A focused mini-review on high-entropy intermetallics for hydrogen storage

Abstract

Intermetallic compounds (IMCs) are considered desirable materials for hydrogen storage. However, traditional hydrogen-storage IMCs have many shortcomings. High-entropy alloys (HEAs), which are composed of multiple metallic elements, exhibit significant lattice distortion and large interstitial sites, making them a promising class of hydrogen storage materials. Among the HEAs used for hydrogen storage, high-entropy intermetallics (HEIs) have shown great potential for hydrogen absorption kinetics and cycling stability, particularly for room-temperature hydrogen storage. This review systematically summarizes research progress on HEIs for hydrogen storage. It first presents a statistical analysis of composition design methods for these alloys, including empirical criteria based on parameters such as valence electron concentration and atomic size mismatch, as well as thermodynamic calculations such as the calculated phase diagram (CALPHAD) method. It then summarizes the characteristics and hydrogen storage performance of the reported alloys, with a detailed discussion of their phase compositions, microstructures, and the corresponding effects on hydrogen storage properties. Particular emphasis is placed on the critical roles of phase boundaries, multiphase synergy, and specific microstructural features (e.g., networked/eutectic morphologies) in enhancing activation performance and improving hydrogen diffusion kinetics. Although the current hydrogen storage capacity of HEIs (approximately 1 H/M (hydrogen-to-metal atomic ratio)) remains lower than that of body-centered cubic (BCC)-structured HEAs, their exceptional reversible hydrogen absorption/desorption capability at room temperature, lack of activation requirements, and remarkable cycling stability make them highly promising for applications in which a moderate capacity is sufficient, such as mobile hydrogen storage. This review provides a systematic summary of research progress on HEIs for hydrogen storage, focusing on the effects of alloy design strategies, phase composition, and microstructural regulation on hydrogen storage properties. The primary challenge currently facing HEIs is their relatively low hydrogen storage capacity. Accordingly, this paper outlines future development directions to address this issue. This review provides a theoretical basis and guidance for the development of next-generation high-performance hydrogen storage materials.

1. Introduction

Solid-state hydrogen storage is considered the most promising technology in the field of hydrogen storage because of its high safety and high hydrogen storage density, achieved by storing hydrogen within materials. Solid-state hydrogen storage can be realized either physically or chemically, depending on how the storage medium interacts with hydrogen. Physical methods primarily rely on the adsorption of hydrogen onto the surfaces of various carbon-based materials, including activated carbon, carbon nanotubes, and carbon fibers [1–2]. However, their low hydrogen storage capacity at ambient temperature and pressure (<2wt% (mass capacity, the mass ratio of absorbed hydrogen molecules to the initial alloy matrix)) hinders their application in solid-state hydrogen storage systems. In chemical methods, diverse hydrogen storage materials can reversibly interact with hydrogen to form hydrides [3–5]. Owing to the advantages of chemical reactions in terms of hydrogen storage density and cycling performance, research has mainly focused on chemical hydrogen storage methods.

Chemical solid-state hydrogen storage materials can be classified into complex hydrides [6] (e.g., Na(AlH4)4, Li(BH4)4, Mg(NH2)2-LiH, and NH3BH3), liquid organic hydrides [7] (e.g., monocyclic aromatic hydrocarbons, benzene, and toluene), and metallic materials [8]. Among the aforementioned hydrogen storage materials, metallic materials are among the most promising hydrogen carriers owing to their favorable cost, high safety, and high volumetric energy density [9]. In general, metallic materials used for hydrogen storage are composed of metal elements with negative binding energies for hydrogen (A-type elements) and those with positive binding energies for hydrogen (B-type elements). At present, the main types of hydrogen storage alloys include Mg-based (A2B-type), Ti-based (AB-type), V-based (body-centered cubic), rare-earth-based (AB5-type), and Zr-based (AB2-type) alloys, distinguished by their alloy compositions and elemental ratios [8,10–14]. Because of the moderate conditions required for hydrogen absorption and release, metallic compounds such as LaNi5 and TiFe have been used to some extent [15]. However, further improvement of the hydrogen storage performance of these traditional metallic materials remains highly challenging.