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In the era of rapid technological development, miniaturized energy storage devices (MESDs) are crucial for the development of portable electronic devices, wearable devices, and micro - sensors. Carbon nanofibers (CNFs) have become promising materials for MESDs due to their high specific surface area, excellent electrochemical performance, low internal resistance, and good durability. However, traditional CNFs face numerous challenges during the preparation process, such as difficulties in controlling the nanofiber structure, issues related to the scalability of the preparation process, cost, energy efficiency, process control, uniformity, and reproducibility. These problems limit their widespread application in fields like micro - supercapacitors.
The laser - induced graphitization method and electrospinning technology (enabled by electrospinning machines) offer new ideas for solving these problems. Laser - induced graphitization can prepare customized porous graphene structures through laser technology, but it still faces challenges in achieving the optimal micropore/mesopore ratio in polymer - based films. Nanofibers prepared by electrospinning have a high specific surface area, excellent mechanical properties, and customizable porosity, which can promote electrolyte transport and ion accessibility in micro - supercapacitors. Combining the two is expected to prepare graphene nanofibers (GNFs) with excellent performance and promote the development of miniaturized energy storage devices.
This paper introduces an integrated manufacturing method for synthesizing GNFs by electrospinning (using an electrospinning device) and laser graphitization of fluorinated polyimide nanofibers (fPI NFs) (Figure 1). Firstly, fluorinated poly(amic acid) nanofibers (fPAA NFs) are prepared by electrospinning and then thermally imidized to form fPI NFs. In this process, the solution concentration plays a crucial role in the morphology of fPI NFs. A 3.00m fPAA solution can prepare smooth and uniform fPAA NFs, and then ideal fPI NFs can be obtained.
Figure 1: Integrated Manufacturing of Graphene Nanofibers (GNFs) by Electrospinning and Laser Photothermal Treatment
Subsequently, GNFs are synthesized by laser photothermal treatment of fPI NFs. Compared with the traditional chemical vapor deposition (CVD) method, laser graphitization is faster and simpler. It is found that parameters such as laser power, speed, and pulses per inch (PPI) have a significant impact on the performance of GNFs. When the laser power is 1.8 W, the speed is 3.5 in s⁻¹, and the PPI is 1000, GNFs with the best conductivity can be prepared (Figure 2). Under these optimized conditions, key chemical and structural modifications are achieved. For example, a π - bonded C═C network is formed with an ID/IG ratio of 0.856, and high - quality graphene layers are obtained with an I2D/IG ratio of 0.911. The evaporation of fluorine during the laser treatment further enhances the porosity and specific surface area, constructing a robust framework for energy storage applications.
Figure 2: Optimization of GNFs Fabrication by Controlling Electrospinning and Laser Photothermal Treatment Parameters
To confirm the performance of the prepared GNFs, researchers conducted a variety of characterization analyses. Scanning electron microscope (SEM) and transmission electron microscope (TEM) images show that the electrospun fPI NFs have a smooth surface with an average fiber diameter of 948.18 nm, while GNFs have a rough surface with an average diameter of 930.80 nm and a lattice spacing of 0.36 nm, indicating the formation of a graphene lattice (Figure 3B, C). Raman spectroscopy analysis shows that laser graphitization successfully converts the amorphous carbon in fPI NFs into high - quality single - layer graphene structures. As the laser power increases, the intensity of the D peak first increases slightly and then decreases, the intensity of the G peak increases, the ID/IG ratio decreases, and both the full width at half - maximum of the 2D peak and the I2D/IG ratio decrease, indicating the formation of ordered single - layer graphene (Figure 3D).
Figure 3: Morphological and Spectroscopic Characterizations of fPI NFs and GNFs
X - ray diffraction (XRD) analysis shows that the XRD pattern of GNFs exhibits distinct peaks. The (002) peak corresponds to an interlayer spacing of approximately 0.34 nm, indicating a high degree of graphitization and complete interlayer stacking. The (100) peak is related to the hexagonal lattice structure of graphitic carbon, and its sharpness and intensity increase with the increase of laser power, reflecting the improvement of in - plane crystallinity (Figure 3E). Brunauer - Emmett - Teller (BET) analysis shows that the BET specific surface area of GNFs is 134.1074 m² g⁻¹, with a high degree of porosity and a wide surface area. Laser photothermal treatment effectively increases the surface area, enhancing its porosity and adsorption capacity (Figure 3F). Thermogravimetric analysis (TGA) shows that GNFs have better thermal stability than fPI NFs (Figure 3G). These characteristics comprehensively reflect the excellent physical properties of GNFs and lay a foundation for their application in energy storage and other fields.
GNFs have a high specific surface area, excellent electrical conductivity, electron mobility, and thermal stability, making them ideal materials for energy storage applications. Researchers prepared GNF - based micro - supercapacitors (GNFs - MSCs) and compared their performance with laser - induced polyimide film - based micro - supercapacitors (LIPI - MSCs). Cyclic voltammetry (CV) measurements show that both GNFs - MSCs and LIPI - MSCs exhibit ideal electric double - layer capacitor (EDLC) behavior within the 0.0 - 1.0 V potential window, but GNFs - MSCs have a higher current density and a larger specific capacitance (Figure 4C, D, E).
Figure 4: Electrochemical Characterizations of GNFs - MSCs and LIPI - MSCs
Galvanostatic charge - discharge (CC) measurements show that GNFs - MSCs have a high specific capacitance of 11.41 mF cm⁻² at a current density of 0.52 mA cm⁻², which is 33 times higher than that of traditional LIPI - MSCs. They can still retain 73% of their capacitance at a high current density of 12.5 mA cm⁻², demonstrating excellent rate retention ability (Figure 4G, H). By comparing with Ragone plots, the areal energy density of GNFs - MSCs reaches 0.002 mWh/cm², and the areal power density is 0.54 mW/cm², which is nearly two orders of magnitude higher than that of LIPI - MSCs and much higher than that of EDLC - type micro - supercapacitors (Figure 4I). These excellent electrochemical properties highlight the important role of the hierarchical porous structure of GNFs in optimizing electrolyte diffusion, charge transfer, and charge storage.
This paper successfully prepared GNFs with uniform fibrous, mesoporous, and microporous structures by combining electrospinning and laser - induced graphitization. The optimized laser photothermal treatment conditions achieved key chemical and structural modifications, forming a π - bonded C═C network (ID/IG ratio of 0.856) and high - quality graphene layers (I2D/IG ratio of 0.911). The evaporation of fluorine during the laser treatment enhanced the porosity and specific surface area. GNFs exhibit excellent physical properties, including high thermal stability and good electrical conductivity. The GNF - based micro - supercapacitors have excellent performance, with specific capacitance, areal energy density, and areal power density significantly superior to traditional LIPI - MSCs.
This integrated manufacturing method is scalable, and GNFs have great potential as materials for the next - generation of miniaturized energy storage devices. Their unique combination of porosity, conductivity, and mechanical stability provides innovative opportunities to meet the growing energy demands, paving the way for efficient and sustainable energy storage technologies. They are expected to be widely applied in various fields, from portable electronic products to flexible and wearable devices.
Article Source: https://doi.org/10.1002/advs.202414607
