Preparation Of Three Dimensional Porous Composite Material And Its Application In Supercapacitors

With the seriously increasing energy crisis and the gradual depletion of traditional fossil fuels, the development of new energy sources such as solar power, wind energy source and geothermal energy attracts extensive attentions. However, there still are many problems abount the efficient use and conversion of these clean energy. Therefore, the development of sustainable high energy storage and conversion devices cause widespread concerns, such as lithium ion batteries, sodium-ion battries and supercapacitors. Among all the storage devices, supercapacitor between the conventional electric double layer capacitor and battery is becoming a relatively promising energy storage for power devices and electronic devices, due to its high specific energy density, long cycle stability, wide temperature range, environmentally friendly as well as its rapid charge-discharge characteristics.Energy and power density, conductivity and cycling stability of the supercapacitor electrode materials play an important role in the practical application of supercapacitors. Therefore, numerous efforts have been devoted to improving the electrochemical properties and achieving the high power/nergy densites and excellent cycling stability. The effective methods is as follows:firstly, preparing graphene/carbon nanotube composites through physical and chemical interaction to improve their conductivity and electrochemical properties; secondly, synthesizing hierarchical porous electrode materials with the two dimensional or three dimensional structure to achieve large specific surface area?short ion diffusion transfer path for high capacity and good cycling stability; Lastly, preparing the heteroatoms-doped electrode materials through biomass to achieve large voltage windows and additional pseudocapacitance for high energy energy supercapacitors. Herein, we successfully prepared three kinds of different three-dimensional (3D) porous composite materials with high capacitance, good conductivity and high energy/power energy in practical supercapacitors. The main research work is as follows:(1) Preparation and electrochemical properties of the self-support vertically cross-linking Nickel hydroxide/graphene (Ni(OH)2-CFG) with good conductivity and high energy density. Herein, self-supporting structure Ni(OH)2-CFG composite has been successfully prepared through a simple hydrothermal method. The resulting Ni(OH)2-CFG composites exhibit a special vertical and cross-linked network structure with a large specific surface area (425.9 m2 g-1, higher than that of the pure nickel hydroxide,366.9 m2 g-1. The Ni(OH)2-CFG composites exhibit excellent rate performance and superior cycling stability, which attributes to the good conductive substrate of the graphene. The Ni(OH)2-CFG composite as a binder-free electrode has high mass capacitance of 2276 F g-1 at 1 A g-1,and good rate capability and excellent cycling stability(no capacitance decay after 5,000 cycles at a high current density of 5 A g-1) in three-system electrode. In addition, an asymmetric Ni(OH)2-CFG//activated carbon supercapacitor exhibits a high cell-voltage of 1.6 V and a maximum specific capacitance of 191.3 F g"1 with an energy density up to 15.0 W h kg-1. The excellent performances of the Ni(OH)2-CFG composite demonstrate its promising potential for future graphene-based energy storage and conversion. Herein, the self-support vertically cross-linking Nickel hydroxide/graphene composite effectively resolve the poor conductivity of additional binder and the conductive graphene support can improve the poor cycling stability and lower energy density, which provides a meaningful guidance for other transition metal oxide/hydroxides graphene-based composite.(2) The preparation and the electrochemical properties of the 3D rGCP composite. Polypyrrole nanospheres with excellent rate performance and cycling stability were successfully embedded in wrinkled graphene layer (rGCP) through the electrostatic interaction of the cationic surfactant cetyl trimethyl ammonium bromide and graphene oxide with the help of DL-aspartic acid. The layered graphene cannot only improve the conductivity of the material, but also resolve the poor cycling stability of the rGCP composite during the repeated charge and discharge process. The prepared 3D porous structure with high specific surface area and short ion diffusion is important for high rate capability and cycle stability. As a result, the hybrid rGCP composite exhibited excellent electrochemical performance with a specific capacitance of almost 400 F g-1 at the scan rate of 1 mV s-1. Furthermore, even at the high current density of 10 A g-1, it still retained the capacitance of 243.2 F g-1, and excellent cycling stability (with 91.8% capacitance retention even after 5,000 cycles). Therefore, The chemical method (the ?-? conjugated and electrostatic interaction between cationic surfactant-modified nano-polypyrroly and wrinkled graphene) contributes to prepare other graphene-based electrode materials with the good cycle stability and excellent rate performance supercapacitors.(3) The preparation and electrochemical properties of dual-doped nitrogen and oxygen-enriched layered sedimentary rocks structure activated carbon with high capacitance and ultra-high rate performance. Gelatin is a natural polypeptide biomass with abundant amino functional groups for nitrogen-doped. Citric acid as a organic acid is full of oxygen-containing groups, attributing to enriched oxygen-doped in activated carbon. What is more, the viscosity of a certain concentration aqueous solution is beneficial for the layered sedimentary rocks structure. Herein, gelatin and citric acid as the natural biomass was selected to successfully prepare an oxygen-enriched carbon with layered sedimentary rocks structure. The dual-doping nitrogen and oxygen atoms greatly improve the wettability and pseudocapacitance of the activated carbon materials apart from the traditional electric double layer energy storage. The specific capacitance reached 272.6 F g-1 at 1 A g-1 and still retained 197.0 F g-1 even at 100 A g-1 (with ultrahigh-rate capacitance retention of 72.3%). Moreover, it delivered an energy density of 25.3 Wh kg-1 even with a high power density of 34.7 kW kg-1 and ultralong cycling stability (with no capacitance decay even over 10,000 cycles at 2 A g-1) in a symmetric supercapacitor, which are highly desirable for their practical application in energy storage devices and conversion. Therefore, we have successfully obtained the high capacitance and specific capacity supercapacitors through simple calcinations of the gelatin and critic acid, which is significant for other carbon-based electrode materials.