Preparation Of Modified Lignite-based Porous Carbon And Their Performance For Electric Double-layer Capacitor

The electrochemical capacitors?ECs also called supercapacitors?attracted the attention of scientists from all over the word,which is owing to their fast charge-discharge,high power densities and long cycling life.The electrochemical performances of ECs are determined by the electrode material.Porous carbon?PC?are regarded as a preferred material for electrode because of the large specific surface area?SSA?,moderate cost and good conductivity.Research and development of PC with high SSA and appropriate pore size distribution is a research hotspot for electrode materials.In this paper,Shengli lignite?SL?was selected as the carbon source to prepare modified lignite-based PCs through two different types of methods and the modified lignite-based PCs were used as an electrode material for ECs.The structure and performance of PCs were investigated by N₂ isothermal adsorption,X-ray diffraction?XRD?,X-ray photoelectron spectroscopy?XPS?,Raman,scanning electron microscopy?SEM?,Fourier transform infrared spectroscopy?FTIR?and transmission electron microscope?TEM?.Moreover,the electrochemical performances such as Galvanostatic charge-discharge?GCD?,cyclic voltammetry?CV?and electrochemical impedance spectroscopy?EIS?of PCs as electrode materials are studied in detail.The main research contents and results are list as following:?1?The modified lignite-based PCs were prepared by co-carbonization of lignin and lignite.The results show that adding lignin caused the increase of SSA due to the wide range of weight loss of lignin is benefit to expansion and extension of pore structure.However,the SSA decreased when excessive lignin was added because of that expanding of existing pores may take the domination in the reaction and lead some pore collapse.The PCs which are obtained from different preparation parameters as electrode for EDLC have good electrochemical performance and exhibit typical EDLC characteristic.The specific capacitance?C_s?increased with increasing the SSA,but the relationship between C_s and SSA is not simply linear proportional.The C_s is also effected by the pore size distribution and increasing the volume of pore smaller than 2 nm is conducive to the increase of C_s.Raising the activation temperature promoted the development of mesopores.Due to the larger SSA and appropriate pore size distribution,the C_s of PCA2?the ratio of lignin and lignite of 5:5,carbonization temperature of 500 ^oC,activated temperature of700 ^oC and KOH-coke ratio of 3:1?reached 341 F g^-1 at the current density of 40 mA g^-1.?2?The modified lignite-based PCs were obtained from SL via hydrothermal carbonization?HTC?pre-treatment and followed by activation with KOH.The SSA of PCs with HTC pre-treatment are larger than that without HTC treatment.Moreover,the PCs are mainly micropores structure.The pore structure of PCs could be tailored effectively by changing the HTC temperature,KOH-hydrochar ratio and activation temperature.The 200-HTC-800-3 with the SSA of 3069 m² g^-1 and total pore volume of 1.68 cm³ g^-1 is achieved at HTC temperature of200 ^oC,KOH-hydrochar ratio of 3:1 and activation temperature of 800 ^oC,which has excellent performance.The C_s decreased from 295 F g^-1 to 210 F g^-1 when the current density was increased from 40 mA g^-1 to 10 A g^-1,indicating that the PC of 200-HTC-800-3 has good rate performance.In addition,the 200-HTC-800-3 shows excellent electrochemical cycle performance that after 15000,which cycles the C_s drop rate only 6.2%.