Assembly And Capacitance Properties Of All-solid Flexible Manganese Dioxide/hole Graphene Composite Fiber Supercapacitors

With the continuous development of science and technology and social progress, traditional energy storage devices cannot meet people’s requirements for high energy density and power density. Traditional energy storage devices include lithium-ion batteries, lead-acid batteries, nickel-cadmium batteries and supercapacitors and so on. In compare with the lithium-ion batteries and nickel-cadmium batteries, the traditional supercapacitor has high charge-discharge speed, good stability, and environmental friendly, it has been widely applied in power-type storage field. However, the conventional supercapacitors show large mass and volume, nonflexible and do not be bent, these disadvantages limit their wide applications in the fields of micro-electronics and wearable electronic devices. Therefore, it is expected to prepare the new solid-state flexible supercapacitors with both good flexibility and superior performance.Carbon fibers, especially graphene fiber, show an important position in building strong strength, good flexibility and light weight electronic devices. However, because there is an unavoidable aggregation in the preparation process of graphene fiber, it leads to the much lower specific capacitance of graphene fiber than its theoretical value, and even lower than that of graphene with 3D structure. Therefore, it is expected to build the holey structure of graphene fiber in order to promote the diffusion rate of ions in electrolyte. Manganese dioxide, due to its high theoretical specific capacitance, environmental friendly, abundant resources and other excellent properties, it is widely used as supercapacitor electrode materials. However, manganese dioxide has small specific surface area and low conductivity, which limits its application and development as electrode materials in supercapacitor. As a result, the manganese dioxide/holey graphene flexible fiber composite electrodes are expected to be prepared.In this thesis, flexible holey graphene fiber with enhanced electrochemical performance is obtained by activating graphene fiber in H₃PO₄ solution and followed by depositing δ-MnO₂ on its surface. An all-solid-state fiber supercapacitor is assembled by two intertwined δ-MnO₂/HRGO fiber electrodes, both of which were solidified in an H₃PO₄ polyvinyl alcohol （PVA） gel electrolyte. This all-solid-state fiber supercapacitor shows good flexibility and enhanced capacitive performance. The thesis includes four chapters, introduction part （Chapter 1） focuses on the preparation, structure and property of both graphene fiber and manganese dioxide, and the characteristics of the all-solid-state flexible supercapacitor. Chapters 2 and 3 are experimental parts, which consist of the H₃PO₄ activation of graphene fiber and deposition of δ-MnO₂ on the surface of holey graphene fiber, and the holy graphene/δ-MnO₂ hybrid fiber is prepared and their morphology, structure and the electrochemical property are also investigated. Chapter 4 is the full text summary.By using a wet-spinning method, GO homogenous suspension was injected into a coagulation bath composed of 5 wt% NaOH/methanol solution using a syringe pump with an injection speed of 100 μL min-1. After immersing for 30 minutes in a coagulation bath, the obtained fiber was transferred into a methanol bath to wash away the residual coagulation solution, GO fiber was obtained and collected onto the bracket. Then GO fiber was soaked in 0.5 M H₃PO₄ solution for 12 h, and calcined at 650 ℃ for 2 h in N2, washed with deionized water for 3 times, the holey reduced graphene oxide fiber was fabricated （HRGO）. The HRGO fiber was then followed by soaking in 4.0 mM KMnO4 solution, and stirred at 65 ℃ for 2 h,δ-MnO₂ was coated on the fiber surface, and δ-MnO₂/HRGO fiber electrode is finally prepared. By TEM and BET characterization techniques, manganese dioxide/hole graphene composite fibers are systematically investigated. The prepared δ-MnO₂/HRGO fiber electrode shows a larger specific capacitance （245 F g-1） at a current density of 1 A g-1 in 1 M Na2SO4 electrolyte, and its capacitance retention is 81% after 1000 cycles.An all-solid-state fiber supercapacitor is assembled by two intertwined δ-MnO₂/HRGO fiber electrodes, both of which are solidified in a H₃PO₄ polyvinyl alcohol （PVA） gel electrolyte. This all-solid-state δ-MnO₂/HRGO fiber supercapacitor not only shows good flexibility, but also gives enhanced capacitive performance. The optimum all-solid-state δ-MnO₂/HRGO fiber supercapacitor gives a high area specific capacitance （16.3-16.7 mF cm-2） at a current density of 0.05 mA cm-2, the enhanced rate capability （64% capacitance retention from 0.05-0.6 mA cm-2）, and relative good stability （80% of initial capacitance values after 1000 consecutive cycles）. This method can be used to improve the conductivity and capacitance for all-solid-state carbon-based fiber supercapacitor with good flexibility and light weight.These research results indicate that graphene fiber holey treatment not only increases its surface area, but also increases the diffusion rate of ions in the electrode material. At the same time, manganese dioxide can be added to the holey grapheme fiber, and makes the capacitance of holey graphene fiber improve while maintaining their good flexibility. This method can be expanded to the preparation and assembly of other metal oxides and graphene composite fiber for all-solid flexible capacitor.