Understanding Gravimetric and Volumetric Hydrogen Cryo-Adsorption Trade-Off in Metal-Organic Frameworks (MOFs) and its Link to Material Properties

With the recent release of hydrogen‑powered fuel cell vehicles that store hydrogen at 700 bar, alternatives are being explored to reduce storage pressure, including the use of an adsorbent-filled tank with operating adsorption/desorption cycles between 100 bar/77 K and 5 bar/160 K. Metal-organic frameworks (MOFs) are promising adsorbents for cryo-adsorbed hydrogen storage due to their highly tunable structure and chemistry, which can be potentially optimized to obtain the highest possible material performance. MOF volumetric and gravimetric hydrogen capacities are key factors that need to be maximized to attain a MOF-filled tank that is neither too large nor too heavy for practical application. Because the goals of maximizing MOF volumetric and gravimetric adsorption individually are incompatible, an in depth understanding of the trade-off between MOF volumetric and gravimetric loadings is necessary to achieve the best compromise between these two properties.

Here we first study, both experimentally and computationally, the trade-off between volumetric and gravimetric cryo-adsorbed hydrogen deliverable capacity in a set of well-known MOFs, including the highly stable zirconium MOFs, NU-1101, NU-1102, and NU-1103, among which we identified the best performers. On the grounds of excellent agreement between simulated and measured hydrogen isotherms, we use molecular simulation to elucidate the mechanism of hydrogen cryo-adsorption and its connection to materials properties such as volumetric and gravimetric surface areas, and pore volume and void fraction. We found that the higher variability in gravimetric deliverable capacity, in contrast to volumetric capacity, occurs due to the proportional relation between gravimetric surface area and pore volume in contrast to the inverse relation between volumetric surface area and void fractions. Using an “inverse molecular design” approach, we explored textural property combinations that could potentially allow attaining specific hydrogen adsorption targets.

Continuing with the inverse design philosophy, then we explored the impact of pronounced alterations in MOF chemistry on hydrogen cryo-adsorption performance. Specifically, we studied the introduction of “blanket” metal catecholate sites in sixty yet-to-be-synthesized MOFs based on a dozen topologies, which were constructed using a topologically-based crystal constructor (ToBaCCo) code. Using molecular simulation, we explored to what extent tuning the interaction strength between hydrogen and MOF metal catecholate sites could enhance volumetric and gravimetric hydrogen cryo-adsorption, depending on the operating conditions.

References:

1. DA Gómez-Gualdrón, TC Wang, P García-Holley, RM Sawelewa, E Argueta, RQ Snurr, JT Hupp, T Yildirim, OK Farha, ACS Appl. Mater. Interfaces, DOI:10.1021/acsami.7b01190