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In response to the DOE’s quest for sorption-based hydrogen storage materials that meet the 2017 volumetric storage capacity target of 40 g hydrogen/L systemfor light-duty fuel cell vehicles, we have undertaken a large-scale experimental investigation of intrapore hydrogen densities—total mass of hydrogen stored pervolume of pore space—in high-surface-area carbons with variable nanopore structure. Intrapore density includes the adsorbed film and non-adsorbed gas, andconsequently lies between the density of bulk hydrogen gas and the density of the adsorbed film at the respective pressure and temperature. Intrapore densitiesup to 85 g/L were observed at 77 K and 200 bar, well in excess of 71 g/L, the density of liquid hydrogen at 20 K and 1 bar, and well above the liquid-gas criticalpoint of hydrogen, 33 K (“supercritical condensation”). Densities of the adsorbed film at saturation were found to vary between 100 and 120 g/L, which is over50% higher than in liquid hydrogen. These are exceptionally high film densities. They are consistent with the finding that, even in carbons with pore widthsexclusively centered at 7 Å (“ultramicroporous carbon”), the ratio of the saturated film volume to total pore volume is only between 0.5 and 0.6, i.e., that the filmdoes not fill the pores. We established that the saturated film is a liquid by demonstrating that the film has a constant volume as a function of gas pressure over awide pressure range (incompressible fluid). From the experimental film volume and surface area, we infer that the saturated film has a thickness of 3.0-3.5 Å at 77K and thus amounts to a monolayer, consistent with that supercritical adsorption does not allow for multilayer films. Implications for experimental determination ofenthalpies of adsorption (isosteric heats) will be presented.Samples have exceptionally low void fractions (0.49-0.70), maxima of gravimetric excess adsorption at exceptionally low pressures (~25 bar), and exceptionallyhigh binding energies (8-10 kJ/mol) consistent with deep potential wells in narrow, 7-Å pores. The resulting saturated film densities outperform previous saturatedhydrogen film densities, 51-69 g/L, in metal-organic frameworks at 50-55 K, by as much as 20% at a 40-50% higher temperature. The exceptionally high filmdensities in our investigation are attributed to the strong hydrogen-carbon adsorption potential in narrow pores. Grand canonical Monte Carlo simulations ofadsorption in slit-shaped pores of variable width (variable binding energy) will be presented to examine the dependence of the film density as a function of bindingenergy and compare computed and experimental film densities, and likewise intrapore densities. In slit-shaped pores, the adsorbed film may be viewed astwo-dimensional liquid with a critical temperature way above 33 K, while in the direction perpendicular to the graphitic planes the film remains a supercriticalmonolayer with critical temperature of 33 K.This material is based on work supported by the U.S. Department of Energy under contracts DE-FG36-08GO18142 and DE-FG02-07ER46411.