Study on the mechanism of Ni and Pr doping to improve the hydrogen absorption and desorption properties of Mg95Ni2Pr3 hydrogen storage alloy

In order to improve the slow kinetic performance and high thermodynamic stability of Mg-based hydrogen storage alloys, the Mg-Ni-Pr hydrogen storage alloy system was constructed. By introducing Pr and Ni elements, the way to optimize the kinetic performance of the alloy and destroy its thermodynamic stability was obtained. The composition, phase composition and microstructure of Mg-based Mg95Ni2Pr3 hydrogen storage alloy prepared by vacuum induction melting were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The gaseous hydrogen absorption and desorption properties of the alloy were measured by PCT hydrogen storage tester. The structural stability and hydrogen adsorption on the surface of the alloy were studied by density functional theory. The results show that the as-cast alloy is mainly composed of Mg matrix phase, Mg2Ni phase and PrMg12 phase. After hydrogen absorption, the phase changes into MgH2, Mg2NiH4 and Pr8H18.96, and after hydrogen desorption, it is reversibly restored to Mg, Mg2Ni and PrH2 phases. The reversible hydrogen storage capacity of the alloy is about 5.6 wt % within 120 minutes at 633 K, and the apparent activation energy of the hydrogen desorption reaction is 66.82 kJ / mol, showing excellent kinetic properties. The microstructure analysis shows that the introduction of Ni and Pr produces a ' hydrogen pump effect ' and a nano-catalytic effect by forming a nano-scale second phase and phase interface. The calculation results show that there is a significant hybridization between Ni and H, Mg, and the strong force between Ni and H, Mg provides a fast diffusion channel for hydrogen. Pr, with its large atomic radius and low electronegativity, introduces local lattice strain to weaken the Mg-H bond, and at the same time, it becomes the stable site of hydrogen atoms in the bulk phase through a strong charge transfer effect, forming a strong polar Pr-H bond, which effectively regulates and weakens the covalent strength of the Mg-H bond, thereby improving the thermodynamic stability of the alloy. The calculation confirms that the hydrogen adsorption energy of the doped alloy increases significantly, and the Fermi level of the total density of states increases from 12.93 eV of pure magnesium to 21.9 eV, indicating that the electron transfer rate is accelerated. In this study, the synergistic mechanism of Ni and Pr in Mg-based hydrogen storage alloys was clarified by combining experiment with theory.