Nanofilms based on superstructured niobium tungsten oxide have high dual-functional potential for electrochromism and energy storage

Niobium-tungsten oxide represents a novel type of bimetallic cathode electrochromic material. The superstructure is capable of accommodating a greater number of ions, which has been a hot material for research in recent years. In this paper, composite niobium-tungsten oxide materials were prepared on fluorine-doped tin oxide (FTO) substrates using the sol–gel spin-coating method. The particle size, band gap and response time of the electrochromic films were improved by regulating the film annealing temperature. As the annealing temperature was increased, the particle size of the Nb18W16O93 film increased from 7.54 nm to 31.8 nm. Concurrently the band gap decreased, which enhanced the electrical conductivity of the material. The electrochemical and optical measurements show that the film has excellent properties, and by choosing the appropriate annealing temperature, the coloring efficiency reaches 55.3 cm2 C−1 from 34.1 cm2 C−1, the switching time reaches tc = 4.3 s, tb = 3.5 s, and the energy storage capacity reaches 21.086 mF cm−2 at a surface current density of 0.05 mA cm−2. The present work demonstrates that adjusting the annealing temperature can enhance the electrochromic and energy storage performance of the proposed films, which have for the potential to be utilized in electrochromic bifunctional applications.