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Preparation And Application Of Carbon Electrode Materials For Supercapacitor

Posted on:2008-07-05Degree:MasterType:Thesis
Country:ChinaCandidate:G J S ChengFull Text:PDF
GTID:2132360212980996Subject:Analytical Chemistry
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Supercapacitor is a new kind of energy storage device in-between conventional capacitor and second battery. It has been widely applied in military and civil products or equipments. The electrode is an important factor influencing the performance of supercapacitor. Therefore, the research of high performance electrode materials has been hot topic in this filed. The supercapacitors constructed by carbon electrodes are referable to the electric double layer capacitor (EDLC). Carbon electrode materials have attracted great attention to academe and industry, because of their abundant sources, commercial available, low price, excellently physical and chemical stability and feasibility of manufacture.The purpose of this study is to prepare low cost and high performance carbon electrode material for supercapacitor and to study the relation between carbon property and performance of the carbon electrode base supercapacitor. The nitrogen adsorption was used to study the specific surface area, pore volume, average pore diameter and pore size distributions. The properties of EDLCs, such as specific capacity, equivalent series resistance (ESR), leakage current were studied by electrochemical methods, such as cyclic-voltammetry, constant current charge/discharge, A.C. impedance etc. Main work was done as following :Activated carbon samples were prepared by means of simultaneous ZnCl2-CO2 physical-chemical activation using wood charcoal as precursor. The specific surface area, pore size distribution was studied according to the nitrogen adsorprion-desorption isotherms through BET model, t-plot and BJH method. As a result, the specific surface area is ranged between 400~800 m2/g, the average pore diameter is about 2 nm approximately and there is an obvious pore size distribution peak in 3~5.3 nm.The resultant carbon electrodes were prepared and two-electrode sandiwich type EDLCs were assembled using KOH solution as electrolyte. The cells exhibited good electrochemical stability, high charge-discharge efficiency, equivalent series resistance of 0.3~0.5O, leakage current about 300~600μA, suggesting that the resultant carbon materials are suitable as electrode material for supercapacitor application.To obtain optimal conditions for preparing activated carbon and electrode preparation, the L34 orthogonal experiment was used in the study. As a result, the optmial experiment conditions are: the mass ratio of C/ZnCl2 of 1:1, activation for 3 hours at 800℃, pressed current collectors at 10 MPa. Hereby, the mass specific capacitance of carbon is 183 F/g at discharge current of 10 mA, and 163 F/g at 80 mA. The leakage current of EDLC is 283μA, equivalent series resistance is 0.40 and charge-discharge efficiency is 98%.The cyclic charge-discharge life is an important parameter for supercapacitor. The result showed that after 10000 times constant charge-discharge at 40 mA, the capacitance retention of the cell was up to 83%, indicating a good serving life.Activated carbons based on coconut shell were also prepared by means of ZnCl2-CO2 physical-chemical activation. The pore size distribution exhibited multiple peaks. The specific capacitance of carbon materials increases with increasing the dosage of activating agent ZnCl2, when the mass tatio of C/ZnCl2 is 2:5, the specific capacitance of resultant carbon is high as high 260 F/g at the discharge current of 5mA, and 153 F/g at 80 mA. The corresponding volumetric specific capacitance and area specific capacitance is 172F/cm3, 42.2μF/cm2, respectively. It has a great potential for supercapcitor applications. However, the leakage current and equivalent series resistance are relatively large, further improvement is needed.In order to increase the energy and power density, organic electrolyte instead of KOH solution was used in the carbon-based EDLC. The result showed that the non-woven fabric is a suitable separator for the organic electrolyte EDLC device. Increasing electrolyte concentration is beneficial to acquire high capacitance. Using 1.0mol/L MeEt3NBFa/PC as electrolyte, at the discharge current of 8 mA, the specific capacitance of a lab-made mesoporou carbon sample is 133.8 F/g; The cell has an equivalent series resistance of 6.4 O, the energy density is above 25 W.h/kg. The capacitance of cell has a few changes with different discharge current, indicating that it can be used in wide range of discharge current.The relation between the capacitance and porosity was studied using 1.0mol/L MeEt3NBF4/PC as electrolyte. The results showed that the specific capacitance increases regularly with mesoporous surface area and mesopore volume, but no evidences showed the capacitance dependence on the specific surface area and the total pore volume. These results have important significance to the reseach and development of activated carbon electrode material for supercapcitor.
Keywords/Search Tags:Activated carbon, Physico-chemical activation, Carbon electrode, Electric double layer capacitor, Volumetric specific capacitance
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