Morphology Regulation Based On Anodic Titanium Oxide Nanotubes For Supercapacitors

Supercapacitor is a new type of energy storage device with the advantages of high power density,high energy density,small size and long cycle life.It is regarded as one of the most promising energy storage devices and has wide applications in hybrid electric vehicles and other high-power energy fields.Anodic titanium oxide(TiO₂)nanotubes is a promising electrode material for supercapacitors because of its simple preparation,high specific surface area and good chemical stability.In this paper,anodization in fluoride-containing electrolyte with different concentrations of H₃PO₄ and H₃PO₄-soaking at room temperature were attempted to regulate the morphology of TiO₂ nanotubes.Then,TiO₂ nanotube/nanopetal composite structure was prepared by corrosion treatment.Finally,TiN nanotube/nanopetal composite structure was prepared by nitridation of TiO₂ nanotube/nanopetal composite structure.FeOOH and Mn O₂ were deposited on TiN composite structure to prepare negative and positive electrode materials,respectively,so as to build a high-performance asymmetric supercapacitor.Firstly,TiO₂ nanotube arrays with double-layer wall were successfully prepared by constant voltage anodization in the mixed electrolyte containing different concentrations of H₃PO₄.The nanotube diameter of TiO₂ nanotubes was 200?400 nm and the thickness of the nanotube wall was 60?160 nm.After soaking in H₃PO₄ solution for a short time(40 min),the electrochemical performance of TiO₂nanotubes were significantly improved.After soaking in H₃PO₄ solution for a long time(3 days),the surface of TiO₂ nanotubes prepared in the conventional fluoride-containing electrolyte without H₃PO₄ was completely covered by the novel nanopetal structure.While TiO₂ nanotubes prepared in the fluoride-containing electrolyte with H₃PO₄ exfoliated due to the weakened interfacial adhesion between TiO₂ nanotubes and Ti substrate after soaking for a long time.Then,by regulating the corrosion conditions of TiO₂ nanotubes in H₃PO₄ solution,such as improving treatment temperature,TiO₂ nanotube/nanopetal composite structure was successfully prepared.This method is more controllable and less time-consuming than the long-time soaking treatment at room temperature.The formation of nanopetal structure could be accelerated by increasing the concentration of H₃PO₄ solution and the corrosion temperature.However,when the concentration of H₃PO₄ solution exceeded 15 wt%,TiO₂ nanotubes peeled off from Ti substrate.The optimal corrosion condition is treating nanotubes for 5 h at 75°C in10 wt%H₃PO₄ solution.The study showed that the nanopetal structure was formed by dissolution and recrystallization of TiO₂ nanotubes in H₃PO₄ solution,accompanied by the crystal transition of TiO₂ from amorphous into rutile and the P doping of TiO₂.Compared TiO₂nanotubes,the electrochemical performance of TiO₂ nanotube/nanopetal composite structure was significantly improved.Finally,TiO₂ nanotube/nanopetal composite structure was nitridation at high temperature to prepare TiN nanotube/nanopetal composite structure.TiN/FeOOH negative electrode and TiN@Mn O_2`positive electrode were prepared by depositing FeOOH and Mn O₂ on TiN composite structure,respectively.Thus,a asymmetric supercapacitor with the two electrode was fabricated in 1 M Na OH solution.At a working voltage of 1.7 V,the supercapacitor showed a specific capacitance of 16.82 m F cm^–2 at a current density of 0.2 m A cm^–2,which is much higher than that of the graphite-based supercapacitor(1.13 m F cm^–2).When the maximum power density of the supercapacitor is 97.21 k W L^–1,the energy density is 15.63 Wh L^–1 and when the maximum energy density of the supercapacitor is 22.51 Wh L^–1,the power density is56.24 k W L^–1.After 5000 times of charging and discharging tests at a current density of 1 m A cm^–2,the supercapacitor exhibited the capacitance retention of 80.2%,showing good cyclic stability.