Page 302 - Battery Reference Book

Aluminium-air secondary reserve batteries 26/5

Cell reactions in alkaline electrolytes. Parasitic side reaction:
At the aluminium electrode:
2A1+ 6HzO + 2Al(OH)3 + 3H2 (26.13)
A1 + 40H- -+ Al(OH), + 3eC. E" = -2.35 V (26.8)
At the air electrode: The aluminium anode is alloyed with an element
such as indium or tin which activates the aluminium
02 + 2H20 + 4e- -+ 40H-, E" = +0.40V (26.9)
so that it displays an extremely negative potential
Overall: in alkaline electrolytes (potassium hydroxide). The
activation effect can also be achieved by adding an
4A1+ 302 + 6H20 + 40H- + 4A1 (OH),, E" = 2.75 V element, e.g. sodium stannate to the alkaline elec-
(26.10) trolyte when using an aluminium gallium alloy as
Typically, the actual voltage achieved falls in the anode.
The addition of sodium stannate (0.06 M) to the elec-
range 1 .O- 1.5 V. Eventually the solution becomes
supersaturated with the formation of aluminium trolyte increases the current density of the cells from
hydroxide precipitate: 520mAcm-' obtained with a pure aluminium anode
to 650 mA ern-'. Corrosion rates are also reduced with
Al(OH), + Al(OH), + OH- the alloy anode.
A typical air cathode comprises oxygen reduction
As well as the above current production reaction a
parasitic heat producing reaction occurs with the for- catalyst and carbon substrate coated on either side (Le.
the electrolyte side and the air side) of an expanded
mation of hydrogen:
nickel mesh framehrrent collector.
2A1f 6Hz0 + 20H- -+ 2Al(OH), + 3HzH (26.1 1) In saline aluminium-air batteries, to prevent clog-
ging of cells by an aluminium hydroxide precipitate
Cell reactions in saline electrolytes. it is necessary to pump the electrolyte continuously
Overall: up and down the cell or stir the electrolyte by gas
4A1+ 6H2O + 302 -+ Al(OH)3, E" = 2.06 V (26.12) injection.