In the preceding article we looked at industrial commodities whose supply is inherently limited—once they’re used up, that’s it. There are other critical substances, though, with constraints of a different kind. Hydrogen, for instance, is the most abundant chemical in the universe, and there’s no danger of running out. The problem is that isolating hydrogen from the compounds that contain it, including water, is both expensive and dependent on huge inputs of energy. And speaking of…

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