Electrochemical evaluation of various commercial expanders for lead–acid battery application
The performance of the negative plate in lead–acid batteries is strongly influenced by the presence of organic substances, usually referred to as ‘expanders’. The latter are generally added to the negative active-material during the manufacturing process. The effects of such additives mainly concern the performance at high rates of discharge and charge acceptance; additionally, they exert other beneficial actions on the overall behaviour of the negative plate.
During discharge process, passivation of the electrode starts due to the formation of a PbSO4 layer. When using an expander, passivation of the surface is prevented by the organic part increasing the current passed through the negative plate during discharge process and the cranking ability of the battery. Thus, the discharge process is more effectively undertaken when a good expander is used. During the charge process, the effects of expanders are simply explained by adsorption of expander onto the metallic surface that decreases the surface area. An organic expander impedes the process of PbSO4 reduction to lead and thereby decreases charge-acceptance ability. Among the various electrochemical techniques that may be used to evaluate different commercial expander, those based on cyclic voltammetry are of particular interested.
Voltammetric curves provide reliable information about the amount of charge stored in the anodic part of the cycle, i.e., on the formation of sponge lead, but they do not give direct kinetic information about single processes. The difference between the maximum value of the amount of charge obtained in the presence of the expander and that obtained without the expander, has been related to the activity of the organic substance, while the difference between the number of cycles for which maximum values are observed has been correlated to the stability of the expander.
In this study, five types of commercial expanders have been considered and then evaluated by 15-cycle potentiodynamic polarization test, which was undertaken between –725 to –325 mV (vs. SCE) at scan rate of 120 mV per min. The working electrode was pure lead. To prepare the solution, 6 g of expander was directly added to 300 cc of 4.8 M H2SO4 solution, followed by mechanically stirring for 2 hr. The results compare the influence of expanders on the maximum current for passivation of the lead surface that leads to capacity improvement and their effect on the PbSO4 to lead reduction process. The effectiveness of the electrochemical technique was evaluated by conducting separate tests of the performance of each expander in a battery.
Ali Asghar Alagheband Hosseini holds BSc and MSc degrees. Between 2006 and 2018, he was the Director of the R&D and Process Engineering Department of the Nirougostaran Khorasan Company, the largest SLI battery manufacturer in eastern Iran. At present, he is the Technical Manager at Sarv Sanat Toos Co.