ABSTRACT
Therefore, it is important to emphasize that to achieve the system-level targets, the gravimetric and volumetric capacities of the storage material should be considerably higher than the established system-level improvements. In addition to weight, volume and cost, there are also several important DOE targets defined by vehicular requirements, such as hydrogen charging and discharging rates, durability, safety, and operability over temperatures and pressures.At present, the hydrogen on-board storage is realized mainly by the use of rather expensive composite vessels with molecular gaseous hydrogen under pressure of about 70-80 MPa. Obviously, these systems are not suitable in terms of costs and safety requirements [1-3]. For the last 10 years various storage strategies and technologies have been proposed and tested, but to date none of the approaches have fulfilled all of the DOE requirements and targets for either transportation or utility use. As concluded in a paper on the DOE National Hydrogen Storage Project [1], DOE agrees with the National Academies’ recent recommendation [2] that new concepts and ideas should be elicited, because success in overcoming the major drawbacks that are blocking the on-board storage is crucial for the future of fuel cells in transportation systems. The development of hydrogen-fueled vehicles and portable electronics will require new materials, and especially, nanomaterials that can store large amount of hydrogen at ambient temperature and at relatively low pressures (not higher than 15 MPa) providing safety, small volume, low weight, and fast kinetics for recharging.During the last decade graphite, carbon nanotubes, and nanofibers have been both theoretically and experimentally investigated as potential adsorbent structures (cf. reviews in Refs. [4, 5]). Even though some reports claim very high storage capacity, such findings have not been supported by the majority of investigators. However, recently hydrogen storage in nanostructured carbon has attracted renewed interest because of new developments in nanotechnology research and to the significant advantage in terms of light weight and of reasonably inexpensiveness of such materials.According to a physical-chemical analysis [5], the most appropriate technology for efficient hydrogen storage, that would satisfy the majority of DOE requirements [1], could be hydrogen multilayer intercalation in various carbonaceous nanostructures (Fig. 2.1). These structures would have physical and chemical
features suitable for the development of super-adsorbents to be used in fuel-cell-powered vehicles.In this case, the carbonaceous super-adsorbents (the carbon-based nanomaterials) could be stored into steel vessels at hydrogen pressure of about 10-15 MPa (100-150 atm). Such working conditions are much more acceptable with respect to both cost and safety requirements [1], in comparison with the composite vessels requiring high hydrogen pressure (700-800 atm). This technology could be used for an efficient hydrogen storage, which remains one of the main technical issues to be resolved to promote a clean, environmentally friendly hydrogen economy. It could also have positive fall out on other industries based on hydrogen generation from water splitting, using either nuclear or tidal energy.On the other hand, a number of researchers do not yet believe in the possibility of a hydrogen multilayer intercalation with carbonaceous nanostructures at ambient pressures and temperatures, even if it is out of physics and chemistry predictions.
