Computational Optimization of Room Temperature Usable Capacity for Hydrogen Storage in MFU-4-Type Metal–Organic Frameworks via Pairwise Metal Substitutions

The
efficient storage of hydrogen is a critical challenge in the
quest for sustainable energy solutions. Current adsorbent-based methods
achieve satisfactory storage densities predominantly under cryogenic
temperatures and/or high pressures, which imposes problems with cost-efficient
and safe implementation of this technology. Materials that can bind
hydrogen gas reversibly at ambient temperatures and more moderate
pressures could play a pivotal role in enabling hydrogen-powered technologies.
In this study, we use reliable computational modeling to investigate
two synthetically feasible paths for tuning the enthalpy of H2 binding in MFU-4-type metal–organic frameworks (MOFs),
aiming to maximize usable capacity. This study examines MIM4IICl3(bta)6 (bta– = benzotriazolate) Kuratowski-type clusters as a model
for strong binding sites in MFU-4l frameworks. We
systematically evaluate the impact of separately tuning the central
MII metal ion (which plays a structural role) and the peripheral
MI metal ion (which binds the substrate) on the energetics
of H2 binding. Our computational study reveals that H2 binding at an MI site mostly follows the trend
AgI < CuI < NiI < CoI < AuI while a larger central MII site generally weakens the H2 binding at a MI site. Importantly, we have identified three new combinations of
MI and MII to achieve high fractional usable
capacities of the total H2 adsorbed under a pressure swing
from 5 to 100 bar at room temperature. Additionally, we examine the
nature of the binding interaction between the peripheral metal atom
and the hydrogen molecule. While charge transfer predominantly induces
this interaction, for several atom combinations, a change in the polarization
(associated with variations in the ionic radius of the MI binding atom) is another important factor for adjusting the strength
of the interaction. We suggest that the proposed compositions of Kuratowski-type
clusters are highly desirable synthetic targets for future laboratory
study.