The Preparation And Performance Of Electrodeposited MnO₂ Films For Supercapacitors

Supercapacitors have been established an effective electrochemical energy storage system for renewable energy storage and mobile auxiliary storage.As a widely-used supercapacitor electrode material,MnO₂ has drawn considerable attentions for its low cost,natural abundance,high theoretical capacity(1370 F g^-1),wide potential window and capability in mild aqueous electrolytes.To meet the requirements of commercialization of supercapacitors,the performance of MnO₂electrode has to be improved,especially in specific capacitance and cycling performance.Therefore,improving the specific capacitance and cycling performance is one of the major research topics of MnO₂ electrode materials and is of great significance.Electrodeposition is one of the principal methods for preparing film electrodes,and has the advantages of convenient operation,easy process control,less pollution and capability in preparing large area films,etc.However,there is no systematic study on the structure control of electrodeposited films so far,and the specific capacitance and cycling performance of electrodeposited MnO₂ film cannot meet the requirements of commercialization yet.In this thesis,nanostructured MnO₂ films were prepared on stainless steel and nickel foam substrates by electrodeposition,and the effects of electrodeposition parameters on the different nanostructures and performance were studied.Meanwhile,ultrasonic treatment was introduced to improve the morphology,structure and electrochemical performance of MnO₂ films.The main contents of this thesis include:?1?Various structured MnO₂ films were prepared on stainless steel substrates by adjusting various electrodeposition parameters,and the effects of electrodeposition parameters on the structure,morphology and electrochemical performance of MnO₂films were studied.The results showed that electrodeposition voltage has a major influence on the formation of MnO₂ films.Uniform and complete MnO₂ films with a microsphere structure can be obtained when the voltage is in the range of 1.2-1.8 V.The concentration polarization of electrolyte during electrodeposition results in inhomogeneous microstructure,which can be reduced by means of pulse electrodeposition,agitation electrodeposition and increasing the area of substrate.The MnO₂ nanowires and nanosheets can be formed with enhanced electrochemical performance.During the process of charge-discharge cycles,the mass of MnO₂ film gradually reduced with the attenuation of specific capacitance,indicating that the dissolution of MnO₂ film is one of the main reasons for the attenuation of specific capacitance.?2?MnO₂ nanowires and nanosheets were prepared on nickel foam substrate by ultrasonic assisted electrodeposition.Through the analyses of SEM,TEM,EDS and electrochemical measurements,it is found that ultrasonic treatment can effectively improve the structure and morphology of electrodeposited MnO₂ film,enhance the structural stability and reduce the resistance of MnO₂ film,leading to significant improvements in the specific capacitance and cycling performance of MnO₂ films.The specific capacitance of the ultrasonic treated MnO₂ film achieves a maximum value of 369 F g^-1,and the capacitance retention rate after 1000 cycles is 97%whereas the MnO₂ film obtained by direct electrodeposition has a specific capacitance of 230 F g^-1 and a capacitance retention rate after 1000 cycles of 65%.?3?MnO₂ nanosheet films were prepared on stainless steel and nickel foam substrates by magnetron sputtering.Compared with stainless steel substrate,MnO₂films deposited on nickel foam substrate by magnetron sputtering show a better electrochemical performance and obtains a highest specific capacitance of 244 F g^-1.In comparison with the method of electrodeposition,magnetron sputtering for MnO₂film preparation has a stricter experimental condition,a lower deposition rate and a high target damage rate.Ag/MnO₂ composite films were prepared by the combination of magnetron sputtering and electrodeposition.The results showed that adding a small amount of Ag can improve the electrical conductivity of MnO₂ film and effectively enhance the performance of MnO₂ film.