Preparation and Supercapacitive Performances of Mn3O4 Nanocrystal/Nitrogenous Porous Carbon Electrode Materials

Author:Cheng Yin Feng

Supervisor:li bao qiang

Database:Doctor

Degree Year:2018

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Pages:162

Size:10998K

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At present,the main problem for supercapacitors is their low energy density,which is hard to satisfy the needs of practical applications.Therefore,how to improve the energy density of the supercapacitor without sacrificing its power density and cycle life is an urgent problem to be solved.In this dissertation,aiming to improve the specific capacitances of electrode materials,realize the optimization of the performance of the electrode materials by regulating the pore structure and morphology,nitrogenous porous carbon materials,Mn3O4 nanocrystals and Mn3O4nanocrystal/porous carbon composites with high specific capacitances were prepared.Then,in order to achieve high working potential window,the asymmetric supercapacitor with working potential window and aqueous electrolyte was fabricated by using Mn3O4 nanocrystal/porous carbon composite and nitrogenous porous carbon as positive and negative electrode materials,respectively.The main research contents are as follows:Biomass-derived nitrogenous carbons with hierarchically porous structures were prepared by rapid microwave carbonization,with low-cost carrot as raw material.In microwave carbonization,ZnCl2 acts as microwave absorber,chemical activator and porogen.The pore structure of carbon materials can be regulated by changing the mass ratio of ZnCl2/carrot.The specific surface area of obtained nitrogenous biomass-derived carbon was arrived to 1899 m2 g-1,with the pore volume of 1.16 cm3 g-1,and the mesoporous ratio of 70%.As the electrode material for supercapacitors,the specific capacitance of nitrogenous biomass-derived carbon is as high as 272 F g-1 at 0.2 A g-1 in 6 mol L-1 KOH electrolyte.When the current density increases to 10.0 A g-1,the specific capacitance is matained at 220 F g-1.To obtain porous carbon materials with higher specific capacitance,nitrogenous porous carbons with high nitrogen content were prepared via microwave carbonization with porous chitosan hydrogel as a raw material,ZnCl2adsorbed in hydrogel as microwave absorber,chemical activator and porogen.The pore structure of carbon materials can be regulated by changing the concentration of ZnCl2 solution.The nitrogen content of nitrogenous porous carbon reached 9.4wt%,the specific surface area was 1170 m2 g-1,the pore volume and the mesoporous ratio were 1.32 cm3 g-1 and 78%,respectively.As the electrode material for the supercapacitors,the specific capacitance reaches 435 F g-1 at 0.2 A g-1 in the three-electrode system with 6 mol L-1 KOH electrolyte.When the current density increases to 10.0 A-1,the specific capacitance is still 309 F g-1.For the fabricated symmetric supercapacitor based on the obtained carbon materials,its working potential window is 1.0 V,the enery density reaches 9.4 Wh kg-11 at the power density of 50 W kg-1.More importantly,its specific capacitance has little decay after 10000 charge-discharge cycles at 5.0 A g-1,indicating excellent cycling stability.To obtain Mn3O4 nanocrystals with high specific capacitance and rate capability,Mn3O4 nanocrystals with various shapes were syntheized in p-xylene,using Mn(CO2CH3)2?4H2 O as precursor,oleic acid as ligand.The shape effect on the supercapacitive performance was also investigated.It is found that oleic acid is selective adsobed on the{101}facets of the crystal structure of Mn3O4,the crystal will grow along the{101}facets when increasing the concentration of oleic acid,which leads to the transformation of the shapes Mn3O4 nanocrystals from square prisms to tetragonal bipyramids.Comparing the supercapacitive performances of Mn3O4 nanocrystals with different shapes,it is found that the Mn3O4 tetragonal bipyramids with its surfaces enclosed by only{101}facets have the highest specific capacitance,rate capacity and cycling stability.Their specific capacitance reaches321 F g-1 at 0.5 A g-1 in 1 mol L-1 Na2SO4 electrolyte.When the current density increases to 10.0 A g-1,the specific capacitance is still 260 F g-1.The specific capacitance can maintain 97%of its initial value after 10000 charge-discharge cycles at 5.0 A g-1.In order further improve the performance of Mn3O4 nanocrystals,Mn3O4/nitrogenous porous carbons composites(Mn3O4/NPC)with different contents of Mn3O4 were prepared via self-assembly by using nitrogenous porous carbon(NPC).It is found that the composite has the highest specific capacitance when the content of Mn3O4 is 32%.The specific capacitance of Mn3O4/NPC-32%composite reaches366 F g-1 at 0.5 A g-1 in 1 mol L-1 Na2SO4 electrolyte.When the current density increases to 10.0 A g-1,the retention rate of specific capacitance is matained at 279F g-1.The asymmetric supercapacitor was fabricated with Mn3O4/NPC-32%composite as positive electrode material and NPCs as negative electrode materials.The working potential window of the as-fabricated supercapacitor is as high as 1.8V,and its enery density reaches 34.7 Wh kg-1 at the power density of 450 W kg-1.The specific capacitance of the supercapacitr can still maintain 97%of the initial value after 10000 charge-discharge cycles at 5.0 A g-1,exhibiting good cycling stability.