Nitrogen-Enriched Porous Polyacrylonitrile-Based Carbon Fibers for

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Materials and Interfaces

Nitrogen-enriched porous polyacrylonitrilebased carbon fibers for CO2 capture Li Li, Xue-Fei Wang, Jun-Jun Zhong, Xin Qian, Shu-Lin Song, Yong-Gang Zhang, and De-Hong Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01836 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on August 4, 2018

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Nitrogen-enriched porous polyacrylonitrile-based carbon fibers for CO2 capture Li Li1,2, Xue-Fei Wang1,*, Jun-Jun Zhong1, Xin Qian1, Shu-Lin Song1, Yong-Gang Zhang1,*, De-Hong Li1 1 National Engineering Laboratory for Carbon Fiber Technology, Ningbo Institute of Material Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, China 2 Nano Science and Technology Institute, University of Science and Technology of China, Suzhou 215123, China

Correspondence to: FAX: +86 574 87911382. E-mail address: [email protected] (Xue-Fei Wang), [email protected] (Yong-Gang Zhang)

ABSTRACT: Utilizing porous polyacrylonitrile (PAN) fibers as the precursors, porous carbon fibers were obtained by crosslinking of precursor fibers with hydrazine hydrate and subsequent heat treatment. A nitrogen content more than 14 wt% was achieved in the carbon fibers. The porous carbon fiber that was prepared at low concentration of hydrazine hydrate (5 wt%) showed an optimal BET surface area of 277.4 m2/g with micro-/meso-/macropores. The CO2 adsorbed amount of this porous carbon fiber was 101 mg/g at 25 oC under atmospheric pressure, which was 2.1 times that of the fiber without crosslinking with hydrazine hydrate. In the simulated flue gas environment (10% CO2/90% N2), the adsorption capacity of the above mentioned porous fiber was 32 mg/g at 25 oC, which was 1.4 times that of the fiber without crosslinking. These CO2 adsorption results demonstrated that nitrogen functionalities and porous structure of the porous carbon fiber played an equivalent important role in the


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adsorption of CO2. The porous carbon fiber also owned an excellent CO2 reusability, and 96% of the adsorption capacity was maintained after 20 cycles of CO2 adsorption and desorption. The porous carbon fibers enriched with nitrogen could thus be a potential material for CO2 capture.



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1. Introduction The rapid cumulation of CO2 in atmosphere is considered to be the major contributor to environmental crisis, such as global warming and ocean acidification.1 The atmospheric CO2 level has increased by 40%, from 280 ppm before the industrial revolution to the record high level of 400 ppm in May 2013.2 In order to reduce the amount of CO2 in the atmosphere,

post-combustion CO2 capture technologies are massively developed during the latest twenty years, including chemical absorption, adsorption, membrane diffusion, cryogenic.3,

4

Alternative to the other methods, adsorption is an economic and promising technique, which has attracted increasing interest.5,

6

Porous solid sorbents such as zeolites, metal organic

frameworks, silica, porous carbons, or porous organic polymers have been utilized as potential candidates for CO2 adsorption.7-10 Among the various solid sorbents, porous carbon materials are advantageous on account of vast precursors, easy preparation, low cost, high surface area and pore volume, controllable surface functionalities, and high thermal and chemical stability.11-15 In particular, porous carbon fibers have higher mechanical properties, and also are knittable and flexible to construct felt, fabric, etc.16-18 The fibrous shape of the porous carbon fibers make these fibers easy to be handled over granular and powdered porous carbons. In this regard, porous carbon fibers are explored to capture CO2. Porous carbon fibers can be obtained by chemical activation of commercially available activated carbon fibers (ACFs).19 With the modulation of KOH/ACF weight ratio, the total pore volume and specific surface area were both improved. Rather than total or micropore volume and specific surface area, ultra-micropores ranging from 0.5 to 0.7 nm were proposed


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to be closely in charge of the CO2 adsorption capacity. Activated carbon fibers with hierarchical pores were prepared from polyacrylonitrile fibers through pre-oxidation at 500 o

C and chemical activation.20 Higher CO2 capacity of the fiber was mainly ascribed to the

large amount of nitrogen-containing groups and consequent basic sites. The hierarchical pore structure of the fiber provided channels for CO2 diffusion, which facilitated the adsorption of CO2.21 Basic groups also played an important role in the adsorption of acidic gas, which was dominated by the amount of nitrogen present.22 Basic groups were brought into ACFs by reaction of ammonia with ACFs at high temperatures or by directly activating precursor fibers with ammonia. 22 For porous carbon materials, whether the pore diameter or the nitrogen playing the dominant role in the adsorption of CO2 is a controversial topic for a long time. Ultra-micropores are considered to be playing a leading role in the high CO2 adsorption.23-25 Comparatively, corporation of nitrogen atoms also exert a positive effect on the adsorption of CO2. Porous carbon materials always have plenty of micropores. However, the nitrogen contents are usually below ~5%, which may be insufficient to define the influence of nitrogen on the adsorption of CO2. Polyacrylonitrile has a nitrogen content as high as ~26% in weight. It is convenient for preparation of porous carbon fibers with high nitrogen content from PAN fiber by activation or template methods. Carbonized or partial carbonized fibers were directly treated by chemical or physical activation, and amounts of micropores were obtained on the fiber surface.26, 27 Polymer template technique is another way to design fine carbon materials, consisting of spinning, stabilization, and carbonization of a blend of polymers with and without carbon



residue.28 Soft template like hydrophilic agent can also be extracted by water after the preparation of precursor nanofibers, and carbon fibers with nanoporous structures were obtained.29 Hard templates such as meso-size particles were removed after carbonization, and mesoporous carbon fibers were prepared.30 Besides, meso-/macropores can be introduced in the precursor fiber by phase separation, and the resulting carbon fiber with 3D connected porous structures was obtained without template removal.31 The porous structure of the fiber can also be modulated by hydrophilic pore-forming agents such as polyethylene oxide and polyvinylpyrrolidone (PVP). Jong et al. used PVP as the pore-foaming agent to modify PAN film and the porous PAN membrane was obtained by dry-jet wet-spinning.32 The addition of PVP would form micropores in the casting film, and the pore size of the membrane could be adjusted by the molecular weight of the PVP.33 In the present work, PAN fiber with high nitrogen content was chosen as a precursor fiber. Porous PAN precursor was firstly made with the assistant of PVP in the phase separation, which had a higher specific surface area than that of the fiber without PVP (Figure S1). In order to reduce the internal stress of the fibers in the heat treatment process, the fibers were crosslinked by hydrazine hydrate before pre-oxidation. The porous carbon fibers with numerous nitrogen functional groups were thus obtained when the precursor fibers were treated subsequently with crosslinking, pre-oxidation, and carbonization. Nitrogen contents of the carbon fibers were higher than 14 wt%, which could be beneficial for the adsorption of carbon dioxide. The contributions of the nitrogen functionalities, as well as that of the porous structure to the CO2 capture at pure CO2 and stimulated flue gas were discussed in detail.


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Finally, reusability performance of porous carbon fiber was also monitored. This work shows insights in the effect of nitrogen functionalities and textural properties on the adsorption of CO2, which is believed to be valuable for the designing of carbon fibers in the field of CO2 capture. 2. Experimental 2.1. Materials. Acrylonitrile (AN) was supplied by Sinopec Shanghai Petrochemical Co., Ltd., China and further distilled before use to remove polymerization inhibitor. Itaconic acid (IA) was obtained from Acros Organics and used as received. 2,2’-Azobisisobutyronitrile (AIBN) was obtained from Shanghai No.4 Reagent & H.V. Chemical Co., Ltd., China and used as an initiator after recrystallization in ethanol. Dimethyl sulfoxide (DMSO) and hydrazine hydrate (HH) with a purity of 85% were supplied by Sinapharm Chemical Reagent Co., Ltd., China and used as received. Polyvinylpyrrolidone (PVP, Mw 1,300 kDa) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., China. The control sample of the nonporous PAN fiber (NF) was supplied by Ningbo Institute of Material Technology & Engineering, Chinese Academy of Sciences. 2.2. Preparation of Porous Carbon Fibers. AN and IA (98/2 wt/wt) was copolymerized in DMSO at 60 oC for 22 hours under nitrogen atmosphere (60 ml/min) by using AIBN as an initiator. AN/IA copolymer was obtained from copolymer solution after precipitation in water, rinsing, and drying at 60 oC in vacuum. PAN spinning solution was prepared by dissolving 20 wt% copolymer in DMSO, and deaerated at 60 oC for overnight. PAN/PVP spinning solution was prepared by adding PVP (20 wt% to the copolymer) into the PAN spinning solution. The dope solution was spun through spinneret of 50 holes (100 µm in hole diameter) in DMSO/H2O with a DMSO concentration of 20% at 60oC at a pressure of 0.3 MPa, as schematically shown



in Figure 1. The fiber was then washed with deionized water, dried at 60 oC in vacuum, and the obtained fiber was donated as “PF”.

Figure 1. Schematic diagram of wet spinning setup. The precursor fibers were stabilized in an oven in air under temperature gradients of 200 oC, 220 oC, 250 oC, 260 oC, and 275 oC for 10 min, 20 min, 40 min, 10 min and 10 min, respectively. The stabilized fibers were carbonized to 750 oC with a heating rate of 10 oC/min from room temperature to 750 oC and stayed at 750 oC for one hour in a ceramic tube under nitrogen atmosphere with a flow rate of 1.5 L/min, and PCF was obtained. NCF was prepared by treating NF under the same subsequently thermal treatments as that of PF. The PF fiber was also treated by HH by soaking the fiber in 80 oC HH solution for 30 hours. The concentration of HH solution was 5% and 10%, respectively. The HH-treated fiber was taken out, washed five times with deionized water, dried at 60 oC. The fiber then suffered the same pre-oxidation and carbonization under the same conditions, and the samples were named as PCF-HX (X=5 and 10), and X represented the percentages of concentration of hydrazine hydrate solution for fiber crosslinking. 2.3. Charaterization. Mass contents of carbon, nitrogen, and hydrogen of the fibers were monitored by Elmentar Vario EL III. The mass content of oxygen was calculated by difference.


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X-ray photoelectron spectroscopy (XPS) of carbon fiber surfaces was carried out using an AXIS ULTRA DLD (Kratos Inc., Japan) spectrometer equiped with an achromatic Mg Kα X-ray source. XPS subpeaks were deconvolved by employing a non-linear least squares curve fitting program with a Gaussian/Lorentzian mix function and background subtraction. Hitachi S4800 field emission scanning electron microscope (FE-SEM) was used to characterize the morphologies of surfaces and cross section of the fibers. The samples were sputter-coated with platinum prior to observation. XRD patterns of the fibers were obtained using a Bruker X-ray diffractometer (D8 Advance) with Cu Kα radiation (λ= 0.15406 nm). Transmission electron microscopy (TEM) images were characterized by FEI Tecnai F20, a 200 kV field emission TEM. Micromeritics ASAP2020-HD88 instrument was used to monitor nitrogen adsorption/desorption isotherms of fibers at -196oC. The multipoint Brunauer-Emmett-Teller (BET) method was employed to calculate the surface area (SBET) in partial pressure (p/p0) range of 0.05-0.2. Pore volume was obtained from the amount of liquid nitrogen adsorbed at p/p0 of 0.99. Based on the nonlocal density functional theory (NLDFT) equilibrium model as well as Barret-Joyner-Halenda (BJH) model, pore size distribution was calculated from the desorption branch of nitrogen isotherm. 2.4. CO2 Capture Performance. CO2 adsorption isotherms were performed by static volumetric technique using an instrument from Micromeritics (ASAP 2020M). Prior to each measurement, sample was degassed at 200 oC for 8 hours. The CO2 capture capacities of the fiber samples were measured at 25 oC. The mass of each sample used is around 0.10 g. CO2 adsorption-desorption experiments were carried out by using a NETZSCH STA 449 F3 thermal analyzer under a stream of 10% CO2 balanced with N2. 15 mg (±1 mg) of the fiber



sample was loaded in a ceramic pan, which was then degassed under a N2 stream at 110 oC for 120 min before the adsorption analysis. The temperature was then decreased to 25 oC with a rate of 10 oC/min and maintained for 60 min. A stream of CO2 with a partial pressure of 10% (50 L/min) was introduced into the chamber and maintained for 1 hours to allow the CO2 adsorption of the sample. The desorption process started when the gas was switched to N2 gas. The cycling adsorption and desorption on PCF-H5 was also monitored by the thermal analyzer. The sample was evacuated and stabilized at the same condition as above mentioned. The adsorption of CO2 was performed at 25 oC in a 10% CO2/90% N2 atmosphere for 15 min, then the gas was transfered to N2 for another 15 min to remove the adsorbed CO2, and the adsorption/desorption process was repeated for 20 runs. 3. Results and discussion 3.1 Chemical Properties. As shown in Table 1, each elemental content of nitrogen, carbon and hydrogen are comparable in NCF and PCF. A high nitrogen content more than 16% is achieved in the two fibers, indicating that most of the nitrogen is retained in the fiber after pre-oxidation and carbonization. When the precursor fibers are treated with hydrazine hydrate (HH) before pre-oxidation, the nitrogen contents of the carbon fibers decrease slightly. The nitrogen content decreases as the concentration of HH increases, but PCF-H10 still has a nitrogen content of 14.17%. Hydrazine hydrate has been utilized to crosslink PAN molecules. The crosslinking extent of the precursor fiber is elevated with HH concentration (Figure S2). Reactions take place between the nitrile groups, and new bonds are formed.34, 35 Nitrogen in the bonds between PAN and hydrazine is less stable than that in ladder polymers, indicating by a large loss in weight


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observed in HH treated fibers (Figure S3). Similar weight loss was also observed in hydrazine modified PAN.34 Hence, it is easier to eliminate the former kind of nitrogen than the latter at high temperature treatment. When the concentration of hydrazine hydrate increases, more nitrile groups on the polymer chains react with hydrazine, thus more nitrogen is eliminated at the carbonization process. Hence, with the increasing of the concentration of HH, the nitrogen content of the carbon fibers decreases. Besides, a high oxygen content in the range of 7.65~10.67 wt% exists in all of the fibers (Table 1). However, the presence of oxygen-containing functional groups is unfavorable for CO2 adsorption.36 Carbonization is a complicated process due to the formation of graphite-like structure with the removal of nitrogen and oxygen.37 When the carbonization temperature is 750 oC, excess oxygen can’t be entirely eliminated at this temperature.38 Table 1 Elemental contents of carbon fibers. N

C

H

Oa

wt%

wt%

wt%

wt%

NCF

16.86

73.16

1.38

8.60

PCF

16.74

73.35

1.52

8.39

PCF-H5

16.48

74.34

1.53

7.65

PCF-H10

14.17

73.28

1.88

10.67

Sample

a

oxygen content was obtained by difference. Nitrogen-containing groups on the surface of four fiber samples NCF, PCF, PCF-H5, and

PCF-H10 were probed by XPS. N 1s spectra are resolved into individual component peaks by deconvolution strategy, as shown in Figure 2. Three types of nitrogen-containing groups are



identified, pyridinic-N (N-6) at 398.0-398.2 eV, pyrrolic-/pyridonic-N (N-5) at 399.7-399.9 eV, and quaternary-N (N-Q) at 400.9-401.0 eV. Amine groups have been reported in other carbon materials obtained at low temperature, which might not exist in these carbon fibers.39 The quantitative concentrations of each peak are shown in Table 2. Nitrogen amount of the form N-5 for PCF is 39.8%, higher than that of NCF. With the crosslinking of PF with HH, N-5 amount is elevated, and the value is as more as 52.5% for PCF-H10. The evolution of nitrogen functionalities in PAN char highly depended on the temperature of heat treatment.39 The difference between the nitrogen functionalities of the four carbon fibers is mainly ascribed to the composition of the precursor fiber. It has been reported that, in general, N-5 usually makes a more significant contribution to CO2 capture than N-6 and N-Q.25, 40, 41 Considering that the nitrogen contents of the carbon fibers are higher than 14%, there is a considerably high amount of N-5 in the fibers, which is beneficial for CO2 capture.

Intensity (a.u.)

b

Intensity (a.u.)

a

N-Q N-6

394

396

398

N-5

400

402

404

N-Q N-6

394

396

Binding energy (eV)

398

N-5

400

402

404

Binding energy (eV)

c

Intensity (a.u.)

d

Intensity (a.u.)

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

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N-Q N-6

394

396

398

N-5

400

402

404

N-Q N-6 N-5

394

396

Binding energy (eV)

398

400

402

404

Binding energy (eV)

Figure 2. XPS spectra of the carbon fibers (a) NCF, (b) PCF, (c) PCF-H5, and (d) PCF-H10.


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Table 2 Concentration of nitrogen functionalities of carbon fibers. The relative content of nitrogen-containing groups (%) Samples pyridinic-N (N-6)

pyrrolic-/pyridonic-N (N-5)

quaternary-N (N-Q)

NCF

45.7

34.6

19.7

PCF

43.8

39.8

16.5

PCF-H5

41.7

45.1

13.3

PCF-H10

32.2

52.5

15.3

3.2 Fiber Morphology and phase structure. Figure 3(a-h) shows the SEM images of surfaces of NF, PF, PF-H5, and PF-H10, as well as those of corresponding carbon fibers. There are some shallow grooves on the surface of NF [Figure 3(a)]. Many meso-/macro-pores exist on the surfaces of PF [Figure 3(b)], resulting from phase separation of the PAN in wet spinning by using PVP as a pore-forming agent. Most of the meso-/macro-pores can be observed on the surface of HH treated fibers [Figure 3(c,d)]. Moreover, many pores remain on the fiber surface of PF-H5 and PF-H10 after stabilization, as shown in Figure S4, indicating that the pores on the surface of precursor fibers can be considerably preserved by intermolecular crosslinking introduced by HH. There are not any pores on the surfaces of pre-oxidized fibers of NF and PF (Figure S4), neither on the surfaces of NCF and PCF [Figure 3(e,f)]. However, the pores on the surface of PF-H5 and PF-H10 can’t be inherited by carbon fibers (PCF-H5 and PCF-H10), as shown in Figure 3(g,h), which may have a relationship with crosslinking degree, stabilization extent, carbonization temperature, etc. Cross sections of NCF and PCF-H10 were also examined by SEM. A smooth plane is



observed on the cross section of NCF [Figure 3(i)], while mesopores are easily observed on the cross section of PCF-H10 [Figure 3(j)], which are considered to be inherited from the precursor fibers. High-resolution TEM image of PCF-H5 in Figure 3(k) shows a high amount of mesopores and a low amount of micropores, indicating that HH treatment before pre-oxidation may have a positive effect on the porosity and pore diameter of the porous carbon fibers. a

b

c

d

e

f

g

h

i

j

k

Figure 3. (a)-(d) Surface images of NF, PF, PF-H5, PF-H10, resprctively. (e)-(h) Surface images for NCF, PCF, PCF-H5, PCF-H10, respectively. (i)-(j) Cross section images of NCF and PCF-H10, respectively. (k) TEM images of PCF-H5. The phase structure of carbon fibers were monitored by XRD and the spectra of carbon fibers are shown in Figure 4. Peaks at around 25o and 43.5o represent (002) and (100) diffraction patterns of turbostratic graphite structure of carbon fibers, respectively.42 The four fibers have almost the same d spacing deduced from the peak position at 25o. Graphite structures exist in all the four carbon fibers, but the extents of graphitization are low, suggesting by the broad FWHM of the two peaks. The XRD data of the PCF-H5 is in agreement with TEM observation shown in Figure 3(k).


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Figure 4. XRD spectra of NCF, PCF, PCF-H5, and PCF-H10. 3.3 N2 adsorption/desorption isotherms. BET specific surface area (SBET) and porous structure of the four carbon fibers were determined by nitrogen adsorption/desorption measurements at -196 oC. Figure 5(a) shows the typical nitrogen adsorption/desorption isotherms for NCF and PCF. Adsorbed nitrogen amount of NCF is low in the whole pressure range, and that of PCF is improved little. The isotherm for PCF primarily exhibits a type IV curves with H3 hysteresis, indicating the presence of mesopores in the fibers.43 However, the mesopore content is very low in PCF. In addition, few micropores exist in PCF, suggested by the low quantity adsorbed at low pressure. PCF-H5 and PCF-H10 fibers show a combination of type I and type IV curves, as shown in Figure 5(b). The adsorbed nitrogen amount is augmented very much at low pressure and high pressure, which indicates a high amount of micropores and mesopores in the two fibers, respectively. The pore width distribution shows that there are almost no micropores or mesopores in NCF, while there are some pores with pore width larger than 40 nm in PCF [Figure 5(c)]. High amount of micropores can be obtained in PCF with the treatment of HH. A few of micropores (

Nitrogen-Enriched Porous Polyacrylonitrile-Based Carbon Fibers for

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