De-/hy-drogenation Mechanism And Hydrogen Storage Properties Of PrMg₁₂-based Alloys

Based on the review of researches and developments of metal hydrides,the PrMg₁₂-based alloys were selected as the subject of this thesis.The structure characteristics and hydrogen storage properties of the samples were systematically investigated by means of X-ray Diffraction analysis（XRD）,transmission electron microscope（TEM）,and P-C isotherms.The related research contents and conclusions are as follows:（1）.On the basis of Mg-Pr samples prepared via ball-milling,Al was introdued to prepare Mg-Pr-Al samples for hydrogen storage.It was found that the dehydrogenation kinetics of Mg-Pr-Al samples was superior to that of Mg-Pr samples.Upon dehydrogenation at 350 ^oC,the intermetallic phase Pr₃Al₁₁1 was formed,which was stable during dehydrogenation/hydrogenation and failed to contribute hydrogen storage capacity,but offered a remarkble catalytic effect for the dehydrogenation of MgH₂,3.39 wt.%hydrogen could be released from the Mg-Pr-Al samples in 20 min at 250 ^o C,while only0.02 wt.%hydrogen was released from the Mg-Pr samples in the same conditions.Moreover,Mg（Al）solid solutions and Mg₁₇Al₁₂2 phase were reversibly formed upon dehydrogenation in the Mg-Pr-Al samples,thus reducing the thermal stability and the dehydriding enthalpy change of MgH₂,the dehydriding enthalpy change of Mg-Pr-Al samples was 76.33 kJ/mol,while it was 78.16 kJ/mol for that of Mg-Pr samples.（2）.Furthermore,based on the theory that Al and In can dissolved together into Mg lattice to form Mg（Al,In）solid solutions,it was reasonable to prepare Mg-Pr-Al-In samples via ball milling.Upon dehydrogenation,it was found that In preferred to react with PrH₃ and Al to form pseudo binary intermetallics Pr-Al（In）,which crystallize with space group of binary Pr-Al compounds,and the Pr-Al（In）compounds would decompose into binary Pr-Al compounds and PrIn₃ compound after hydrogenation.Due to the introduction of In,more Pr-Al compounds and Pr-In compounds were formed in the Mg-Pr-Al-In samples and the dehydriding activation energy of MgH₂ was thus obviously reduced.（3）.In the work of chapter 5,Cr and Mn were employed to substitute some Al and Mg-Pr-Al-M（Mn,Cr）samples were thus obtained via ball milling.It was found that the free Cr,Mn were always stable in the Mg-Pr-Al-M samples as they didn’t react with other phases during dehydrogenation/rehydrogenation.Owing to the less content of Al in the Mg-Pr-Al-M samples in comparison to that of Mg-Pr-Al samples,only Mg（Al）solid solutions were formed in the dehydrogenated Mg-Pr-Al-M samples with the absence of Mg₁₇Al₁₂2 phase.It is interesting that the content of Pr₃Al₁₁1 in the Mg-Pr-Al-M samples is almost as more as that in the Mg-Pr-Al samples and they exhibit similar dehydrogenation kinetics.These results indicated it was Pr₃Al₁₁1 phase other than Mg₁₇Al₁₂2 phase that play important roles on catalyzing the dehydrogenation of MgH₂ in the Mg-Pr-Al samples.