Electrochemical studies of sealed lead-acid battery electrode reactions

Abstract

Sealed lead-acid battery (SLA), also called valve-regulated lead-acid battery (VRLA), is the subject of considerable research efforts at the present time. There are, however, a few outstanding problems to be solved to ensure a safe, long-life operation possible for SLA. These are minimisation of corrosion of lead and lead alloys in sulfuric acid, minimisation of the deleterious impact of dissolved antimony (III) on the stability of lead in sulfuric acid, and irreversible crystal growth of lead sulfate in concentrated sulfuric acid. The thesis deals with these problems, especially those related to deep-discharge cycle life and to the corrosion of lead and lead alloys in the battery electrolyte. One of the approaches to solve these problems is to have suitable additives in the electrolyte and elimination of impurities, such as antimony, which have low overpotential for hydrogen evolution reaction (HER). Some of the problems of Sb-free positives are eliminated, at least partially, by the addition of H₃PO₄. However, re-optimisation of H₃PO₄ additive is required because of the use of higher than normal concentration of electrolyte in sealed cells. Moreover, there is little information available on the effects of H₃PO₄ additive on the performance of the negative (Pb/PbSO₄) electrode. In this study, the corrosion/hydrogen evolution rate is obtained by electrochemical polarisation studies in the cathodic Tafel domain for lead and lead alloys (Pb, Pb-Ca-Sn, Pb-Sb-Se) with and without H₃PO₄ additive, both in the presence as well as in the absence of antimony (III) [0–10 mg·L⁻¹] in the sulfuric acid electrolyte. The results clearly demonstrate a decrease in the antimony- poisoning effect on the negative electrode as also a decrease in the open-circuit corrosion of lead alloys in the battery environment. The effects observed are optimum at about 20 mg·L⁻¹. These findings are supported by other investigations such as open-circuit potential-time transients, galvanostatic potential-time transients, gasometry, and UV spectrophotometry. The results are explained using a model based on adsorption of H₃PO₄ at the electrode/electrolyte interface.