Adsorption-Based Hydrogen Storage in Activated Carbons and Model Carbon Structures

The experimental data on hydrogen adsorption on five nanoporous activated carbons (ACs) of various origins measured over the temperature range of 303-363 K and pressures up to 20 MPa were compared with the predictions of hydrogen density in the slit-like pores of model carbon structures calculated by the Dubinin theory of volume filling of micropores. The highest amount of adsorbed hydrogen was found for the AC sample (ACS) prepared from a polymer mixture by KOH thermochemical activation, characterized by a biporous structure: 11.0 mmol/g at 16 MPa and 303 K. The greatest volumetric capacity over the entire range of temperature and pressure was demonstrated by the densest carbon adsorbent prepared from silicon carbide. The calculations of hydrogen density in the slit-like model pores revealed that the optimal hydrogen storage depended on the pore size, temperature, and pressure. The hydrogen adsorption capacity of the model structures exceeded the US Department of Energy (DOE) target value of 6.5 wt.% starting from 200 K and 20 MPa, whereas the most efficient carbon adsorbent ACS could achieve 7.5 wt.% only at extremely low temperatures. The initial differential molar isosteric heats of hydrogen adsorption in the studied activated carbons were in the range of 2.8-14 kJ/mol and varied during adsorption in a manner specific for each adsorbent.