Crystal Structural Investigations for Understanding the Hydrogen Storage Properties of YMgNi₄-Based Alloys

Abstract

The hydrogen storage properties and crystal structures of YMgNi4-based alloys, which were synthesized from (2 – x)­YNi2 and xMgNi2 (0.6 ≤ x ≤ 1.2), were investigated by pressure–composition–temperature measurements and powder neutron diffraction at a deuterium gas pressure to understand the hydrogen absorption and desorption reactions viewed from atomic arrangements around H atoms. Reducing the amounts of MgNi2, which was utilized as a Mg source in YMgNi4-based alloys, has been observed to lower the hydrogen absorption and desorption pressures and increase the hydrogen storage capacities. However, the reversible hydrogen capacity attained a maximum value of 1.2 mass % at x = 0.8 because of the formation of a thermodynamically stable hydride in which hydrogen was not released at x = 0.6. In the case of x = 0.6, the presence of excessive Y atoms around the H atoms in the hydrogen-absorbed phase would lead to the formation of a hydride with stronger interaction between Y and H because of the affinity between them. Moreover, the presence of small amounts of D atoms with short interatomic D–D distances (1.6 and 1.9 Å) in the deuterium-absorbed phase (Y0.81Mg1.19Ni4.00D3.35 and Y1.06Mg0.94Ni4.00D3.86) at <5 MPa and 323 K was proposed by the crystal structural investigations. The D atoms with short D–D interatomic distances were located in the same local atomic arrangements of D atoms in a deuterium-absorbed phase, which were formed at a higher-pressure range, and had higher hydrogen storage capacities than the deuterium-absorbed phases in this study.

The Crystal Structures in Hydrogen Absorption Reactions of REMgNi4-Based Alloys (RE: Rare-Earth Metals)

“…In fact, the design of intermetallic compounds that do not form ionic or complex hydrides is one approach to obtain efficient hydrogen storage materials. So far, REMgNi 4 -based alloys, such as RE (2−x) Mg x Ni 4 (RE: rare-earth metals; 0 < x < 2), have been reported to exhibit reversible hydrogen absorption and desorption reactions at ambient temperatures [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. In these reactions, hydrogen atoms are located at interstitial sites in the lattice of RE (2−x) Mg x Ni 4 and three hydride phases are formed [19][20][21][22][23][24][28][29][30][31][32][33] the α-, β-, and γ-hydride phases, respectively.…”

“…So far, REMgNi 4 -based alloys, such as RE (2−x) Mg x Ni 4 (RE: rare-earth metals; 0 < x < 2), have been reported to exhibit reversible hydrogen absorption and desorption reactions at ambient temperatures [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. In these reactions, hydrogen atoms are located at interstitial sites in the lattice of RE (2−x) Mg x Ni 4 and three hydride phases are formed [19][20][21][22][23][24][28][29][30][31][32][33] the α-, β-, and γ-hydride phases, respectively. The experimentally reported hydride phases with their unit cell parameters, formation enthalpy of the hydrides, and hydrogen contents are listed in Table 1.…”

“…The experimentally reported hydride phases with their unit cell parameters, formation enthalpy of the hydrides, and hydrogen contents are listed in Table 1. In RE (2−x) Mg x Ni 4 , the atomic radii of the components, which are influenced by the addition of Mg with a smaller atomic radius than RE atoms [34], selection (radius) of the RE element, and compositional ratios, are crucial parameters for reversible hydrogen absorption and desorption reactions [14][15][16][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] and for avoiding hydrogen-induced amorphization (HIA) [35,36]. Absorbed hydrogen in RENi 2 without Mg is hardly released at ambient temperature because of the considerably lower hydrogen desorption pressure than the ambient pressure.…”

“…Absorbed hydrogen in RENi 2 without Mg is hardly released at ambient temperature because of the considerably lower hydrogen desorption pressure than the ambient pressure. By contrast, Mg in RENi 2 , which forms RE (2−x) Mg x Ni 4 , leads to increased equilibrium hydrogen absorption and desorption reaction pressures [20]. The atomic radii of components can also be adjusted by the selection (radius) of RE elements and compositional ratios because the unit cell parameters also depend on the RE element [17].…”

“…In the Refs. [18][19][20][21][22], the hydrogen storage properties and crystal structures focusing on RE = Y have been described. In the RE (2−x) Mg x Ni 4 alloys, RE = Y is expected to have a higher gravimetric hydrogen density because Y is the lightest RE element that forms REMgNi 4 .…”