Fabrication Of Anodic Films On Titanium Alloys And Their Supercapacitive Performances

Supercapacitor is a new type of energy storage device with high energy density and high power density.It has many advantages such as short charge/discharge time and long cycle life,and has been widely applied in many fields.Anodic TiO₂ nanotubes?ATNTs?grown vertically on titanium?Ti?substrate possess many advantages,such as large specific surface area,excellent chemical stability,unique electronic transmission path and simple fabrication method,and have become a promising electrode material for supercapacitor.However,the specific capacitance of ATNTs is generally low.Preparation of composite oxide nanotubes?CONTs?by anodization of titanium alloy is an effective way to improve the supercapacitive performance of ATNTs.First,the ordered CONTs were successfully prepared on five kinds of titanium alloy substrates by anodization in fluorine-containing ethylene glycol electrolyte.The growth rule of CONTs had been studied.The diameter and growth rate of the CONTs increases with the NH₄F concentration in the electrolyte.As the anodization time increases,the tube length of the CONTs becomes longer.However,the growth rate of the CONTs decreases with time.When the anodization time reached 2 h,the top of the nanotubes collapsed and nanograss appeared.The diameter of the nanotubes also increases with voltage.However,when the voltage reached 80V,the current density continued to increase and could not reach a relatively stable state.Then,the as-prepared CONTs were etched in 10 wt%H₃PO₄ aqueous solution at 75°C for8 h in a hydrothermal reactor.The motivation of this etching treatment was to selectively remove Al₂O₃ in CONTs and improve its electrical conductivity.After etching treatment,Al₂O₃was successfully removed,besides,the morphology and structure of the CONTs changed greatly.The original nanotubes were partially transformed into petal-like architectures,a new nano-flower/nanotube composite architecture TiO₂ mixed oxide?FTTMO?was prepared.Compared with CONTs,the electrochemical properties of FTTMOs are significantly improved.Finally,the electrochemical properties of FTTMOs in acid,alkali and salt electrolytes were studied.The FTTMOs exhibit the highest area specific capacitance in H₂SO₄ electrolyte,and the highest area specific capacitance of the FTTMOs reaches 67.40 mF cm^-2.However,the cycle stability is not satisfactory.When KOH aqueous solution was used as the electrolyte,the cycle stability was the best.The specific capacitance is about 90%of the original value after3000 cycles,but the specific capacitance measured in alkaline electrolyte is the lowest.In Li₂SO₄ electrolyte,the FTTMOs exhibit good capacitance performance and high rate performance.Although the specific capacitance is slightly lower than that in H₂SO₄ electrolyte,the cycle stability is significantly improved.The FTTMOs exhibit a specific capacitance of33.23 mF cm^-2 at a current density of 0.1 mA cm^-2,about 6-fold improvement over the CONTs.