Determination of saturated film densities and volumes for adsorbed hydrogen, and application to the calculation of the enthalpy of adsorption at room temperature

Abstract

The development of high-performance materials for hydrogen storage by physical adsorption requires understanding of detailed microscopic properties of the adsorbed film. In this work, we show that adsorbed hydrogen films in activated carbons have saturation densities [about]100 g/L, well in excess of liquid hydrogen, and remarkably independent of sample characteristics and temperature (a property of the adsorbate only). We propose a reliable method to determine the volume of the adsorbed film at cryogenic or room temperatures by extrapolation of the low-coverage adsorption isotherms using the Ono-Kondo model and the saturation film density as a fixed point. Remarkably, film volumes are only [about]40% or [about]12% of the total pore volume at 77 K and 296 K, respectively (the reduction of pore volume with temperature is explained in terms of population of adsorption sites of different depths). By reliably determining the film volume, absolute adsorption isotherms for an activated carbon are calculated at 273 K and 296 K and used with the Clausius-Clapeyron relation to obtain the enthalpy of adsorption (8.3 kJ/mol, within 1.2% agreement of the low-coverage cryogenic determination for the same adsorbent, in concordance with the fact that at high temperatures deeper adsorption sites are dominant). This methodology should facilitate reliable calculations of the enthalpy of adsorption for room temperatures for weakly adsorbing gases. This report presents the first investigation of a 5.3-liter tank, filled with 2.86 kg of University of Missouri monolithic carbon, under operation at 23 [degree]C (room temperature), 0 [degree]C (ice bath), and -79 [degree]C (dry-ice bath) and pressures 0-100 bar. Storage and fast charge/discharge data, including temperature and pressure profiles as a function of time are reported. These storage data agree within 2% of small-scale measurements on the commercial instrument, Hiden HTP1-V. Remarkably, the tank can be filled in 3-5 minutes, the DOE target for 2020 and later. The table below shows that there is a large temperature rise and drop, of 30-50 [degree]C, during filling and discharging even at room temperature. These are unexpectedly large temperature excursions because binding of H[2] on carbon, with a typical heat of adsorption of 5.0 kJ/mol (H[2] on graphite), is considered weak and a source of major temperature excursions only at liquid nitrogen temperature. It shows, as expected, that equilibration is faster at high temperature than at low temperature.