Behavior of the Evaporated  WO 3 Film in Li‐Salt PC Electrolytes

Electrochemical properties of evaporated amorphous WO 3 films on conductive glass substrates have been studied in propylene carbonate electrolytes, and the degradation of the electrochromic activity of the WO 3 film on storage in the electrolyte and the prevention of the degradation of the film have been investigated by cyclic voltammetry. The WO 3 films have also been analyzed by XPS, SIMS, and FT‐IR spectra. We have confirmed that the thermodynamic degradation is accelerated by H 2 O and LiClO 4 in the electrolyte. It is proposed that: first, H 2 O contributes to the change of WO 3 into W ( OH ) x ; second, Li + exchanges with the H + of W ( OH ) x ; and then these reactions result in the formation of Li 2 WO 4 ; and, consequently, in the negative shift of the cathodic reaction potential of the film. We have also confirmed that the degradation is prevented by addition of an acid, such as H 3 PO 4 or acrylic acid, to the LiClO 4 PC electrolyte and by using LiBF 4 as a supporting electrolyte in place of LiClO 4 .