Browsing by Author "Mzimela, Zimele N. T."

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    Indium-based thiospinels via solid-state pyrolysis of metal-organic precursors for water splitting and supercapacitance
    (University of Zululand, 2023) Mzimela, Zimele N. T.; Revaprasadu, Neerish and Khan, Malik D.
    The ever-increasing global energy demands emanating from population growth and technological advancements have propelled the research community into modelling plausible breakthroughs in the fabrication of electrochemical energy storage and conversion technologies with improved performances. As a result, the past few years have witnessed significant developments in the synthesis of electrode materials with refined electrochemical activities for water splitting and supercapacitance applications. The work outlined in this thesis focuses on the synthesis and evaluation of various indium-based thiospinel nanomaterials for enhanced electrochemical energy applications. Firstly, a comparative evaluation of the energy application capabilities of bare and oleylamine-capped NiIn2S4 nanosheets fabricated from dithiocarbamate and xanthate molecular precursors is outlined. Experimental results showed that the best energy storage activities were achieved with NiIn2S4 synthesized from the xanthate mixture through the colloidal route, which presented the highest Csp of 40 F/g at a current density of 0.5 A/g. In terms of electrocatalysis for water splitting, the highest performance was observed for NiIn2S4 synthesized from the xanthate mixture through the solventless thermolysis route, which presented an HER performance with an η of 138 mV at 10 mA/cm2 and a Tafel slope of 118 mV/dec. In terms of OER performance, an η of 382 mV at 10 mA/cm2 was obtained along with a Tafel slope of 145 mV/dec. Furthermore, the scalable fabrication of compositionally-tuned Ni1-xCoxIn2S4 solid solutions through the solventless thermolysis approach is outlined. All the alloyed nanomaterials outperformed the parent compounds in terms of both supercapacitance and water splitting applications. The best OER electrocatalytic performance was obtained when x = 0.6, which displayed an overpotential of 340 mV at 10 mA/cm2 along with a Tafel slope of 87 mV/dec. In terms of HER, the highest performance was obtained when x = 0.2, which reached an overpotential of 110 mV and a Tafel slope of 176 mV/dec. For supercapacitance applications, the nanoparticles synthesized at x = 0.8 yielded the highest specific capacitance of 118 F/g at 1 A/g, along with a maximum power density of 3450 W/kg at an energy density of 2.4 Wh/kg. The work also dwells into the amplification of the electrochemical properties of MnIn2S4 through the fabrication of Mn0.5M0.5In2S4 (where M= Ni/Co) solid solutions/alloys via the solventless thermolysis of compositionally-tuned xanthate precursors. Experimental results showed that the Ni2+ and Co2+-modified MnIn2S4 outperformed pure MnIn2S4 for both water splitting and supercapacitance applications. Overall, Mn0.5Co0.5In2S4 displayed the best electrocatalytic water splitting and supercapacitance activities amongst all the synthesized nanomaterials. We further describe the synthesis of Cd1-xZnxIn2S4 composites and their subsequent evaluation for electrochemical water splitting and supercapacitance applications. Structural analyses revealed the formation of cubic and hexagonal CdIn2S4 and ZnIn2S4 thiospinels with the Fd3m and P63mc spacegroups, respectively. Furthermore, a range of stoichiometric nanostructured composite materials, whose diffraction patterns are located between those of the two thiospinels were formed between x = 0.2 and x = 0.8. Morphological evaluation showed the formation of nanosheets. All alloyed materials displayed improved supercapacitive properties, with Cd0.2Zn0.8In2S4 achieving the best results. In terms of electrocatalytic hydrogen generation, the alloyed materials displayed lower capabilities than the parent compounds. Evidently, the ternary thiospinels, solid solutions and composite nanomaterials displayed improved performances for water splitting and supercapacitance applications. This is primarily due to the synergistic effects arising from the influence of different elements. Other factors, such as the morphology proved to play a significant role in the suitability of the materials for electrochemical water splitting and supercapacitance.