Here we report direct physical evidence that confinement of molecular hydrogen (H2) in an optimized nanoporous carbon results in accumulation of hydrogen with characteristics commensurate with solid H2 at temperatures up to 67 K above the liquid-vapor critical temperature of bulk H2. This extreme densification is attributed to confinement of H2 molecules in the optimally sized micropores, and occurs at pressures as low as 0.02 MPa. The quantities of contained, solid-like H2 increased with pressure and were directly evaluated using in situ inelastic neutron scattering and confirmed by analysis of gas sorption isotherms. The demonstration of the existence of solid-like H2 challenges the existing assumption that supercritical hydrogen confined in nanopores has an upper limit of liquid H2 density. Thus, this insight offers opportunities for the development of more accurate models for the evaluation and design of nanoporous materials for high capacity adsorptive hydrogen storage.

Experimental hydrogen isotherms on several metal-organic frameworks (IRMOF-1, IRMOF-3, IRMOF-9, ZIF-7, ZIF-8, ZIF-9, ZIF-11, ZIF-12, ZIF-CoNIm, MIL-101 (Cr), NH2-MIL-101 (Cr), NH2-MIL-101 (Al), UiO-66, UiO-67 and HKUST-1) synthesized in-house and measured at 77 K and pressures up to 18 MPa are presented, along with N2 adsorption characterization. The experimental isotherms together with literature high pressure hydrogen data were analysed in order to search for relationships between structural properties of the materials and their hydrogen uptakes. The total hydrogen capacity of the materials was calculated from the excess adsorption assuming a constant density for the adsorbed hydrogen. The surface area, pore volumes and pore sizes of the materials were related to their maximum hydrogen excess and total hydrogen capacities. Results also show that ZIF-7 and ZIF-9 (SOD topology) have unusual hydrogen isotherm shapes at relatively low pressures, which is indicative of "breathing", a phase transition in which the pore space increases due to adsorption. This work presents novel correlations using the modelled total hydrogen capacities of several MOFs. These capacities are more practically relevant for energy storage applications than the measured excess hydrogen capacities. Thus, these structural correlations will be advantageous for the prediction of the properties a MOF will need in order to meet the US Department of Energy targets for the mass and volume capacities of on-board storage systems. Such design tools will allow hydrogen to be used as an energy vector for sustainable mobile applications such as transport, or for providing supplementary power to the grid in times of high demand.

BACKGROUND The effect of framework topology on the guest‐induced flexibility of several crystalline zeolitic imidazolate frameworks (ZIF‐7, ZIF‐9, ZIF‐11 and ZIF‐12) was investigated via analysis of experimental nitrogen (N2), carbon dioxide (CO2) and argon (Ar) isotherms at 77 K (N2 and Ar) and 273 K (CO2) for gas pressures up to 0.13 MPa. RESULTS The experimental isotherms of these frameworks were analysed in order to investigate their structural flexibility using gases with kinetic diameters equal to or larger than the diameters of their static pore apertures. The results of gas sorption measurements indicate guest‐induced phase changes for ZIF‐7 and ZIF‐9 (SOD topologies). ZIF‐12 (RHO topology) also shows uptake for gases, despite its pore‐limiting diameter being smaller than the kinetic diameters of the adsorbed molecules. CONCLUSIONS This work highlights the ability of ZIFs with different framework topologies to change their structure and increase their pore aperture upon interaction with certain gases. These findings are key in the development of more selective ZIF‐based materials for important industrial applications including low‐energy gas separations, catalytic nanoreactors and sensor technology. © 2019 The Authors. Journal of Chemical Technology & Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

A sound understanding of any sorption system requires an accurate determination of the enthalpy of adsorption. This is a fundamental thermodynamic quantity that can be determined from experimental sorption data and its correct calculation is extremely important for heat management in adsorptive gas storage applications. It is especially relevant for hydrogen storage, where porous adsorptive storage is regarded as a competing alternative to more mature storage methods such as liquid hydrogen and compressed gas. Among the most common methods to calculate isosteric enthalpies in the literature are the virial equation and the Clausius-Clapeyron equation. Both methods have drawbacks, for example, the arbitrary number of terms in the virial equation and the assumption of ideal gas behaviour in the Clausius-Clapeyron equation. Although some researchers have calculated isosteric enthalpies of adsorption using excess amounts adsorbed, it is arguably more relevant to applications and may also be more thermodynamically consistent to use absolute amounts adsorbed, since the Gibbs excess is a partition, not a thermodynamic phase. In this paper the isosteric enthalpies of adsorption are calculated using the virial, Clausius-Clapeyron and Clapeyron equations from hydrogen sorption data for two materialsactivated carbon AX-21 and metal-organic framework MIL-101. It is shown for these two example materials that the Clausius-Clapeyron equation can only be used at low coverage, since hydrogen's behaviour deviates from ideal at high pressures. The use of the virial equation for isosteric enthalpies is shown to require care, since it is highly dependent on selecting an appropriate number of parameters. A systematic study on the use of different parameters for the virial was performed and it was shown that, for the AX-21 case, the Clausius-Clapeyron seems to give better approximations to the exact isosteric enthalpies calculated using the Clapeyron equation than the virial equation with 10 variable parameters.

Experimental results for methane adsorption on two high-surface area carbons (TE7-20 and AX-21) and one metal-organic framework (MIL-101(Cr)) are presented, with isotherms obtained at temperatures ranging from 250 to 350 K and at pressures up to 15 MPa. The isotherms were analysed to determine if these materials could be viable alternatives for on-board solid-state storage of methane. The results show a very high adsorbate density in the pores of all materials, which for some can even exceed liquid methane density. At moderate pressures below 5 MPa, the calculated total energy densities are close to the energy density of methanol, and are almost 40 % of the energy density of gasoline (petrol). Compared with standard compression at the same conditions, the results show that adsorption can be a competitive storage alternative, as it can offer equal volumetric capacities at much lower pressures, hence reducing the energy penalty associated with compression. It is shown that the optimal conditions for adsorptive methane storage in these materials are at moderate pressure ranges, where the gains in amounts stored when using an adsorbent are more pronounced when compared to cylinders of compressed methane gas at the same operating conditions. Finally, a study on deliverable capacities for adsorbed methane was carried out, simulating two charging pressure scenarios of 3.5 and 6.5 MPa and discharge at 0.5 MPa. The results show that some of the tested materials have high working volumetric capacities, with some materials displaying more than 140 kg m-3 volumetric working capacity for charging at 6.5 MPa and delivery at 0.5 MPa.