Electrochromism (EC) is a phenomenon in which the color of a material changes reversibly under the action of an external electric field. Many transition metal oxides have been found to have electrochromic properties. These metal oxides can be classified into a reduction process by cathodic coloring. Materials (such as oxides of W, Mo, V, Nb, and Ti) and anodic coloring materials (such as oxides such as Ir, Rh, Ni, and Co) during oxidation. Among them, WO3 is the most studied cathode coloring electrochromic material.

Regarding the electrochromic mechanism of WO3 film, the most widely cited is the theory of inter-valency transition proposed by Faughnan. That is, due to the action of external electric field, electrons and cations are simultaneously injected into WO3 from both sides of the film, and electrons are trapped by W atoms. In the local state, the metal ion M+ resides in this region to form a deep blue tungsten bronze compound MxWO3". There are different valence W ions in MxWO3, and the transition of electrons between different valence W atoms causes the color of WO3 film to change. The electrochemical process is as follows:

WO3 (colorless) +xM++xe→MxWO3 (dark blue)

Using the electrochromic effect of WO3, researchers have applied it to various electrochromic devices for practical life. The electrochromic device has many excellent characteristics, such as the light transmittance can be continuously changed over a wide range (applicable to 4:1 or more), and can be manually adjusted; the driving voltage is low (1~2V), and the power supply is simple. The power consumption is saved; there is no viewing angle limitation on the display, there is a storage function and no power is consumed during storage. These characteristics make tungsten trioxide electrochromic materials have broad application prospects in optical information and storage displays, military camouflage, special smart windows and the like. In addition, under the action of the applied pulse voltage, WO3 can be reversibly changed between colorless and dark blue, and thus can also be used as a photo-induced switch.