High-temperature anodization route to nanoporous niobium oxides with enhanced supercapacitive properties in aqueous electrolytes

Relatively little work has been done on Nb 2 O 5 pseudocapacitive behaviors in aqueous solutions. Here, the galvanostatic anodization of niobium in K 2 HPO 4 -containing glycerol electrolyte at high temperature is fully explored. The anodically grown films on niobium have ordered nanoporous morphology and are composed of a low-crystalline orthorhombic Nb 2 O 5, which can be directly employed as supercapacitor electrodes without any posttreatment process. To further improve their supercapacitive performances, a facile technique to generate Fe-doped Nb 2 O 5 films is developed, where the as-anodized films are treated by an electrochemical doping in FeCl 3 ethylene glycol solution. The Fe-doped Nb 2 O 5 films show a wider potential window and higher capacitance than that of the as-anodized films in aqueous electrolytes. They present a maximum areal capacitance of 168.1 mF cm −2 at 1 mV s −1 and excellent cycling stability with 85.8 % capacitance retention after 10,000 cycles. The charge storage in the Fe-doped Nb 2 O 5 is believed to occur by the incorporation of Li + ions with concomitant reduction of Nb 5+ to Nb 4+ in aqueous lithium-containing solutions. Unlike nonaqueous media, Li + intercalation/deintercalation in aqueous media is a diffusion-controlled process, leading to the comparatively slower pseudocapacitive processes. Nevertheless, in terms of an industrial applicability the Fe-doped Nb 2 O 5 films deserve special attention.