Effect Of In-situ Formation Of Pr₃Al₁₁ On Hydrogen Storage Performance Of Magnesium-based Composites

Based on the review of the research status and classification of hydrogen storage alloys,MgH₂–Pr H₂–Al,MgH₂–PrF₃–Al and MgH₂–PrF₃–Al–Ni samples were prepared by mechanical ball milling with Mg-based hydrogen storage alloys as the research object.The hydrogen storage performances of the composites were studied by utilizing X-ray diffraction technology(XRD),transmission electron microscope(TEM)and Pressure–Composition–Temperature(P–C–T)curve test.The major research contents and results are described below:(1)MgH₂–PrH₂–Al composite was prepared by mechanical ball milling method.MgH₂sample was ball-milled under the same conditions,and hydrogen storage characteristics of the two samples were compared.The results indicate that the hydrogen storage performance of MgH₂–Pr H₂–Al composite is better than that of MgH₂sample.At1st cycle,the MgH₂–Pr H₂–Al composite desorbs 5.30 wt%of hydrogen for 5 h at 300°C,and the hydrogen desorption is completely completed.After 30th cycle,the hydrogen release capacity retention rate is 95%.At 1st cycle,the MgH₂sample desorbs 3.10 wt%of hydrogen for 5 h at 300°C.After 40 h,the dehydrogenation is basically completed and the hydrogen capacity is 6.50 wt%.After 30th cycle,the capacity retention rate is 63%.The kinetic properties and cycle durability of MgH₂–Pr H₂–Al composite are significantly better than that of MgH₂sample.The improvement of hydrogen storage performance is attributed to the in-situ formation of Pr₃Al₁₁nanoparticles during hydrogen absorption and desorption,which are pinned to the grain boundaries of Mg/MgH₂,hindering grain boundary movement and inhibiting the growth of Mg/MgH₂.Moreover,Pr₃Al₁₁nanoparticles are uniformly disperses on the surface of Mg/MgH₂,which enhances the oxidation resistance of MgH₂–Pr H₂–Al composite,thereby promoting the improvement of kinetic properties and cycle stability.(2)MgH₂–PrF₃–Al composites was prepared by mechanical ball milling.The results show that compare with MgH₂–Pr H₂–Al composite,the hydrogenation and dehydrogenation rate of MgH₂–PrF₃–Al composite is significantly accelerated.At 1st cycle,the MgH₂–PrF₃–Al composite material desorbs 3.65 wt%of hydrogen for 1 h at 300°C,and desorbs 4.81 wt%of hydrogen after 5 h and the hydrogen desorption is completely completed.After 30th cycle,the hydrogen retention rate of capacity reaches 95%,and the kinetic performance and cycle stability are much better than the MgH₂sample.In the process of absorbing and releasing hydrogen,Mg F₂and Pr₃Al₁₁nanoparticles are formed to inhibit the growth of Mg/MgH₂crystal grains,and Mg F₂is always stable in the process of hydrogen absorption and desorption.It catalyzes the hydrogen absorption and desorption of Mg/MgH₂to accelerate the initial activation of the composite and reduces the apparent activation energy of the material,thus further improving the dynamic performance and cyclic stability of MgH₂–PrF₃–Al composite.(3)MgH₂–PrF₃–Al–Ni composite was prepared by mechanical ball milling.The results show that the MgH₂–PrF₃–Al–Ni composite desorbs 4.67 wt%of hydrogen for the first time within 1 h at 300°C,and the hydrogen desorption is completely completed.After30th hydrogen cycles,the hydrogen capacity retention rate is 95%.The hydrogen absorption of MgH₂–PrF₃–Al–Ni composite is fully saturated within 10 min at 300°C.The kinetic characteristics are significantly improved in comparison with MgH₂,MgH₂–Pr H₂–Al and MgH₂–PrF₃–Al samples,mainly due to the formation of Pr₃Al₁₁,Mg F₂,Pr H₃and Mg₂Ni H₄during the hydrogen absorption and desorption of MgH₂–PrF₃–Al–Ni composite,which have a synergistic mechanism to jointly improve the hydrogen storage performance of the composite material.