Intermetallic compound hydrides represent a critical class of materials for hydrogen storage, offering unique advantages due to their reversible absorption and desorption characteristics. Among these, AB2 and AB5-type alloys are prominent due to their tunable properties, which make them suitable for various applications. These materials form hydrides through a chemical reaction with hydrogen, storing it in their crystal lattice. The ability to modify their composition via alloying allows precise control over hydrogen storage properties, including capacity, kinetics, and thermodynamic stability.

AB5-type intermetallic hydrides, such as LaNi5, consist of a rare-earth element (A) and a transition metal (B). These compounds exhibit a hexagonal crystal structure and demonstrate moderate hydrogen storage capacities, typically around 1.4 to 1.6 wt%. Their key advantage lies in their favorable kinetics and relatively low desorption temperatures, often between 20°C and 80°C at moderate pressures. The plateau pressure, which defines the equilibrium between hydrogen absorption and desorption, can be adjusted by substituting elements in the A or B sites. For example, replacing lanthanum with mischmetal or cerium alters the plateau pressure, while substituting nickel with aluminum or manganese increases cycle stability but may reduce capacity.

AB2-type alloys, such as TiMn2 or ZrV2, have a cubic Laves phase structure and generally offer higher hydrogen storage capacities, ranging from 1.8 to 2.5 wt%. These materials exhibit higher plateau pressures compared to AB5-types, making them suitable for applications requiring rapid hydrogen release. Alloying adjustments in AB2 compounds often involve substituting titanium or zirconium with other transition metals to optimize hysteresis effects—the difference between absorption and desorption pressures. Hysteresis is a critical factor in energy efficiency, as excessive hysteresis increases the energy penalty for hydrogen cycling.

The tunability of intermetallic hydrides is achieved through strategic alloying, which modifies the electronic and structural properties of the host lattice. For AB5 alloys, partial substitution of nickel with cobalt improves corrosion resistance, while aluminum additions enhance cycle life by reducing pulverization. In AB2 systems, vanadium or chromium substitutions can fine-tune plateau pressures to match specific operational requirements. These modifications enable the design of hydrides tailored for either stationary or mobile applications.

In stationary hydrogen storage systems, such as those for renewable energy buffering or industrial use, AB5-type hydrides are often preferred due to their lower desorption temperatures and excellent reversibility. Their moderate storage capacity is less of a limitation in fixed installations where weight and volume constraints are relaxed. Additionally, their ability to operate near ambient temperatures reduces the need for extensive thermal management systems, lowering overall costs. The long cycle life and stability of AB5 hydrides make them ideal for applications requiring frequent hydrogen cycling, such as backup power systems or grid balancing.

For mobile applications, including fuel cell vehicles, AB2-type hydrides offer advantages due to their higher gravimetric and volumetric storage densities. The ability to operate at higher pressures allows for more compact storage systems, a critical factor in transportation where space and weight are at a premium. However, the higher hysteresis and desorption temperatures of some AB2 alloys necessitate efficient thermal management, increasing system complexity. Advances in alloy design have mitigated these issues, with newer compositions exhibiting reduced hysteresis and improved kinetics, making them increasingly viable for automotive use.

Plateau pressure is a defining characteristic of intermetallic hydrides, dictating the operational conditions for hydrogen release. A flat plateau indicates consistent hydrogen equilibrium over a range of concentrations, ensuring predictable performance. Deviations from flat plateaus, such as sloping or multiple plateaus, can result from inhomogeneities in alloy composition or lattice strain. Alloying strategies aim to minimize these deviations to enhance efficiency. For instance, in AB5 alloys, homogeneity in lanthanum distribution ensures a single, well-defined plateau, while in AB2 systems, precise control of titanium and manganese ratios prevents phase separation during cycling.

Hysteresis effects in intermetallic hydrides arise from energy losses during absorption and desorption, often due to lattice expansion and contraction. AB5 alloys generally exhibit lower hysteresis compared to AB2 types, contributing to higher round-trip efficiency. Reducing hysteresis involves optimizing the alloy's elastic properties to minimize mechanical energy dissipation. For example, adding small amounts of silicon to AB5 compounds reduces lattice strain, thereby lowering hysteresis losses. In AB2 systems, nanostructuring or introducing secondary phases can mitigate hysteresis by accommodating volume changes more effectively.

Practical challenges remain in deploying intermetallic hydrides for large-scale hydrogen storage. Degradation mechanisms such as pulverization, oxidation, and impurity poisoning can reduce cycle life. AB5 alloys are more susceptible to surface oxidation, particularly in humid environments, while AB2 systems may suffer from phase segregation after prolonged cycling. Protective coatings and optimized operating conditions can extend material lifetimes. For instance, encapsulating AB5 particles with thin polymer films prevents oxidation without significantly impeding hydrogen diffusion.

In summary, intermetallic compound hydrides like AB2 and AB5-types provide versatile solutions for hydrogen storage, with properties that can be finely adjusted through alloying. Their distinct characteristics make them suitable for different applications—AB5 alloys excel in stationary systems requiring durability and moderate conditions, while AB2 alloys are better suited for mobile applications demanding higher storage densities. Continued advancements in material design aim to further improve their performance, addressing challenges such as hysteresis, degradation, and system integration. The ongoing development of these materials will play a pivotal role in enabling broader adoption of hydrogen-based energy systems.