Preparation And Electrochemical Performance Of MOFs-Based Manganese Oxides And Composite Carbon Materials

In recent years,dramatic increases of fossil fuel consumption and environmental pollution have already become severe challenges,so it is urgent to develop sustainable energy storage and conversion devices.Supercapacitors are regarded as clean energy-storage devices,which have attracted great attention due to rapid charging and discharging,high power density and long-term cycling stability.In this thesis we used manganese-based metal-organic frameworks as precursors to synthesis different manganese oxides,manganese oxide composite carbon materails and spinel structure(Co_1.5Mn_1.5O₄)materials,and the electrochemical properities were also explored.The main studies are as follows:1.Mn₃O₄ hollow microcubes and solid nanospheres were selectively fabricated from a metal formate framework?[CH₃NH₃][Mn?HCOO?₃]?by employing the ion exchange reaction in alkaline solution under different hydrothermal conditions.The relationship between Mn₃O₄ morphology and reaction conditions,such as reaction molar ratio and reaction time,were investigated.When used as the electrode materials for supercapacitor,Mn₃O₄ hollow microcubes and solid nanospheres had specific capacitances of 176 and 150 F g-1 respectively at a current density of 0.3 A g-1 in 1 M Na2SO4.The specific capacitance retentions of Mn₃O₄ hollow microcubes and solid nanospheres were 95% and 86% respectively after 8000 charge-discharge cycles at a current density of 2 A g-1.Electrochemical results show that the Mn₃O₄ hollow microcubes and solid nanospheres exhibit acceptable specific capacitance and excellent cycle stability.2.Mn₃O₄/CNT composites were synthesized by the alkaline solution treatment of [CH₃NH₃][Mn?HCOO?₃] and carbon nanotubes hybrid under the hydrothermal conditions.Mn₃O₄ nanoparticles with 10 nm in size were loaded on the carbon nanotubes surface perfectly.The BET surface area of Mn₃O₄/CNT was calculated to be 87.8 m2 g-1 and the corresponding pore size distribution reveals uniform pore size distribution?6.5 nm?.The electrochemical results incidate that Mn₃O₄/CNT had specific capacitances of 206 F g-1 at a current density of 0.5 A g-1 and the specific capacitance retention of Mn₃O₄/CNT was 77.3% after 8000 charge-discharge cycles at a current density of 5 A g-1.Obviously,the specific capacitance and the cycle stability of Mn₃O₄/CNT are better than pure Mn₃O₄.3.Plate like MnO₂/CNT composite were synthesized by ammonium sulfate oxidation at the air condition with Mn₃O₄ obtained through calcination of the metal organic frameworks [CH₃NH₃][Mn?HCOO?₃] and carbon nanotubes as the starting materials,The relationships between the different amount of carbon nanotubes on the influence of the composite structure and electrochemical properties were investigated.In composite MnO₂/CNT-10,the carbon nanotubes were interspersed with plate like MnO₂,which possesses with high specific area?253.8 m2 g-1?and uniform pore size distribution?5 nm?.Electrochemical results show that the specific capacitance of MnO₂/CNT-10 prohibit best electrochemical properties.The specific capacitance of MnO₂/CNT-10 is 215 F g-1 at a current density of 0.5 A g-1,and the specitance capacitance retentions is 109% after 8000 charge-discharge cycles at a current density of 5 A g-1.4.Take the advantages of metal formate frameworks are easily controlled and different metal ions can be uniformly doped in the structure of the compound,Co_1.5Mn_1.5O₄ was obtained by calcined the double metal formate framework precursor [?CH₃NH₃?Mn_0.5Co_0.5?HCOO?₃] at the air condiction.The product pocesses high specific area?32.5 m2 g-1?and uniform pore size distribution?4.5 nm?.Electrochemical results show that the specific capacitance of Co_1.5Mn_1.5O₄ is 310 F g-1 at a current density of 0.5 A g-1.The specific capacitance retention of Co_1.5Mn_1.5O₄ is 113% after 2000 charge-discharge cycles at a current density of 1 A g-1.Compared with the single metal manganese oxide Mn₃O₄ prepared by the same method,the composite oxide has higher specific capacitance and excellent cycle stability.