Cathodic Electrodeposition of Mixed Molybdenum Tungsten Oxides from Peroxo-polymolybdotungstate Solutions^†

Mixed molybdenum tungsten trioxide films of varying stoichiometry (Mo_xW_1 - xO₃, 0 < x < 1) were prepared by cathodic electrodeposition on indium tin oxide (ITO)-coated glass substrates from aqueous peroxo-polymolybdotungstate solutions. Electrochemical quartz crystal microbalance (EQCM), cyclic voltammetry, and chronocoulometry were used to gain insight into the electrodeposition mechanism. The compositional and structural properties were characterized for Mo_xW_1 - xO₃ films deposited at intermediate potentials (−0.35 V vs Ag/AgCl) and sintered at 250 °C using energy-dispersive spectroscopy, X-ray diffraction, and Raman spectroscopy. These studies reveal that films consist of homogeneously mixed Mo_xW_1 - xO₃, with an enriched Mo content ranging in composition from 0.4 < x < 0.7 depending upon the mol % Mo present in the deposition solution. Chronoamperometry and spectroelectrochemical measurements were conducted to estimate lithium ion diffusion coefficients and coloration efficiencies for the mixed metal oxide films in 1 M LiClO₄/propylene carbonate. The subtle interplay between structural and compositional properties due to the uniform mixing of Mo and W oxide components shows that electrochromic and lithium ion transport properties are moderately enhanced relative to those of single-component WO₃ and MoO₃ and demonstrate improved structural stability over pure MoO₃ polymorphs during electrochemical cycling.