| Electrical energy storage (EES) plays an important role in people's daily lives, with devices such as rechargeable batteries for cell phones, computers, and electric cars. Electrochemical, thermal, and mechanical storage systems are just a few of the many types of energy storage systems. In all these systems, a supercapacitor is critical for energy storage. Due to a supercapacitor's high power density, outstanding cycle stability, and long life span, it is widely used in memory backup, hybrid cars, electric vehicles and emergency systems; it plays an essential role to replace tradition fossil fuels in the car markets. Unlike lithium-ion batteries (Libs), the energy density of supercapactior is very low (about 1/10 of Libs) which limits its applications greatly. How to increase the energy density of a supercapacitor without altering beneficial properties is an essential topic in the energy community and attracts numerous attentions for decades. Energy density E=l/2 CV2, C here is the specific capacitances, it mainly relies on the materials of the electrodes, V is the voltage window which is confined by the stability of the electrodes. Many types of electrode materials have been investigated for their ability to increase energy density, among these materials, carbonaceous materials,(activated carbon, carbon nanotube, carbon cloth, graphene, graphite, reduces graphene oxide (rG0), carbon monolith and et al.) are used most commonly as electrodes or as a supporting matrix for metal compounds in electrodes. Due to the easy synthesis process, low cost, large surface area, easily controllable porosity and thermal and chemical stability, activated carbon (AC) is the most popular form of carbon electrode material. Besides, the source to get AC is very rich, it mainly can be derived from biomass and bio-waste. The nano-structures of carbon sources are inherited in the AC, such as layered structure, sponge-like structure and diamond structure. The reviews of different carbon as the electrode materials were given in Chapter I .;The carbon sources used in this research are algae and oil tea shells separately, with ZnCl2 as the activation agent. For algae-derived-carbon (ADC), a high capacitance (325F/g 0.5A/g) and a large surface (>1500m 2/g) were obtained. However, the rate capability is poor; we further study the storage mechanism by dividing the surface areas into mircropore ( 50nm) areas. Generally, The contributions of micropores, mesopores and macropores to the capacitances are different, using the specific capacitances measured from the electrochemical tests, combing the data of surface areas and pore volumes obtained from the N2 adsorption and desorption data at 77K; the capacitance of each single nanopore is calculated, assuming the cylindrical structures of all the nanopores; the results are identical to the published works. Metal compounds . especially for the transition metal oxides are other types of common electrode materials due to the high theoretical capacitances, revisable redox reactions happen during the charge and discharge cycles, which help to transfer/store more electrons than the carbon materials. However, the electrical conductivities and efficient active surface areas for redox reactions of metal oxides themselves are low, which reduce their performances in specific capacitance, rate capability and cycle stability. Normally, metal oxides need to combine with carbon materials, such as activated carbon, carbon nanotube, graphene and et al. to improve the conductivities and expand the active surface areas. The ADC shows a sponge-like structure which is ideal to be used a scaffold to support the metal oxides. The activated carbon supported CoO was synthesized by treating algae with Co(OH)2, then assessed as an electrode material. It was observed that the capacitance, rate stability, and cycle stability were enhanced significantly, as compared to the pure CoO as the electrode materials, applying this simple method, it is easy to scale up the productions from laboratory level to industry level. In chapter 2, we used ADCs to study the pore size and depth effects on capacitances, and further increased the capacitance significantly by the composite of CoO/A DC.;AC is also obtained from the bio-waste oil tea shells, the characterizations show that a certain amount of graphitic structure carbons exist, and the BET surface area is up to 2800m2/g, using this large surface; MnO 2 particles in the solution were adsorbed to form a film `like coating on the surface, a specific capacitance of 1600F/g 0.5A/g is achieved. An asymmetric capacitor cell is set up (Mn02/AC//AC), an energy density 24Wh/kg g275W/kg was reached, which shows great enhancement according to the published works; the details were given in Chapter 3.;Carbon nanotube (CNT) is another important material for application. It possesses high electron conductivity, high thermal conductivity, large tensile strength, huge current tolerance and flexibility. Ml these merits are based on the growth of CNTs; in Chapter 4, we discussed about the CNT growth by 26 factorial designs; the main factor was found. The obtained CNT arrays were further pulled in yarns and doped with Au particles, which enhance the electron conductivity greatly. |
