Conclusions

The results obtained at 298 K for carbon materials indicate that the hydrogen adsorption capacities in gravimetric basis up to 20 MPa depend on both micropore volume and micropore size distribution. The results obtained with carbon nanofibers and nanotubes fit into the tendencies obtained with the other type of carbon materials, indicating that hydrogen adsorption on these materials is also taking place by a physisorption process. At 298 K and 20 MPa, the sample with the best hydrogen adsorption capacity is an activated carbon, which has both high micropore volume and narrow micropore size distribution.

Hydrogen adsorption capacity in gravimetric basis at 77 K and at pressures up to 4 MPa follows a good correlation with the total micropore volume for all the samples, including carbon nanofibres and carbon nano-tubes. These results indicate that hydrogen adsorption depends on the porosity of the sample and does not depend on its morphology. The results obtained for the series of carbon materials analyzed point out that, at these adsorption conditions, micropore size distribution does not play an important role, contrary to what happened at room temperature.

From an application point of view, not only the gravimetric target has to be achieved but also the volumetric one. For this reason it is necessary to know the packing density of the materials. The packing density of the carbon materials decreases with increasing the porosity development. Therefore, the hydrogen adsorption capacity, expressed per unit of volume, goes through a maximum. Thus, the samples with the highest porosity do not present the best volumetric performance, but the samples with a relatively high porosity and packing density give the best values. Therefore, a good balance between porosity development and packing density is needed.

The highest total hydrogen (adsorbed plus compressed) stored at room temperature and 20 MPa is 16.7 g H2/L (3.2 wt.%). The best value of total hydrogen storage capacity obtained in the present work at 77 K and 4 MPa was 38.8 g H2/L (8 wt.%). These high values obtained remark the importance of developing materials with suitable porosity and high packing density for this application.

It has been shown that highly activated carbons have a great potential as carriers of hydrogen. On the other hand, carbon nanotubes and carbon nano-fibers are not promising storage media for hydrogen from an application point of view, contrary to what is concluded from many results published in the literature.

0 0

Post a comment