Structure Control And Electrochemical Performances Of Biomass-based Porous Carbon

Developing stable,efficient and low-cost energy storage devices is a great significance for the development of renewable energy and green energy.As a new type of green energy storage device,supercapacitor has attracted wide attention due to its high power density,fast charge and discharge rate,long cycle life.In addition,supercapacitor is also considered as one of the most promising energy storage devices.The electrochemical performances of supercapacitor is affected by the electrode materials.Development of electrode materials with high performance have become one of research hotspot to study the supercapacitor.Waste biomass are common waste resources in life.It can not only solve environmental problems,but also realize the high value-added utilization of biomass resources by using the waste biomass to prepare porous carbon materials with high porosity.In this paper,hierarchically porous carbon,spherical porous carbon and nitrogen doped porous carbon had been derived from waste biomass,such as solid(soybean shell and lignin)and liquid(waste liquid produced in the process of vitamin C production).The preparation and application of porous carbon have been researched emphatically.It provides a theoretical basis for the development of supercapacitor electrode materials with excellent capacitance performance and the high value-added utilization of waste biomass resources.This thesis mainly contains the following content:(1)Soybean shell is converted into porous carbon with high specific surface area and well-developed pore structure via hydrothermal carbonization with H₃PO₄ followed by KOH activation.The product obtained from the hydrothermal carbonization with H₃PO₄possesses more carbon content,which is beneficial to the KOH activation reaction.The specific surface area of activated porous carbon(PC)after H₃PO₄ treatment is 1.95 times of that of untreated porous carbon(SSC).And the PC also has more pore in the range of 2?5 nm,which is conducive to electrolyte transport.Compared with the SSC,PC used as the electrode materials for supercapacitor has better electrochemical performance.In the two-electrode system,the specific capacitance of PC can reach upto 301 F g^-1at the current density of 0.1 A g^-1 in 6 M KOH.And PC also exhibits excellent cycling stability with the specific capacitance remaining at 93.8%after15000 cycles.(2)The porous carbon with honeycomb structure was obtained by hydrothermal carbonization of Zn Cl₂ solution followed KOH activation.The porous carbon obtained by Zn Cl₂solution hydrothermal carbonization show honeycomb structure,while the porous carbon without Zn Cl₂ show block structure,indicating that adding Zn Cl₂ solution affects the morphology of porous carbon.Specific surface area of porous carbon increase and the pore towards to the width pore with the increasing the activation temperature and activation ratio.However,excessive high temperature and activation ratio will cause the decrease of specific surface area of porous carbon due to over activation.The sample obtained at 600 ^oC activation temperature and activation ratio of 5(0.5-HPC-600-5)has the high specific capacitance of 384F g^-1(0.04 F g^-1)and the specific capacitance is still maintain 239 F g^-1 at the current density of5 A g^-1.Moreover,0.5-HPC-600-5 possesses high energy density of 10.48 Wh kg^-1 and specific capacitance remaining at 96.96%after 15000 cycles.(3)The interconnected network structure pore with super high specific surface area was obtained by using melamine foam as template and waste liquid(by-product from the process of vitamin C production)through carbonization and activation two step method.The porous carbon used as electrode materials for supercapacitor show outstanding electrochemical performances in EMIMBF₄ and the working voltage can reach 3.8 V.The porous carbon shows a high specific capacitance(217 F g^-1 at the current density of 0.1 A g^-1),excellent rate performance(the specific capacitance is as high as 94 F g^-1 at 30 A g^-1),excellent cycle stability(specific capacitance remaining at 89%after 10000 cycles in 5 A g^-1)and ultra-high energy density(107 Wh kg^-1).In addition,the porous carbon also has electrochemical performance in all-solid-state symmetric supercapacitor.(4)The porous carbon spheres are obtained from waste liquid via hydrothermal treatment followed with K₂Fe O₄ activation.The results show that the prepared samples have spherical structure and hierarchical pore structure(rich micropores and a certain of mesopores).The pore size tends to the direction of pore width with the increase of K₂Fe O₄ content.The prepared porous carbon spheres show high specific capacitance,excellent cycle stability and good rate performance in aqueous electrolyte.At the same time,in the solid-state capacitor with PVA/KOH as electrolyte,the porous carbon spheres also showed high specific capacitance(196 F g^-1 at 0.1A g^-1)and good cycle stability(82.6%specific capacitance retention after 10000 cycles).In order to improve the energy density of the supercapacitor,the symmetric supercapacitor is assembled with ionic liquid(EMIMBF₄)as electrolyte and the working voltage of supercapacitor reaches3.8 V.The energy density of supercapacitor is 47.2 Wh kg^-1 and excellent cycle performance(the specific capacitance retention is 88.9%after 10000 cycles).(5)The N-doped can improve the capacitive performance.In order to promote the electrochemical performance of porous carbon spheres,urea(as N source)was introduced during the K₂Fe O₄ activation process to prepare porous carbon spheres with high N content.The results show that the N has been successfully doped in the porous carbon spheres.The N content in the NPCS-700 is as high as 3.9 wt.%.In addition,urea maybe convert into carbon nitride,which is decomposes to generate gas and released from carbon materials under high activation temperature during the activation process.Therefore,the specific surface area of NPCS-700 is reach upto 2796 m² g^-1.In addition,NPCS-700 also possesses abundant large micropore(the pore side in the 1?2 nm)and the mesopore in the pore range of 3?5 nm.When NPCS-700 used as electrode material for supercapacitor,it shows good electrochemical performance in different electrolyte.In 6 M KOH,NPCS-700 exhibits excellent cycle stability(the specific capacitance retains 97.1%of initial capacitance after 6000 cycles)and the specific capacitance is as high as316 F g^-1.The working voltage in 1 M Na₂SO₄ and EMIMBF₄ can reach 1.8 V and 3.8 V,respectively.The energy density of NPCS-700 is as high as 20.2 Wh kg^-1 and 85.5 Wh kg^-1 in 1M Na₂SO₄ and EMIMBF₄,respectively.The electrochemical performance of NPCS-700 in different electrolytes is different owing to the different of ion size,ionic conductivity,the matching degree between pore structure of porous carbon and electrolyte ions.This thesis contains 100 Figures,26 Tables and 270 references.