Diffusivity of anion vacancies in WO3 passive films

The WO3 films were grown in 0.1 M HClO4 aqueous solution, at different formation potentials (Ef) in the range of 2.0–7.0 V versus sce, on W electrode. The anion diffusion coefficient (DODO) of WO3 films was calculated from EIS spectra, following the surface charge approach (at high-field limit approximation), the Point Defect Model and the Mott–Shottky analysis. Among the parameters necessary to evaluate DODO, the half-jump distance (a) is very relevant, given that a small variation in a   has a great impact in the calculation of DODO. In this work, it is proposed the half-jump distance (a) should be evaluated from spectroscopic data (available in the literature). The value of a (∼1.9 Å) is taken from lattice constants of a-WO3 (amorphous-WO3), with different values of N (coordination number), and the lattice constants of m-WO3 (monoclinic-WO3). The calculated value of DODO was ∼3 × 10−17 cm2/s.