Metallic Hydrogen can also be a liquid and even a gas, the metallic part indicates it's an electric conductor, not a solid.
The "idea" behind metallic hydrogen is that since hydrogen is above the alkali metals in the periodic tables there should be a phase where it behaves like a metal, the state of the phase is not defined as a solid/gas/liquid in fact in all honesty it would exist as all 3 and more.
The first production of metallic oxygen was a liquid, this is the first solid one, tho at these pressures it's really unlikely that it would effective as fuel store.
One of the more interesting parts about metallic hydrogen is the information we learn also applies to hydrides (hydrogen alloys) especially things like Lithium-Hydrogen since basically you can stabilize a large amount of hydrogen atoms (6 or more) around a single atom of Lithium, this might actually be very interesting for a lot of applications including energy storage.
Hydrides should be considerably more stable and more importantly at considerably lower pressures which would mean that they can actually be used for various applications since you are not going to find something that only can exist at 400 gPA levels of pressure or higher very useful for high stress applications.
This is the principle behind atomic hydrogen welding. An electric arc converts the hydrogen to atomic form, which recombines to molecules when it hits the surface. The hydrogen is also strongly reducing so you don't need flux. It still has some niche uses but it's not used much now because of risk of hydrogen embrittlement.
Technically it's degenerate matter since the atoms do not interact due to the pressure and other conditions, this happens because the electrons would have to follow the exclusion principle under those pressures/densities the electrons basically separate from the hydrogen proton as they cannot occupy the same space and the hydrogen becomes a degenerate matter; a sea of electrons flowing freely through a lattice of protons, because of this none of the "atoms" can combine into molecules, but they aren't technically atoms at that point either.
The problem with hydrogen as fuel storage (mainly for space travel) is the pressure, operating a fuel tank at 400 gPA or higher is going to be tricky, I've edited my previous post and added the bit about Hydrides which might actually considerably more viable for both terrestrial and space applications.
If we can actually make LiH6 alloys and can find an easy way to break them apart it can be a great fuel source, yes we might have to carry some "dead weight" but Li has an atomic number of 3 Oxygen has an atomic number of 8, considering that Hydrogen-Oxygen reaction is pretty much 1 to 1 (well "2") we have to carry considerably more Oxygen now than we would have to carry Lithium.
If a Hydrogen-Hydrogen reaction could be harvested as a propellant it would still be more effective than what we have even if we don't find any clever ways of catalyzing the lithium we already carry to push the reaction even further, we might be able to carry a smaller amount of oxygen also to catalyze the lithium tho I'm not sure if the energy from that reaction would be worth the added weight of the oxygen and the parts of the fuel and propulsion systems that would support it and the O-Li process.
P.S.
Metallic hydrogen is also of "interest" to those who still study cold fusion, even tho you can't call it cold fusion any more due to the stigma, at certain conditions there is a high likelihood of Inertial Confinement Fusion and Pycno Nuclear Fusion occurring, this often brought up as "superconductor fusion" simply because you can't call anything cold fusion and get it published.
That might also be a quite interesting fuel source, especially if you simply need to push it to a certain limit and once the fusion kicks in it would transition to other fusion stages.
And lastly this might be especially interesting if we can make Li-Deuterium alloys since D-Li fusion produces Helium 4 and over 20 MeV of energy :)
This (last) type of fusion is extremely interesting because not only it's highly energetic but it's aneutronic, which means that less than 1% of the energy is released via neutrons vs 80-90% in other forums of fusion, aneutronic fusion produces tons of energy an pretty much no ionizing radiation which means if you can make it work it would be in theory a safe enough battery to power a cellphone without frying your brain.
"Moreover, SMH is predicted to be metastable so that it may exist at room temperature when the pressure is released (10). If so, and superconducting, it could have an important impact on mankind’s energy problems and would revolutionize rocketry as a powerful rocket propellant (11)."
Whether or not MH is metastable in the real universe, it -- or at least some form of it -- is metastable in Alastair Reynolds "Revalation Space" universe, and there ohh boy is it explosive.
