Supercapacitor Based On Freeze-dried Renewable Biomass Materials

With increasing challenge of energy,resource and environment,it has become more and more urgent to develop renewable energy and corresponding efficient energy storage technologies basing on reproducible materials.Recently,using biomass to prepare electrode materials for energy storage devices is gaining growing attentions.However,in existing researches,biomass is generally directly carbonized at high temperature which would inevitably cause the collapsing of the potential porous structure in biomass.Therefore,the element and structure advantages of biomass cannot be fully protected for the produced electrode material and the resulted energy storage properties are required to be further improved.In this thesis,freeze-drying pretreatment was employed to protect the original composition and structure advantages of biomass.The resulted porous biomass was then carbonized at high temperature to produce porous carbon.Through modifying the carbonization temperature and time and simple,clean activation process,interconnected hierarchical porous carbon with excellent energy storage properties was obtained.This method can be widely applied to other energy storage devices.And thus,we developed a new simple and efficient method to prepare biomass based electrode materials with excellent energy storage performances.The main research and conclusion are as follows:(1)Celery was used as material resource for the preparation of supercapacitor electrode material because it contains variety of elements and has long fibrous stalk which can result doping and macroscopically ordered porous structure.Freeze-drying process was applied to dry the fresh celery so as to protect the original structure and produce highly porous biomass.Then the freeze-dried biomass was carbonized at high temperature to prepare graphitized porous carbon.Through modifying the carbonization temperature and time,not only the porous structure originated from the cell and nutrient transport channels of biomass was well protected,but also new interconnected pores between original pores were properly formed.This novel and simple method can fully exploit the advantages of biomass and the produced graphitized carbon possesses three-dimentional(3D)interconnected hierarchical porous structure.Being used as supercapacitor electrode,the resulted graphitized 3D interconnected hierarchical porous carbon(IHPC)exhibited excellent energy storage properties.The specific capacitance of the resulted IHPC reached 350 F/g at current density of 1 A/g and the power density reached 8 kW/kg with the energy density maintaining 16.3 Wh/kg.(2)To turn waste into wealth,this thesis further used pomelo peel to prepare electrode material.Similarly,pomelo peel was first freeze-dried and then carbonized to prepare porous carbon(PC).The sample contained few nano-scale pore structure,and the specific capacitance was only 5.3 F/g at a current density of 1 A/g.In order to improve the specific capacity of the sample,several activating processes were researched.On one hand,pomelo peel was soaked by KOH aqueous solution before freeze-drying and carbonization.Most of the pores in the produced PCK are micropores which are not available for the ion insertion and extraction.Therefore,the specific capacity at 1 A/g was only increased to 26.9 F/g.Replacing KOH aqueous solution with phosphoric acid to soak pomelo peel the produced PCP contains more mesopores,so its specific capacitance at 1 A/g was increased to 94.4 F/g.On the other hand,without any corrosive chemicals,eco-friendly air activation was applied C-900 after freeze-drying and carbonizing processes.The average pore size of the produced PCAx(activated at x ?)was significantly increased.The PCA-350 has maximum specific capacitance of 160 F/g at 1 A/g due to the moderate size and proper proportion of pores with different sizes.And at a power density of 9 kW/kg,the energy density can be maintained at 8.81 Wh/kg.The charge-discharge test was carried out at a current density of 20 A/g,after 100 000 cycles,97.24% of the specific capacity was still maintained,indicating that the electrode material has excellent lifetime and cyclic stability.