Complex hydrides represent a promising class of materials for high-capacity hydrogen storage due to their high gravimetric and volumetric hydrogen densities. Among these, borohydrides such as lithium borohydride (LiBH4) and magnesium borohydride (Mg(BH4)2) have attracted significant attention. These materials can theoretically store large amounts of hydrogen, with LiBH4 offering a gravimetric capacity of 18.5 wt% and Mg(BH4)2 reaching up to 14.9 wt%. However, their practical application is hindered by complex decomposition pathways, high thermodynamic stability, and kinetic limitations.

The decomposition of complex hydrides occurs through multi-step reactions, often involving intermediate phases. For LiBH4, the process begins with melting at around 280°C, followed by the release of hydrogen through the breakdown of the [BH4]− anion. The initial step yields LiH and boron, with intermediate phases like Li2B12H12 forming during the reaction. Similarly, Mg(BH4)2 decomposes in stages, first releasing hydrogen to form MgH2 and boron, followed by further decomposition into Mg and additional hydrogen. These pathways are influenced by temperature, pressure, and the presence of catalysts or additives.

Thermodynamic tuning is critical for making complex hydrides viable for reversible hydrogen storage. The high stability of these materials often necessitates temperatures above 300°C for dehydrogenation, which is impractical for many applications. Strategies to destabilize these hydrides include the introduction of reactive additives or the formation of composite systems. For example, combining LiBH4 with MgH2 lowers the dehydrogenation temperature due to the formation of more stable intermediate compounds, shifting the equilibrium toward hydrogen release at milder conditions. Another approach involves using metal oxides or halides to modify the reaction energetics, as seen in systems where SiO2 or TiCl3 enhances the decomposition kinetics of Mg(BH4)2.

Despite these advances, irreversibility remains a major challenge. The decomposition products, such as boron and metal hydrides, often recombine incompletely during rehydrogenation, leading to capacity loss over cycles. Additionally, the slow kinetics of hydrogen uptake and release necessitate prolonged cycling times and elevated pressures, further complicating practical deployment. To address these issues, researchers have explored nanocomposite engineering, where nanoconfinement of complex hydrides in porous scaffolds like carbon aerogels or metal-organic frameworks (MOFs) improves both kinetics and reversibility. The high surface area and tailored pore structures of these scaffolds facilitate better hydrogen diffusion and prevent particle agglomeration.

Additive engineering has also shown promise in enhancing the performance of complex hydrides. Transition metals, their oxides, and fluorides act as catalysts, lowering activation barriers for hydrogen release. For instance, the addition of NbF5 to LiBH4 reduces the dehydrogenation temperature by 50°C while improving reversibility. Similarly, nanoscale nickel or cobalt dispersed within Mg(BH4)2 significantly accelerates hydrogen sorption rates. These additives work by promoting the nucleation of intermediate phases or by providing alternative reaction pathways with reduced energy requirements.

Recent advances in material design have further expanded the potential of complex hydrides. Core-shell nanostructures, where active hydride materials are coated with protective or catalytic layers, enhance stability against degradation. Another innovative approach involves the use of reactive hydride composites (RHCs), where two or more hydrides interact synergistically to improve overall performance. For example, the LiBH4–MgH2 system exhibits a lower enthalpy of reaction compared to its individual components, enabling more efficient cycling.

While challenges persist, ongoing research continues to refine these materials for real-world applications. The development of in-situ characterization techniques, such as neutron scattering and synchrotron X-ray diffraction, has provided deeper insights into the mechanistic pathways of hydrogen release and uptake. Computational modeling, including density functional theory (DFT) calculations, aids in predicting optimal composite formulations and destabilization strategies.

In summary, complex hydrides like LiBH4 and Mg(BH4)2 offer substantial potential for high-capacity hydrogen storage, but their practical implementation requires overcoming thermodynamic and kinetic barriers. Advances in nanocomposites, additive engineering, and reactive composite systems are paving the way for more efficient and reversible storage solutions. Continued innovation in material design and process optimization will be crucial for integrating these materials into future hydrogen energy systems.