Potential Fuels
The battery has a very close relative, the fuel cell. Unlike the battery the fuel cell has a continuous supply of fuel.
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.