Page 284 - Lindens Handbook of Batteries

12.4 PRIMARY BATTERIES

Direct solution of the zinc electrode in the alkaline solution on open circuit is minimized by dissolv-
ing zinc oxide in the electrolyte and amalgamating the zinc in the electrode. Mercury levels used
in zinc electrodes were usually in the range of 5 to 15% w/w. Great attention was also paid to the
impurity levels in the zinc, since minor cathodic inclusions in the electrode can drive the hydrogen
generation reaction despite the precautions indicated. 5,6

12.3.3 Cadmium Anode
The reaction at the anode is

–
Cd + 2OH → Cd(OH) + 2e
2
This implies the removal of water from the electrolyte during discharge, necessitating an adequate
quantity of electrolyte in the cell and the desirability of a high percentage of water in the electro-
lyte. Cadmium has a high hydrogen overvoltage in the electrolyte, and so amalgamation is neither
necessary nor desirable, since the electrode potential is some 400 mV less electropositive than
zinc.
Cadmium metal powders as produced conventionally were unsuitable for use as electrode
materials. Activated cadmium anodes were produced by (1) electroforming the anode, (2) electro-
forming powder by a special process followed by pelleting, or (3) precipitating by a special process
as a low-nickel alloy and pelleting. All of these processes have been used by different manufacturers
to give cells with various performance parameters. 7

12.3.4 Mercuric Oxide Cathode
At the cathode, the overall reaction may be written
–
HgO + H O + 2e → Hg + 2(OH)
2
Mercuric oxide is stable in alkaline electrolytes and has a very low solubility. It is also a nonconduc-
tor, and adding graphite is necessary to provide a conductive matrix. As the discharge proceeds, the
ohmic resistance of the cathode falls and the graphite assists in the prevention of mass agglomeration
of mercury droplets. Other additives that have been used to prevent agglomeration of the mercury are
manganese dioxide, which increases the cell voltage to 1.4 to 1.55 V, lower manganese oxides, and
silver powder, which forms a solid-phase amalgam with the cathode product.
Graphite levels usually range from 3 to 10% and manganese dioxide from 2 to 30%. Silver
powder was used only in special-purpose cells because of cost considerations, but may be up to
20% of the cathode weight. Again, great care was taken to obtain high-purity materials for use in
the cathode. Trace impurities soluble in the electrolyte are liable to migrate to the anode and initi-
ate hydrogen evolution. An excess of mercuric oxide capacity of 5 to 10% was usually maintained
in the cathode to “balance” the cell and prevent hydrogen generation in the cathode at the end of
discharge.

12.3.5 Materials of Construction
Materials of construction for the zinc/mercuric oxide cells were limited not only by their ability to
survive continuous contact with strong caustic alkali, but also by their electrochemical compatibility
with the electrode materials. External contacts were chosen for their corrosion resistance, galvanic
compatibility with the equipment interface and, to some degree, cosmetic appearance. Metal parts
may have been homogeneous, plated metal, or clad metal. Insulating parts may have been injection-,
compression-, or transfer-molded polymers or rubbers.