Browsing by Author "Mathumba, Penny"

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    Aluminium and gold functionalized graphene quantum dots as electron acceptors for inverted Schottky junction type rainbow solar cells
    (University of Western Cape, 2020) Mathumba, Penny; Iwuoha, Emmanuel
    The main aim of this study was to prepare band gap-engineered graphene quantum dot (GQD) structures which match the different energies of the visible region in the solar spectrum. These band gap-engineered graphene quantum dot structures were used as donor materials in rainbow Schottky junction solar cells, targeting all the energies in the visible region of the solar spectrum for improved solar-to-electricity power conversion efficiency. Structural characterisation of the prepared nanomaterials under solid-state nuclear magnetic resonance spectroscopy (SS-NMR) showed appearance of bands at 40 ppm due to the presence of sp3 hybridised carbon atoms from the peripheral region of the GQD structures. Other bands were observed at 130 ppm due to the presence of polycyclic aromatic carbon atoms from the benzene rings of the GQD backbone, and around 180 ppm due to the presence of carboxylic acid carbons from oxidation due to moisture. Fourier-transform infrared resonance (FTIR) spectroscopy further confirmed the presence of aromatic carbon atoms and oxidised carbons due to the presence of C=O, C=C and -OH functional groups, concurrent with SS-NMR results.
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    Metal oxide (Co3O4 and Mn3O4) impregnation into S, N-doped graphene for oxygen reduction reaction (ORR)
    (MDPI, 2020) Mathumba, Penny; Fernandes, Diana M.; Matos, Renata
    To address aggravating environmental and energy problems, active, efficient, low-cost, and robust electrocatalysts (ECs) are actively pursued as substitutes for the current noble metal ECs. Therefore, in this study, we report the preparation of graphene flakes (GF) doped with S and N using 2-5-dimercapto-1,3,4-thiadiazole (S3N2) as precursor followed by the immobilization of cobalt spinel oxide (Co3O4) or manganese spinel oxide (Mn3O4) nanoparticles through a one-step co-precipitation procedure (Co/S3N2-GF and Mn/S3N2-GF). Characterization by different physicochemical techniques (Fourier Transform Infrared (FTIR), Raman spectroscopy, Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD) of both composites shows the preservation of the metal oxide spinel structure and further confirms the successful preparation of the envisaged electrocatalysts. Co/S3N2-GF composite exhibits the best ORR performance with an onset potential of 0.91 V vs. RHE, a diffusion-limiting current density of-4.50 mA cm-2 and selectivity for the direct four-electron pathway, matching the results obtained for commercial Pt/C. Moreover, both Co/S3N2-GF and Mn/S3N2-GF showed excellent tolerance to methanol poisoning and good stability.