Study On The Characterization Of Needle Coke, Activated Carbon And Their Composite For Electrochemical Capacitor

In recent years, electrochemical capacitors (ECs) have been become a new developing electrochemical energy storage device. ECs can be used in many areas with great value and market potential because of its higher energy density and fast charge and discharge performance. As ECs electrode materials, carbon materials have a lot of advantages, such as large specific surface area, good electrical conductivity and thermal conductivity, stable chemical performance, low coefficient of thermal expansion, cheap, easy to achieve industrial production. So the carbon materials are the most practical electrode materials of EC now. Compared with single carbon materials, the composition materials have been become research focus because of that the synergistic effect between components can improve their overall performance and meet the performance requirements in all aspects of ECs.In this paper, needle coke and activated carbon, as raw materials, were oxidation-treated with nitric acid of chemical activation method at the temperature of80℃.The influence of nitric acid on the physical properties and electrochemical performance of needle coke and activated carbon was studied respectively. And then the property differences between needle coke and activated carbon were compared. After that, the needle coke and activated carbon were compounded. The complementary performance of needle coke and activated carbon was discussed. In addition, the best mass ratio of needle coke and activated carbon was researched by comparing the electrochemical performance of the composite. The crystal structure and morphology of carbon materials were analyzed by X-ray diffraction and scanning electron microscopy. The surface chemical properties of carbon materials were analyzed by Fourier transform infrared spectroscopy. At last, the cyclic voltammetry and AC impedance measurements of electrode materials were tested by electrochemical workstation. The results show that:(1) The graphite interlayer distances (d002) of needle coke and activated carbon oxidation-treated with nitric acid do not change. The crystallite sizes Le(002) of them decreased and the impurities on the surface of them reduced. Compared with NC, the needle-like fibers of ONC got thicker. And, compared with AC, the surface of OAC got uneven.(2) With the content of activated carbon in electrodes reduced where the needle coke increased, the diffraction peak (002) intensity at20=26°of the composite of NC and AC oxidation-treated with nitric acid got higher and the width of half-peak got narrow and the crystallite sizes Lc(002) of composite improved.(3) Compared with NC and AC, the oxygen functional groups-OH,-COOH of ONC and OAC increased. The oxygen functional groups-OH,-COOH of composite OANC-3were more than other composites.(4) The capacitances per weight of NC, AC, ONC and OAC are2.8F/g,4.8F/g,4.5F/g and51.6F/g respectively. The capacitance per weight of OAC is more than10times as large that of ONC,(5) Compared with NC and AC, the charge-transfer resistance on the surface of electrode and the gap resistance within electrode of ONC and OAC decreased and the capacitance characteristics of that improved. The electric double-layer capacitance characteristic of ONC was better than that of OAC.(6) After the composite of NC and AC were oxidation-treated by nitric acid, the capacitance characteristic and impedance characteristic of OANC-3are the best by cyclic voltammetry and AC impedance measurements. In other words, the electrochemical performances of composite electrodes are the best when the weight ratio of activated carbon and needle coke is1:1.