Preparation And Electrochemical Properties Of TNTs/C/MnO₂ Composite Films

Due to energy crisis and environmental pollution caused by unprecedented fast fossil fuel consumption in the process of social development in recent years,it is of great significance to develop highly-efficient energy storage devices.Among various energy storage devices,the supercapacitor has attracted people’s interests and been increasingly investigated as a viable charge storage technology because of its excellent characteristics of pollution-free,rapid charge-discharge and high efficiency.TiO₂ nanotubes is widely applied in binder-free supercapacitors due to its high specific surface area,highly order structure that shortened the ion and electron transport pathway and the substrate Ti plate directly as a current collector.But its poor electrical conductivity of TiO₂ nanotubes limits its electrochemical performances.Carbon materials have good electrical conductivity and double layer capacitance,which can improve its electrical conductivity.The theoretical capacitance of MnO₂ is as high as 1370 F/g,when combined with other materials,the capacitance of the material can be improved.In this paper,TiO₂ nanotube arrays（denoted as TNTs）were prepared by anodic oxidation on pure Ti plate.Then TNTs/C film was prepared by gas thermal penetration with methanol as drop agent.Finally,MnO₂ was loaded on TNTs/C film by two methods of hydrothermal and constant current electrodeposition.The structure and composition of composite film was characterized by SEM,XRD,EDS,RAMAN and XPS,respectively.The electrochemical performances were evaluated by electrochemical impedance spectroscopy（EIS）,cyclic voltammetry（CV）and galvanostatic charging/discharging（GCD）tests in a three-electrode system.After gas thermal penetration,C was successfully loaded on the surface of the nanotube in two forms of amorphous carbon and graphitized carbon.The internal resistance of the TiO₂ nanotube was 3 orders of magnitude lower,and the capacitance is76 times larger than that of TNTs,reaching 2.30 mF/cm² at scan rate of 10 mV/s.Based on the reducibility of carbon and the oxidation of KMnO₄,TNTs/C/MnO₂ were obtained by hydrothermal method with KMnO₄ solution.MnO₂ grain was dispersed in the wall and nozzle of the nanotube and the content of C decreased.The carbon that reacted with KMnO₄ was mainly inorganic carbon.The results revealed the best electrochemical properties of TNTs/C/MnO₂ film prepared at hydrothermal temperature of 90℃,0.3 g KMnO₄ and hydrothermal time of 60 min was achieved with 25.98 mF/cm² areal capacitance at discharge current density of 0.25 mA/cm²,27.35 times than that of TNTs/C film.After 1000 times galvanostatic charge/discharge tests,the capacitance rentention was up to 93.24%.In the study of the preparation of TNTs/C/MnO₂ by electrodeposition,the layered MnO₂ nanosheets were uniformly distributed on the TNTs/C composite film.With the increase of oxidation current,the thickness and porosity of MnO₂ nanosheets increased and the distance between nanosheets increased.MnO₂ is birnessite and with the increase of oxidation current and oxidation time,the crystal form and the valence state had not changed,which only caused the change of the load.When the electrodeposition temperature is 70℃,the oxidation current is 0.7 mA/cm² and the oxidation time is 120 min,the electrochemical property of the composite is the best,reaching 335 mF/cm² at discharge current of 0.5 mA/cm²,353 times of that of the TNTs/C film.After 1000 times galvanostatic charge/discharge tests,the capacitance was increased,reaching 108%,indicating excellent reversibility and cycle stability.