Mg-containing multi-principal element alloys for hydrogen storage: A study of the MgTiNbCr_0.5Mn_0.5Ni_0.5 and Mg_0.68TiNbNi_0.55 compositions

Highlights

• Two new Mg-containing multicomponent alloys were investigated for hydrogen storage.

• Multicomponent hydrides were synthesized directly via reactive milling.

• In-situ synchrotron XRD showed a phase transition during hydrogen sorption.

• Different hydrides were formed during synthesis.

• The Mg_0.68TiNbNi_0.55 alloy is suggested as a good candidate for hydrogen storage.

Abstract

Recently, there has been growing interest in multi-principal element alloys for hydrogen storage. However, most of the papers published so far report compositions based only on transition metal elements, which limit the gravimetric storage capacities due to their densities. Since Mg is a low-density element promising for hydrogen storage, the study of Mg-containing multi-principal element compositions is opportune. In the present work, we report for the first time the structural characterization and hydrogen storage properties of the A₂B type MgTiNbCr_0.5Mn_0.5Ni_0.5 alloy and its derivative Mg_0.68TiNbNi_0.55 alloy. These Mg-containing multi-principal element alloys form major BCC phase (W-type, Im$\overline{3}$m) and major FCC hydride (MH₂ with CaF₂-type structure) when synthesized by mechanical alloying (MA) and reactive milling (RM), respectively. Hydrogen is desorbed from both RM samples in two steps, with some overlap, from different hydrides formed during synthesis. The microstructure of the Mg_0.68TiNbNi_0.55 composition is more homogeneous (less secondary phases), but both alloys present a total gravimetric capacity of around 1.6 wt% H₂.

Introduction

Currently, hydrogen is considered an ideal energetic vector for exploiting the benefits of renewable energy sources and reducing the pollution from fossil fuels combustion, as it can be used in fuel cells that generate only water as a by-product. However, safe and efficient hydrogen storage is still challenging. Several strategies to synthesize materials for hydrogen storage have been reported, such as melting methods for obtaining bulk alloys [1], soft- and hard-templating for nanoporous/mesoporous materials [2,3], and high-energy ball milling (HEBM) for nanostructured materials and solid solutions [4]. Metal hydrides have been extensively studied as an alternative for storing hydrogen [[5], [6], [7]]. Among a large number of metal hydrides investigated so far, alloys and intermetallic compounds are the most important ones due to volume effectiveness, high energy density, reversibility, and safety [1,8]. The AB₅ and AB₂ Laves-phase compounds and some body-centered cubic (BCC) alloys are candidates that have shown high capacity of storing hydrogen [9,10]. Conventional BCC alloys based on 3d transition elements such as Ti, V, and Cr are one of the most promising classes of materials considering the large storage capacity up to 2 hydrogen over metal atom ratio (H/M = 2) [[11], [12], [13]]. Nevertheless, none of these promising alloys fulfills all the requirements for practical applications, which indicates that the field of hydrogen storage in the solid-state still needs more investigations. Recently, a new alloying strategy based on the idea of using multi-principal elements has emerged and gained attention. In this strategy, significant atom fractions of four or more alloying elements are mixed in nearly equal concentrations (non-equal concentrations are also used). High entropy alloys (HEAs) are within this new alloying concept, and according to Yeh et al. [14], HEAs can be defined as alloys with five or more alloying elements mixed in atomic concentrations between 5% and 35%. These multi-principal element alloys can crystallize into simple single-phase solid solutions with BCC, face-centered cubic (FCC), and hexagonal close-packed (HCP)-types structures. The fundamentals behind the formation and stability of such phases are still under consideration, but it seems that several chemical and physical aspects like configurational entropy, mixing enthalpy, atomic misfit, and valence electron concentration play an essential role [15,16].

More recently, several HEAs have been reported as good candidates for hydrogen storage applications. Kao et al. [17] reported that the CoFeMnTi_xV_yZr_z alloy, whose structure is single C14 Laves phase, absorbs hydrogen reversibly for x, y, and z in the range of 0.5 ≤ x ≤ 2.5, 0.4 ≤ y ≤ 3.0 and 0.4 ≤ z ≤ 3.0. According to the authors, the reaction kinetics and maximum storage capacity are related to the lattice parameters and reaction enthalpy, respectively. Kunce et al. [18] proposed another HEA with a dominant C14 Laves matrix. The obtained maximum hydrogen capacity exhibited by the ZrTiVCrFeNi alloy is 1.81 wt% H₂ after synthesis and 1.56 wt% H₂ after additional treatment; however, the hydrogen desorption was incomplete due to low equilibrium pressure. Sahlberg et al. [19] investigated the equiatomic TiVZrNbHf. This alloy crystallizes into a BCC structure and can absorb much higher amounts of hydrogen than its constituents, reaching an H/M of 2.5. The authors proposed that the large hydrogen storage capacity is due to the lattice strain that makes favorable the absorption of hydrogen in both tetrahedral and octahedral interstitial sites. To test the latter hypothesis, the TiVZrNbHf was studied further by in-situ and ex-situ neutron diffraction. It was verified that the BCC phase undergoes a phase transition to a body-centered tetragonal (BCT) hydride with hydrogen occupying both tetragonal and octahedral interstitial sites. However, in this case, the alloys could not reach H/M = 2.5 again due to different experimental conditions (temperature and H₂ pressure) [20]. Zlotea et al. [21] investigated another equiatomic composition. The TiZrNbHfTa alloy forms a stable BCC phase after arc melting and homogenization by induction heating. This alloy undergoes a two-stage hydrogen absorption reaction to an FCC dihydride with an intermediate tetragonal monohydride, very similar to the V–H system, although V is absent in the composition. Zepon et al. [22] studied an A₂B type HEA system, namely, MgTiZrFe_0.5Co_0.5Ni_0.5. The A elements are dihydride formers such as Mg, Ti, and Zr. B elements are transition metals that form complex hydrides with Mg in an A₂B stoichiometry. In this study, the alloy was processed by HEBM under both argon and hydrogen atmospheres. This alloy forms a BCC structure when milled under argon, and this phase absorbs up to 1.2 wt% H₂ (H/M = 0.7) before it undergoes a phase transition to FCC hydride during absorption kinetics. The full transformation to the dihydride phase (H/M = 2) was not complete due to kinetics and thermodynamics hindering, but if it had happened, the gravimetric capacity of the alloy would have been 3.5 wt% H₂. Interestingly, HEBM could directly synthesize the FCC dihydride under hydrogen pressure. Motivated by the results presented by Sahlberg et al. [16], Nygård et al. [23] investigated the structure and hydrogen storage properties of a series of HEAs based on Ti, V, Zr, Nb, and Ta. The authors intended to evaluate the influence of the varying degree of local lattice strain on the hydrogen storage properties. The alloys crystallize into a BCC structure and form FCC hydrides with H/M close to 2. However, no correlation between the hydrogen storage capacity and the local lattice strain was observed. Recently, the same group has studied a group of HEAs related to the ternary system TiVNb. These alloys also have BCC crystal structure and form FCC hydrides with H/M close to 2. The authors found that a larger expansion of the lattice during phase transition (BCC to FCC) may destabilize the metal hydrides, and the destabilization can be tuned by changing the valence electron concentration. Based on this, the group proposed the TiVCrNbH₈ as a suitable material for hydrogen storage [24]. More recently, Montero et al. [25] reported the synthesis optimization, physicochemical, and hydrogen storage properties of Ti_0.325V_0.275Zr_0.125Nb_0.275 multi-principal element alloy. The alloy was synthesized both by arc melting and ball milling under Ar, forming a BCC phase that transforms into a BCT dihydride with a maximum hydrogen uptake of 1.7 H/M (2.5 wt% H₂). The dihydride could be directly synthesized by reactive ball milling under hydrogen pressure, and this material presented the best hydrogen sorption properties.

Hitherto, multi-principal element alloys that crystallize into BCC or Laves phase structures have shown high potential for the development of new metal hydrides with excellent storage properties for practical applications. However, most of the compositions proposed are based just on transition metal elements, which might limit the hydrogen storage gravimetric capacities due to their densities. Mg is a promising element in terms of hydrogen storage, and the study of Mg-containing compositions may be fundamental for the development of low-density multi-principal element alloys for hydrogen storage. Here, we present for the first time the structural characterization and hydrogen storage properties of the A₂B type MgTiNbCr_0.5Mn_0.5Ni_0.5 alloy and its derivative Mg_0.68TiNbNi_0.55 alloy. In the A₂B type composition, A and B represent dihydride forming elements (Mg, Ti, Nb) and non-hydride forming elements (Cr, Mn, Ni), respectively. The study of these Mg-containing multi-principal element alloys was conducted by using different techniques to analyze the formation and stability of phases after synthesis processes, the structural behavior during hydrogen absorption/desorption, and the capacity of the alloys to store hydrogen.