Study On Activatied Carbon Electrode Material Of Ionic Liquid Electrolyte Supercapacitor

Environmental pollution,energy crisis and global warming have become a huge challenge to mankind,beause of mass use of fossil energy resources including coal and oil.The imminent needs for the development of clean and efficient energy conversion/storage device are very essential and important.Compared with other energy storage devices,the supercapacitor has been widely applied in standby power supply,portable electronic devices,electric tools,electric vehicles,intelligent power grid and other fields,due to such advantages as wider operating temperature range,higher safety,higher charge and discharge efficiency and longer cycle life.However,its lower energy density has limited the application of the supercapacitor.There are two feasible ways to improve the energy density of the capacitor,one is to improve the specific capacitance of the electrode material and another is to increase the potential window of the capacitor.For broadening the application of the supercapacitor and acceleratingthe industrialization process,we did research on the influences of the pore structures of activated carbons and the types of ionic liquids on the electrochemical performances of the supercapacitor,aiming to improve the energy density of the supercapacitor and thereby contributing to produce the supercapacitor with high electrochemical performances.Here,we applied two processes,phosphoric acid?H₃PO₄?and water vapor activation which were both widely applied in the industry,to produce activated carbons for the subsequent studying.The H₃PO₄ activation process used H₃PO₄ as activator and Chinese fir sawdust as raw materials to produce activated carbon electrode materials with anadjustable pore by altering impregnation ratio and activation time.The water vapor activation used coconut shell activated carbons as raw materials to produce optimized activated carbon electrode materials by altering activation temperature and time.We then characterized the elemental composition and pore structure and morphology of activated carbon electrode materialsby using nitrogen adsorption/desorption isotherms,scanning electron microscopy and X-ray photoelectron spectroscopy.And,we also performed cyclic voltammetry,galvanostatic charge/discharge and electrochemical impedance spectroscopyto evaluate the electr ochemical performances of the supercapacitor based on activated carbon electrodes and ionic liquid electrolytes.To study the effects of pore structure of activated carbons on the electrochemical performances of the supercapacitor,we used the 1-butyl-3-methyl-imidazolium hexafluorophosphate?[BMIM]PF6?as electrolyte.We prepared activated carbons with thevolume of mesoporeover 66% using the H₃PO₄ activation process with the activation temperature of 800-950 oC and an appropriate impregnation ratio.Under the circumstance of impregnation ratio being 3:1,the single electrode of the prepared activated carbons had a mass specific capacitance of 162 F/g and an area specific capacitance of 10.29 ?F/cm2 with an energy density of 22.5 Wh/kg.The specific capacitance remains 86% after 5000 cycles at 5A/g,suggesting high rate capability and good cycling stability.Comparison with other activated carbons with lower mesopore volume,the activated carbons with a pore size of 2-5 nm produced by the H₃PO₄ activation process hadbetter capacitance performances and higher rate capability,and were thus more suitable for the ionic-liquid-based supercapacitor.To study the effects of the type of ionic liquids on the electrochemical performances of the activated-carbon-based supercapacitor,we used the water vapor activatedcoconut shell?W-AC?as the electrode material.Comparison analysis suggested that the ionic liquids comprised of EMIM+,BMIM+ and BF4-were more suitable for the W-AC electrodes.The specific capacitance of the W-AC electrodes reached 153 F/g when using [EMIM]BF4 as electrolyte,and the supercapacitor reached a high energy density of 57 Wh/kg with a potential window of 3.5Vwhen using [BMIM]BF4 as electrolyte.These results suggested that the supercapacitor with high specific capacitance,wide electrochemical window and high energy density would be obtained by using activated carbons with large micropore volume and large mesopore number as electrodes and ionic liquids comprised of [EMIM]BF4 and [BMIM]BF4 as electrolytes.In order to optimize the electrochemical performances of the water vapor activated carbon electrodes,we used coconut shell activated carbons as raw materials to produce the W-AC electrodes withan optimum electrochemical performance by changing the activation time and temperature.The [EMIM]BF4 ionic liquid,whichwas suitable for the water vapor activation,was selected as the electrolyte.After optimization,the activated-carbon-based supercapacitor had a stable potential window of 3V,a high mass specificcapacitance of 247 F/g and anarea specific capacitance of 12.11?F/cm2.In addition,the energy density of the supercapacitor was increased to 77.19 Wh/kg and the specific capacitance still remained 90% after 2000 charge and discharge cycles at 2A/g under room temperature environment.