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.