Carbon nanotubes (CNTs) have emerged as a promising material for hydrogen storage due to their unique structural properties, high surface area, and ability to adsorb hydrogen through physisorption and chemisorption mechanisms. Their cylindrical nanostructure, composed of rolled graphene sheets, provides an ideal framework for hydrogen interaction, making them a subject of extensive research in the field of energy storage.

Structurally, CNTs can be single-walled (SWCNTs) or multi-walled (MWCNTs), with diameters ranging from less than one nanometer to several nanometers. The high surface-to-volume ratio of SWCNTs, in particular, allows for greater hydrogen adsorption per unit mass. The hydrogen storage capacity of CNTs depends on factors such as tube diameter, chirality, purity, and the presence of defects or functional groups. Theoretical and experimental studies suggest that hydrogen molecules can adsorb to the exterior walls, interior cavities, and interstitial spaces between bundled nanotubes.

Hydrogen adsorption in CNTs occurs primarily through two mechanisms: physisorption and chemisorption. Physisorption involves weak van der Waals forces between hydrogen molecules and the carbon surface, typically requiring cryogenic temperatures (77 K) for significant uptake. Chemisorption, on the other hand, involves the dissociation of hydrogen molecules and the formation of covalent bonds with carbon atoms, which can occur at higher temperatures but often requires energy input for hydrogen release. The balance between these mechanisms determines the practical storage capacity and release kinetics of CNT-based systems.

Experimental studies have reported hydrogen storage capacities ranging from 0.1 to 10 wt% for CNTs under varying conditions. The highest values are typically achieved with purified and functionalized SWCNTs at cryogenic temperatures and moderate pressures (up to 100 bar). However, achieving high storage capacities at ambient temperatures remains a challenge due to the weak binding energy of physisorbed hydrogen.

Synthesis methods for CNTs include arc discharge, laser ablation, and chemical vapor deposition (CVD). CVD is the most scalable and widely used technique, allowing for controlled growth of CNTs with specific diameters and lengths. The quality and hydrogen storage performance of CNTs depend heavily on synthesis parameters such as catalyst type, temperature, and carbon feedstock. Post-synthesis treatments, including purification to remove amorphous carbon and metal residues, are critical for optimizing hydrogen adsorption.

Functionalization techniques have been explored to enhance hydrogen uptake in CNTs. These include doping with heteroatoms (e.g., nitrogen or boron), decorating with metal nanoparticles (e.g., palladium or platinum), and introducing defects or sidewall modifications. Metal-doped CNTs, for example, can facilitate hydrogen spillover, where dissociated hydrogen atoms migrate from metal sites to the carbon surface, increasing storage capacity. Chemical functionalization with oxygen or hydrogen groups can also alter the electronic properties of CNTs, improving their interaction with hydrogen molecules.

Despite their potential, CNT-based hydrogen storage faces several challenges. Reproducibility of storage performance across different batches of CNTs is a major issue due to variations in synthesis and post-processing conditions. Scalability of high-quality CNT production remains another hurdle, as large-scale synthesis often introduces defects or impurities that reduce storage efficiency. Additionally, the cost of producing and functionalizing CNTs must be reduced to make them competitive with conventional storage methods.

When compared to other nanostructured materials, such as metal-organic frameworks (MOFs) or graphene, CNTs exhibit distinct advantages and limitations. MOFs offer higher surface areas and tunable pore sizes but often suffer from lower stability under repeated hydrogen cycling. Graphene shares similarities with CNTs in terms of carbon-based structure but lacks the confined tubular geometry that enhances hydrogen adsorption in nanotubes. Conventional storage methods, such as compressed gas or liquid hydrogen, provide higher volumetric densities but require heavy tanks or energy-intensive cooling, respectively.

Safety considerations for CNT-based hydrogen storage include the risk of hydrogen leakage, flammability, and material degradation under cyclic loading. The mechanical robustness of CNTs is generally high, but long-term exposure to hydrogen can lead to embrittlement or structural changes. Proper encapsulation and system design are necessary to mitigate these risks.

Recent advancements in CNT research include the development of hybrid materials combining CNTs with other nanostructures, such as MOFs or metal hydrides, to synergistically enhance hydrogen storage. Innovations in in-situ characterization techniques, such as neutron scattering or X-ray diffraction, have provided deeper insights into hydrogen adsorption mechanisms. Computational modeling and machine learning are also being employed to predict optimal CNT configurations for maximum hydrogen uptake.

Future research directions for CNT-based hydrogen storage focus on improving material design, optimizing functionalization strategies, and addressing scalability challenges. Exploring new synthesis routes, such as plasma-enhanced CVD or bio-inspired assembly, could yield CNTs with superior properties. Integration of CNTs into practical storage systems, including tanks and portable devices, will require collaboration between materials scientists and engineers.

In conclusion, carbon nanotubes represent a compelling candidate for hydrogen storage, offering a combination of high surface area, tunable chemistry, and mechanical stability. While significant progress has been made in understanding their hydrogen adsorption behavior, overcoming challenges related to reproducibility, scalability, and ambient-temperature performance will be critical for their widespread adoption. Continued research and technological innovation hold the key to unlocking the full potential of CNTs in the hydrogen economy.