I've heard one somewhat far fetched theory that the zeroth energy level of the hydrogen atom - the one in which the electron orbit would have zero radius and which doesn't exist - might be split by the immense electric fields that exist inside crystals. This would create an orbit just outside the nucleus which, the theory goes, could be occupied by an electron. The energy released by an electron falling into this orbit would be very large by chemical reaction standards. Whether this could happen or not, the fact remains that you would either have to put energy back in to release the hydrogen from the metal or else replace the metal. Palladium is an expensive fuel!