Rare earth-based metal hydrides represent a critical class of materials for hydrogen storage due to their unique ability to absorb and desorb hydrogen under moderate conditions. Among these, intermetallic compounds such as LaNi5 and mischmetal-based MmNi5 have been extensively studied for their favorable thermodynamic and kinetic properties. These materials exhibit high volumetric hydrogen storage densities, reversible hydrogenation behavior, and relatively low operating temperatures and pressures compared to other hydride systems. Their development has been driven by the need for efficient, safe, and compact hydrogen storage solutions, particularly for stationary and mobile applications.

The crystal structure of rare earth-based hydrides plays a fundamental role in their hydrogen storage capabilities. LaNi5, for instance, crystallizes in a hexagonal CaCu5-type structure, where lanthanum atoms occupy the larger atomic sites, and nickel atoms fill the smaller interstitial positions. When exposed to hydrogen, the material forms LaNi5Hx, where x can reach up to approximately 6.5 hydrogen atoms per formula unit, corresponding to a hydrogen storage capacity of around 1.4 wt%. The hydrogen atoms occupy interstitial sites within the lattice, leading to minimal structural distortion during cycling. MmNi5, which substitutes lanthanum with a naturally occurring mixture of rare earth elements (mischmetal), exhibits similar behavior but often with improved cycle life due to the synergistic effects of multiple rare earth components.

One of the defining characteristics of rare earth-based hydrides is their moderate thermodynamic stability, which allows for hydrogen release at practical temperatures and pressures. The equilibrium pressure for LaNi5 at room temperature is typically in the range of 1 to 2 bar, making it suitable for near-ambient applications. The enthalpy of formation for these hydrides generally falls between -30 and -40 kJ/mol H2, striking a balance between stability and ease of dehydrogenation. This property is particularly advantageous for systems requiring frequent hydrogen cycling without excessive energy input for release.

The kinetics of hydrogen absorption and desorption in these materials are also noteworthy. Rare earth-based hydrides demonstrate rapid reaction rates due to the catalytic effect of nickel, which facilitates hydrogen dissociation and recombination at the surface. Activation energy barriers are relatively low, enabling full hydrogen capacity to be achieved within minutes under optimal conditions. Surface treatments and alloy modifications, such as partial substitution of nickel with aluminum or manganese, can further enhance kinetic performance by improving resistance to oxidation and poisoning.

Synthesis of rare earth-based hydrides typically involves metallurgical processes such as arc melting, induction melting, or mechanical alloying. Arc melting is the most common method, where stoichiometric amounts of high-purity rare earth metals and transition metals are melted under an inert atmosphere to form homogeneous intermetallic compounds. Subsequent annealing may be applied to eliminate compositional inhomogeneities. Mechanical alloying, a solid-state powder processing technique, offers an alternative route that can produce nanocrystalline or amorphous phases with potentially improved hydrogen storage properties. However, achieving precise stoichiometry and phase purity remains a challenge with this method.

Performance metrics for these hydrides highlight their advantages and limitations. Gravimetric hydrogen storage capacities for LaNi5 and MmNi5 are modest, usually below 2 wt%, which is a drawback for weight-sensitive applications such as automotive storage. However, their volumetric capacities are competitive, reaching up to 115 kg H2/m3, making them attractive for space-constrained systems. Cycle stability is another critical parameter; rare earth-based hydrides can typically withstand thousands of absorption-desorption cycles with minimal degradation, provided that impurities such as oxygen and moisture are minimized. The presence of contaminants can lead to surface passivation, reducing effective storage capacity over time.

Cost and resource availability are significant limitations for widespread adoption. Lanthanum and other rare earth elements are subject to supply chain fluctuations and geopolitical factors, leading to concerns about long-term material sustainability. Mischmetal-based alloys offer a partial solution by utilizing less refined rare earth mixtures, but they still rely on critical raw materials. Additionally, the weight of these hydrides remains a challenge for mobile applications where energy density is paramount.

When compared to other metal hydride families, rare earth-based systems occupy a distinct niche. Unlike complex hydrides such as sodium alanate or magnesium-based hydrides, they do not require high temperatures for hydrogen release, nor do they suffer from slow kinetics. In contrast to interstitial hydrides like those based on titanium or zirconium, rare earth-based materials operate at more practical pressure ranges without the need for extreme thermal management. However, they are generally outperformed by lightweight hydrides in terms of gravimetric capacity, limiting their use in applications where weight is a primary constraint.

Modification strategies have been explored to optimize the properties of rare earth-based hydrides. Partial substitution of nickel with elements such as cobalt, iron, or copper can fine-tune thermodynamic parameters and improve corrosion resistance. For example, substituting 20% of nickel with aluminum in LaNi5 increases plateau pressure and enhances cycle life by reducing lattice strain during hydrogen cycling. Similarly, MmNi5 alloys with adjusted mischmetal compositions can exhibit tailored properties for specific operating conditions. These adjustments highlight the versatility of rare earth-based hydrides in meeting diverse application requirements.

Degradation mechanisms in these materials primarily involve surface oxidation, phase segregation, and microstructural fatigue. Exposure to air or moisture leads to the formation of oxide layers that impede hydrogen absorption. Repeated cycling can induce lattice defects and particle fracturing, gradually reducing storage capacity. Advanced encapsulation techniques and protective coatings have been investigated to mitigate these effects, though they often add complexity and cost to the system.

In industrial applications, rare earth-based hydrides have found use in niche areas where their moderate operating conditions and reliability are prioritized over absolute storage capacity. Examples include hydrogen compressors, purification systems, and stationary storage units where weight is less critical than safety and longevity. Research continues to explore new compositions and hybrid systems that combine rare earth hydrides with other storage materials to overcome inherent limitations.

The future of rare earth-based metal hydrides hinges on advancements in material science and engineering. Efforts to reduce reliance on critical raw materials through recycling and alternative sourcing are essential for sustainable deployment. Meanwhile, innovations in nanostructuring and composite formation may unlock further improvements in performance, pushing these materials toward broader adoption in the hydrogen economy. Their established role in hydrogen storage underscores the importance of continued research to address remaining challenges while leveraging their unique advantages.