Manipulation of the Crystalline Phase Diagram of Hydrogen through Nanoscale Confinement Effects in Porous Carbons

Condensed phases of molecular hydrogen (H2) 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 H2 within nanoporous materials is shown to significantly manipulate the hydrogen phase diagram leading to preferential stabilization of unusual crystalline H2 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 H2 when confined inside both meso- and microporous carbons, we conclusively demonstrate the preferential stabilisation of face-centred cubic (FCC) solid ortho-H2 in microporous carbons, at temperatures five times higher than would be possible in bulk H2. Through examination of nanoconfined H2 rotational dynamics, preferential adsorption and spin trapping of ortho-H2, 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 H2 phase diagram via 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.

a Calculated from Rouqerol BET method applied to N 2 adsorption data at 77 K for 0.007 < P/P 0 < 0.05 b Calculated from Rouqerol BET method applied to N 2 adsorption data at 77 K for 0.05 < P/P 0 < 0.3 c Micropore surface area calculated from the statistical t-method using carbon black STSA model thickness range 3. exhibiting a certain degree of amorphicity compared to graphite [7] , offset in the ydirection, for clarity.When comparing the molar volumes of the nanoconfined crystal phases to the bulk solids at similar temperatures (~10 K) and pressures (~200 and 2000 bar) [1][2][3] .In general, a larger volume was found for the confined phases.For HCP H 2 confined in the highly microporous TE7 at 200 bar and 2 kbar, the molar volume was 8% and 13 % larger than the equivalent bulk, respectively, and HCP-H 2 confined in mesoporous OLC at 2 kbar, having molar volumes 22% larger than the calculated bulk.
For the observed FCC H 2 phase, the results were compared to the known unit cell molar volumes observed by Mills et al [1,2] , while noting that these were recorded under significantly different pressure and temperature conditions (1.3 K, 1 bar) -as the FCC H 2 phase has not been observed at 10 K before.
The results show that FCC H 2 confined in microporous TE7 at 200 bar, 10 K has a 3% larger molar volume than that observed in bulk FCC H 2 (1.3 K, 1 bar).Noting that the molar volume of bulk HCP decreases by ~24% with increasing pressure, it is reasonable to assume that the molar volume of FCC H 2 would follow a similar trend, thus FCC H 2 in TE7 at 2 kbar is likely still larger than the bulk FCC H 2 molar volume at the same pressure, if it formed at this temperature in the bulk.
The comparatively lower density than the bulk may be a function of the extremely high pressure used in this experiment.Similar results are seen for CO 2 sequestered in coal, whereby above a threshold pressure, the density of the confined phase becomes less than the bulk. [4]This may relate to the "break-even" threshold pressure observed in H 2 storage materials [5] , and the flexibility of the porous substrate used.Lower pressure diffraction studies are required to confirm this hypothesis.

Figure S3 .
Figure S3.Temperature-dependent neutron powder diffraction for microporous TE7 dosed to 200 bar H 2 at 77 K. Neutron diffraction data from detector bank 5 (a) and bank 3 (b) of the GEM diffractometer, respectively.Stars represent reflections for a solid H 2 HCP phase, filled squares represent reflections for a solid H 2 FCC phase.Reflections from the aluminium cell are shaded in grey and patterns offset in the ydirection for clarity.Dotted lines are added as a guide to the eye to show the HCP unit cell getting smaller with increasing temperature (negative thermal expansion).

Figure S4 .
Figure S4.Temperature-dependent neutron powder diffraction data for microporous TE7 dosed to 2 kbar H 2 at 77 K. Neutron diffraction data from detector bank 5 (a) and bank 3 (b) of the GEM diffractometer, respectively.Stars represent reflections for a solid H 2 HCP phase, filled squares represent reflections for a solid H 2 FCC phase.Reflections from the aluminium cell are shaded in grey.Patterns offset in the ydirection, for clarity.Dotted lines are added as a guide to the eye to show insignificant change to unit cell dimensions with temperature.

Figure S5 .
Figure S5.Temperature-dependent elastic neutron diffraction data for mesoporous OLC dosed to 2 kbar H 2 at 77 K. Neutron diffraction data from detector bank 5 (a) and bank 3 (b) of the GEM diffractometer, respectively.Stars represent reflections for a solid H 2 HCP phase.Reflections from the aluminium cell are shaded in grey.Patterns offset in the y-direction, for clarity.Dotted lines are added as a guide to the eye to show unit cell dimension show insignificant change with temperature.

Figure S7 :
Figure S7: Diffraction of H 2 adsorbed in microporous TE7 at 2 kbar, 77 K plotted in momentum transfer (Q) units showing oscillations in the background signal.If diffuse scattering was coming from short-range ordering of hydrogen molecules, the background oscillations should match at the same Q value.

Figure S8 .
Figure S8.STEM EDX analysis of microporous TE7 after acid wash pretreatment.Revealing no trace metals or residual chlorine in the sample.Copper, silicon and oxygen were observed due to the copper TEM grid, silicon oxide grease contamination in the STEM and adsorbed water in the hygroscopic pores from sample transfer in air.

Table S4 . Refined unit cell dimensions for HCP and FCC H 2 solids confined in TE7 and OLC porous carbons across the range of pressures and temperature observed. Full Rietveld analysis was conducted using TOPAS v6-Academic software.
a HCP unit cell formula units Z = 2 b FCC unit cell formula units Z = 4

Table S5 . Refined unit cell dimensions for confined HCP and FCC H 2 solids observed via neutron diffraction at 10 K in this experiment compared to unit cell parameters taken from references [1-3] .
a HCP unit cell formula units Z = 2 b FCC unit cell formula units Z = 4

Table S6 . Rotational peak parameters from the J = 0 →1 and J = 1 →0 transitions and the calculated mean squared displacements of each individual peak.
a Peak