Microwave-initiated Synthesis of Metal Chalcogenides on Graphene Support for Sustainable Energy Applications

Abstract

To meet the increasing demand on energy, new types of renewable resources along with advanced energy conversion and storage systems are being developed. Nanotechnology plays a vital role to solve the existing energy issues. Nanomaterials, such as carbon materials, metal oxides, metal chalcogenides, carbides, phosphides, polymers, etc. possess superior mechanical, thermal and electrical properties, leading to broad applications in composite materials, smart structures, chemical sensors, energy storage and nano-electronic devices. However, the high cost and difficulty in getting large scale, high quality nanomaterials remain challenges. This research proposes to demonstrate an ultrafast, facile, and reliable microwave-initiated synthesis approach for the direct growth of metal chalcogenides (MCs) on graphene support. In this regard, a series of suitable combinations of metals and chalcogens have been selected, such as molybdenum disulfide (MoS2), molybdenum ditelluride (MoTe2), molybdenum sulftotelluride (MoSTe), cobalt doped MoS2 (Co-MoS2) and other hybrids. To confirm the successful synthesis of desired products, the scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), EDS mapping, transmission electron microscope (TEM), Raman spectroscopy, x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS) analyses have been performed. Moreover, electrochemical characterizations i.e., cyclic voltammogram (CV), linear sweep voltammogram (LSV), galvanostatic charge-discharge (GCD), electrochemical impedance spectroscopy (EIS), and other tests reveal that the as-produced nanocomposites can be used for hydrogen evolution reaction (HER) to generate useful hydrogen fuel and for electrical energy storage (EES) such as hybrid-capacitor applications. This research also intends to optimize the reaction parameters to improve the electrocatalytic and energy storage behaviors of MC/graphene nanocomposites. This single-step microwave approach can be universally employed to produce other useful MCs and their hybrids, that will catalyze substantial development in more widespread uses of MC-based nanocomposites for successful energy applications. The first chapter (Chapter 1) discussed the background, state-of-arts, and motivations for this research work. Current energy issues have been described and the potential solution for these issues through electrocatalysis and energy-storage systems were proposed, where the mechanisms were thoroughly reviewed. Further, the microwave-initiated synthesis of advanced energy materials, such as metal chalcogenides (MCs), were compared with other conventional methods. As well, the physicochemical and electrochemical characterization techniques were explained. The last section of this chapter emphasized on the motivation and objectives of this research. The second chapter (Chapter 2) focuses on the microwave-initiated synthesis of molybdenum disulfide (MoS2) on graphene and their characterizations for employing as electrocatalysts for hydrogen evolution reaction (HER). The microwave reaction conditions and the electrocatalytic performances were optimized. Additionally, the issues were addressed that occurred from using the platinum (Pt) counter electrode. Hence, for other projects described in the following chapters, the Pt electrode was replaced with graphite rod electrode. The third chapter (Chapter 3) described the results obtained for electrocatalytic activities of microwave synthesized MoTe2/graphene nanocomposites. Along with the experimental findings, a collaborative computational study was performed to understand the HER mechanisms and to identify the catalytic active sites in MoTe2/graphene catalyst. The fourth chapter (Chapter 4) portrayed a comprehensive study on the HER activities of different combinations of metal chalcogenides and their hybrids. Three major parameters (overpotential, Tafel slope, exchange current density) were compared for as-produced nanocomposites. The fifth chapter (Chapter 5) discussed the synthesis methods and the effects of metal-rich hybrid MCs (MoS0.46Te0.58/graphene) on the improvement of HER activities. Based on the combined experimental and computational studies, this work revealed that the excess amount of molybdenum (Mo) in comparison to stoichiometry enhanced the number of active sites in the as-produced nanocomposites, which further improved their HER performances. The sixth chapter (Chapter 6) focused on studying the impact of cobalt (Co) doping effect on the HER performance of MoS2/graphene nanocomposite. The doping amount was optimized and revealed the enhancement on electrocatalytic behavior of Co-doped MoS2/graphene comparing to the undoped counterpart. The seventh chapter (Chapter 7) depicted the energy storage behaviors of MoTe2/graphene nanocomposite. Based on this study, it was shown that, besides utilizing as electrocatalysts, the as-produced MCs and their hybrids possess promising charge storage abilities, which revealed their potential applications on developing supercapacitors, batteries, and hybrid-capacitors. Finally, the eighth chapter (Chapter 8) summarized the results and outcomes from all the studied projects. It also described the possible future works to advance this current research.