Magnesium-based hydrides, particularly magnesium dihydride (MgH2), have emerged as promising candidates for solid-state hydrogen storage due to their high gravimetric capacity, abundance, and relatively low cost. With a theoretical hydrogen storage capacity of 7.6 wt%, MgH2 surpasses many other metal hydrides, making it attractive for applications where weight is a critical factor, such as in mobile and portable energy systems. However, the practical deployment of MgH2 faces challenges related to its thermodynamic stability, slow kinetics, and cycling degradation. Recent advances in material engineering, including nanostructuring, alloying, and catalytic doping, have shown potential to overcome these limitations.

The high hydrogen storage capacity of MgH2 stems from the ability of magnesium to reversibly absorb and desorb hydrogen. The reaction proceeds as Mg + H2 ↔ MgH2, with an enthalpy of formation around -75 kJ/mol H2, indicating a highly stable hydride. This thermodynamic stability necessitates high temperatures, typically above 300°C, for hydrogen release, which poses energy efficiency challenges for practical applications. Additionally, the kinetics of hydrogen absorption and desorption are sluggish due to the low diffusion rate of hydrogen in magnesium and the high activation energy required for breaking Mg-H bonds. These factors have driven research into modifying MgH2 to lower desorption temperatures and accelerate reaction rates.

Nanostructuring has proven effective in improving the kinetics and thermodynamics of MgH2. Reducing particle size to the nanoscale increases surface area and shortens hydrogen diffusion pathways, significantly enhancing absorption and desorption rates. Ball milling is a widely used technique to produce nanocrystalline MgH2, often resulting in particle sizes below 100 nm. Studies have demonstrated that ball-milled MgH2 can release hydrogen at temperatures 50-100°C lower than bulk material while achieving faster kinetics. However, nanosized particles are prone to agglomeration and oxidation, which can degrade performance over multiple cycles. Encapsulation with protective coatings or dispersion in porous matrices has been explored to mitigate these issues.

Alloying magnesium with other metals is another strategy to destabilize MgH2 and improve its hydrogen storage properties. Elements such as nickel, aluminum, and transition metals form intermetallic compounds or solid solutions with magnesium, altering the enthalpy of hydride formation. For instance, Mg2Ni, when hydrogenated to Mg2NiH4, exhibits a lower decomposition temperature compared to pure MgH2. Similarly, ternary systems like Mg-In and Mg-Al have shown reduced thermodynamic stability while maintaining reasonable hydrogen capacities. However, alloying often leads to a trade-off between thermodynamic improvement and gravimetric capacity, as additive metals dilute the magnesium content. Careful optimization of composition is necessary to balance these factors.

Catalytic doping is a complementary approach to enhance the kinetics of MgH2 without significantly affecting its thermodynamics. Transition metals, metal oxides, and carbon-based materials have been investigated as catalysts to facilitate hydrogen dissociation and recombination on the MgH2 surface. Titanium-based additives, such as TiCl3 and TiO2, are particularly effective, reducing desorption temperatures by up to 70°C and improving absorption rates. Carbon nanotubes and graphene have also been employed as supports to prevent particle aggregation and provide conductive pathways for electron transfer during hydrogenation. The synergistic effects of combining multiple catalysts, such as Ni and Nb2O5, have further improved performance, achieving near-ambient hydrogen absorption under moderate pressures.

Despite these advancements, scalability and cycling stability remain critical challenges for MgH2-based storage systems. Industrial-scale production of nanostructured or catalyzed MgH2 requires cost-effective and reproducible methods. Ball milling, while effective for research purposes, may not be easily scalable due to energy consumption and contamination risks. Alternative synthesis routes, such as vapor deposition or solution-based methods, are being explored but face their own limitations in yield and purity. Cycling stability is another concern, as repeated hydrogenation and dehydrogenation can lead to particle coarsening, phase segregation, and capacity loss over time. Strategies such as in-situ regeneration and the use of resilient scaffold materials are under investigation to extend the operational lifespan of MgH2 storage systems.

The environmental and economic aspects of MgH2 also warrant consideration. Magnesium is abundant and non-toxic, making it a sustainable option compared to rare-earth or heavy-metal hydrides. However, the energy input required for high-temperature desorption and the need for catalytic additives may offset some of these advantages. Life cycle assessments indicate that the overall carbon footprint of MgH2-based storage depends heavily on the hydrogen production method and the energy source used for material processing. Pairing MgH2 with renewable energy systems could enhance its sustainability profile.

In summary, magnesium-based hydrides offer a compelling combination of high hydrogen capacity and material availability, but their practical implementation hinges on overcoming thermodynamic and kinetic barriers. Nanostructuring, alloying, and catalytic doping have demonstrated significant improvements in performance, though challenges in scalability and cycling stability persist. Future research directions may focus on integrated material designs that combine multiple modification strategies while optimizing cost and durability. As hydrogen storage technologies evolve, MgH2 remains a strong contender for lightweight applications, provided that these technical hurdles can be addressed effectively.