The Development of a Fullerene Based Hydrogen Storage System

This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). The project objective was to evaluate hydrogen uptake by fullerene substrates and to probe the potential of the hydrogedfullerene system for hydrogen fuel storage. As part of this project, we have completed and tested a fully automated, computer controlled system for measuring hydrogen uptake that is capable of handling both a vacuum of 1 x 10-6 torr and pressures greater than 200 bars. We have fist established conditions for significant uptake of hydrogen by fullerenes. Subsequently, hydrogenation and dehydrogenation of pure and catalyst-doped C60 was further studied to probe suitability for hydrogen storage applications. C600H18.7 was prepared at 100 bar H2 and 400°C, corresponding to hydrogen uptake of 2.6 wt % . Dehydrogenation of C600H18.7 was studied using thermogravimetric and powder x-ray diffraction analysis. The C600H18.7 molecule was found to be stable up to 430°C in Ar, at which point the release of hydrogen took place simultaneously with the collapse of the fullerene structure. X-ray diffraction analysis performed on C600H18.7 samples dehydrogenated at 454"C, 475"C, and 600°C showed an increasing voiume fraction of amorphous material due to randomly oriented, single-layer graphine sheets. Evolved gas analysis using gas chromatography and mass spectroscopy confirmed the presence of both H, and methane upon dehydrogenation, indicating decomposition of the fullerene. The remaining carbon could not be re-hydrogenated. These results provide the fxst complete evidence for the irreversible nature of fullerene hydrogenation and for limitations imposed on the hydrogenatioddehydrogenation cycle by the limited thermal stability of the molecular crystal of fullerene. Background and Research Objectives The acute need for an effective mode of hydrogen storage has been very clear to our team, having been involved for more than ten years in polymer electrolyte (PEM) fuel cell R&D for transportation applications. One daunting engineering challenge that remains to be solved in the context of transportation applications of PEM-type fuel cells is effective "Principal Investigator, e-mail: [email protected] hydrogen storage, preferably in solid state and at weight percent exceeding the 1% level offered by metal hydrides. The other features required include reversibility of charge / discharge cycles and relatively low temperatures for such cycles of hydrogen uptake / release so that the waste heat from the fuel cell can be effectively tapped for hydrogen release. Prototype hydrogen fuel cell vehicles unveiled recently all use hydrogen gas stored in highly pressurized ( 3000-5000 psig ) gas cylinders -the only way known at present to achieve storage levels of 5% hydrogen by weight. Safety concerns would generate high barriers for introduction of such pressurized cylinders to replace fuel tanks in passenger vehicles. Known and much safer hydrogen storage instruments, such as metal hydrides, are associated with energy densities that are too low for transportation applications and, hence, with a limited range per single refueling for a vehicle fueled by hydrogen and powered by a fuel cell vs. gasoline fueled vehicles powered by internal combustion engines. Unavailability of a solid, reversible mode of hydrogen storage at weight percent hydrogen well in excess of 1 % is perhaps the most important reason for having to resort to on-board liquid fuel processing for generation of hydrogen-rich gas mixtures. Such hydrogen rich mixtures generated on board have served as fuel feed stream to the fuel cell stack in all demonstrations (or disclosures) made recently by the automotive industry -a clear indication of the failure to date to develop an effective mode of hydrogen storage. graphite nanotubes and nanofibers, up to 300 weight percent, have stirred strong renewed interest in using various forms of carbon as substrate for hydrogen storage ( I ) . Previous to this recent work, suggestions have been made that C,, and C,, fullerene carbons could serve as substrates for hydrogenatioddehydrogenation. Preparation of C,,*H, (x=2, 18, 36) molecules have been reported by a variety of methods which include Birch reduction (2), direct hydrogenation of the solid phase (3-5), aqueous electrochemical (6) and organic chemical (7). However, there have been no complete reports regarding the reversibility of the process, specifically reports on conditions for dehydrogenation of hydrogenated fullerenes. The exception have been some claims of reversibility of hydrogen uptake by fullerenes, made particularly by one industrial laboratory (8). The purpose of this work has been to examine the suitability of fullerenes as hydrogen storage medium, particularly for potential transportation applications. We have completed and tested, as part of this project, a fully automated, computer controlled system for measuring hydrogen uptake that is capable of handling both a vacuum of 1 x and pressures greater than 200 bars. We have established conditions for significant uptake of hydrogen by fullerenes. During the last year of the project, hydrogenation and dehydrogenation of pure and catalyst-doped C , was further studied to probe suitability for Recent (still controversial) claims that suggest high hydrogen storage capacity in