Abstract
The processes of anodic oxidation of bismuth in neutral aqueous halide solutions have been investigated. It has been found by X-ray diffraction analysis that in KCl, KBr and KI solutions, porous polycrystalline anodic bismuth oxyhalide (BiOHal) films are formed on the bismuth electrode, while in KF solutions, a bismuth trifluoride (BiF 3) film with an admixture of tysonite-type BiO x F 3−2 x phases (0 < x ⩽ 0.5) is formed. The formation of BiOHal films on the electrode surface proceeds by a stepwise mechanism through intermediate formation of bismuth halide complexes BiHal n 3− n , followed by their hydrolysis. In moderately concentrated solutions of KCl and KBr (0.2 mol/l), the anodic potentiodynamic formation of a BiOHal film is fairly well described by a model assuming gradual filling of the electrode surface with a BiOHal layer and simultaneous growth of its thickness with ionic migration in the solution as the rate-determining stage. Then, as the entire electrode surface is covered with the BiOHal film, the limiting stage changes from ionic migration in the solution bulk to ionic diffusion of Hal − in pores in the BiOHal.
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