I don't know everything about chemistry, but I can't think of why you think hydrogen would be cold. Perhaps you're thinking of refrigerated liquid nitrogen? Or maybe you're thinking of dry-ice, frozen CO2. In either of these cases, the medium has to be frozen first, so you don't 'save' any energy, you just have to do the refrigeration first.

But you could do the refrigeration part with grid-power instead of gasoline-- that would be some benefit. Grids run far more efficiently than engines.