Preparation And Characterization Of Mg-Ni-Mm Hydrogen Storage Materials

Development and utilization of hydrogen can effectively solve the energy crisis caused by shortage to ease pressure on the environment. However, popularization and application of hydrogen energy must first deal with the safety of hydrogen storage issues. Among many hydrogen storage material systems, Mg-based hydrogen storage alloys have received considerable interest in the last few years due to their low cost, high capacity, lightweight, great abundance. However, their high desorption temperature and relatively slow H-absorption/desorption kinetics make them still inapplicable for practical application. Presently, the reaction kinetics of Mg-H is improved by the addition of transition and earth elements as well as microstrutuctural refinement by rapid solidification method.In the present study, Mg-10Ni-xMm （x=1, 2, 3 at.%） and Mg-yNi-2Mm（y=8, 10, 12 at.%） alloys were developed and prepared with vacuum induction melting method, and the hydrogen storage properties were investigated by focusing on the catalytic effect of Ni and Mm additions. The as-cast alloys were successfully obtained using a simple and low-cost two-step of pre-alloying and vacuum induction melting. Nanocrystalline and amorphous alloys were obtained by rapaid solidification method with the surface velocity of the copper wheel of 10.5 and 20.9 ms^-1, respectively. The effect of the different microstrutures on absoring/desorpting hydrogen behaviors of the alloy was particularly studied.The composition and phase structure of the alloys were analyzed by X-ray fluorescence （XRF） and X-ray diffraction （XRD）, respectively. The non-hydrogenated and hydrogenated microstructures of the alloys were observed by scanning electron microscopy （SEM） and transmission electron microscopy （TEM）. Hydrogen storage properties and absorption and desorption rate were tested by pressure-composition- temperature （PCT）. The values ofΔH andΔS were calculated by Van’t Hoff equation.The hygrogen storage capacities of the as-cast Mg-Ni-Mm systems with the main hydrogen absorption phases of Mg and Mg₂Ni are 5⁶wt.%H, and the corresponding hydrided phases are MgH2 and Mg₂NiH₄, respectively. MmMg12 phase as a catalyst is to promote the dissociation of hydrogen. The hydrogen absorption/desorption processes are the process of nucleation and growth, which are dominated by nucleation initially and later controlled by diffusion of hydogen. The addition of Mm makes appear in the lattice volume expansion and amorphous phenomenon, which help hydride nucleation and diffusion of hydrogen. Uniform distribution of hydrogen diffusion of "channel" provides a convenience process for the diffusion of hydrogen. Longer time of effective diffusion of hydrogen increases the hydrogen storage capacity. An increase of Ni content results in more lamellar eutectic structure, providing a large number of continuous boundaries, and promoting the nucleation and hydrogen diffusion. However, when excess Ni is added, Mg₂Ni play an important role, the overall hydrogen storage capacity declines due to its low capacity of hdyrogen. Phase transformation of Mg₂NiH₄ directly affects the absorption/desorption kinetics of alloys. In all the as-cast alloys, the Mg-10Ni-3Mm alloy shows the best hydrogen storage performance. The best conditions for hydrogen absorption and desorption is T=325℃and P=1.0MPa. The maximum absorption capacity of hydrogen, 5.16wt.%H, is obtained. Complete hydrogen releases within 6.2min with the hydrogen desorption rate of 95.6%. Mg₂NiH₄ nucleates easily due to its lower stability. Mm-rich Mg-based phases improves its activation properties, MgH2 is generated during decomposition of MmMg12 as well. Thus, Ni and Mm offer a large number of nucleation centers, to speed up the overall alloy hydride nucleation rate and to reduce the thermal stability. The thermodynamic properties of hydrogen absorption alloy improved: the values ofΔH andΔS of Mg₂NiH₄ of the as-cast Mg-10Ni-2Mm alloy are -56.9 kJ / mol H₂ and -110.7 J / mol H₂, respectively, which are much lower than the literature values of -64.5 kJ / mol H₂ and -122 J / mol H₂. The formation enthalpy of MgH2 is consistent with those for pure Mg.The grian size of rapidly solidified amorphous and nancrystalline alloys are greatly reduced. The largest one is not more than 20nm, and it remains about the size of 35nm after hydroagenation. Fine grains increase the specific surface area and grain boundary densities, which help nucleation and reduce the distance of diffusion of hydrogen, and the rate of hydrogen absorption/desorption is therefore improved. As a lot of strains exist in the amorphous and nanocrystalline Mg-10Ni-2Mm alloy, the hydrogen storage capacity decreases a little. The values of enthalpy and entropy of Mg₂NiH₄ phase for amorphous and nanocrystalline Mg-10Ni-2Mm alloy areâ–³H= -49.5KJ/mol H₂,â–³S= -99.9J/mol H₂ andâ–³H= -55.6KJ/mol H₂,â–³S= -109.1J/mol H₂, respectively, indicating that thermodynamic performance of the alloy is improved. The value of enthalpy of amorphous alloy is the lowest among the all alloys. The nanocrystalline alloy shows the best overall hydrogen storage properties. The optimum condition of hydrogen absorption/desorption processes for nanocrystalline alloy is obtained: T=325℃and P=1.0MPa. The hydrogen storage capacity is 5.09wt.% H, and the amount of hydrogen desorption is 4.86wt.%H. The hydrogen desorption rate of 95.5% in the alloy is available.