Critical Factors in Computational Characterization of Hydrogen Storage in Metal–Organic Frameworks

Inconsistencies
in high-pressure H2 adsorption data
and a lack of comparative experiment–theory studies have made
the evaluation of both new and existing metal–organic frameworks
(MOFs) challenging in the context of hydrogen storage applications.
In this work, we performed grand canonical Monte Carlo (GCMC) simulations
in nearly 500 experimentally refined MOF structures to examine the
variance in simulation results because of the equation of state, H2 potential, and the effect of density functional theory structural
optimization. We find that hydrogen capacity at 77 K and 100 bar,
as well as hydrogen 100-to-5 bar deliverable capacity, is correlated
more strongly with the MOF pore volume than with the MOF surface area
(the latter correlation is known as the Chahine’s rule). The
tested methodologies provide consistent rankings of materials. In
addition, four prototypical MOFs (MOF-74, CuBTC, ZIF-8, and MOF-5)
with a range of surface areas, pore structures, and surface chemistries,
representative of promising adsorbents for hydrogen storage, are evaluated
in detail with both GCMC simulations and experimental measurements.
Simulations with a three-site classical potential for H2 agree best with our experimental data except in the case of MOF-5,
in which H2 adsorption is best replicated with a five-site
potential. However, for the purpose of ranking materials, these two
choices for H2 potential make little difference. More significantly,
100 bar loading estimates based on more accurate equations of state
for the vapor–liquid equilibrium yield the best comparisons
with the experiment.