Layered Molybdenum-Based Oxide and Dichalcogenide Nanomaterials and their Composites for Energy Storage Applications
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Nanostructured materials with hybrid compositions are the most suitable candidates for replacement of existing supercapacitor electrode materials. They can have additional advantages due to synergistic effects of hybrid materials obtained through individual components and more importantly their structural effects which can impart significance improvement in their electrochemical applications. Hybrid materials could be based on combination of different materials mainly consisting of carbon materials and some pseudocapacitive materials like metal oxides or dichalcogenides. This thesis consists of molybdenum based oxide and dichalcogenide hybrid materials with graphen and carbon nanotubes and their applications in to supercapacitor and Li ion battery as energy storage devices. The low energy density is the bottle neck problem of supercapacitors which has been overcome through the incorporation of molybdenum trioxide and molybdenum disulfide in the hybrid materials. Hierarchical structures of transition metal oxides with well-defined compositions is crucial for achieving advanced electrodes for energy storage devices particularly two and three dimensional hierarchical hetero nano structures are the emerging electrode materials for energy storage devices and are yet to be explored, especially for molybdenum trioxide (MoO3) and molybdenum disulfide (MoS2) layered materials with reduced graphene oxide and carbon nanotubes. Structural effects are very important due to the dependence of the amount of charge stored on the surface area of the electrode, diffusion of electrolyte ions in to the interior of the electrode, access of electrolyte ions to the redox active cites and high thermal, chemical and mechanical stabilities. In this research firstly we demonstrate the hierarchically structured MoO3 assembled by twisted nanoribbons with hybrid composition for improved rate capability and cycle stability of pseudocapacitor. The hierarchical, flower-like structures of MoO3 assembled by hybrid nanoribbons were induced by the specific interactions of MoO3 interlayers with ionic liquids (ILs) as proven by spectroscopic and electrochemical analyses. The facilitated ion diffusion is attributed to hierarchical nanostructure for short diffusion length and ion accessibility, the high ion mobility in hybrids, and the interlayer modification of MoO3 by IL coating. Secondaly we demonstrate the sonochemical synthesis of multi-walled carbon nanotubes (MWCNTs) and MoO3 hybrids for an application in supercapacitor electrodes. The MoO3 nanodots with the diameter of 90% retention up to 2.12 A g-1 and cyclic stability of 80% retention during 1000 cycles of charge/discharge because of the mechanical stability of the MWCNTs and the good contact between the MoO3 and MWCNTs. Thirdly we demonstrate the liquid phase exfoliation of bulk materials into mono or few layers of MoS2 nanosheets through two different methods, chemical exfoliation method through Li intercalation and tip sonication method by utilizing the high power ultrasonic waves to exfoliate the layers of MoS2. Each method has its own advantages and disadvantages. Tip sonication method gives 100 % pure 2-H polytype structures but has long processing time, low yield and the use of nonvolatile organic solvents which hinder the use of these materials in many applications. On the other hand Li intercalation method gives high yield, uses aqueous solvents but great changes in physical structures in to 1-T polytype occures. Fourthly we demonstrate the transition of MoS2 from intercalation (battery like) into surface redox capacitive (pseudocapacitor like) mechanism. The capacitive behaviour of MoS2 becomes more prominent when bulk MoS2 is scaled down to nanoscale and hybridized with reduced graphene oxide (rGO) on a nanometre level. We observed the transformation of 2H MoS2 phase into 1T phase at the interface, where MoS2 strongly interact with functional groups of rGO to create the lattice distortion and defects, on an atomic level. This hetero-structure and composition of nanoscale MoS2 at the bulk and interface contribute to greatly improve charge storage performance due to highly reversible and fast surface redox charge transfer. Lastly we demonstrate for the first time surface-exposed 2D MoS2 nanosheets in to a 3D hierarchical heterostructures by ice templating method and its application in to Li ion batteries. This research offers new insight into the rational design of advanced electrode materials on a basis of the hierarchical complex structures of molybdenum trioxides and dichalcogenides with well-defined hybrid compositions for future applications in energy conversion and storage.