Nanocrystalline Mg <sub> 2 </ Sub> Feh <sub> 6 </ Sub> Hydrogen Storage Materials, Preparation, Structure And Performance

Hydrogen as an alternative energy to fossil fuel energy supply is becoming to be recognized day to day.However,the hydrogen storage remains unsolved.There has been increasing interest in the metal and intermetallic alloys for hydrogen storage in the last ten years,which offer a safe alternative to storage in compressed or liquid form.Their high storage capacity,coupled with a low price,suggests that magnesium and magnesium alloys could be advantageous for use as gaseous-hydrogen storage systems.Magnesium can reversibly store about 7.7wt%hydrogen.However,the use of pure magnesium is prevented by its very poor hydrogen absorption and desorption kinetics.For that reason a large number of investigations have been aimed at improving the kinetics characteristics of hydriding and dehydriding of magnesium.Various Mg-based alloys have been studied in regard to decreasing the working temperature and increasing hydrogen sorption/desorption kinetics.In the present work,we synthesize the nanostructured complex hydrides Mg₂FeH₆ by mechano-chemical method.We pay our attention on improving the yields of Mg₂FeH₆,hydrogenation kinetics and lowering the working temperature of Mg.The research work we have carried out mainly including the following three aspects:A Uni-Ball-Mill 5 and a XQM-4 type planetary ball mill were adopted respectively in order to compare effect of milling method on synthesis of Mg₂FeH₆ hydrogen storage materials.Hydrogen pressure loss,X-ray diffraction(XRD), scanning electron microscopy(SEM)and thermoanalysis were conducted to determine the structure and performance of the milled powders.The results show that the yield and H-storage capacity of Mg₂FeH₆ hydride synthesize by XQM-4 type planetary were better than which synthesized by Uni-Ball-Mill 5.The difference may be attributed to the fact that the planetary ball mill has higher milling efficiency than the Uni-Ball-Mill 5.Mg₂FeH₆ hydrogen storage materials were synthesized with difference milling parameters like the starting powder ratio of Mg to Fe as 2:1 and 3:1. X-ray diffraction(XRD)measurement shows that the highest Mg₂FeH₆ yield ratio, 83.7%,is acquired in the sample with n(Mg):n(Fe)=3:1 and directly milled in H₂.The sample milled with Mg to Fe being 2 to 1 and directly milled in H₂ has the minimum hydrogen desorption starting temperature 204.4℃.Simultaneity,the effect of difference milling modes on the synthesis of nanostructured ternary complex hydride Mg₂FeH₆ was studied by controlled reactive mechanical alloying(CRMA),the formation of MgO was suppressed with the continuous milling mode.And the Schematic of the formation process of Mg₂FeH₆ was also be investigated.Absorption/desorption properties of the milled nanostructured ternary complex hydride Mg₂FeH₆ were investigated by Sievert apparatus.It was shown that the mixture exhibits a fast hydrogen absorption/desorption kinetics by completely absorbing 4.42 wt.%H₂ at 653K under 4.0 MPa H₂ and can absorb/release hydrogen directly without any activation step.The composite phases Mg₂FeH₆-Ti,Mg₂FeH₆-TiO₂ were synthesis by the addition of Ti and TiO₂ in the 3Mg+Fe mixture. The absorption/desorption properties and catalysis were investigated.