Metal hydrides represent a critical class of materials for hydrogen storage, offering unique advantages in safety, volumetric efficiency, and reversibility. These materials function by chemically bonding hydrogen atoms within their crystal structures, enabling compact storage at moderate pressures and temperatures. The underlying mechanisms of hydrogen absorption and desorption, governed by thermodynamics and kinetics, make metal hydrides suitable for applications ranging from stationary storage to mobile fuel systems.

The storage process in metal hydrides begins with the dissociation of molecular hydrogen (H2) into atomic hydrogen at the material's surface. This step is facilitated by catalytic properties inherent to certain metals or surface treatments. Once dissociated, hydrogen atoms diffuse into the bulk material, occupying interstitial sites within the crystal lattice. The nature of these sites depends on the host metal's structure, with common configurations including tetrahedral and octahedral voids in cubic or hexagonal arrangements. The occupation of these sites forms a metal-hydrogen solid solution, often referred to as the α-phase. As hydrogen concentration increases, a phase transition occurs, leading to the formation of a hydride phase (β-phase), accompanied by significant lattice expansion.

Thermodynamics plays a central role in determining the equilibrium conditions for hydrogen absorption and desorption. The pressure-composition-isotherm (PCI) curve characterizes this behavior, revealing plateaus where the α and β phases coexist. The plateau pressure, typically ranging from 0.1 to 10 bar at room temperature for practical hydrides, defines the operational window for reversible storage. Van't Hoff analysis relates this pressure to temperature, following the equation ln(P) = ΔH/(RT) - ΔS/R, where ΔH and ΔS represent the enthalpy and entropy changes of hydride formation. High-enthalpy hydrides, such as magnesium-based systems (ΔH ≈ -75 kJ/mol H2), require elevated temperatures for hydrogen release, while low-enthalpy hydrides like LaNi5 (ΔH ≈ -30 kJ/mol H2) operate near ambient conditions but exhibit lower gravimetric capacity.

Kinetic factors determine the rate at which hydrogen is absorbed or released. Surface reactions, including dissociation and recombination, often limit performance unless optimized through catalytic additives like platinum or nickel. Bulk diffusion, governed by Fick's laws, depends on temperature and the energy barriers between interstitial sites. Nanostructuring and alloying can enhance these kinetics by reducing diffusion path lengths and modifying electronic structures. For example, ball-milled magnesium hydride with niobium oxide additives shows absorption times reduced from hours to minutes at 300°C.

Crystal structure dictates hydrogen storage properties. AB5-type alloys (e.g., LaNi5) adopt a hexagonal CaCu5 structure with rapid kinetics but limited capacity (1.4 wt%). Body-centered cubic (BCC) alloys, such as Ti-V-Cr systems, achieve higher capacities (2.5-3.0 wt%) through multiple interstitial site occupancies. Magnesium-based hydrides (MgH2) offer high theoretical capacity (7.6 wt%) but face challenges with slow kinetics and high desorption temperatures due to their stable rutile structure. Recent advances in complex hydrides, such as sodium alanate (NaAlH4), demonstrate reversible storage through catalyzed decomposition pathways, though these systems require careful management of side reactions.

Key performance metrics for metal hydrides include gravimetric capacity (wt% H2), volumetric density (kg H2/m³), reversibility, and cycle life. Commercial AB5 hydrides typically achieve 1-2 wt% and 100-150 kg H2/m³, while advanced magnesium systems reach 5-7 wt% in laboratory settings. Volumetric densities often exceed liquid hydrogen (70 kg/m³), making metal hydrides attractive for space-constrained applications. Reversibility depends on structural stability during cycling; some alloys withstand thousands of cycles with minimal degradation, while others suffer from disproportionation or phase segregation. Cycle life is influenced by factors like purity of hydrogen gas, operating temperature range, and mechanical stress from lattice expansion.

Material modifications address limitations in metal hydride performance. Alloying adjusts thermodynamic properties; partial substitution of lanthanum with cerium in LaNi5 lowers plateau pressure. Composite formation with carbon scaffolds improves thermal conductivity and prevents particle agglomeration. Catalytic doping with transition metals or metal oxides enhances surface reactions. For instance, adding 5% FeTiO3 to MgH2 reduces desorption temperature by 50°C while maintaining capacity over 100 cycles.

Practical considerations for metal hydride systems include heat management during hydrogenation, which is exothermic, and dehydrogenation, which is endothermic. Efficient heat exchangers are necessary to maintain reaction rates, especially for high-enthalpy hydrides. System designs often incorporate porous metal foams or heat pipes to distribute thermal energy uniformly. Pressure vessels must accommodate lattice expansion, which can exceed 20% in volume for some alloys.

Environmental and safety aspects favor metal hydrides over compressed gas or cryogenic storage. The inherent stability of bonded hydrogen minimizes explosion risks, and low operating pressures reduce mechanical failure modes. However, material costs and weight remain challenges for widespread adoption, particularly in automotive applications where lightweight solutions are critical.

Ongoing research focuses on destabilized hydride systems, where additives modify reaction pathways to improve thermodynamics without sacrificing capacity. For example, combining lithium borohydride with magnesium hydride lowers dehydrogenation temperatures through intermediate compound formation. Another direction explores high-entropy alloys, where configurational disorder creates a distribution of interstitial site energies, potentially enabling tunable plateau pressures. Computational materials science accelerates discovery by predicting new stable compositions and their hydrogenation properties using density functional theory and machine learning.

In summary, metal hydrides provide a versatile platform for hydrogen storage through precise engineering of atomic-scale interactions. Their development balances fundamental material science with practical engineering constraints, offering solutions that bridge the gap between high-capacity but irreversible chemical hydrides and low-capacity physisorption systems. Continued advances in alloy design, nanostructuring, and system integration will determine their role in enabling a hydrogen-based energy economy.