Microstructures and Hydrogen Desorption Properties of the MgH2-AlH3 Composite with NbF5 Addition

Niobium fluoride (NbF5) is introduced into the MgH2 + 1/4AlH(3) hydride composite by ball milling to improve the hydrogen desorption properties of the Mg-Al-H system. It is found that after being ball milled with 1 mol % of NbF5, AlH3 in the composite has almost fully decomposed and forms metallic Al, which indicates that NbF5 can significantly destabilize AlH3. DSC results show that NbF5 addition also helps reduce the peak desorption temperature of MgH2 in the composite from 324 to 280 degrees C. Isothermal desorption measurements demonstrate that MgH2 in the doped composite can rapidly release 98% of its hydrogen after desorption at 300 degrees C for 1 h, while the valued is only 46% for the undoped composite. The activation energy for hydrogen desorption of MgH2 in the doped composite is calculated to be 104.5 kJ/mol, much lower than that in the undoped composite (127.4 kJ/mol). These results suggest that NbF5 addition dramatically improves the hydrogen desorption kinetics of the MgH2 + 1/4AlH(3) composite. Dehydrogenation hydrogenation measurements reveal that the hydrogen desorption kinetics of the undoped composite declines with cycle number, whereas the NbF5-doped composite maintains good cycling stability. Microstructure studies indicate that the decline of the kinetics is attributed to the grain growth and particle agglomeration of MgH2 during hydrogen sorption cycling. However, NbF5 addition can suppress this grain growth through the formation of Nb/NbH layers surrounding the particles of MgH2 and acting both as the impediment to grain growth of Mg/MgH2 and as the gateway for hydrogen diffusion. Finally, the role that AlH3 plays in the hydrogen desorption process of the Mg-Al-H composites is discussed.