying large scale in silico screening methods to this process is a promising way to obtain new candidate materials. In this study we report the first such large scale screening of oxide materials for oxyfuel combustion, utilising the Materials Project database of theoretically determined structures and ground state energies. From this screening several promising candidates were selected due to their predicted thermodynamic properties and subjected to initial experimental thermodynamic testing, with SrFeO3−δ emerging as a promising material for use in CLC. SrFeO3−δ was further shown to have excellent cycling stability and resistance to carbonation over the temperatures of operation. This work further advances how in silico screening methods can be implemented as an efficient way to sample a large compositional space in order to find novel functional materials.

Complex 5d Magnetism in a Novel S = 1/2 Trimer System, the 12L Hexagonal Perovskite Ba4BiIr3O12

The 12L hexagonal perovskite Ba4BiIr3O12 has been synthesized for the first time and characterized using high-resolution neutron and synchrotron X-ray diffraction as well as physical properties measurements. The structure contains Ir3O12 linear face-sharing octahedral trimer units, bridged by corner-sharing BiO6 octahedra. The average electronic configurations of Ir and Bi are shown to be +4(d(5)) and +4(s(1)), respectively, the same as for the S = 1/2 dimer system Ba3BiIr2O9, which undergoes a spin-gap opening with a strong magnetoelastic effect at T* = 74 K. Anomalies in magnetic susceptibility, heat capacity, electrical resistivity, and unit cell parameters indeed reveal an analogous effect at T* ≈ 215 K in Ba4BiIr3O12. However, the transition is not accompanied by the opening of a gap in spin excitation spectrum, because antiferromagnetic coupling among S = 1/2 Ir(4+) (d(5)) cations leads to the formation of a S = 1/2 doublet within the trimers, vs S = 0 singlets within dimers. The change in magnetic state of the trimers at T* leads to a structural distortion, the energy of which is overcompensated for by the formation of S = 1/2 doublets. Extending this insight to the dimer system Ba3BiIr2O9 sheds new light on the more pronounced low-temperature anomalies observed for that compound.

In situ studies of materials for high temperature CO2 capture and storage

Carbon capture and storage (CCS) offers a possible solution to curb the CO2 emissions from stationary sources in the coming decades, considering the delays in shifting energy generation to carbon neutral sources such as wind, solar and biomass. The most mature technology for post-combustion capture uses a liquid sorbent, amine scrubbing. However, with the existing technology, a large amount of heat is required for the regeneration of the liquid sorbent, which introduces a substantial energy penalty. The use of alternative sorbents for CO2 capture, such as the CaO-CaCO3 system, has been investigated extensively in recent years. However there are significant problems associated with the use of CaO based sorbents, the most challenging one being the deactivation of the sorbent material. When sorbents such as natural limestone are used, the capture capacity of the solid sorbent can fall by as much as 90 mol% after the first 20 carbonation-regeneration cycles. In this study a variety of techniques were employed to understand better the cause of this deterioration from both a structural and morphological standpoint. X-ray and neutron PDF studies were employed to understand better the local surface and interfacial structures formed upon reaction, finding that after carbonation the surface roughness is decreased for CaO. In situ synchrotron X-ray diffraction studies showed that carbonation with added steam leads to a faster and more complete conversion of CaO than under conditions without steam, as evidenced by the phases seen at different depths within the sample. Finally, in situ X-ray tomography experiments were employed to track the morphological changes in the sorbents during carbonation, observing directly the reduction in porosity and increase in tortuosity of the pore network over multiple calcination reactions.

A high temperature gas flow environment for neutron total scattering studies of complex materials

We present the design and capabilities of a high temperature gas flow environment for neutron diffraction and pair distribution function studies available at the Nanoscale Ordered Materials Diffractometer instrument at the Spallation Neutron Source. Design considerations for successful total scattering studies are discussed, and guidance for planning experiments, preparing samples, and correcting and reducing data is defined. The new capabilities are demonstrated with an in situ decomposition study of a battery electrode material under inert gas flow and an in operando carbonation/decarbonation experiment under reactive gas flow. This capability will aid in identifying and quantifying the atomistic configurations of chemically reactive species and their influence on underlying crystal structures. Furthermore, studies of reaction kinetics and growth pathways in a wide variety of functional materials can be performed across a range of length scales spanning the atomic to the nanoscale.

Supporting Data for 'Large scale in silico screening of materials for carbon capture through chemical looping'

This data was generated as part of the EPSRC grant EP/K030132/1. It contains: 1. MaterialsProject-ScreenedOxidationReactions.xlsx: A spreadsheet with all the screened oxidation reaction obtained from the Materials Project database (www.materialsproject.org). Description of each column: a. Composition - Formula of reactant material b. O2 chemical potential - Chemical potential under which the reaction takes place. c. dE_EV - Change in energy of the reaction (calculated by DFT at 0 K) d. O2 capacity - gravimetric O2 capacity of the reaction, based on the stoichiometry e. Temp - temperature at which the equilbrium constant was calculated (here chosen to be 298.15 K, room temperature) f. Kp at T - calculated equilibrium constant g-j. T_oxidation at p_XXX - Temperature at which the oxidation reaction takes place under different partial pressures of O2 k. Reaction - Screened oxidation reaction (this is separated across several columns). 2. XRD.zip - folder containing raw x-ray diffraction data for the compounds presented in the paper. 3. SI-TGA.zip - folder containing raw TGA data for Figure S1 4. TGA.zip - folder containing raw TGA data (ForPaper.xlsx) and Matlab file (ForPaper.m) to produce figures.

Research data supporting "Probing Oxide-Ion Mobility in the Mixed Ionic-Electronic Conductor La

17O solid-state MAS variable-temperature NMR spectra of La2NiO4+δ at 4.7 T, 7.05 T, and 16.4 T, at variable temperature between 35°C and 140°C; pseudo-2D T1 saturation recovery data. Additional 17O solid-state MAS NMR spectra of a mixture of mixture of La2NiO4+δ, La3Ni2O7 and La4Ni3O10 at 7.05 T at room temperature; and of La2O3 at 16.4 T at variable temperature between 35°C and 135°C. Raw XRD data as plotted in Figure S1 and S13 (SI). Raw TGA data as plotted in Figure S2 (SI). Calculations of NMR parameters as output from the solid-state density functional theory (DFT) code CRYSTAL09, using the geometry-optimised structure files. All experimental and computational parameters are given in the article and/or data files.

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Matthew Dunstan1, Cindy Lau2, Can Kocer1, Wenting Hu3, John Dennis4, Andrew Morris5, Stuart Scott2, Clare Grey1 1Department Of Chemistry, University Of Cambridge, Cambridge, United Kingdom, 2Department of Engineering, University of Cambridge, Cambridge, United Kingdom, 3School of Chemical Engineering and Advanced Materials, Newcastle University, Newcastle, United Kingdom, 4Department of Chemical Engineering and Biotechnology, University of Cambridge, Cambridge, United Kingdom, 5Department of Physics, University of Cambridge, Cambridge, United Kingdom E-mail: mtd33@cam.ac.uk