The line that Bron spat out sounded like some PR shit, what with making it from seawater. But storing hydrogen inside of a crystal isn't that far off. That's one of the main areas of research for metal-organic frameworks (MOFs). MOFs have incredibly high surface areas (+1000 m^(2) / g), and their linkers can be tuned to preferentially attract certain gasses. That being said, this is usually done under incredibly high pressure. Furthermore, MOFs can be very sensitive to shock, causing void collapses and loss of surface area. This could explain the hesitation lionel had about it, but they never get into exact detail in the story about what it is, so take this as just one sci-fi plausible explanation.

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I found it incredibly on point; an idiot billionaire who thinks he's an expert disregards the expertise of leading scientists to rush a dangerous product to market.