Carbon fibers from electrospinning

Carbon fiber has numerous applications as reinforcement in composites and electrodes due to its conductive nature. Reduction of its size to the nanometer scale may potentially enhance its performance and open up new applications [Mao et al 2013]. Electrospinning is a simple method of spinning polymer solution to form nanofibers. Since carbon fibers are commonly derived from carbonizing polymer, electrospinning is an attractive method for producing polymer nanofibers for carbonizing. Polyacrylonitrile (PAN) is the most commonly used polymer in carbon fiber production. Electrospinning of PAN nanofibers have also been shown to be a simple process as it dissolves readily in N,N-dimethylformamide (DMF) and does not require any modification to the setup to form fibers [Gu et al 2005]. Electrospinning has also been used on other polymers to form fibers for carbon conversion.

With growing awareness and effort to reduce carbon footprint, researchers have explored using plant-based sources for production of carbon fibers. Various source of lignin has been electrospun for conversion into carbon fibers. However, their molecular structure differs depending on the source of lignin. Nar et al (2016) electrospun nanofibers from C-lignin and compare it to a Kraft lignin in terms of their carbonized fibers characteristic. The C-lignin derived electrospun carbon fibers have a higher graphitic structure compared to Kraft lignin derived carbon fibers. The transverse moduli of C-lignin derived carbon fibers found to be better than Kraft and commercial carbon fibers. For a more environmentally friendly method of obtaining carbon fibers using electrospinning, ethanol based-lignin may be used. García-Mateos et al (2018) prepared an ethanol solution with 40 wt% Alcell® lignin. To facilitate the electrospinning of the lignin solution, a co-axial configuration was used where additional flow of pure ethanol is added in the outer needle for stabilization purpose. The resultant electrospun lignin fibers were subsequently carbonised to form carbon fibers. Using ethanol-based lignin, metal precursors may be added for a one-pot synthesis of electrocatalyst. Considering the difficulty in preparing and electrospinning of lignocellulosic, Chen et al (2019) demonstrated the feasibility of using electrospinning on sugarcane bagasse (SCB) to produce nanofibers. A homogeneous esterification process was carried out on SCB to form esterified products (SCB-A). This is then mixed with 50% or less of polyacrylonitrile (PAN) so that the mixture can be electrospun. Following the carbonization process of the electrospun fibers, the amount of graphitization is the same as pure carbonized PAN fibers. With SCB-A/PAN carbon nanofibers, nitrogen species of pyridinic-N, pyrrolic-N, and graphitic-N can be found while pyrrolic-N was absent in PAN carbon nanofibers. The presence of both pyridinic-N and pyrrolic-N would contribute to accumulation or electric charge during the charge-discharge process. This led to better conductivity of SCB-A/PAN carbon nanofibers compared to PAN carbon nanofibers. Worarutariyachai et al (2020) investigated an alternative strategy for electrospinning alkali lignin (AL) (kraft lignin) to produce carbon fibers. Their objective is to use pure AL solution without any carrier polymers for electrospinning. To prepare the AL solution, they used water and glycerol as the co-solvent to improve electrospinnability. Glycerol helps to reduce the surface tension of the solution although too much of it would reduce the conductivity and increase the viscosity of the solution. The electrospinning solution was heated to 80 °C during electrospinning to further enhance electrospinnability. They were able to obtain beadless electrospun microfibers and following carbonization, the resultant carbon fibers have an average diameter of 21.05 µm with rough, uneven surfaces. Such irregular surfaces have been attributed to the presence of inorganic contaminants which may be removed by sulfuric acid (H₂SO₄) washing. The carbon fibers showed a smooth surface following the washing process with a conductivity of 6.07 mS/cm.

SEM images of carbon fibers from AL solutions: (a) in water (lignin:water = 1:1.5 w/w); and (b) in mixed glycerol/water solvent (lignin:solvent = 1:1.25 w/w, glycerol:water = 0.5:1 v/v); numbers 1 and 2 refer to before and after H2SO4 washing, respectively [Worarutariyachai et al 2020].

The versatility of electrospinning has enabled the incorporation of other materials in the resultant carbon fibers. To improve the performance of electrospun carbon fibers in electrical double layer capacitors (EDLC), Kim et al (2009) mixed PAN with pitch in solution for electrospinning. This takes advantage of the high carbon content and conductivity of pitch and the electrospinnability of PAN to form fibers. The pitch/PAN electrospun fibers activated at 900 °C demonstrated a high specific capacitance (143.5 Fg^-1) and power density of 1300 Wkg^-1. Another way of increasing specific capacitance of electrospun derived carbon fiber membrane is to use sacrificial polymers. Wang et al (2018) showed that electrospinning a blend of polysulfone (PSF) and PAN solution to form a fibrous membrane was able to yield specific capacitance of 272 F.g^-1 and conductivity of 13.85 S.cm^-1 after carbonization, values that are twice that of carbonised pure electrospun PAN fibrous membrane. Such superior performance has been attributed to microporous and mesoporous structure, well interconnected carbon nanofibrous network with good graphitic carbon contents of the PAN/PSF nanofibrous membrane. Carbon membrane obtained from carbonised pure PAN electrospun fibers showed smooth surface, non-connected fibers and poorer graphitic carbon contents. Microporous and mesoporous carbonised PAN/PSF nanofibrous membrane has been attributed to decomposition of PSF and release of SO₂ gas from the nanofibers. Interconnection is probably due to melting of PSF at temperatures above 350°C before decomposition at temperatures above 500°C. Since the presence of interconnecting networks may enhance mechanical and electrochemical properties of carbon mesh, Ma et al (2021) used hot pressing to consolidate the electrospun PAN fibers prior to carbonization. For electrospinning, they used a mixture of polyacrylonitrile blends PAN150 and PAN85 with molecular weights (MWs) of 150,000 and 85,000 mol^-1 respectively. A mixture of PAN150 7:3 PAN85 was found to give optimal performance compared to blends with higher ratios of PAN85 after carbonization. The optimal carbonised PAN fibers showed a good specific capacitance of 689 F g^-1 with energy density of 1.74 W h k^-1 (0.38 W kg^-1) and a cyclic stability of 83% after 2000 continuous charge-discharge cycles at a current density of 2 A g^-1. Compared to carbonised fibers of the same composition without going through hot pressing, hot-pressed carbonised nanofibers membrane showed a higher current density which demonstrated the benefits of forming networked structure for improving electrochemical performance.

Additives such as silicon may also be added to the electrospun carbon fibers to improve its mechanical properties. Faccini et al (2015) constructed a carbon nanofiber (CNF)/Si composite membrane by electrospinning a solution of PAN and tetraethoxyorthosilicate (TEOS). After carbonizing at temperature from 800 to 1000 °C, SNF/Si was formed. The resultant membrane showed good flexibility and can be bent without breaking. Huang et al (2016) was able to construct an aerogel made of elastic carbon nanofibrous composite containing graphene. They first blend graphene oxide (GO) into PAN solution followed by electrospinning to form PAN nanofibers containing GO. An preoxidation process was carried out to prevent the PAN fibers from fusing together during carbonization. The resultant composite aerogel showed interconnected macropores and integrated conductive networks which make it useful in various applications such as energy storage and conversion.

Digital picture showing the flexibility of the CNF/Si mats [Faccini et al 2015]

PAN is one of the most commonly used polymer for conversation to carbon fibers and there are many studies on the carbonizing process and behavior of PAN. Schierholz et al (2019) studied the carbonation behaviour of electrospun PAN nanofiber using in situ transmission electron microscopy in temperatures up to 1000°C. During the initial oxidative stabilisation at temperatures between 200 to 300°C, fiber shrinkage of up to 20% of its initial diameter was observed. The subsequent heating up to 1500°C under inert gas atmosphere is the carbonation stage. As the temperature rises to 600°C, their TEM showed the transition from amorphous contrast to diffraction contrast. At 600°C, ordering of the carbon atoms takes place with their (002)-planes preferentially arranged parallel to the fiber axis on the lateral surfaces. However, only a few planes are stacked and the lateral size (L₁₀) are only a few nm. At 800°C, lateral and stacking size is around 5 nm. At 1000°C, the turbostratic regions become as large as 10 nm.

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