Hydrogen adsorption at high pressure in carbon nanostructures: equilibrium and kinetic properties

H2 is recognized as an ideal energy carrier even though it is not yet commercially used. A major limitation in this direction is the development of an easy and efficient storage process. Scientific research is focusing its efforts in the synthesis and characterization of nanoporous materials for H2 storage. The use of these materials, instead of a H2 storage as a cryogenic liquid or a extremely compressed gas, would resolve the safety problems and reduce the costs of the H2 transport systems. The use of nano-structured materials is based on the use of physical interactions between adsorbent and adsorbate (van der Walls forces) to achieve an easy sorption and release of the gas. However, to reach the H2 density target values indicated by the Department of Energy (DOE) of the United States of America, which sets the gravimetric capacity of hydrogen as 6 weight%, we need the 'tuning' of porous materials (pores size and distribution, specific active surface area), the use of lightweight materials and the functionalization of the pore walls for an effective hydrogen adsorption and release.

 In this framework, the hydrogen adsorption properties of graphite-based materials have been investigated using a Sievert-type volumetric apparatus with which we collect the adsorption isotherms and the relative kinetic behavior. The investigated carbon-based samples have low (activated carbon and cellulose), medium (carbon nanotubes) or high (graphene) specific surface areas. The isotherms were recorded up to 8 MPa and at different temperatures (77 K and ambient temperature). The asymptotic adsorbed H2 amount seems to be mainly related to the specific surface of samples. However, in graphene-based samples at high pressures, we find a selective adsorption allowing the access to the internal surfaces. The results of repetitive adsorption/desorption cycles show the mechanical stability of these materials. Furthermore, different adsorption kinetics are observed according to the samples porosity. In particular, we observed much faster molecular hydrogen diffusion in graphene samples than in the carbon nanotubes and activated carbon/cellulose ones. The equilibrium adsorption properties and the relative kinetics are linked to the morphological and structural properties revealed through the Raman, SEM, XRD and BET techniques.