Solid-state hydrogen storage using metal hydrides offers a promising solution for safe and compact hydrogen containment. Unlike gaseous or liquid storage, metal hydrides bind hydrogen within a solid matrix, enabling high volumetric storage density and eliminating the need for high-pressure or cryogenic conditions. The technology relies on reversible hydrogen absorption and desorption within metallic or intermetallic compounds, governed by thermodynamic and kinetic principles.
Metal hydrides form when hydrogen atoms occupy interstitial sites within a metal lattice or chemically bond to form complex compounds. The process involves dissociative adsorption of hydrogen molecules on the surface, followed by diffusion into the bulk material. Absorption is exothermic, releasing heat, while desorption requires energy input to break metal-hydrogen bonds. The equilibrium pressure and temperature for these reactions determine practical usability.
Key classes of metal hydrides include AB5, AB2, and complex hydrides. AB5-type alloys, such as LaNi5, exhibit moderate storage capacities (1.4–1.5 wt%) and favorable kinetics due to their crystalline structure. They operate near room temperature and ambient pressure, making them suitable for stationary applications. AB2-type alloys, like TiFe, offer higher capacities (1.8–2.0 wt%) but require activation treatments to overcome surface oxide barriers. Complex hydrides, such as sodium alanate (NaAlH4), store hydrogen through multi-step decomposition reactions, achieving capacities up to 5.6 wt%. However, they suffer from high desorption temperatures and sluggish kinetics without catalysts.
Thermodynamics dictate the stability of metal-hydrogen bonds, often described using pressure-composition-temperature (PCT) curves. The enthalpy of formation determines the energy required for hydrogen release; ideal hydrides balance moderate bond strength to enable reversible operation at practical conditions. For example, LaNi5 has a formation enthalpy of -30 kJ/mol H2, while MgH2, with -75 kJ/mol H2, requires excessive heating (>300°C) for desorption. Kinetics depend on surface reactivity, hydrogen diffusion rates, and nucleation barriers. Slow kinetics in complex hydrides arise from solid-state phase transformations and mass transport limitations.
Recent advancements focus on alloy design, nanostructuring, and catalytic doping to enhance performance. Alloying modifies thermodynamic properties by altering electronic structures and lattice parameters. Partial substitution in AB5 alloys (e.g., replacing Ni with Co or Al) adjusts plateau pressures and cycling stability. Nanostructuring reduces diffusion path lengths and increases surface area, improving kinetics. Ball milling MgH2 with transition metal oxides (e.g., Nb2O5) decreases desorption temperatures by facilitating nucleation. Catalytic doping with titanium or carbon accelerates surface reactions; for instance, adding TiCl3 to NaAlH4 lowers decomposition temperatures by 50°C and enables rehydrogenation under milder conditions.
Metal hydrides offer inherent safety advantages, as hydrogen remains chemically bound, minimizing leakage risks. Their volumetric density (up to 150 kg/m³) surpasses compressed gas or liquid storage, benefiting space-constrained applications. However, challenges persist. Gravimetric capacity remains low for many alloys, hindered by the weight of host metals. Degradation from cyclic expansion and impurity sensitivity reduces lifespan. Complex hydrides face irreversibility issues without additives.
Ongoing research explores metastable hydrides, composite materials, and machine learning-guided discovery. Lightweight alloys like Mg-based systems are optimized through nano-confinement in porous scaffolds. Multi-component hydrides combine elements to tailor thermodynamics, such as LiBH4-MgH2 composites destabilizing each other’s bonds. These innovations aim to meet targets for automotive and grid-scale storage, where rapid refueling and energy efficiency are critical.
In summary, metal hydrides present a viable pathway for solid-state hydrogen storage, balancing density and safety. While kinetic and thermodynamic hurdles remain, material engineering continues to expand their feasibility for a hydrogen-powered future.