Different approach to estimation of hydrogen-binding energy in nanospace-engineered activated carbons

Abstract

Binding energy between adsorbent and adsorbate strongly affects the mechanism of adsorption. Porous systems are usually characterized by a distribution of this energy, which is not easy to determine experimentally. A coupled experimental-simulation procedure to estimate binding energy directly from experimental adsorption isotherms is proposed. This new approach combines experimental information (pore size distribution determined from nitrogen adsorption at 77 K) and numerical data (grand canonical Monte Carlo simulations of adsorption in pores) to explain an influence of binding energy on adsorption isotherms. The procedure has been validated by analysis of hydrogen adsorption in a series of carbons activated with KOH:C ratio varying from 3 to 6. These carbons show high capacity of hydrogen storage both at 80 and 303 K (115 gH2/kgC and 23 gH2/kgC at p = 100 bar, respectively, for carbon activated during 1 h at T = 790 C (T = 1361 K) with KOH:C ratio equal to 3, having the surface area above 2600 m2/g, 0.77 porosity, and large fraction (31%) of pores with average width below 1 nm). An additional energetic parameter has been introduced into the conventional fitting procedure to account for the distribution of adsorption energy in measured samples. The observed high consistency between experimental and simulated results validates/correlates the characterization procedures and proves the coherence and robustness of both the experimental results and the numerical simulations.