Preparation And Electrochemical Performance Of MnO₂-MOFs Derived Composites

Supercapacitor has been widely studied and applied as energy storage devices due to their important features including high power density,long cycle life and fast charge-discharge capability.Its performance is mainly based on their electrode efficiency and activity,therefore,electrode materials with a well-engineered structure allowing the efficient utilization of materials and compositions for an effective redox process are highly necessary.Metal organic framework?MOFs?derived carbon is an ideal choice for the electrode materials because of its designable pore structure,high surface area,relatively good conductivity and high mechanical stability.In this paper,ZIF-8 was used as the precursor system to prepare hollow nitrogen doped carbon materials,and a series of manganese dioxide/carbon composite electrode materials were designed and prepared based on the respective advantages of manganese dioxide and carbon materials,and their structures and electrochemical properties were studied.The main research contents are as follows:?1?The hollow nitrogen-doped carbon?HNC?was prepared by chemical etching and pyrolysis with ZIF-8 as the precursor and tannic acid as the etchant.By adjusting the morphology and structure of the initial ZIF-8,rhombic dodecahedral and cubic the hollow nitrogen-doped carbon?RDHNC and CHNC?were prepared.The synthetic carbon has obvious cavity structure and largely retains the morphology of the initial ZIF-8.The prepared carbon material has a large surface area,a wide pore size distribution,and doping of nitrogen heteroatoms,which provides a basis for preparing composite materials.?2?Hollow cubic carbon with a core derived from ZIF-8 serves as a support template and conductive material for the MnO₂ shell to constructed composite CHNC@MnO₂.The electrochemical properties of the composite material are better than those of the single component MnO₂ and CHNC,and the unique double-shell structure makes it have better electrochemical properties than solid nitrogen-doped carbon CNC@MnO₂;It was found that the capacitance characteristics of amorphous MnO₂?a-MnO₂?composites are better than ?-MnO₂ composites by changing MnO₂ crystal;By discussing the relative proportion of reactants,it is concluded that the obtained sample CHNC@a-MnO₂-1.4 showed excellent electrochemical performance when used as electrode materials for supercapacitors with hollow rhombic dodecahedron,mass ratio KMnO₄:CHNC=1.4 and amorphous MnO₂.In the 1M Na₂SO₄ electrolyte,the electrode of CHNC@MnO₂ exhibited high specific capacitance of 199 F g^-1at a current density of 0.5 A g^-1and about 82% capacitance retention still can be maintained after 1000 times of charge/discharge cycling at the high current density of 5 A g^-1.?3?The RDHNC@MnO₂ composite material was prepared with RDHNC as the substrate.RDHNC@a-MnO₂-2.1 exhibited the best electrochemical performance by modulating the hollow structure of the material,the MnO₂ crystal type and the ratio of reactants.In the 1M Na₂SO₄ electrolyte,the electrode of RDHNC@a-MnO₂-2.1 exhibited high specific capacitance of 247.9 F g^-1at a current density of 0.5 A g^-1and about 82.7% capacitance retention still can be maintained after 2000 times of charge/discharge cycling at the high current density of 5 A g^-1.RDHNC@MnO₂ with a unique hollow rhombic dodecahedron structure has a larger specific surface area,exhibits better capacitance characteristics than CHNC@MnO₂,and can become a strong candidate for future supercapacitor electrode materials.