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Electrospinning is a commonly used method for fabricating nanofibers. An electrospinning machine or electrospinning device is simple to operate, highly scalable, and compatible with a wide range of materials. However, the fibers obtained from traditional electrospinning are mostly randomly oriented, which limits their performance. To address this issue, researchers have attempted various methods for aligning fibers, which can be mainly divided into two categories. One category is to suppress the whipping motion of the fibers, such as using rotating collectors, introducing centrifugal force, shortening the collection distance, or adopting near-field electrospinning. The other category is to utilize the whipping motion and achieve fiber alignment through means like auxiliary templates, patterned collectors, and electrode regulation. Nevertheless, most of these methods can only achieve uniaxial alignment, making it difficult to construct complex structures and control the interfiber porosity. To overcome this limitation, researchers have developed various nanofiber alignment techniques, among which the insulating block-assisted electrospinning technique stands out for its high efficiency and scalability.
This article reviews the latest progress in the preparation of biaxially aligned nanofibers using the insulating block-assisted electrospinning technique. This technique enables precise alignment of nanofibers by adjusting the electric field between the nozzle and the substrate, combined with the movement of the substrate in an electrospinning device. The article details the influence of key parameters of this technique, such as the gap between insulating blocks, nozzle movement speed, substrate motion period, etc., on the fiber alignment and structure, and summarizes its applications in fields such as air filtration, lithium-ion battery electrodes, gel polymer electrolytes for aqueous batteries, and reinforced composite membranes for fuel cells.
(1) Preparation Method and Experiments
The key of the insulating block-assisted electrospinning technique lies in adjusting the electric field distribution through insulating blocks in an electrospinning machine. In the experiment, researchers placed two insulating blocks around the nozzle to restrict the electric field distribution along the x-axis, causing the fibers to move mainly in the y-axis direction. By adjusting parameters such as the gap between insulating blocks, nozzle movement speed, and substrate motion period, researchers can precisely control the alignment and structure of nanofibers (such as linear, wave-like, grid-like, etc.).
Figure 1: Schematic illustration and photograph images of a) conventional electrospinning and b) insulating block-assisted electrospinning for the jet using a polymer solution
In traditional electrospinning, the polymer solution is ejected from the nozzle under the action of a high-voltage electric field in an electrospinning device, forming a Taylor cone. After the solvent evaporates, the fibers are randomly deposited on the substrate, forming a disorderly non-woven structure (Figure 1a). In insulating block-assisted electrospinning, two plastic insulating blocks are placed around the nozzle (Figure 1b), which restricts the electric field in the x-axis direction and makes the fibers move mainly in the y-axis direction. Combined with the movement of the rotating collector, a biaxially aligned nanofiber mat can be fabricated.
During this process, multiple factors can affect the structure and alignment quality of the fibers, such as the gap between the insulating blocks, the nozzle movement speed, the collector motion period, the distance between the nozzle and the substrate, and the applied voltage. By adjusting these parameters, researchers have obtained various fiber patterns, such as linear, wave-like, cross-rectilinear, and cross-wave-like.
Figure 2: Electrospun polyethylene oxide (PEO) nanofibers display: a) linear, b) wave-like, c) cross rectilinear shape, and d) cross wave-like shape configuration
(2) Application in Air Filtration
The research team developed a partially aligned dual-nanofiber air filtration material (ATRN) using the insulating block-assisted electrospinning technique. This material consists of a randomly arranged thin nanofiber base layer and a uniformly aligned thick nanofiber spacer layer. ATRN is composed of a randomly oriented thin nanofiber base layer and a uniformly oriented thick nanofiber spacer layer. By controlling the electric field and the concentration of the polymer solution in the electrospinning device, the fiber diameter and pore structure can be adjusted. Compared with traditional melt-blown filters (MBF), ATRN has a 97% increase in filtration efficiency, a 50% reduction in pressure drop, and a record-high quality factor of 0.0781. This achievement provides a new approach for the development of high-performance filtration materials and has broad application prospects in fields such as face masks and protective clothing.
(3) Application in Lithium-Ion Battery Electrodes
The research team further explored the application of the insulating block-assisted electrospinning technique in lithium-ion battery electrodes. Biaxially aligned carbon nanofibers (CA-CNF) were prepared by carbonizing polyacrylonitrile (PAN) nanofibers. Experimental results show that the CA-CNF electrodes are significantly superior to traditional randomly arranged carbon nanofibers in terms of cycle stability and high-rate performance.
(4) Application in Gel Polymer Electrolytes for Aqueous Batteries
Zinc-air batteries (ZABs) based on gel polymer electrolytes (GPEs) are ideal power sources for wearable devices, but the problem of electrolyte evaporation limits their service life. In aqueous zinc-air batteries, the research team developed a biaxially aligned nanofiber composite electrolyte based on polyacrylic acid (PAA), which forms efficient ion transport channels. Experimental results show that compared with pure PVA electrolytes, the ionic conductivity of this composite GPE has increased nearly 6 times, reaching 235.7 mS/cm. The cycle stability and charging performance of the ZAB battery based on this composite GPE have been significantly improved, and it can stably cycle for more than 80 hours.
(5) Application in Composite Membranes for Fuel Cells
The research team also explored the application of the insulating block-assisted electrospinning technique in composite membranes for fuel cells. By preparing biaxially aligned polytetrafluoroethylene (PTFE) nanofiber-reinforced membranes, the mechanical stability and proton conduction performance of the composite membranes were significantly improved. Experimental results show that this reinforced membrane exhibits excellent proton conduction performance under high humidity conditions.
The insulating block-assisted electrospinning technique provides an efficient and scalable solution for the precise alignment of nanofibers. By adjusting the electric field distribution and substrate motion in an electrospinning device, researchers can achieve biaxial alignment of nanofibers and demonstrate excellent performance in fields such as air filtration, lithium-ion battery electrodes, aqueous battery electrolytes, and fuel cell composite membranes. This technique has not only achieved remarkable results at the laboratory scale but also has the potential for large-scale production, providing a new direction for the development of high-performance nanofiber materials.
Electrospinning Nanofibers Article Source:
https://doi.org/10.1002/marc.202400888
