Journal of Nano Research Vol. 44

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Authors: Roxanne Gostynski, Marilé Landman, Jeanet Conradie
Abstract: A comparative electrochemical study of a series Fischer carbene complexes (FCCs) of Cr and W containing an ethoxy and an aryl group as carbene substituents, showed that the Cr-FCCs generally exhibit two one-electron oxidation processes, namely Cr (0) to Cr (I) to Cr (II) while W-FCCs are oxidized in two two-electron oxidation processes, namely W(0) to W(II) to W(IV). The first one-electron oxidation processes of Cr-FCCs is generally reversible, while the first two-electron oxidation processes of all W-FCCs are found to be irreversible. The first reduction process observed for both the W-FCCs and Cr-FCCs is a one-electron process located on the carbene ligand. Both the metal oxidation, as well as the carbene reduction of Cr-FCCs, occurs at lower potentials than for W-FCCs. Substitution of carbonyls coordinated to the metal, or the group attached to the carbene ligand, led to similar trends related to the ease of oxidation or reduction of Cr-FCC and W-FCCs. Density functional theory calculations support and led to a better understanding of experimental electrochemical behaviour.
Authors: Natasha Ross, Ntuthuko Wonderboy Hlongwa, Chinwe O. Ikpo, Miranda M. Ndipingwi, Priscilla Gloria Lorraine Baker, Emmanuel Iheanyichukwu Iwuoha
Abstract: The migration of lithium (Li) ions in electrode materials affects the rate performance of rechargeable Li ion batteries. Therefore, the application of LiMn2O4, which is an appealing cathode material in high power systems, requires fast electron transfer kinetics which is possible through the use of nanostructured morphologies and conductive material. Nanowires offer the advantage of a large surface to volume ratio, efficient electron conducting pathways and facile strain relaxation. In this study, LiMn2O4 nanowires with cubic spinel structure were synthesized by using a α-MnO2 nanowire-template-based method. LiMn2O4 nanowires have diameters less than 10 nm and lengths of several micrometers. Fe-Au nanoparticles were synthesized and used as coating material to improve both the catalytic activities and stability of the LiMn2O4 nanowires. The Li[Fe0.02Au0.01]Mn1.97O4 nanowires with modified architecture effectively accommodates the structural transformation during Li+ ion charge and discharge. Hence, the Li[Fe0.02Au0.01]Mn1.97O4 nanowire cathode system shows outstanding stability and enhanced electrocatalytical properties. Microstructural analysis of Li[Fe0.02Au0.01]Mn1.97O4 linked its composition and processing to its properties and performance. High resolution transmission electron microscope (HR-TEM) of the nanomaterial showed good crystallinity which contributed towards good reversibility. XRD analysis revealed a pure cubic spinel structure without any impurities. Structural information provided by Raman and solid state spectroscopy further corroborated these findings. The improved rate and cycling performance is related to the conductive particles infused within the nanowires which make up the electrode.
Authors: Malefetsane Khesuoe, Mangaka Matoetoe, Fredrick Okumu
Abstract: Modification of commercial platinum (Pt) and glassy carbon (GC) electrodes with polyaniline (PANI) and silver nanoparticles doped polyaniline (PANI/Ag NPs) through electropolymerization of aniline in the absence and presence of Ag NPs in 1 M hydrochloric acid (HCl) was interrogated. Fourier transform infrared (FTIR) and transmission electron microscope (TEM) techniques were used for structural, compositional and morphological elucidation. FTIR spectra for PANI and PANI/Ag NPs had the characteristic PANI functional groups as well as desired bands for the conducting emeraldine (EM) form. The predominance of the PANI pattern in the spectra is indicative of the intact PANI structure in the presence of Ag NPs while the slight band shifts are signify interfacial interactions between PANI and Ag NPs. TEM micrograms depicts different size one dimensional nanofibric tubes of the supramolecular structures of PANI. Ag NPs functionalized PANI had larger smoother tubes, suggesting organized morphology arrangement. An increased energy dispersive spectroscopy (EDS)-TEM count from 256 to 277 confirms incorporation of Ag NPs in PANI. GC/PANI/Ag NPs exhibited outstanding electroactivity (higher conductivity and rate of electron transfer).This might be a result of the large surface coverage, film thickness and diffusion coefficient as a result of the large GC surface area. Possibly, the improvement might be due to the GC electrode properties. The electroactivity of the electrodes increased in the order: Pt < GC < Pt/PANI < Pt/PANI/Ag NPs < GC/PANI < GC/PANI/Ag NPs. The effect of Ag NPs in the polymer was demonstrated by ultimate band gap reduction of PANI and enhanced magnitudes of current response per electrode.
Authors: Chima Benjamin Njoku, Patrick Gathura Ndungu
Abstract: In this study, Ce0.8Sr0.2Fe0.9Ir0.1O3-δ (CSFI) perovskite type material was prepared by sol-gel technique, characterised, and then tested as a cathode material for solid oxide fuel cells operating between 300 – 500 °C. The materials were studied using X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis. The morphology was examined using scanning electron microscopy and high resolution transmission electron microscopy. Samples showed changes in the overall structure and defect chemistry with an increase in calcination temperature. When tested as cathode materials, the material calcined at 1000 °C had the greatest performance at a test temperature of 500 °C, with a current density of 774.47 mA/cm2, a power density of 483.07 mW/cm2 and an area specific resistance (ASR) of 0.342 Ω/cm2.
Authors: Lisebo Phelane, Siyabulela Hamnca, Priscilla Gloria Lorraine Baker, Emmanuel Iheanyichukwu Iwuoha
Abstract: Innovation in nanoscience depends fully on the ability to synthesize nanomaterials as well as to assemble them efficiently into complex architectures. The discovery of graphene and graphene polymer nanocomposites is playing a key role in modern nanoscience and nanotechnology. Graphene oxide (GO), is a derivative of graphene obtained by the use of strong oxidizing agents to obtain graphene oxide, a nonconductive hydrophilic carbon material. Synthesis of graphene oxide was performed using the Hummers method. Graphene oxide was integrated into the polysulfone (PSF) matrix to form polysulfone/graphene oxide nanocomposite. Polysulfone casting suspension was prepared by dissolving polysulfone in N,N-dimethyl acetamide. The polymer composites consisted of homogeneously blended polysulfone and graphene oxide casting solutions and drop coated onto boron-doped diamond electrodes (BDD). Interfacial electrochemical dynamics were characterised using cyclic voltammetry (CV), square wave voltammetry (SWV), and electrochemical impedance spectroscopy (EIS). Scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR) and Raman spectroscopy were used to study the morphology and structure of the prepared materials. Modifying with polysulfone blocks electron transfer in the redox reaction of K3Fe(CN)6. PSF /GO enhances electron transfer even when compared to bare BDD. The diffusion coefficient and sensitivity for PSF-GO/BDD electrode was calculated to be 2.660 x10-4 cm2 s-1 and 6.7587 x10-6 mV s-1 / A.
Authors: Xolani Terrance Ngema, Meryk Ward, Siyabulela Hamnca, Priscilla Gloria Lorraine Baker, Emmanuel Iheanyichukwu Iwuoha
Abstract: Polyamic acid (PAA) thin films were prepared by electrodepositing PAA onto indium tin oxide (ITO) electrode and characterized using electrochemical methods (cyclic voltammetry, square wave voltammetry), Ultraviolet Visible spectroscopy and Ultraviolet Visible/Spectro-electrochemistry (UV/vis Spectro-electrochemistry). The electrodeposited PAA thin films were observed to have two redox couples with a formal of 118 mV and 274 mV. The diffusion coefficient (De) determined from cyclic voltammetry was found to be 7.9x10-6 cm2/s and provide a measure of how fast charge is transported through the thin film. PAA showed a broad absorption peak at 214 nm due to the carbonyl chromophores within the polymer and shoulder peak at 293 nm from a quinoid-type chromophore. The calculated band gap of 4.23 eV suggested the polymer was optically transparent between 300 nm to 800 nm. This indicated that the PAA thin films has emerged as a very promising and cost effective alternative material to ITO with good transparent and conductive properties. PAA thin films were further applied for the detection of anthracene. The analytical response of anthracene was studied at the ITO/PAA using spectro-electrochemistry. The characteristic analytical absorbance signal for anthracene was clearly identified at 375 nm when ITO/PAA electrode was polarised at -800 mV (vs Ag/AgCl). The calibration curve for anthracene showed a linear response from 4.95x10-4 to 1.15x10-2 M. The ITO/PAA showed a low detection limit of (0.0068 g/L) and high sensitivity for anthracene, making it a suitable platform for spectro-electrochemical analysis of polycyclic aromatic hydrocarbons.
Authors: Nomxolisi Dywili, Njagi Njomo, Chinwe O. Ikpo, Anne Lutgarde Djoumessi Yonkeu, Suru Vivian John, Ntuthuko Wonderboy Hlongwa, Naledi Raleie, Emmanuel Iheanyichukwu Iwuoha
Abstract: Nanostructured anilino-functionalized reduced graphene oxide intercalated with Pt metal nanoparticles was successfully synthesized. Graphene oxide nanosheets were synthesized using a modified Hummers method with simultaneous in-situ functionalization with aniline and ionic Pt reduction and dispersion through sonication. The nanomaterial was characterised with FTIR, UV-visible, SEM, TEM, EDX, XRD and Raman spectroscopy to ascertain surface, chemical, elemental and crystalline properties, composite structures, size, morphology and successful entrapment of metal nanoparticles while the electro-conductivity of the nanomaterial was interrogated using CV. The graphene oxide was successfully functionalized with aniline with new peaks belonging to the N-H and C-N group being present and calculated band gaps of 5.35 eV and 4.39 eV which are attributed to functionalization of graphene oxide. The functionalized graphene oxide was successfully loaded with platinum nanoparticles as TEM revealed that the Pt particles are spread out on the graphene sheets and when magnified a uniform distribution of the nanoparticles can be observed. The material (functionalized graphene oxide loaded with platinum nanoparticles) was used in the design of an asymmetric supercapacitor cell using 6M KOH aqueous electrolyte. On testing by galvanostatic charge/discharge, a high specific capacitance value of 605 F/g with a corresponding energy and power densities of 0.021kWh/Kg and 0.372kW/Kg respectively, were obtained.
Authors: Ntuthuko Wonderboy Hlongwa, Chinwe O. Ikpo, Natasha Ross, Myra Nzaba, Miranda M. Ndipingwi, Priscilla Gloria Lorraine Baker, Emmanuel Iheanyichukwu Iwuoha
Abstract: Abstract. Pristine LiMnPO4 and LiMnPO4/Mg-Au composite cathode materials were synthesized and their electrochemical properties interrogated using voltammetric, spectroscopic and microscopic techniques. The composite cathode exhibited better reversibility and kinetics than the pristine LiMnPO4. This was demonstrated in the values of the diffusion coefficient (D) and the charge and discharge capacities determined through cyclic voltammetry. For the composite cathode, D = 2.0 x 10-9 cm2/s while the pristine has a D value of 4.81 x 10-10 cm2/s. The charge and discharge capacities of LiMnPO4/Mg-Au at 10 mV/s were 259.9 mAh/g and 157.6 mAh/g, respectively. The corresponding values for pristine LiMnPO4 were 115 mAh/g and 44.75 mAh/g, respectively.. A similar trend was observed in the results obtained from electrochemical impedance spectroscopy measurements. These results indicate that LiMnPO4/Mg-Au composite has better conductivity and will facilitate faster electron transfer and better electrochemical performance than pristine LiMnPO4.
Authors: Nolubabalo Matinise, Noluthando Mayedwa, Chinwe O. Ikpo, Ntuthuko Wonderboy Hlongwa, Miranda M. Ndipingwi, Lerato Molefe, Nomxolisi Dywili, Anne Lutgarde Djoumessi Yonkeu, Tesfaye Waryo, Priscilla Gloria Lorraine Baker, Emmanuel Iheanyichukwu Iwuoha
Abstract: Symmmetrically oriented Pd (100) and its bimetallic Pd (100)Ru electrocatalysts were chemically synthesized and their conductive properties employed in the electrochemical oxidation of ammonia. Electrochemical data based on EIS, SWV and CV revealed that the Pt/Pd (100)Ru electrode showed a better conductivity and higher catalytic response towards the electrooxidation of ammonia compared to Pt/Pd (100) electrode. This was demonstrated by the EIS results where Pt/Pd (100)Ru gave a charge transfer resistance (Rct) of 48.64 Ω, high exchange current and lower time constant (5.2738 x 10-1A and 3.2802 x 10-7 s /rad) values while the Pt/Pd (100) had values of 173.2 Ω, 1.4811 x 10-1A and 4.8321 10-7 s /rad. The drastic drop in Rct highlights the superiority of the Pt/Pd (100)Ru over the Pt/Pd (100) and confirms that facile interfacial electron transfer processes occur on the Pt/Pd (100)Ru electrode during the electrocatalytic ammonia oxidation. Investigations through voltammetry revealed that the Pt/Pd (100)Ru had a higher peak current density and a shift in potential to more negative values at ≈ -0.2 V and ≈ -0.4 V. The EASA value of Pt/Pd (100)Ru was found to be 119.24 cm2 whereas Pt/Pd (100) had value of 75.07 cm2. The high electrochemically active surface area of Pd (100)Ru at 119.24 cm2 compared to the 75.07 cm2 for Pd (100) strengthened this observation in performance between the two catalysts for ammonia electrooxidation.

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