Catalyzed alanates for hydrogen storage

The discovery that hydrogen can be reversibly absorbed and desorbed from complex hydrides (the alanates) by the addition of catalysts has created an entirely new prospect for lightweight hydrogen storage. Unlike the interstitial intermetallic hydrides, these compounds release hydrogen through a series of decomposition/recombination reactions e.g.: NaAlH{sub 4} {Leftrightarrow} 1/3Na{sub 3}AlH{sub 6} + 2/3Al + H{sub 2} {Leftrightarrow} NaH + Al + 3/2H{sub 2}. Initial work resulted in improved catalysts, advanced methods of preparation and a better understanding of the hydrogen absorption and desorption processes. Recent studies have clarified some of the fundamental material properties as well as the engineering characteristics of catalyst enhanced sodium alanate. Phase transitions observed real-time through in situ X-ray powder diffraction demonstrate that the decomposition reactions occur through long-range transport of metal species. SEM imaging and EDS analysis verify aluminum segregation to the surface of the material during decomposition. The equilibrium thermodynamics of decomposition have now been measured down to room temperature. They show a plateau pressure for the first reaction of 1 atm at 33 C, which suggest that, thermodynamically, this material is ideally suited to onboard hydrogen storage for fuel cell vehicles. Room temperature desorption with slow but measurable kinetics has been recorded for the first time. Studies at elevated temperatures (125-165 C), approaching that found in fuel cell operations, were performed on a scaled-up test bed. The bed demonstrated surprisingly good kinetics and other positive material properties. However, these studies also pointed to the need to develop new non-alkoxide based catalysts and doping methods to increase capacity and reduce the level of hydrocarbon impurities found in the desorbed hydrogen. For this reason, new Ti-Cl catalysts and doping processes are being developed which show higher capacities and improved kinetics. An overview of the current state-of-the-art will be presented along with our own studies and the implications for the viability of these materials in on-board hydrogen storage applications