The electrochromic response of tungsten bronzes M(x)WO(3) with different ions and insertion rates

The electrochemical intercalation of Li+ and Na+ into thin WO3 films prepared by evaporation (130 nm thick) has been performed starting from liquid electrolytes with very low water content (< 20 ppm). Transmittance and reflectance measurements have been performed 'in-situ' during the electrochemical formation of the tungsten bronzes under controlled current. For inserted charges larger than 20 mCcm(-2) there is an optical transition responsible for a pale yellow coloration instead of the typical dark blue. This transition is irreversible since the subsequent bleaching of the film is not complete and a part of the inserted charge cannot be extracted again. When the insertion rate is high (0.4 mAcm(-2)) the irreversible transition occurs earlier (around 10 mCcm(-2)) in the case of Na+, which is known to diffuse slowly into the film. This limit composition for reversible electrochromic reaction is therefore reached earlier near the film surface, where the accumulation of the Na ions is responsible for an optical transition seen in the reflectance and for a phase change shown by a typical inflection in the electrode potential curve.