A Study Of The Hydrogen Storage Properties Of(2-x)La-xCe-78Mg-20Ni Alloys

This study investigates the impact of Ce substitution for La on the hydrogen storage properties of magnesium-based alloy.Two sets of alloys,1La-79Mg-20Ni and(2-x)La-xCe-78Mg-20Ni(x=0,0.5,1,1.5 wt.%)were prepared through vacuum induction melting,while another set of(2-x)La-xCe-78Mg-20Ni(x=0,0.5 wt.%)alloys was prepared through rapid quenching and ball milling.The as-cast alloy underwent SEM,EDS,and XRD characterization,revealing that the main phase was La2Mg17,along with Mg2Ni and La2Ni3.There was no new phase produced by cerium substitution.During the activation experiments at 375℃,the hydrogen storage properties of the alloy weakened with increasing percentage of Ce substitution,a 0.5 wt.%Ce substitution for La led to significant improvements in the first hydrogen absorption capacity,absolute hydrogen release,and hydrogen release ratio of the experimental alloy,the latter two up to 5.59 wt.%and 94.59%,respectively.However,the maximum hydrogen uptake and the rate of hydrogen release will be reduced at the same time.Meanwhile,over-substituting La with 1.0 wt.%and 1.5 wt.%of Ce accelerated the hydrogen uptake and release rate,but reduced the number of activations required.Notably,the 0.5 wt.%Ce substituted La group demonstrated the best thermodynamic performance,with a plateau pressure of about 0.55 MPa.Moreover,the maximum hydrogen absorption capacity at 350℃ and 4 MPa activation was up to 6.151 wt.%,while the enthalpy of hydrogen uptake became-80.36 kJ/mol and the enthalpy of hydrogen release became 81.12 kJ/mol.Using JMAK to fit the kinetic test curve at 350℃,the rate constant of hydrogen uptake increased from 0.729 to 2.303 as the mass ratio of Ce increases,highlighting the significant improvement in hydrogen storage kinetic performance caused by Ce replacing La in the alloy.After comparing the fast-quenched samples with the as-cast samples,it was observed that the activation curves indicated the former was easier to activate and the activation cycle was completed by the third cycle.The PCT test showed that the fast-quenched group could easily absorb hydrogen at low pressure,but the maximum hydrogen uptake was slightly lower due to the short hydrogen uptake plateau and low plateau pressure.However,at 270℃,310℃ and 350℃,the hydrogen storage thermomechanical properties were not entirely superior to those of the as-cast alloys,potentially due to the slower cooling rate and reduced amorphous ratio during the fast-quenching process.Regarding C2,the absolute values of the enthalpy-entropy of the fast-quenched group are smaller than those of the as-cast group,indicating an improvement in the material’s thermomechanical properties through fast quenching.Kinetic tests revealed that both fast-quenching and Ce substitution could increase the hydrogen absorption rate of the alloy in the early stages,but both methods reduced the maximum hydrogen absorption.The maximum velocity constant was 4.179 for the fast-quenched C2 group,emphasizing the significant improvement of hydrogen storage kinetics induced by fast quenching.