Phase Structure And Hydrogen Storage Properties Of Re-Mg-Ni Alloys

The catalysis of rare earth hydride compound and the synergetic effect of multiphaseof Rare earth-Mg-Transitional metal system alloys have been investigated for the pastdecades. As a part of our continuous efforts in this line of research, according to theternary alloy phase diagram of RE-Mg-Ni system, herein we report the preparation of anovel Rare earth-Mg-Transitional metal alloy LaMg_3.93Ni_0.21and single phase alloyREMg₂Ni (RE=La, Pr), their phase structure and hydrogen storage properties.LaMg₂Ni alloy was prepared by inductive melting under Ar atmosphere. X-raydiffraction (XRD) shows that LaMg₂Ni alloy decomposes to LaH3phase and Mg₂NiH4phase during hydriding process, and LaH3phase transforms to La₃H₇phase in dehydridingprocess. Compared with Mg₂Ni alloy, due to the existence of La hydride compound andthe phase transition from La₃H₇phase to LaH3phase in hydriding process, LaMg₂Ni alloyshows better hydriding kinetic, it could be reached the90%of the maximum hydrogenabsorption capacity within280s, while pristine Mg₂Ni alloy needs1200s. La hydridecompound is beneficial not only to the enhancement of the hydriding kinetic but alsoimproves the thermodynamic properties of LaMg₂Ni alloy. The enthalpy and entropy forthe hydriding Mg₂Ni phase in the LaMg₂Ni alloy were calculated to be-53.02±0.89kJ/mol H2,-84.96±1.58J/K mol H2, the Mg₂NiH4in LaMg₂Ni alloy is less stable thanpristine Mg₂NiH4phase (-64.50kJ/mol and-123.10J/K mol H2). Pressure-Composition-Temperature (P-C-T) measurement exhibits that the hydrogen storage capacity ofLaMg₂Ni alloy is about1.95wt.%and keep stably from603K to523K, while thehydrogen storage capacity of pristine Mg₂Ni alloy declined distinctly form4.09wt.%to3.13wt.%with reduction of temperature from603K to523K, which suggests that Lahydride compound could improve the hydriding/dehydriding properties of the LaMg₂Nialloy at low temperature. PrMg₂Ni alloy is also single phase structure alloy, its hydrogenstorage capacity is about1.85wt.%and keep stably with the temperature reduction from603K to523K. The hydrogen absorption rate of PrMg₂Ni alloy is faster than that ofLaMg₂Ni alloy, and its hydrogen absorption kinetic becomes better with the reduction oftemperature. It is found that the hydrogen absorption process of LaMg₂Ni alloy can be best fitted to Jander rate equation, the hydrogen absorption process can be described bythe one-dimensional diffusion mechanism; the hydrogen absorption process of PrMg₂Nialloy is best agreement with Avrami-Erofeev equation, the hydriding process of theLaMg₂Ni compound follows the nucleation and growth mechanisms.The phase structure and hydrogen storage property of the LaMg_3.93Ni_0.21alloy havebeen studied. The XRD and SEM results exhibit that the alloy consists mainly of LaMg3,La2Mg17and LaMg₂Ni phases; after hydriding/dehydriding process, all three phasestransformed, LaHxphase existed and the actual hydrogen absorption phases are Mg andMg₂Ni phases. The P-C-T measurement shows that the reversible hydrogen storagecapacity of LaMg_3.93Ni_0.21alloy is2.54wt.%at523K. The enthalpy and entropy for thehydriding and dehydriding reactions were calculated to be-56.38±1.10kJ/mol H2,-100.96±1.96J/K mol H2and58.50±3.87kJ/mol H2,98.28±5.48J/K mol H2,respectively. A Comparison of these data with those of MgH2(-74.50kJ/mol H2,-132.30J/K mol H2) and Mg₂NiH4(-64.12kJ/mol H2,-118.49J/K mol H2) suggests that thehydride of LaMg_3.93Ni_0.21alloy is less stable than MgH2and Mg₂NiH4. The existence ofLa hydride and synergetic effect of multiphase lead to the higher reversible hydrogenstorage capacity and better kinetic property at lower temperature for LaMg_3.93Ni_0.21alloy.