Physisorption, chemisorption and spill-over contributions to hydrogen storage

Abstract

Activated carbons (ACs), derived from resorcinol–formaldehyde xerogels and activated with KOH, were doped with nickel in supercritical carbon dioxide. Five Ni-doped ACs were thus prepared with nickel contents ranging from 6.6 to 31.9 wt.% and Ni nanoparticles diameters from 19 to 37 nm. Nitrogen and carbon dioxide adsorption allowed a complete textural characterisation; surface area decreased from 1933, for non-doped AC, to 917 m2 g−1 for the most highly doped AC. Such a surface decrease was explained by increase of materials density but also by progressive pore blocking. However, the average micropore size was found to be constant, around 1.3 nm, ensuring similar specific physisorption properties at the surface of all materials. Hydrogen adsorption isotherms were obtained up to 10 MPa at 298 K with a volumetric device. Physisorbed, chemisorbed and spilt-over contributions to hydrogen storage were calculated and allowed concluding that the fraction of hydrogen engaged in spill-over is around 25% at 298 K and 0.5 MPa for the AC doped with 31.9 wt.%Ni.