The partial substitution of M （M=Sm, Nd, Pr） for La was performed in order to ameliorate the electrochemical hydrogen storage performance of RE–Mg–Ni-based A2B7-type electrode alloys. The La0.8–xMxMg0.2Ni3.35Al0.1Si0.05 （M=Sm, Nd, Pr;x=0-0.4） electrode alloys were fabricated by casting and annealing and their microstructures were characterized by X-ray diffraction （XRD） and scanning electron microscopy （SEM）. The major phases （La, Mg）2Ni7 with the hexagonal Ce2Ni7-type structure and LaNi5 with the hexagonal CaCu5-type structure make up the basic microstructure of the experimental alloys. The discharge capacities of the as-cast and annealed alloys all gain their maximum values with the M （M=Sm, Nd, Pr） content varying. The electrochemical cycle stability of the as-cast and annealed alloys clearly rises with the M （M=Sm, Nd, Pr） content growing. Furthermore, the electrochemical kinetics of the alloys, including the high rate discharge ability, charge transfer rate, limiting current density and hydrogen diffusion coefficient, all present a increase trend at first and then decrease with the rising of M （M=Sm, Nd, Pr） content.
The as-cast Mg2Ni-type Mg20–xYxNi10 (x=0, 1, 2, 3 and 4) electrode alloys were prepared by vacuum induction melting. Subsequently, the as-cast alloys were mechanically milled in a planetary-type ball mill. The analyses of scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM) reveal that nanocrystalline and amorphous structure can be obtained by mechanical milling, and the amount of amorphous phase increases with milling time prolonging. The electrochemical measurements show that the discharge capacity of Y0 alloy increases with milling time prolonging, while that of the Y-substituted alloys has a maximum value in the same condition. The cycle stabilities of the alloys decrease with milling time prolonging. The effect of milling time on the electrochemical kinetics of the alloys is related to Y content. Whenx=0, the high rate discharge ability, diffusion coefficient of hydrogen atom, limiting current density and charge transfer rate all increase with milling time prolonging, but the results are exactly opposite whenx=3.
In order to ameliorate the electrochemical hydrogen storage performance of La-Mg-Ni system A2B7-type electrode alloys, a small amount of Si was added. The La0.8Mg0.2Ni3.3Co0.2Six （x=0-0.2） electrode alloys were prepared by casting and annealing. The effects of adding Si on the structure and electrochemical hydrogen storage characteristics of the alloys were investigated systematically. The results indicate that the as-cast and annealed alloys hold multiple structures, involving two major phases of （La, Mg）2Ni7 with a Ce2Ni7-type hexagonal structure and LaNi5 with a CaCu5-type hexagonal structure as well as one residual phase LaNi3. The addition of Si results in a decrease in （La, Mg）2Ni7 phase and an increase in LaNi5 phase without changing the phase structure of the alloys. What is more, it brings on an obvious effect on electrochemical hydrogen storage characteristics of the alloys. The discharge capacities of the as-cast and annealed alloys decline with the increase of Si content, but their cycle stabilities clearly grow under the same condition. Furthermore, the measurements of the high rate discharge ability, the limiting current density, hydrogen diffusion coefficient as well as electrochemical impedance spectra all indicate that the electrochemical kinetic properties of the electrode alloys first increase and then decrease with the rising of Si content.
Abstract： The nanocrystalline and amorphous Mg2Ni-type electrode alloys with a composition of Mg20-xYxNi10 （x=0, 1, 2, 3 and 4） were fabricated by mechanical milling. Effects of Y content on the structures and electrochemical hydrogen storage performances of the alloys were investigated in detail. The inspections of X-ray diffraction （XRD）, transmission electron microscopy （TEM） and scanning electron microscopy （SEM） revealed that the substitution of Y for Mg brought on an obvious change in the phase composition of the alloys. The substitution of Y for Mg resulted in the formation of secondary YMgNi4 phases without altering the major phase Mg2Ni when Y content x≤1. But with the further increase of Y content, the major phase of the alloys changed into YMgNi4 phase. In addition, such substitution facilitated the glass forming of the Mg2Ni-type alloy. The discharge capacities of the as-milled alloys had the maximum values with Y content varying, but Y content with which the alloy yielded thc biggest discharge capacity was changeable with milling time varying. The substitution of Y for Mg had an insignificant effect on the activation ability of the alloys, but it dramatically improved the cycle stability of the as-milled alloys. The effect of Y content on the electrochemical kinetics of the alloys was related to milling time. When milling time was 10 h, the high rate discharge ability （HRD）, diffusion coefficient of hydrogen atom （D） and charge transfer rate all had the maximum value with Y content increasing, but they always decreased in the same condition when milling time increased to 70 h.
Ce1-xBixO2-δ （x = 0.00, 0.03, 0.05, 0.07, 0.10, 0.15, 0.30） solid solutions were synthesized via a hydrothermal method. The structure, spectra and electrochemical transport properties of the samples were characterized systematically. The powder X-ray diffraction analysis showed that all of the doped samples exhibited single phase fluorite structure. The particle sizes decreased from 18 to 9 nm and the lattice parameters increased gradually with the dopant content increasing from x = 0.03 to x = 0.30. The Bi^3＋ doping also induced the F2g Raman peak to shift from 463 to 455 cm^-1, and caused a red shift of the band gap energies calculated from UV-Vis spectra. The impedance plots at different temperature demonstrated that the boundary resistance was much larger than the grain resistance, and two activation energy values were obtained in different temperature range.
Melt spinning technology was used to prepare the Mg2 Ni-type（Mg24 Ni10 Cu2）100–x Ndx（x=0,5,10,15,20） alloys in order to obtain a nanocrystalline and amorphous structure.The effects of Nd content and spinning rate on the structures and electrochemical hydrogen storage performances of the alloys were investigated.The structure characterizations of X-ray diffraction（XRD）,transmission electron microscopy（TEM） and scanning electron microscopy（SEM） linked with energy dispersive spectroscopy（EDS） revealed that the as-spun Nd-free alloy displayed an entire nanocrystalline structure,whereas the as-spun Nd-added alloys held a nanocrystalline and amorphous structure and the degree of amorphization visibly increased with the rising of Nd content and spinning rate,suggesting that the addition of Nd facilitated the glass forming of the Mg2 Ni-type alloy.The electrochemical measurements indicated that the addition of Nd and melt spinning improved the electrochemical hydrogen storage performances of the alloys significantly.The discharge capacities of the as-cast and spun alloys exhibited maximum values when Nd content was x=10,which were 86.4,200.5,266.3,402.5 and 452.8 mAh/g corresponding to the spinning rate of 0（As-cast was defined as the spinning rate of 0 m/s）,10,20,30 and 40 m/s,respectively.The cycle stability（S20,the capacity maintain rate at 20thcycle） of the as-cast alloy always rose with the increasing of Nd content,and those of the as-spun alloys exhibited the maximum values for Nd content x=10,which were 77.9%,83.4% 89.2% and 89.7%,corresponding to the spinning rate of 10,20,30 and 40 m/s,respectively.
The La-Mg-Ni-based A2B7-type La0.5Mg0.2Ni3.3Co0.2Six （x=0-0.2） electrode alloys were prepared by casting and annealing. The influences of the additional silicon and the annealing treatment on the structure and electrochemical performances of the alloys were investigated systemically. Both of the analyses of XRD and SEM reveal that the as-cast and annealed alloys are of a multiphase structure, involving two main phases （La, Mg）2Ni7 and LaNi5 as well as one minor phase LaNi3. The addition of Si and annealing treatment bring on an evident change in the phase abundances and cell parameters of （La, Mg）2Ni7 and LaNi5 phase for the alloy without altering its phase structure. The phase abundances decrease from 74.3% （x=0） to 57.8% （x=0.2） for the （La, Mg）2Ni7 phase, and those of LaNi5 phase increase from 20.2% （x^0） to 37.3% （x=0.2）. As for the electrochemical measurements, adding Si and performing annealing treatment have engendered obvious impacts. The cycle stability of the alloys is improved dramatically, being enhanced from 80.3% to 93.7% for the as-annealed （950 ℃） alloys with Si content increasing from 0 to 0.2. However, the discharge capacity is reduced by adding Si, from 399.4 to 345.3 mA.h/g as the Si content increases from 0 to 0.2. Furthermore, such addition makes the electrochemical kinetic properties of the alloy electrodes first increase and then decrease. Also, it is found that the overall electrochemical properties of the alloys first augment and then fall with the annealing temperature rising.
