Deposition of 2D Materials by Vacuum Kinetic Spray Method and Its Application in Supercapacitors

Abstract

The developments in mobile/portable electronics and alternative energy vehicles prompted engineers and researchers to develop electrochemical energy storage devices called supercapacitors, as the third-generation type capacitors. Most of the research and development on supercapacitors focus on electrode materials, electrolytes, and hybridization. Some attempts have been directed towards increasing the energy density by employing electroactive materials, such as metal oxides and conducting polymers. However, the high cost and toxicity of applicable metal oxides and poor long-term stability of the conducting polymers paved the way for alternative electrode materials. The electroactive materials with carbon particles in composites have been used substantially to improve the stability of supercapacitors. Furthermore, the use of carbon particles and metal oxides could significantly increase the energy density of supercapacitor electrodes compared to metal oxides. Recent developments in carbon materials, such as carbon nanotubes (CNTs), activated carbon, reduced graphene oxide, and graphene, have found applications in supercapacitors because of their enhanced double-layer capacitance due to the large surface area, electrochemical stability, and excellent mechanical and thermal properties. Several approaches have been used to develop binder-free supercapacitor hybrid electrode materials, such as electrochemical deposition, chemical bath deposition, chemical vapor deposition, and the sol-gel method. These approaches have been shown some drawbacks, such as complicated processes, long processes time, expensive equipment, high vacuum, volatile precursors and toxic chemicals. On the contrary, the nanoparticle deposition system (NPDS) offers low vacuum, room temperature, eco-friendly, and binder-free deposition method. The main objective of this thesis is to study the deposition of some electroactive materials using the NPDS technique. Promising electroactive materials, such as few-layer graphene nano-flakes, molybdenum disulfide, nickel hydroxide, and graphene-based composites have been selected to be deposited by the NPDS. The deposition was carried out with different deposition parameters, and different contents of the electroactive materials. By studying the structural and electrochemical performance of these materials, we have established a complete set of deep understandings on the concepts, structures at varying lengthy scales and electrochemical performance. The research carried out in this thesis are briefly summarized as follows: (1) The effect of the deposition parameters on the formation of few-layer graphene nano-flakes and their related electrochemical performance are examined by changing the scan speed of the deposition. The faster scan speed of deposition shows higher degree of fragmentation and better electrochemical performance. The electrochemical performance of the few-layer graphene nano-flakes symmetric supercapacitor was further improved by increasing the different electrolytes concentration. (2) The influence of MoS2 content on the electrochemical performance of MoS2-graphite hybrid electrode is studied. The MoS2-graphite hybrid electrode deposited with 5, 10, 15, 20, 25 and 30% wt. of MoS2 by the NPDS on stainless-steel substrates. The capacitance of the MoS2-graphite hybrid electrodes based symmetric supercapacitor demonstrates maximum areal capacitance at 15% of MoS2 content used in the deposition (5.1 mF cm-2 @ 2 mV s-1). (3) Ni(OH)2 deposited by the NPDS on nickel sheets and nickel foam. The structural and electrochemical performance of the Ni(OH)2 deposited with different parameters on nickel sheets are compared. The deposited Ni(OH)2 on nickel sheets with 5 mm SoD and 0.3 MPa carrier gas pressure demonstrates better electrochemical performance. The Ni(OH)2 deposition on nickel foam with 5 mm SoD and 0.3 MPa showed superior specific capacitance (2092 F g-1 @ 1 A g-1) and good cyclic stability. (4) The effect of Ni(OH)2 content in Ni(OH)2-graphene hybrid electrodes on the electrochemical performance is examined. Ni(OH)2 and graphite powders is mixed with 20, 40, 60, and 80% wt. of Ni(OH)2 and deposited on nickel foam substrate. The structural and electrochemical performance of the Ni(OH)2-graphene hybrids is investigated. The Ni(OH)2-graphene hybrids demonstrates a maximum specific capacitance at 10% the content of the Ni(OH)2 in the hybrid electrode. An asymmetric supercapacitor is fabricated based on 10% Ni(OH)2-graphene hybrid electrodes as a positive electrode and rGO as a negative electrode. The Ni(OH)2-Gr//rGO ASC displays an energy density of 64 W h kg-1 and a high power density of 8230 W kg-1.