Electrospun carbon nanofibers represent a class of one-dimensional nanostructured materials with unique structural and electrochemical properties, making them highly suitable for energy storage applications. Their synthesis involves a combination of electrospinning techniques followed by thermal treatments, resulting in fibers with tunable porosity, high surface area, and excellent electrical conductivity. These characteristics position them as promising candidates for electrodes in supercapacitors and lithium-ion batteries, where performance metrics such as capacitance, cycling stability, and rate capability are critical.

The synthesis process begins with the selection of precursor materials, typically polymer-based solutions that can be electrospun into continuous fibers. Common precursors include polyacrylonitrile (PAN), polyvinyl alcohol (PVA), polyimide (PI), and pitch-based polymers. PAN is the most widely used due to its high carbon yield and ability to form stable fibers during electrospinning. The precursor solution is prepared by dissolving the polymer in a suitable solvent, such as dimethylformamide (DMF) or dimethyl sulfoxide (DMSO), to achieve a homogeneous mixture with optimal viscosity for spinning.

Electrospinning parameters play a crucial role in determining the morphology and diameter of the resulting nanofibers. Key variables include applied voltage, solution flow rate, needle-to-collector distance, and environmental conditions such as humidity and temperature. A typical setup involves applying a high voltage (10–20 kV) to the polymer solution, which forms a Taylor cone at the needle tip, ejecting a charged jet that elongates and thins as it travels toward the grounded collector. The flow rate is usually maintained between 0.5–2 mL/h to ensure continuous fiber formation without bead defects. The needle-to-collector distance, typically 10–20 cm, influences fiber diameter and alignment. Controlled humidity (30–50%) prevents rapid solvent evaporation, which can lead to uneven fiber formation.

After electrospinning, the as-spun polymer nanofibers undergo stabilization and carbonization to convert them into carbon nanofibers. Stabilization is performed in an oxidative atmosphere (air) at temperatures between 200–300°C for several hours. This step induces cross-linking and cyclization of the polymer chains, enhancing thermal stability for subsequent high-temperature treatment. Carbonization follows in an inert atmosphere (nitrogen or argon) at temperatures ranging from 800–1200°C, depending on the desired graphitic structure and conductivity. Higher carbonization temperatures promote greater graphitization, improving electrical conductivity but potentially reducing surface area due to pore collapse.

The structural properties of electrospun carbon nanofibers are characterized by high surface area (200–1500 m²/g), tunable porosity (micro- and mesopores), and a fibrous morphology that facilitates electrolyte penetration and ion transport. The presence of heteroatoms (nitrogen, oxygen) from the precursor or post-treatment modifications can further enhance electrochemical activity. Their conductivity ranges from 1–100 S/cm, depending on the carbonization conditions and any additional conductive additives.

In supercapacitors, electrospun carbon nanofibers serve as binder-free electrodes due to their self-supporting structure and interconnected pore network. Their high surface area and porosity enable efficient double-layer charge storage, while pseudocapacitance can be introduced through surface functionalization or incorporation of redox-active materials. Specific capacitances of 100–300 F/g in aqueous electrolytes have been reported, with excellent cycling stability (>90% retention after 10,000 cycles). The fibrous architecture also reduces ion diffusion resistance, enabling high-rate performance.

For lithium-ion batteries, carbon nanofibers act as conductive scaffolds for active materials such as silicon, sulfur, or transition metal oxides. Their porous structure accommodates volume changes during lithiation/delithiation, mitigating mechanical degradation. When used as anodes, they exhibit reversible capacities of 300–500 mAh/g, surpassing conventional graphite (372 mAh/g). In composite cathodes, they improve charge transfer kinetics and reduce polarization, leading to enhanced rate capability and cycle life.

Compared to other carbon-based materials like activated carbon or carbon black, electrospun carbon nanofibers offer distinct advantages. Their continuous fibrous network provides mechanical flexibility and eliminates the need for binders or conductive additives, simplifying electrode fabrication. Unlike graphene or carbon nanotubes, which require complex processing and face challenges in scalability, electrospinning is a versatile and scalable technique. However, their performance is highly dependent on synthesis conditions, and achieving uniform fiber diameters and pore distributions remains a challenge.

In summary, electrospun carbon nanofibers combine tunable morphology, high surface area, and excellent electrochemical properties, making them versatile materials for energy storage. Their synthesis via electrospinning and thermal treatment allows for precise control over structural features, while their applications in supercapacitors and lithium-ion batteries demonstrate superior performance metrics. Ongoing research focuses on optimizing precursor compositions, spinning parameters, and post-treatment methods to further enhance their functionality in next-generation energy storage devices.