The Preparation, Structure And Hydrogen Storage Properties Of RE-Mg-Ni Alloys

Hydrogen plays an important role in world’s future energy system becauseit is clean and high efficient. However, how to storage hydrogen energy hasbecame the bottleneck of hydrogen development and application. Solidhydrogen storage material show great advantages on volumetric and masshydrogen storage density, safety and environmental protection. Mg-baseRE-Mg-Ni type hydrogen storage alloys are superior in hydrogen storagecapacity, discharge capacity, activation and cost. These materials show goodpotential application in Ni-MH batteries with high capacity and output power.Although many achievements were made in developing RE-Mg-Ni typehydrogen storage alloys, problems such as preparation, performanceenhancement and application still remain to be solved.First of all, we investigate the influence of different preparation processingon the hydrogen storage properties of RE-Mg-Ni alloys. Results show that thehydrogen absorption amount and early time desorption rate of alloys preparedby casting were similar to that by rapid quenching. The alloy prepared by rapidquenching owns higher hydrogen desorption plateau, hydrogen amount and laterdesorption rate. The La_0.7Mg_0.3Ni_3.5ingot casting alloy is composed of LaNi₅and （LaMg）₂Ni₇phases, while the rapid quenching alloy is composed of LaNi₅,（LaMg）Ni₃, and （LaMg）₂Ni₇phases. Heat-treatment can improve the plateaupressure properties of La_0.7Mg_0.3Ni_3.5, increase the valid hydrogen desorptionamount and change the relative amount of composed phases. La_0.7Mg_0.3Ni_3.5after900°C heat-treatment shows best comprehensive properties with maximumhydrogen desorption amount and kinetics. More LaNi₅phase converts to（LaMg）₂Ni₇phase with the increase of heat-treatment temperature.The hydrogen absorption plateau pressure of RE-Mg-Ni type hydrogenstorage alloy prepared by rapid quenching method reduces with the increase ofMg amount. The lag between ab-/adsorption process becomes larger, while the plateau pressure of absorption reaction becomes broder and flat. However, partof stored hydrogen cannot be released due to the over stability of the hydrideand hence reduces the overall dehydrogenation capacity. La_xMg_{（1-x）}Ni_3.5（x=0.70，0.85） alloy shows best dehydrogenation kinetics and capacity afterheat-treatment at900°C. The plateau pressure of absorption reaction becomeshigher and broader with the increase the ratio of B side of RE-Mg-Ni typehydrogen storage alloy. La_0.7Mg_0.3Ni_3.5alloy has better plateau properties withmoderate pressure. The dehydrogenation capacity can reach1.45wt%. TheLa_xMg_{（1-x）}Ni_3.5（x=0.70，0.85） is composed of LaNi₅,（LaMg）Ni₃and（LaMg）₂Ni₇phases. More （LaMg）Ni₃phase can be found with the increase of xvalue. Part of LaNi₅and （LaMg）Ni₃converts to （LaMg）₂Ni₇afterheat-treatment.The plateau pressure of （RE, Mg）Niyalloy prepared by casting methodshows no obvious difference with the increase of y value, but becomes higherand broader after heat-treatment. The hydrogen absorption amount achieve1.60wt%, while the adsorption amount achieve1.40wt%. XRD results show thatthe （La, Mg）Niy（y=3） alloys are composed by LaNi₅,（LaMg）Ni₃and（LaMg）₂Ni₇phases. The diffusion of Mg is enhanced with the increase ofannealing temperature, which also makes the composition of the alloy becomemore homogenous. More LaNi₅and （LaMg）Ni₃phases convert to （LaMg）₂Ni₇with the increase of annealing temperature. But reverse reaction happens whenthe temperature reach certain level. The lattice volume of each phase changesafter heat-treatment. The lattice volume of （LaMg）₂Ni₇becomes smaller afterheat treatment, while that of LaNi₅becomes larger. However, the c/a ratio of（LaMg）₂Ni₇phase has obvious change after heat treatment, but that of LaNi₅and （LaMg）Ni₃does not change much.We also study the electrochemistry properties of RE-Mg-Ni type hydrogenstorage alloys prepared by ingot casting and rapid quenching methods. The results show that the La_0.88Mg_0.12Ni_3.38Co_0.63alloy prepared by casting iscomposed of LaNi₅and La2Ni7phases. The heat treatment can enhance theelectrochemistry performance of the alloy. Samples show good cyclingproperties after heat treatment at950°C for4hours. The La_0.85Mg_0.15Ni_3.15Co_0.60alloy, which was prepared by rapid quenching at a linear speed of roll of25m/s,shows better comprehensive electrochemistry properties. The discharge capacityof the alloy increases up to380.8mAh/g after heat treatment at900°C for4hours from331.5mAh/g before heat treatment. The cycling life reaches556cycles. The addition of Mg leads to the reduction of discharge capacity andcycle life. The cycle stability of the La-Mg-Ni hydrogen storage alloy will beimproved by using Ce, Pr, Nd to substitute certain amount of La. The lowself-discharge rate Ce2Ni7-type hydrogen storage material,Nd_0.85Mg_0.15Ni_3.65Al_0.1, prepared by rapid quenching method can achieve adischarge capacity of334mAh/g after heat treatment at850°C. AA2000mAhbatteries assembled with this alloy retain69.5%capacity after being placed at60°C for30days. Cyclic voltammetry and alternating current impedance testingresults show that the electrode reaction is mainly controlled by H atomsdiffusion among the alloy.