Thermodynamic and kinetic destabilization of Mg solid-solution alloys with nanosized grains for hydrogen storage

Abstract

Solid-solution magnesium-based alloys have garnered significant attention for hydrogen storage applications. However, their practical implementation has been limited by their stable thermodynamic properties and sluggish kinetics. Herein, we report a nanoengineering approach to simultaneously enhance the kinetic and thermodynamic properties of Mg-based solid-solution alloys. Using Mg(In) alloys as a model system, we demonstrate this positive size effect through a two-step fabrication process. First, the Mg0.9In0.1 alloy was synthesized via ball milling combined with absorption/desorption cycles. Subsequently, the alloy was subjected to high-pressure milling under a 4 MPa H2 atmosphere with immiscible Mn at a controlled molar ratio, resulting in Mg(In) nanograins uniformly embedded within the Mn-composite matrix (denoted as (Mg0.9In0.1)xMn1−x). The (Mg0.9In0.1)0.25Mn0.75 nanocomposite, with an average grain size of ~61 nm, demonstrated superior hydrogen storage properties. Compared with pure MgH2, this material exhibits much lower onset and peak temperatures for hydrogen release, at ~120 and ~240°C, respectively. Moreover, enhanced kinetic performance, with a significantly lower activation energy of ~78.34 kJ/mol, and improved cycling stability, with 97% retention after 50 cycles, are achieved due to the Mg(In) nanograins, which remain well-preserved even upon multiple cycles. This study highlights that the synergistic combination of solid-solution formation and nanoscale engineering can effectively modify the thermodynamic and kinetic properties of Mg-based hydrogen storage alloys, offering a promising approach for the development of high-performance magnesium-alloy hydrogen storage materials.

1. Introduction

Hydrogen energy is considered a core secondary energy source for achieving carbon neutrality goals because of its unique advantages, including a gravimetric energy density of up to 142 MJ/kg and a combustion product consisting only of water [1–3]. However, the volumetric energy density of hydrogen under standard conditions is only 0.01079 MJ/L, making the development of efficient hydrogen storage technologies a key bottleneck in the hydrogen energy industry [4–5]. Among numerous solid-state hydrogen storage materials, magnesium hydride (MgH2) has become one of the most promising candidates for large-scale applications because of its theoretical hydrogen storage capacity of 7.6wt%, excellent cycle reversibility (>2000 cycles with >95% capacity retention), and crustal abundance advantage (Mg element accounts for approximately 2.3% of the Earth’s crust) [6–8].

Nevertheless, the practical application of MgH2 is constrained by its strong Mg–H covalent bonding (~75 kJ/mol), which results in high thermodynamic stability (dehydrogenation temperature > 300°C) and kinetic limitations (hydrogen diffusion coefficient: 10−14–10−13 cm2/s).

Over the past few decades, extensive efforts have been made to enhance the hydrogen storage performance of MgH2. Although the use of catalysts and nanoparticles can significantly improve the dehydrogenation kinetics of MgH2, these approaches remain far from satisfactory for practical applications because catalyst addition has little effect on the thermodynamic properties of MgH2 [9–11]. The high thermodynamic stability of the Mg/MgH2 system primarily originates from the strong Mg–H covalent bonds. However, this stability can be modulated through structural and compositional modifications [12–14]. Partial substitution of Mg with other metallic elements reduces the strength of the Mg–H bond, thereby destabilizing MgH2. Alloying treatments have proven particularly effective in weakening these bonds and improving the hydrogen storage properties [15–17]. Generally, alloying refers to the introduction of additional metallic elements to form a new alloy with Mg. Many metal elements can combine with Mg to form fixed phases and absorb hydrogen to generate complex hydrides such as Mg2FeH6 [18] and Mg2CoH5 [19]. Shao et al. [20] successfully synthesized nanostructured Mg3CoH5 using plasma technology. Owing to the introduction of Co, the enthalpy of formation of the nano-material (−73.16 kJ/mol) was lower than that of pure MgH2. In addition, Wang et al. [21] prepared Mg–Fe nanocrystals by ball milling under a 0.55-MPa hydrogen atmosphere. The resulting Mg2FeH6 nanocrystals began to release hydrogen at 62°C and completed hydrogenation at 340°C, with a final dehydrogenation capacity of 5.15wt%, thereby maintaining a reasonable hydrogen storage capacity while reducing the hydrogen release temperature. Mg-based solid-solution alloys exhibit higher equilibrium hydrogen pressures than pure Mg. Mechanical alloying of Mg with metals such as Zn, Al, Cd, and In to form Mg-based solid solutions increases the plateau pressure for hyd