TbMgNi_4-xCo_x–(H,D)₂ System. I: Synthesis, Hydrogenation Properties, and Crystal and Electronic Structures

Abstract

Series of cubicTbMgNi 4-x Сo x (x = 0-4) pseudo binary compounds have been synthesized either by solid state reaction or mechanical alloying with further annealing treatment. Their hydrogenation properties have been studied by Pressure-Composition-Temperature (PCT) measurements, showing the formation of α-, βand γ-hydrides. The Co for Ni substitution yields a lowering of the equilibrium pressure and an increase of hydrogen capacity. An improvement of the kinetic of hydrogen absorption was observed for increasing Co content. The β-TbMgNi 4-x Сo x H 4 (x = 0-3) hydrides show an orthorhombic distortion (Pmn2 1 space group) whereas β-TbMgСo 4 H 3.3 hydride crystallizes in a monoclinic structure (Pm space group) derived from the orthorhombic structure. Sensitivity of the formation of the β-TbMgСo 4 H 3.3 hydride to the temperature has been observed in PCT curves: it exists below 50 °C. γ-TbMgNi 4-x Сo x H y (x = 2-4, y > 5) hydrides preserve the parent cubic structure (F-43m space group) with an hydrogen induced volume expansion of 17.9-22.3 %. The deuterium for hydrogen substitution in TbMgNiCo 3 -(H,D) 2 system prevents fast desorption at room temperature and ambient pressure The positions of deuterium atoms in the γ-TbMgCo 4 D 6 deuteride were determined by neutron powder diffraction. Two types of interstitial sites were found occupied: 87 % of deuterium atoms (D1 site) are located inside Tb 2 MgCo 2 octahedra and the remaining 13 % (D2 site) are inside Co 4 tetrahedra. The increase of Co content allows to improve the thermodynamic stability of TbMgNi 4-x Сo x hydrides as demonstrated by the lowering of the desorption plateau pressure, the increase of the desorption temperature and of the enthalpy of formation versus Co content. This last experimental result is confirmed by first principles calculations where electronic structures of intermetallic compounds and their cubic TbMgNi 4x Сo x H 6 hydrides are compared for x = 0, 2 and 4.

Solid gas and electrochemical hydrogenation of the selected alloys (R’,R’’)2-Mg Ni4-Co (R’, R’’ = Pr, Nd; x = 0.8–1.2; y = 0–2)

“…Recently, we conducted a detailed study of several 1:1:4 alloy systems and have shown the element substitution effects on structural, hydrogen sorption and electrochemical properties [19][20][21][22][23][24][25]. In some cases the characteristics of the alloys were improved.…”

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.…”

“…Shtender et al reported that the γ-hydride phase was formed by Co doping in Ni atomic sites as in YMgNi 2 Co 2 or YMgCo 4 [19]. Although most studies on RE (2−x) Mg x Ni 4 have mainly focused on Ni [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28], Co doping at Ni atomic sites or replacement of Ni atoms with Co have also been attempted in RE = La [25], Ce [29], Pr [30], Nd [30,31], and Tb [32,33], in which, Co could lead to increased hydrogen storage capacities and decreased equilibrium hydrogen absorption reactions. A new hydride phase with a monoclinic structure has been reported [33].…”