(519c) Metal-Organic Frameworks with High Volumetric Hydrogen Storage Capacities

Current fuel cell vehicles utilize gaseous hydrogen compressed to 700 bar in expensive and bulky pressure vessels. An alternative hydrogen storage concept is to pack high-surface area adsorbents into low pressure tanks (7 times lower) which are held at cryogenic temperatures. Many adsorbent materials have been well studied to evaluate their potential storage capability ranging from activated carbon to the highly tunable metal-organic framework (MOF) materials. The metal-organic framework, Zn₄O(BDC)₃ (known as MOF-5 or IRMOF-1), is still considered as a benchmark material for hydrogen adsorbents due to its ability to adsorb hydrogen at both high gravimetric and volumetric capacities. However, further improvements to the hydrogen storage capacities of adsorbents are required in order to match 700 bar compressed storage. In particular, the design and testing of sub-scale cryo-adsorbent systems based on MOF-5 has revealed that volumetric capacity (i.e., the mass of hydrogen stored within a given volume) is a key material property of adsorbents which currently limits system performance. While the gravimetric hydrogen storage capacities of recently-synthesized MOFs now exceed 9 wt.% (with BET surface areas exceeding 6000 m²/g), the low crystal densities of these MOFs limit or negate any corresponding increase in volumetric capacity. Inefficient packing of low-density MOF powders within sorbent beds further erodes volumetric capacity. This work focuses on developing MOFs with volumetric hydrogen storage capacities higher than that of MOF-5. We have applied a computational screening process to databases of existing MOF crystal structures in order to identify candidates with excellent volumetric capacity. Our high-throughput calculations have effectively predicted hydrogen volumetric capacities at cryogenic temperatures by employing grand canonical Monte Carlo simulations with a pseudo-Feynman-Hibbs interatomic potential for H2-host interactions. We have synthesized the top candidates from the screening, and have found a small set of MOFs which match or exceed the MOF-5 benchmark for volumetric capacity. At the same time, many of the candidates identified from the computational screening have failed to exhibit the high surface areas and hydrogen uptakes after synthesis, a result we attribute to structural instability and pore collapse. With our successful material candidates, we have applied system models to estimate the volumetric capacities and other parameters for comparison with 700 bar compressed hydrogen storage at a full scale system level.