Since the beginning of human space missions, hydrogen–oxygen
fuel cells have been used for electric energy production onboard
spacecraft because of their high specific energy storage
capabilities compared to batteries. In the Gemini space program
(1962–1965) the acidic ion exchange membrane IEM fuel cell was
used, providing an average power of 620 W and a maximum
power of 1 kW. The Apollo mission to the moon (1968–1972) used
alkaline fuel cells to supply electric energy. Three modules supplied
a maximum power of 1.5 kW. Alkaline fuel cells remained
the main energy source in the Space Shuttle. One may note that
the specific power of the fuel cells has shown major improvement
(about 4 fold) of specific power from Apollo to the Space Shuttle,
from about 14–23 W/kg to 63–100 W/kg, respectively