Dissolution and precipitation reactions of lead sulfate in positive and negative electrodes in lead acid battery

Several studies in the author's former laboratory at Kyoto University, have been reviewed on the dissolution–precipitation reactions on the electrodes in the lead acid battery. At the discharges of β-PbO2 in the positive electrode and Pb in the negative electrode, PbSO4 deposited on both electrode surfaces through the large supersaturation of Pb2+ ion. Thus, the discharge reactions of the positive and the negative electrodes proceeded smoothly, and the largest crystal size of PbSO4 was obtained in 0.5–1.0 M H2SO4 with the largest Pb2+ ion concentration on the surface. The size of its PbSO4 crystals became smaller at a higher current discharge through the formation of many of nuclei on the electrode surface. On the other hand, the charge reactions, which are the anodic oxidation of PbSO4 at the positive electrode and the cathodic reduction of PbSO4 at the negative electrode, did not proceed in the same way as the discharge reactions, because PbSO4 is a large ionic crystal without electronic or ionic conductivities and its solubility is very low in sulfuric acid solutions. The reaction site is considered to be the interface between β-PbO2 of the positive active material, or Pb of the negative active material, and PbSO4. At such an interface, Pb2+ ions can be adequately supplied from the PbSO4 crystal, and a charge-transfer reaction can occur on β-PbO2 or Pb. The reaction rate depends on the electrochemical kinetics parameters, e.g., exchange charge-transfer rate, real area of the interface between PbO2 or Pb and PbSO4, and the mass-transfer in the narrow gap can be assumed to be like that of a thin layer cell. For the oxidation of PbSO4 to ß-PbO2, the charge-transfer process was the rate-determining step. For the reduction of PbSO4 to Pb, the mass-transfer process was the rate-determining step. For both processes, it was concluded that a large crystal size of PbSO4 gives a smaller current because of the smaller reaction site area per unit volume.