Study On The Electrochemical Performance Of Aqueous Supercapacitors With High Energy Density Based On Biomass-derived Porous Carbons

Energy density is an important index to evaluate the performance of supercapacitors.Expanding the operating voltage of supercapacitor and increasing the specific capacitance of supercapacitor are considered as effective strategies to harvest high energy density.Aqueous electrolytes are more likely to meet the power capacity and cycling stability requirements of supercapacitors due to their high ionic conductivity,low viscosity and small ionic radius.However,aqueous supercapacitors are limited in their development and commercial applications on account of their narrow operating voltages,which result in suboptimal energy densities.Therefore,it is essential to improve the energy density of supercapacitors by means of increasing the operating voltage of aqueous supercapacitors and finding matched electrode materials.In this thesis,the voltage window of aqueous supercapacitors was increased by developing a novel high-voltage aqueous electrolyte,while a hierarchical porous carbon with excellent performance was obtained by using inexpensive and readily available biomass materials as precursors and matched with the developed high-voltage aqueous electrolyte to obtain aqueous supercapacitors with high power density,high energy density and high cycle life.（1）A new alkaline aqueous electrolyte（MAA）with a mixture of KOH and choline hydroxide（Ch OH）was developed,and a matching porous carbon with interconnected hierarchical porous structure,large specific surface area(up to 3463 m² g^-1),and high conductivity(up to 8.7 S cm^-1)derived from phoenix tree fruit was developed as an electrode material.The obtained hierarchical porous carbon demonstrates a high operating voltage of 1.3 V and a high specific capacitance of up to 462 F g^-1 in MAA electrolyte.The assembled symmetrical supercapacitor can operate stably at 1.3 V with excellent capacitance performance.The device has a high specific capacitance of 84 F g^-1 at a current density of 0.5 A g^-1 and an energy density of up to 19.7 Wh kg^-1 at a power density of 203 W kg^-1.Meanwhile,the supercapacitor based on MAA electrolyte and the prepared interconnected hierarchical porous carbon exhibit good rate capability and excellent cycle stability,up to 89%of initial specific capacitance is retained even at a large current density of 20 A g^-1,and more than 91.4%of initial specific capacitance is retained after 30000 cycles.This work will provide a new approach to broadening the operating voltage of aqueous supercapacitors,which can significantly increase the energy density of aqueous supercapacitors without sacrificing their charge storage capacity,high power density and long cycle life.（2）How to simultaneously achieving high energy and power density is a key challenge for ultrafast-charging supercapacitors.Herein,a composite alkaline electrolyte with widened operation voltage is utilized to improve the energy density of aqueous supercapacitors,while a well-designed semi-graphitic hierarchical porous carbon nanosheet electrode material is prepared to ensure the high power density by effectively alleviating polarization during the charging/discharging process.The symmetrical supercapacitor device built by the obtained semi-graphitic hierarchical porous carbon nanosheet and the composite electrolyte can be reliably operated at 1.3V for a long time and exhibits excellent capacitive performance even under ultra-large current density,which specific capacitance reaches 52.5 F g^-1 at 0.5 A g^-1,and more than 85%of initial specific capacitance is retained at 50 A g^-1,demonstrating excellent rate capability.A high energy density of 12.2 Wh kg^-1 is recorded at 203 W kg^-1,which value is still retained at 10.2 Wh kg^-1 even under an ultrahigh power density of 20.4k W kg^-1.The capacitance retention rate of the symmetrical supercapacitor exceeds 94.1%after 10,000 cycles,demonstrating excellent cycling performance.Furthermore,the assembled device can be charged to 100%within 3.4 s at 20 A g^-1 and then delivers a specific capacitance of 48.9 F g^-1 at 0.5 A g^-1,which value reaches 93.1%of the specific capacitance recorded by using the same charging-discharging current density(0.5 A g^-1).After 10000 times cycles at a charging current density of 20 A g^-1 and a discharging current density of 2 A g^-1,more than 89.0%of the initial specific capacitance is maintained,indicating the reliable ultrafast-charging performance of the assembled device in practical application.（3）A hierarchical porous carbon electrode material with hierarchical pore size,controlled ultra-large specific surface area and interconnected pore structure was prepared by KOH activation using Amygdalus triloba fruit as the carbon source.The material was matched with a newly developed trichloroacetic acid（TCA）electrolyte to obtain an aqueous supercapacitor with both high power density,high energy density and high cycle life.The carbon material can operate at 1.3 V in the TCA acidic aqueous electrolyte and exhibits a high specific capacitance of up to 410 F g^-1.The symmetrical supercapacitor assembled from the prepared hierarchical porous carbon and TCA electrolyte achieves a device specific capacitance of 80.3 F g^-1 at a current density of0.5 A g^-1.The energy density is as high as 18.8 Wh kg^-1 at a power density of 203 W kg^-1 and is still satisfying at a high power density of 8.1 k W kg^-1 with an energy density of 10.4 Wh kg^-1,showing remarkable energy density at high power density.After20,000 cycles,the symmetrical capacitor retains more than 95.6%of the initial specific capacitance,indicating the ideal cycling performance of the supercapacitor.In addition,the performance of the supercapacitor in practical applications has been studied.The device is charged at a current density of 20 A g^-1 and then discharged at a current density of 0.5 A g^-1,providing a specific capacitance of 56.9 F g^-1.After 20,000 cycles at a charging current density of 20 A g^-1 and a discharge current density of 2 A g^-1,the capacitance retention rate of the supercapacitor is as high as 95.0%,and it exhibits excellent cycle stability in practical applications.