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    Solventless Synthesis of Nanostructured Spinel Ferrite Solid Solutions for Supercapacitance and Electromechenical Water Splitting Application
    (University of Zululand, 2021) Malima, Nyemaga Masanje
    Metal oxide nanocrystals that adopt the spinel crystal structure, such as spinel ferrites exhibit a variety of interesting electronic, magnetic, and optical properties, which render them suitable for numerous technologically relevant applications. Interestingly, tuning the composition of spinel nanoferrites via the design of solid solutions is recognized as an effective way to improve their electrochemical properties towards supercapacitance and water splitting. In this regard, achieving synthetic control over the composition is critical to tuning the properties of spinel ferrite nanocrystals. Efforts to find sustainable approaches to nanoparticle synthesis have focused on green chemistry principles, including reducing waste, improving yield and atom economy, and minimizing auxiliaries and reaction steps. The solventless approach, in which the synthesis of nanomaterials proceed by thermal decomposition of precursors has attracted considerable research interest and proven to be simple, economical, time-effective, scalable, and eco-friendly. The work described in this thesis demonstrates the suitability of the solventless thermolysis route for the fabrication of a series of nanostructured spinel ferrite solid solutions using metal acetylacetonate precursors. Investigation on the efficacy of the synthesized ferrite solid solutions for supercapacitance and water splitting applications is also described. The thesis is organized into seven chapters as described hereunder. The first chapter presents the introduction and literature review which are the foundations upon which the entire research work is based. This chapter gives insight into electrochemical energy systems with a special focus on the theory behind electrocatalytic water splitting and the mechanism of hydrogen production in both acidic and alkaline electrolytes. Similarly, the description, classification and working principles of supercapacitors are described. It also shades light into the concept and potential applications of spinel ferrites and their corresponding solid solutions. The first chapter is culminated by highlighting the research justification and establishes the working scope and objectives of the study. The work described in chapter two entails the scalable synthesis of nanostructured Ni1-xCoxFe2O4 solid solutions via a solventless thermolysis method. The physicochemical analysis of the as-prepared solid solutions is established by a suite of characterization techniques, while the procedures of materials fabrication and electrochemical analysis are also presented. The p-XRD analysis confirmed the formation of a series of monophasic cubic spinel ferrites with space group Fd3m. Investigation of the synthesized materials for vi supercapacitance revealed that the nanospinel Ni0.4Co0.6Fe2O4 electrode demonstrated a longer charge-discharge time, signifying superior charge storage capacity. For efficient HER electrocatalysis, the Ni0.6Co0.4Fe2O4 electrode showed high performance manifested by low overpotential of 168 mV and Tafel slope of 120 mV/dec. Similarly, Ni0.8Co0.2Fe2O4 exhibited a lower overpotential of 320 mV with a low Tafel slope of 79 mV/dec, indicating enhanced OER activity. Chapter three describes scalable nanofabrication of composition-tuneable spinel Co1xZnxFe2O4 solid solutions via a solvent-free thermolysis approach. The discussion of the experimental results regarding the materials’ structural, compositional, morphological and optical properties is provided. Experimental results revealed that incorporation of diamagnetic Zn2+ in the crystal lattice of CoFe2O4 significantly enhanced both the physicochemical and electrochemical properties of the resultant material. Higher discharge time displayed by Co0.4Zn0.6Fe2O4 is indicative of higher specific capacitance of the material compared to the pristine CoFe2O4. For OER, the Co0.8Zn0.2Fe2O4 solid solution exhibited higher performance reflected by low overpotential of 317 mV along with a small Tafel slope of 56 mV/dec. As for HER in alkaline electrolyte, Co0.6Zn0.4Fe2O4 displayed decent performance with a low overpotential of 169 mV and Tafel slope of 136 mV/dec compared to other electrode compositions. Chapter four demonstrates that by regulating the molar composition of Mg and Ni in the preparation of Ni1-xMgxFe2O4 solid solutions, the physicochemical and the electrochemical performance of the material were tuned. The Ni1-xMgxFe2O4 (x = 0.6) nanoparticles exhibited the best electrocatalytic activity for HER with an overpotential of only 121 mV which is much smaller compared to its analogues, at current density of 10 mA/cm2 and the electrode exhibits good stability during long-term electrolysis. Meanwhile, Ni0.2Mg0.8Fe2O4 showed the best OER activity, requiring an overpotential of 284 mV to deliver the same current density within the window of potential examined. In chapter five, a series of Ni1-xZnxFe2O4 (0 ≤ x ≤ 1) solid solutions with varying amounts of zinc and nickel have been efficaciously fabricated via a solventless pyrolysis method. The p-XRD and EDX analyses confirmed the formation of homogeneous phase-pure Ni1-xZnxFe2O4 (0 ≤ x ≤ 1) nanoparticles. In comparison, the incorporation of zinc in the crystal lattices of nickel ferrite endowed a larger benefit on HER and OER than on supercapacitance. Specifically, the Ni1-xZnxFe2O4 (x = 0.8) nanocatalyst displays excellent HER performance with superior activity which is manifested by a small overpotential of 87 vii mV, whereas Ni1-xZnxFe2O4 (x = 1) catalyst exhibited superior OER performance with a small overpotential of 330 mV. The main aim of the sixth chapter was to employ the same solventless pyrolysis approach to afford uniform Co1-xMgxFe2O4 (0 ≤ x ≤ 1) nanoparticles using metal acetylacetonate precursors. Structural analysis showed that all samples exhibited a cubic spinel ferrite structure with space group Fd3m. All samples showed the same morphology irrespective of the amount of Mg being incorporated in the CoFe2O4 system. Considering the band gap value of pristine cobalt ferrite, a blue shift was observed for all compositions except for x = 0.2 and 1, which were red shifted. The results and findings of this chapter are of profound significance for the design of novel electronic and optoelectronic devices. Chapter 7 culminates the entire research project by presenting a brief summary of the work and possible areas to be considered for future work. Overall, it was observed in this study that compared to the parent spinel ferrites, their corresponding solid solutions demonstrated improved physicochemical and electrochemical activity, except for Ni1-xZnxFe2O4 where the parent ZnFe2O4 exhibited higher OER activity than the solid solutions.
University of Zululand