Preparation And Electrochemical Properties Of Lignin-based Carbon Nanotubes/Carbon Composites

Lignin is the most abundant renewable aromatic polymer on earth,and its carbon content is significant.The preparation of lignin based porous carbon with high specific surface area has innate advantages and considerable ecological and economic benefits.However,lignin based porous carbon has high resistivity and poor electrical conductivity,so it is difficult to realize its value in the field of electrochemistry.If it is combined with carbon nanotubes with low internal resistance and high conductivity,the advantages of both can be combined to obtain electrode materials with excellent electrochemical performance.In this paper,lignin-based carbon nanotube/carbon（L-CNT/C）composites were prepared by mobile chemical vapor deposition method using renewable lignin as the main carbon source.Then they were activated to obtain high specific surface area and developed porous structure,and were used as electrode materials to explore their electrochemical performance.In this paper,the preparation conditions of L-CNT/C are explored,and the morphology,structure,thermal properties and electrochemical analysis of the composite materials are combined to select the optimal preparation methods.Firstly,the morphologies and element contents of the initial pyrolysis products prepared by different catalyst/lignin ratios were analyzed.It was found that Fe and Co metal catalysts were distributed evenly in the initial pyrolysis products,and there were certain pores between the particles,which was conducive to the growth of carbon nanotubes.Subsequently,L-CNT/C composites prepared with different catalyst/lignin ratios were characterized and analyzed.It was found that when the catalyst/lignin ratio was low,amorphous carbon generated by lignin decomposition deposited and coated the catalyst rapidly,which deactivated a large number of catalysts and only a few carbon tubes grew.With the increase of catalyst/lignin ratio,carbon tubes began to grow on catalysts under appropriate carbon source concentration.Compared with the L-CNT/C composites prepared with lower catalyst/lignin ratio,the L-CNT/C composite prepared with catalyst/lignin ratio of 2:1 had more carbon nanotubes.L-CNT/C composites were prepared at different reaction temperatures with the ratio of catalyst/lignin at 2:1,and was characterized and analyzed.The results showed that increasing temperature was beneficial to the formation of carbon tubes.When the temperature was 850℃,there were more carbon tubes in the L-CNT/C composite.However,too high reaction temperature led to catalyst deactivation,which was not conducive to the growth of carbon tubes,and reduced the graphitization degree of the L-CNT/C composites.L-CNT/C composites were activated to obtain high specific surface area and developed porous structure.The pore size analysis showed that the number of intermediate pores large significantly,which was conducive to the increase of specific surface area.Subsequently,the activated L-CNT/C composites were prepared as electrodes to study their electrochemical properties.It was found that the excellent electrical conductivity of carbon nanotubes in L-CNT/C and the porous structure of lignin-based carbon（L-C）could improve the ability of charge transport and storage.In the three-electrode system,when the current density was 0.2 A·g^-1,the mass specific capacitance of L-CNT/C electrode reached 234.04 F·g^-1,while the specific capacitance of L-C electrode was only 61.22 F·g^-1.At the same time,the L-CNT/C electrode maintained 95.2%of the initial capacitance after 3000 constant-current charge-discharge tests,showing excellent electrochemical performance.L-C and L-CNT/C electrodes were assembled into L-C symmetric supercapacitors（L-C SC）and L-CNT/C symmetric supercapacitors（L-CNT/C SC）,respectively.The mass specific capacitance of the L-CNT/C SC at 0.2 A·g^-1 was 141.71 F·g^-1,while the specific capacitance of the L-C SC was only 37.47 F·g^-1.At the power density of 219.99 W·kg^-1,the L-CNT/C SC had an energy density of 95.26 Wh·kg^-1,while the L-C SC had an energy density of 25.18 Wh·kg^-1.At the current density of 0.4 A·g^-1,the L-CNT/C SC maintained 93.92%of the initial capacitance after 6000 cycles of charging and discharging,showing good cycle stability.