Preparation And Performance Of Activated Carbon Materials For Supercapacitors

As new energy storage device in-between conventional capacitor and seeond battery, supercapacitor is becoming a hotspot in the field of electrochemical materials. Activated carbon(AC) is widely used as the eletrode material of supercapacitors and its performance is great importance for the supercapacitor performance. In this dissertation, the preparation of AC materials with different structures and surface properties derived from biomass and their electrochemical performance were in vestigated. The AC was prepared with phosphoric acid as activator by one-step high-temperature carbonization and activation process. Different structures and surface properties for the AC were obtained by changing the biomass, impregnation ratio, activation temperature and activation time to improve the structure and electrochemical performances of the biomass-derived AC. The dependence of the electrochemical performance of the AC on its structure(such as specific surface area, pore volume, and mesoporosity) and surface properties was investigated.The structure and surface properties of obtained AC were characterized by Automatic surface area and porosity analyzer(BET), FTIR and scanning electron microscope(SEM). The eletrochemical performances of obtained AC materials was evaluate by electrochemical workstation. The main work and results are as follows:Firstly, the ACs that prepared by one-step high-temperature carbonization and activation process from different biomass feedstock species including fir sawdust, pine sawdust, sycamore and rice husk, have different specific surface area and pore structure.Fir sawdust, pine sawdust and Firmianae activated carbon has a larger surface area, respectively 1625.08 m2/g,1826.94 m2/g and 1769.19 m2/g. Testing the electrochemical performance of AC electrodes. The results showed that fir sawdust AC electrode has the highest specific capacitance and excellent charge and discharge performance. The specific capacitance and energy density of fir sawdust AC electrode can reach to 249 F/g in 6 mol/L KOH at 0.3 A/kg. Therefore, this dissertation will use fir sawdust as raw material to research the correspondence between activation process, activated carbon performance and activated carbon electrode supercapacitor performance.The results showed that the pore structure and surface chemical properties of activated carbon can be tailored effectively to changing technological conditions, such as impregnation ratio(the weight raio of H3PO4 to fir sawdust), activation temperature and so on. It is conducive to the development of the pore structure of activated carbon by reasonable improving impregnation ratio and activation temperature. But excess impregnation ratio and activation temperature would exacerbate the activation process, resulting in excessive reaming, causing part of the activated carbon matrix pore collapse, thereby reducing the yield of activated carbon, the specific surface area, total pore volume and mesopore rate, destroying AC surface morphology. Increasing activator content will promote more carbon matrix activation reaction with phosphoric acid, and increase carbon surface functional groups content. But it will enhance deoxygenation activation process by increasing activation temperature, to remove the AC surface oxygen functional groups effectively. Activation time has little effect on the performance of AC. Testing the electrochemical performance of AC electrodes. The results showed that there’s no direct relationship between specific capacitance and specific surface area of carbon, however, the pore size distribution played an important role. Mesopore was favorable to the formation of electric double layer in KOH electrolyte. The existence of surface oxygen functional groups will lead to decreased the specific energy and the stability of the capacitor. As a result, the carbon eleetrod materials with high specific surface area, high ratio of mesopore and low oxygen content ware obtained, which had excellentel ectrochemical performance in KOH electrolyte. In the best activation condition that impregnation ratio, activation temperature, activation time of 2,500℃ and 120 minutes, respectively, the Fir sawdust AC electrode capacitance and energy density as high as 249 F/g and 49.8 Wh/kg, even better than some ACs for supercapacitor in market.In summary, this dissertation compares the electrochemical properties of AC electrode derived from different biomass. The preparation process, pore structure, surface properties and electrochemical performance of AC materail were systematically investigated. It provide a reference for the development of AC materials for supercapacitors and optimized preparation of AC.