Carbon nanotubes (CNTs) have emerged as a promising material for hydrogen storage due to their unique structural and chemical properties. Both single-walled (SWCNTs) and multi-walled (MWCNTs) variants exhibit characteristics that make them suitable candidates for adsorbing and releasing hydrogen efficiently. The high surface area, tunable porosity, and ability to functionalize their surfaces enable CNTs to interact with hydrogen molecules through various mechanisms, including physisorption, chemisorption, and spillover effects. However, challenges such as low volumetric density and stringent purity requirements must be addressed to realize their full potential in practical applications.
Hydrogen storage in CNTs primarily occurs through physisorption, where hydrogen molecules weakly bind to the nanotube surfaces via van der Waals forces. The curvature of CNTs enhances the binding energy compared to flat graphene sheets, making them more effective for hydrogen adsorption. SWCNTs, with their uniform cylindrical structure, offer a higher surface-to-volume ratio than MWCNTs, leading to improved gravimetric storage capacity. Theoretical and experimental studies suggest that SWCNTs can achieve hydrogen uptake of 1-3 wt% under moderate temperatures and pressures. MWCNTs, while less efficient per unit mass, provide structural stability and easier scalability, making them attractive for bulk storage applications.
Beyond physisorption, defect-mediated adsorption plays a critical role in enhancing hydrogen storage in CNTs. Introducing defects, such as vacancies or Stone-Wales defects, creates localized sites with higher binding energies. These defects disrupt the sp² hybridization of carbon atoms, generating dangling bonds that can chemically interact with hydrogen. Functionalization with metal nanoparticles, such as platinum or palladium, further improves hydrogen uptake through spillover effects. In this process, hydrogen molecules dissociate on the metal surface and migrate onto the CNT framework, increasing storage capacity. Research indicates that spillover can enhance hydrogen adsorption by up to 30% in metal-decorated CNTs.
The diameter and chirality of SWCNTs also influence hydrogen storage performance. Narrower tubes exhibit stronger interactions with hydrogen due to increased curvature, while metallic nanotubes facilitate charge transfer, promoting chemisorption. Bundling of SWCNTs, however, reduces accessible surface area and can hinder hydrogen adsorption. Strategies such as intercalation with alkali metals or creating hierarchical porous structures have been explored to mitigate this issue. MWCNTs, with their concentric layers, offer additional interlayer spacing for hydrogen storage, but the inner layers often remain inaccessible without aggressive activation treatments.
Despite these advantages, CNT-based hydrogen storage faces significant limitations. The volumetric density of stored hydrogen remains low compared to compressed or liquid hydrogen systems. Achieving high gravimetric capacity often requires cryogenic temperatures or high pressures, which are energy-intensive and impractical for many applications. Purity requirements for CNTs are stringent, as impurities like amorphous carbon or metal catalysts can block adsorption sites or catalyze unwanted reactions. Additionally, the cost of producing high-quality CNTs at scale remains a barrier to widespread adoption.
Efforts to optimize CNTs for hydrogen storage focus on material engineering and system design. Chemical treatments, such as acid oxidation or plasma etching, can introduce functional groups that enhance hydrogen binding. Doping with heteroatoms like nitrogen or boron modifies the electronic structure of CNTs, improving their interaction with hydrogen. Advances in synthesis techniques, such as chemical vapor deposition (CVD), enable better control over nanotube morphology and alignment, which is critical for maximizing storage performance. Hybrid systems combining CNTs with metal hydrides or porous polymers are also being investigated to leverage synergistic effects.
The long-term stability and reversibility of hydrogen storage in CNTs are essential for practical deployment. Repeated adsorption-desorption cycles can lead to structural degradation or loss of active sites, reducing efficiency. Studies show that SWCNTs maintain over 80% of their initial capacity after 100 cycles, while MWCNTs exhibit superior mechanical resilience but lower cyclic stability due to layer delamination. Environmental factors, such as exposure to moisture or oxygen, can further degrade performance, necessitating protective coatings or encapsulation.
In conclusion, carbon nanotubes present a compelling avenue for hydrogen storage, with SWCNTs and MWCNTs offering distinct advantages and challenges. Their ability to adsorb hydrogen through multiple mechanisms, including spillover and defect-mediated binding, underscores their potential. However, overcoming limitations related to volumetric density, purity, and cost is critical for their integration into real-world hydrogen storage systems. Continued research into material modifications, hybrid architectures, and scalable production methods will be pivotal in unlocking the full capabilities of CNTs for this application.