Carbon nanofibers (CNFs) have emerged as a promising class of nanomaterials for gas storage and adsorption applications due to their unique structural and surface properties. Their physisorption capacity, particularly when enhanced via spillover mechanisms or doping with elements such as lithium (Li) or platinum (Pt), has been extensively studied. The effectiveness of CNFs in these applications is closely tied to their Brunauer-Emmett-Teller (BET) surface area and pore size distribution, which differentiate them from other porous materials like metal-organic frameworks (MOFs) and carbon nanotubes (CNTs).

The physisorption capacity of CNFs is primarily governed by their textural properties. BET surface area measurements reveal that CNFs typically exhibit surface areas ranging from 100 to 500 m²/g, depending on synthesis conditions and post-treatment methods. Activation processes, such as chemical or physical treatments, can further increase surface area by introducing microporosity. For instance, steam activation has been shown to enhance the surface area of CNFs to over 1000 m²/g by creating additional micropores. The presence of these micropores (<2 nm) is critical for gas adsorption, as they provide high binding energy sites for physisorbed molecules.

Pore size distribution plays an equally important role in determining adsorption performance. CNFs often exhibit a hierarchical pore structure, combining micropores, mesopores (2–50 nm), and macropores (>50 nm). This multimodal porosity facilitates rapid diffusion of gas molecules while maintaining high adsorption capacity. Unlike MOFs, which possess highly ordered and uniform pores, CNFs offer a more heterogeneous pore network that can be advantageous for certain applications where flexibility in adsorbate size is required.

Doping CNFs with metals such as Li or Pt can significantly enhance their physisorption capacity through spillover mechanisms. Spillover involves the dissociation of gas molecules (e.g., hydrogen) on metal nanoparticles, followed by migration of atomic species onto the carbon support. Platinum-doped CNFs, for example, have demonstrated improved hydrogen storage capacity due to the spillover effect. Studies indicate that Pt nanoparticles dispersed on CNFs can increase hydrogen uptake by up to 1.5 wt% at room temperature, compared to undoped CNFs. Similarly, lithium doping enhances hydrogen adsorption via polarization effects, where Li ions create strong electrostatic interactions with hydrogen molecules.

The distinction between CNFs and other carbon-based materials like CNTs lies in their structural morphology and surface chemistry. While CNTs are characterized by their cylindrical graphene walls and high aspect ratios, CNFs consist of stacked graphene layers arranged in a less ordered, often conical or platelet-like structure. This structural difference results in varying surface properties; CNFs generally exhibit more edge sites and defects, which can be beneficial for functionalization and doping. Additionally, CNFs tend to have a higher density of surface oxygen groups compared to CNTs, further influencing their adsorption behavior.

In contrast to MOFs, which rely on coordinatively unsaturated metal sites and ultrahigh porosity for gas adsorption, CNFs offer superior mechanical stability and thermal resistance. MOFs often suffer from structural degradation under humid conditions or elevated temperatures, whereas CNFs maintain their integrity in harsh environments. This makes CNFs more suitable for applications requiring long-term stability, such as industrial gas storage or filtration systems.

The role of surface chemistry in CNF adsorption cannot be overlooked. Functional groups such as carboxyl, hydroxyl, and carbonyl groups introduced via oxidative treatments can enhance interactions with polar gases like CO₂ or NH₃. However, excessive functionalization may reduce surface area by blocking pores, highlighting the need for optimization. Thermal treatments in inert atmospheres can be used to selectively remove unstable oxygen groups while preserving porosity.

Comparative studies between CNFs and activated carbons reveal that while activated carbons often exhibit higher surface areas, CNFs provide better control over pore structure and mechanical properties. The graphitic nature of CNFs also contributes to higher electrical conductivity, making them suitable for applications where both adsorption and charge transport are desired, such as in supercapacitors or catalytic supports.

Recent advances in CNF synthesis have enabled precise tuning of their properties for specific applications. Electrospinning of polymer precursors followed by carbonization is a common method to produce continuous CNF mats with adjustable diameters. Chemical vapor deposition (CVD) techniques allow for the growth of vertically aligned CNFs with tailored porosity. Post-synthesis modifications, including acid treatments or plasma etching, further refine surface characteristics.

In summary, carbon nanofibers present a versatile platform for physisorption applications, with their performance being highly dependent on BET surface area, pore size distribution, and doping strategies. Their structural advantages over MOFs and CNTs, combined with the ability to enhance adsorption via spillover mechanisms, position them as a competitive material for gas storage, environmental remediation, and energy-related applications. Future research directions may focus on optimizing dopant dispersion and developing hybrid systems that leverage the strengths of CNFs alongside other nanomaterials.