Electrochemistry of the porous lead electrode

The electrochemistry of lead and porous lead electrodes has been examined using the techniques of linear sweep voltammetry, potential step and alternating current impedance at static and rotating disG electrodes. Investigations were made using sulphuric acid solutions at ambient and low temperatures. The behaviour of porous electrodes containing additives conventionally used to improve lead-acid battery redox processes were studied. Complementary data has been obtained by Scanning Electron Microscopy and galvanostatic cycling experiments. The solution reaction at the porous electrode of typical commercial thickness, controlled by factors affecting the diffusion layer in a direction away from the porous fsce, was found to be insignificant in comparison with resctions occurring within the porous matrix. The thickness of PbS04 films developed on the electrode is potentialdependent, with thicker films st lower potentials. The development of PbS04 on the solid lesd electrode is controlled by nucleation and growth processes. At low overpotentials the process is three-dimensional but becomes two-dimensional at higher overpotentials. The behaviour of the porous electrode can be interpreted in terms of well-established porous electrode theory, assuming the same crystallisation processes are observed in the qaae of solid electrodes. On reduction at both solid and· porous lead sulphate electrodes, the electrode process has a finite depth of penetration into the electrode. The kinetics of the formation of metallic lead from lead sulphate on both types of electrodes appear to be by instantaneous nucleation and two-dimensional growth, with subsequent current limitations owing to overlap of growing lead and PbS04 depletion. The current limitation processes are complex; the subsequent current decay rate varies with the porosity of the electrode. Ambient temperature investigations of the additives used in the commercial lead electrode demonstrated that lignosulphonate facilitated the nucleation of lead on recharge, and effected a progressive increase in surface area/porosity of the electrode. These effects promoted an increased utilisation of-,tbe electrode active material on discharge. BaS04 was found to provide nucleation centres for PbS04 formation. The low-temperature electrochemistry of solid and porous lead has been investigated and the effects of additives are discussed.