Ultra-large optical modulation of electrochromic porous WO₃ film and the local monitoring of redox activity

Porous WO₃ films with ultra-high transmittance modulation were successfully fabricated on different substrates by a novel, facile and economical pulsed electrochemical deposited method with 1.1 s interval time between each pulse. The near ideal optical modulation (97.7% at 633 nm), fast switching speed (6 and 2.7 s), high coloration efficiency (118.3 cm² C^-1), and excellent cycling stability are achieved by the porous WO₃ on ITO-coated glass. The outstanding electrochromic performances of the porous WO₃ film were mainly attributed to the porous structure, which facilitates the charge-transfer, promotes the electrolyte infiltration and alleviates the expansion of the WO₃ during H⁺ insertion compared to that of the compact structure. In addition, the relationships between the structural and electrochemical activity of the electrochromic WO₃ films were further explored by the scanning electrochemical microscopy. These results testify that the porous structure can promote the infiltration of electrolyte and reduce the diffusion path, which consequently enhance the electrochemical activity.