Synthesis and electrochemical characterization of porous carbon nanosheets/metal silicide nanowires based electrode materials

Abstract

Due to the increasing demand of the portable electronic devices, hybrid electric vehicles, and renewable energy production, development of energy storage devices with high energy, high power, safe operation, and long cycle life is important. Supercapacitor which has lower energy density but higher power density and better cycle life than battery has received great attention as energy storage device. It is important to study new electrode materials to develop supercapacitor with high energy density, high power density, and good cycle life.In this thesis, we report the synthesis of porous carbon nanosheets/metal silicide nanowire based electrode materials and their electrochemical performance for supercapacitor application. In chapter 1, we demonstrate the synthesis of porous carbon nanosheets with controllable pore size and their supercapacitor application. Porous carbon nanosheets have emerged as promising candidate for electrode materials for supercapacitor because of their large specific surface area, high electronic conductivity, and short diffusion distance. We synthesize porous carbon nanosheets with controllable pore size distribution ranging from predominantly microporous (< 2 nm) to mesoporous (2-50 nm) by controlling the type and the ratio of alkali metal citrates. Among them, the hierarchical micro/mesoporous carbon nanosheets derived from the mixture of potassium and sodium citrates in a weight ratio of 8:2 exhibit high specific capacitance (~200 F $g^{-1}$ at the scan rate of 5 mV $s^{-1}$), excellent rate capability and cyclability. Also, we suggest that the alkali metal carbonates formed in situ during pyrolysis act not only as activation agents but also as templates for the formation of carbon nanosheets from the observation of the structural changes in the carbon materials produced during pyrolysis. The synthetic method of porous carbon nanosheets with controllable pore size distribution and the understanding of the mechanism of porous carbon nanosheets are expected to find use in various applications like energy storage, water purification, and catalytic reactions. In chapter 2, we report the electrochemical performance of porous cabon nanosheets surface-functionalized with polydopamine and $Fe^{3+}$/tannic acid. Polydopamine can be coated on the surface of various materials by simple method and provide functional groups for redox reactions. After coating polydopamine on the porous carbon nanosheet electrode, the specific capacitance is increased by ~40% ($~185 F g^{-1} at 5 mV s^{-1}$) as compared to that of unmodified electrode. Furthermore, the electrode coated with both polydopamine coating and $Fe^{3+}$/tannic acid layer-by-layer deposition shows ~83% higher value of specific capacitance ($~244 F g^{-1} at 5 mV s^{-1}$) than that of unmodified electrode. This is attributable to the introduction of more hydroxyl groups for additional redox reactions. Also, the electrode coated with both polydopamine coating and $Fe^(3+)$/tannic acid layer-by-layer deposition show good cyclability (~92% after 1000 cycles) due to the strong interaction between the $Fe^{3+}$ ions and the catechol groups. Given that the proposed surface modification method of electrode is simple, eco-friendly, and effective for increasing the capacitance while ensuring excellent cyclability, this work can be utilized to fabricate high-performance supercapacitors or other promising energy storage devices such as Li- and Na-ion batteries. In chapter 3, we present the synthesis of cobalt silicide nanowires on silicon substrate and the electrochemical performance as electrode materials for supercapacitor. We synthesize $Co_{2}Si$ NWs on silicon substrate with high density by a simple chemical vapor tansport method without using any catalyst or etching solution. When tested in an ionic liquid electrolyte (EMIM TFSI), the $Co_{2}Si$ NWs-based supercapacitor exhibits high areal capacitance ($~983 \muF cm^{-2} at 2 \muA cm^{-2}$), high energy density ($~629 \muJ cm^{2} at 2 \muA cm^-{2}$), and excellent cyclability (~94% after 4000 cycles). This could be mainly due to high electrical conductivity, high surface area, and good adherence to the Si substrate of $Co_{2}Si$ NWs. The simplicity of the fabrication method and the ease of integration on Si substrate together with good electrochemical performance make the $Co_{2}Si$ NWs promising electrode materials for on-chip microsupercapacitor.