title: Understanding Volumetric and Gravimetric Hydrogen Adsorption Trade-off in Metal-Organic Frameworks.
abstract: Metal-organic frameworks (MOFs) are porous crystalline materials that are promising for adsorption-based, on-board storage of hydrogen in fuel-cell vehicles. Volumetric and gravimetric hydrogen capacities are the key factors that determine the size and weight of the MOF-filled tank required to store a certain amount of hydrogen for reasonable driving range. Therefore, they must be optimized so the tank is neither too large nor too heavy. Because the goals of maximizing MOF volumetric and gravimetric hydrogen adsorption loadings 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 properties. Here we study, both experimentally and computationally, the trade-off between volumetric and gravimetric cryo-adsorbed hydrogen deliverable capacity by taking an isoreticular series of highly stable zirconium MOFs, NU-1101, NU-1102, and NU-1103 as a case study. These MOFs were studied under recently proposed operating conditions: 77 K/100 bar →160 K/5 bar. We found the difference between highest and lowest measured deliverable capacity in the MOF series to be ca. 40% gravimetrically, but only ca. 10% volumetrically. From our molecular simulation results, we found hydrogen "monolayer" adsorption to be proportional to the surface area, whereas hydrogen "pore filling" adsorption is proportional to the pore volume. Thus, we found that the higher variability in gravimetric deliverable capacity in contrast to the volumetric capacity, occurs due to the proportional relation between gravimetric surface area and pore volume in the NU-110x series in contrast to the inverse relation between volumetric surface area and void fraction. Additionally, we find better correlations with geometric surface areas than with BET areas. NU-1101 presents the highest measured volumetric performance with 46.6 g/L (9.1 wt %), whereas NU-1103 presents the highest gravimetric one with 12.6 wt % (43.2 g/L).
authors.affiliation: Department of Chemical and Biological Engineering, Colorado School of Mines , Golden, Colorado 80401, United States.
authors.forename: Diego A
authors.initials: DA
authors.lastname: Gómez-Gualdrón
authors.forename: Timothy C
authors.identifier: http://orcid.org/0000-0002-2736-2488
authors.initials: TC
authors.lastname: Wang
authors.forename: Paula
authors.initials: P
authors.lastname: García-Holley
authors.affiliation: Department of Chemical and Biological Engineering, Colorado School of Mines , Golden, Colorado 80401, United States.
authors.forename: Ruth M
authors.initials: RM
authors.lastname: Sawelewa
authors.forename: Edwin
authors.initials: E
authors.lastname: Argueta
authors.forename: Randall Q
authors.identifier: http://orcid.org/0000-0003-2925-9246
authors.initials: RQ
authors.lastname: Snurr
authors.forename: Joseph T
authors.identifier: http://orcid.org/0000-0003-3982-9812
authors.initials: JT
authors.lastname: Hupp
authors.affiliation: NIST center for Neutron Research, National Institute of Standards and Technology , Gaithersburg, Maryland 20899, United States.
authors.forename: Taner
authors.initials: T
authors.lastname: Yildirim
authors.affiliation: Department of Chemistry, King Abdulaziz University , Jeddah 22254, Saudi Arabia.
authors.forename: Omar K
authors.identifier: http://orcid.org/0000-0002-9904-9845
authors.initials: OK
authors.lastname: Farha
keywords: cryoadsorption; energy storage; molecular modeling; nnoporous materials; zirconium MOFs