Design And Performance Of Manganese (Molybdenum) Oxide-Based Flexible Solid-State Supercapacitors

Recently, much effort has been devoted to flexible, bendable thin-film supercapacitor to meet the growing demands for small, flexible, lightweight and even roll-up portable electronic devices. To this end, it becomes critically important to develop matchable flexible and lightweight supercapacitors to power them. Compared with the supercapacitors using liquid electrolyte, all-solid-state supercapacitor have many advantages such as lightweight, high flexibility, high safety and environmentally benign nature, which are suitable for flexible and portable electronics. On the basis of charge storage mechanism, supercapacitor can be classified into electric double layer capacitors (EDLCs) and pseudocapacitors. Electrode materials in EDLCs are active carbon materials because of the high specific surface area of these materials. Transition-metal oxides and conducting polymers are typical electrode materials for pseudocapacitors. Although flexible supercapacitors have gained new improvement, they still suffer some restrictions such as low energy density. In this thesis we have been focused on the design of new type of electrode materials and the balance of the positive/negative electrode to improve the electrochemical performance of the flexible supercapacitors.(1) Flexible solid-state symmetric supercapacitor was fabricated using MnO2 nanofilms growing directly on carbon cloth as the electrodes and PVA/H3PO4 gel as the electrolyte/separator. The device can be operated at a stable cell-voltage up to 1.4 V, obviously larger than that of conventional solid-state symmetric supercapacitors (≤1V). It exhibit excellent rate capability with a scan rate as high as 20 V s-1 and a long cyclability (～60000 cycles) even under severe mechanical deformation. The charge mechanism at different scan rates was also quantitatively analyzed.(2) In this section, we present an asymmetric flexible, wire-shaped and all-solid-state supercapacitor with high energy and power density by twisting a ahierarchical metal oxide MnO2 core/shell nanorod array fiber-electrode and a mesoporous MoO2 nanofilm fiber-electrode. The hierarchical MnO2 core/shell nanorod fiber-electrode is synthesized by a carbon layer-assisted two-step hydrothermal method, while MoO2-C nanofilm negative electrode is produced via a facile electrochemical deposition. The asymmetric flexible, wire-shaped and all-solid-state supercapacitor with a high operating voltage of 2 V can gain a high specific capacitance of 31.7 mF cm-2. It exihibits a high energy density of 7.036 mWh cm-3 at a high power density of 50 mW cm-3. Even at a high power density of 8011.4 mW cm-3, the device still has an energy density of 1.335 mWh cm-3. Furthermore, the capacitance retention of the supercapacitor is also 97.37% over the 100000 cycles, indicating a good cyclic life.