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Electrochem 2021, 2 242 storage applications [98,99]. Moreover, the modification of CNFs to obtain an increased capacitance can be done in two ways. First, the electrospinning process is performed by tuning the electrospinning solution parameters and processing parameters. Second, modifications can be done by post-CNF synthesis processes [32,59]. The post-modification approaches for increasing capacitance are more or less similar to those of CC. The most commonly used polymer to synthesize carbon nanofibers is PAN due to its high carbon yield compared to other polymers. Other polymers used for that purpose are shown in Table 2. Various processing parameters, such as the voltage, flow rate, and collector-to-tip distance, along with solution parameters, such as the viscosity and con- ductivity of a polymer solution, can play a role in controlling the morphology and fiber diameter of the final product. Pure, as-spun polymeric nanofibers generally have low conductivity; therefore, stepwise thermal treatment processes, such as stabilization and subsequent carbonization, must be performed to make the carbon fibers more conduc- tive. The potential of electrospun carbon nanofiber-based materials in diverse advanced technologies has been increasingly studied, and progress has been summarized in a few reviews. However, works related to electrospun carbon nanofiber-based negative electrode materials for supercapacitors have been rarely summarized. We believe this review can serve as a resource for further studies on the development of negative electrode materials for high-performance energy storage devices. Typically, ECNFs are solid and have a small surface area that results in low capacitance and energy density. To this end, it is necessary to design carbon fibers to increase the surface area and make composites with materials that can contribute to a high capacitance, while not hampering the inherent features of carbon fibers. Nanofibrous materials with a high specific surface area, controllable porosity, good conductivity, and flexibility are promising features for next-generation technologies. Therefore, different strategies have been adopted to fabricate nanofibers with such properties. Coaxial electrospinning produces sheath-core nanofibers. It uses a coaxial needle that consists of inner and outer hollow needles that are arranged concentrically and dispense two different solutions. By using less volatile or washable core polymers, hollow nanofibers can be obtained [83]. Coaxial electrospinning using poly(methyl methacrylate) (PMMA) as the core solution and a PAN solution as the shell solution, thereby producing PMMA/PAN core-shell nanofibers. Regarding the PMMA/PAN nanofibers after carbonization, a more volatile PMMA portion is removed; therefore, hollow carbon nanofibers are obtained (Figure 3b) [88]. Electrospinning PAN/PMMA blends with different ratios produce multiporous nanofibers (Figure 3c) [100]. Using block copolymer-based precursors as an approach is significant for con- trolling the porosity of the resulting material and may revolutionize the synthesis of PCNFs [91]. Zhou et al. synthesized dual-doped PCNFs with well-controlled bi- modal pores, namely, mesopores (10 nm) and micropores (0.5 nm), by electrospinning poly(acrylonitrile-block-methyl methacrylate) (Figure 3f) [91]. There are some other notable reports. Yan et al. prepared highly porous sponge-like carbon nanofibers by elec- trospinning poly(tetrafluoroethylene) and poly(vinyl alcohol) with boric acid as the cross- linking agent. These nanofibers possessed well-controlled macro/meso/micropores and an ultrahigh porosity (>80%) and outstanding conductivity (980 S cm−1), while being triple-doped with B-F-N (Figure 3g) [103]. Yang et al. prepared necklace-like hollow carbon nanofibrous materials with an abundance of micro/meso/micropores and an ultrahigh content of doped N (Figure 3h) [104]. One of the common strategies for fabri- cating porous nanofibers is the selective removal of a sacrificial phase from the as-spun nanofibers by washing, leaching, or heating. The sacrificial phase may be small nanopar- ticles or another polymer. Wang et al. fabricated silicon oxide (SiO2)- and Sb-entrapped nanofibers by electrospinning antimony trichloride (SbCl3), polyvinylpyrrolidone (PVP), and tetraethylorthosilicate (TEOS). After carbonization and etching with HF, a highly porous carbon nanofibrous structure was obtained (Figure 3d) [102]. They later used this electrode as a highly stable electrode for Li batteries.PDF Image | Review of Electrospun Carbon Nanofiber-Based Negative Electrode Materials
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