Fullerenes, particularly C60 and C70, have emerged as promising candidates for hydrogen storage due to their unique molecular structures and tunable chemical properties. These carbon-based nanomaterials exhibit a hollow cage-like architecture, which allows for various hydrogen storage mechanisms, including hydrogenation, doping, and encapsulation. Research into fullerene-based hydrogen storage has explored both theoretical capacities and experimental validation, though challenges such as cost, temperature requirements, and reversibility remain significant hurdles.

Hydrogenation is one of the most studied approaches for hydrogen storage in fullerenes. This process involves the covalent attachment of hydrogen atoms to the carbon framework, forming fullerane derivatives (C60Hx). Theoretical studies suggest that C60 can store up to 60 hydrogen atoms, achieving a gravimetric capacity of approximately 7.7 wt%. However, experimental results have shown lower values due to steric hindrance and incomplete hydrogenation. For instance, C60H36 has been synthesized with a hydrogen content of around 4.8 wt%, while higher hydrogenation levels (C60H48) have demonstrated storage capacities near 6.2 wt%. The hydrogenation process typically requires high pressures (above 60 bar) and elevated temperatures (200–400°C), which complicates practical applications. Additionally, the release of hydrogen often demands even higher temperatures, reducing energy efficiency.

Doping fullerenes with metal atoms is another strategy to enhance hydrogen storage performance. Alkali and alkaline earth metals, such as lithium and calcium, have been incorporated into fullerene structures to improve hydrogen adsorption through spillover mechanisms or Kubas interactions. Theoretical models predict that metal-doped fullerenes, such as Li12C60, could achieve hydrogen storage capacities of up to 9 wt% under moderate conditions. Experimental studies on Ti-decorated fullerenes have shown physisorption of hydrogen molecules at near-ambient temperatures, with capacities reaching 3–4 wt%. However, metal aggregation and oxidation remain critical challenges, often leading to reduced cycling stability.

Encapsulation of hydrogen within fullerene cages, known as endohedral hydrogen storage, offers a different mechanism. Molecular hydrogen (H2) can be trapped inside C60 or C70 cages under high-pressure and high-temperature conditions. Theoretical simulations indicate that each C60 molecule could encapsulate up to 29 H2 molecules, corresponding to a remarkable gravimetric capacity of 13 wt%. However, experimental realizations have been limited by the difficulty of achieving high encapsulation yields. The highest reported experimental value for endohedral hydrogen storage in C60 is around 5 wt%, achieved under extreme conditions (2000 bar, 400°C). The release of hydrogen from endohedral fullerenes also requires significant energy input, often involving laser irradiation or thermal decomposition.

Comparing fullerenes with other carbon-based nanomaterials reveals distinct advantages and disadvantages. Carbon nanotubes (CNTs) and graphene have been extensively investigated for hydrogen storage, primarily through physisorption on their high-surface-area structures. While CNTs can achieve hydrogen uptake of 1–3 wt% at cryogenic temperatures (77 K), their performance at ambient conditions is limited to less than 1 wt%. Graphene-based materials, including graphene oxide and reduced graphene oxide, exhibit similar limitations, with most practical storage capacities below 2 wt% at room temperature. In contrast, fullerenes offer the potential for higher capacities through chemical bonding (hydrogenation) or encapsulation, though their practical performance is hindered by temperature and pressure requirements.

The cost of fullerenes is another limiting factor. The synthesis of high-purity C60 and C70 involves complex processes such as arc discharge or chemical vapor deposition, resulting in high production costs compared to other carbon materials. Scaling up fullerene-based hydrogen storage systems would require significant reductions in manufacturing expenses. Additionally, the low-temperature requirements for efficient hydrogen release further increase operational costs, making fullerenes less competitive with conventional storage methods like compressed gas or metal hydrides.

Despite these challenges, research continues to explore chemically modified fullerenes for improved hydrogen storage. Functionalization with heteroatoms (e.g., nitrogen or boron) has been shown to alter electronic properties and enhance hydrogen binding energies. Hybrid systems combining fullerenes with metal-organic frameworks (MOFs) or porous polymers have also demonstrated synergistic effects, improving both capacity and reversibility. For example, fullerene-MOF composites have achieved hydrogen uptake of 4–5 wt% at 77 K, though room-temperature performance remains modest.

In summary, fullerenes present a unique platform for hydrogen storage with theoretical capacities surpassing many other carbon nanomaterials. Hydrogenation, doping, and encapsulation strategies each offer distinct pathways for optimizing storage performance, though practical limitations persist. Experimental validation has yet to match theoretical predictions, and challenges related to cost, temperature, and reversibility must be addressed for fullerenes to become viable for large-scale applications. Compared to CNTs and graphene, fullerenes provide alternative mechanisms for hydrogen interaction, but their adoption will depend on advancements in material synthesis and process engineering. Future research may focus on hybrid materials and novel functionalization techniques to overcome current barriers and unlock the full potential of fullerene-based hydrogen storage.