Activated carbon (aC), compared to other high specific surface porous materials like zeolites, porous silica, metal organic frameworks (MOF), shows many advantages in view of large scale on-board applications: abundance and low cost of raw precursor (coal, coconut shells, woods, bamboo, cellulose and others materials at high carbon content), ease synthesis, stability and tunable pore structure. Commercial aCs exhibit a microporosity which makes them potential hydrogen sorbents. In this perspective, an important role is covered by the pore morphology as the best adsorption performance is observed on micropore (pore width < 20 Å) and ultra-micropore (pore width < 7 Å).
In this work, different standard techniques were performed to characterize the correlation between the structural parameters of aCs, and the hydrogen adsorption. In this regard, hydrogen adsorption isotherms in the range of 77 K up to room temperature were performed. Moreover, for structural and morphological investigation, nitrogen adsorption, enthalpy evaluation, scanning electron microscopy (SEM), and x-ray diffraction (XRD) analysis of the sample were implemented. In fact, some of these aCs showed large fraction of ultra-microporosity which leads to the very effective hydrogen uptake. Moreover, they showed an interesting phenomenon of hydrogen trapping at 77K. To understand the origin of this phenomenon, further complementary techniques were performed.
Probed aC samples show a pore structure ranging from the ultra-micropores up to the mesopore, with distributions varying from mostly microporous to mainly mesoporous. Comparing the pore distribution to the hydrogen adsorption proprieties three different adsorption regimes are found occurring respectively in pores of dimension below 7 Å (narrow pores), between 7 ÷13 Å (useful pores) and above 13 Å (wide pores). Wide pores are important for hydrogen storage at 77 K only when they contribute as the extension of the available surface area. Nevertheless, in view of adsorption ad higher temperatures for technological application, as on-board hydrogen storage, their contribute is negligible because the adsorption enthalpy energies (the average values is 4 ÷ 4.5 kJ/mol) are not strong enough to produce a dense adsorbate film. The effect of the potential overlap is important in narrow and useful pores with a typical dimension below 13 Å. In this case, the exposure to hydrogen molecules leads to adsorption in the pore volumes by micropore filling. Therefore, in this regime, the extension of the pore surface is not determinant as the micropore volume.
Hydrogen micropore filling is a general adsorption mechanism as result from the comparison of the values of hydrogen uptake in the pores with typical dimension up to 10 Å. The density of the hydrogen adsorbed by micropore filling at 77 K is estimated in 30 mmol/cm3, which is comparable to the liquid hydrogen density at 17.9 K.