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

Abstract Transparent and adherent CeO 2 –ZrO 2 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. CeO 2 –ZrO 2 films were polycrystalline with cubic fluorite crystal structure and the crystallinity was improved with increasing ZrO 2 content. Films were highly transparent ( T  ∼ 92%), showing decrease in band gap energy from 3.45 eV for pristine CeO 2 to 3.08–3.14 eV for CeO 2 –ZrO 2 films. The different morphological features of the film obtained at various CeO 2 –ZrO 2 compositions had pronounced effect on the ion storage capacity and electrochemical stability. CeO 2 –ZrO 2 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 LiClO 4  + PC electrolyte due to its film thickness (584 nm) coupled with relatively larger porosity (8%). The optically passive behavior of such CeO 2 –ZrO 2 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 WO 3 thin film is found to enhance from 47 to 107 cm 2  C −1 when CeO 2 –ZrO 2 is coupled as a counter electrode with WO 3 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.