Influence of H2SO4 concentration on the mechanism of the processes and on the electrochemical activity of the Pb/PbO2/PbSO4 electrode

Abstract The aim of the present investigation is to study the influence of H 2 SO 4 concentration on the electrochemical activity, the phase composition and the structure and morphology of the PbO 2 particles. The study is performed through cycling (between 700 and 1600 mV versus Hg/Hg 2 SO 4 electrode) of a Pb/PbO 2 /PbSO 4 electrode immersed in sulfuric acid solutions of various concentrations (ranging within 2 orders of magnitude: 6.0–0.05 M H 2 SO 4 ). In this concentration region, sulfuric acid dissociates in two steps resulting in the formation of HSO 4 − and SO 4 2− ions, respectively. It has been established experimentally that the electrochemical activity of the PbO 2 /PbSO 4 electrode depends on the concentration of HSO 4 − ions in the solution. Three acid concentration regions can be distinguished: (a) active acid concentration region (5.0 M > C H 2 SO 4 > 0.5 M), where the concentration of HSO 4 − ions is the highest and a βPbO 2 phase is formed; PbO 2 particles are drop-like in shape and contain large hydrated (gel) zones; the electrode has the highest capacity; (b) passive high concentration region ( C H 2 SO 4 > 5.0 M), where the concentration of HSO 4 − ions decreases at the expense of formation of H 2 SO 4 molecules; crystal-shaped αPbO 2 particles are formed; the capacity of the electrode declines; (c) passive low concentration region ( C H 2 SO 4 4 − ions decreases at the expense of the formation of SO 4 2− ions; the content of αPbO 2 in the anodic layer increases; PbO 2 particles are crystal-shaped and are interconnected in dendrites; the capacity of the electrode declines. The above electrochemical behavior of the PbO 2 /PbSO 4 electrode is explained by the mechanism of the reactions in the gel zones of the PbO 2 particles and by the influence of HSO 4 − ions on the number of electrochemically active particles. On grounds of the obtained experimental results it has been established that the working interval within which the C H 2 SO 4 may change on cycling is from 5.0 to 1.5 M, i.e. 3.5 M H 2 SO 4 per 1 l of H 2 SO 4 solution with s.g. 1.28 takes part in the reactions on both battery plates. This is the maximum amount of H 2 SO 4 in the solution that would have no detrimental effect on the positive plates of the lead-acid battery.