## Results
### Electrochromic Properties of the Grown WO\({}_{3}\) Film
In the region up to 550 nm, i.e., in the violet and blue parts of the visible spectrum, the transmittance of the film drops significantly less when colored, due to which the colored film shows a blue coloration, which is typical for WO\({}_{3}\)-based electrochromic materials. Figure 8c also shows that the processes of coloration and bleaching are asymmetrical: the coloration is much slower to plateau than the bleaching process, so the response time (during which 90% of the maximum optical contrast during the corresponding half cycle is reached) for coloration is 25.5 s, while for bleaching it is only 1.5 s. Such asymmetry may indicate the predominance of an oxide phase with a highly stable structure in the sample, which makes the processes of tungsten(VI) reduction and lithium ion intercalation difficult. From the obtained data on optical contrast, coloration efficiency (CE) at 600 nm was calculated according to the following formula:  
\[\eta=\frac{\log\left(\frac{T_{\text{bleached}}}{T_{\text{colored}}}\right)}{Q}, \tag{3}\]  
where \(\eta\) is CE, T\({}_{\text{bleached}}\) is transmittance in the bleached state, T\({}_{\text{colored}}\) is transmittance in the colored state, and Q is charge density (charge inserted into the film per area unit). The resultant value for the grown WO\({}_{3}\) film was 15.5 cm\({}^{2}\)/C. In the literature, CE values for WO\({}_{3}\) vary in a large range, reaching up to 141 cm\({}^{2}\)/C [11] and even 193 cm\({}^{2}\)/C [12]. Thus, our value seems to be relatively low. However, it should be noted that in most works, CE and other electrochromic properties are studied in three-electrode cells with liquid electrolyte (usually LiClO\({}_{4}\) in propylene carbonate), which is not the case in our experiment, where a polymeric gel electrolyte was used in a two-electrode cell. Polymeric and liquid electrolytes can differ significantly in their ionic conductivity, which might lead to lower CE in the case of electrolytes with less mobile ions. Other parameters, such as the voltage at which coloration and bleaching is observed, can also differ between two electrolytes. Consequently, it would be more correct to compare the electrochromic characteristics of our material with those reported in other works in which polymeric electrolytes were used (preferably, also based on polyethylene glycol and LiClO\({}_{4}\)). In work [60], WO\({}_{3}\)-PEO composite was used as electrochromic material in a cell with gel electrolyte consisting  
Figure 8: Measurements of the electrochromic properties of the WO\({}_{3}\) film: cyclic voltammetry (**a**), changes in transmittance at 600 nm wavelength together with a sweep over the potential during the CV recording (**b**), optical response of the oxide film upon potential changes (**c**), cell transmittance spectra before measurements, in the colored and bleached states (**d**). of propylene carbonate, PEO, LiClO\({}_{4}\), and ferrocene (which also exhibits electrochromic properties and can act as an anodic species [61]). The CEs for their composites were 24.3-61.4 cm\({}^{2}\)/C at 700 nm, depending on the WO\({}_{3}\):PEO ratio. Considering that we have calculated the CE for 600 nm, and contrast at 700 nm is higher in our case, our CE for 700 nm would most likely be close to the lower values from [60]. The fact that ferrocene also contributed to coloration and thus to CE values in [60] should also be taken into account. It should also be noted that the PEO:LiClO\({}_{4}\) ratio was different in [60] compared to ours. In work [62], the optical contrast for a WO\({}_{3}\) electrochromic reflective device based on polymeric electrolyte was 5-6% in the potential range of \(-\)3-3 V, which is a smaller contrast than ours and a larger potential window. No values for CE were provided by the authors. However, that was a reflective and not a transparent device, and a different polymer was used by the authors of [62].