Enhanced Capacitance of TiO2 Nanotubes with a Double-Layer Structure Fabricated in NH4F/H3PO4 Mixed Electrolyte.

TiO2 is an attractive electrode material in fast charging/discharging supercapacitors because of its high specific surface area. However, the low capacitance of TiO2 nanotubes as-anodized in the classical electrolyte restricts their further application in supercapacitors. Here, we study the performances of larger-diameter nanotubes with a double-layer structure fabricated in an NH4F/phosphoric acid (H3PO4) mixed electrolyte. Results show that the double-layer structure increased the specific surface area of nanotubes owing to the cavities between the double layers and the porous structure on walls. After soaking in H3PO4 aqueous solution for 40 min, the nanotubes anodized in the mixed electrolyte containing 6 wt % H3PO4 show a specific capacitance of 13.89 mF cm-2, ∼3.11 times that of the pristine nanotubes in the classical electrolyte. The specific surface area of the soaked nanotubes is up to 113.2 m2 g-1, which is ∼2.94 times that of the pristine nanotubes. The values of specific surface area of the anodized nanotubes and the soaked nanotubes fabricated in the mixed electrolyte containing 6 wt % H3PO4 are roughly equal. It demonstrated that the specific surface area increased mainly due to the double-layer structure. The double-layer structure reveals a new strategy to enhance the specific capacitance of TiO2 nanotubes.