Browsing by Author "Khotseng, Lindiwe E."

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    Compaction of a zirconium metal–organic framework (UiO-66) for high density hydrogen storage applications
    (Royal Society of Chemistry, 2018) Bambalaza, Sonwabo E.; Langmi, Henrietta W.; Mokaya, Robert; Musyoka, Nicholas M.; Renad, Jianwei; Khotseng, Lindiwe E.
    We report a rare case whereby a metal–organic framework (MOF), namely UiO-66, is compacted at high pressure ( 700 MPa or 100 000 psi) resulting in densification and improved total volumetric hydrogen storage capacity, but crucially, without compromising the total gravimetric uptake attained in the powdered form of the MOF. The applied compaction pressure is also unprecedented for MOFs as most studies have shown the MOF structure to collapse when compacted at very high pressure. The UiO-66 prepared in this study retained 98% of the original surface area and microporosity after compaction at 700 MPa, and the densified pellets achieved a total H2 uptake of 5.1 wt% at 100 bar and 77 K compared to 5.0 wt% for the UiO-66 powder. Depending on the method used to compute the volumetric uptake, the densified UiO-66 attained unprecedented volumetric capacity at 77 K and 100 bar of up to 74 g L 1 (13 g L 1 at 298 K) compared to 29 g L 1 for the powder (6 g L 1 at 298 K) using a conventional method that takes into account the packing density of the adsorbents, or 43 g L 1 (compared to 35 g L 1 for the powder at 77 K and 100 bar) based on a method that uses both the single crystal and skeletal densities of MOFs. However, regardless of the difference in the calculated values according to the two methods, the concept of UiO-66 compaction for improving volumetric capacity without compromising gravimetric uptake is clearly proven in this study and shows promise for the achievement of hydrogen storage targets for a single material as set by the United States Department of Energy (DOE).
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    Underpotential deposition of SnBi thin films for sodium ion batteries: The effect of deposition potential and Sn concentration
    (Elsevier, 2019) Xaba, Nqobile; Modibedi, Remegia M.; Khotseng, Lindiwe E.
    Bimetallic SnBi film was deposited on a Cu foil substrate via the electrochemical atomic layer deposition (E-ALD) technique. The deposition attainment of Sn and Bi were investigated using cyclic voltammetry (CV) and linear sweep voltammetry (LSV). The deposition potential of Bi was varied in the underpotential deposition (UPD) region and the concentration of Sn was varied in the SnBi bimetallic material. The materials were characterised using field emission scanning electron microscopy coupled with energy dispersive spectroscopy (FE-SEM/EDS) for morphology and elemental distribution, focused ion beam scanning electron microscopy (FIBSEM) for thickness, X-ray diffraction (XRD) for crystallinity and inductively coupled plasma mass spectroscopy (ICP-MS) for composition measurements. Bi deposited at different UPD regions was structurally different. The deposits were crystalline SnBi materials containing Sn, Bi and other phases of Cu and Sn. Bi was concentrated on the surface, while Sn was distributed evenly across the film. The SnBi electrodes were tested as anode materials in Na-ion batteries using galvanostatic cycling (GC), CV and electrochemical impedance spectroscopy (EIS). Initial discharge capacities of 1900 mAh g 1 for SnBi (1:1) and 341 mAh g 1 for SnBi (3:1) electrodes at 38.5 mA g 1 were obtained, while the electrodes suffered capacity loss after 10 cycles.