Reserach On The Applications Of SiC Nanowires And Metal Oxide/hydroxide Composites In Supercapacitors

Supercapacitor is a new type of energy-storage device between traditional capacitors and batteries. It has high power density and long cycle lives, but low energy density. To improve the specific capacitance and energy density of supercapacitors, many efforts have been paid on the electrode materials and other aspects. In this dissertation,chemical vapor deposition(CVD) and electrochemical deposition methods have been used temprepare sUicon carbide nanowires cycles NWs,metal oxide/hydroxide nanostructures and their composites. Supercapacitor performances of electrodes made of SiC NWs and metal oxide/hydroxide nanostructures have been carefully studied. The main contents of the dissertation are shown as follows.1.Supercapacitor electrodes with excellent cycle stability made of SiC NWs on carbon fabric are developed. Carbon fabric serve as a highly conductive substrate and Sic NWs, owing diameter around 50 nm and large specific surface area， grow directly on carbon fabric. The maximum capacitance per area of 23 mF cm~-2 is achieved at room temperature, which is due th the high surface area of SiC NWs. The specific capacitance increases with the elevated temperature because of the decreased Warburg diffusion element. No observable decrease of capacitance occurs at room temperature(20) after 10~5 cycles. Further increasing the measurement temperature to 60, 90%of the initial capacitance is still retained after 10~5 cycles. The excellent cycle stabHity of the electrodes is attributed th the thermal and chemical stabdity of SiC NWs and carbon fiber.2.A porous nickel oxide-hydroxide(NiO(OH)) film is deposited on passivated3D Ni foam by anodic electrodeposition. High-temperature annealing with limited oxygen passivates the surface of the Ni foam and suppresses the reaction of the Ni foamsurface in KOH solution, making the Ni foam more suitable for current collectors in nickel oxide-hydroxide supercapacitors. The deposited NiO(OH) film is porous and conductive, beneficial for supercapacitor electrode. The 3D structure ofnickel foam and the nickel oxide-hydroxide nanostructure facilitate the penetration of the electrolyte into the whole electrode and reduce the internal resistance, contributing to a high specific capacitance of 2302 F g-1 and a large energy density of 64.7 Wh kg-1 at a current density of 1 A g-1. Also, the electrode shows good cyclic stability.3. SiC NWs@Ni(OH)2 core-shell nano-composites were prepared by chemical vapor deposition and electrochemical deposition methods. SiC NWs grow directly on the highly conductive carbon fabric and the NWs contact with each other to form a high-surface-area and conductive network, and Ni(OH)2 porous films deposite uniformly on the surface of NWs. A high rate capability is achieved in the electrode made of Ni(OH)2 (deposition time:lmin) on SiC NWs (growth time:30min). The electrode has a high specific capacitance of 1412 F g-1 at an ultrahigh current density of 100 A g-1. A high power density of 27.5 kWkg-1 and energy density of 59.4 Wh kg-1 are achieved at 100 A g-1. The solid-state supercapacitor based on SiC NWs@Ni(OH)2 shows good flexible and cycling properties by replacing aqueous electrolyte with organic gel electrolyte.4. SiC NWs@MnO2 core-shell nano-composites were prepared by chemical precipitation method and electrochemical deposition method. The core-shell structures have smaller charge-transfer resistance and better supercapacitor performance. A large specific capacitance of about 500 F g-1 is achieved at a current density of 1 A g-1 in the composite electrode, larger than that of the electrode by directly depositing MnO2 on the substrate without SiC NWs. SiC NWs, with large slenderness ratio and specific surface area, when composited with pseudocapacitor materails, can improve the electrochemical performances of the composite supercapacitor electrodes.Supercapacitor electrodes made of SiC nanowires and metal oxide/hydroxide composites show good electrochemical performances, promising for high-performance supercapacitors.