Thermodynamics and electronic structure evolution from single-H2 adsorption to stepwise hydrogen loading of Fe metalloporphyrin

Abstract

Designing molecular motifs that can bind hydrogen strongly enough for uptake yet weakly enough for deliverable release remains a central challenge for adsorptive storage. Here, we use dispersion-corrected density functional theory to map the complete hydrogen-loading pathway of an iron porphyrin (Fe-MP, Fe metalloporphyrin) scaffold from n = 1 to 20 H₂, connecting optimized configurations to adsorption energetics, thermodynamics and electronic-structure fingerprints. A clear hierarchy of adsorption environments emerges: the first H₂ binds at the Fe center with a short Fe⋯H₂ contact and an elongated H–H bond (1.65 Å and 0.80 Å, respectively), while subsequent H₂ molecules populate progressively weaker sites surrounding the macrocycle. This transition is reflected in thermodynamics, with the mean adsorption energy collapsing from −0.47 eV per H₂ at n = 1 to −0.03 eV per H₂ at n = 20, accompanied by a marked reduction in desorption temperature from 599 K to 42 K. Despite the increase in gravimetric capacity to an upper bound of 9.97 wt% at full loading, the high-coverage reservoir is therefore intrinsically weakly bound and most relevant under pressure-assisted and/or cryogenic conditions. Charge analysis and projected density of states reveal progressive polarization without disruptive changes to the host electronic backbone, with an essentially invariant frontier gap (5.88–5.93 eV) across loading. Real-space interaction fingerprints from RDG–sign(λ₂)ρ maps confirm a shift from localized attractive contributions at low coverage to dispersive confinement and steric crowding at high coverage. Together, these results separate capacity from usability in a chemically transparent adsorbent and provide a transferable design rule for computational materials discovery: high uptake must be accompanied by the multiplication of intermediate-strength binding motifs to move the storage manifold beyond a purely dispersion-dominated outer shell.