Carbon nanohorns (CNHs) are a unique class of carbon-based nanomaterials characterized by their horn-like tubular structures terminating in conical tips. These nanostructures exhibit high surface area, tunable porosity, and excellent chemical stability, making them promising candidates for gas storage applications, particularly for hydrogen (H2), methane (CH4), and carbon dioxide (CO2). The adsorption capacity of CNHs is influenced by their pore structure, surface area, and the presence of functional groups, which can be tailored to optimize performance.

**Structural Properties and Gas Adsorption Mechanisms**
CNHs are typically synthesized via laser ablation or arc-discharge methods, resulting in aggregates of conical tubules with diameters ranging from 2 to 5 nm and lengths up to 50 nm. These aggregates form spherical assemblies with interstitial pores that contribute to their high surface area, often exceeding 1000 m²/g. The pore structure of CNHs consists of three main types: internal pores within the individual horns, inter-horn voids, and defects or edge sites on the conical tips.

For gas storage, the adsorption mechanism depends on the gas type. Hydrogen physisorbs weakly via van der Waals interactions, requiring high surface area and micropores to enhance uptake. Methane, with its larger molecular size, benefits from a combination of micropores and mesopores to maximize adsorption. CO2, which has a quadrupole moment, interacts more strongly with polar surface sites, making functionalization a key factor in improving capture efficiency.

**Role of Pore Structure and Surface Area**
The gas storage capacity of CNHs is closely tied to their pore size distribution. Micropores (< 2 nm) are critical for hydrogen storage due to the overlapping potential fields from pore walls, which enhance adsorption energy. Experimental studies have shown that CNHs with optimized micropore volumes can achieve hydrogen uptake of 1-2 wt% at 77 K and moderate pressures.

For methane, mesopores (2-50 nm) facilitate higher packing density due to the larger kinetic diameter of CH4 molecules. CNHs with hierarchical pore structures exhibit improved methane storage, reaching up to 15-20 wt% at 298 K and 35 bar, competitive with metal-organic frameworks (MOFs) and activated carbons.

CO2 adsorption is influenced by both pore size and surface chemistry. The presence of narrow micropores increases the adsorption enthalpy, while functional groups such as carboxyl or hydroxyl enhance selectivity. CNHs modified with nitrogen or oxygen functionalities have demonstrated CO2 uptake capacities of 3-5 mmol/g at 298 K and 1 bar, outperforming unmodified counterparts.

**Impact of Functionalization**
Surface functionalization plays a pivotal role in tuning the gas adsorption properties of CNHs. Oxidation treatments introduce oxygen-containing groups, which improve CO2 capture due to dipole-quadrupole interactions. Nitrogen doping enhances hydrogen spillover effects, where dissociated hydrogen atoms migrate from metal catalysts to the carbon surface, increasing storage capacity.

For methane storage, hydrophobic functionalization prevents competitive water adsorption, maintaining high uptake in humid conditions. Additionally, metal-decorated CNHs, such as those loaded with palladium or platinum, exhibit enhanced hydrogen adsorption through chemisorption mechanisms, though this often requires higher temperatures for desorption.

**Comparison with Other Porous Materials**
CNHs offer distinct advantages over traditional porous materials like activated carbons, zeolites, and MOFs. Unlike zeolites, which have rigid pore structures, CNHs provide flexible inter-horn spacing that can adapt to different gas molecules. Compared to MOFs, CNHs exhibit superior thermal and chemical stability, making them more suitable for high-pressure or corrosive environments.

Activated carbons, while cost-effective, often lack the uniformity in pore structure that CNHs possess. The conical morphology of CNHs creates a higher density of edge sites, which can be more readily functionalized for targeted gas adsorption. However, MOFs still outperform CNHs in some cases due to their ultrahigh surface areas and precisely tunable pore geometries.

**Challenges and Future Directions**
Despite their potential, CNHs face challenges in scalability and cost-effective synthesis. The aggregation tendency of CNHs can reduce accessible surface area, necessitating dispersion techniques or templating methods to preserve porosity. Advances in chemical activation or thermal treatments may further enhance their gas storage performance.

Future research should focus on optimizing the balance between pore size distribution and functional group density to maximize gas uptake under practical conditions. Hybrid systems combining CNHs with other nanomaterials, such as graphene or MOFs, could also unlock synergistic effects for improved storage capacities.

In summary, carbon nanohorns represent a versatile platform for gas storage, with their unique pore structure, high surface area, and tunable surface chemistry offering significant advantages for hydrogen, methane, and CO2 adsorption. While challenges remain in large-scale production, their robust properties position them as competitive candidates for next-generation gas storage solutions.