Preparation And Supercapacitive Performances Of Mn₃O₄ Nanocrystal/Nitrogenous Porous Carbon Electrode Materials

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.