"Morgans" wrote:
"Jim Logajan" wrote

Actually that's not strictly true - a class of batteries called fuel
cells could (actually already can) utilize the oxygen in air.

A fuel cell is about as much of a battery as a Honda Generator.

My 5th edition copy of "Physical Chemistry" by Peter Atkins states "A fuel
cell operates like a conventional galvanic cell with the exception that the
reactants are supplied from outside rather than forming an integral part of
its construction."

So from a chemistry standpoint they are batteries. The Honda Generator is a
heat engine that converts the reactants first to heat, then to mechanical
motion, and finally to electrical current.

A fuel cell is an electrical generating device. You put fuel and an
oxidizer in and you get power and water out. They store no energy.

The atmosphere is a handy place to store part of the reactants. But there
generally isn't any need to make an earth-bound fuel cell fully reversable
and self-contained like other batteries, but it has been done.