Page 180 - Lindens Handbook of Batteries

BATTERY ELECTROLYTES 7.5

The addition of ammonium chloride increases the conductance substantially. The actual conductance
in the cell is modified, however, by the inclusion of a gelling agent, such as starch, which immobi-
lizes the electrolyte and minimizes leakage and orientation effects. This is part of the art of manufac-
turing carbon-zinc cells, and the reader is referred to Chap. 9 and references therein for details.
The zinc-bromine rechargeable cell, which is under development for large-scale energy storage
applications, employs a near-neutral zinc bromide solution in the discharged state. During charge,
one electrode provides a surface for zinc plating, while the second generates bromine which is dis-
solved in the electrolyte. An ammonium salt, which may have up to four organic substituents on the
nitrogen (either cyclic or linear), is usually added to form a tribromide ion, which has a much lower
vapor pressure than elemental bromine. The chemistry is rather complex, especially if chlorides are
added, but much of the work on additives is proprietary so it is difficult to interpret the results. An
additional system using neutral electrolytes for energy storage is the so-called polysulfide-bromine
battery. This system uses a sodium bromide electrolyte on the positive electrode side and a sodium
polysulfide electrolyte on the negative electrode side. Again, proprietary additives are common and
analysis is difficult. The reader is referred to Chap. 30 for further information.

7.2.3 Acid Electrolytes

The main electrolyte in this category is sulfuric acid, which, although it has a long history in batter-
ies, is now mainly employed in the lead-acid or more recently in the carbon-lead-acid cell. It can be
argued that this electrolyte is the most important one because of the widespread use of this battery
type and its worldwide economic importance. The lead-acid cell was invented in 1859 by Gaston
Planté and utilized dilute sulfuric acid which today is usually fixed on construction of the cell at a
concentration of 37% by weight in the fully charged condition. As is the case for alkaline batteries,
the electrolyte is a reactant and therefore varies in concentration during charge and discharge; the
electrode reactions given by

-
+
Positive: PbSO + 5 H O  PbO + 3H O + HSO + 2e - E° = 1.685 V (7.2)
4
3
2
2
4
-
-
+
Negative: PbSO + H O + 2e  Pb + HSO + H O E° = -0.356 V (7.3)
3
4
4
2
+
Cell reaction: 2 PbSO + 4 H O  Pb + PbO + 2 H O + 2 HSO 4 - E° = -2.041 V (7.4)
2
4
2
3
where the cell reaction shows that 2 moles of sulfuric acid are used during discharge to produce
2 moles of water. This utilization of sulfuric acid contributes substantially to the weight and volume
of the battery and also results in electrolyte properties changing during the course of the discharge as
the electrolyte concentration changes. The reader is referred to the chapters on lead-acid batteries
(Chaps. 16 and 17) for further details, although we note that the conductivity of sulfuric acid in the
35% concentration range is of the order of 800 mS/cm, one of the most conductive electrolytes used
in batteries at room temperature. It is noted from the individual electrode potentials given in Eqs.
(7.2) and (7.3) that the electrodes are not stable with respect to oxygen evolution [Eq. (7.2)] and
hydrogen evolution [Eq. (7.3)] in the fully charged condition. These reactions cause a steady decline
in capacity of the cell as gassing occurs, which is one of the principle problem areas of the lead-acid
battery. The degree to which hydrogen is evolved causes an irreversible increase in pressure since
the oxygen evolution is balanced to some degree by recombination with the negative electrode mate-
rial, while hydrogen combination with the positive or with oxygen is very slow. In some versions
of VRLA cells, a hydrogen-oxygen recombination catalyst is included in the head space of the cell
to reform water. In the absence of such a catalyst, the cell simply vents the gasses, thus causing a
decrease in the amount of water in the cell (increasing the sulfuric acid concentration and lowering
the electrolyte level) and severe stress on the electrodes. Venting of hydrogen-containing gasses can
also cause hydrogen explosions if a spark is present and the gas mixture is within the explosion limit
of hydrogen content (above 4%).
A variation on the lead-acid battery includes the addition of activated carbon to the lead negative
electrode leading to a combined double-layer, Faradaic redox process. The double-layer capacitance