Solid state electrochemical characterization of tungsten oxides and related heteropoly‐12‐tungstic acid single crystals

Tungsten oxide, dihydrate (WO 3 · 2H 2 O) and monohydrate (WO 3 · H 2 O) pastes, as well as silico‐12‐tungstic acid single crystals (H 4 SiW 12 O 40 · 31H 2 O) have been subjected to electrochemical investigations in solid‐state, i.e., in the absence of deliberately added external liquid electrolyte phase. Mobile protons, which exist in WO 3 · H 2 O and H 4 SiW 12 O 40 · 31H 2 O (but not in WO 3 · H 2 O) bulk materials, are capable to support electron transport between W VI, V mixed‐valence sites (generated in the systems upon partial reduction). Silicotungstic acid can be reduced reversibly to heteropolyblue (H z SiW 12 O   40 4− , where z is 1, 2 or 4), which is analogous to hydrogen tungsten oxide bronze (H x WO 3 , 0 < x < 0.35), i.e., blue nonstoichiometric W VI, V oxide. The values of z = 4 and x ≈ 0.35 are approximately equivalent because each silicotungstate structure consists of twelve WO 3 surrounding the SiO 4 group. There are similarities in the voltammetric behaviors of WO 3 · 2H 2 O and H 4 SiW 12 O 40 · 31H 2 O. Quantitative characterization of H 4 SiW 12 O 40 · 31H 2 O has been achieved with the use of a carbon fiber ultramicrodisk electrode. Upon injection of four electrons per silicotungstate unit, the maximum concentration (6–7 M) of W VI, V redox sites is generated within the diffusional flux in the crystal. An effective diffusion coefficient for charge propagation is high (3–4 × 10 −7 cm 2 s −1 ), Injection of more electrons ( z > 4) leads to irreversible redox transitions in the system. Silico‐12‐tungstic acid single crystal can be considered as a bulk, molecular, model for redox reactions in catalytic tungsten oxides.