Study On Hydrogen Storage Properties Of TiCrV Hydrogen Storage Alloys And Their Composites

As the reversible hydrogen storage materials with high capacity, TiCrV solid solution alloys, Mg based hydrogen storage materials and the NaAlH4 alanates have been extensively studied in recent years. For the hydrogen storage application, the TiCrV solid solution alloys have good thermodynamic and kinetic properties, but a poor cycle life; NaAlH4 analates possess the moderate thermodynamic, but poor dynamic performances; Mg based hydrogen storage materials are required to be improved in the thermodynamic and kinetic properties. To develop hydrogen storage composite materials with both high gravimetric and volumetric hydrogen storage density, and good dynamic performance under mild conditions, many efforts have been made in this thesis by controlling and characterizing composition, microstructure and properties of the hydrogen storage materials. The composition of TiCrV alloys is optimized, and the decay mechanism of hydrogen storage capacity during the cycles of hydrogen absorption and desorption was analyzed. The effects of hydriding ball milling technology on the hydrogen storage properties and microstructure of TiCrV/NaAlH4 and TiCrV/Mg composite materials are investigated. The work and main results obtained are as follows:For TiCrV alloys, absorbing capacity of the alloys is enhanced by growing Ti/Cr ratio due to the increase of lattice parameters. When Ti/Cr ratio is 0.71, the maximum hydrogen absorption capacity is reached. Base on this alloys, the hydrogen desorption plateau pressure is raised and the slope of plateau reduced by the addition of Mn in the alloys. When the content of Mn is 8 at.%, the maximum capacity of effective hydrogen storage is obtained. But the Ti-rich phase is precipitated out with the addition of Mn. In order to eliminate Ti-rich phase, Ce is added in the alloys. The compositional homogeneities of the alloys can be improved by adding Ce in the alloys which induce the preferential formation of CeO2 and considerably inhibit precipitation of Ti-rich second phases in the alloys. The effective hydrogen storage capacity is significantly increase and the plateau slope is reduced. The (Ti0.25Cr0.35V0.40)92Mn8Ce0.4 hydrogen storage alloy is developed and the maximum hydrogen capacity reaches 3.55wt.%, hydrogen desorption capacity 2.0wt.% and 2.55wt.% at 295K and 393K respectively.During the cycles of hydrogen absorption and desorption for the TiCrVMnCe alloys, the hydrogen storage capacity of the alloys declines rapidly in the initial stage of the cycles, and then gradually keep stable. After being treated in vacuum at 573K, the hydrogen storage capacity of the alloys can be restored. The relationship is established between the composition and structure in different regions of the alloy in the process of hydrogen absorption and desorption by the analysis of DSC, XRD, high resolution electron microscopy and energy spectrum. The degradation mechanism of hydrogen storage capacity in cycles of TiCrVMnCe alloys is derived. It is attributed to redistribution of Ti and V atoms due to hydrogen induced strain. The redistribution of Ti and V atoms results in the reduction of lattice constant of BCC phase, the precipitation of Ti-rich stable hydride phase and V-poor Laves phase, which lead to the decrease of the capacity of hydrogen storage alloy. It is the important way to extend cycle life of TiCrV alloys that maintain the stability of the compositional homogeneity in the alloys against cycles of hydrogen absorption and desorption.In order to improve the kinetic properties of hydrogen desorption of NaAlH4 and increase its volumetric density of hydrogen storage, the powder of a TiCrV alloy is added in CeHx doped NaAlH4. The TiCrV/NaAlH4 composites are prepared by hydriding ball milling. The effect of ball milling is improved by the addition of TiCrV alloys in NaAlH4. The results show that the size of powder particles is reduced, the grains refined and phase boundary between TiCrV alloys and NaAlH4 increased, which accelerate the diffusion rate of AlxHy clusters in the composites. The fine particles of TiCrV alloys, homogeneously distributed in the matrix of NaAlH4, play the roles of surface catalysts and hydrogen pumps during the process of hydrogen absorption and desorption which speed up the hydrogen evolution rate in the NaAlH4. The hydrogen desorbing temperature of second step in NaAlH4 is significantly decreased, only 20℃ higher than that of first step, by nano-crystallization in the composites. A composite of CeHx doped NaAlH4/30 mol.% TiCrV possess good hydrogen evolution properties with desorption capacity of 4.40wt.% and desorption time of 24min up to 90% hydrogen capacity at 423K and 0.1MPa.For improving the properties of hydrogen absorption and desorption of Mg and increasing its volumetric density of hydrogen storage, the TiCrV powder is added in the powder of Mg, and the TiCrV/Mg composites are prepared by hydriding ball milling. The fine nano-crystalline TiCrV powders are uniformly distributed in the matrix of Mg in the composite. The hydrogen molecules are predominantly dissociated by the TiCrV powders which play a role of "overflow" in hydrogen absorption process. The hydrogen absorption rate of the composites is accelerated by enormous active interfaces between TiCrV and Mg which provide channels for the rapid diffusion of hydrogen in Mg matrix. The composite of TiCrV/Mg is developed with good hydrogen sorption kinetics characteristics, which show the maximum hydrogen absorption capacity of 3.0wt.% at room temperature and more than 90% of saturation hydrogen absorption capacity in less than 1 min. At the same time, the initial dehydriding temperature of the composites is decreased by 110℃ due to formation of γ--MgH2 and nano-crystallization.