Research On Composite Mn Based Oxides As Supercapacitor Electrode Materials

With the rapid development of the global economy,the rapid consumption of fossil fuels and the increasing of environmental pollution,there is a much higher demand for efficient,clean and sustainable sources of energy,as well as new technologies associated with energy conversion and storage.Electroche mical supercapacitors（ES）have attracted tremendous attentions due to their high power density and long cycle life.At present,the major challenges for supercapacitors include low energy density and high cost.To overcome these challenges,the research and development of supercapacitors mainly focus on the research of new materials and composite electrode materials.In this dissertation,manganese dioxide,the pervoskite type oxides as well as their composites are investigated as supercapacitor electrode materials.Their physical and chemical characters and electrochemical performance are tested and related mechanisms are further revealed.In this dissertation,MnO₂ was synthesized by a facile hydrothermal method,and its phase and electrochemical properties were analyzed.Then MnO₂ was annealed from 100 to 700 ℃.MnO₂ electrodes treated at different annealing temperature were also investigated simultaneously.With the increase of annealing temperature,the physical and chemcial characters and electrochemic al performance of MnO₂ change obviously.Especially,the electrochemical performance of MnO₂ can be greatly improved when the annealing temperature is in the range of 100-300 ℃.Annealing temperature is one of the important factors that affect the physical and chemical characters and electrochemical performance of MnO₂.The pervoskite type oxides strontium doped lanthanum manganite（LSM）has high electronic conductivity and excellent electrochemcial activity.However,there are few reports about using LSM as the electrode materials for supercapacitors.In this thesis,the pseudo-capacitance feature and cycling stability of（La0.75Sr0.25）0.95 Mn O3-δ were preliminary investigated and analyzed.It is proved that LSM can be used as a novel supercapacitor electrode material and applied to practical research.La1-x Srx Mn O3 samples were also synthesized by sol-gel method.The specific capacitances of LSM electrodes are found to increase gradually with increasing Sr-content.Moreover,LSM samples shows the best electrochemical performance when the Sr doping amounts is 50%（x=0.5）,the specific capacitance can reach 134.7 F g-1.Based on the research of MnO₂ and LSM,taking account of the good electrical conductivity,the pseudo-capacitance feature and long cycle stability of LSM,for the first time the LSM and MnO₂ powders were mechanically mixed to form composites with different LSM content to improve the electrochemical performance of MnO₂ in this thesis.Compared to pure LSM and MnO₂ electrodes,LSM（10 wt%）/MnO₂ composite electrode shows excellent electrochemical performance with the specific capacitance of 287.8 F g-1 at 2 m V s-1,higher than that of pure MnO₂（251.7 F g-1）and LSM（108 F g-1）electrode and retaining good cycle stability for 1000 cycles.In order to study the role of LSM in the composite electrodes,C/MnO2 composite electrode was also prepared as the contrast electrode.It is found that LSM can improve the rate capability of MnO₂,especially at high current charge/discharge stages,capacitance improvement can reach 47.1 % at 200 m V s-1.The C/MnO₂ composites,however,don’t have this rate capability improvement effect.The results indicate that LSM can not only improve the electronic conductivity of LSM/MnO₂ composite electrodes,so as to enhance the specific capacitance of composite electrodes,but also its intrinsic pseudo-capacitance characteristics can significantly improve the rate capability of composite electrodes at high current discharge.In order to further study the modification mechanism of LSM on MnO₂ electrode and improve the electrochemical properties of LSM/MnO2 composites,LSM/MnO2 composites were synthesized by one-step hydrothermal method.The results show that,compared with mechanical mixing,LSM/MnO₂ composites synthesized by hydrothermal method have superior electrochemical performance and the specific capacitance can reach 437.2 F g-1.Moreover,LSM can not only significantly improve the specific capacitance of MnO2,but also can greatly improve the rate capability of MnO₂: compared with pure MnO₂ and LSM electrode,the capacitance improvement of LSM/MnO₂ composite electrodes can reach 70.1 % and 416.3 %,respectively.The present results indicate that the LSM/MnO₂ composites have important application value at high current density.The modification mechanism of LSM on MnO₂ electrode was analyzed and the results indicate that the perfect performance of the LSM/MnO₂ composite electrode comes from the high electronic conductivity of LSM at room temperature（5-6 orders of magnitude higher than that of MnO2）,the pseudo-capacitance effect of LSM and MnO₂ and the strong synergistic effect between LSM and MnO₂.