Optimization of Mg-based hydrogen storage materials with multicomponent and high-entropy catalysts

Abstract

Novel hydrogen storage materials have propelled progress in hydrogen storage technologies. Magnesium hydride (MgH2) is a highly promising candidate. Nevertheless, several drawbacks, including the need for elevated thermal conditions, sluggish dehydrogenation kinetics, and high thermodynamic stability, limit its practical application. One effective method of addressing these challenges is catalyst doping, which effectively boosts the hydrogen storage capability of Mg-based materials. Herein, we review recent advancements in catalyst-doped MgH2 composites, with particular focus on multicomponent and high-entropy catalysts. Structure–property relationships and catalytic mechanisms in these doping strategies are also summarized. Finally, based on existing challenges, we discuss future research directions for the development of Mg-based hydrogen storage systems.

1. Introduction

In recent decades, energy shortages and climate change have heightened demand for cleaner and more sustainable energy solutions. Hydrogen is regarded as an ideal substitute for conventional fossil fuels, paving the way for a zero-carbon future [1–3]. Despite its advantages, a hydrogen-based economy will be difficult to achieve [4]. One of the primary obstacles is hydrogen storage [5]. Current options include high-pressure gas, cryogenic liquid, and solid-state storage [6]. High-pressure gas and cryogenic liquid technologies have disadvantages such as low hydrogen storage densities, safety risks, or high energy consumption. By contrast, solid-state technologies have high hydrogen capacities, good safety profiles, and portability [1]. The advantages and disadvantages of these methods are presented in Table 1 [6–9].

Among diverse solid-state hydrogen storage materials, light-metal binary hydrides have been extensively researched owing to their controllable absorption and desorption properties [10]. Magnesium hydride (MgH2) is a promising material for the commercialization of solid-state hydrogen storage technologies. It has an exceptional hydrogen storage density of 7.6wt%, as well as reversible absorption/desorption properties. Furthermore, its constituent elements are in abundant supply. Nevertheless, high temperatures (over 300°C) are required for the complete dehydrogenation of MgH2 owing to the strong Mg–H interactions and high enthalpy of dehydrogenation (74.1 kJ/mol) [11]. Furthermore, the high hydrogen dissociation energy barrier and sluggish hydrogen diffusion result in unsatisfactory hydrogenation kinetics [12]. Therefore, various strategies such as nanostructuring [13–15], alloying [16–18], and catalyst doping [19–20] have been employed to optimize the hydrogen storage performance of MgH2.

Catalyst doping has been extensively studied for modifying the hydrogen storage behavior of Mg-based materials [21]. The catalyst destabilizes the Mg–H bonds during dehydrogenation, provides abundant active sites for hydrogen dissociation, and aids the absorption and diffusion of hydrogen atoms during hydrogenation [22]. Transition metals (TMs), metal oxides, and other compounds (e.g., nitrides and fluorides) have been utilized for this purpose [23]. Nevertheless, single elements and simple compounds have low catalytic activity, hindering their practical application. This has led to the development of numerous multicomponent materials with unique catalytic advantages [24–26]. Multicomponent materials typically consist of three or more chemical components (elements or compounds) combined in specific ways to create composites with unique properties [27–28]. Multicomponent materials are designed to maximize the strengths of each component to achieve superior physical, chemical, or mechanical properties [29–30]. For example, Jiang et al. [31] demonstrated that two multicomponent catalysts, (Cu0.2Ni0.8)O/NiFe2O4 and FeNi3/Fe4Cu3, enhanced the hydrogen storage capacity of MgH2 via the in situ formation of Fe along with the reversible phase transitions of Mg2Ni/Mg2NiH4. High-entropy alloys (HEAs), which comprise five or more principal elements in equiatomic or near-equiatomic ratios, are an innovative type of multicomponent material [32–33]. Their distinct compositional design endows them with high microstructural disorder and homogeneity, resulting in outstanding properties [34–36].