Influence of pH of the H 2SO 4 solution on the phase composition of the PbO 2 active mass and of the PbO 2 anodic layer formed during cycling of lead electrodes

Abstract

During charge and discharge of lead-acid batteries the concentration of the H 2SO 4 solution in the pores of the active material and in the interface grid/active mass varies widely. In this investigation, the influence of pH of the H 2SO 4 solution on the phase composition of the positive active mass (PAM) and of the interface PAM/grid is studied. The influence of pH on the phase composition of the interface is determined indirectly by cycling Pb electrodes between 0.70 and 1.60 V (versus Hg/Hg 2SO 4) in H 2SO 4 solutions of various concentrations and determining the phase composition and the structure of the anodic layer formed. The influence of pH on the phase composition of the PAM is investigated by immersing fully charged PAM samples into H 2SO 4 solutions of various concentrations and determining the phase composition of the PAM and the size of the α-PbO 2, β-PbO 2 and PbSO 4 crystals. It has been found that the outer sub-layer of the anodic layer participates in the cycling processes and its phase composition depends on the pH of the solution and on the potential scan rate. The reduction rate of PbO 2 in this sub-layer depends on the solution pH. If the reduction of PbO 2 proceeds in solutions with pH between −1.0 and −0.50, the rate of the processes is high. When it proceeds at pH>−0.50 the reduction rate is lower. This behaviour of the PbO 2/PbSO 4 electrode influences the power performance of the lead-acid battery when the positive plates are the power limiting component. The rate of oxidation of PbSO 4 to PbO 2 is determined by the pH of the solution because the solubility of PbSO 4 depends on pH. In concentrated solutions the solubility of PbSO 4 is low, the charge process is slow and some unoxidised PbSO 4 may remain in the charged plate. In diluted H 2SO 4 solutions, the solubility of PbSO 4 is high and PbSO 4 crystals are oxidised fully during charge. It has been found that the phase composition of the PAM depends on the pH of the solution since the hydrated part of the PbO 2 particles interacts with the ions in the solution as a result of which the crystal zones/hydrated zones and hydrated zones/solution equilibria are changed. The content of α- and β-PbO 2 crystal zones in the PAM depends on the pH of the solution. In the pH region between −0.75 and 0, SO 4 2− ions which have penetrated into the hydrated gel zones react with Pb 2+ ions from the nonstoichiometric part of the PAM (PbO 2− δ ) forming PbSO 4 molecules, which leave the hydrated zones and enter the solution forming PbSO 4 crystals there. When the PAM is immersed in solutions with pH between 0 and −1.0, the content of the crystal phases of α- and β-PbO 2 decreases. In this pH region the average size of the β-PbO 2 crystals decreases while that of α-PbO 2 remains constant or increases slightly. The results of this investigation evidence that the PAM is a dynamic system, which interacts with the ions in the solution, and hence the phase composition of the PAM depends on the concentration of the H 2SO 4 solution.