Preparation Of Porous Carbon Composite Nickel-cobalt Hydroxide From Licorice Residue And Its Electrochemical Properties

Renewable sources of wind,hydro,and solar energy suffer from intermittency,regionality,and high costs,which in turn limit their widespread use.Therefore,it is essential to develop advanced energy conversion and storage technologies and devices.Supercapacitors are excellent energy storage capacitors with higher capacitance and power density,and shorter discharge times than batteries.Licorice is one of the most famous medicinal plants,widely distributed in northwestern China and Siberia,Russia,and is mainly used for extracting Licorice acid.China produces about 7 million tons of extracted licorice residue annually.Most of the licorice residues are not fully and effectively utilized,but directly discarded or burned as solid waste,which is undoubtedly a great waste of biomass resources and pollutes the environment.The use of cellulose and hemicellulose from licorice residues to prepare biomass-based porous carbon materials with special structures is a strong candidate for supercapacitor electrode materials.In this thesis,we propose a green and scalable strategy to prepare biomass-derived porous carbon from licorice residue,and on this basis,we synthesize composite materials of licorice residue porous carbon and nickel-cobalt hydroxide for supercapacitor applications,as follows.(1)Carbonaceous materials with high electrochemical properties were prepared by the pre-carbonation-KOH activation method using licorice residue as raw material.The prepared graded porous material has a very high specific surface area(1665.2 m~2/g).When used as electrode materials for supercapacitors,the licorice residue carbon materials provided a specific capacitance of 222 F/g at a current density of 1 A/g and maintained 188 F/g at a current density of 10 A/g,with a capacitance retention rate of80.8%.(2)To address the low capacitance of the produced licorice residue-graded porous carbon,a layered substrate-load material structure was formed by in situ growth of nickel-cobalt hydrotalcite(Ni Co LDH)on the surface of nickel foam coated with porous carbon through a simple electrodeposition method.The fabricated composites possess a specific surface area of 40.7 m~2/g and exhibit superior supercapacitor performance in a three-electrode system,with a specific capacitance as high as 811.5 F/g at a current density of 1 A/g and maintaining a low equivalent series resistance(0.60Ω)and charge transfer resistance(0.12Ω).(3)In order to improve the specific surface area of the above composites and further improve the capacitive performance,a three-dimensional core structure composite material with hydrothermally synthesized licorice residue porous carbon as the core and Ni Co LDH as the shell was developed.The material has a high specific surface area(152.1 m~2/g)and exhibits a high specific capacity(1807.8 F/g at 1A/g)and high-rate performance(90.4%capacitance retention at 10 A/g).The assembled asymmetric supercapacitors have high energy density(51.5 Wh/kg at 374.9 W/kg)and excellent electrochemical stability(82.6%retention after 2000 tests).These results indicate that the porous carbon derived from licorice residue is expected to be an excellent electrode material for future practical applications,and this activation-loading process may provide a potential idea for the preparation of carbonaceous metal composites with morphology retention,high specific surface area,and excellent energy storage performance to improve the resource utilization of biomass waste.