A novel double-layer nanotube structure fabricated in high concentration H3PO4 and fluoride-containing mixed electrolyte without annealing

Abstract To increase the specific surface area of nanotubes, we form large diameter nanotubes with double-layer structure by anodizing Ti foils in high concentration H 3 PO 4 and fluoride-containing mixed electrolyte without annealing rather than in pure NH 4 F electrolyte. This double-layer structure consists of the anions-contaminated layer and the barrier layer. Owing to the addition of PO 4 3- ions into electrolyte, the thickness of the anions-contaminated layer and the diameter of the nanotubes increased. Compared to single-layer structure, this double-layer structure exhibits ∼5.81 times increase in specific surface area of nanotubes because of the multi-hole interface between the two layers for supercapacitors.