Hydrogen Storage using Physisorption : Modified Carbon Nanofibers and Related Materials

Abstract

This thesis describes our research on adsorbent systems for hydrogen storage for small scale, mobile application. Hydrogen storage is a key element in the change-over from the less efficient and polluting internal combustion engine to the pollution-free operating hydrogen fuel cell. In general, hydrogen can be stored pressurized, liquefied, absorbed in metals and physisorbed on a suitable material (adsorbent). Our analysis was that storage by physisorption might be the most promising option. We therefore decided to focus our efforts on a further exploration of this alternative. Probably, storage by physisorption has the highest energy efficiency, even when such a system has to be kept at a low temperature of 77 K. Uptake and release of hydrogen can be fast and easily effectuated with relatively small pressure and/or temperature changes. Because the non-specificity of the interaction the amount of adsorbed hydrogen will be mainly related to the specific surface, notably to the pore structure and pore diameter. This opens the opportunity to choose a material, abundantly available, not toxic by itself and safe when used. We concentrated on various carbon materials, notably on carbon nanofibers (CNF).

The work described in this thesis has provided valuable information about the demands on hydrogen adsorbents and the structure and modification of carbon nanofibers. The results contribute to a better understanding of the material demands we place on hydrogen adsorbents. Furthermore we have gained a more fundamental understanding of the structure of carbon nanofibers and the possibilities and impossibillities of their modification. This study describes the importance of the presence of small micropores for high hydrogen adsorption capacities of adsorbents. The possibility to synthesize large, homogeneous batches of carbon nanofibers using a fluidized bed system has been described. It has been shown that fishbone, as well as parallel carbon nanofibers contain a large amount of defects, on the surface of the fiber, as well as inside the fibers. Furthermore it was shown that heat treatments in an inert atmosphere at 2000°C increase the aromatic character of the surface of the fibers, but have no influence on the amount and nature of the defects inside the fibers. It was shown that the intercalation of potassium in carbon nanofibers did significantly increase the hydrogen storage capacity of this material. This is however not enough to meet the DOE target. Additionally, the resulting material is not very suitable for vehicular use, because it is very sensitive to air. The atomically dispersed potassium reacts violently when contacted with air. This is not a very desirable product to use in vehicles, where there is always the possibility of collision and penetration of the tanks. Finally it has been described that the intercalation with FeCl3 in carbon nanofibers did not proceed and intercalation of FeCl3 in graphite did not enlarge the hydrogen storage capacity of the resulting material. It was also shown that subsequent treatment of the intercalated graphite to produce FeCl2 intercalated graphite, did not enlarge the hydrogen storage capacity of the resulting material.