Abstract:

Objective Supercapacitors have shown good application potential in the field of energy storage and conversion due to their long span of life, environmental friendliness, higher security, fast charging and discharging capacity, and high power density, which attracted much research attention. As an important electrode material, the physical and chemical properties of carbon nanofibers affect the electrochemical performance of the electrode. However, the low specific surface area and porosity as well as the non-interconnected structure between fibers lead to the poor electrochemical performance of carbon nanofiber electrodes, which limits their development and application in supercapacitors. Therefore, it is imperative to design and develop good carbon nanofiber electrodes.
Method Porous and interconnected carbon nanofibers were prepared by electrospinning polyacrylonitrile (PAN) and high amylose starch (HAS) blends followed by pre-oxidation and carbonization. The cross-linking reaction between glutaraldehyde and starch was used to construct the interconnected structure of carbon nanofibers. Meanwhile, the thermal degradation of starch was used to create pore structures and increase specific surface areas of carbon nanofibers. During electrospinning, the voltage, distance, and extrusion speed were set as 20 kV, 15 cm, and 1.2 mL/h, respectively. The obtained nanofibers were pre-oxidized in a muffle furnace at 240 ℃ with a heating rate of 2 ℃/min for 2 h. The carbon nanofibers were prepared by carbonizing pre-oxidized nanofibers in a tubular furnace at 1 000 ℃ with a heating rate of 5 ℃/min for 2 h.
Results The addition of HAS and glutaraldehyde crosslinking had a great effect on the morphologies of nanofibers and carbon nanofibers. The prepared PAN/HAS nanofibers and carbon nanofibers had smaller diameters of 570 and 370 nm compared to the pure PAN nanofibers (910 nm) and carbon nanofibers (620 nm) (Fig. 1). After glutaraldehyde crosslinking, PAN/HAS-based carbon nanofibers had N and O co-doping with a high C content of 94.8% (Fig. 2). The presence of N and O elements as shown to improve the hydrophily and conductivity, and also to provide more active sites for carbon nanofibers. These characteristics played a positive role in improving the electrochemical performances of prepared carbon nanofiber electrodes. Moreover, the PAN/HAS-based carbon nanofibers with interconnected structures had better graphitized extents than non-interconnected carbon nanofibers (Fig. 3). The addition of HAS successfully increased the specific surface area, total pore volume, mesoporous volume, and microporous volume of carbon nanofibers, while the pore volume of carbon nanofibers crosslinked by glutaraldehyde was further increased. For carbon nanofiber electrodes, micropores were more conducive to the storage of ions, and mesopores were more conducive to the diffusion of ions. Their synergistic effect could improve the electrochemical performances of the electrodes (Fig. 4). The interconnected carbon nanofiber electrode had the best specific capacitances under cyclic voltammetry and galvanostatic charge-discharge measurements in a three-electrode system. At a scan rate of 10 mV/s, the specific capacitance was as high as 260 F/g, retaining 68% at a high scan rate of 200 mV/s (Fig. 5). The specific capacitance was as high as 255 F/g (1 A/g), retaining 71% at a high current density (20 A/g) (Fig. 6). In addition, after 10 000 charge and discharge cycles, the capacitance retention rate was as high as 99.8%, showing excellent cycle durability (Fig. 7).
Conclusion Porous and interconnected carbon nanofibers for supercapacitor electrodes were proposed by carbonizing electrospun PAN/HAS nanofibers. The fiber diameter, specific surface area, porosity, micro-meso pore content, and interconnected structure of PAN/HAS-based carbon nanofibers were tailored by glutaraldehyde crosslinking reaction. The prepared carbon nanofibers had nano-sized fiber diameter (370 nm), high C element content (94.8%), good graphitized extent, and also exhibited large specific surface area (647 m²/g), high total porous volume (0.60 cm³/g), high micropore content (67%), and small pore size (2.6 nm). These excellent physical and chemical properties could provide conditions for high-performance electrodes in supercapacitors. When interconnected PAN/HAS-based carbon nanofibers were used as active materials to prepare into electrodes, the electrode had a high specific capacitance about 255 F/g, a good rate capability about 71%, a low internal resistance about 0.64 Ω, and an excellent cycling stability about 99.8% after 10 000 charging and discharging cycles. The design of novel carbon nanofibers was expected to be widely applicable for the development of high-performance electrodes for energy storage field.

Key words: electrospinning, carbon nanofiber, polyacrylonitrile, high amylose starch, electrochemical property, supercapacitor