The battery has a very close relative, the fuel cell. Unlike the battery
the fuel cell has a continuous supply of fuel.
[Illustration: Figure 11 _This diagram shows how a hydrogen-oxygen
fuel cell works. The chemical battery works in the same way, except
that the chemicals are different and are not continuously supplied
from outside the cell. The water produced by the H-O cell shown can
be used for drinking on spaceships._]
ANODE H₂ IN
CATHODE O₂ IN
ELECTRONS
LOAD
KOH ELECTROLYTE
K⁺ ION
OH⁻ ION
NEGATIVE ION FLOW
40H⁻ + 2H₂ ⇒ 4H₂O + 4e
O₂ + 2H₂O + 4e ⇒ 40H⁻
The hydrogen-oxygen cell of Figure 11 is typical of all fuel cells. It
essentially burns hydrogen and oxygen to form water. If the hydrogen and
oxygen can be supplied continuously and the excess water drained off, we
can greatly extend the life of the battery. The fuel cell accomplishes
this. Fueled _electrical_ cell would be more descriptive since the
physical principles are identical with those of the battery.
Perhaps the most challenging task contemplated for the fuel cell is to
bring about the consumption of raw or slightly processed coal, gas, and
oil fuels with atmospheric oxygen. If fuel cells can be made to use
these abundant fuels, then the high natural conversion efficiency of the
fuel cells will make them economically superior to the lower efficiency
steam-electric plants now in commercial service.
So far we have dwelt on the fuel cell as a cold energy conversion device
that is _not_ limited by the Carnot efficiency. A variation on this
theme is possible. Take a hydrogen iodide (HI) cell, and heat the HI to
2000°K. Some of the HI molecules will collide at high velocities and
dissociate into hydrogen and iodine: 2HI = H₂ + I₂; the higher the
temperature, the more the dissociation. By separating the hydrogen and
iodine gases and returning them for recycling to the fuel cell where
they are recombined, we have eliminated the fuel supply problem and
created a _regenerative_ fuel cell. We have, however, also reintroduced
the heat engine and the Carnot cycle efficiency. The thermally
regenerative fuel cell is a true heat engine using a dissociating gas as
the working fluid.