Electrochemical study of Bi2O3 and Bi2O2CO3 by means of a cavity microelectrode I. Observed phenomena and direct analysis of results

The physical and chemical processes occurring for the electrochemical reduc tion and oxidation of Bi2O3 and Bi2O2CO3 can be determined through a direct analysis of the voltammograms recorded from the cavity microelectrode. The voltammogram shows an evolution for around 1 h, which corresponds to the f irst SO cycles carried out at 50 mV s(-1). The evolution is due to the sequ ence of the following concurrent processes: (i) impregnation of the powder and its dissolution in the electrolytical solution, (ii) deposit of porous metal-Bi during the cathodic scan, and deposit of Bi(III) species during th e anodic scan. These processes cause a reconfiguration of the architecture of the powder within the cavity by making the material grains smaller. For the second stage, the voltammograms do not vary any more during the cycling . For the fast cathodic scan, the reduction process starts from the bottom of the cavity at the interface Pt/solution only, then it progresses toward the mouth. Contrarily, the oxidation takes place throughout all the grains. During the next cathodic scans, the reduction also starts throughout all t he grains when the grains are not entirely oxidized from the preceding anod ic scan. (C) 2000 The Electrochemical Society. S0013-4651(00)01-077-6. All rights reserved.