Rational Design Of Porous MnO₂ Nanotube For High Performance Supercapacitors

The increasing concerns about the limited fossil fuel supply and global environmental conservation have stimulated considerable research into renewable energy storage and conversion technologies. Among different energy conversion/storage systems in operation or under study, supercapacitors, also called electrochemical capacitors?ECs?, are attracting special attention because of their high power density, excellent pulse charge-discharge characteristics, super-high cycling life and safe operation. The electrochemical properties are often influenced by the characteristics of the electrode materials. Transition metal oxides are considered to be the most promising materials for supercapacitors for their high theoretical specific capacitance and favourable reversibility.In this work, various MnO₂ nanotubes are synthesized by two different template methods and tested as electrode materials. The purposes of these designs are to enhance the electric conductivity and the utilization of manganese atoms. The crystal structure, morphologies and specific surfaces area were investigated by X-ray diffraction, focused ion beam scanning electron microscopy, transmission electron microscopy and N2 adsorption-desorption measurement. Electrochemical properties were characterized by cyclic voltammetry?CV?, galvanostatic charge-discharge and electrochemical impendance spectroscopy?EIS?. The research content and the main conclusion are summarized as below:Three-dimensional?3D? porous MnO₂ array assembled from one-dimensional tubular MnO₂ with ultrathin sheets were fabricated by a one-pot hydrothermal method using polycarbonate membrane as the template and potassium permanganate as source of manganese. The diameter and thickness of nanotubes can be controlled by the membrane and the hydrothermal temperature. In a three-electrode system, the detailed electrochemical characterization reveals that the porous MnO₂ arrays exhibited good rate performance and cycle life?92% remain after 2000 cycles?. The highest specific capacitance was 411.9 F g-1 at a current density of 0.25 A g-1. An asymmetric supercapacitor with porous MnO₂ nanotubes arrays as the positive electrode and activated graphene as the negative electrode yielded an energy density of 22.6 Wh Kg-1 at a power density of 225.3 W Kg-1. The capacitive performance was correlated with the hierarchical structure of the porous MnO₂ nanotubes.The CuO@MnO₂ core-shell nanostructures were prepared by hydrothermal reaction with KMnO4, in which Cu nanowires were utilized as the template. Then the ultrathin MnO₂ nanosheets-built nanotubes with a large specific surface of 179.5 m2 g-1 were fabricated via a large-scale chemical etching method.The electrochemical tests in the three-electrode configuration demonstrated that the MnO₂ nanotubes show a specific capacitance as high as 377.5 F g-1 at a current density of 0.25 A g-1, and have good rate performance. An asymmetric supercapacitor with MnO₂ nanotubes as the positive electrode and activated microwave exfoliated graphite oxide?MEGO? as the negative electrode yields an energy density of 22.68 Wh Kg-1 at a power density of 4.5 KW Kg-1, rendering the MnO₂ nanotubes promising as the electrode materials for the high-performance supercapacitors.In summary, these two facile synthetic strategies of MnO₂ nanotubes could be useful to design other tubblar nanostructures. Simultaneously, the obtained nanocomposites in this work exhibited excellent electrochemical performance and they could be the optimized nanomaterials for supercapacitors.