| Electrochemical supercapacitor(ES)is a new type of energy storage device between conventional dielectric capacitor and battery/fuel cell.It possess unique advantage of fast charge/discharge rate(in seconds),long cycle life(>100000)and high power density.According to the energy storage mechanism,supercapacitors can be divided into electrochemical double-layer capacitors,pseudocapacitors,and hybrid capacitors.Electrode materials play an important role in the supercapacitor,mainly including carbon materials,conductive polymers,transition metal oxides and composites.In this paper,the carbon materials are derived from cheap sword bean shells for supercapacitors application,and further modification were conducted by heteroatom doping or compounding with metal hydroxides to obtain composite electrode material,which possesses higher energy density and excellent cycle stability.Main tasks as follows:(1)Using nitrogen-rich sword bean shell as a precursor,a two-step method of carbonization and KOH activation was used to prepare a nitrogen-doped porous carbon material with high specific surface area.The electrochemical performance of the prepared activated carbon electrode materials were investigated by different alkali-carbon ratios and activation temperatures.The morphological structure and composition of the carbon material were analyzed by SEM,XPS,BET and other characterization methods.The electrochemical performance of the carbon material was evaluated by methods such as cyclic voltammetry,constant current charge and discharge and AC impedance.Nitrogen-doped porous carbon(ACBS14-700)synthesized under the best conditions(KOH/DC mass ratio 4 and activation temperature 700℃)has a good pore structure,a high specific surface area(2282 m2 g-1)and higher nitrogen content(1.71 wt.%).Electrochemical test results show that when the current density is 1 A g-1,the specific capacitance of the ACBS14-700 electrode material reaches 263 F g-1 and has good rate performance.The ACBS 14-700 electrode was assembled into a symmetrical supercapacitor in 6 M KOH electrolyte,it still has a 100%capacity retention rate after 10000 cycles through constant current charge and discharge.This shows that ACBS 14-700 has excellent cycle stability,and when the power density is 200 W kg-1,the energy density is as high as 12.5 Wh kg-1.(2)Using ACBS14-700 as the precursor,ammonium pentaborate tetrawater as the boron source and nitrogen source,the traditional hydrothermal method was used to prepare boron-nitrogen co-doped sword bean shell-based carbon materials.The effects of different quality dopants,reaction temperature and reaction time on the electrochemical behavior of electrode materials were investigated.Under the optimal conditions(the mass of ammonium pentaborate tetrawater is 6 g,the reaction temperature is 190℃ and the reaction time is 30 h),the synthesized boron-nitrogen co-doped porous carbon material(BNACBS6-190-30h)has a higher specific surface area(2859 m2 g-1)and similar pore structure,higher nitrogen content(2.03 wt.%)and boron content(3.12%).In the three-electrode system,the specific capacitance of the BNACBS6-190-30h electrode material is as high as 369 F g-1 when the current density is 1 A g-1,and the capacitance retention rate is 65%when the current density is 20 A g-1.In 6 M KOH electrolyte,BNACBS6-190-30h was assembled into a symmetrical supercapacitor.At a current density of 10 A g-1,the specific capacitance retention rate reached 93%after 10000 cycles through constant current charge and discharge,indicating that this material has good cycle stability.At the same time,when the power density is 200 W kg-1,its maximum energy density reaches 20.9 Wh kg-1.(3)The nickel-cobalt bimetal hydroxide/ACBS 14-700 composite electrode material was prepared by a solvothermal method.The effects of different nickel-cobalt molar ratios on the morphology surface and electrochemical performance of the composite electrode material were investigated.SEM test showed that the nickel-cobalt bimetal hydroxide was uniformly distributed on the surface of the carbon material and aggregated into flakes.XRD showed that the composite was a mixed crystal phase of α-Ni(OH)2 and α-Co(OH)2.BET indicates that the composite has a relatively high specific surface area(590 m2 g-1)and a layered porous structure.The electrochemical test results show that when the nickel-cobalt molar ratio is 1:1,the specific capacitance of Ni0.5Co0.5 LDH/AC at a current density of 1 A g-1 reaches 947 F g-1,and the capacitance retention rate is 63%when the current density is 20 A g-1.Ni0.5Co0.5 LDH/AC and ACBS14-700 are assembled into an asymmetric supercapacitor.At a current density of 10 A g-1,the cycle life reaches 80%after 5000 cycles through constant current charge and discharge,and the power density is 800 W kg-1,its maximum energy density reaches 39.7 Wh kg-1. |
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