Page 311 - Lindens Handbook of Batteries

BUTTOn CELL BATTErIES: SILVEr OxIDE–ZInC AnD ZInC-AIr SYSTEmS 13.17

taBle 13.4 major Strengths and Weaknesses of Zinc/Air Batteries
Strengths Weaknesses
High energy per unit volume responds to environmental conditions
Stable voltage curve Limited shelf life after open to air
Environmentally friendly Flooding in high rH
Economical Poor on intermittent use
Convenient Tape must be removed from air holes to activate cell

a migration to smaller sizes, and a demand for greater power output as circuitry became more sophis-
ticated. new work was demanded of the battery as digital signal processing was added to the chain of
events that takes place between the reception of sound at the microphone and the eventual delivery of
amplified sound at the receiver. The advent of cochlear implants raised the bar for zinc/air in the need
for a carrier signal to inductively power the implanted electronics. This is used to stimulate the nerves
of the cochlea with a superimposed message signal that produces the sensation of sound.
Very low-power applications, where infrequent maintenance and long service life are needed, can
also benefit from the high capacity of zinc/air cells. In these applications, the cells are much more
isolated from their environment. remote railroad signaling, marine navigation systems, and electric
fence applications are examples of this legacy from the earlier incarnations of commercially avail-
able zinc/air products. These batteries are often multicell and prismatic in configuration. Large zinc/
air batteries are currently finding use in military applications (see chap. 33).

13.7 CHEMISTRY

The zinc/air battery consists of three main electrochemical components: zinc metal in a fine powder
form, aqueous alkaline electrolyte, and a catalytic cathode structure that makes oxygen available for
-
conversion to OH ions when an external circuit is established. The zinc is separated from the cath-
ode physically by a wettable microporous membrane, so that only ionic conduction can take place
within the body of the cell. The cathode layer is compressed against an insulating seal gasket to
contain the liquid constituent of the cell and prevent leakage. One or more holes in the cathode can
admit oxygen by diffusion as the internally contained oxygen is consumed during discharge.
The anode reaction as understood from an elementary viewpoint goes as follows:

Zn + OH → 2 - Zn OH) + 2e -
(
2
+
Zn OH) → ZnOH O
(
2 2
The cathode reaction, occurring first at the catalytic sites and then in the electrolyte where the ionic
conduction takes place, is as follows:
-
+
O + 2 H O 4e - → 4 OH
2 2
The oxygen reduction process is predicated on the peroxide–free radical (O H) formation, followed
2
by a subsequent decomposition of the peroxide. This effectively restricts performance of the cell in
practice, as it is the rate limiting electrochemical step
+
O + H O2e - → OH + - OH -
2 2 2
OH → 2 - OH + - 1 2 O 2
Peroxide decomposition can be accelerated in order to increase overall system performance. This is
usually accomplished by catalysts that promote the reaction in the last equation. While one of many
transition metal compounds or a rare earth metal may be appropriate, most commonly it is an oxide
of manganese that finds use in commercially available products. 28–30 Care must be given to the