| As a new type of energy storage element,supercapacitors have attracted wide attention due to their advantages of long cycle life,good reversibility and high power density.As an important part of the super capacitor,the electrode material largely determines the performances of supercapacitors.The porous carbon materials with large specific surface area,high conductivity,proper pore size distribution,regular pore structure and chemical stability can be used as the ideal electrode material.Among different types of porous carbon materials,carbon nanofibers have mechanical flexibility and three-dimensional network structures with good conduction to electrons.Therefore they do not need to any binders and conductive agents to prepare the electrode material,which will reduce unnecessary consumption of internal resistance,etc.These advantages make it an excellent supercapacitor electrode material.In the past,most of carbon nanofibers were made from PAN and pitch as carbon sources,but PAN and pitch are got from fossil energy sources,which are non-renewable and not environmentally friendly.Lignin,as the second most abundant biomass material preceded only by cellulose,is the most abundant natural aromatic polymer.Lignin with high carbon content and a benzene ring structure can be used as carbon fiber raw materials and used in supercapacitors.The alkali lignin is used as a carbon precursor to prepare nanofiber membranes by electrospinning process.After pre-oxidation and carbonization,lignin-based carbon nanofibers are obtained.The results show,the diameter of lignin-based carbon nanofiber is 200-250nm.Lignin as a support framework,with the increase of spinning additive PVP,the specific surface area increases.Without any activation,the specific surface area can reach 600m2/g.At the same time,this lignin-based carbon nanofiber has mechanical flexibility,without grinding and adding any binders,can be directly cut into electrodes for electrochemical performance testing.At a current density of 0.2A/g,the specific capacitance value can reach 183 F/g.In addition,the samples have a small resistance and exhibit good performance.The microporous carbon nanofibers with high specific surface area were prepared by electrospinning,pre-oxidation,carbonization and pickling when the precursors of TEOS silica template were added to the spinning solution above.The amount of silica template agent has an obvious effect on the size of the fiber diameter;With the increase of the amount of template agent,the diameter of nanofibers show the tendency of decreasing and then increasing.The carbon fiber prepared has the smallest fiber diameter of 132nm when the ratio of PVP、lignin、TEOS is 2:1:2.The maximum specific surface area is 1197m2/g and the maximum pore volume is 0.609cm3/g.The obtained lignin-based carbon nanofibers are tested for electrochemical performance and show good electrochemical performance.The results show that the highest specific capacitance is 282 F/g at 0.2 A/g.At 20A/g,182F/g can still be achieved with capacitance retention of 66%.Silicon-based templates require highly corrosive HF acids during pickling.HF acids will corrode equipment and instruments in a certain degree.In order to overcome its strong corrosiveness,magnesia,which requires less corrosive HCl acid during pickling,is used as a hard templating agent to prepare lignin-based porous carbon nanofibers The carbon nanofibers has a high rate of mesoporosity utilizing the nanometer size of magnesia The specific surface area of the material reached 1140 m2/g,which can be comparable with microporous carbon nanofibers The mesoporosity increases to 78%and the pore size appears multi-level distribution.The material presents from the non-mesoporous,to the emergence of large-diameter mesopores,and finally to the small-diameter mesopores.In the three-electrode test system,the capacitance retention rate is about 60%at 20A/g,which is mainly due to the introduction of mesopores to enhance the rate performance of the material. |
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