High density hydrogen films adsorbed in engineered carbon nanospaces
Résumé
The search for sustainable automotive fuels has driven numerous studies of sorption-based hydrogen storage for hydrogen fuel cell vehicles. Storage by adsorption is fully reversible, achieves fast fill/discharge demands by simple pressurization/depressurization, and operates at much lower pressure than compressed hydrogen and at much less demanding temperatures than liquid hydrogen. In support of the DOE 2020 storage capacity target of 40 g hydrogen/L system, we have investigated the density of the adsorbed hydrogen films in a variety of porous carbons synthesized at the University of Missouri. The investigation decomposes storage into a high-density adsorbed film and low-density non-adsorbed gas and determines the fraction of pore volume occupied by the two phases.
We find exceptionally dense H2 films at liquid nitrogen temperature, 77 K. Saturated film densities are 100-120 g/L across all samples at pressures as low as 35-70 bar. This is 1.4-1.7 times the density of liquid hydrogen at its normal boiling point, 71 g/L (20 K). Experimental film thicknesses are 0.30-0.32 nm, and fractions of total pore volume filled with high-density film are 0.25-0.53. Thus high storage capacities, well in excess of the DOE target and even in excess of liquid hydrogen, can be achieved at 77 K in appropriately engineered nanoporous carbons.
The dense films occur at a temperature more than twice the liquid-gas critical temperature of hydrogen, 33 K, above which no bulk liquid exists at any pressure. The high-density film above 33 K does not contradict the non-existence of bulk liquid: the film is not a bulk, 3D phase, but a monomolecular 2D phase. Monte Carlo simulations confirm the observed high density and small film thickness. The film density and volume remain constant up to gas densities ~80% of the film density. A discussion in terms of competing forces acting on adsorbed molecules will be given.