Abstract: A general model for hydrogen adsorption is derived to discuss the compressibility of the hydrogen molecules on the adsorption surface caused by the strong adsorptive potential exerted by the carbon wall. The mono-layer capacity (see Table 1) inside the tube of the MWCNTs tested in our laboratory is evalued by the comparison studies on the adsorptive performance between the tubular pore and the slit-pore, the results show that better adsorptive performance can only be obtained when temperature is getting lower or the diameter of the tube is getting smaller (see Fig.1). From the model, the strong attraction to the surface may even cause hydrogen molecules to attain much higher densities than that of liquid hydrogen, and this will surely make adsorbed hydrogen molecules to behave as repulsion among each other. This phenomenon is discussed in terms of experimental data and the ascertained model (see Fig.3). A linear form of the model is applied to determine the interaction energy between hydrogen molecules in the adsorbed layer from the experimental data. Analysis of the experimental data of hydrogen adsorption over a wide range of pressure and temperature shows that the energies of hydrogen-hydrogen interactions in the adsorbed phase are positive in low temperature region(< 200 K) and obtain the highest value in the temperature range of 160～180 K, but will be negative when the temperature is above 230 K(see Fig.4), indicating the repulsions among the adsorbed hydrogen molecules are prominent in low temperature, however, the attractions among the adsorbed hydrogen molecules at higher temperature are hard to understand because the pressures are also high in those cases. Comments are made that the chemical adsorption should be included to interpret the experimental data at higher temperatures. The results presented here also show that the concept of ‘maximum capacity’ for hydrogen adsorption should differ from the density of the adsorbed hydrogen molecules.

Key words: Hydrogen storage, Adsorption capacity, Multi-walled carbon nanotubes (MWCNTs)