Effects of SnO 2 addition on the structural, electrochemical, and electrochromic properties of WO 3 electrodes were investigated, in which both the WO 3 and SnO 2 -WO 3 were synthesized via a sol-gel method. The SnO 2 -WO 3 composite electrode, whose chemical states were identified through X-ray photoelectron spectroscopy analysis, gave the optical modulation of 38.3-99.5% and faster response times of 0.7 and 3.3 s. The coloration efficiencies of the SnO 2 -WO 3 electrode were greater ͑49.7 cm 2 C −1 ͒ by 14% than the WO 3 alone, together with the improved memory effect and durability during the electrochromic cycling tests up to 300 cycles.Electrochromic materials represent a reversible change in color under an applied electric field; thus, their devices are of considerable interest for the potential applications in information displays, light shutters, smart windows, and variable-emittance radiators with their low power consumption, high coloration efficiency ͑CE͒, and memory effect. [1][2][3] Among various transition metal oxides with the electrochromic properties ͑e.g., W, Ni, V, Mn, Mo, Cr, Ti, Nb, Co, Rh, and Ir oxides͒, tungsten oxide ͑WO 3 ͒ and nickel oxide ͑NiO͒ have been widely used as cathodic and anodic coloration electrochromic materials, respectively. 2-5 In particular, WO 3 has been extensively studied because it has a relatively fast response time and high CE compared to other electrochromic materials. The electronic structure of WO 3 can be readily modified by injection and extraction of ions with electrons, accompanying reversible optical changes from transparent states to visible light absorbing states, as represented by the following electrochemical reactions on the WO 3 in an acidic condition 1,4Various methods, such as sputtering, chemical vapor deposition, and electroplating, have been employed to improve electrochromic properties of oxide-based materials. For example, Avendano et al. 6 used a reactive dc magnetron sputtering system to prepare Ni-based oxides with compositions of Ni, NiV 0.08 , NiAl 0.56 , and NiMg 0.8 . They showed that Al-and Mg-containing Ni oxide films produced significantly increased luminous transmittances as compared to the Ni oxide alone, making them suitable for applications requiring a high bleached-state transmittance. Meda et al. 7 reported that porous nanocrystalline W oxide films prepared by organometallic chemical vapor deposition showed a fast response time with CE of 22 cm 2 C −1 at 630 nm. Also, a radio frequency ͑rf͒ magnetron cosputtering to prepare NiO-Ta 2 O 5 nanocomposite electrodes was demonstrated with high optical-transmittance modulation of 41-95%, high CE ͑ca. 30.5 cm 2 C −1 at 633 nm͒, and good durability. 3 A sol-gel technique has been regarded as an efficient method to synthesize ceramics via intermediate sol and gel states; 8 thereby, the sol-gel prepared electrochromic materials have received much attention because of simple fabrication with low cost. For example, a sol-gel prepared W oxide film was demonstrated by Biswas et al. 9 that ...
