Effects And Mechanisms Analysis Of Initial Phase Structure On Hydrogen Storage Properties Of La-Mg-Ni Alloy

La-Mg-Ni-based hydrogen storage alloy has been considered as the potential hydrogen storage material due to its fine reversible hydrogen storage performance in relatively mild conditions. However, the high temperature of reversible hydrogen storage and poor kinetic properties restrict its practical application. Research shows that the onset phase of La-Mg-Ni hydrogen storage alloy can decompose at first hydriding process, which play an important role in absorption/desorption performance of alloy. According to the ternary alloy phase diagram of La-Mg-Ni system, we report the preparation of a novel La-Mg-Ni alloy LaMg4 Ni and La0.1Mg9.4Ni0.5 their phase structure and hydrogen storage properties by vacuum induction melting technology, copper-mould-cast technology(rapid cooling) and ultra-high pressure technology. Inductive coupled plasma(ICP), X-ray diffraction(XRD), Differential scanning calorimeter(DCS), Scanning electron microscope(SEM)-Energy dispersive spectrometer(EDS), Hydrogen absorption/desorption performance test were utilized to characterize their phase structure and hydrogen storage properties. And, the relevance and the influence on hydrogen storage property between onset phase of La-Mg-Ni-based multiphase hydrogen storage alloys and the phase distribution(the phase size, unidormity) are discussed in detail.The results for copper-mould-cast LaMg4 Ni alloys indicated that the dehydriding onset temperature and peak temperature of the as-cast alloy hydride were higher than theose of the copper-mould-cast one. Moreover, the copper-mould-cast alloy exhibited better hydriding/dehydriding kinetics compared with the as-cast one. The reversible hydrogen storage capacities and plateau hydrogen pressures of the two alloys were close, indicating copper-mould-casting hardly affected the thermodynamic property of the LaMg4 Ni alloy. Similar thermodynamic properties of the two alloys should be ascribed to the same hydrogen storage phases, Mg and Mg2 Ni. The better hydrogen sorption kinetics of copper-mould-cast alloy should be ascribed to the more uniform phase distribution compared with that of the as-cast one.The results for ultra-high pressure(UHP) LaMg4 Ni alloys indicated that as-cast LaMg4 Ni alloy consists of La2Mg17, LaMg2 Ni and Mg2 Ni phases. The LaMg4 Ni alloys after ultrahigh pressure treatment consist of LaMg3, LaMg2 Ni and Mg2 Ni phases. Moreover, the alloys with ultra-high pressure treatment exhibits more uniform microstructure than the as-cast one, wherein the microstructure of the UHP alloy is much finer at 973 K than at 823 K. The reversible hydrogen storage capacities and plateau hydrogen pressure of the alloys with ultra-high pressure treatment are close to that of the as-cast alloy, indicating the treatment hardly affects the thermodynamic properties of the LaMg4 Ni alloys. Both the dehydriding onset temperature and the dehydriding peak temperature are lower in the alloys with ultrahigh pressure treatment than in the as-cast one. Moreover, the treated alloys show faster hydriding and dehydriding kinetics compared with the as-cast one. The improvement of hydriding/dehydriding kinetics is ascribed to better catalytic effect of La hydride and Mg2NiH4 or Mg2 Ni and more phase boundary as hydrogen diffusion channel due to the more homogeneous distribution of La hydride, Mg2NiH4 and MgH2 phases, which is related with the more homogenous phase distribution and much finer microstructure of bulky alloys after ultra-high pressure treatment.The results for ultra-high pressure La0.1Mg9.4Ni0.5 alloys indicated that the dehydriding onset temperature and peak temperature of the as-cast alloy are higher than that of the UHP alloy, respectively. Additionally, in the heating process, the UHP alloy has a faster dehydriding rate compared with the as-cast alloy hydride. The as-cast alloy is composed of Mg, Mg2 Ni and La2Mg17 phases, and the UHP alloy consists of Mg, Mg6 Ni and La2Mg17 phases. An UHP treatment makes Mg2 Ni and Mg transfer to Mg6 Ni phase, resulting in the decrease of Mg phase. The improvement of hydriding/dehydriding kinetics is ascribed to better catalytic effect of Mg2NiH4 and more phase boundary as hydrogen diffusion channel due to a more homogeneous distribution of Mg2NiH4 phase, which is related with the formation of Mg6 Ni phase after ultra-high pressure treatment.