Electrospinning Equipment: One-Step Synthesis of Lignin-Derived CNFs without Stabilization

1. Research Background

Carbon nanofibers (CNFs) have shown broad application prospects in fields such as aerospace, energy storage, and environmental remediation due to their excellent mechanical properties, electrical properties, and chemical stability. Currently, the preparation of CNFs typically involves electrospinning, followed by two key steps: low-temperature stabilization and high-temperature carbonization. The stabilization process introduces oxygen functional groups at temperatures ranging from 250 to 300°C to promote fiber cross-linking and prevent fiber fusion and structural collapse during carbonization. However, this process is time-consuming and energy-intensive, accounting for 80% of the total production time and significantly increasing manufacturing costs. Moreover, traditional CNF precursors, such as polyacrylonitrile, pitch, and rayon, pose challenges related to high costs, poor scalability, and environmental issues. In recent years, lignin has garnered attention as a green precursor due to its naturally occurring oxygen functional groups, which may accelerate stabilization or even eliminate this step, thereby reducing costs and time.

2. Abstract

This study explores the feasibility of omitting the stabilization step and its impact on the properties of lignin-PVA-derived electrospun carbon nanofibers (CNFs). Electrospun mats were subjected to carbonization with (St-CNF) and without thermal stabilization (NSt-CNF). The results show that even without stabilization, the fibrous structure remains intact without fusion. However, stabilization significantly affects carbon yield, graphitic content, and defect density. Compared to NSt-CNF, St-CNF exhibits higher carbon yield, specific surface area (381.5 m²/g), and pore volume (0.40 cc/g), leading to enhanced electrochemical properties, such as specific capacitance (191.5 F/g), energy density (26.6 Wh/kg), and power density (520.5 W/kg). This study provides valuable insights into the properties of lignin-based CNFs and explores their potential in energy storage applications.

3. Research Content

In this study, lignin and PVA (90:10) were used as the polymer matrix, with sodium dodecyl sulfate (SDS) added as a surfactant to modulate the viscosity and surface tension of the solution, thereby improving fiber spinnability. Electrospun fiber mats were prepared using an electrospinning device and subsequently subjected to thermal stabilization and carbonization to produce St-CNF and NSt-CNF samples.

Figure 1: Preparation Process and Chemical Structural Changes of CNFs

Figure 1(a) illustrates the preparation process of CNFs, including both unstabilized (NSt-CNF) and stabilized (St-CNF) pathways. It is evident that the addition of SDS significantly reduces the viscosity and surface tension of the solution, enhancing fiber spinnability. The interaction between SDS and PVA increases the solution's conductivity, lowering the voltage required for electrospinning.

Figure 2: Thermogravimetric Analysis (TGA) and XPS Analysis Results

The TGA curves of the two types of CNFs (St-CNF and NSt-CNF) in Figure 2 show that stabilization significantly enhances fiber thermal stability, reduces mass loss during carbonization, and increases carbon yield. XPS analysis indicates that oxygen functional groups (such as carboxyl groups) introduced during stabilization promote fiber cross-linking and form a more stable carbon structure. Sulfur elements are successfully doped into CNFs, and the stabilization process does not alter the final chemical environment of the CNFs.

Figure 3: Morphological Analysis of CNFs

As shown in Figure 3, the fiber morphology indicates that even without stabilization, the fibrous structure remains intact without fusion. Samples with SDS exhibit smaller and more uniform fiber diameters, indicating that SDS improves fiber morphology. HR-TEM shows that both types of CNFs have a disordered porous fibrous morphology, indicating a low degree of graphitization.

Figure 4: Structural and Porosity Characterization of CNFs

XRD and Raman spectroscopy reveal that St-CNF has a higher degree of graphitization and lower defect density. The specific surface area and pore volume of St-CNF are significantly higher than those of NSt-CNF, indicating that stabilization promotes the formation of micropores and mesopores. The differences in porosity are mainly attributed to the release of gases from oxygen functional groups introduced during stabilization, forming more pores during carbonization.

St-CNF exhibits higher specific capacitance, energy density, and power density, mainly due to its higher specific surface area, pore volume, and electrical conductivity. EIS analysis shows that St-CNF has lower equivalent series resistance and charge transfer resistance, indicating better ion diffusion performance. Both types of CNFs show good cycling stability, but St-CNF maintains a higher capacitance value after 5000 cycles.

4. Conclusion

This study successfully prepared lignin and PVA-derived electrospun CNF mats with the addition of SDS surfactant using an electrospinning machine, eliminating the stabilization step. The resulting CNF mats retained their fiber geometry with no significant changes in diameter distribution. TGA analysis indicates that stabilization enhances the thermal stability of CNFs, increasing carbon yield by 11.7% compared to NSt-CNF samples. Stabilization also significantly impacts specific surface area and pore volume, with St-CNF having a 57.3% higher specific surface area and 62.5% higher pore volume than NSt-CNF. These advantages contribute to St-CNF's higher specific capacitance (191.5 F/g at 1 A/g), energy density, and power density (26.6 Wh/kg at 520.5 W/kg). In terms of electrode stability, both St-CNF and NSt-CNF show good stability after 5000 cycles, with stable capacitance values. Since the stabilization step accounts for up to 50% of the total CNF production cost, eliminating this step can significantly improve cost and time efficiency. Although the structural and thermal properties of CNFs without stabilization are compromised, the economic benefits are substantial. The decision to retain or eliminate the stabilization step should be based on specific application requirements, balancing material properties and production costs.

Electrospinning Nanofibers Article Source: 

https://doi.org/10.1016/j.cartre.2025.100456