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C.M. Jensen; K.J. Gross


The dehydriding of sodium aluminum hydride, NaAlH(4), is kinetically enhanced and rendered reversible in the solid state upon doping with selected titanium compounds. Following the initial reports of this catalytic ef feet, further kinetic improvement and stabilization of the cyclable hydrogen capacity have been achieved upon variation in the method of the introduction of titanium and particle-size reduction. Rapid evolution of 4.0-wt%2hydrogen at 100 degreesC has been consistently achieved for several dehydriding/rehydriding cycles. An improved, 4.8-wt%2cyclable capacity has been observed in the material doped with a combination of Ti and Zr alkoxide complexes. Doping the hydride with Ti(OBu(n))(4) and Fe(OEt)(2) also produces a synergistic effect, resulting in materials that can be rehydrided to 4 wt%2at 104 degreesC and 87 atm of hydrogen within 17h. The improved kinetics allowed us to carry out constant-temperature. equilibrium-pressure studies of NaAlH(4) that extended to temperatures well below the melting point of the hydride. The 37-kJ/mol value determined for enthalpy of the dehydriding of NaAlH(4)(s) to Na(3)AlH(6) and Al and the hydrogen plateau pressure of 7 arm at 80 degreesC are in line with the predictions of earlier studies. The nature of the active catalyst and the mechanism of catalytic action are unknown. The catalytically enhanced hydrides appear to be strong candidates for development as hydrogen carriers for onboard proton exchange membran (PEM) fuel cells. However, further re search and development in the areas of rehydriding catalysts, large-scale, long-term cycling, safety and adjust ment of the plateau hydrogen pressure associated with dehydriding of AlH(6)(-) are required before these materials can be utilized in commercial onboard hydrogen-storage systems.






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