Preparation And Electrochemical Performance Of Biomass-based Supercapacitor Electrode Materials

Energy shortages and environmental problems caused by excessive use of fossil fuels have prompted people to attach great importance to the development of environmental protection and low-cost energy storage technologies.In recent years,supercapacitors have been widely used in portable electronic devices,backup power sources,electric vehicles,and other fields due to their excellent cycle durability,rapid charging and discharging rates,and environmental friendliness.However,the main disadvantage of supercapacitors is the limited energy density,which becomes a major obstacle to its further application in various fields.Therefore,it is necessary to design high-capacitance electrode materials and use of electrolytes with a wide voltage window.Among the various electrode materials for supercapacitors,carbon materials prepared from biomass have attracted the most attention owing to their low cost,environmental friendly,sustainable,and better electrochemical performance.Ionic liquids with low vapor pressure,non-flammable and higher thermal stability can provide a wider voltage（2.5～4V）and show their great potential as electrolyte for fabricating SCs in comparison with water-based and organic-based electrolytes.In this paper,three biomasses are used as raw materials,and the simple preparation process is used to obtain porous carbon materials with high specific surface area and well-developed pore structures.Ionic liquid 1-ethyl-3-methylimidazole tetrafluoroborate（EMIMBF₄）is used as electrolytes to study the electrochemical properties of materials.The specific research contents are as followed:1.Using honeysuckle flowers as the raw material,a one-step carbonization and activation method was used to successfully prepare a three-dimensional interconnected honeycomb porous carbon material（HC-x）with a high specific surface(2589m~2 g^-1)and hierarchical porosity.The electrode is applied to symmetrical ionic liquid-based supercapacitors.Through the characterization of the microstructure and morphology of the material,the capacitance performance of the corresponding device of the honeysuckle-derived carbon material was systematically measured under the voltage window of 3.8V.The results show that the specific capacitance of the carbon material prepared at a current density of 1 A g^-1 is 186 F g^-1;the corresponding device can provide an energy density of 93 Wh kg^-1 when the power density is 954W kg^-1.The assembled device still remained a 83%capacitance retention after 7500 charge-discharge tests at 3 A g^-1.Thanks to the three-dimensional interconnected frame structure of the honeysuckle-derived carbon material and suitable pore size distribution characteristics,the corresponding device shows good coulombic efficiency and rate performance.2.Collect the waste from boat-fruited sterculia seed and tackle it with activator KOH and urea solution after freeze-drying treatment for hydrothermal pre-carbonization.The obtained mixture was then subjected to secondary carbonization to prepare 3D cross-linked nitrogen-doped carbon material（PDH）with abundant hierarchical pores.Meanwhile,the morphology,crystal structure,element composition,specific surface area and pore size distribution of the PDH materials were extensively characterized.The nitrogen-doped PDH shows good electrical conductivity.And the large specific surface and abundant multi-level micropores improves the utilization of electrolyte ions to pores with different diameters,which makes PDH exhibit an enhanced capacitance performance in Emim BF4 electrolyte(201 F g^-1).The assembled device still remained a 79%capacitance retention after 8000 charge-discharge tests at 3 A g^-1.Moreover,an energy density of 101Wh kg^-1 can be obtained at a power density of 962 W kg^-1by the device.3.The biomass cattail as raw material was hydrothermally treated with KOH solution to obtain a homogeneous mixture of alkali and biomass.After freeze-dried under vacuum,the mixture was then carbonized to obtain carbon material with multi-level pores.The cattail-derived material（XP-x）was used as an electrode material for electrochemical tests.XP-3 has a specific surface area of 2214 m~2 g^-1.This material is used as a supercapacitor electrode material for electrochemical testing.At a current density of 1 A g^-1,the specific capacitance of XP-3 material is 167 F g^-1,the symmetrical supercapacitor based on XP-3 has a power density of 967 W kg^-1 and an energy density of up to 84 Wh kg^-1.