(326c) Phase Behavior and Dynamics of Hydrogen Adsorbed In Activated Carbon Nanopores: A Neutron Scattering Investigation

We studied the phase behavior and dynamics of hydrogen adsorbed on activated carbon derived from polyfurfuryl alcohol (PFAC) using in-situ small angle neutron scattering (SANS) and quasi-elastic neutron scattering (QENS). In-situ SANS was performed at the general purpose SANS (GP-SANS) instrument of the High Flux Isotope Reactor (HIFR) at Oak Ridge National Laboratory (ORNL). Hydrogen was adsorbed on PFAC at the ambient temperature and pressures ranging from 5 to 200 bar. The SANS instrument has been modified to allow data acquisition over a broad momentum transfer range (0.01 Å^-1 < Q < 0.9 Å^-1) which provided information on hydrogen adsorbed in nanopores with sizes as small as 9 Å. The results revealed that the density of adsorbed hydrogen was reaching the density liquid hydrogen at 200 bar pressure and room temperature in the narrowest pores accessed by our measurements. This extreme densification resulted in an enormous increase of the internal pressure of hydrogen adsorbed in 9 Å pores, up to 1600 bar in equilibrium with an external pressure of ~200 bar. Both density and internal pressure were the highest in the narrowest pores and decreased with the increase in pore width. The QENS study was performed at the backscattering spectrometer (BASIS) of the Spallation Neutron Source (SNS) at ORNL. In the temperature range from 10 K to 37 K, we measured the broadening of elastic peak of scattered neutrons caused by inelastic collisions with H₂ molecules. At temperatures below 20 K, the carbon surface acts as a contact catalyst which converts ortho hydrogen to para hydrogen. Analysis of QENS patterns revealed that hydrogen confined in narrow nanopores has very low mobility below 19-22 K. At higher temperatures the mobility increases rapidly and translational diffusion follows the fixed length jump model. However the melting” temperature of solid state of hydrogen is elevated by 8 – 10 K compared to bulk hydrogen as a result of extreme confinement.

Research sponsored by the Materials Sciences and Engineering Division, and by the Scientific User Facilities Division, Office of Basic Energy Sciences, U. S. Department of Energy.