Nanoengineered borohydrides such as lithium borohydride (LiBH₄) and sodium borohydride (NaBH₄) represent a promising class of materials for high-capacity hydrogen storage due to their high gravimetric and volumetric hydrogen densities. LiBH₄, for instance, boasts a theoretical hydrogen capacity of 18.5 wt%, while NaBH₄ offers 10.8 wt%. However, their practical application is hindered by high thermodynamic stability, slow kinetics, and irreversibility under moderate conditions. Recent advancements in nanoscale engineering have introduced innovative strategies to overcome these challenges, including destabilization techniques, nanoconfinement, and additive-enhanced kinetics.

### Destabilization Strategies
The strong covalent B-H bonds in borohydrides result in high decomposition temperatures, often exceeding 400°C for LiBH₄ and 500°C for NaBH₄. Destabilization approaches aim to lower these temperatures by altering the reaction pathways through chemical modifications or composite formation. One effective method involves combining borohydrides with reactive hydrides or metal oxides to form intermediate compounds with reduced thermodynamic stability. For example, mixing LiBH₄ with MgH₂ lowers the dehydrogenation temperature to around 200–250°C due to the formation of MgB₂ and LiH intermediates. Similarly, adding SiO₂ to NaBH₄ reduces the decomposition temperature by facilitating the release of hydrogen at lower energies.

Another destabilization strategy involves cation or anion substitution. Partial replacement of Li⁺ with Mg²⁺ in LiBH₄ destabilizes the crystal lattice, leading to lower desorption temperatures. Anion substitution, such as introducing halides (e.g., Cl⁻ or F⁻), can also weaken B-H bonds by inducing lattice strain. These modifications have demonstrated reversible hydrogen capacities in the range of 8–12 wt%, making them more viable for practical applications.

### Nanoconfinement Effects
Nanoconfinement involves embedding borohydride particles within porous scaffolds, such as carbon aerogels, mesoporous silica, or metal-organic frameworks (MOFs), to restrict particle size and enhance surface interactions. By reducing the diffusion path lengths for hydrogen and providing a high surface area for reactions, nanoconfinement improves both kinetics and reversibility. For instance, infiltrating LiBH₄ into carbon nanofibers with pore sizes below 10 nm has been shown to lower the dehydrogenation temperature to 150–200°C while maintaining a capacity of 10–12 wt%.

The porous matrix also prevents particle agglomeration during cycling, a common issue leading to performance degradation. Additionally, nanoconfinement can alter the thermodynamic properties of borohydrides by introducing interfacial energy effects. Studies indicate that the enthalpy of decomposition for nanoconfined LiBH₄ can be reduced by 10–20 kJ/mol H₂ compared to bulk material, significantly improving the energy efficiency of hydrogen release.

### Additive-Enhanced Kinetics
Catalysts and additives play a crucial role in accelerating the reaction kinetics of borohydrides. Transition metals, metal oxides, and metal halides have been extensively studied for their catalytic effects. For example, adding TiCl₃ to LiBH₄ reduces the activation energy for hydrogen desorption from 160 kJ/mol to 100 kJ/mol, enabling faster release at lower temperatures. Similarly, Ni nanoparticles dispersed in NaBH₄ have been shown to enhance rehydrogenation rates by facilitating H₂ dissociation and surface diffusion.

Another approach involves using reactive additives that participate in the dehydrogenation process. For instance, combining LiBH₄ with Al forms a Li-Al-B-H intermediate phase, which decomposes at lower temperatures than pure LiBH₄. These reactive systems often exhibit improved cycling stability, with some composites retaining over 80% of their initial capacity after 20 cycles.

### Thermodynamic Barriers and Recent Breakthroughs
The primary thermodynamic challenge for borohydrides lies in the high enthalpy of dehydrogenation, typically exceeding 60 kJ/mol H₂ for LiBH₄ and NaBH₄. This necessitates energy-intensive conditions for hydrogen release, limiting their practicality for mobile applications. Recent breakthroughs have focused on tailoring the reaction thermodynamics through multicomponent systems. For example, the LiBH₄-MgH₂ composite exhibits a two-step decomposition process with an effective enthalpy reduction to 40–45 kJ/mol H₂, making it more compatible with fuel cell operating temperatures.

Another advancement involves using mechanochemical synthesis to create metastable borohydride phases with altered thermodynamics. High-energy ball milling of LiBH₄ with metal hydrides or oxides induces structural disorder, leading to lower-temperature hydrogen release. Some mechanochemically processed composites have demonstrated reversible capacities of 5–8 wt% at temperatures below 150°C.

### Conclusion
Nanoengineered borohydrides hold significant potential for high-capacity hydrogen storage, with recent progress in destabilization, nanoconfinement, and additive-enhanced kinetics addressing longstanding challenges. While thermodynamic barriers remain a critical hurdle, innovative material designs and processing techniques continue to push the boundaries of performance. Future research is expected to focus on optimizing composite formulations, scaling up nanoconfinement methods, and integrating these materials into real-world hydrogen storage systems. The development of borohydride-based storage solutions could play a pivotal role in enabling a sustainable hydrogen economy.