Gas-diffusion approach to the kinetics of oxygen recombination in lead-acid batteries

Diffusion of oxygen through the thin liquid film (TLF) covering the surface of the negative active mass (NAM) crystals and impeded charge transfer through the electric double layer (EDL) were assumed to be the rate limiting stages of the closed oxygen cycle in VRLAB. Based on the theory of colloid systems, the thickness of the TLF was determined and its dependence on pore radius and surface tension at the gas/electrolyte interface was estimated. The initial and boundary conditions were defined and Fick’s equation was solved for this case. The diffusion kinetics results obtained explain the dependence of the recombination current on the metal surface on which the electrochemical reaction takes place, the thickness of the TLF, the concentration of oxygen at both film surfaces, and the time of polarization. Using the film thickness equation derived by the colloid chemistry theory, a general equation was developed which determines the oxygen recombination rate dependence on the thin liquid film parameters, NAM structure and polarization time. The validity of this equation was experimentally proved for low and high electrolyte saturation levels of the plates, as well as for negative plates produced employing various technologies. This equation makes it possible to draw conclusions about the possible ways of improvement of the efficiency of the oxygen cycle and about the technologies for negative plate production for VRLAB applications.