Study On The Improvement Of Hydrogen Storage Performance Of NaAlH₄Complex Hydride

Light metal complex hydrides, whose typical representive is NaAlH4, is one of the research hot for its high theoretical hydrogen storage capacity in high capacity hydrogen storage materials area. However, the sluggish kinetics as well as high dehydrogenation and hydrogenation temperature hinder it from practical applications. Based on an overall review of the research and development of sodium alanate (NaAlH4), NaAlH4is selected as the subject of this paper. KH or NaF was added into CeCl3-doped NaAlH4complex hydrides to improve its hydrogen storage properties. Sievert hydrogen storage properties tester, X-Ray Diffraction (XRD), synchronized Differential Scanning Calorimetry-Thermogravimetry (DSC/TG), Scanning Electron Microscopy (SEM) and other materials modern analysis methods were used to detailedly investigate the influence of the addition of KH or NaF to the hydrogen storage performances and microstructure of CeCl3-doped NaAlH4as well as their modification mechanism.The research results of preparation, de-/rehydrogenation performances and microstructure for (NaH/Al)-CeCl3/xKH composites show that the doped sodium aluminum hydride (NaAlH4) can be directly synthesized by ball milling NaH/Al-0.02CeCl3/xKH (x=0,0.02,0.04) composites, using NaH, Al as raw materials and CeCl3as dopant while adding different amount of KH, under a hydrogen pressure of3MPa at room temperature. It is found that the addition of KH can effectively improve the dehydrogenation dynamics properties (especially the second step reaction) of NaAlH4system. And the composite shows the best dehydrogenation performances when the amount of KH is2mol%, which starts to release hydrogen from87℃and completes dehydrogenation process within20mins at170℃(dehydrogenation campacity is4.33wt%) with good cycling stability. It is also found that the addition of KH can swell the unite cell of Na3AlH6since K+can partially replace Na+in Na3AlH6to form Na3-xKxAlH6(0<x<3) sosolid, then decrease the dehydrogenation temperature as well as dehydrogenation apparent acticvation energy and increase the hydrogenation plateau pressure of second dehydrogenation reaction step, finally improve its dehydrogenation kinetics. Moreover, the reaction kinetics model analysis shows that both decomposition steps of0.98NaH/Al+0.02CeCl3/0.02KH composite were conformed to two-dimension boundary growth mechanism. The research results of preparation, de-/rehydrogenation performances and microstructure for (NaH/Al)-CeCl3/yNaF composites show that the doped sodium aluminum hydride (NaAlH4) can be directly synthesized by ball milling NaH/Al-0.02CeCl3/yKH (y=0.04,0.06,0.08) composites, using NaH, Al as raw materials and CeCl3as dopant while adding different amount of NaF, under a hydrogen pressure of3MPa at room temperature. The addition of NaF to NaAlH4complex hydrides can effectively decrease its dehydrogenation temperature, and the best hydrogen storage capacity and dehydrogenation dynamic performance could be acheived when the NaF anount is6mol%. Microstructure analyses found no F contained phase and obvious appearance change during the cycling of NaF-doped system, therefore modification mechanism needs further research.CeAl4can be formed when carrying out vacuum-pumping to dehydrogenated doped NaAlH4complex hydrides, and the formed CeAl4shows better catalytic effect than CeH2.(NaH/Al)-0.02CeAl4complex hydrides can obtain the best de/rehydrogenation performance when added the optimum amount of KH (2mol%) and NaF (6mol%), finishing the first step in2.8mins and completing dehydrogenation in less than20mins, with a hydrogen storage amount of more than4.9wt%.The co-doped KH and NaF can decrease the apparent activation energy of CeAl4-NaAlH4system, thus improving its dehydrogenation kinetics.