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Laser Carbonization of PAN-Nanofiber Mats with Enhanced Surface Area and Porosity Dennis Go, Philipp Lott, Jochen Stollenwerk, Helga Thomas, Martin Moeller, and Alexander J. C. Kuehne ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09358 • Publication Date (Web): 13 Sep 2016 Downloaded from http://pubs.acs.org on September 15, 2016

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ACS Applied Materials & Interfaces

Laser Carbonization of PAN-Nanofiber Mats with Enhanced Surface Area and Porosity Dennis Goa, Philipp Lottb, Jochen Stollenwerkb, Helga Thomasa, Martin Möllera and Alexander J. C. Kuehnea* a

DWI

−

Leibniz

Institute

for

Interactive

Materials,

RWTH

Aachen

University,

Forckenbeckstraße 50, 52074 Aachen, Germany. bFraunhofer Institute for Laser Technology (ILT), Steinbachstraße 15, 52074 Aachen, Germany. KEYWORDS: carbon fibers, porosity, graphene, electro spinning, polyacrylonitrile

ABSTRACT: Here we present a novel laser process to generate carbon nanofiber nonwovens from polyacrylonitrile. We produce carbon nano-fabrics via electrospinning followed by infrared laser induced carbonization, facilitating high surface area and well controlled hierarchical porosity. The process allows precise control of the carbonization conditions and provides high nanoscale porosity. In comparison with classical thermal carbonization, the laser process produces much higher surface areas and smaller pores. Furthermore, we investigate the carbonization performance and morphology of polyacrylonitrile nanofibers compounded with graphene nano-platelet fillers.

ACS Paragon Plus Environment

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Nonwovens from carbon fibers are a powerful class of materials with a broad spectrum of applications ranging from reinforcement materials in polymer composites to conductive materials for energy applications and separation materials in filters and membranes. Today, carbon fiber materials are predominantly obtained via pyrolysis of precursor fibers prepared from polyacrylonitrile (PAN).1 Critical parameters need to be controlled to generate specific properties, such as high tensile strength, high conductivity or high specific surface area.2 At optimized fabrication parameters, high-strength (> 4 GPa) with ultra-high modulus (> 500 GPa) can be achieved in carbon fibers.3 Materials reinforced with such carbon fibers facilitate high performance materials, making carbon fiber composites suitable for applications in lightweight structural design.3,4 Depending on the pyrolysis conditions and optional posttreatment, such as chemical activation, the porosity and surface area of carbon fibers can be adjusted.4–7 High porosities make carbon fiber materials highly relevant for use in membrane applications for filtration and separation, as well as in vapor sensing, enzyme immobilization and for use in tissue engineering.4,8–10 Furthermore, carbon materials are inert to a variety of chemicals, solvents, electrolytes and biological entities.2,11–13 The electrical conductivity of carbon materials can be enhanced by thermal treatment or addition of conductive nanofillers.2,11–13 High porosity and large specific surface areas together with high electrical conductivity allow application of carbon fibers as electrodes in energy storage devices.14 Here, charged electrolyte ions can be screened more efficiently upon adsorption to a porous carbon fiber electrode, leading to increased charge carrier density at the electrode and hence improved capacities.15 One straightforward way to access carbon fibers with high surface area per weight is by reducing the fiber diameter. PAN can be spun from solution using electro- or rotational spinning leading


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to fibers with diameters on the micro- and nanometer size regime.8,16,17 The spinning-, stabilization and carbonization conditions for PAN are well understood, making PAN precursors highly convenient for creating 3D fiber fleece networks.5,9,18,19 Precise control over the surface area and pore size would allow further improvement of the carbon fiber performance as electrodes as well as for filtration and separation applications. However, there are several problems that arise during the fabrication processes, deteriorating the performance of carbon fiber materials.4 First, the pore size is mainly determined during the pyrolysis process, where low molecular weight carbonization byproducts diffuse out of the carbon fiber and produce nucleation points for pores, which can be accessed during subsequent activation. PAN is a very poor heat conductor (~ 0.2 W/(m×K))20,21 and thermal gradients during the high temperature carbonization process lead to varying carbonization conditions across the fiber material and the pore size distribution becomes inhomogeneous and broad. Second, the chemical or physical activation processes are difficult to control as morphological preconditions in the fibers (from extrusion, stretching or shrinking processes) affect the carbonization and activation and the resulting porosity.22 Third, thermal carbonization processes are time consuming and require energy consuming ovens at high temperature. Furthermore, chemical activation processes require large amounts of corrosive liquids. These conditions represent unfavorable drawbacks for the production of porous carbon fiber mats and their scale up. To circumvent these problems one requires a fast localized heating method, which can be scanned over the carbon fiber textile to circumvent the problem of thermal gradients. Fast heating could induce fast expulsion of carbonization by-products affording an in-situ activation process, circumventing corrosive substances for the activation process.


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In this manuscript we develop a laser carbonization process to circumvent the problems associated with inhomogeneous pyrolysis and activation. We locally apply an IR-laser on one side of the stabilized PAN-fiber mats to induce carbonization. The laser-spot can be fine-tuned and allows fast, homogeneous heating rates. Thus, causing explosive blowout of the carbonization by-products, generating fibers with high porosity making subsequent activation no longer required. We investigate the necessity for compounding with IR absorbing graphene nano-platelets (GnPs) and their effect on the fiber surface characteristics.13,23 Our novel technique allows generation of nonwovens with hierarchical porosity control. The fleeces have mesh sizes on the micrometer scale, with fiber diameters on the order of hundreds of nanometers and surficial pores with diameters on the nanometer scale. Our laser carbonized fiber mats have higher surface areas and smaller pore sizes than carbon fibers, which are thermally pyrolyzed and chemically activated. The established laser process is fast, allows facile control over the carbonization conditions by variation of the laser intensity and the irradiation time and is easily scalable. The process produces improved carbon fiber materials with high specific surface area

Figure 1: Images of the fiber fleeces. (a) as-spun PAN fleece, (b) stabilized fleece, (c) thermally carbonized and (d) laser carbonized fleeces (scale bars indicate 1 cm). (e) Corresponding FTIR spectra supporting stages of the as spun (blue squares), stabilized (red diamonds), thermally carbonized (black circles) and laser carbonized fibers (dark gray triangles), (f) Comparison of the different laser treatments, whereat a 50 K/s heating rate and an irradiation for 60 s. (dark gray triangles) fits the best with the thermal carbonization process in comparison to heating rates of 5 K/s (light gray squares), 25 K/s (gray diamonds) and 50 K/s (dark gray pentagons) without additional holding time. The inset shows the IR absorption of stabilized PAN with 0% GnP (dark gray diamonds), 1.0 wt% GnP (gray diamonds), 1.5 wt% GnP (light gray diamonds). The 968 nm and 998 nm laser lines are indicated by a red line. ACS Paragon Plus Environment

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and porosity. We start by producing our carbon-fiber precursors via electro-spinning of a poly(acrylonitrile-coitaconic acid) copolymer solution in DMF. The itaconic acid co-monomer allows stabilization at lower temperatures than pristine PAN.19,24,25 We collect the nanofiber fleece on a grounded target lined with aluminum foil. The 8 wt% polymer solution is spun under an applied voltage of 17 kV at a distance of 15 cm between nozzle and collector. These optimized parameters allow production of densely packed fiber meshes with good mechanical stability.8 The nonwovens can be detached from the aluminum support as thin white sheets (see Figure 1a). To increase the IR absorption we spin composite fibers by first exfoliating GnPs in DFM using a tip-sonicator.

Figure 2: SEM characterization of the morphology of the PAN-fibers with different GnP loadings (from left to right: 0 wt%, 0.1 wt%, 0.5wt%, 1.0 wt% and 1.5 wt%) at different stages: (a-e) freshly-spun PANfibers with an average diameter of 220 ± 20 nm, (f-j) under air thermally stabilized PAN-fibers with an increased, average diameter of 250 ± 20 nm, (k-o) under nitrogen thermally carbonized and (p-t) laser carbonized PAN-fibers. Scale bars indicate 1 µm.


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Different concentrations of GnP in PAN (0.1% - 1.5%) are prepared by addition of PAN solutions in DMF to the GnP stock solution. We expose the free standing PAN mats to thermal stabilization in a convection oven in air. The stabilization comprises an intramolecular cyclisation step on the molecular scale, which is always accompanied with material shrinkage. This shrinkage can be reduced at low temperatures and the itaconic acid co-monomer acts as a cyclization accelerator enabling full stabilization at 250 °C

9,19,25

. We submit the PAN fiber nonwovens to an equilibration step at 100 °C for 2 h to

remove residual solvent and resolve stresses in the fibers and facilitate fission at points of contact between the fibers to obtain a truly connected 3D fiber network (cf. Figure 2a and f). The nonwoven is then heated to 250 °C for 30 min to induce stabilization. The color of the fleece changes from white to brown during this process due to the formation of chromophores in the cyclized nitrile structure (see Figure 1b).26 To verify full conversion we monitor the process using IR spectroscopy. We observe that the nitrile band (C≡N) at 2243 cm-1 vanishes almost completely and gives way to an imine band (C=N) for the cyclization product at 1750 cm-1 (see Figure 1e). This substantiates complete stabilization of the PAN fiber fleeces. To generate carbon fibers the stabilized PAN-fibers need to be pyrolyzed further, leading to fission of the stabilized polymer ribbons to form graphitic carbon. To choose suitable laser wavelengths for carbonization we determine the absorption of the stabilized fleeces in the near IR and IR regime. We observe a slight increase in absorption for stabilized materials with compounded GnPs between 800 and 1200 nm (see inset Figure 1f). We use a diode laser with emission wavelengths of 968 and 998 nm where the absorption of the fleece is between 60 and 70 % indicating efficient coupling of laser energy into the stabilized textile. To develop a suitable laser carbonization process,20 which achieves full carbonization of the stabilized PAN


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fiber fleeces, we vary several laser parameters. The laser carbonization is performed in an inert nitrogen atmosphere (oxygen < 20 ppm) and heating rates are varied from 5-50 K/s and the heating time is varied from 1 – 60 s. After carbonization, the color of the fleeces changes to black, as the conjugated polymer ribbons condensate and graphitic structures emerge (see Figure 1c and d). We monitor the carbonization results using IR spectroscopy. For all material compositions we obtain the best results for a heating rate of 50 K/s and an irradiation time of 60 s. For these conditions we observe an almost identical IR spectrum compared to a thermally carbonized material treated in inert atmosphere at 1300 °C (see Figure 1e). Fiber fleeces, which are carbonized at lower heating rates show residual amine NH2 groups, visible as a broad band around 3250 cm-1. Residual amine groups are detrimental to the carbon fiber material, as full graphitization has not been achieved. Shorter irradiation times lead to incomplete fission of the polymer ribbons and smaller sized graphitic domains as can be seen by a relatively high methine (CH) signal occurring at the graphene grain boundaries (cf. Figure 1e and f). By contrast, complete carbonization is associated with the loss of carbonyl, imine and methine signals (bands between 1500 and 1700 cm-1). Only the well resolved aromatic C=C signals remain for the laser and the thermally carbonized fiber fleeces (see region between 1700 and 1400 cm-1 in Figure 1e).27,28 Having established laser carbonization conditions, we move on to investigate the morphology of the carbonized nonwovens using electron microscopy. We compare carbon fleeces produced from pristine PAN and those with admixed exfoliated GnPs to investigate the effect on the resulting material constitution.29 The as-spun PAN-fibers show an average diameter of 220 ± 20 nm (see Figure 2a-e) for all formulations, independent of the GnP loading. The small diameter is beneficial for the subsequent stabilization treatment, avoiding temperature gradients


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across the diameter of the fiber.19,20 We investigate the morphology by SEM and find no obvious influence of GnP content on the appearance of the fibers. During the drying and stabilization step the PAN-fibers fuse together at the points of fiber contact, morphing the material from a fiberon-fiber fleece into a favorable three-dimensionally continuous fiber network (see Figure 2f-j). Generally, the fibers gain in diameter during the stabilization process (250±20 nm). This is a well-established effect of stress relaxation as a result of the rigidification of individual flexible PAN chains into stiff, π-conjugated polymer ribbons.19 The subsequent carbonization is performed either thermally or via laser carbonization. The morphologies of the resulting carbon fiber nonwovens are compared using SEM. In both cases the fiber diameter shrinks on average by about 50 nm, while the fiber mesh maintains its shape. Here, fiber shrinkage is an effect of the desired material loss during the condensation reactions of the polymer ribbons to form graphitic carbon.3 This process entails the expulsion of gaseous CH4, H2, HCN, H2O, CO2, NH3 byproducts.

4,8,19

The thermally (∆) carbonized fibers appear to have a smooth surface (see

Figure 3a and Figure 2k-o). The laser (hν) carbonized fibers have a similar appearance with no clear difference to the thermally carbonized materials and there is no apparent change in appearance with increasing GnP nanofiller content (see Figure 3c and Figure 2p-t). However, when we perform nitrogen adsorption (BET) analysis to determine the specific surface area of the carbonized specimen, we observe much greater specific surface areas as of the laser carbonized as compared to the thermally carbonized carbon fiber mats (see Table 1). The laser carbonized specimen without GnP exhibits a as of almost 500 m2/g with of about 4 nm. We account this high surface area to the fact that the laser induced heating during the laser carbonization process is very fast, leading to an explosive expulsion of the above mentioned gaseous carbonization by-products, which fracture the fiber surface.


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The

thermally

carbonized

fibers

with

compounded GnP exhibit increasing surface areas with rising GnP content. By contrast the GnP addition is not beneficial for increased surface areas in case of the laser carbonized fibers. Addition of even small amounts of GnP hinder the creation of large surface areas and high porosity and the Figure 3: (a) High resolution SEM of a thermally carbonized fiber fleece, exhibiting smooth surfaces compounded fibers exhibit an average as of and (b) its surface after being subjected to KOH activation. (c) laser carbonized fiber fleece 140±12 m2/g. We account this limitation to exhibiting porosity due expulsion of gaseous carbonization by-products before and (d) after the large aspect ratios of GnPs with KOH activation. The scale bars represent 500 nm. graphene

sheets

diameters

on

the

micrometer scale. These large sheets might impede efficient expulsion of the carbonization byproducts, thus lessening the effect of fracturing and impeding higher surface areas. The thermal carbonization process delivers mean pore diameter of 6-8 nm, while pore diameters are smaller in laser carbonized samples with 2-4 nm. The amount of compounded GnP does not seem to have an effect on the pore diameter. To improve the porosity of carbon fibers, several physical and chemical activation methods have been developed.7 We perform a well-established chemical treatment with KOH at elevated temperatures.30–32 The process entails etching of the carbon fiber surface and intercalation of the potassium ions into the graphite galleries, thus exfoliating graphene sheets and opening up micro-pores and small meso-pores.30 This generation of microand meso-pores is clearly evidenced by a strongly roughened surface when comparing high resolution SEM images of thermally carbonized and activated fleeces in Figure 3a and b.


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The surface of the activated fibers is cavernous, rough and appears to be highly meso-porous. To put numbers on the evidently increased porosity we perform BET analysis on the thermally carbonized and KOH activated fibers. The KOH treatment yields a 12-fold increase of the specific surface as to about 140 m2/g and about a 60% decrease of the mean pore diameter to about 5 nm (see Table 1). By contrast, this value is still not as high as the specific surface areas obtained using the laser carbonization process. When exposing the laser-carbonized nonwoven to KOH activation the specific surface area does not increase further but drops to the value of the thermally pyrolyzed and activated carbon fleeces (see Table 1). We account this effect to etching away of the highly porous surface and effectively returning to an overall morphology comparable to the thermally carbonized and activated samples (cf. Figure 3b and d). Table 1 – Surface Area Characterization: Specific surface area as and average pore size for thermally ∆ and laser hν carbonized nanofibers with various amounts of added graphene nanoplatelets. 0.1 wt% GnPs

0 wt% GnP ∆

hν

∆

1.0 wt% GnPs

hν

∆

hν

1.5 wt% GnPs ∆

hν

as 11.6 [m2/g]

492.5 13.1

125

21.5

143.9 48.9

148.3

[nm]

3.9

2.4

7.5

3.6

2.2

8.4

7.9

5.9

KOH activated as 139.7 [m2/g]

143.3

[nm]

1.8

4.9


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We have presented a novel method for carbonization fiber nonwovens prepared from stabilized PAN precursors. The controlled laser treatment allows a precise transport of heat into the fleece reducing thermal gradients. While admixed GnPs increase the IR absorption, they do not have a beneficial effect on the fiber surface morphology. However, GnPs might have a positive effect on the electrical conductivity of the carbon fiber mats. The presented laser carbonization process is fast, facilitating high specific surface areas and small pore sizes by explosive expulsion of gaseous by-products. The laser carbonization process effectively combines conventional carbonization and activation treatments into one process. Laser carbonization is less energy and time consuming compared to state-of-the-art thermal carbonization methods. These promising results will facilitate new application of PAN derived carbon fibers for application as electrodes in batteries, (super)capacitors, filters and reinforcement materials.

Electro-spinning of PAN fleeces as precursors for carbon fiber fleeces The precursor is a co-polymer based on polyacrylonitrile (PAN) with a small amount of itaconic acid comonomer and is provided by Dralon GmbH. The PAN copolymer is dissolved in N,Ndimethylformamide (DMF) to obtain 8 wt% polymer solution. Graphene Nanoplatelets (GnPs) are obtained from xgscience and used as nanofillers. To prevent aggregation of the GnPs during the spin process, a stock solution of sonicated and well-dispersed GnPs in DMF is prepared. 100 mg of GnPs are added to 40 ml DMF and the dispersion is sonicated for two hours using a tip sonicator at 5 W. The dispersion is then centrifuged at 12,000 rpm and the supernatant is separated and again centrifuged until no more GnPs sediment to obtain a stable GnP dispersion. The exact volume fraction of GnPs inside the solution is determined by thermogravimetric


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analysis. Depending on the concentration of GnP the stock solution is added to the polymer solution and different loadings of nanofillers compounded to the PAN-fibers can be prepared. Fiber mats are prepared by electro-spinning of 2.5 ml polymer solution from a plastic syringe with a stainless steel needle, which serves as a nozzle. The flow rate of the syringe pump is adjusted to 0.5 mL/min. and the applied high voltage is 17 kV. Such high voltages are required to overcome the surface tension of the polymer solution to be able to form a jet. The fibers are collected on a grounded aluminum foil target, with a distance of 15 cm from the nozzle. Stabilization of the PAN fiber mats The stabilization of the PAN-fibers is conducted in a Nabertherm furnace. In a first step the fleece is heated to 100 °C at a rate of 5 °C min-1 and held for two hours to remove residual solvent and equilibrate the PAN fibers. During the stabilization step, the sample is heated to 250 °C at the same heating rate and kept this temperature for 30 minutes. Thermal Carbonization of the thermally stabilized PAN fiber fleeces Thermal carbonization is conducted in a tube furnace with N2 supply at a continuous flow rate of 2 L/min. After purging the furnace five times with N2, the fiber fleece is heated with a rate of 5 K/min up to a temperature of 1300 °C, which is held for 30 minutes. Laser Carbonization of the thermally stabilized PAN fiber fleeces The laser carbonization is conducted in a process chamber with an IR transparent window and N2 supply at a flow rate of 4 L/min. The experiments are carried out in an atmosphere with less than 20 ppm oxygen. In all laser experiments a diode laser with wavelengths of 968 nm and 998 nm and a maximum laser power of 800 W was applied. The laser spot has a size of approx. 30 x


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30 mm² and a Top-Hat intensity profile. In order to control the shrinkage of it the fleece is positioned between two masks which have a cutout of approx. 35 x 35 mm². With spacers the distance and therefore the friction of the fleece between the masks can be varied (normally between 0.5 and 1.5 mm). During the experiments the laser spot is stationary and the laser power is controlled to achieve a heating rate of approx. 50 K/s. Scanning Electron Microscopy To observe the morphological appearance of the fibers in their as-spun, stabilized and carbonized states, SEM is performed on a Hitachi FE-SEM S4800 and Hitachi UHR FE-SEM SU9000 for high-resolution analysis. The as-spun and stabilized nonwovens were sputtered with a ~6 nm Pd/Au layer to prevent charging during imaging. The carbonized fleeces are conductive by themselves and do not require metalation, allowing to examine the pristine fiber surface. Carbon Fiber Activation The activated carbon fibers are prepared from the thermally and laser carbonized fibers by chemical activation based on previous research.32 The mass of KOH, which correspond to the threefold amount of fiber mass, which has to be activated, is dissolved in 40 ml of deionized (DI) water. The fibers are soaked in the prepared solution and treated in the oven for 12 hours at 120 °C under inert gas atmosphere. Afterwards the nonwoven is heated up in a crucible with a rate of 7 °C min-1 up to 800 °C under N2 and kept at this temperature for 30 minutes. After the heat treatment the sample is washed with DI water until the wash solution has a neutral pH. Finally the fiber fleece is dried in a vacuum oven at 110 °C. Specific Surface Area characterization


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To determine the specific surface area a Micromeritics ASAP 2020 BET analysis is used. The relative pressure interval was maintained between 0.05–0.30, with nitrogen as adsorbent. All samples are prepared by degassing them at 90 °C for 12 hours and subsequent heating for 6 hours at 200 °C before analysis. The micropore size distributions were calculated by applying density functional theory (DFT) to the resulting N2 isotherms.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

ACKNOWLEDGMENTS We thank the AiF for financial support under the project number 17973 BG. We thank Sibel Ciftci for the help with SEM imaging, Nadine Jansen for support with fiber-spinning experiments (both at DWI) and Dr. Andreas Wego at Dralon GmbH for supplying PANcopolymers. This work was performed in part at the Center for Chemical Polymer Technology CPT, which was supported by the EU and the federal state of North Rhine-Westphalia (grant EFRE 30 00 883 02). REFERENCES (1)

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Laser Carbonization of PAN-Nanofiber Mats with ... - ACS Publications

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