Abstract:

Objective As a key component of supercapacitors, the separator's main function is to prevent direct contact between the positive and negative electrodes, avoid short circuits, and ensure the safe and stable operation of supercapacitors. The separator also plays a role in providing a transport channel for electrolyte ions in supercapacitors, which facilitates the rapid movement of ions and improves the charging and discharging efficiency and performance of supercapacitors. The performance of the separator directly affects the cycle life, charge and discharge performance, energy density, and power density of supercapacitors. However, in practical applications, nanofiber-based supercapacitor separators still face the following problems. It's difficult to balance ion permeability and mechanical strength. Costs are increased while improving the performance of fiber membranes. It's difficult to meet environmental adaptability and durability requirements in specific applications. Therefore, the development of polyacrylonitrile (PAN) nanofiber-based supercapacitor separator materials with low cost and high performance is of great research significance.

Method Porous and crosslinked electrospun nanofibers were prepared using polyacrylonitrile, high amylose starch, and polyvinylpyrrolidone (PVP) as precursors followed by water dissolution and glutaraldehyde cross-linking treatment. PVP dissolves in water to form a porous structure, while glutaraldehyde crosslinks with starch to form a bonding structure within fibers. During electrospinning, the voltage, distance, extrusion speed, and rotary speed were set as 15 kV, 15 cm, 1.0 mL/h, and 250 r/min, respectively. The electrospun nanofibers were immersed in deionized water for 5 h under room temperature. Then, the treated nanofibers were placed in a vacuum reactor and crosslinked in glutaraldehyde vapor for 5 h to obtain a porous and crosslinked nanofiber membrane.

Results It was found that the addition of starch and glutaraldehyde crosslinking had a great effect on the morphologies of nanofibers. After adding a certain amount of starch and crosslinking with glutaraldehyde, obvious bonding points appeared at the fiber intersections, with the increase of starch content, the average fiber diameter increased from 290 nm to 310 nm. For the blended nanofiber membrane, all characteristic absorption peaks of the three polymers PAN, starch, and PVP were observed on its FT-IR curve. The above results prove the successful preparation of ternary polymer nanofibers. As the temperature increased, none of the five samples showed shrinkage, and the original square size was maintained even when the temperature was raised to 200 ℃. The results demonstrate that the prepared nanofiber membrane exhibits excellent heat stability, providing a guarantee for the subsequent application of supercapacitor separators. After adding starch and performing vacuum glutaraldehyde cross-linking treatment on the fiber membrane, bonding points were formed between the fibers to reinforce the fiber network, resulting in an increase in the tensile strength and thickness of the fiber membrane. With the addition of PVP and starch, the contact angle of the fiber membrane decreased, further improving the hydrophilicity of the fiber membrane. With an increase in starch content, the hydrophilic properties of the fiber membrane were improved, resulting in more complete dissolution of PVP. However, excessive starch content and cross-linking led to a further decrease in porosity. The supercapacitor device was prepared using nanofiber membrane as separator and tested under cyclic voltammetry and galvanostatic charge-discharge in a two-electrode system. Under a current density of 0.25 A/g, the specific capacitance of device was 25.72 F/g, retaining 47.1% at a high current density of 2 A/g. In addition, after 5 000 charge and discharge cycles, the capacitance retention rate was as high as 96.22%, showing excellent cycle durability.

Conclusion After adding PVP, the fiber membrane thickness decreased to 0.03 mm, resulting in a decrease in tensile strength to 8.15 MPa. Addition of starch improved, the effect of PVP on the tensile strength of the fiber membrane, and with the increase of starch content, the contact angle of the fiber membrane decreased, indicating improvement the hydrophilic performance. By sequentially dissolving in water and crosslinking with glutaraldehyde, the thickness of the fiber membrane was decreased from 0.15 mm to 0.12 mm, while its tensile strength increased from 9.22 MPa to 11.86 MPa. When m(PAN)∶m(starch)∶m(PVP)=5∶4∶1 (mass ratio), the fiber membrane had a high porosity (88.02%). The assembled supercapacitor had a specific capacitance of 25.72 F/g at a current density of 0.25 A/g, an energy density of 3.57 W·h/kg at a power density of 124.45 W/kg, and a specific capacitance retention rate of 96.22% after 5 000 charge discharge cycles, demonstrating potential applications in supercapacitor separator materials.

Key words: electrospinning, nanofiber, supercapacitor, separator, glutaraldehyde cross-linking, polyacrylonitrile, starch, polyvinylpyrrolidone