There are some really interesting chemical methods of creating hydrogen gas using like, aluminum and sodium ( or potassium? ) in water that might be able to allow for on demand gas generation with otherwise solid/condensed fuel sources;
it's "simple" enough of a concept that creators[1] are doing it to power their own systems, though I imagine scaling it up carries its own set of difficulties ( e.g., using aluminum nanoparticles for greater surface area to more rapidly generate gas, but keeping it cool enough to avoid problematic side effects, or like, you know, exploding in general )
Edit: it looks like MIT is actually actively working[2] on this type of clean hydrogen production from the viewpoint of creating a scalable system!
it's "simple" enough of a concept that creators[1] are doing it to power their own systems, though I imagine scaling it up carries its own set of difficulties ( e.g., using aluminum nanoparticles for greater surface area to more rapidly generate gas, but keeping it cool enough to avoid problematic side effects, or like, you know, exploding in general )
Edit: it looks like MIT is actually actively working[2] on this type of clean hydrogen production from the viewpoint of creating a scalable system!
1: https://youtu.be/LKfbZvpoQ0g?t=5m23s
2: https://energy.mit.edu/news/using-aluminum-and-water-to-make...