Analysis of adsorption equilibrium of hydrogen on graphene sheets

Abstract

The adsorption equilibrium of hydrogen on graphene sheets (GS) was studied based on a sample of GS with SBET = 300 m2/g at the temperatures of 77.15 K–293.15 K and the pressures of 0 MPa–6 MPa. In the meantime, the adsorptions (Excess adsorption measurements) of hydrogen on granular coconut shell SAC-02 activated carbon (SBET = 2074 m2/g) and carbon nanofiber (CNFs, SBET = 205 m2/g) were investigated at the pressures of 0–8 MPa and the temperature of 77.15 K. The outcomes from experiments were used to determine the parameters in Toth equation by way of Non-linear fit. The absolute adsorption amounts of hydrogen on the GS, which were calculated from the equation, were used to calculate the isosteric heat of hydrogen adsorption by use of adsorption isosteres.It shows that, under the experimental conditions, the excess adsorption amount of hydrogen on the GS increases monotonically and correlatively as pressure increases, and the mean relative deviation between the experimental data and those predicted from Toth equation is less than 1%. The result also shows that the storage density of hydrogen on the GS is 1.75 wt% and 0.168 gH2/L at pressure 5.4 MPa and temperature 77.15 K, which is lower than that of hydrogen on the activated carbon but is higher than that of hydrogen on the CNFs. The isosteric heat of hydrogen adsorption on the GS falls within 5.14 kJ/mol–6.37 kJ/mol, which is comparable to that of hydrogen adsorption on the activated carbon. It suggests that interaction between hydrogen molecules and the graphene layer is stronger than that between hydrogen and carbon surface, and the hydrogen storage capacity of GS is closely related to its physical properties.