Preparation Of Plant-based Porous Carbons For High Performance Supercapacitor

With the gradual depletion of fossil energy and the deterioration of environmental problems,the development of renewable energy and new energy storage devices has become a top priority.Supercapacitor,a new type of energy storage device with high energy density,high power density,low maintenance cost,excellent cycle stability,and fast charging and discharging rate,is widely used in transportation,smart grid,computer,medical,military,industrial,and power,uninterruptible power supply system.The electrochemical performance of supercapacitors mainly depends on the electrode materials and electrolyte.Porous carbon material is the most widely used electrode material for supercapacitor due to its high conductivity,large specific surface area,developed pore structure,abundant surface heteroatoms,low cost and simple prepation process.With the rapid development of economy,the overexploitation fossil energy leads to the problem of resource depletion.Hence,useing the recycled,low cost,green environmental,sustainable and abundant biomass as carbon source preparation of porous carbon materials has received the widespread attention.In this thesis,different methods were used to prepare hierarchical porous carbon materials with unique structure by direct activation of different biomass precursor.The effect of preparation methods and precursor characteristics on the specific surface area,pore structure,surface chemical state,electrical conductivity and electrochemical properties of porous carbon materials were mainly studied.The specific research contents are as follows:（1）A dual-activation strategy of KOH and KMn O₄was developed to prepare the cotton stem-based porous carbon with large specific surface area(1634 m²g^-1),interconnected pore structure,and reasonable mesopore volume(0.44 cm³g^-1)and micropore volume(0.59 cm³g^-1).The high specific capacitance of 335 F g^-1(at a current density of 1 A g^-1)in a three-electrode system was obtained due to the rich electrochemical active sites and rapid transfer channels for electrolyte ions.When the current density was increased to 100 A g^-1,the capacitance retention still reaches 67%,suggesting excellent rate performance of the obtained samples.The obtained samples showed a high areal capacitance of 3.8 F cm^-2under a high mass loading of 16 mg cm^-2.After 10000 cycles,the capacitance loss is only 2%,indicating outstanding cycle perforamance of the obtained sample.In the symmetric two-electrode system with 1 M Na₂SO₄as the electrolyte,the energy density of the sample is up to 19.9 Wh kg^-1at 397W kg^-1.This work provides a new research idea for large-scale conversion of abundant plant wastes into carbon materials.（2）Hierarchical porous carbons with large surface area and interconnected pore structure were prepared by KMn O₄and KOH dual-activation using mungbean bark,lavender stem,hemp stem and pine tower as precursors.The obtained BPC,LPC,HPC,and PPC show 333.6 F g^-1,321.9 F g^-1,316.5 F g^-1and 317.8 F g^-1,respectively in the three-electrode system at a current density of 1 A g^-1.When the current density was increased from 1 A g^-1to 50 A g^-1,all four samples showed more than 70%capacitance retention.In the symmetrical two electrode system,the capacitance retention is more than 95%and the coulombic efficiency is almost 100%after 10000 cycles at the current density of 5 A g^-1.Moreover,in the two-electrode system using 1 M Na₂SO₄as electrolyte,the energy density of the sample LPC is 18.11 Wh kg^-1at 397 W kg^-1and6.88 Wh kg^-1at 13757 W kg^-1.（3）The porous carbon was prepared from cotton stalk with potassium formate as activator.Potassium formate is a green and environmentally friendly organic potassium salt,which contains high content of potassium and has low corrosion.The large specific surface area of the prepared porous carbon can provide abundant active sites for the accumulation of ions,and appropriate mesopores can provide channels for the rapid transport of electrolyte ions.The sample shows a high specific capacitance of 364 F g^-1at 1 A g^-1in a three-electrode in KOH electrolyte,and a high capacitance retention of73.9%when the current density is increased to 100 A g^-1.In the two-electrode KOH electrolyte system,the capacitance retention reachs to 82.2%when the current density is increased from 1 A g^-1to 50 A g^-1,showing an excellent rate performance.After 10000cycles in 6 M KOH in a two electrode system,the capacity retention rate is 98%and the coulomb efficiency is almost 100%.These results indicate that the as-prepared porous carbons prepared with different plant precursors show broad application prospects as electrode materials for supercapacitors.