Hydrogen adsorption capacity

Fast adsorption/desorption kinetics and relatively small (<10 kj/mol) adsorption enthalpies are observed for hydrogen adsorption on many porous materials, which indicates that physisorption on porous materials is suitable for fast recharging with hydrogen [81,82], The narrowest pores make the biggest contribution to hydrogen-adsorption capacity, whereas mesopores contribute to total pore volume, but little to hydrogen capacity, and are detrimental for the overall volumetric capacity. Hence, porous materials with very narrow pores or pore-size distributions are required for enhanced hydrogen capacity at low pressures. 

Contrary to what happened in the case of the hydrogen adsorption capacity at 20 MPa, if hydrogen adsorption capacity at 50 MPa is plotted versus the narrow micropore volume, Vnjip (CO2), (see Figure 2(b)), a very high dispersion of the results is obtained. 

As it has been discussed in previous section, from a practical point of view1, and also for comparison between adsorbents, hydrogen adsorption capacities should be reported in a volumetric basis, which makes necessary to know the sample density. Unfortunately, papers reporting hydrogen adsorption capacities of MOFs in volumetric basis use the crystal density of the materials, which is not realistic for this application because it does not include the inter-particle space. Crystal densities of MOFs can vary between 0.2 and 1.3 g cm 3 36 39, and similar to what happens with tap and packing densities of carbon materials, crystal densities of MOFs decreases when porosity increases. Therefore, as in the case of carbon materials (see Figure 5) a maximum is observed when the hydrogen uptake in volumetric basis is plotted versus the porosity of the MOFs samples, and a compromise between density and porosity is necessary from a practical point of view. 

Effect of Sulfur on the Hydrogen Adsorption Capacity and the Hydrogen Binding Energy of Platinum- and Iridium-Supported Catalysts... 

To date, Ref. [65] provides probably the most reliable data on a series of chemically activated carbons and also on other types of carbon materials, such as activated carbon fibers, CNTs, and CNFs. The best values measured for adsorption capacity at 298 K were 1.2 and 2.7 wt% at 20 and 50MPa, respectively. At 77K, the hydrogen adsorption capacity reached 5.6wt% at 4MPa. Such values demonstrate that nanoporous carbons are not worst than other kinds of materials studied at the moment for hydrogen storage. 

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