Preparation Of Bamboo-based Activated Carbon Used For Supercapacitor And The Research On Its Electrochemical Properties

In this thesis,electrode materials of supercapacitors, the bamboo-based activated carbon with high specific surface,was prepared from biomaterial bamboo,by the process of carbonization,and activation by KOH.The basic properties of bamboo and bamboo charcoal were analyzed by means of thermogravimetric analysis, nitrogen adsorption-desorption test ,SEM,XRD ect. The influences of the carbonization temperature,the ratio of KOH to bamboo charcoal, the activating temperature and activating time on yield, specific surface area,pore structure and morphology of the high specific surface area activated carbon(HSAAC) had been investigated. Based on these, the technologic methods and conditions of controlling structure and properties of HSAAC had been established. All bamboo-based activated carbon were adapted to electrode materials for supercapacitors. The effects of different activating parameters on galvanostatic charge-discharge performance, cyclic voltammetry and AC impedance of the electrodes were discusssed in 1 mol/L LiPF6 electrolyte and the optimal technologic conditions of preparing bamboo-based activated carbon were established.The electrodes of supercapacitors were obtained by ultrasonic mixing of bamboo-based activated carbon into the fibrosis film with that activated carbon. The influences of activated carbon particle size, the amount of bonding agent, conductive agent content and electrolyte type on the electrochemical performances of the electrodes were investigated.Experiment results show that bamboo is a kind of biomass raw materials, its main ingredient is lignin, cellulose, water and so on. The carbonization process includes four stages: precipitation of water, maintaining, pyrolysis and slow loss of weight. The major changes occurred in the 220-400℃and the weight loss reaches up to 69%. Bamboo charcoal remains amorphous carbon and inherites the original microstructure of bamboo.Carbonization temperature is an important factor that affects morphology and apparent quality of bamboo charcoal.The shrinkage of bamboo charcoal cross-section increases with increase of carbonization temperature, charcoal graphite crystallites in the size and number showed an increasing trend, the specific surface area was first increased and then decreased.The structure and performance of activated carbon are influenced mainly by the carbonization temperature,the ratio of KOH to bamboo charcoal, the activating temperature and activating time.The yield of bamboo-based activated carbon increases with increase of carbonization temperature,and its yield decreases with the increase of KOH, activating temperature and activating time while the BET specific surface area, total pore volume of HSAAC increase firstly and then decrease.The effect of carbonization temperature on pore size distribution of bamboo-based activated carbon is great,the activated carbon with larger diameter and a wide pore size distribution can be obtained by controlling lower carbonization temperature;the BET specific surface area, total pore volume of HSAAC increase with the increase of KOH;the pore size distribution as a whole moves toward the direction of increasing diameter with the increase of activating temperature;the activated carbon with a larger aperture,wider pore size distribution can be obtained by controlling and reducing the activation time reasonably.The bamboo-based activated carbon electrodes Which are prepared with different carbonization temperature,different ratio of KOH to bamboo charcoal,the activating temperature and activating time have a similar charge and discharge curves.The capacitance of bamboo–based HSSAC,when using 1mol/L LiPF6 as nonaqueous electrolyte, increase firstly and then decrease with the increase of carbonization temperature,the ratio of KOH to bamboo charcoal,activating temperature and activating time.When the carbonization temperature is 500℃,the ratio of KOH to bamboo charcoal is 4, activation temperature is 800℃and activation time is 1h, the specific capacitance of the obtained bamboo-based activated carbon reaches up to 183.6F/g. The shape of cyclic voltammetry curve is similar to rectangle and obviously symmetrical without the peak of Faraday, which demonstrates that capacitance of HSSAC is mainly supplied through the electric double-layer formed at the the electrode/electrolyte interfaces.The Nyquist plot is an semi-circle at high frequency and is a Warburg bias at intermediate frequency while it is a vertical line to X-axes at low frequency. The impedance characteristics of activated carbon is closely related to pore structure, It is advantageous to improve the transfer speed of electrolyte ions and reduce the resistance through the increase of mesopores in the bamboo–based HSSAC.It is indicated that activated carbon particle size, binder content, the content of conductive carbon black and electrolyte types are important factors which affect the electrochemical properties of the electrodes of bambooecreases, retention rate of capacitance shows a downward-based HSSAC. As the activated carbon particle size decreases, the capacitance of the bamboo-based activated carbon electrode increases firstly and decreases afterwards, leakage current increases.While the capacitance d trend,the transfer resistance Rct of the charge increases gradually With the increase of adhesive PTFE content. With the increase of the amount of conductive carbon black, the specific capacitance of bamboo-based activated carbon electrode increases significantly, the leakage current decreases, the transfer resistance Rct of the charge decreases quickly.When the average particle size of activated carbon is 3.88μm, the binder content is 5%, conductive agent content is 10%, the specific capacitance of bamboo-based activated carbon electrode reaches as high as 187.9F/g.The first charge curve of bamboo-based activated carbon electrode is bent in different electrolytes,while the discharge curves showed good linearity.The discharge specific capacitance decreases with the increase of current density.It has high retention rate of capacitance in 30% KOH solution and DLC306 solution.In the same time,It show good cycle stability in 30% KOH solution、DLC307 solution and LBC305 solution,but show Poor cycle stability in DLC306 solution.The best electrochemical properties of the bamboo-based activated carbon are shown in30% KOH solution.