In this answer there is discussion of some research on the potential, future use of a solid metallic hydrogen phase that would be metastable at ambient pressure as a rocket propellant. The work is futuristic of course, but there is interest.
There is more information in the following questions and their answers:
...and the BBC news article Claim made for hydrogen 'wonder material', quoted extensively in the second question.
Is the thinking is that it would be used as a monopropellant? Is it possible that the much heavier LOX needed for a conventional LH2/LOX engine would not even be needed?