Aluminum hydride, or alane, is a chemical hydride with significant potential for hydrogen storage due to its high gravimetric and volumetric hydrogen densities. It contains approximately 10.1% hydrogen by weight, which is higher than many conventional metal hydrides, and a volumetric density of around 148 kg/m³, making it an attractive candidate for applications where space and weight are critical factors. These properties position it as a promising material for aerospace and automotive industries, where energy density and efficiency are paramount.

The decomposition of aluminum hydride is a key aspect of its utility. It releases hydrogen through a thermal decomposition process, which begins at temperatures around 150-200°C, depending on the polymorph and doping strategies employed. The reaction proceeds as AlH₃ → Al + 1.5H₂, yielding hydrogen gas and aluminum metal. Unlike reversible metal hydrides, this process is irreversible under moderate conditions, meaning that the spent aluminum must be reprocessed off-site to regenerate the hydride. This irreversibility presents a logistical challenge for closed-loop systems but does not preclude its use in applications where one-time or intermittent hydrogen release is acceptable.

Synthesis of aluminum hydride is complex and has historically limited its widespread adoption. Traditional methods involve the reaction of lithium aluminum hydride with aluminum chloride in ether-based solvents, but these processes are sensitive to moisture and require careful handling. More recent advances have explored direct synthesis routes using hydrogen gas and aluminum at high pressures, but these methods are energy-intensive and not yet economically viable at scale. Stabilizing the hydride against premature decomposition during storage is another challenge, as some polymorphs of AlH₃ are metastable and can release hydrogen slowly at room temperature.

Doping strategies have been investigated to control the decomposition kinetics of aluminum hydride. Introducing additives such as titanium or magnesium can lower the onset temperature of hydrogen release or modify the rate of decomposition. These dopants interact with the AlH₃ matrix to alter the activation energy required for hydrogen desorption, enabling more precise control over the release profile. Such modifications are critical for tailoring the material to specific applications, such as fuel cell systems that require a steady hydrogen supply.

In aerospace applications, the high hydrogen density of aluminum hydride makes it suitable for propulsion systems where minimizing mass is essential. Rocket propulsion and unmanned aerial vehicles could benefit from the compact storage of hydrogen, reducing the weight penalty associated with traditional gas or liquid hydrogen tanks. Similarly, in automotive fuel cell systems, AlH₃ could serve as a solid-state hydrogen source, eliminating the need for high-pressure cylinders or cryogenic storage. However, the irreversibility of its decomposition means that the material must be replaced or regenerated after use, which could increase operational costs.

Compared to other chemical hydrides like sodium borohydride or ammonia borane, aluminum hydride offers superior hydrogen capacity by both weight and volume. Sodium borohydride, for example, carries a higher theoretical hydrogen content (10.8 wt%), but its practical capacity is lower due to the need for hydrolysis and the mass of required catalysts. Ammonia borane has a high hydrogen density (19.6 wt%) but suffers from slow kinetics and the release of volatile byproducts. Metal hydrides like magnesium hydride (7.6 wt%) or lithium hydride (12.7 wt%) are reversible but often require high temperatures for hydrogen release and have lower volumetric efficiencies.

The primary limitations of aluminum hydride include its irreversibility, synthesis complexity, and long-term stability issues. Unlike reversible metal hydrides, which can be recharged with hydrogen in situ, AlH₃ requires off-site regeneration, making it less suitable for continuous cycling applications. Scaling production to meet industrial demand remains a hurdle due to the sensitivity of synthesis methods and the need for high-purity precursors. Additionally, some polymorphs exhibit slow decomposition at ambient conditions, which could lead to unintended hydrogen loss during storage.

Research continues to explore methods to improve the practicality of aluminum hydride. Advances in nanostructuring, for instance, may enhance the kinetics of hydrogen release by reducing diffusion barriers within the material. Composite systems combining AlH₃ with other hydrides or catalysts could also provide synergistic effects, improving overall performance. Meanwhile, efforts to develop more efficient regeneration processes could mitigate the irreversibility drawback, making it more viable for recurring-use scenarios.

In summary, aluminum hydride stands out among chemical hydrides for its high hydrogen density and favorable decomposition characteristics. While challenges remain in synthesis, stability, and reversibility, ongoing research into doping, nanostructuring, and composite materials may unlock its full potential. For industries where weight and space efficiency are critical, such as aerospace and automotive sectors, AlH₃ presents a compelling option despite its current limitations. As hydrogen storage technologies evolve, aluminum hydride could play a pivotal role in enabling compact, high-capacity solutions for a low-carbon energy future.