Negative plates hydrogen evolution

During corrosion of the positive grids, antimony diffuses to the electroljrte and is deposited on the negative plates. Hydrogen evolution on antimony proceeds at low overpotentials. Hence, decomposition of water from the electrolyte is accelerated. The battery has to be topped up with water periodically. 

Two other reactions must be taken into account during the charging of a VRLA cell. These are, the evolution of hydrogen at the negative plate ... 

Formation of the plates containing expander is completed within 10 h, while those without expander require 12 h of formation. Hydrogen evolution starts on plates with expander at an overpotential which is 160 mV more negative than that on plates with no expander. 

Fig. 9.2. Model depicting mixed-potential condition at negative-plate surface due to simultaneous functioning of hydrogen-evolution zones and oxygen-recombination gas paths.

It has been reported that silver decreases the rate of oxidation of lead, particularly in the Barton-pot process [25], by up to 10%. In the active materials, silver increases oxygen evolution (at the positive plate) more than it does to hydrogen evolution (at the negative plate). The promotion of gassing by silver, however, does not appear to be as serious a problem as was once thought, at least for materials with silver contents up to 0.01 wt.% [25]. 

Fig. 17.8. Build-up of lead sulfate and concomitant evolution of hydrogen in negative plates after 1735 cycles of simulated HRPSoC duty (battery initially discharged at Ci rate to 50% state-of-charge duty cycle charge and discharge at 2Ci = 3% state-of-charge) [10].

In 1957, Otto Jache from Sonnenschein introduced the gel electrolyte and patented the sealed lead—acid battery [18]. In this battery construction, the oxygen evolved at the positive plates passes through cracks in the gel and reaches the negative plates, where it is reduced and thus restores the lost water and suppresses hydrogen evolution. The German company Sonnenschein started mass production of gel-sealed lead—acid stationary batteries. 

In order to minimise, or even eliminate, the need for maintenance, the battery should operate with minimum water loss. This is achieved when the over-voltage of hydrogen and oxygen evolution on Ph and Pb02 surfaces, respectively, is very high. This is of utmost importance for VRLA and maintenance-free batteries. Hence, it is essential to reduce the rate of hydrogen evolution at the negative plates and to find appropriate methods to oxidize the evolved... 

Inhibitors of hydrogen evolution slow down the self-discharge processes and improve the charge acceptance of the negative plates. 

Anisaldehyde, in combination with Indulin-C and Na-l-naphthol-1-sulfonate in amounts of 700 ppm, is a highly efficient inhibitor of the hydrogen evolution reaction and increases the capacity of the negative plates. It can be introduced during operation of the battery, when the water loss increases [44]. 

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