Nanoconfinement has emerged as a promising strategy to enhance the performance of chemical hydrides such as magnesium hydride (MgH2) and ammonia borane (NH3BH3) for hydrogen storage applications. By embedding these materials within porous scaffolds, researchers have achieved improvements in reaction kinetics, thermodynamic stability, and cycling durability. The approach leverages size effects, interfacial catalysis, and thermal management to overcome intrinsic limitations of bulk hydrides, offering a pathway toward practical hydrogen storage solutions.

Chemical hydrides are attractive for hydrogen storage due to their high gravimetric and volumetric capacities. However, their widespread adoption has been hindered by slow kinetics, high desorption temperatures, and poor reversibility. For instance, MgH2 exhibits a theoretical hydrogen capacity of 7.6 wt%, but its practical use is limited by a high decomposition enthalpy (∼75 kJ/mol H2) and sluggish sorption rates. Similarly, NH3BH3, with a hydrogen content of 19.6 wt%, suffers from uncontrolled dehydrogenation and irreversibility under moderate conditions. Nanoconfinement addresses these challenges by altering the material’s physical and chemical behavior at the nanoscale.

Size effects play a critical role in enhancing the properties of confined hydrides. When MgH2 or NH3BH3 is dispersed within nanoporous scaffolds such as carbon aerogels, metal-organic frameworks (MOFs), or mesoporous silica, the reduced particle size shortens hydrogen diffusion pathways and increases surface area. Studies show that MgH2 confined in carbon scaffolds with pore sizes below 10 nm exhibits dehydrogenation temperatures up to 100°C lower than bulk MgH2. The smaller particle size also mitigates pulverization and agglomeration during cycling, improving long-term stability. For NH3BH3, confinement within porous matrices suppresses undesirable side reactions, such as borazine formation, by restricting molecular mobility and preventing phase segregation.

Interfacial catalysis further enhances the performance of nanoconfined hydrides. The scaffold material often acts as a catalyst or supports catalytic nanoparticles that lower activation barriers for hydrogen release and uptake. For example, when MgH2 is embedded in a scaffold decorated with transition metals like nickel or titanium, the metal-hydride interface facilitates hydrogen dissociation and recombination. Experimental results demonstrate that Ni-doped carbon nanofibers reduce the onset dehydrogenation temperature of MgH2 to below 200°C, compared to 300°C for pure MgH2. Similarly, confining NH3BH3 in MOFs with open metal sites accelerates dehydrogenation kinetics by promoting B-H bond cleavage. The intimate contact between the hydride and catalyst in these systems ensures efficient charge transfer and minimizes energy losses.

Thermal management is another advantage of nanoconfinement. The high thermal conductivity of certain scaffold materials, such as graphene or metallic frameworks, helps dissipate heat generated during exothermic hydrogenation, preventing localized overheating and degradation. Conversely, during endothermic dehydrogenation, the scaffold facilitates heat transfer to the hydride, ensuring uniform temperature distribution and faster kinetics. This thermal regulation is particularly important for applications requiring rapid hydrogen release, such as fuel cell vehicles. Studies on MgH2 confined in copper-modified mesoporous carbon show that the composite maintains stable performance over 50 cycles with minimal capacity loss, attributed to efficient heat dissipation and reduced sintering.

Experimental studies provide compelling evidence for the benefits of nanoconfinement. In one investigation, MgH2 infiltrated into a porous carbon template exhibited a hydrogen release rate 20 times faster than bulk MgH2 at 300°C. Another study on NH3BH3 confined in a MOF reported a 70% reduction in dehydrogenation temperature, with hydrogen release occurring at 80°C instead of 150°C. These improvements are attributed to the combined effects of size reduction, catalytic interfaces, and thermal optimization. However, challenges remain in scaling up these systems for commercial use.

Scalability hurdles include the cost and complexity of synthesizing uniform porous scaffolds with precise pore sizes and chemical functionality. Many high-performance scaffolds, such as MOFs or graphene aerogels, are expensive to produce in large quantities. Additionally, achieving complete infiltration of hydrides into nanopores without clogging or phase separation requires careful control of processing conditions. The long-term stability of nanoconfined systems under real-world operating conditions, including exposure to moisture and mechanical stress, also needs further validation.

Despite these challenges, progress in nanoconfinement techniques continues to advance. Recent developments include the use of low-cost templating methods and scalable deposition techniques to produce composite hydride-scaffold materials. For example, researchers have demonstrated the feasibility of melt infiltration for embedding MgH2 in porous carbon derived from biomass, offering a more sustainable and economical route. Similarly, advances in additive manufacturing may enable the production of tailored scaffold architectures with optimized pore networks for hydrogen storage.

In summary, nanoconfinement of chemical hydrides within porous scaffolds represents a transformative approach to overcoming the limitations of bulk materials. By exploiting size effects, interfacial catalysis, and thermal management, researchers have achieved significant improvements in hydrogen storage performance. While scalability and cost remain critical challenges, ongoing innovations in materials synthesis and processing hold promise for realizing practical nanoconfined hydrogen storage systems. The continued refinement of these technologies will be essential for enabling the transition to a hydrogen-based energy economy.