Electrochemical behavior of silica gel electrolyte in lead-acid cell

Abstract

Gel electrolyte with lead electrodes is the core mechanism of valve-regulated lead-acid batteries (VRLA batteries). In this study, the lead acid gel electrolyte system and effect of additive addition have been studied using electrochemical methods, such as impedance spectroscopy and cyclic voltammetry. The bulk conductivity of electrolyte decreased with the increase of silica contents. The conductivity was poorer as the molecular weight and the amount of added binder were larger. This is because additive addition lowered a relative amount of sulfuric acid in the solution and made the electrolyte more viscous to resist ionic motion. Silica addition increased the solution resistance and charge transfer resistance. Also, the formation of $PbSO_4$ increased charge transfer resistance with the decreased of applied potential. Under a potential of 0.9V for 6000 sec, a gel electrolyte system brought in poorer electrode surface morphology than sulfuric acid solution. The lower concentration gradient and larger viscosity of the system provide one of possible explanation for the less uniform crystal growth. The reduction of active area by gas evolution and the consequent separation of electrode-electrolyte interface influenced the peak current density in cyclic voltammetric measurements of positive electrode. Photographic investigation of electrode surface was helpful to identify the reacted and unreacted areas in the last cycle of cyclic voltammetry. The peak current density of gel electrolyte system reached 80% of sulfuric acid solution. In cyclic voltammetric measurement of negative electrode, silica addition enhanced the efficiency of charge/discharge. However, gel system was less stable than sulfuric acid system. Smaller concentration gradient of sulfate ions and higher viscosity explain the suppressed thin compact layer on the negative electrode of an electrolyte system with silica in potentiostatic analysis.