Electrochemical and morphological investigation of electrodeposited lead dioxide
During the course of this work two fundamental aspects of the electrochemistry of the lead acid battery system have been investigated. The first area of study concerns the respective roles of alpha and beta lead dioxide in the positive plate, and more specifically the interconversion behaviour of the two polymorphs. The second research area concerns the occurence of a passivating layer of lead monoxide between substrate lead and the active material of the electrode. A subject of extreme importance to the lead acid battery manufacturer, since passivating layers of this type are acknowledged as being one of the major failure modes. This work has differed from most before it, in that use has been made of electrodeposits of the two modifications to study both the interconversion products, and the conditions necessary for lead monoxide formation. A survey of some of the recommended methods of preparation for the two forms of lead dioxide has been carried out and compared to the data given in the Powder Diffraction File. X-ray diffraction has been the major investigative tool utilised, and electrodes have been subjected to analysis in both the powder and unground state, after both galvanostatic and potentiodynarnic cycling. Cyclic voltarnmetry was used to study the potentiodynarnic conversion products of deposits swept between the hydrogen and oxygen gas evolution regions in sulphuric acid. The galvanostatic cycling showed that a conversion of alpha lead dioxide to the beta does occur, although evidence for conversion of the beta form to the alpha was not found. The potentiodynamic study revealed that in order for lead monoxide to be formed under deposits of beta lead dioxide, the presence of substrate lead is required beneath a lead sulphate film or membrane. Under these conditions it was discovered that the lead monoxide itself is a precursor to alpha lead dioxide formation.