Synthesis and characterization of highly stable optically passive CeO2–ZrO2 counter electrode

Transparent and adherent CeO2–ZrO2 thin films having film thicknesses ∼543–598 nm were spray deposited onto the conducting (fluorine doped tin oxide coated glass) substrates from a blend of equimolar concentrations of cerium nitrate hexahydrate and zirconium nitrate having different volumetric proportions (0–6 vol.% of Zr) in methanol. CeO2–ZrO2 films were polycrystalline with cubic fluorite crystal structure and the crystallinity was improved with increasing ZrO2 content. Films were highly transparent (T ∼ 92%), showing decrease in band gap energy from 3.45 eV for pristine CeO2 to 3.08–3.14 eV for CeO2–ZrO2 films. The different morphological features of the film obtained at various CeO2–ZrO2 compositions had pronounced effect on the ion storage capacity and electrochemical stability. CeO2–ZrO2 film prepared at 5 vol.% Zr concentration exhibited higher ion storage capacity of 24 mC cm−2 and electrochemical stability of 10,000 cycles in 0.5 M LiClO4 + PC electrolyte due to its film thickness (584 nm) coupled with relatively larger porosity (8%). The optically passive behavior of such CeO2–ZrO2 film (with 5 vol.% Zr) is affirmed by its negligible transmission modulation irrespective of repeated Li+ and electron insertion/extraction. The coloration efficiency of spray deposited WO3 thin film is found to enhance from 47 to 107 cm2 C−1 when CeO2–ZrO2 is coupled as a counter electrode with WO3 in an electrochromic device (ECD). These films can be used as stable ‘passive’ counter electrodes in electrochromic smart windows as they retain full transparency in both the oxidized and reduced states and ever-reported longevity.