Absorption and Desorption Hydrogen Kinetic of Mg-Y-Ni Based Hydrogen Storage Alloy

With the advent of a new round of industrial revolution, the profound changes had taken place in energy development and utilization technologies, and the green and low-carbon had become the general trend of global energy development. Therefore, the hydrogen energy became a clean and renewable energy for human sustainable development in the 21st century. In large-scale application of hydrogen energy, Mg-based hydrides were one of the most promising hydrogen storage materials because of their relatively high storage capacity, abundance, and low cost. However, overly stable thermodynamics and sluggish kinetics hindered the practical applications of Mg-based hydrogen storage alloys. In views of this, the Mg-Y-Ni system was constructed in order to solve the puzzle of slow hydrogen absorption and desorption kinetics of Mg-based hydrogen storage alloy by introducing Y and Ni catalytic elements.The as-cast Mg95-xY5Nix(x=5, 10, 15) alloy were prepared by vacuum induction melting. X-ray diffraction analysis (XRD) was used to analytical phase composition of the as-cast, hydrogenation, dehydrogenation alloy, respectively. Besides, scanning electron microscope (SEM) and transmission electron microscope (TEM) were also used to characterize the microstructure and crystalline state of these samples. Meanwhile, the kinetic properties of isothermal hydrogen adsorption and desorption at different temperatures also were tested by the semi-automatic pressure-capacity-temperature (PCT) device which used the Sievert isometric volume method. In addition, the dehydrogenated activation energy was calculated based on combining the Jonhson-Mehl-Avrami-Kolmogorov (JMAK) model and the Arrhenius equation. The results indicated that the as-cast Mg-Y-Ni samples was a polyphase structure composed of Mg, Mg2Ni and YNi3. Besides, with the increase of Ni content, Mg2Ni and YNi3 phases which evenly dispersed in Mg gradually increased and became more and more fine. This was conducive to dividing large area Mg into smaller blocks and obtaining more phase boundaries, which provided more rapid and convenient channels for hydrogen diffusion in the alloy. The hydrogenated sample was nanocrystalline structure composed of MgH2, Mg2NiH4 and YH3 phases. The nanocrystalline structure could introduce a large number of grain boundaries and provide more channels for hydrogen diffusion, which had an obvious effect on improving the kinetic properties of hydrogen storage alloys. After dehydrogenation, MgH2 and Mg2NiH4 phases decomposed into corresponding Mg and Mg2Ni phases and released hydrogen. However, the in-situ formed YH3 phase did not decompose in the process of hydrogen desorption, which dispersed in the mother alloy, acted as a positive "hydrogen pump" catalytic effect for the reversible cyclic reaction of Mg and Mg2Ni phases. In the process of hydrogen absorption, Ni still showed a positive catalytic effect. With the increase of Ni content, the hydrogen absorption and desorption kinetics of Mg-Y-Ni alloy were improved obviously, especially for Mg80Y5Ni15 alloy, the optimal hydrogenated temperature was reduced to 200 ℃, and 90% of the maximum hydrogen storage capacity could be absorbed within 1 min. This was mainly due to the change of microstructure caused by Ni element. With the increase of the content of Ni alloy, the microstructure of the alloy became more finest and uniform, thus Sample Ni15 which processed a smallest slice-layer structure shown the optimum kinetic performance of hydrogen absorption. In addition, with the increase of Ni element, the "optimum hydrogen absorption temperature" of Mg-Y-Ni alloy was also reduced, which was great significance for practical application. In the process of hydrogen desorption, with the increase of Ni content, the desorption kinetics of the sample was improvedsignificantly, especially for Ni15 alloy, which could not only release hydrogen completely at 320 ℃, but also only take 5 min, which almost reached the desorption capacity of Ni5 alloy at 360 ℃. This was mainly due to the formation of Mg2Ni compound with Mg and the formation of Mg2NiH4 hydride with high platform pressure after hydrogenation. Then, in the process of hydrogen desorption, Mg2NiH4 phase had priority over MgH2 phase to separate hydrogen and cause lattice shrinkage, which will lead to the increase of interfacial gap between MgH2 and Mg2NiH4, thus reduced the concentration of hydrogen and further inducedthe decomposition of MgH2. In addition, the dehydrogenation activation energies of Ni5, Ni10 and Ni15 were calculated by JMAK model and Arrhenius equation, and the values were 91.8, 71.6 and 67.0 kJ·mol-1, respectively. These values decreased with the increase of Ni content, indicated that the addition of Ni could significantly reduce the energy barrier required by the desorption reaction of Mg-Y-Ni based hydrogen storage alloys, and further reduce the desorption temperature, and improve the desorption kinetic performance. © 2022, Youke Publishing Co., Ltd. All right reserved.