| The increased utilization of proton-exchange membrane (PEM) fuel cells as an alternative to internal combustion engines is expected to increase the demand for hydrogen, which is used as the energy source in these systems. The objective of this work is to develop and test new methods for the storage and production of hydrogen for fuel cells.; Six ligand-stabilized hydrides were synthesized and tested as hydrogen storage media for use in portable fuel cells. These novel compounds are more stable than classical hydrides (e.g., NaBH4, LiAlH4) and react to release hydrogen less exothermically upon hydrolysis with water. Three of the compounds produced hydrogen in high yield (88 to 100 percent of the theoretical) and at significantly lower temperatures than those required for the hydrolysis of NaBH4 and LiAlH4. However, a large excess of water and acid were required to completely wet the hydride and keep the pH of the reaction medium neutral. The hydrolysis of the classical hydrides with steam can overcome these limitations. This reaction was studied in a flow reactor and the results indicate that classical hydrides can be hydrolyzed with steam in high yields at low temperatures (110 to 123°C) and in the absence of acid. Although excess steam was required, the pH of the condensed steam was neutral. Consequently, steam could be recycled back to the reactor.; Production of hydrogen for large-scale transportation fuel cells is primarily achieved via the steam reforming, partial oxidation or autothermal reforming of natural gas or the steam reforming of methanol. However, in all of these processes CO is a by-product that must be subsequently removed because the Pt-based electrocatalyst used in the fuel cells is poisoned by its presence. The direct cracking of methane over a Ni/SiO2 catalyst can produce CO-free hydrogen. In addition to hydrogen, filamentous carbon is also produced. This material accumulates on the catalyst and eventually deactivates it. The Ni/SiO2 catalyst can be repeatedly regenerated with steam for at least 10 successive cracking/regeneration cycles with no loss of activity during repeated cycles. Kinetic studies of the gasification of filamentous carbon indicate that several independent catalytic reactions take place during this process. These include the reactions of filamentous carbon with steam and with the CO2 produced in the previous step, as well as the reverse water-gas shift and methanation reactions. Each of these reactions was studied independently to determine the effect of the operating conditions (i.e., temperature, space velocity and gas composition) on its rate. Of particular interest is the effect of these parameters on the rate of formation of CO.; A mathematical model was developed based on the proposed sequence of independent reactions. Kinetic parameters extracted from the study of the independent reactions were used in this model. Furthermore, two kinetic rate constants for reactions that could not be independently studied were used as adjustable parameters. An excellent agreement was achieved between model predictions and experimental results obtained at different temperatures, space velocities and steam concentration. |
