Preparation And Properties Of Manganese Dioxide Electrode Materials Based On Porous Carbon Nanotubes

Supercapacitors have attracted widespread attention due to their advantages such as high power density,fast charge and discharge rates,long cycle life,good stability,and environmental friendliness.The electrode material is the"core"of the supercapacitor,which directly determines the performance of the supercapacitor.Therefore,current research on supercapacitors focus in developing high-performance electrode materials.Carbon nanotubes are promising supercapacitor electrode materials because of their unique one-dimensional tubular cavity structure,excellent electrical conductivity,large specific surface area and stable physical and chemical properties.However,the application of CNTs is hindered because of their hydrophobic surface,difficulty of dispersing tube bundles and low capacitance.The main contents of this thesis are as follows:A special kind of porous carbon nanotube with porous structure was developed by hydrogen peroxide etching of carbon nanotubes.Porous carbon nanotubes?PCNTs?not only retain the advantages of CNTs,but also increase the wettability of CNTs,providing an effective way for electrolyte penetration and ion transport.The effect of compound material on the electrochemical performance of supercapacitors was studied by combining PCNTs with manganese dioxide materials which has large theoretical specific capacitance and the effects of MnO2 loading on the structure and properties of MnO2@PCNT/MnO2composites were studied.The nanoparticles in MnO₂@PCNT/MnO₂ composites are not only loaded on the surface of the tube,but also filled into the nanocavity of CNTs compared with CNT/MnO₂ composites.In addition,the valence of MnO₂ on the surface of nanotubes is IV,while there is Mn?III?and Mn?IV?in the nanocavity of nanotubes.Electrochemical tests show that the specific capacitance of MnO₂@PCNT/MnO₂composite is 298.7 F g^-11 at the scanning speed of 10 mV s^-1,which is 38.6%higher than that of CNT/MnO2 composite and higher than most reported results so far.The specific capacity retention of MnO₂@PCNT/MnO₂ composites is 67.7%,which is higher than that of CNT/MnO₂ composites from 5 m V s^-11 to 100 mV s^-11 indicating good magnification properties.In addition,the capacity retention rate is 98%after 6000 cycles at 5 A g-l.The reason for the improvement of the performance of this novel electrode material is that we creatively make use of the porous properties of carbon nanotubes and the synergistic effect of PCNTs and MnO₂ inside and outside the tube wall,which not only increases the electrical conductivity of PCNTs,shortens the ion transport distance,but also increases the capacity storage of Mn?IV?/Mn?III?redox electron pairs,and the porous structure of PCNTs is beneficial to ion exchange.MnO₂@PCNT/MnO₂ nanocomposites has good capacitance and cycle stability because it can adapt to the volume expansion and contraction in the process of charge and discharge.Secondly,a series of heteroatom-doped X-PCNTs?X=B,N,B/N,F?were prepared and compounded with manganese dioxide materials,and the effects of different atom doping on their electrochemical properties were further studied.The B dopping into the carbon lattice changes the electronic structure of doped carbon,increases the surface polarity,enhances the electrostatic and ion-?interaction between anions and cations and the surface of carbon nanotubes,which strengthens the adsorption of electrolyte ions on the surface of carbon tubes,and so increases the specific capacitance of MnO₂@B-PCNT/MnO₂.The specific capacitance of MnO₂@B-PCNT/MnO₂ is 368.2 F g^-ll at a scan rate of 10 mV s^-1,which is 23.3%higher than that of MnO₂@PCNT/MnO₂.However,the specific capacity retention is reduced owing to the B-doping destroys the?telectron pairs and so reduces the conductivity of PCNTs.N or B/N co-doping does not destroy the?electron pairs in the structure,and so there is little change in the conductivity of PCNTs,so the specific capacity retention of the prepared composites is close to that of undoped PCNTs.The electrochemical performances of the composites based on F-doping PCNTs need to be further studied.