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Grex Science Item 50: Aluminum-Air Batteries
Entered by drew on Wed Jul 14 21:17:29 UTC 1999:

    What is the chemistry of an aluminum-air battery? What electrolyte
is used? How is it physically constructed? How does one get air to
be an "electrode"? And, can it be recharged, like lead-acid cells,
by supplying current when partially or heavily discharged, rather than
by replacing the aluminum core? How quickly can they be charged?

    http://members.accessus.net/~voltek/ mentions them briefly but
doesn't seem to have any specifics, except that a byproduct of the
reaction is Al(OH)3.

3 responses total.

#1 of 3 by russ on Thu Jul 15 00:42:28 1999:

1.)     Al + 3 OH- -> Al(OH)3 + 3e-,   2 H2O + O2 + 4e- -> 4OH-
2.)     Generally aqueous.  More I can't tell you.
3.)     Widely varied.  There was at least one automotive version
        where the aluminum electrode was composed of aluminum filings
        on a graphite backing; the metal was carried onto the graphite
        by a stream of electrolyte, making it rechargeable almost instantly.
4.)     No.  Aluminum has a stronger affinity for oxygen than hydrogen
        does; you will dissociate water before you can regenerate
        aluminum metal.  To regenerate metallic Al, you need different
        chemistry (and generally a molten salt bath).

#2 of 3 by russ on Fri Jul 16 01:59:53 1999:

Oh, as for how one gets air to an electrode:

Generally, you have some kind of catalyst-coated conductor, and you
apply air to it.  In the case of the automotive battery I read about,
ISTR they used platinum-coated graphite and ran air through or past
the graphite.

#3 of 3 by rcurl on Wed Jul 21 21:06:38 1999:

I don't know about an aluminum-air battery, but the zinc-air battery
uses a powdered zinc slurry in 30% KOH. The "anode" is the zinc in the
slurry, but a metal 'can' is used to act as the anode terminal. The
cathode terminal is a steel 'can' with a membrane through which oxygen
can diffuse, and the cathode space is a thin (0.5 mm) space filled with
30% KOH solution, and separated from the anode space by another porous
membrane. The byproduct of the reaction is potassium zincate.

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