NASM--Apollo to the Moon--Fuel Cells

Both Gemini and Apollo spacecraft obtained electrical power from hydrogen-oxygen fuel cells.
A fuel cell is like a battery. It converts energy released in a chemical reaction directly to electrical power. Unlike a storage battery, a fuel cell continues to supply current as long as chemical reactants are available or replenished (even while the cell is operating).
For space applications, fuel cells have another advantage over conventional batteries: they produce several times as much energy per equivalent unit of weight.
When oxygen and hydrogen combine to form water, energy is released because the electrons in the water molecule are in a lower energy state than those in the gas molecules. In a combustion reaction, as in a rocket engine, the energy appears as heat. In a fuel cell some of it --about 50-60%-- is converted directly to electrical energy.
As fuel cells operate, oxygen and hydrogen combine to produce water as well as electrical power. Apollo crews used this water for drinking.
The fuel cell models displayed here contain many individual fuel cells along with the plumbing and sensors required to supply reactants and keep the cell at the proper temperature. The reactants were stored in separate tanks in liquid form to reduce space. This required keeping the oxygen at -173°C (-280°F) and at a pressure of 63.26 kilograms per square centimeter (245 pounds per square inch). Waste heat from the fuel cells was used to bring the reactants to gaseous form before they entered the cell. The Apollo fuel cell operated at a temperature of about 206°C (400°F) and the Gemini cell at about 65°C (150°F).

Apollo Fuel Cell Section