Storage Capacity of Metal−Organic and Covalent−Organic Frameworks by Hydrogen Spillover

We determine the saturation storage density for hydrogen on several metal−organic framework (MOF) and covalent−organic framework (COF) materials by spillover. We use density functional theory on periodic frameworks to achieve reliable and accurate predictions for these materials. We find that one hydrogen can be stored at each C atom of the linker, and an additional H for each CO2 group. For IRMOF-1 and IRMOF-8, we find reasonable agreement with the experimental results. For other materials, such as COF-1 and MOF-177, we find that the experiments could be dramatically improved. We also predict the gravimetric and volumetric storage densities for several new materials, including IRMOF-9, IRMOF-993, MIL-101, PCN-14, COF-1, and COF-5, which appear very promising. We find gravimetric storage densities up to 5.5 wt % and volumetric storage densities up to 44 g/L. ■ INTRODUCTION Hydrogen storage remains one of the main challenges in the implementation of a hydrogen-based energy economy. Although several different approaches are being pursued, sorption onto a porous high-surface-area material is one contender. There is great interest in finding porous solid materials that can store hydrogen for use in fuel cell vehicles. Ideally, these materials would adsorb large amounts of hydrogen gas reproducibly at room temperature and moderate pressure. Recent experiments using the spillover method are operating at room temperature and are approaching the realworld 2010 gravimetric targets as set by the U.S. Department of Energy for potential use in fuel cell cars. The spillover process works using nanoscale metal catalysts distributed through the porous substrate material to break the molecular hydrogen gas into physisorbed atomic hydrogen. The atomic H then diffuses across and chemisorbs to the substrate. As H covers the nearby surface area, further H diffuses across the saturated areas and spills over onto remaining areas. The best results have been on substrates based on metal−organic framework materials. An important open question remains: how to design improved substrates for hydrogen spillover. The sample preparation for these hydrogen spillover experiments is quite complex, and there has been significant scatter in the experimental results. Without clear and accurate predictions for saturation storage capacities, it has been difficult to evaluate the experimental results. For this and other reasons, it has also been difficult to improve the early results. In this paper, accurate predictions for saturation storage density at room temperature for several metal−organic frameworks (MOFs) and covalent−organic frameworks (COFs) will be made using quantum chemistry calculations of binding energies for individual and multiple hydrogen atoms on full periodic models of the crystals. Instead of estimates based on surface area, we count specific binding sites on the crystal surface. This work shows that many of the experimental results are a factor of 3 below theoretical predictions. Therefore, these materials require further improvement in sample preparation in order to achieve their full hydrogen storage potential. These MOF and COF materials are relatively easy to fabricate, porous, and lightweight and have extremely high surface areas. Hydrogen storage in general in MOF materials was recently reviewed by Murray et al. Spillover has been a topic of investigation for many years; we will concentrate only on hydrogen storage by spillover onto COF and MOF materials. Hydrogen storage by spillover has been reviewed by Wang and Yang. Li and Yang found that IRMOF-8 with bridged Pt catalysts can reversibly store 4 wt % hydrogen at room temperature and 100 bar pressure by spillover. Ingeniously, they used commercial Pt catalysts, mounted on amorphous carbon substrates (Pt/AC). These were then mixed with IRMOF-8 crystals, and sucrose, and annealed to form amorphous carbon bridges between the Pt/AC and the IRMOF-8. Yang’s group has also tested several other high-surface-area substrates, but none of these have yet reached the storage capacity of bridged IRMOF-8. Recently, Yang’s group has published the details of the bridging methods and tested variations. Independently, Tsao et al. have extended the IRMOF-8 result to 4.7 wt % in equilibrium at 70 bar. This is the largest value to date for hydrogen storage by spillover at room temperature and is close to the 2010 DOE gravimetric storage targets of 6 wt %. One issue for practical use of the IRMOF materials is that many of them are sensitive to water contamination. Wang and Received: November 11, 2011 Revised: January 10, 2012 Published: January 12, 2012 Article