Activated Carbon Nanofiber Nonwovens: Improving Strength and

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Materials and Interfaces

Activated Carbon Nanofiber Nonwovens: Improving Strength and Surface Area by Tuning Fabrication Procedure Basma I. Waisi, Seetha S Manickam, Nieck Edwin Benes, Arian Nijmeijer, and Jeffrey R. McCutcheon Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05612 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on February 7, 2019

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Industrial & Engineering Chemistry Research

Activated Carbon Nanofiber Nonwovens: Improving Strength and Surface Area by Tuning Fabrication Procedure Basma I. Waisi1,2,3, Seetha S. Manickam1, Nieck E. Benes2, Arian Nijmeijer2 and Jeffrey R. McCutcheon*,1

*Corresponding Author Tel.: +1 (860) 486-4601. E-mail: [email protected]

1

Department of Chemical &Biomolecular Engineering. Center for Environmental Sciences and Engineering, University of Connecticut, Storrs, CT, United States 2

Inorganic membranes, Department of Science and Technology. MESA+ Institute for Nanotechnology, University of Twente, Enschede, The Netherlands.

3

Department of Chemical Engineering, College of Engineering, University of Baghdad, Baghdad, Iraq

Keywords: electrospinning, carbon nanofiber, steam activation, mechanical strength, surface area.

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Electrospun based activated carbon nanofiber nonwovens (ACNFN) are interesting candidate materials for adsorption processes due to their high surface area and low flowthrough resistance. However, the mechanical properties of these materials must be sufficient to withstand the conditions for use and to prevent breakage and fiber shedding. Improvements in the mechanical properties of the ACNFNs should not be accompanied by deterioration of other beneficial properties, however. In this research, improving the mechanical properties of ACNFN based on 14 wt. % PAN/DMF was done by tuning the fabrication conditions; in the carbonization step was at 600 oC for 2 h and the steam activation step was at 750 oC using 60 g steam for 1 h. We demonstrated the capability to generate ACNFN with high accessible surface area reaches 520 m2/g and acceptable mechanical strength (The break strength is 0.9 MPa and 75 MPa Young’s modulus) for improved handling and using in different applications.


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1.

Introduction Activated carbon nanofiber nonwovens have been attracting increased attention due to

their unique physical properties, such as a high electrical conductivity, a high specific surface area, and biocompatibility 1. Due to the nanosize of the fibers and existing micropores on the surface of ACNFs, they showed a high efficiency in removing volatile organic compounds (VOCs) by adsorption 2. Also, the well-built pore structure of ACNFs creates excellent electrochemical properties; as a result, they were used as an electrochemical capacitor 3. In addition, due to the controlled pore structure and high bioavailable surface area, ACNFs could be used as an electrode in a microbial fuel cell

4

and it is considered a promising material for high-performance

rechargeable batteries such as lithium-ion batteries because of the high specific surface area 5. Fabrication of activated carbon materials requires a precursor material that is carbonized and activated through sequential thermal and chemical treatments. To make an activated carbon nanofiber nonwoven (ACNFN), the precursor must be provided in the desired architecture (a nanofiber nonwoven). Commonly, nonwoven nanofibers are fabricated via the electrospinning technique which has been developed for the manufacturing of fibrous membranes by extruding the polymer using an electrically forced fluid jet and a grounded collector drum 6. The electrospun fiber membranes can be fabricated as nanofibers or as microfibers resulting unique features such as high surface-to-volume ratio, intrinsically high porosity, fully interconnected pore structures, low hydraulic resistance, and ease of scalable synthesis. Thus, the electrospun nanofiber membranes are increasingly considered to be good candidates for the applications that need high permeability and low energy cost such a water treatment 7,8. This fabricated precursor will then undergo thermal treatments to carbonize and



activate the fibers. The properties of the precursor fibers, as well as the carbonation and activation conditions, determine the final properties of the ACNFN 9. Polyacrylonitrile (PAN) serves as a popular carbon precursor material due to its thermal stability and high carbon yield 10. Carbonizing electrospun PAN nanofiber requires a moderate heating in an air atmosphere followed by high-temperature carbonization in an inert gas. The result is a carbon nanofiber nonwoven (CNFN). This material can be further treated with steam or other chemical exposure to produce an activated carbon nanofiber nonwoven (ACNFN) 11– 13

. Fundamental changes in both chemical composition and physical properties occur during

each of these fabrication steps. For instance, stabilization, which occurs in an oxidizing atmosphere at temperatures of 180 - 300 °C, causes a number of chemical reactions including cyclization, dehydrogenation, aromatization, oxidation, and crosslinking formation among linear polymer chains 14–16. These various reactions can result in mechanical weakening of the fiber

15

. During the early stages of carbonization, intermolecular dehydrogenation and

crosslinking reactions occur in the oxidized PAN. Then by dehydration and denitrogenation, the cyclized structure is created

14

. A variety of gases (e.g., H2O, N2, HCN, and others) are

evolved, and the carbon content increases to higher than 85 wt.% leading to fiber diameter reduction 17,18. The reduction in fiber size can weaken individual fibers and detach fibers from each other at their junction points. CNFN is activated to produce meso- and microporosity, which yields high specific surface area and oxygen groups content 19. Carbon activation can be done in a number of ways, but steam

20

and carbon dioxide 21 are common. Steam is often used because of its low cost

and low environmental impact 22. At a temperature higher than 700 oC, the steam reacts with


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carbon atoms of CNFN, thereby eliminating heteroatoms and releasing CO and H2 gasses. Removal of carbon atoms creates small and large defects on the CNFN surface that result in an increase in the specific surface area 14,23. Although the unique characteristics of PAN-based ACNFN such as the high accessible surface area and high permeability, it suffers from brittleness and rigidity due to the significant weight loss and shrinkage during the various thermal treatment steps. The poor mechanical strength limits ACNFN to be used in various applications which expose the material to viscous flow, such as microbial fuel cells

4,24

and water filtration

20,25

. Mass loss and shrinkage are

common during carbonization and activation and each step can negatively impact the mechanical strength of an ACNFN 12,20. Understanding how steps in the fabrication approach impact the mechanical properties is the first step to fabricate a material with the necessary strength for certain applications 26. This work seeks to elucidate how various processing steps of ACNFN fabrication impact the mechanical properties. We examine how the polymer precursor concentration, the carbonization temperature and time, the activation time, and the steam quantity impact the fundamental characteristics of an ACNFN, such as the fiber size, surface morphology, mechanical strength, flexibility, and specific surface area. 2.

Materials and Methods

2.1 Electrospinning of Polymer Precursors Polyacrylonitrile (PAN) from Scientific Polymer Products Inc. (MW avg. 150,000 g/mol) and Dimethylformamide (DMF) from Acros Organics were used to prepare 10, 14, and 16 wt. % PAN in DMF solution by constant stirring at 60 °C for 2 h. A high-pressure syringe pump (KD Scientific) was used to dispense the charged solution at a constant rate of 1 ml/h through a 20 gauge blunt end needle. The fiber was spun onto a collector (grounded) rotating at 70



rpm. The applied voltage was 27, 24 and 21 kV (for 10, 14 and 16 wt. % PAN respectively) and the tip to collector distance was held constant at 18 cm. The precursor mats were all spun at room temperature under a relative humidity of 10 –20%. 2.2 Fabrication of CNFN and ACNFN To make CNFN, the PAN nanofiber mats were stabilized in air at 280 °C for 1 h in a muffle furnace (Carbolite) using a ramp rate of 1 °C/min. The stabilized nanofibers were then carbonized in a tube furnace (Lindberg Blue M, Thermo Scientific) in an inert nitrogen atmosphere with a ramp rate of 3 °C/min. The 14 wt. % PAN fibers were carbonized for 1 h at three different temperatures (600, 750, and 850 °C). The 600 °C carbonization was conducted for three lengths of time (1, 2, and 3 h). To make an ACNFN from this CNFN, it was activated in the same furnace using steam in an inert nitrogen atmosphere at 750 °C. The activation process carried at different times (30 and 60 min) using different steam flow rates (30 and 60 g/h). Figure 1 shows a scheme of the process used to produce CNFN and ACNFN from PAN showing the optimum conditions to fabricate the base samples.


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Figure 1 A scheme of the fabrication process of carbon and activated carbon nanofiber nonwovens 2.3 Nonwoven Characterization 2.3.1 Fiber Morphology and Size CNFN and ACNFN samples were sputter coated with gold and imaged using an FE-SEM from FEI Company (Hillsboro, OR) to observe the surface structure/morphology. To obtain fiber size distributions and the average fiber diameter, thirty individual fiber sizes were measured using Image J software (National Institutes of Health, USA). 2.3.2 Mechanical Properties The tensile strength and flexibility of the samples were used as parameters to quantify the mechanical strength of the carbonized and activated nonwovens. A Dynamic Mechanical Analyzer (DMA) from TA Instruments was used to derive the tensile strength and Young’s modulus. Sample sizes of 3 cm x 6 mm were used for the tests which were all performed at 25



°

C and ambient humidity. All results presented were the average of three individual tests from

different samples. 2.3.3 Surface Area Measurements Specific surface areas were measured using an ASAP 2020 Physisorption Analyzer (Micromeritics Instrument Corporation). The samples were degassed at 300 °C for 2 h and then analyzed for nitrogen sorption at 77 K. Adsorption isotherms were used to calculate the specific surface area through application of the BET model. All results presented were the average of three individual tests from different samples. 2.3.4 Surface Chemistry (FT-IR) The surface chemistry of the carbonized samples analyzing was done using Fourier transform-infrared (FT-IR), Nicolet iS10 FT-IR (Thermo Scientific). Infrared spectra were recorded in the wave number range of 500 – 4000 cm-1 with a resolution of ±4 cm-1 and a total of 16 scans per sample. The attenuated total reflection mode with a diamond crystal was used to scan the samples. 3.

Results and Discussion

3.1 Fiber Morphology Figure 2 (a-c) show the SEM images and the corresponding fiber size distributions of PAN-based CNFN carbonized at 600 °C for 2 h as a function of PAN concentration (10, 14, and 16 wt. %). The average fiber diameter increased from 160 nm at 10 wt. % PAN to 760 nm at 16 wt. % PAN. The increases in fiber diameter are due primarily to the higher viscosity of the spinning solution, which reduces the drawdown of the fiber prior to solidification 27. This result has been well documented in other studies on electrospinning 28–30.


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Figure 2 d shows SEM images and the corresponding fiber size distribution of 14 wt. % PAN-based CNFN after an activation step at 750 °C for 1 h using 60 g of steam. The activation decreases the average fiber size from 350 nm (CNFN) to 250 nm (ACNFN). This is consistent with previous work 20 and is attributed to the weight loss and compacting of the fibers 25.

Figure 2 - SEM images and fiber size distributions of (a) 10 wt. % PAN-based CNFN, (b) 14 wt. % PAN-based CNFN, (c) 16 wt. % PAN-based CNFN, and (d) 14 wt. % PAN-based ACNFN.



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3.2 Surface Chemistry A comparison of FT-IR spectra of 14 wt. % PAN-based stabilized, carbonized (at 600 oC for 2 h), and activated (at 750 oC for 1h using 60 g of steam) nanofiber nonwoven samples is shown in Figure 3 a. The effect of carbonization temperature and time on the surface chemistry of 14 wt. % PAN-based CNFN is shown in Fig 3 b and c, respectively. In Figure 3 a, the main peaks groups in the FT-IR curve of the stabilized nanofiber are observed at 1270 cm-1, 1350 cm-1, and 1450 cm-1 for the aliphatic CH groups, at 1590 cm-1 for a mixture of C꞊N, C꞊C, and N-H groups, and at 1732 cm-1 for the C꞊O group

31

. During

stabilization (at 280 oC), various reactions occur, such as cyclization, dehydrogenation, and oxidation. These reactions produce C꞊N, C꞊C, and C꞊O groups, respectively 20,31. Also, a small peak can be noticed at 2242 cm-1, which is attributed to C≡N group in the stabilized nanofibers. The intensity of all these groups in the curve of the stabilized sample was decreased significantly by doing the carbonization step (at 600 oC) due to the cyclization and crosslinking reactions 32. Exposing the CNFN to a higher temperature (at 750 oC) during steam activation results in shifting the peak from 1350 -1450 cm-1 (aliphatic CH groups peaks) to 1160 cm-1 (O-H bond in the phenolic group) after steam activation. The peak at 1580 cm-1 becomes more pronounced due to an increase in the number of carbonyl groups existing during steam activation 20,31. Figure 3 b shows the effect of carbonization temperature on CNFN surface chemistry. A reduction of the intensity of the C꞊N and C꞊O peaks can be observed with increasing the carbonization temperature. At a carbonization temperature between 400 °C and 600 °C, intermolecular dehydrogenation occurs linking the cyclized sequences. The remaining linear segments either become cyclized or suffer chain scission with the evolution of gaseous


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materials. At a temperature greater than 600 °C, denitrogenation reactions take place, resulting in the removal of structural defects and non-carbon elements and forming the sheet-like structures

14

.

Figure 3 c shows the effect of carbonization time on the two main permanent peaks height differences. A very small peak of C≡N can be noticed at 2242 cm-1, which indicates that 1 h carbonization at 600 oC is not enough to convert all C≡N to C꞊N through cyclization and crosslinking. This peak no longer exists when the carbonization continues for a longer time (2 and 3 h).



Figure 3- FT-IR analysis of 14 wt. % PAN/DMF based nonwoven (a) Stabilized (280 oC for 1 h), CNFN (600 oC for 2 h), and ACNFN (750 oC for 1 h using 60 g of steam) (b) CNFN at various carbonization temperatures for 1 h, and (c) CNFN at 600 °C for various carbonization times

3.3 Specific Surface Area Figure 4 a shows the surface areas of CNFN fabricated from different concentrations of PAN precursors and carbonized at 600 oC for 2 h. It is found that an increase in polymer concentration correlates to a decrease in surface area, as a result of an increase in fiber size (


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as shown in Figure 2) 33. The specific surface area of CNFN decreases from 45 to 5 m2/g upon increasing the polymer concentration from 10 to 16 wt. % PAN. Figure 4 b shows the surface areas of 14 wt. % PAN-based ACNFN using a steam at 750 o

C for various activation times and steam delivery rates. Steam exposure creates micro- and

mesoporosity in the carbon nanofiber mat due to the reactions of water molecules with carbon atoms. These reactions are strongly affected by the activation temperature and time

23

. The

specific surface area of the carbon nonwoven mat indeed substantially increases during steam activation: 60 g/h of steam for 30 min results in an increase of the specific surface area from 19 m2/g (CNFN) to 380 m2/g (of ACNFN). The specific surface area is further increased to 520 m2/g by doubling the activation time as a result of increasing the created defects and pores This is consistent with previous work 23,34.

Figure 4 - Specific surface areas of (a) CNFN (at 600 oC for 2 h) from different PAN concentrations and (b) 14 wt. % PAN-based ACNFN fabricated using various steam mass and activation time.

3.4 Mechanical Strength Figure 5 a shows the tensile strength and Young’s modulus (both averaged from three individual samples) as a function of polymer concentration. These results show that the 16



wt. % PAN-based CNFN has the highest tensile strength and Young's modulus, while the 10 wt. % PAN-based CNFN has the lowest tensile strength and Young's modulus. Increasing polymer concentration increases the polymeric nanofibers size and strengthens the nonwoven (Young’s modulus and tensile strength)

35

. This is attributed to the fact that at higher

concentration the number of polymer chains per fiber which increases chain interaction and entanglement 36. Larger fibers also contain a more residual solvent in their core than smaller fibers. The evaporation of the solvent facilitates the bonding of fibers, which improves the strength of the mat. The effects of the carbonization step on the mechanical properties are shown in Figure 5 b. The strength and Young's modulus decrease with increasing carbonization temperature. For the strength, both the temperature and time appear to have a threshold value. Carbonization at 600 oC for 1 and 2 hours results in a similar mechanical strength, whereas treatment at the same temperature for 3 hours results in a pronounced decrease in mechanical strength. Keeping the treatment time fixed at 1 hour, and increasing the temperature from 600 oC to 750 o

C has a pronounced effect on the mechanical strength, but a further increase in temperature

to 850 oC does not substantially further reduce the strength. The mechanical properties of the ACNFN are shown in Figure 5 c. The mechanical strength decreases substantially upon activation from CNFN to ACNFN for all activation conditions. This decrease in strength is attributed to an increase in meso- and micro-porosity during activation

37

which can weaken individual fibers and the junction between fibers.

Increasing this porosity is, however, essential to having the high specific surface areas shown in Figure 4 b. The variation in the strength of the fibers obtained after activation is not strongly dependent on the exact activation conditions in the ranges of independent variables (steam


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amount, activation time) tested here. A more pronounced result is noted with changing Young’s modulus. The decrease of in this modulus is approximately twice as large when the activation time is doubled, as compared to when the amount of steam is doubled. A combined increase in activation time and amount of steam results in a further decrease of Young’s modulus. A decrease in the modulus without a substantial decrease in tensile strength could be beneficial in applications involving the need for flexible materials.



Figure 5 - Mechanical properties (Tensile strength and Young’s modulus) of (a) CNFN based on different polymer concentration, (b) 14 wt. % PAN/DMF based CNFN at different carbonization temperatures and time, and (c) 14 wt. % PAN/DMF based ACNFN activated at different activation time using different amounts of steam.

4.

Conclusions In this work, we have demonstrated that ACNFN with improved mechanical strength and

specific surface area reaches to 520 m2/g can be fabricated by adjusting the conditions of fabricating the precursor (14 wt. % PAN/DMF) and subsequent thermal treatments (carbonization at 600 oC for 2 h, and activation using 60 g of steam for 60 min). Our results show that changing the fabrication conditions can allow for predictable adjustment of critical properties of the material, including specific surface area and strength. This tunability in the fabrication approach provides scientists with the tools needed to make these activated carbon nanofiber mats for a variety of applications, ranging from water treatment to electrodes.

Acknowledgments The authors acknowledge financial support from the Higher Committee for Education Development (HCED) in Iraq (Grant Number is 1000324). The authors also acknowledge the University of Connecticut Center for Clean Energy Engineering for the use of the nitrogen adsorption analyzer. References (1)

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Activated Carbon Nanofiber Nonwovens: Improving Strength and

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