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

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.