In order to improve the electrochemical hydrogen storage performance of the Mg2Ni-type electrode alloys, Mg in the alloy was partially substituted by La, and the nanocrystalline and amorphous Mg2Ni-type Mg20-xLaxNi10 (x=0, 2) alloys were synthesized by melt-spinning technique. The microstructures of the as-spun alloys were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The electrochemical hydrogen storage properties of the experimental alloys were tested. The results show that no amorphous phase is detected in the as-spun Mg20Ni10 alloy, but the as-spun Mg18La2Ni10 alloy holds a major amorphous phase. As La content increases from 0 to 2, the maximum discharge capacity of the as-spun (20 m/s) alloys rises from 96.5 to 387.1 mA·h/g, and the capacity retaining rate (S20) at the 20th cycle grows from 31.3% to 71.7%. Melt-spinning engenders an impactful effect on the electrochemical hydrogen storage performances of the alloys. With the increase in the spinning rate from 0 to 30 m/s, the maximum discharge capacity increases from 30.3 to 135.5 mA·h/g for the Mg20Ni10 alloy, and from 197.2 to 406.5 mA·h/g for the Mg18La2Ni10 alloy. The capacity retaining rate (S20) of the Mg20Ni10 alloy at the 20th cycle slightly falls from 36.7% to 27.1%, but it markedly mounts up from 37.3% to 78.3% for the Mg18La2Ni10 alloy.
The La-Mg-Ni-based A2B7-type La0.8-xNdxMg0.2Ni3.35 Al0.1Si0.05(x=0,0.1,0.2,0.3,and0.4) electrode alloys were prepared by casting and annealing. The influence of the partial substitution of Nd for La on the structure and electrochemical performances of the alloys was investigated. The structural analysis of X-ray diffraction and scanning electron microscopy reveals that the experimental alloys consist of two major phases: (La,Mg)2Ni7 with the hexagonal Ce2Ni7-type structure and LaNi5 with the hexagonal CaCu 5-type structure as well as some residual phases of LaNi3 and NdNi5 . The electrochemical measurements indicate that an evident change of the electrochemical performance of the alloys is associated with the substitution of Nd for La. The discharge capacity of the alloy first increases then decreases with the growing Nd content, whereas their cycle stability clearly grows all the time. Furthermore, the measurements of the high rate discharge ability, the limiting current density, and hydrogen diffusion coefficient all demonstrate that the electrochemical kinetic properties of the alloy electrodes first augment then decline with the rising amount of Nd substitution.
The nanocrystalline and amorphous Mg2Ni-type Mg2Ni1-xCox (x = 0, 0.1, 0.2, 0.3, 0.4) alloys were synthesized by melt quenching technology. The structures of the as-cast and quenched alloys were characterized by XRD, SEM and HRTEM. The gaseous hydrogen storage kinetics of the alloys was measured using an automatically controlled Sieverts apparatus. The alloy electrodes were charged and discharged with a constant current density in order to investigate the electrochemical hydrogen storage kinetics of the alloys. The results demonstrate that the substitution of Co for Ni results in the formation of secondary phases MgCo2 and Mg instead of altering the major phase Mg2Ni. No amorphous phase is detected in the as-quenched Cofree alloy, however, a certain amount of amorphous phase is clearly found in the as-quenched alloys substituted by Co. Furthermore, both the rapid quenching and the Co substitution significantly improve the gaseous and electrochemical hydrogen storage kinetics of the alloys, for which the notable increase of the hydrogen diffusion coefficient (D) along with the limiting current density (IL) and the obvious decline of the electrochemical impedance generated by both the Co substitution and the rapid quenching are basically responsible.
