Structure–property relationships in metal-organic frameworks for hydrogen storage

Abstract

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.

Flexible ZIFs: probing guest‐induced flexibility with CO2, N2 and Ar adsorption

“…The CO 2 adsorption isotherms of both ZIF‐7 and ZIF‐9 (Figs and ) show a sudden CO 2 uptake between 10 and 20 kPa. ZIF‐7 and ZIF‐9 both show hysteresis in their desorption isotherms, and a step that is compatible with a sorbate‐induced gate‐opening phenomenon, a feature that has been widely reported in the literature . Regarding the N 2 or Ar isotherms of ZIF‐7 and ZIF‐9 in Figs and (both with similar structural dimensions), these barely show any adsorption, which can be due to the gases' high kinetic diameters (0.364 and 0.354 nm, respectively, for N 2 and Ar) compared to their pore window size (0.29 nm) .…”

“…This reversible transition also was assigned to the flexibility of the benzimidazole linkers, which enable a large‐pore phase that can allow molecules of 0.52 nm diameter to enter the main cavities. In addition to CO 2 and CH 4 , other molecules have been shown to induce a phase‐change in ZIFs – in our past work we have shown this effect for hydrogen (H 2 ) adsorption in ZIF‐7 and ZIF‐9, and other reports showed phase changes upon adsorption of ethane, ethylene, propane, propene, butane and butene in ZIF‐7 . Other ZIF materials have been investigated for the phase transition seen in ZIF‐7 using CO 2 adsorption, such as ZIF‐11, ZIF‐93 and ZIF‐94, a group of ZIFs that combine RHO and SOD topology, but the only ZIF material that showed a phase transition was ZIF‐7 .…”

“…Measurements were done in flat plate mode at 298 K on wet samples (methanol for ZIF‐12, and ethanol for ZIF‐7, ZIF‐9 and ZIF‐11), with the solvents used in several cycles to wash the samples. Experimental powder XRD spectra were compared against the original CIF files obtained from the Cambridge Crystallographic Database as well as the literature, in order to ensure the correct synthesis of the materials, details of which can be found in our previous work . The XRD spectra for ZIF‐7 and ZIF‐9 are shown in Fig.…”

“…We previously investigated the stability of the synthesized materials in order to determine degassing conditions . These were performed on a Setaram TGA 92 16.18 (Setaram, Caluire, France), with the materials heated at 5 K min −1 from 293 to 873 K in flowing, dry N 2 at 1 bar.…”

“…Other ZIF materials have been investigated for the phase transition seen in ZIF‐7 using CO 2 adsorption, such as ZIF‐11, ZIF‐93 and ZIF‐94, a group of ZIFs that combine RHO and SOD topology, but the only ZIF material that showed a phase transition was ZIF‐7 . In our previous work, we showed that ZIF‐9 (SOD topology), which only differs from ZIF‐7 in the nature of the metal cluster (Co rather than Zn), also showed the same features as ZIF‐7 upon the adsorption of hydrogen at 77 K and at 1 MPa . To our knowledge, no CO 2 adsorption has been measured on this material.…”