Two competing storage mechanisms run side by side on the same nanoporous scaffold. Physisorption (cyan) is a weak van-der-Waals attraction: intact H2 molecules stick to the outer pore surface without breaking apart. It needs almost no activation energy, so it happens fast even in the cold — but that same weak bond means raising the temperature shakes molecules loose again almost as fast. Chemisorption (orange) only happens where an H2 molecule lands on a catalytic metal nanoparticle (the larger gold spheres). There it can dissociate into two separate H atoms, which then diffuse into interstitial sites deep inside the lattice — exactly how a metal hydride forms. That dissociation step has a real activation barrier, so it only turns on once temperature climbs past a threshold; but the resulting metal-hydrogen bond is strong, so those atoms stay locked in place until temperature climbs much further still.
P(physisorb) ~ (1 − T/Tmax) · pressure
P(chemisorb) ~ max(0, T − Tact)/(Tmax − Tact) · pressure · catalyst_load
P(desorb_phys) ~ T/Tmax (fast, weak bond)
P(release_chem) ~ only if T > Trelease (slow, strong bond)
- Temperature — low temperature favors fast, reversible physisorption; only high temperature unlocks the chemisorption pathway, and only very high temperature releases it again.
- H2 pressure — sets how often gas molecules reach the surface at all; both pathways compete for the same flux.
- Catalyst loading — how much of the scaffold surface is coated with dissociation-active metal nanoparticles; more catalyst means more chemisorption capacity, but does not affect physisorption.
Real materials trade off exactly this: carbon nanotubes and MOFs physisorb H2 reversibly but only reach ~1-2 wt% at cryogenic temperature, while metal hydrides chemisorb H atoms into their bulk lattice for 5-7 wt% capacity at the cost of needing heat to release the fuel again.