RESUMO
A series of CeH2.73/CeO2 composites with different ratios of hydride and oxide phases are prepared from the pure cerium hydride via oxidation treatments in the air at room temperature, and they are subsequently doped into Mg2NiH4 by ball milling. The desorption properties of the as-prepared Mg2NiH4+CeH2.73/CeO2 composites are studied by thermogravimetry and differential scanning calorimetery. Microstructures are studied by scanning electron microscopy and transmission electron microscopy, and the phase transitions during dehydrogenation are analyzed through in situ X-ray diffraction. Results show that the initial dehydrogenation temperature and activation energy of Mg2NiH4 are maximally reduced by doping the CeH2.73/CeO2 composite with the same molar ratio of cerium hydride and oxide. In this case, the CeH2.73/CeO2 composite has the largest density of interface among them, and the hydrogen release effect at the interface between cerium hydride and oxide plays an efficient catalytic role in enhancing the hydrogen desorption properties of Mg2NiH4.
RESUMO
The three-component Mg(NH2)2-LiNH2-4LiH composite reversibly stores hydrogen exceeding 5 wt% at a temperature as low as 150 °C. In this work, a number of additives such as CeF4, CeO2, TiCl3, TiH2, NaH, KBH4 and KH are added to the Mg(NH2)2-LiNH2-4LiH composite in order to improve its kinetics, thermodynamics and cycling properties. Addition of 3 wt% of KH reduces the dehydrogenation onset temperature of the Mg(NH2)2-LiNH2-4LiH composite to below 90 °C without emission of NH3 during the whole dehydrogenation process up to 450 °C. Moreover, the dehydrogenation kinetics and cycling ability are remarkably enhanced upon KH-addition. The reaction model of the Mg(NH2)2-LiNH2-4LiH composite is altered upon KH-addition with the active molecule density improved by about 200 times. In addition, by optimization of the ratio of Mg2+ to Li+ in the Mg(NH2)2-LiNH2-LiH system, several novel composites, e.g., Mg(NH2)2-2LiNH2-5.9LiH-0.1KH and Mg(NH2)2-LiNH2-5.9LiH-0.1KH, with the hydrogen storage capacity exceeding 6 wt% without emission of NH3 below 250 °C are developed. Our study demonstrates that there are various undiscovered candidates with promising hydrogen storage properties in the three-component Li-Mg-N-H system.