On the question of electrochemical activity of differently formed lead dioxides

Electrochemically and chemically formed lead dioxides show varying electrochemical properties, which has often been discussed in terms of hydrogen content and amorphous phases. These differences in electrochemical activity have always interested research related to their behaviour as the positive active material (PAM) in the lead-acid battery. In this work, the crystallinity and composition of different lead dioxide samples were examined with respect to preparation method. Two electrochemically oxidised samples, one originating from a pure lead foil and the other one from tetra basic lead sulphates (TBLS), were investigated and compared with a commercial chemically prepared lead dioxide. The samples were investigated by X-ray diffraction (XRD) and thermogravimetric analysis (TGA) as a function of temperature in the range of 25-600 degreesC. The experimental results show that only surface bound water is present in the different lead dioxide phases investigated. However, a clear difference between chemically and electrochemically formed oxide is revealed from high temperature XRD (HT-XRD). At high temperatures, a lead oxide sulphate phase is formed in the electrochemically prepared samples, showing the existence of sulphate ions bound to internal surfaces. These ions are believed to facilitate the reduction process of lead dioxide and hence contribute to the electrochemical activity. (C) 2000 Elsevier Science S.A. All rights reserved.