Metallic hydrogen is metastable could be used as superlightweight structural material for floating cities

Metallic hydrogen has been created in a diamond anvil in a Harvard lab.

Diamond anvil cells can use only vanishingly small sample sizes. A typical amount is about 160 cubic micrometers

If metallic hydrogen is metastable then there are a lot of potential applications.

Metastable would mean that the phases could retain their high-pressure forms for an indefinite period once external forces are removed, much as diamonds formed by high temperatures and pressures deep inside Earth remain diamonds even after they reach the surface, instead of immediately reverting to carbon’s more stable form, graphite. Nellis and others have imagined a host of applications for metastable metallic hydrogen, ranging from

* super-lightweight structural materials that would allow entire cities floating on the sea to be built

* to rocket fuel that packs nearly four times as much propellant power per kilogram as the liquid hydrogen used in the most powerful rockets today

Hydrogen sulphide becomes a superconductor at the surprisingly high temperature of 203 K (–70 °C), when under a pressure of 1.5 million bar. This suggests that the theoretical predictions of room temperature superconductivity for metallic hydrogen are correct.

Lithium Hydrogen metals

In 2009, hydrogen-rich metallic compounds, such as LiH2, LiH6 and LiH8 became stable under high pressure. Many of their properties would be similar to those of the long-sought metallic hydrogen, but conditions of synthesis were readily achieved in the lab. A study showed a way to prepare metallic almost-hydrogen for possible practical use.

PNAS – A little bit of lithium does a lot for hydrogen. From detailed assessments of electronic structure, they found that a combination of significantly quantal elements, six of seven atoms being hydrogen, becomes a stable metal at a pressure approximately 1/4 of that required to metalize pure hydrogen itself. The system, LiH6 (and other LiHn), may well have extensions beyond the constituent lithium. These hypothetical materials demonstrate that nontraditional stoichiometries can considerably expand the view of chemical combination under moderate pressure.

Metastable Metal Hydrides

Similar to ammonia borane, the metastable metal hydrides offer high capacities and low temperature hydrogen release. However, unlike ammonia borane, thermal decomposition does not result in the formation of gaseous species that can degrade capacity and poison the fuel cell. Of particular interest are the aluminum-based metastable hydrides including aluminum hydride (AlH3), lithium aluminum hydride (LiAlH4 and Li3AlH6), magnesium aluminum hydride (Mg(AlH4)2), among others. These materials exhibit a low decomposition enthalpy, which reduces the heat required to release the hydrogen at practical pressures. In addition, these materials exhibit rapid H2 evolution rates at low temperature (80–100°C), due to the large driving force for decomposition.