15-12-2012, 04:01 PM
On the hydrogen balance in sealed lead/acid batteries and its effect on battery performance
[attachment=43939]
Abstract
An overview is provided on the basic principles of decreasing hydrogen pressure in sealed
lead/acid batteries. Approaches that are based on present technology, as well as other
possibilities for suppressing hydrogen evolution and oxidizing hydrogen, are studied and
discussed. The results emphasize the relevance of so-called hydrogen-evolution inhibitors
for industrial applications.
Introduction
In the lead/acid battery, besides the charging/discharging processes, undesirable
side reactions also occur (Fig, 1). During overcharging at the end of charging and
during open circuit, water is decomposed and results in gas evolution. The basic design
principles for sealed lead/acid (SLA) batteries are, therefore, measures for gas removal
(Fig. 2) and the suppression of gas evolution (Fig. 3).
Gas recombination in SLA batteries is mainly based on an efficient oxygen cycle.
Oxygen reduction at the negative electrode proceeds at a sufficiently high rate, provided
there is a rapid transfer of oxygen from the positive to the negative electrode. This
can be achieved by immobilizing the electrolyte and by limiting the amount of electrolyte
(see a in Fig. 2).
Experimental
Investigation of the processes of hydrogen evolution and hydrogen oxidation were
conducted mainly in sealed test cells that comprised one positive (3 A h) electrode,
two negative electrodes of the same dimension, and gelled electrolyte, as described
in ref. 14. The gas pressure during overcharging and cycling was recorded and served
as a criterion for the oxygen-cycle efficiency and the extent of hydrogen evolution
2141. The following investigative techniques were also employed: linear sweepvoltammetry
(LSV); A h capacity determination; special testing probedures, e.g., for characterizing
Hz/O* recombination catalysts, as described in ref. 7.
Suppression of hydrogen evolution at the negative electrode
Znfluence of the processes of oxygen reduction and PbS04 reduction
The rate of hydrogen evolution is influenced by the rates of both the competing
reactions at the negative electrode, oxygen reduction and PbS04 reduction. As shown
recently [14], the oxygen-recombination rate can be increased by increasingp,, and
improving the oxygen-recombination conditions, both these result in suppression of
hydrogen evolution. The substantial influence of the oxygen-recombination conditions,
primarily a measure for the reactive contact area lead/electrolyte film/oxygen [17], is
confirmed by the following measurements.
Figure 7 shows the influence of the state-of-charge of the negative electrode. It
is clear that a decrease in the state-of-charge leads to coverage of the surface by
discharge products and diminishes the area of free-lead sites. This favours the PbS09
reduction reaction and hinders oxygen recombination.
Influence of hydrogen evolution inhibitors
Contrary to the antimony-poisoning effect (see Fig. 4, ref. lo), there are many
additives that are able to increase the hydrogen overvoltage at the negative electrode.
The effect on hydrogen evolution of a wide variety of inorganic and organic substances
has been described in the literature, e.g., refs. [20-291. Organic additives that show
a substantial inhibition effect have been found to be derivatives of benzaldehyde [26-281
and benzoic acid 125, 291. The structure of these additives is similar to that of the
substructure of the expander lignin. Indeed, expanders are sometimes reported to
exhibit a slight inhibition effect [25, 261, but this appears to be true only for the
unpurified state [26].
[attachment=43939]
Abstract
An overview is provided on the basic principles of decreasing hydrogen pressure in sealed
lead/acid batteries. Approaches that are based on present technology, as well as other
possibilities for suppressing hydrogen evolution and oxidizing hydrogen, are studied and
discussed. The results emphasize the relevance of so-called hydrogen-evolution inhibitors
for industrial applications.
Introduction
In the lead/acid battery, besides the charging/discharging processes, undesirable
side reactions also occur (Fig, 1). During overcharging at the end of charging and
during open circuit, water is decomposed and results in gas evolution. The basic design
principles for sealed lead/acid (SLA) batteries are, therefore, measures for gas removal
(Fig. 2) and the suppression of gas evolution (Fig. 3).
Gas recombination in SLA batteries is mainly based on an efficient oxygen cycle.
Oxygen reduction at the negative electrode proceeds at a sufficiently high rate, provided
there is a rapid transfer of oxygen from the positive to the negative electrode. This
can be achieved by immobilizing the electrolyte and by limiting the amount of electrolyte
(see a in Fig. 2).
Experimental
Investigation of the processes of hydrogen evolution and hydrogen oxidation were
conducted mainly in sealed test cells that comprised one positive (3 A h) electrode,
two negative electrodes of the same dimension, and gelled electrolyte, as described
in ref. 14. The gas pressure during overcharging and cycling was recorded and served
as a criterion for the oxygen-cycle efficiency and the extent of hydrogen evolution
2141. The following investigative techniques were also employed: linear sweepvoltammetry
(LSV); A h capacity determination; special testing probedures, e.g., for characterizing
Hz/O* recombination catalysts, as described in ref. 7.
Suppression of hydrogen evolution at the negative electrode
Znfluence of the processes of oxygen reduction and PbS04 reduction
The rate of hydrogen evolution is influenced by the rates of both the competing
reactions at the negative electrode, oxygen reduction and PbS04 reduction. As shown
recently [14], the oxygen-recombination rate can be increased by increasingp,, and
improving the oxygen-recombination conditions, both these result in suppression of
hydrogen evolution. The substantial influence of the oxygen-recombination conditions,
primarily a measure for the reactive contact area lead/electrolyte film/oxygen [17], is
confirmed by the following measurements.
Figure 7 shows the influence of the state-of-charge of the negative electrode. It
is clear that a decrease in the state-of-charge leads to coverage of the surface by
discharge products and diminishes the area of free-lead sites. This favours the PbS09
reduction reaction and hinders oxygen recombination.
Influence of hydrogen evolution inhibitors
Contrary to the antimony-poisoning effect (see Fig. 4, ref. lo), there are many
additives that are able to increase the hydrogen overvoltage at the negative electrode.
The effect on hydrogen evolution of a wide variety of inorganic and organic substances
has been described in the literature, e.g., refs. [20-291. Organic additives that show
a substantial inhibition effect have been found to be derivatives of benzaldehyde [26-281
and benzoic acid 125, 291. The structure of these additives is similar to that of the
substructure of the expander lignin. Indeed, expanders are sometimes reported to
exhibit a slight inhibition effect [25, 261, but this appears to be true only for the
unpurified state [26].