Manipulation of the crystalline phase diagram of hydrogen through nanoscale confinement effects in porous carbons.

Condensed phases of molecular hydrogen (H) are highly desired for clean energy applications ranging from hydrogen storage to nuclear fusion and superconductive energy storage. However, in bulk hydrogen, such dense phases typically only form at exceedingly low temperatures or extremely high (typically hundreds of GPa) pressures. Here, confinement of H within nanoporous materials is shown to significantly manipulate the hydrogen phase diagram leading to preferential stabilization of unusual crystalline H phases. Using pressure and temperature-dependent neutron scattering at pressures between 200-2000 bar (0.02-0.2 GPa) and temperatures between 10-77 K to map out the phase diagram of H when confined inside both meso- and microporous carbons, we conclusively demonstrate the preferential stabilisation of face-centred cubic (FCC) solid -H in microporous carbons, at temperatures five times higher than would be possible in bulk H. Through examination of nanoconfined H rotational dynamics, preferential adsorption and spin trapping of -H, as well as the loss of rotational energy and severe restriction of rotational degrees of freedom caused by the unique micropore environments, are shown to result in changes to phase behaviour. This work provides a general strategy for further manipulation of the H phase diagram nanoconfinement effects, and for tuning of anisotropic potential through control of confining material composition and pore size. This approach could potentially provide lower energy routes to the formation and study of more exotic non-equilibrium condensed phases of hydrogen that could be useful for a wide range of energy applications.