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The 2LiNH 2 /MgH 2 system has been identified as an attractive system for hydrogen storage, but suitable catalysts are needed to reduce the hydrogen desorption temperature and improve the rates of reaction. One of the most effective catalysts for lowering the desorption temperature and improving the desorption rate from this system has been KH. In this work a new catalytic additive, rubidium hydride (RbH), was synthesized, and its effects on hydrogen desorption from the 2LiNH 2 /MgH 2 system were studied and compared to those of KH. Temperature-programmed desorption measurements showed that the addition of approximately 3 mol % RbH lowered the desorption temperature of the system by 94 °C, which is somewhat better than KH. Desorption enthalpies for the catalyzed samples were found to be approximately 42 kJ/mol, which is significantly lower than the 65 kJ/mol that was found for the uncatalyzed mixture. The hydrogen desorption rate of the RbH-doped sample was found to be approximately twice as fast as the KH-doped sample and about 60 times faster than the uncatalyzed sample, making RbH one of the best catalytic additives to date for the 2LiNH 2 /MgH 2 system. Modeling studies were done using two approaches, and both indicated that diffusion controlled the rate of hydrogen desorption from 2LiNH 2 / MgH 2 in the two-phase plateau region.

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Lithium amideDesorption kinetics Potassium hydride Rubidium hydride Cesium hydride a b s t r a c t In this study, the effectiveness of several alkali metal hydrides (KH, RbH and CsH) for improving the hydrogen desorption properties of a 2LiNH 2 /MgH 2 mixture was studied.Results showed that the relative effectiveness of these additives in decreasing the hydrogen desorption temperature, lowering the activation energy and increasing desorption rates from the mixtures is in the order: RbH > KH > CsH > Un-catalyzed. Modeling studies showed that diffusion through a Li 2 Mg(NH) 2 product layer is the rate-controlling process. It is believed that the alkali elements: K, Rb and Cs partially replace the Li in the product layer. This may have an inductive effect in which the NeH bond is weakened thus leading to lower desorption enthalpies. The lattice expansion caused by substitution of the larger alkali elements for Li may also allow for faster diffusion and increased desorption rates.

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