Hydrogen is the most common element in the universe, but no so much on Earth, not in its elemental state. The only viable source is the oceans, but you need energy to crack water, and you won't get it all back by burning it. There's a possibility of using genetically engineered bacteria to crack it enzymatically, but really the best use of hydrogen is controlled nuclear fusion. Though the current versions of fusion require the rarer isotopes of hydrogen: Deuterium (Hydrogen-2) and Tritium (Hydrogen-3). Some of which can be acquired as products of other nuclear processes (breeding Tritium from Lithium, for example) but would probably be best acquired from the moon.
Also Tritium is radioactive and, like Hydrogen, very difficult to contain. The Deuterium-Tritium fuel cycle has 100 times the neutron flux as current fission power reactors, which is a problem for material design. The free neutrons produced by many of the proposed fusion paths are liable to alter the atoms of the reactor structure itself, changing them into radioactive isotopes. Certainly, the first series of D-T tests at the Joint European Torus left the vacuum vessel sufficiently radioactive to warrant remote handling for the following year. This wasn't contamination by radioactive materials like what you get when exposed to fissionable materials and waste, but the metal itself had been changed.
Nuclear reactors are focuses of true alchemy.
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