Activated porous carbon nanofibers using Sn segregation for high-performance electrochemical capacitors

Activated porous carbon nanofibers (CNFs) with three different types of porous structures, which were controlled to contain 1, 4, and 8 wt% of Sn–poly(vinylpyrrolidone) (PVP) precursors in the core region and 7 wt% polyaniline (PAN)–PVP precursors in the shell region during electrospinning, were synthesized using a co-electrospinning technique with H₂-reduction. The formation mechanisms of activated porous CNF electrodes with the three different types of samples were demonstrated. The activated porous CNFs, for use as electrodes in high-performance electrochemical capacitors, have excellent capacitances (289.0 F/g at 10 mV/s), superior cycling stability, and high energy densities; these values are much better than those of the conventional CNFs. The improved capacitances of the activated porous CNFs are explained by the synergistic effect of the improved porous structures in the CNF electrodes and the formation of activated states on the CNF surfaces.     

Polyacrylonitrile and Carbon Nanofibers with Controllable Nanoporous Structures by Electrospinning

In this work, polyacrylonitrile (PAN) and carbon nanofibers with controllable nanoporous structures were successfully prepared via electrospinning technique. For the preparation of porous PAN nanofibers, two kinds of polymers of PAN and polyvinylpyrrolidone (PVP) were used as electrospun precursor materials, and then the bicomponent nanofibers of PAN and PVP were extracted with water to remove the PVP in the composite polymer nanofibers. By altering the ratio of PAN/PVP in the precursor, the pore size and pore distribution of porous PAN nanofibers could be easily controlled. By using the porous PAN nanofibers as structures directing template and through heat treatment, carbon nanofibers with nanoporous structures were obtained. The porous nanofibers were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT‐IR), differential thermal analyses (DTA), Brunauer–Emmett–Teller (BET) nitrogen adsorption, X‐ray diffraction (XRD), and Raman spectra.

     

A comparison of coagulation and gelation on the structures and stabilization behaviors of polyacrylonitrile fibers

The fiber spinning methods determine the formation of the physical structures of polyacrylonitrile (PAN) fibers which further affect stabilization reactions and the mechanical performances of the resultant carbon fibers. In this study, PAN fibers were prepared by both dry-jet gel spinning (g-PAN) and dry-jet wet spinning (w-PAN), and their stabilization behaviors were compared. While the stabilized w-PAN fibers show sheath-core structures, the stabilized g-PAN fibers exhibit relatively uniform stabilized structures along the radial direction. Additionally, the stabilization reactions of g-PAN fibers occur faster than that of w-PAN fibers, and the cyclization, oxidation, and crosslinking reaction activation energies of g-PAN fibers are lower than that of w-PAN fibers, respectively. Moreover, the carbon yield of g-PAN is higher than that of w-PAN fibers. We believe that above changes are possibly ascribed to the formation of different PAN sheath structures and oriented chain structures during dry-jet wet spinning and dry-jet gel spinning. It is concluded that gel spinning could significantly reduce the sheath-core difference of PAN fibers and the stabilized fibers as compared with wet spinning, which leads to a faster stabilization and more uniform stabilized structures. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 137, 48671.     

Thermo-chemical reactions occurring during the oxidative stabilization of electrospun polyacrylonitrile precursor nanofibers and the resulting structural conversions

Thermo-chemical reactions occurring during the oxidative stabilization of electrospun polyacrylonitrile (PAN) precursor nanofibers with diameters of approximately 300 nm were investigated as well as the resulting structural conversions, and the results were compared to those of conventional SAF 3K (Courtaulds) precursor fibers. The study revealed that: (1) the nitrile groups in the electrospun nanofibers possessed a higher reactivity than those in the SAF 3K fibers; (2) the macromolecules in the electrospun nanofibers predominantly underwent inter-molecular cyclization/crosslinking while those in the SAF 3K fibers underwent intra-molecular cyclization during the early stages of stabilization; and (3) under the same stabilization conditions, the structural conversion from linear macromolecules to aromatic ring/ladder structures in the electrospun nanofibers occurred faster and more thoroughly than in the SAF 3K fibers. These characteristics combined with other properties, including small diameter and high degree of structural perfection, suggest that electrospun PAN precursor nanofibers may be used to develop continuous nano-scale carbon fibers with superior mechanical strength, especially if the electrospun nanofibers could be further aligned and stretched.     

Characterization on oxidative stabilization of polyacrylonitrile nanofibers prepared by electrospinning

Ultrafine polyacrylonitrile (PAN) fibers, as a precursor of carbon nanofibers, with diameters in the range of 220–760 nm were obtained by electrospinning of PAN solution using N,N-dimethyl formamide (DMF) as solvent. Morphology of the nanofibers for varying concentration and applied voltage was investigated by field emission scanning electron microscopy (FESEM). The thermal properties and structural changes during the oxidative stabilization process were primarily investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and Fourier transform infrared (FT-IR) and Raman spectroscopy. The nanofiber diameters increase as the applied voltage is increased and they also increase with an increase in the concentration of the polymer solution. It was also concluded that the electrospun fibers displayed a very sharp exothermic peak at 297.34 °C. A transition temperature observed by FT-IR and Raman was approximately 300 °C, which was closely consistent with the results of DSC and TGA studies. It was also found that oxidative stabilization in air was accompanied by a change in color of nanofibers webs.     

Fabrication and electrochemical characterization of polyimide-derived carbon nanofibers for self-standing supercapacitor electrode materials

We report the electrochemical performance of aromatic polyimide (PI)-based carbon nanofibers (CNFs), which were fabricated by electrospinning, imidization, and carbonization process of poly(amic acid) (PAA) as an aromatic PI precursor. For the purpose, PAA solution was electrospun into nanofibers, which were then converted into CNFs via one-step (PAA-CNFs) or two-step heat treatment (PI-CNFs) of imidization and carbonization. The FTIR and Raman spectra demonstrated a successful structural evolution from PAA nanofibers to PI nanofibers to CNFs at the molecular level. The SEM images revealed that the average diameter of the nanofibers decreased noticeably via imidization and carbonization, while it decreased slightly with increasing the carbonization temperature from 800 °C to 1000 °C. In case of PI-CNF carbonized at 1000 °C, a porous structure was developed on the surface of nanofibers. The electrical conductivity of PI-CNFs, which was even higher than that of PAA-CNFs, increased significantly from 0.41 to 2.50 S/cm with increasing the carbonization temperature. From cyclic voltammetry and galvanostatic charge/discharge tests, PI-CNF carbonized at 1000 °C was evaluated to have a maximum electrochemical performance of specific capacitance of ~126.3 F/g, energy density of ~12.2 Wh/kg, and power density of ~160 W/kg, in addition to an excellent operational stability. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47846.     

Magnetic‐field‐assisted electrospinning highly aligned composite nanofibers containing well‐aligned multiwalled carbon nanotubes

Composite nanofiber meshes of well‐aligned polyacrylonitrile (PAN)/polyvinylpyrrolidone (PVP) nanofibers containing multiwalled carbon nanotubes (MWCNTs) were successfully fabricated by a magnetic‐field‐assisted electrospinning (MFAES) technology, which was confirmed to be a favorable method for preparation of aligned composite nanofibers in this article. The MFAES experiments showed that the diameters of composite nanofibers decreased first and then increased with the increase of voltage and MWCNTs content. With the increase of voltage, the degree of alignment of the composite nanofibers decreased, whereas it increased with increasing MWCNTs concentration. Transmission electron microscopy observation showed that MWCNTs were parallel and oriented along the axes of the nanofibers under the low concentration. A maximum enhancement of 178% in tensile strength was manifested by adding 2 wt % MWCNTs in well‐aligned composite nanofibers. In addition, the storage modulus of PAN/PVP/MWCNTs composite nanofibers was significantly higher than that of the PAN/PVP nanofibers. Besides, due to the highly ordered alignment structure, the composite nanofiber meshes showed large anisotropic surface resistance, that is, the surface resistance of the composite nanofiber films along the fiber axis was about 10 times smaller than that perpendicular to the axis direction. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41995.     

Effect of polymer and additive on the structure and property of porous stainless steel hollow fiber

Porous stainless steel hollow fiber has been widely used due to its high mechanical strength, excellent thermal conductivity and good sealing properties compared with other porous supports. We successfully prepared porous stainless steel hollow fibers using polyacrylonitrile (PAN) as polymer via dry-wet spinning followed by sintering through temperature programming method. The PAN concentration had an obvious impact on the structure and property of porous stainless steel hollow fiber even if it would be burned off during sintering. The results showed that the morphology could be tuned by adjusting the concentration of PAN. With increasing PAN concentration in casting solution for spinning, the viscosity was increased dramatically, resulting in much compact structures with high pure water flux (higher than 3×10⁵ L·m^?2·h^?1·Pa^?1). A more dense structure could be obtained by adding additive polyvinylpyrrolidone (PVP) as viscosity enhancer.     

Preparation, structure and supercapacitance of bonded carbon nanofiber electrode materials

Interconnected carbon nanofibrous membranes were prepared by conventional electrospinning and bicomponent electrospinning to produce polyvinylpyrrolidone (PVP)/polyacrylonitrile (PAN) blend nanofibers and PVP/PAN side-by-side bicomponent nanofibers, followed by a direct pyrolysis treatment. The inter-fiber connection was highly affected by the PVP/PAN ratio and electrospinning method. The carbon nanofibers prepared from the side-by-side PVP/PAN nanofibers were found to have higher electrochemical capacitance than those from the PVP/PAN blend nanofibers.     

Comparative investigation of the formation of polytetrafluoroethylene nanoparticles on different solid substrates through the adsorption of tetrafluoroethylene

The effects of different solid substrates, including carbon nanofibers (CNFs), activated carbon, alumina, silica, molecular sieves, and poly(N‐vinylpyrrolidione) (PVP), were compared for the high‐pressure synthesis of polytetrafluoroethylene [PTFE or (CF₂)_n] nanoparticles via the adsorption of thermally synthesized tetrafluoroethylene (C₂F₄) as the monomer. Scanning electron microscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis (TGA) were used for the characterization of the PTFE nanoparticles on different solid substrates. The results demonstrate that the average diameters of the PTFE nanoparticles were about 90 nm for the CNFs, 130 nm for PVP, 150 nm for alumina, and about 200 nm for silica. Also, TGA showed that the amounts of PTFE nanoparticles synthesized on each solid substrate were 3.53 ± 0.09% for CNFs, 2.31 ± 0.10% for PVP, 2.11 ± 0.12% for silica, and 0.97 ± 0.16% for alumina. Depending on the active surface area and the morphology of nanomaterials, such as CNFs, different capacities were evaluated for each solid support in the formation of the PTFE nanoparticles. The quantities and the size of the synthesized PTFE nanoparticles relied on the characteristics of the solid substrate. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

A cost effective and facile approach to prepare beadless polycarbonate nanofibers with ultrafine fiber morphology

Preparing defect free nanofibers with average diameter well below 100 nm is a challenge to researchers by electrospinning technology. In the present contribution, the electrospinning method was utilized to prepare beadless polycarbonate (PC) nanofibers with average diameter 90 nm using comparatively less toxic and suitable solvents in a convenient way. Spinning PC with pure dichloromethane (DCM) and also with 1:1 mixture of DCM and N,N dimethylformamide under the same spinning parameters with varying PC concentration has very much helped to establish the effect of solvents on fiber formation. This study also proved the impact of solution concentration, viscosity, and solution conductivity on the formation of beadless ultrafine PC fibers and subsequently on the bead density and average fiber diameter. The appropriate proportion of solvents under suitable spinning parameters has helped to minimize the quantity of PC during the formation of bead free nanofibers by electrospinning. The ultrafine, uniform, and beadless morphology of the electrospun PC fibers can be utilized for various nanotechnology advancements. POLYM. ENG. SCI., 59:1799–1809, 2019. © 2019 Society of Plastics Engineers     

Polyacrylonitrile-based nanofibers—A state-of-the-art review

Polyacrylonitrile (PAN), a well-known polymer with good stability and mechanical properties, has been widely used in producing carbon nanofibers (CNFs) as these have attracted much recent attention due to their excellent characteristics, such as spinnability, environmentally benign nature and commercial viability. Among the various precursors to produce CNFs, PAN has been extensively studied due to its high carbon yield and flexibility for tailoring the structure of the final CNFs as well as the ease of obtaining stabilized products due to the formation of a ladder structure via nitrile polymerization. In view of this, they have applications in areas such as electronics, tissue engineering, membrane filtration and high performance composites. This review presents various combinations of PAN and PAN-based precursors in producing CNFs from the PAN homopolymer or its modified precursors, copolymers, blends and various composites. Various modifications of PAN and their future prospects in different scientific and technological disciplines are addressed.     

Electrospun polyacrylonitrile/zinc chloride composite nanofibers and their response to hydrogen sulfide

In this work, we explore the electrospinning of polyacrylonitrile (PAN)/zinc(II) chloride (ZnCl₂) composite nanofibers and the response of these nanofibers to hydrogen sulfide (H₂S). Solution properties, including surface tension, viscosity, and conductivity, have been measured and integrated with the results of a variety of other analytical techniques to investigate the effects of ZnCl₂ salt on the structure and thermal properties of electrospun nanofibers. It is found that the addition of ZnCl₂ reduces the diameter and inhibits the instantaneous cyclization reaction of these nanofibers. Additionally, exposing PAN/ZnCl₂ fibers to H₂S leads to the formation of PAN/zinc sulfide (ZnS) composite nanofibers that contain ZnS crystals on the surface. These results indicate that PAN/ZnCl₂ composite nanofibers could find applications in H₂S sensing and removal, or as precursors for semiconductor ZnS-coated polymer nanofibers.     

Structure and thermo-chemical properties of continuous bundles of aligned and stretched electrospun polyacrylonitrile precursor nanofibers collected in a flowing water bath

Continuous bundles of aligned and stretched electrospun polyacrylonitrile (PAN) precursor nanofibers were prepared in an attempt to develop carbon nanofibers with superior strength. The bundles were prepared through collection of electrospun nanofibers with a flowing water bath followed by stretching in water at 97 °C. Their morphologies, structures, and thermo-chemical properties were characterized by SEM, XRD, and DSC. The shrinkages in boiling water and the amounts of residual solvent were also measured. The results indicated that, the nanofibers in the bundles were uniform with smooth surfaces and small variations in diameters; after stretching the bundles by 4 times, the average fiber diameter was reduced to 56%, while the crystallinity of PAN was improved by 72%. The post-spinning stretching process facilitated the stabilization of PAN, as evidenced by the shift of the cyclization reaction to a lower temperature with smaller activation energy and larger enthalpy change. In comparison with the commonly adopted nanofiber collection method of a rotating drum, the flowing water bath method results in higher degree of uni-axial alignment and more desired structures of nanofibers.     

Graphitic carbon nanofibers developed from bundles of aligned electrospun polyacrylonitrile nanofibers containing phosphoric acid

Graphitic carbon nanofibers (GCNFs) with diameters of approximately 300 nm were developed using bundles of aligned electrospun polyacrylonitrile (PAN) nanofibers containing phosphoric acid (PA) as the innovative precursors through thermal treatments of stabilization, carbonization, and graphitization. The morphological, structural, and mechanical properties of GCNFs were systematically characterized and/or evaluated. The GCNFs made from the electrospun PAN precursor nanofibers containing 1.5 wt.% of PA exhibited mechanical strength that was 62.3% higher than that of the GCNFs made from the precursor nanofibers without PA. The molecules of PA in the electrospun PAN precursor nanofibers initiated the cyclization and induced the aromatization during stabilization, as indicated by the FT-IR and TGA results. The stabilized PAN nanofibers possessed regularly oriented ladder structures, which facilitated the further formation of ordered graphitic structures in GCNFs during carbonization and graphitization, as indicated by the TEM, XRD, and Raman results.     

Desirable electrical and mechanical properties of continuous hybrid nano-scale carbon fibers containing highly aligned multi-walled carbon nanotubes

The continuous highly aligned hybrid carbon nanofibers (CNFs) with different content of acid-oxidized multi-walled carbon nanotubes (MWCNTs) were fabricated through electrospinning of polyacrylonitrile (PAN) followed by a series of heat treatments under tensile force. The effects of MWCNTs on the micro-morphology, the degree of orientation and ordered crystalline structure of the resulting nanofibers were analyzed quantitatively by diversified structural characterization techniques. The orientation of PAN molecule chains and the graphitization degree in carbonized nanofibers were distinctly improved through the addition of MWCNTs. The electrical conductivity of the hybrid CNFs with 3 wt% MWCNTs reached 26 S/cm along the fiber direction due to the ordered alignment of MWCNTs and nanofibers. The reinforcing effect of hybrid CNFs in epoxy composites was also revealed. An enhancement of 46.3% in Young’s modulus of epoxy composites was manifested by adding 5 wt% hybrid CNFs mentioned above. At the same time, the storage modulus of hybrid CNF/epoxy composites was significantly higher than that of pristine epoxy and CNF/epoxy composites not containing MWCNTs, and the performance gap became greater under the high temperature regions. It is believed that such a continuous hybrid CNF can be used as effective multifunctional reinforcement in polymer matrix composites.     

Formation of PVP hollow fibers by electrospinning in one-step process at sub and supercritical CO2

It was well known that electrospinning is one of the simple technical methods for the production of polymer nanoparticles and nanofibers. Various polymers have been successfully electrospun into ultrafine particles and fibers in recent years mostly in solvent solution and some in melt form. In this work, hollow fibers with walls made of organic polymer composites have been formed by electrospinning in a single processing step under pressurized carbon dioxide (CO₂). The experiments were conducted at 313 K and ∼8 MPa. The capability and feasibility of this technique was demonstrated by the production of polyvinylpyrrolidone (PVP) fibers whose size and wall thickness could be independently varied by controlling a set of experimental parameters. The PVP fibers had an average pore diameter 2–4 μm. At low pressures (<5 MPa; subcritical conditions), the solid fibers were formed, the baloon-like structures of PVP was formed with increasing pressure of CO₂ at 8 MPa (supercritical condition)     

Kinetics of the cyclization and isomerization reactions in polyacrylonitrile based carbon fiber precursors during thermal-oxidative stabilization

The kinetics of reactions in polyacrylonitrile (PAN) based carbon fiber (CF) production should be of significance to the guidance of process control, fiber structure formation. PAN precursor fibers were isothermally stabilized at 210, 225, 240, 255, and 270 °C, respectively, for 10 to 100 min in an air oven to study the kinetics of the cyclization and isomerization reactions. The structural evolution of PAN precursor fibers during thermal-oxidative stabilization was characterized by Fourier transform infrared (FTIR) spectroscopy and solid state ¹³C nuclear magnetic resonance (¹³C NMR). The results indicate that the FTIR absorbance of  CN (the resultant of the cyclization) in PAN shows a trend of first increasing and then decreasing. And then the NMR peak assigned to the carbon atoms linking imino groups ( NH ) proves the isomerization of  CN into  NH in pyridone structure. Based upon the FTIR absorbance method, the entire process of the cyclization and isomerization reactions is considered as a consecutive first-order reaction. A kinetic model for the consecutive reaction has been established via the evaluation of the reaction rate constants of two single reactions. According to the model, the simulated kinetic curves of the characteristic groups ( CN,  CN , and  NH ) conform to the FTIR absorbance trends of these groups based on experimental data. This study is expected to furnish in-depth information on the crucial reaction kinetics during stabilization of PAN precursors, which is of advantage to the process optimization of the CF production. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48819.     

Investigation of nanoyarn preparation by modified electrospinning setup

Higher ordered structures of nanofibers, including nanofiber‐based yarns and cables, have a variety of potential applications, including wearable health monitoring systems, artificial tendons, and medical sutures. In this study, twisted assemblies of polyacrylonitrile (PAN), polyvinylidene fluoride trifluoroethylene (PVDF‐TrFe), and polycaprolactone (PCL) nanofibers were fabricated via a modified electrospinning setup, consisting of a rotating cone‐shaped copper collector, two syringe pumps, and two high voltage power supplies. The fiber diameters and twist angles varied as a function of the rotary speed of the collector. Mechanical testing of the yarns revealed that PVDF‐TrFe and PCL yarns have a higher strain‐to‐failure than PAN yarns, reaching 307% for PCL nanoyarns. For the first time, the porosity of nanofiber yarns was studied as a function of twist angle, showing that PAN nanoyarns are more porous than PCL yarns. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44813.     

Mechanism and kinetics of oxidation during the thermal stabilization of polyacrylonitrile fibers

The thermal behavior and structural evolution during the thermal stabilization of polyacrylonitrile (PAN) fibers in N₂ and air were investigated using differential scanning calorimetry and solid‐state ¹³C nuclear magnetic resonance. It was found that an oxidation reaction, that generated carbonyl (C?O) groups could occur at 160°C which has not been reported in the literature. It is proposed that the cyclized structures in the PAN macromolecule chains are a prerequisite for the oxidation. Further investigations indicate that with more cyclized structures in the PAN macromolecule chains, the oxidation proceeds more readily, which is consistent with the proposed mechanism. The kinetic parameters for the oxidation and cyclization reactions were estimated using the Kissinger method. The activation energies for the reactions of oxidation and cyclization for PAN fibers are about 96.4 kJ/mol and 190.0 kJ/mol, respectively, which implies that the cyclization is the rate determining step during the thermal stabilization of PAN fibers. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013