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theses

In the past few decades, there has been a tremendous ongoing effort to understand the nature of nanomaterials. Shrinking dimensions of the materials to the nanoscale regime reveal various intriguing properties of the nanomaterials due to quantum confinement effects. Especially, electron confinement in two dimensional materials enables compelling electronic properties as compared to the other nanostructured materials. Besides, thinning 2D materials down to monolayer thicknesses increases the surface-to-volume ratio to such an extent that makes them appealing for electrochemical energy conversion and storage technologies. Moreover, atomically thin 2D materials are flexible with good performance under bending, making them attractive materials for flexible electronics, energy storage devices and actuators. Recent advancements in energy storage technologies using 2D materials, particularly in materials such as graphene and its analogues, would finally make supercapacitors viable to complement or replace batteries. High surface-to-volume ratio and good ionic transport in addition to excellent electrical conductivity enables high charge storage capacities with fast energy uptake and delivery. For instance, graphene possess very high capacitive charge storage performance thanks to pure electrostatic attraction of ions (electrochemical double layer effect) on the highly porous carbon surface. Besides, pseudocapacitive 2D materials such as transition metal carbides (Mxenes) possess good electrical conductivity and capacitive performance concurrently because of the presence of transition metal in the structure. Transition metal dichalcogenides (LTMDs) such as molybdenum disulphide (MoS2) are also studied candidate materials for electrochemical charge storage as well. However, naturally occurring MoS2 has a 2H phase crystal structure with 1.9 eV band gap, which renders it semi-insulating and therefore not immediately attractive as an electrode material for energy storage. Despite, the 1T phase of MoS2 is metallic and 107 time more conductive that 2H phase. The aim of this work is to use the metallic 1T phase of MoS2, which can be obtained from the semiconducting 2H phase of MoS2 during chemical exfoliation of the bulk material. In so doing, the ultimate goal is to exploit the phase transformation of MoS2 and successfully utilize it as supercapacitor electrode. The results show that that chemically exfoliated nanosheets of MoS2 containing a high concentration of the metallic 1T phase can electrochemically intercalate ions such as H+ , Li+ , Na+ and K+ with extraordinary efficiency and achieve capacitance values ranging from ∼400 to ∼700 F cm−3 in a variety of aqueous electrolytes. We also demonstrate that this material is suitable for high-voltage operation in non-aqueous organic electrolytes, showing prime volumetric energy and power density values, coulombic efficiencies in excess of 95%, and stability over 5,000 cycles. As we show by X-ray diffraction analysis, these favourable electrochemical properties of 1T MoS2 layers are mainly a result of their hydrophilicity and high electrical conductivity, as well as the ability of the exfoliated layers to dynamically expand and intercalate the various ions. The obtained layer expansion behavior can indeed be utilized to transform the energy to mechanical energy. Our findings indicate that charge storage induces a reversible elongation of electrodes, generating enough mechanical force to bend bimorph actuator and lift masses 100 times heavier than its own weight. This study also includes a detailed experimental work on the synthesis of the metallic MoS2 phase, fabrication of supercapacitor and actuator electrodes and, their electrochemical and electrochemomechanical performance in various electrolytes.

ETD_7644

Ph.D.

Includes bibliographical references

by Muharrem Acerce

doi:10.7282/T3N018TF

ETD doctoral

The author owns the copyright to this work.