Characteristics of A₂B₇-type LaYNi-based hydrogen storage alloys modified by partially substituting Ni with Mn

Highlights

• A₂B₇-type LaYNiMn alloy belongs to a system of new hydrogen storage materials.

• Discharge capacities of x = 0.5, 1.0 alloys exceed theoretical capacity of LaNi₅ alloy.

• LaYNi alloy can be easily prepared compared with the LaMgNi alloys.

Abstract

LaY₂Ni_10.5−xMn_x (x = 0.0, 0.5, 1.0, 2.0) alloys are prepared by a vacuum induction-quenching process followed by annealing. The structure, as well as the hydriding/dehydriding and charging/discharging characteristics, of the alloys are investigated via X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), pressure-composition isotherms (PCI), and electrochemical measurement. The alloys have multiphase structures mainly composed of Gd₂Co₇-type (3R) and Ce₂Ni₇-type (2H) phases. Partial substitution of Ni by Mn clearly increases the hydrogen storage capacity of the alloys. The x = 0.5 alloy exhibits a maximum hydrogen storage capacity of 1.40 wt % and a discharge capacity of 392.9 mAh g⁻¹, which are approximately 1.5 and 1.9 times greater than those of the x = 0.0 alloy, respectively. The high-rate dischargeability (HRD) of the x = 0.5 alloy is higher than that of the other alloys because of its large hydrogen diffusion coefficient D, which is a controlling factor in the electrochemical kinetic performance of alloy electrodes at high discharge current densities. Although the cyclic stability of the x = 0.5 alloy is not as high as that of the other alloys, its capacity retention ratio is as high as 56.3% after the 400th cycle. The thermodynamic characteristics of the x = 0.5 alloy satisfy the requirements of the hydride electrode of metal hydride–nickel (MH–Ni) batteries.

Introduction

Intermetallic compounds RMn (R = rare earth; M = transition metal; 1 ≤ n ≤ 5) can reversibly store a large amount of hydrogen and are therefore important energy storage materials [1], [2], [3]. According to the LaNi binary phase diagram [4], phases such as RNi₃ and R₂Ni₇ may form during heating through peritectic reactions. Among the RNi₃-type compounds (R = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Y), YNi₃ exhibits the largest cell volume, lowest density, and most satisfactory electrochemical characteristics [5].

The partial substitution of Y in YNi₃ compounds with La and/or Ce leads to the formation of ternary yttrium-based alloys having the general formula La_1−xCe_xY₂Ni₉ (0 ≤ x ≤ 1) [1], [6], [7], [8]. Similar to the R–MgNi system superlattice alloys [9], these R–YNi compounds adopt a rhombohedral structure of the PuNi₃ type, which can be described as an intergrowth between RNi₅- and RNi₂-type structures and induces greater receptivity to hydrogen than commercial RNi₅-type alloys. Furthermore, the Y element and Mg element have similar effects in the two series of hydrogen storage alloys. Both Mg in the LaMgNi alloy and Y in the LaYNi alloy increase the structural stability of the corresponding alloys in the hydrogenation/dehydrogenization process and avoid or delay the hydrogen-induced amorphous (HIA) of the alloys [10], [11], [12]. The lanthanum compound LaY₂Ni₉ absorbs 12.3 H/f.u. (formula unit), which corresponds to 380 mAh g⁻¹ in equivalent electrochemical units under 1 bar of hydrogen gas, and its plateau pressure for absorption is 0.06 bar at 298 K [1]. Owing to these characteristics, ternary yttrium-based alloys are among the most promising negative electrode materials for metal hydride–nickel (MH–Ni) batteries.

In recent years, R–MgNi system hydrogen storage alloys have been investigated extensively, and their discharge capacities have been found to be as high as 414 mAh g⁻¹ [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. However, the relatively high volatility of Mg makes the preparation process difficult and expensive [14], [18], [19], [20], [21], thereby restricting the application of this family of alloys. In contrast, R–YNi alloys can be easily prepared by induction melting, which is also preferable for R–MgNi alloys. However, the discharge capacity of the AB₃-type LaYNi alloy is only 260 mAh g⁻¹ [6], [7], [8], which requires further enhancement. In our previous work [27], [28], [29], we found that the electrochemical properties of AB₃-, A₂B₇-, and A₅B₁₉-type R–YNi system alloys can be improved significantly by adjusting the alloy composition using the method of element substitution. The partial substitution of Ni by metals such as Mn and Al can significantly increase the discharge capacity of the alloys and provide an optimal trade-off between high hydrogen capacity and good cyclic stability.

Mn is an indispensable element in conventional mischmetal-based AB₅-type alloys for maintaining cycle stability and high-rate dischargeability (HRD) [30]. Nevertheless, the substitution of the Ni in the mischmetal-based superlattice alloys by Mn decreases both the gaseous phase and electrochemical capacities due to a reduction in the abundance of the main A₂B₇ phase. It additionally adversely affects alloy properties such as the phase homogeneity, capacity, cycle stability, HRD, and surface reaction due to deterioration in the surface catalytic ability as the Mn content increases [30]. However, the R–MgNi system superlattice alloy family (AB₃, A₂B₇, A₅B₁₉) tends to focus on La- and Nd-only alloys, for which the partial substitution of Mn for Ni can increase the lattice parameters and the cell volume, decrease the plateau equilibrium pressure, increase the discharge capacity and improve the electrochemical catalytic activity of the alloys [22], [23], [24]. In practice, there exists an optimum content of Mn for improving the hydrogen storage property and overall electrochemical properties of the alloys. Mn might be suitable for dual tuning of the thermodynamic and kinetic properties of the Mn-containing hydrogen storage alloys [31].

A study of the effect of the partial substitution of Ni by Mn on the structure and properties of the A₂B₇-type LaY₂Ni_10.5 superlattice alloy is therefore necessary and is presented in the present work.