# Application of Tungsten-Oxide-Based Electrochromic Devices for Supercapacitors
## 2 Tungsten Oxide Electrodes
### One Dimension
The optical contrast of the device was 60%, the fast color response was 4.9 s, and the highest color efficiency was 61.5 cm\({}^{2}\)/C. At a current of 0.5 mA/cm\({}^{2}\), the maximum area capacitance of 38.75 mF/cm\({}^{2}\) was obtained. In addition, even after 5000 charge/discharge cycles, the capacitance retention was still 78.5%. The unique morphology of nanostructures, the feasible redox reaction caused by the existence of active sites, and the high charge-transfer rate are the main factors to improve the electrochromism and electrochemical energy storage properties [95; 96; 97]. In addition to the low-dimensional tungsten oxide structures, the tungsten oxide nanostructures of two-dimensional tungsten oxide nanosheets and three-dimensional tungsten oxide have also been reported, as shown in Figure 8, but the porosity and lifetime of the electrodes composed of both do not reach the low-dimensional level; therefore, the device efficiency and lifetime are low. However, there are some reports in the field of sensors and batteries with low life requirements. Figure 7: (**a**) Schematic illustration of the fabrication process of WO\({}_{3}\) nanotubes and PEDOT:PSS layer. Characterization of Ag-nanowire-embedded PDMS substrate and WO\({}_{3}\) nanotube-PEDOT:PSS thin layer. Scanning electron microscope images of (**b**) Ag-nanowire-embedded PDMS, (**c**) WO\({}_{3}\) nanotube coated on Ag-nanowire-embedded PDMS, (**d**) cross-section image of a WO\({}_{3}\) nanotube, and (**e**) PEDOT:PSS thin layer coated on a WO\({}_{3}\) nanotube. (**f**) Transmission electron microscope cross-section image of a PEDOT:PSS thin layer coated on a WO\({}_{3}\) nanotube. (**g**) X-ray diffraction results of the WO\({}_{3}\) nanotube [85]. Table 1 compares many related reports on tungsten-oxide-based ECSCs; mesoporous tungsten oxide electrodes also appear in the ECSC report. Although it does not have the high specific surface area of nanostructure, a mesoporous structure also gives the electrode extremely high porosity; the high-density electrode also gives the device higher energy density, and the cycle stability of the device is higher than that of a loose tungsten oxide electrode. To increase capacitance, carbon materials are widely used. Carbon and graphene oxide, which are widely used in the battery field, are also used in ECSCs. The carbon materials can increase the electric double-layer capacitance of the device. Organic coatings are widely used in tungsten oxide nanostructures, which mainly play a role in two respects: The first is to maintain the stability of the nanostructures and prolong the electrode's life, i.e., preserve the protective layer outside the quantum dots and the organic-inorganic core-shell structure. However, this protective layer will still be damaged in the process of ion entry and exit; thus, in the long run, a new method is needed to improve the stability of the nanostructure. The other is the protective layer between the electrode and the substrate, necessary to meet the needs of flexible devices, e.g., a PEDOT protective layer. It is undeniable that ECSCs are developing in the direction of flexible wearability.