Electrochemical preparation of electrodes modified with non-stoichiometric mixed-valent tungsten(VI, V) oxides

Abstract

Non-stoichiometric tungsten(VI, V) oxide films were grown on carbon substrates by cycling the potential between −0.40 and 0.80 V for 0.5–3 h in a colloidal suspension of Na 2SO 4 · 2H 2O in 2 M H 2SO 4. Different thicknesses and extents of hydration were obtained by varying the experimental parameters during the modification step. Longer cycling times in the freshly prepared modification solution resulted in thicker films. With a gradual increase of temperature from 15 to 90° C, the film converted from dihydrate to monohydrate. Redox transitions appeared to be faster in more fully hydrated films but the stability was poorer. The cathodic processes are believed to yield the hydrogen tungsten oxide bronzes, H xWO 3 (0 < x < 1), or substoichiometric lower tungsten oxides with the formula WO 3-y (0 < y < 1). The bronzes prevailed with coatings of large water content. The anodic response results from the reversible oxidation of hydrogen bronze/W(V) centers (or perhaps W(IV) in more reduced coatings), to W(VI). Cyclic voltammetric experiments were also carried out together with optical absorption measurements in the UV-visible region, and they confirmed the reversible electrochromic behavior of the system. Further information was gained about the chemical composition and purity of the film from Auger electron spectroscopy and secondary ion mass spectrometry. The presence of W, O and S (from H 2SO 4) was detected as the major components, and Na and Mo were found as impurities.