High-throughput, low-cost production chemistries for nanoporous carbons offer exciting opportunities for development of novel materials for on-board storage and reaction of hydrogen in fuel cells. When dispersed with Pd or Pt nanoclusters, single-wall carbon nanohorns (SWNHs) have shown exceptional promise as fuel cell catalysts. SWNHs are single-layer carbon cylinders 2-3 nm in diameter and ~50 nm in length, with cone-shaped end caps; the nanohorns coalesce to form spherical aggregates with interstitial surface area of ~300 m2/g. SWNHs can be produced at a rate (10-50 g/h), yield (70-80%) and purity (>98%) exceeding those of more costly single-wall carbon nanotubes. When oxidized at modest temperature (573-693 K), subnanoscale windows are opened on the nanohorn walls, increasing the surface area to ~1000 m2/g for small molecules (e.g. H2, O2) with access to the internal pore space. Deposition of Pd nanoclusters on SWNHs sharply increases hydrogen sorption capacity. Moreover, SWNH-supported Pd nanoclusters have higher intrinsic fuel cell reactivity than Pd dispersions on activated carbons or carbon blacks. In this paper, molecular simulation results are reported for interstitial and internal hydrogen adsorption in SWNHs at supercritical temperatures. The effect of nanowindow size on H2 adsorption is discussed.