Rune E. Johnsen

Structural Changes and Coordinatively Unsaturated Metal Atoms on Dehydration of Honeycomb Analogous Microporous Metal-Organic Frameworks

Porous metal-organic framework compounds with coordinatively unsaturated metal sites on the inner surface of the pores promise to be valuable adsorbents and catalyst systems, either in industrial applications or as model systems to study interactions with guest molecules. The dehydration process of two isostructural microporous coordination polymers, [M2(dhtp)(H2O)2].8 H2O, termed CPO-27-M (M=Co, Zn; H(4)dhtp=2,5-dihydroxyterephthalic acid) was investigated by in situ variable temperature X-ray diffraction. Both compounds contain accessible coordination sites at the metal after complete removal of the solvent. However, despite the analogy of their crystal structures, they behave differently during dehydration. For CPO-27-Co, water desorption is a smooth topotactic process of second order with no concomitant space group change and no increase in microstrain, which is beneficial for the applicability of the material. Removal of the water propagates from the center of the channels outwards. The coordinating water molecule at the metal desorbs only when almost all the bulk water in the pores has disappeared. In contrast, discontinuities in the powder pattern of CPO-27-Zn indicate the occurrence of first-order transitions. The crystal structures of four of the five individual phases could be determined. The structure of the intermediate phase occurring just before the framework is completely evacuated was elusive in respect to full structure solution and refinement, but it is most probably related to the removal of the axis of threefold symmetry. The zinc-based material experiences a significant amount of strain.

Capillary based Li–air batteries for in situ synchrotron X-ray powder diffraction studies

For Li–air batteries to reach their full potential as energy storage system, a complete understanding of the conditions and reactions in the battery during operation is needed. To follow the reactions in situ a capillary-based Li–O2 battery has been developed for synchrotron-based in situ X-ray powder diffraction (XRPD). In this article, we present the results for the analysis of 1st and 2nd deep discharge and charge for a cathode being cycled between 2 and 4.6 V. The crystalline precipitation of Li2O2 only is observed in the capillary battery. However, there are indications of side reactions. The Li2O2 diffraction peaks grow with the same rate during charge and the development of the full width at half maximum (FWHM) is hkl dependent. The difference in the FWHM of the 100 and the 102 reflections indicate anisotropic morphology of the Li2O2 crystallites or defects along the c-axis. The effect of constant exposure of X-ray radiation to the electrolyte and cathode during charge of the battery was also investigated. X-ray exposure during charge leads to changes in the development of the intensity and the FWHM of the Li2O2 diffraction peaks. The X-ray diffraction results are supported by ex situ X-ray photoelectron spectroscopy (XPS) of discharged cathodes to illuminate non-crystalline deposited materials.

Structural and microstructural changes during anion exchange of CoAl layered double hydroxides: an in situ X-ray powder diffraction study

Anion-exchange processes in cobalt–aluminium layered double hydroxides (LDHs) were studied by in situ synchrotron X-ray powder diffraction (XRPD). The processes investigated were CoAl–CO3 → CoAl–Cl → CoAl–CO3, CoAl–Cl → CoAl–NO3 and CoAl–CO3 → CoAl–SO4. The XRPD data show that the CoAl–CO3 → CoAl–Cl process is a two-phase transformation, where the amount of the CoAl–CO3 phase decreases exponentially while that of the CoAl–Cl phase increases exponentially. Energy-dispersive X-ray spectroscopy (EDXS) studies of a partially chloride-exchanged CoAl–CO3 LDH sample along with in situ XRPD data suggested that the individual particles in the CoAl–CO3 sample are generally anion-exchanged with chloride one at a time. In contrast with the CoAl–CO3 → CoAl–Cl transformation, the XRPD data show that the reverse CoAl–Cl → CoAl–CO3 process is a one-phase transformation. Rietveld refinements indicate that the occupancy factors of the carbon and oxygen sites of the carbonate group increase, while that of the chloride site decreases. In the CoAl–Cl → CoAl–NO3 anion-exchange reaction, the XRPD patterns reveal the existence of two intermediate phases in addition to the initial CoAl–Cl and final CoAl–NO3 phases. The in situ data indicate that one of these intermediates is a mixed nitrate- and chloride-based LDH phase, where the disorder decreases as the nitrate content increases. The XRPD data of the partial CoAl–CO3 → CoAl–SO4 anion-exchange reaction show that the process is a two-phase transformation involving a sulfate-containing LDH with a 1H polytype structure.

Capillary-based micro-battery cell for in situ X-ray powder diffraction studies of working batteries: a study of the initial intercalation and deintercalation of lithium into graphite

A novel capillary-based micro-battery cell for in situ X-ray powder diffraction (XRPD) has been developed and used to study the initial intercalation and deintercalation of lithium into graphite in a working battery. The electrochemical cell works in transmission mode and makes it possible to obtain diffraction from a single electrode at a time, which facilitates detailed structural and microstructural studies of the electrode materials. The micro-battery cell is potentially also applicable for in situ X-ray absorption spectroscopy and small-angle X-ray scattering experiments. The in situ XRPD study of the initial intercalation and deintercalation process revealed marked changes in the diffraction pattern of the graphitic electrode material. After the formation of the solid electrolyte interphase layer, the d spacing of the diffraction peak corresponding to the 002 diffraction peak of graphite 2H changes nearly linearly in two regions with slightly different slopes, while the apparent half-width of the diffraction peak displays a few minima and maxima during charging/discharging. DIFFaX+ refinements based on the initial XRPD pattern and the one after the initial discharging–charging cycle show that the structure of the graphite changes from an intergrown structure of graphite 2H and graphite 3R to a nearly ideal graphite 2H structure. DIFFaX+ was also used to refine a model of the stacking disorder in an apparent stage III compound with AαAB- and AαAC-type slabs.

XRPD/TEM studies of model high-temperature shift catalysts

The combination of transmission electron microscopy (TEM) and in situ X-ray powder diffraction (XRPD) for the investigation of four model high-temperature shift catalysts makes it possible to obtain and compare information concerning the crystallite and particle shapes and sizes before, during and after the reduction of the synthesized hematite-based model catalyst to the active magnetite-based catalyst. Two chromium-containing iron oxide model catalysts and two pure iron oxide model catalysts were synthesized from hydrated chloride or nitrate salts, resulting in particles with different shapes and sizes. The average crystallite sizes of four model catalysts were determined by XRPD using the Scherrer equation before and after the reduction. The crystallite sizes determined before the reduction were compared with particles sizes determined from TEM images of the same samples. These sizes were generally in good agreement. By using the Rietveld method combined with the Scherrer equation and the Lorentzian Scherrer broadening parameters, the development of the average crystallite size during the in situ reduction was demonstrated. This showed that the average crystallite size of the remaining hematite increases when the reduction begins. Additionally, the average crystallite sizes of the reduced samples showed that the chromium-containing model catalysts have the smallest increase in the overall crystallite size.

A comparative in situ Rietveld refinement study: thermal decomposition and transformation of CoAl and CoZnAl layered double hydroxides

Rietveld refinement based on in situ X-ray powder diffraction (XRPD) data was combined with thermogravimetric analysis (TGA) and mass spectrometry (MS) to study and compare the phase transformations, thermal stability, microstructural and structural changes of two cobalt-containing nitrate-based layered double hydroxides (LDHs) upon heating in a controlled inert atmosphere of nitrogen. The XRPD data were collected, using synchrotron X-ray radiation, with a time resolution of 107 s, which made it possible to carry out detailed structural studies of the initial layered double hydroxides as well as their decomposition products: spinel for a CoAl–NO3 LDH and spinel/zincite for a CoZnAl–NO3 LDH. Correlating these data with those from the TGA–MS analyses gives us information about the transformation mechanisms. Rietveld refinements of the two spinel phases reveal remarkable differences. The a axis of the spinel formed by decomposition of the CoAl–NO3 LDH increases almost linearly from approximately 598 to 1163 K, mainly due to the dominating thermal expansion, whereas the a axis of the spinel formed by decomposition of the CoZnAl–NO3 shows a more complex temperature dependency. Between approximately 698 and 1073 K, the a axis is almost constant due to pronounced chemical interaction with an additional amorphous phase and the zincite phase, whereas from 1073 up to 1163 K it increases linearly. Calculations, based on the results of the Rietveld refinements, of the size of the octahedral and tetrahedral coordination polyhedra in the spinel show that the octahedra shrink and the tetrahedra expand with increasing temperature. The unusual thermal behaviour of the octahedra is discussed and attributed to the low formation temperature of the cobalt aluminium spinel phase. Finally, the intensity of a low-angle scattering (LAS) signal observed in the XRPD patterns was correlated with the decomposition of the LDH, and determination of the specific surface areas gave the temperature-dependent BET surface areas.

Microstructural changes in porous hematite nanoparticles upon calcination

This combined study using small-angle neutron scattering (SANS), X-ray powder diffraction (XRPD), transmission electron microscopy (TEM) and adsorption isotherm techniques demonstrates radical changes in the microstructure of porous hematite (α-Fe2O3) nanoparticles upon calcination in air. TEM images of the as-synthesized hematite sample show that it consists of subrounded nanoparticles [50 (8)–61 (11) nm in average minimum and maximum diameters] with an apparent porous structure of nanosized pores/channels or cracks. SANS data confirm the presence of two characteristic sizes, one originating from the particle size and the other from the pore/void structure. Furthermore, the TEM images show that the particle sizes are nearly unaffected by calcination at 623 K, whereas their pore/void structure changes radically to an apparently pitted or spongy microstructure with cavities or/and voids. The change in microstructure also causes a reduction in the surface area as calculated by gaseous adsorption. The XRPD and SANS data show that the crystallite and SANS particle sizes are virtually unchanged by calcination at 623 K. Calcination at 973 K induces a significant alteration of the sample. The XRPD data reveal that the crystallite size increases significantly, and the SANS and adsorption isotherm studies suggest that the specific surface area decreases by a factor of ∼5–6. The TEM images show that the particles are sintered into larger agglomerates, but they also show that parts of the porous microstructure observed in the sample calcined at 623 K are retained in the sample calcined at 973 K.

Design of Nickel-Based Cation-Disordered Rock-Salt Oxides: The Effect of Transition Metal (M = V, Ti, Zr) Substitution in LiNi0.5M0.5O2 Binary Systems

Cation-disordered oxides have been ignored as positive electrode material for a long time due to structurally limited lithium insertion/extraction capabilities. In this work, a case study is carried out on nickel-based cation-disordered Fm3 ̅m LiNi0.5M0.5O2 positive electrode materials. The present investigation targets tailoring the electrochemical properties for nickel-based cation-disordered rock-salt by electronic considerations. The compositional space for binary LiM+3O2 with metals active for +3/+4 redox couples is extended to ternary oxides with LiA0.5B0.5O2 with A = Ni2+ and B = Ti4+, Zr4+, and V+4 to assess the impact of the different transition metals in the isostructural oxides. The direct synthesis of various new unknown ternary nickel-based Fm3̅ m cation-disordered rock-salt positive electrode materials is presented with a particular focus on the LiNi0.5V0.5O2 system. This positive electrode material for Li-ion batteries displays an average voltage of ∼2.55 V and a high discharge capacity of 264 mAhg-1 corresponding to 0.94 Li. For appropriate cutoff voltages, a long cycle life is achieved. The charge compensation mechanism is probed by XANES, confirming the reversible oxidation and reduction of V4+/V5+. The enhancement in the electrochemical performances within the presented compounds stresses the importance of mixed cation-disordered transition metal oxides with different electronic configuration.