Hydrogen evolution impacts battery performance as a secondary and side reaction in Lead–acid batteries. It influences the volume, composition, and concentration of the electrolyte. Generally accepted hydrogen evolution reaction (HER) mechanisms in acid solutions are as follows:
This hydrogen evolution, or outgassing, is primarily the result of lead acid batteries under charge, where typically the charge current is greater than that required to maintain a 100% state of charge due to the normal chemical inefficiencies of the electrolyte and the internal resistance of the cells.
In the past two decades, there has been a significant increase in the research and development of external recombinant catalyst technology as a primary mechanism for reducing the problems associated with hydrogen gas evolution in flooded lead acid batteries.
Figure 1 shows the single electrode potentials of flooded lead acid batteries at the x-axis of the diagram, the positive electrode range on the right (+1.7 V), and the negative-electrode range on the left side (-0.23V).
In normal operation (float voltage), flooded lead acid batteries are kept in a state of maximum voltage potential in order to maintain maximum power reserve.
Therefore understanding the phenomenon of hydrogen evolution is an important part of the engineering for any battery system. While it is particularly critical for flooded lead acid battery systems, even VRLA batteries will vent hydrogen gas under certain conditions.