# Application of Tungsten-Oxide-Based Electrochromic Devices for Supercapacitors
## 2 Tungsten Oxide Electrodes
### One Dimension  
The one-dimensional (1D) tungsten oxide nanostructure is one of the most widely used WO\({}_{3}\) nanostructures. Nanowires, nanorods, and nanotubes are all kinds of one-dimensional materials, as shown in Figure 6. Hydrothermal, electrochemical, vapor deposition, and electrostatic spinning methods can all be used to synthesize 1D tungsten oxide nanostructures [73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85], and the hydrothermal method is the most used and the simplest one. The synthesis mechanism is that during the formation of tungsten oxide, the guiding agent is used to wrap the material to make the material grow in one direction, and nanostructures with different aspect ratios can be obtained by controlling the temperature, time, and material ratio. [PERSON] [86] prepared WO\({}_{3}\) fibers with lengths of 50-200 nm and widths of 20-60 nm by a hydrothermal method, and deposited them electrophoretically on a conductive substrate. Au nanoparticles and PEDOT were coated on the surface of WO\({}_{3}\) to obtain PEDOT-Au\(\mathrm{\ddot{e}}\)WO\({}_{3}\) composite electrodes, and it was shown that this hybrid electrode can effectively replace the expensive Pt-based counter-electrode solar cells, while the one-dimensional tungsten oxide material can form a functional framework on the flexible substrate at low temperatures or even room temperature, so flexible electrochromic device supercapacitors are an inevitable direction of development [7]. [PERSON] [84] prepared composite membranes with graded porous structures via vacuum-assisted filtration of mixed dispersions containing W\({}_{18}\)O\({}_{49}\) nanowires and single-walled carbon nanotubes, and assembled a flexible, asymmetric electrochemical capacitor based on aluminum ions. The high degree of connectivity of nanowires and nanotubes formed an interpenetrating nanonetwork that ensured fast ion transport and high conductivity of the electrodes (1626 S/cm). The flexible devices need to be fabricated according to the suitability of thin films and flexible substrates, and after fabrication, their use under bending and stretching should be considered. [PERSON] [85] found that the stability of the ECD supercapacitor was further improved on the basis of the flexible and stretchable WO\({}_{3}\) nanotubes combined with PEDOT:PSS in a double stack, as shown in Figure 7 The WO\({}_{3}\) nanotubes provided a very high specific capacitance and electrochromic properties, while the PEDOT:PSS stacked on top improved the mechanical properties of the device. The final device had a maximum specific capacitance of 471.0 F/g, a capacity retention of 92.9% after 50,000 charge/discharge cycles, and a color rendering efficiency of 83.9 cm\({}^{2}\)/C. The use of tungsten oxide nanostructures in the paper effectively improved the film's mechanical properties, the use of PEDOT:PSS improved the compatibility of the film with the flexible substrate, and the use of gel electrolytes effectively solved  
Figure 6: Different one-dimensional tungsten oxide nanostructures: (**a**) nanotubes [73], (**b**) nanorods [74], (**c**) nanowires [75]. the impact of liquid leakage on the lifetime of the device. One-dimensional tungsten oxide electrodes are characterized by high density of stacked materials, but the stacked materials lead to more nodes and uneven impedance distribution, so it is important to order the 1D structure. WO\({}_{3}\)-Ag nanowire lattices were constructed using the L-B method. The WO\({}_{3}\) lattices were arranged in an orderly fashion on a flexible substrate, and this method of large-area co-assembly of nanowires can be used for a variety of flexible nanowire devices [86]. ECSCs fabricated on flexible substrates have great application prospects, while ECSCs on rigid substrates can obtain very high device parameters [87; 88; 89; 90; 91; 92; 93]. [PERSON] [94] mixed tungsten oxide (WO\({}_{3}\)) and vanadium oxide (V\({}_{2}\)O\({}_{5}\)) to prepare an ECSC.