| Supercapacitors have the characteristics of fast charging and discharging rates,long service life,wide operating temperature range,and environmental-friendly.Therefore,they have been widely employed in the portable electronic equipment and energy storage devices.It is known that the electrode material is a key factor determining the performance of supercapacitors.Biomass-derived porous carbon is considered to be an ideal potential electrode material for supercapacitors due to its abundant resources,low cost,good renewability,and controllably porous structure.In this thesis,a series of hierarchical porous carbon materials were prepared from the biomass wastes poplar catkin by controlling different activation agents(e.g.H3PO4,KOH,and K2CO3),different activation agent amounts and different calcination temperatures.The structure-performance relationships of the obtained materials were elucidated based on the analysis results of various techniques(such as SEM,TEM,XRD,Raman,N2 adsorption-desorption and XPS)and supercapacitive performance in three-and two-electrode systems.The specific contents are as follows:(1)A series of mesopore-dominant porous carbon materials(denoted as PCC-XY)were prepared with the equal volume impregnation combined with direct calcination under N2 atmosphere by using H3PO4 as the activation agent,which is more milder than traditional strong acid activators(such as concentrated sulfuric acid).Among them,PCC-3-600,which was fabricated under the conditions of H3PO4 solution 3.0 mL and calcination temperature 600℃,possessed a specific surface area of 2011 m2 g-1 and a total pore volume of 1.215 cm3 g-1.The specific capacitance of PCC-3-600 was 251.8 F g-1 at a current density of 1.0 A g-1 in the three-electrode system.The PCC-3-600based symmetrical device depicted an energy density of 25.9 Wh kg-1 when the power density was 240 W kg-1.Moreover,the assembled configuration delivered a 103.7%capacitance retention in the 30000th charge/discharge cycle at 1.0 A g-1.(2)The pre-carbonized material(denoted as AC)was prepared by the calcination of poplar catkins at 600℃ for 2 h under N2 atmosphere.Then,a series of microporedominant porous carbon materials(labeled as HAPC-X-Y)were prepared by using KOH as the activation agent.Among them,HAPC-700-3,which was prepared at 700℃with a mass ratio of 3:1 for KOH to AC,demonstrated a specific surface area of 1693 m2 g-1 and a total pore volume of 0.866 cm3 g-1.The specific capacitance of HAPC700-3 was 248.0 F g-1 at 1.0 A g-1 in the three-electrode system.The energy density of the assembled symmetrical device based on HAPC-700-3 was 35.8 Wh kg-1 when the power density was 240 W kg-1.Moreover,the assembled configuration delivered a 94.0%capacitance retention after the 30000th charge/discharge cycle at 1.0 A g-1.(3)Also,a series of micropore-dominant porous carbon materials(denoted as KAPC-X-Y)were prepared via the same strategy for the above case of KOH by using environmentally friendly K2CO3 as the activation agent.Specifically,KAPC-700-2,which was fabricated at 700℃ with a mass ratio of 2:1 for K2CO3 to AC,exhibited a specific surface area of 1410 m2 g-1 and a total pore volume of 0.602 cm3 g-1.The specific capacitance of KAPC-700-2 was 278.0 F g-1 at 1.0 A g-1 in the three-electrode system.The energy density of the assembled symmetrical device based on KAPC-7002 was 40.5 Wh kg-1 when the power density was 270 W kg-1.After 30000 charge/discharge cycles,the initial capacitance retention of the assembled device was as high as 105.8%.(4)A series of large micropore and mesopore-dominant porous carbon materials(denoted as HKPC-X-Y)were prepared from poplar catkins with a step-by-step activation strategy by using H3PO4 and K2CO3.Among them,HKPC-1-2.0,which was prepared under the conditions of H3PO4 solution(15 wt.%)1.0 mL,a mass ratio of 2:1 for K2CO3 to the corresponding H3PO4 pre-activated carbon PCC-1-600,and a calcination temperature of 700℃ demonstrated a specific surface area of 2133 m2 g-1 and a total pore volume of 1.004 cm3 g-1 HKPC-1-2.0 displayed a specific capacitance of 296.2 F g-1 at 1.0 A g-1 in the three-electrode system.The energy density of the assembled symmetrical device based on HKPC-1-2.0 was 53.7 Wh kg-1 when the power density was 270 W kg-1.Moreover,the assembled configuration delivered a 100.1%capacitance retention in the 40000th charge/discharge cycle at 1.0 A g-1.Based on the concepts of reducing experimental cost,improving electrochemical performance and "green chemistry",in the present thesis,HKPC-1-2.0,which was prepared with the step-by-step activation method by using H3PO4 and K2CO3 was optimized to be the most suitable electrode material for supercapacitors.Benefiting from its suitable hierarchical porous structure,large specific surface area and high heteroatom contents(N 1.64 at.%,O 10.93 at.%,P 0.19 at.%),HKPC-1-2.0 exhibits superior supercapacitive performance(such as ultra-high energy density and outstanding charge/discharge cycling stability)to many previous results.This work contributes to the theoretical study to a certain extent for the effective utilization of biomass wastes and develops advanced electric energy storage configurations,offering an available preparation technology as well as an applicable electrode material. |
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