Application progress of two-dimensional materials in magnesium-based solid-state hydrogen storage

Magnesium hydride (MgH2) is broadly considered one of the most promising solid-state hydrogen storage materials for large-scale applications owing to its abundance, excellent reversibility, cost-effectiveness, and high theoretical hydrogen storage density (7.6 wt%). However, the application of MgH2 in solid-state hydrogen storage is considerably hindered by its strong thermodynamic stability (ΔH = 74.7 kJ mol–1 H2), sluggish kinetics, and inevitable particle agglomeration and grain growth during cycling. To address these challenges, numerous studies have focused on modifying the thermodynamic and kinetic properties of MgH2, yielding remarkable results. Two-dimensional (2D) materials, with their large surface areas, abundant surface ends, and excellent chemical and physical stability, serve as effective catalysts for hydrogen storage in MgH2 and as ideal nano-confined scaffolds. Based on the current research status of magnesium-based solid hydrogen storage materials worldwide, this paper summarises the progress made in utilising various 2D materials such as graphene, 2D metals and metal oxides, as well as MXenes, as catalysts or carriers to enhance the hydrogen absorption/desorption kinetics of MgH2. The influence mechanisms of these 2D materials on the hydrogen storage performance of MgH2 are elaborated. For example, as a high-efficiency catalyst, graphene functions as a buffer during the ball milling process, effectively preventing grain growth and particle agglomeration caused by excessive refinement of MgH2 particles. Moreover, it serves as a nucleation and growth centre for MgH2/Mg during hydrogenation/dehydrogenation, facilitating the diffusion of H atoms and thereby promoting the dehydrogenation/rehydrogenation of MgH2. In addition, graphene’s large 2D surface allows it to form composites with transition metal–based catalysts, enabling synergistic enhancement of hydrogen absorption and desorption of Mg/MgH2. It can also anchor Mg/MgH2 to realise nanosizing and thermodynamic destabilisation of Mg/MgH2. The efficient catalytic activity of 2D transition metals and transition metal oxides for MgH2 hydrogen storage is generally attributed to the ‘hydrogen pump’ effect, which aids in the dissociation of H2 and diffusion of H atoms, as well as the in-situ formation of transition metal monomers that provide high catalytic activity or a multivalent environment that accelerates electron transfer during the dehydrogenation/rehydrogenation of MgH2. Furthermore, density functional theory calculations reveal that transition metal–based catalysts interact with the H 1s electron orbitals of Mg through their unsaturated d electron orbitals, weakening the Mg–H bond, reducing the dissociation energy of hydrogen molecules on the Mg surface, and ultimately improving the hydrogen adsorption/desorption kinetics of Mg/MgH2. MXenes, which offer abundant metal active sites, are excellent catalysts for Mg/MgH2 hydrogen storage. They not only provide abundant catalytically active sites and hydrogen diffusion pathways but also effectively inhibit the growth and agglomeration of Mg/MgH2. Notably, the in-situ formation of metal-based active substances substantially contributes to the high catalytic activity of most MXenes. In addition, MXenes’ large specific surface area with easy-to-modify surface groups makes them an ideal catalyst carrier/nano-confined Mg/MgH2 scaffold. Finally, this paper summarises the challenges associated with 2D layered catalysts for magnesium-based solid hydrogen storage, offering insights into the future design of high-performance magnesium-based composite hydrogen storage materials. More comprehensive investigations are necessary to adequately exploit the potential of 2D layered materials in catalysing Mg/MgH2-based hydrogen storage and to overcome the remaining challenges in the practical application of magnesium-based solid-state hydrogen storage.