# Application of Tungsten-Oxide-Based Electrochromic Devices for Supercapacitors
## 2 Tungsten Oxide Electrodes
### Zero Dimensions  
The zero-dimensional (0D) material refers to materials with three dimensions in the nanoscale range (0-100 nm), or consisting of them as the basic unit. Due to the significant proportion of surface atoms, their surface density of states is greatly increased at the same time; for such small particles, certain quantum effects (e.g., quantum size effect, quantum limitation effect, quantum tunneling effect, quantum interference effect, etc.) are notable. The small size of 0D materials in the electrochromic device significantly shortens the diffusion path of the intercalated ions in the solid phase, (1)which helps to achieve fast charge/mass transfer; (2) the large surface-to-volume ratio of the quantum dots facilitates close contact between the electrode material and the electrolyte/collector, thus providing fast charge-transfer and electron-transfer kinetics; and (3) the significantly higher ratio of surface atoms makes the quantum dot electrodes more active in electrochemical reactions. [PERSON] [69] reported tungsten oxide quantum dot electrochromic supercapacitors, where tungsten oxide quantum dots of 3 nm in size were synthesized by a hydrothermal method, as shown in Figure 4. Compared to bulk tungsten oxide, the quantum dot tungsten oxide has a wider forbidden band, and the Faraday electrochemical process is accelerated, showing excellent electrochemical and electrochromic behavior. Another major task of the paper was to maintain the dispersion stability of the quantum dots, which were wrapped in an octane diamine ligand and modified with a conductive pyridine ligand on the surface. The final device had a fading time of 1 s and a coloring efficiency of 154 cm\({}^{2}\)/C. The synthesis of tungsten oxide quantum dots by hydrothermal methods is difficult, mainly because the material tends to agglomerate into clusters or lumps during the synthesis process. However, in the synthesis of other dimensional materials, low concentrations of tungsten oxide quantum dot byproducts can be obtained, which can be purified to obtain a high-concentration solution [70]. [PERSON] [71] prepared oxygen-enriched nanometer tungsten oxide by adjusting magnetron sputtering parameters. The particle size of tungsten oxide was less than 10 nm, as shown in Figure 5, and a large voltage window (1.4 V) was obtained, with a specific capacitance of 228 F/g at 0.25 A/g, maintaining 75% of the initial capacitance for 2000 cycles, and a color rendering efficiency of 170 cm\({}^{2}\)/C. It was verified that oxygen-rich tungsten oxide can accelerate the modulation speed of the device, because the excess oxygen element introduces more defects into the microstructure and achieves the purpose of containing more Li+ ions. This is also similar to the reason why amorphous tungsten oxide films have faster response speed than crystalline tungsten oxide films, both of which increase the porosity of the films [72]. Figure 4: Comparison of bulk (green) and quantum dot (black) tungsten materials [66]. Figure 5: **(a)** TEM and **(b)** HR-TEM images with 10 and 2 nm scale bars, respectively. The inset shows the X-ray nature of the synthesized film. The SAED pattern of the corresponding film is shown in (**c**), and the inset shows that the monoclinic WO\({}_{3}\) phase is mostly composed of corner-linked octahedra of 4-membered channels along all crystallographic directions. The 002 and 022 crystallographic planes obtained from the SAED patterns are projected in (**d**). All crystallographic projections were made with the VESTA structure drawing software [71]. There is little research on the working mechanism of 0D tungsten oxide particles, and the quantum effect of tungsten oxide in quantum dots has not been observed in the discoloration process, while the synthesis of nano-tungsten oxide particles is also difficult, and the life of the electrode is short. However, its coloring efficiency and specific capacitance are outstanding.