Most known MOFs are included in a larger database of known organic crystalline materials called the Cambridge Structure Database (CSD). The database contains more than 600,000 entries, most of which aren’t MOFs and aren’t relevant for hydrogen storage. However a vast catalog of existing MOFs does reside within the CSD; many of the gas uptake properties of these had not yet been assessed.

Goldsmith and his colleagues employed data mining and automated structure analysis to identify, to “cleanup,” and to predict rapidly the hydrogen storage properties of these compounds. The process and the results were described in an open access paper published in the ACS journal Chemistry of Materials in 2013.

Approximately 20,000 candidate compounds were generated from the CSD using an algorithm that removes solvent/guest molecules. These compounds were then characterized with respect to their surface area and porosity. Employing the empirical relationship between excess H₂ uptake and surface area (Chahine rule), the U-M researchers then predicted the theoretical total hydrogen storage capacity for the subset of ∼4,000 compounds exhibiting nontrivial internal porosity.

Theoretical total (adsorbed + gas phase H2 at 77 K and 35 bar) volumetric and gravimetric density of stored H2 in ∼4000 MOFs mined from the CSD. The data account only for the mass and volume of the MOF media; mass and volume contributions from the system are neglected. For comparison, the region bounded by the dashed lines represents the DOE 2017 targets for H2 storage systems. Crossed circles represent common MOFs with incomplete or disordered crystal data in the CSD; structures for these compounds were constructed by hand. Credit: ACS, Goldsmith et al. Click to enlarge.

Our screening identifies several overlooked compounds having high theoretical capacities; these compounds are suggested as targets of opportunity for additional experimental characterization. More importantly, screening reveals that the relationship between gravimetric and volumetric H₂ density is concave downward, with maximal volumetric performance occurring for surface areas of 3100–4800 m²/g. We conclude that H₂ storage in MOFs will not benefit from further improvements in surface area alone. Rather, discovery efforts should aim to achieve moderate mass densities and surface areas simultaneously, while ensuring framework stability upon solvent removal.

—Goldsmith et al.

The several known, yet overlooked compounds recommended for further investigation had high hydrogen storage densities exceeding 10 wt % (g H₂/g MOF basis) and 58 g/L (total H₂, at 77K and 35 bar).

Crystal structures, CSD identifiers, and MOF names (if known) for four of the top performing MOFs identified by screening. Credit: ACS, Goldsmith et al. Click to enlarge.

The next phase of the project will be funded by the DOE Fuel Cell Technologies office and aims to synthesize and more completely characterize the promising MOFs identified earlier. The group aims to explore MOFs that, at least on paper, could meet the DOE’s 2017 targets for hydrogen storage systems.

The exploratory research was initiated with the aid of a $40,000 seed grant from the U-M Energy Institute’s Partnerships for Innovation in Sustainable Energy Technology program. Additional support came from the DOE Hydrogen Storage Engineering Center of Excellence, which has sponsored Siegel’s hydrogen-related research since 2009.

Resources

• U-M MOF Database

• Jacob Goldsmith, Antek G. Wong-Foy, Michael J. Cafarella, and Donald J. Siegel (2013) “Theoretical Limits of Hydrogen Storage in Metal–Organic Frameworks: Opportunities and Trade-Offs” Chemistry of Materials 25 (16), 3373-3382 doi: 10.1021/cm401978e