A Study On Hydrogen Storage Characteristics Of NaAlH₄ Catalyzed With Metal Hydrides

Based on the review of the research and development of NaAlH₄ complex hydrides as hydrogen storage material,NaAlH₄ complex hydrides doped with LaH_3.01,CeH_2.51, PrH_2.92,NdH_2.27,TiH₂ and LaNi₅H₆ powders as catalysts were prepared by hydrogenation of the ball-milled(NaH/Al+x mol%RE-H)(RE=La,x=2,4,6;RE= Ce,Pr,Nd,x=6),(NaH/Al+6 mol%TiH₂) and(NaH/Al+14 wt%LaNi₅H₆) composites.Their microstructures and hydrogen storage properties were systematically investigated by means of XRD,SEM/EDS,DSC analyses and the gaseous hydrogen storage properties test methods.Then explored the metal hydride influence on hydrogen storage properties of NaAlH₄.The following results have been obtained:The study on the hydrogen storage properties and microstructures of(NaH/Al+x mol%LaH_3.01)(x=0,2,4,6) composites shows that,as increasing the catalyst's concentration in the composites,the hydriding/dehydriding kinetics is improved, whereas the hydrogen absorption/desorption capacity within 10 h is decreasing.Its hydrogen absorption capacity decreases from 4.78 wt%(x=2) to 3.68 wt%(x=6) at 120℃.Accordingly,its hydrogen desorption capacity decreases from 4.68 wt%(x=2) to 3.38 wt%(x=6) at 170℃.It is found that the remains of Na₃AlH₆ phase in the hydrided and dehydrided composite samples indicate the incompletion of the hydriding/dehydriding reactions within 10 h.The LaH_3.01 particles remain on the NaH/Al matrix surface acting as the catalytic active sites on the surface of hydride matrix,and improving the H atoms to diffuse into the matrix.The(NaH/Al+x mol% LaH_3.01)(x=2,4,6) composites' dehydriding activation energy are decreasing as the increasing of catalyst's concentration.The investigation on the hydrogen storage properties and microstructure of (NaH/Al+6 mol%RE-H)(RE=Ce,Pr,Nd) composites shows that,doping with CeH_2.51,PrH_2.92 and NdH_2.27 can obviously improve the NaH/Al composite's hydriding/dehydriding properties.The(NaH/Al+6 mol%CeH_2.51) composite has best hydriding kinetics,whereas its hydriding/dehydriding capacities are low.The hydriding/dehydriding capacity of(NaH/Al+6 mol%PrH_2.92) composite reaches highest at first cycle,and its hydriding/dehydriding capacity is 3.66 wt%at 120℃and 2.94 wt%at 170℃,respectively.The(NaH/Al+6 mol%NdH_2.51) composite has nearly the same hydrogen storage properties.The microstructure analyses indicate the ball-milled and hydrided/dehydrided composites samples remain Na₃AlH₆,phase,and the composite particles grow bulky with pore on the surface after 5 cycles.By Kissinger plot calculation,the dehydriding activation energies of(NaH/Al+6 mol%RE-H)(RE= Ce,Pr,Nd) composites are 299.9 kJ/mol(RE=Ce),277.4 kJ/mol(RE=Pr) and 334 kJ/mol(RE=Nd),respectively.The hydrogen storage properties and microstructure of(NaH/Al+6 mol%TiH₂) and(NaH/Al+14 wt%LaNi₅H₆) composites have been studied.The(NaH/Al+6 mol%TiH₂) composite has higher hydriding capacity of 4.29 wt%at 120℃and dehydriding capacity of 3.92 wt%at 170℃in 10 h.However,the hydriding/ dehydriding capacities of(NaH/Al+14 wt%LaNi₅H₆) composite are 3.21 wt%and 3.14 wt%,respectively.Both composites have good hydriding/dehydriding cycling stablity.The microstructure analysss indicate Na₃AlH₆ phase is produced after ball-milling and hydriding/dehydriding reactions.After 5 cycles,the composite particles congregate bulky with pore on the surface.By Kissinger plot calculation,the dehydriding activation energies of(NaH/Al+6 mol%TiH₂) and(NaH/Al+14 wt% LaNi₅H₆) composites are 388.2 kJ/mol and 430.2 kJ/mol,respectively.