Cobalt sulfide nanostructures : synthesis and application in supercapacitors

Electrical energy storage technologies have been profoundly impacted by nano-science and technology, particularly in the engineering of bespoke electrode materials for batteries and supercapacitors. Redox-based supercapacitors with nanostructured electrode materials are promising candidates. They can achieve high energy density comparable with that of batteries and at the same time maintain the excellent power capabilities of conventional capacitors at the laboratory scale, although they face important challenges related to durability, processability, cost and environmental effects for technological impact. The size and shape-dependent properties of nanostructured materials have stimulated significant effort towards the synthesis of nanostructures in a controllable manner. Thus novel nanomaterial systems must be explored for electrochemical supercapacitors. To enhance the performance of supercapacitors, the morphology and size of the nanostructures must be carefully designed. And chemistry and physical properties of nanomaterials can be tailored and investigated. To realize these aims, the following researches have been conducted. First, a novel carbon coated Co9S8 nanodandelion was synthesized in oil phase, in which Dodecanethiol was acting as sulfur source and also a soft template. When sulfur powder was used instead of Dodecanethiol, uniform 20 nm Co9S8 nanoparticles without carbon coating were produced. The growth mechanism of Co9S8 nanodandelions and nanoparticles was proposed and investigated. As a supercapacitor electrode, the Co9S8 nanodandelion can deliver a specific capacitance of 308 F g-1 at current density of 0.5 A g-1. The value is much larger than that of Co9S8 nanoparticles (131 F g-1 at 0.5 A g-1). Furthermore, the Co9S8 nanodandelion also exhibits good rate capability, e.g. 250 F g-1 at a high current density of 5 A g-1 and 237 F g-1 at an even higher current density of 10 A g-1. Such synthesis method is attractive for the preparation of sulfide electrode materials with high specific capacitance. Second, a novel cosolvent solvothermal method was used to generate interconnected Co3S4 nanosheets in a mixed solvent of water and n-propanol. It has recently been confirmed that alcohols such as n-propanol and 2-propanol show hydrophobic interactions among themselves although they are miscible with water. Thus it is possible that the solution inhomogeneity in cosolvent systems can be utilized to induce the formation of the 2D structures. It was found that the composition of the solvent mixture is critical in determining the morphology of resulting Co3S4 products and introduction of a certain amount of water was indispensable for the formation of nanosheet-shaped morphology. Further electrochemical evaluations reveal that these Co3S4 nanosheets manifest attractive supercapacitive performance, which is related to their high surface area and unique interconnected porous sheet-like morphology. Third, a facile synthesis was developed to synthesize mesoporous CoS2 hollow spheres through a solution-based route. The hollow spheres are composed by secondary structure of thin nanosheets with a thickness of about 5 nm. The growth process of CoS2 hollow spheres goes through oil droplets evaporation and self-assembly crystallization processes. The CoS2 hollow spheres with a high surface area exhibit excellent supercapacitive performances, which is superior compared with most of sulfide materials reported previously. This work offers a new route for the design of other sulfide materials with enhanced properties for energy, microelectronics, and other applications.Doctor of Philosophy (MSE