# Application of Tungsten-Oxide-Based Electrochromic Devices for Supercapacitors
); [EMAIL_ADDRESS] (Q.Z. ); [EMAIL_ADDRESS] (Q.F. ); [EMAIL_ADDRESS] (J.Z. ); [EMAIL_ADDRESS] (Q.F. ); [EMAIL_ADDRESS] (J.P.)  
Xinglin Li  
1 State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China; [EMAIL_ADDRESS] (M.L. ); [EMAIL_ADDRESS] (H.Y. ); [EMAIL_ADDRESS] (C.L. ); [EMAIL_ADDRESS] (Q.Z. ); [EMAIL_ADDRESS] (Q.F. ); [EMAIL_ADDRESS] (J.Z. ); [EMAIL_ADDRESS] (Q.F. ); [EMAIL_ADDRESS] (J.P.)  
Xinglin Li  
1 State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China; [EMAIL_ADDRESS] (M.L. ); [EMAIL_ADDRESS] (H.Y. ); [EMAIL_ADDRESS] (C.L. ); [EMAIL_ADDRESS] (Q.Z. ); [EMAIL_ADDRESS] (Q.F. ); [EMAIL_ADDRESS] (J.P.)  
[PERSON]  
1 State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China; [EMAIL_ADDRESS] (M.L. ); [EMAIL_ADDRESS] (H.Y. ); [EMAIL_ADDRESS] (Q.F. ); [EMAIL_ADDRESS] (J.Z. ); [EMAIL_ADDRESS] (Q.F. ); [EMAIL_ADDRESS] (J.Z. ); [EMAIL_ADDRESS] (Q.F.F. ); [EMAIL_ADDRESS] (J.P.)  
Xingbonds, with a few covalent bonding components, a large number of dangling bonds, and defects all over the WO\({}_{3}\) structure, which are very favorable for electrons to enter the crystal to accelerate the reaction. The electrochromic reaction of tungsten oxide follows the following chemical formula:  
\[\text{WO}_{3}(\text{bleached})+\text{xM}^{+}+\text{xe}^{-}\leftrightarrow\text{ M}_{\text{x}}\text{WO}_{3}(\text{coloured})\]  
where M\({}^{+}\) is H\({}^{+}\), Li\({}^{+}\), K\({}^{+}\), Na\({}^{+}\), etc., and the color change is achieved by reversible ion injection/extraction in the WO\({}_{3}\) film. Figure 1 shows a diagram of the continuous electron-transfer process in electrochromism, which can be described as two steps:  
\(\text{W}^{6+}+\text{e}^{-1}\leftrightarrow\text{W}^{5+}\)  
\(\text{W}^{5+}+\text{e}^{-1}\leftrightarrow\text{W}^{4+}\)  
Tungsten oxide has a high intrinsic density with a rich skeletal diversity, and is therefore an extremely promising material for energy-storage electrodes [22; 23; 24; 25; 26]. Tungsten oxide with different morphologies can be obtained via magnetron sputtering, as well as by hydrothermal, electrochemical, or vapor phase deposition [27; 28; 29]. General Electric patented the first supercapacitor in 1958, which used activated carbon as a substrate, where charge was not exchanged between the electrode and the electrolyte, and where the electrode simply stored charge. Carbon materials and their derivatives, which have a large specific surface area and good electrical conductivity, therefore have a wide range of applications in supercapacitors [29; 30; 31; 32; 33]. The output power versus stored charge is significant. According to Figure 2, output power and stored energy density are important parameters of energy storage devices, which can be classified as capacitors, supercapacitors, batteries, fuel cells, etc. [1; 8; 33; 34; 35; 36]. Compared to batteries, which have a higher energy density but lower transmission density, supercapacitors have a lower energy density but higher transmission density, which comes from the large specific surface area of their electrodes but lower densification. Electrochromic devices have many similarities to supercapacitors and, therefore, have the potential to be integrated. Structurally, both are sandwich structures consisting of an electrode layer and an electrolyte layer, as shown in Figure 3. In terms of operating principles, both electrochromic devices and pseudocapacitors are based on redox reactions for energy transfer and storage [37; 38; 39; 36]. In terms of parameters, both supercapacitors and electrochromic devices have fast switching times and low operating voltages. The electrode is the core component of a capacitor, and its performance is influenced by its strengths and weaknesses.