高性能スーパーキャパシタのためのナノ材料合成とその応用

Supercapacitor, as a new energy storage device, has been widely used in aerospace industry and heavy machinery areas due to the high power density and long cycling life. However, the low energy density limits their widespread application. It is well known that the energy density depends on specific capacitance (Cs) of the electrode and working potential. Therefore, one effective way is to develop nanostructured materials of electrodes for increasing the specific capacitance. The other approach is to fabricate asymmetric supercapacitors, which can effectively broaden the working potential. Hence, we mainly focus on developing electrode materials and assembling asymmetric supercapacitors with high specific capacitance. In chapter 1, the research background is introduced. Supercapacitor structure and its working principle are also described. In order to get an understanding of research progress in materials, the material classification and research progress are also given and concluded. Lastly, the research motivation is proposed based on the previous description. In chapter 2, the equipment and chemicals used in this thesis are listed. The characterization methods and electrochemical measurements are also described. The characterization methods of materials and devices mainly include X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) are the most widely used electrochemical measurements. In chapter 3, Ni3S2/MnS composite with unique 3D morphology was prepared by in-situ hydrothermal method. The Ni3S2/MnS composite shows an enhanced electrochemical performance compared to the single material of Ni3S2 and MnS. The Cs is increased to 6.7 mAh cm-2. Moreover, the Cs remains 97% after 1000 cycles. In chapter 4, in order to improve the electrochemical performance of supercapacitors in terms of flexibility and safety, 1D nanowire WO2.72 with high conductivity (2.58 Ω-1 cm-1) was firstly in-situ grown on carbon cloth (NW WO2.72/CC) by a simple solvothermal reaction. The NW WO2.72/CC electrode shows a high Cs of 398 F g-1 at a current density of 2 A g-1. Furthermore, flexible solid-state asymmetric supercapacitor devices were assembled based on the WO2.72/CC as the positive electrode and activated carbon/carbon cloth (AC/CC) as the negative electrode. The device displays a high energy density of 28 Wh kg-1 at a power density of 745 W kg-1. More impressively, the Cs of the device remains 81% after 10000 cycles. In chapter 5, a novel 2D layered material of Nb2SnC was prepared at relatively low temperature. After etching, the Cs is increased to 128 F g-1 at a scan rate of 2 mV s-1, which is much higher than the pristine sample (25 F g-1). To further improve the electrochemical performance, the etching sample is exfoliated. When the obtained sample was used as an electrode, the Cs has an increase (140 F g-1). Furthermore, symmetric supercapacitors (SSCs) were assembled. The energy density of the device is 14.5 Wh kg-1 within the electrochemical window of 0 V-0.8 V, higher than the energy density (4.4 Wh kg-1) tested within 0 V-0.5 V. In the final chapter, the general conclusions are described. Meanwhile, future prospects are proposed. In this thesis, we synthesized 3 kinds of nanomaterials of the 3D Ni3S2/MnS, 1D NW WO2.72, and 2D Nb2SnC. The electrochemical performance was also investigated. They show great potentials for application of supercapacitors. Future study should focus on the materials development with high conductivity, substrate with high mechanical strength and conductivity, as well as devices with high performance.