Hydrogen atom distribution and hydrogen induced site depopulation for the La2−xMgxNi7–H system

An alternative for the anode materials of nickel metal hydride batteries: an AB3-type La0.6Gd0.2Mg0.2Ni2.6Co0.3Al0.1 hydrogen storage alloy

In order to satisfy the demand for the cyclic stability of commercial Ni-MH anodes, a PuNi3-type La0.6Gd0.2Mg0.2Ni2.6Co0.3Al0.1 alloy with excellent overall electrochemical properties was prepared by annealing the as-cast alloy sample at different temperatures for a week. The alloy had the highest PuNi3-type content of 86.9 wt% (1073 K), which offered a capacity retention of 69.6% after 100 cycles. However, 23.7 wt% PuNi3 type phase of the alloy constantly converted into the Ce2Ni7 type phase within a temperature increase of 50 °C, which improved the capacity retention by 12.1% under the same discharge capacity. We found that the addition of Gd did not change the stacked [LaMgNi4]/[LaNi5] superlattice and it maintained the structural stability of the crystal as well as its anti-corrosion, which is also a key factor to improve cyclic stability. These findings imply that alloys with both PuNi3-type and Ce2Ni7-type multiphase structures can be considered as a new choice for hydrogen storage.

Effect of Substitutional Elements on the Thermodynamic and Electrochemical Properties of Mechanically Alloyed La1.5Mg0.5Ni7−xMx alloys (M = Al, Mn)

The A2B7-type La-Mg-Ni-M-based (M = Al, Mn) intermetallic compounds were produced by mechanical alloying and annealing. The thermodynamic and electrochemical properties of these materials were studied. The nickel substitution by aluminum and manganese in the La-Mg-Ni system improves the kinetics of hydrogen absorption. The hydrogen desorption capacity of Mn substituted compounds is improved significantly, and it reaches the value of 1.79 wt.% at 303 K when the composition is La1.5Mg0.5Ni6.80Mn0.20. On the other hand, the La1.5Mg0.5Ni6.85Al0.15 shows a much higher reversible electrochemical capacity than the La1.5Mg0.5Ni7 materials at the 50th cycle. The electrochemical discharge capacity stability increases with the increasing value of Al and Mn up to x = 0.2 and 0.3, respectively. Additionally, a reduction in the discharge capacity was measured for the Al and Mn content above x = 0.25 and 0.5, respectively. From the practical aspect, only La1.5Mg0.5Ni6.80Mn0.20 has a potential in the application as a hydrogen storage material.

Synergistic effects of Gd and Co on the phase evolution mechanism and electrochemical performances of Ce2Ni7-type La-Mg-Ni-based alloys

The influences of Gd and Co co-substitution for La and Ni on phase structures, and electrochemical properties of La_0.83-xGd_xMg_0.17Ni_3.35-2xCo_2xAl_0.15 (x = 0-0.83) alloys were investigated. All the alloys contained A₂B₇-type (Ce₂Ni₇- and Gd₂Co₇-type) phase, Pr₅Co₁₉-type phase, PuNi₃-type phase and CaCu₅-type phase. The partial replacement of Gd and Co for La and Ni increased the phase abundance of the Ce₂Ni₇-type superstructure and decreased cell volumes, which contributed to a better hydrogen absorption capacity, cyclic stability and HRD. Compared to those of single Gd- or Co-substitutions, the synergistic effects of Gd and Co on the overall electrochemical properties of alloys were significant. Such a superior overall electrochemical performance may result from appropriate cell volumes and anti-pulverization abilities.

Crystal Structure of Pr3MgNi14Dx Studied by in Situ Neutron Diffraction

The crystal structure of Pr₃MgNi₁₄D₁₈ was determined by neutron diffraction. The determined structure of Pr₃MgNi₁₄D₁₈ consisted of 89.0% Gd₂Co₇-type structure and 11.0% PuNi₃-type structure. The lattice parameters of a and c of Gd₂Co₇-type structure were refined at 0.52903(7) nm and 3.90179(1) nm. The deuterium atoms were distributed among nine deuterium sites in both the CaCu₅-type and MgZn₂-type cells. The D2 occupancy in the Pr₂Ni₄ octahedral sites of the CaCu₅-type cell was the largest (0.75) when compared with the other deuterium sites (<0.49). The deuterium content of the CaCu₅-type cell showed 0.75 D/M, but the D/M value of the MgZn₂-type cell was 1.53. The volume expansions during deuteration of the CaCu₅-type and MgZn₂-type cells were nearly equal. The cyclic hydrogenation property of Pr₃MgNi₁₄ is comparable to that of LaNi₅. It is inferred that the similar expansion behavior of the CaCu₅-type and MgZn₂-type cells during deuteration is the origin of this cyclic stability.

Crystal Structure Analysis of La2Ni6CoD(x) During Deuterium Absorption Process

The crystal structures of La2Ni6CoD(x) (x = 5.2 and 9.6) were determined by in situ neutron diffraction along the P-C isotherm. La2Ni6CoD(5.2) (phase I) was found to be orthorhombic with lattice parameters a = 0.500670(2) nm, b = 0.867211(4) nm, and c = 2.99569(7) nm. The 10 deuterium sites were located in the MgZn2-type and CaCu5-type cells, with deuterium contents of 0.95 D/M and 0.39 D/M, respectively. The full deuteride La2Ni6CoD(9.6) (phase II) was monoclinic with lattice parameters a = 0.516407(3) nm, b = 0.894496(6) nm, c = 3.11206(1) nm, and β = 90.15(1)°. The phase II had 11 sites for deuterium occupation. The deuterium contents of the MgZn2-type and the CaCu5-type cell were 1.63 D/M and 0.78 D/M, respectively. The sequence of phase transformation of La2Ni6Co was hexagonal, followed by orthorhombic (phase I), and then monoclinic (phase II), for the first absorption process. The phase transformation resulted in lowered symmetry and the variation of deuterium atom occupation.