CC BY
9
UDC 677.494
DOI: 10.18698/1812-3368-2023-2-160-172
POLYACRYLONITRILE-BASED COMPOSITE CARBON NANOFIBERS WITH TAILORED MICROPOROSITY
E.S. Vtyurina Ig.I. Ponomarev A.G. Buyanovskaya I.I. Ponomarev K.M.Skupov
ves1809@yandex.ru
gagapon@ineos.ac.ru
analyst@ineos.ac.ru
ivan.ponomarev84@gmail.com
kskupov@ineos.ac.ru
A.N. Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences, Moscow, Russian Federation
Abstract
Carbon nanofibers are currently used in many applications including electrochemical power sources, particularly, fuel cells. Their properties are highly dependent on the micro- and mesoporous structure. Here we provide a porosimetric analysis of the polyacrylonitrile-based electrospun composite Zr- and Ni-containing carbon nanofiber mats by N2 and CO2 adsorption methods for the first time. It was found that pyrolysis temperature affects specific surface area and volume: the values increase for the sample pyrolyzed at 900 °C compared with the initial stabilized nanofibers (300 °C, air) according to the Dubinin — Radushkevich, non-local density functional theory (NLDFT) and grand canonical Monte-Carlo methods (GCMC). For higher pyrolysis temperatures (1000 and 1200 °C), the porosimetric parameters decrease compared with the one pyrolyzed at 900 °C. According to the NLDFT and GCMC pore size distribution, the difference for pyrolyzed samples is mostly related to a sharp decrease in the specific surface area for pores with a size of ~ 0.5 nm and an increase for pores at 0.55-0.8 nm compared with the initial stabilized sample. The study demonstrates a way to adjust porosimetric parameters depending on the pyrol-ysis conditions of the nanofiber mats, since it can improve characteristics of such type of carbon materials in electrochemical devices
Keywords
Polyacrylonitrile, polymer nanofibers, carbon nanofibers, CO2 adsorption, specific surface area, electrospinning, micropores
Received 08.06.2022 Accepted 23.08.2022 © Author(s), 2023
The study was financially supported by Russian Science Foundation (grant no. 22-13-00065)
Introduction. The shape and geometry of pores as well as their specific volume, specific surface area and pore size distribution are extremely important for the materials used in the electrochemical power sources. In general, the porous structure may play an even more important role than the electrocatalyst activity since it is strongly related to the mass transfer optimization [1-4]. The hydrogen-air high-temperature polymer electrolyte membrane (HT-PEM) fuel cells are a very important type of fuel cell because it allows operation with hydrogen contaminated with CO at 150-200 °C [5, 6]. HT-PEM fuel cell on polybenzimidazole/o-phosphoric acid (PBI/H3PO4) membrane usually operates with electrodes with platinum nanoparticle electrocatalyst dispersed on carbon black due to its high specific surface area and relevant porous structure. However, carbon black is unstable under long-term high temperature (150-200 °C) acidic operating conditions. So that, for further HT-PEM fuel cell development, its replacement with more stable carbon nanostructured materials, for example, carbon nanofibers (CNF), is required [7-23].
Materials capable of high CO2 uptake are becoming important due to the need to develop CO2 capture and transfer technologies to prevent the climate change. Some activated carbons are known for the mentioned purpose [24]. Nevertheless, finding ways to tailor the CNF porous structure and, consequently, their properties is highly important.
Earlier [25-35] we have shown the possibility of an HT-PEM fuel cell operation with polyacrylonitrile (PAN) or polyheteroarylene based electrospun pyrolyzed CNF electrodes and PBI-OPhT membrane developed in our laboratory. Addition of small amounts (~ 5 wt. % relative to PAN) of carbon black (Vulcan® or Ketjenblack®) into PAN electrospinning solution significantly increased the uniformity of the material, improved the electrospinning process and increased the mechanical strength of pyrolyzed mats, but had almost no effect on the specific surface area of the material. The replacement of such types of carbon black with cheaper and more affordable in terms of import substitution technical carbon (carbon black) UM-76 (UM) is of great importance.
It has been established [36, 37] that optimization of the porous structure, in particular, reducing the excessive microporosity, positively affects the performance of the fuel cell (FC). The obtained results indicate the emerging need to find ways of tuning of the porosimetric characteristics such as specific surface area and volume of the pores and their pore size distribution.
In the given study, the CO2 adsorption method (at 273 K) was applied to the novel Zr and Ni nanoparticle containing PAN-UM composite pyrolyzed carbon nanofiber mats in order to calculate porosimetric characteristics according
to the Dubinin — Radushkevich, non-local density functional theory and grand canonical Monte-Carlo methods for the first time.
Data and methods for solving problems, accepted assumptions. The composite polymer nanofibers were obtained by the string electrospinning [38-40] method (where multiple jets are generated from the string under high voltage) from an electrospinning polymer solution which contained 3.25 g of PAN with Mw = 150-103 Da, 0.1 g of UM-76 (~3 wt. % relative to PAN), 0.03 g of zirconium (IV) chloride and 0.37 g of nickel (II) acetate well dispersed in 50 mL of DMF during 3 h in ultrasonic bath. The electrospinning was carried out on NS Lab Nanospider™ setup (Elmarco, Czech Republic) at relative humidity of 8 % with the distance between electrodes of 190 mm at the voltage of 69 kV.
The obtained polymer nanofiber mat was stabilized (oxidized) at 300 °C in air (2 h) to make the material suitable for pyrolysis (1) [41]. In order to obtain CNF [42, 43], on the next step, the material was pyrolyzed under vacuum (2 • 10-5 mbar) at 900, 1000 and 1200 °C (CNF mats 2, 3 and 4 respectively).
The elemental composition was determined using a CHN analyzer equipped with a Vario Micro Cube thermal desorption column (Elementar, Germany). Zirconium and nickel were determined by the X-ray fluorescence analysis using a VRA-30 X-ray fluorescence spectrometer (Carl Zeiss, Germany).
The in-plane electrical conductivity of the pyrolyzed mats was measured with a RLC E7-8 setup (Belarus) equipped with a four-point probe. The sample thickness was determined using an eXacto ElektroPhysik thickness gage (Germany) and comprised 51 ± 5 ^m.
Scanning electron microscopy (SEM) images were obtained with a Hitachi TM4000 SEM tabletop microscope in the back-scattered electron mode at 15 kV of accelerating voltage.
Nitrogen and carbon dioxide adsorption isotherms of the samples were obtained at 77 and 273 K respectively using a 3P Micro 200 Surface Area and Pore Size Analyzer (3P Instruments GmbH and Co. KG, Germany). The Brunauer — Emmett — Teller (BET) equation was applied to nitrogen adsorption isotherms according to the Rouquerol criteria [44]. For microporosity studies, the Dubinin — Radushkevich (DR), non-local density functional theory (NLDFT) [44] and grand canonical Monte-Carlo (GCMC) [44] methods were applied to carbon dioxide adsorption isotherms using NovaWin (version 11.04), Quantachrome Instruments, considering CO2 cross-sectional area A of 0.21 nm2, adsorbed CO2 density pads = 1.044 g • mL-1 and p0 = 3.485 MPa, the affinity coefficient P was taken as 0.35 [45]. CO2 uptake was calculated at standard temperature and pressure (according to IUPAC) of 273 K and 1 bar [46].
Results. After pyrolysis, the shape of the CNF mats was saved. Also, it was found that the materials possess a nanofiber structure (Fig. 1), as confirmed by scanning electron microscopy.
Fig. 1. SEM images of 1 (300 °C, air, 1 atm), 2 (300 °C, air, 1 atm; 900 °C, vacuum, 2 ■ 10-5 mbar), 3 (300 °C, air, 1 atm; 1000 °C, vacuum, 2 ■ 10-5 mbar) and 4 (300 °C, air, 1 atm; 1200 °C, vacuum, 2-10-5 mbar)
More precise observation of the SEM images of the samples reveals slightly lower nanofiber diameters for the pyrolyzed samples 2-4 50-300 nm compared with 80-400 nm for the initial stabilized sample 1. However, precise determination of the nanofiber diameters is difficult due to not very high resolution of the SEM setup. Elemental analysis and electrical conductivity data for the samples are shown in Table 1.
Table 1
In-plane electrical conductivity a of the nanofiber mats measured by a four-point probe and their elemental analysis, %
Sample ст, S ■ cm 1 C N H Zr Ni
1 - 54.7 18.2 2.6 0.6 3.7
2 14.2 83.9 3.8 0.6 0.5 7.9
3 16.2 86.6 2.3 0.6 0.6 6.6
4 29.3 82.0 0.7 0.3 0.5 6.8
As can be seen from Table 1, an increase in the pyrolysis temperature from 900 to 1200 °C leads to a decrease in the nitrogen content from 3.8 to 0.7 wt. % and an increase in the electrical conductivity from 14.2 to 29.3 S • cm1 which could be explained by higher degree of graphitization at higher temperature. Attempts to determine the specific surface area and total pore volume by low-temperature N2 adsorption (77 K) method using the BET equation were unsuccessful due to the inaccessibility of micropores for N2 molecules at such a low temperature. As a result, the measurable specific surface area was only 5-15 m2 • g-1. Anyways, the result indicates the limited applicability of the N2 adsorption method for such type of CNF. The CO2 adsorption method avoids the kinetic limitations of the adsorbate penetration into micropores, since it is carried out at a higher temperature of 273 K. The micropore specific surface area and specific volume were calculated from the CO2 adsorption isotherm curves according to the DR, NLDFT and GCMC methods. The isotherm curves and DR plots as well as specific surface area Ssp and specific volume Vsp data are shown in Fig. 2 and Table 2.
V, cm3- g"1 (STP)
Fig 2
lg2(p/p0)
. 2. CO2 adsorption isotherms (a) and DR plots (b) for 1 at 300 °C, air, 1 atm (•), at 300 °C, air, 1 atm; 900 °C, vacuum, 2-10-5 mbar (▲), 3 at 300 °C, air, 1 atm; 1000 °C, vacuum, 2-10-5 mbar (■), 4 at 300 °C, air, 1 atm; 1200 °C, vacuum, 2-10-5 mbar (♦)
Different values of for Ssp and Vsp obtained by different methods for the same samples are expected since they are related to different physical and mathematical approaches applied in these methods. Slightly higher values for Ssp and Vsp obtained by the DR method are expectable since the NLDFT and GCMC data are limited by the size of micropores of 1.5 nm, however, the trends
of changes for all the values depending on temperature are the same for all three methods. CO2 uptake was calculated at standard temperature and pressure (STP) defined as 273 K and 100 kPa according to IUPAC [46].
Table 2
Specific surface area and specific volume by the DR, DFT and GCMC methods and CO2 uptake data for samples 1-4
Sample DR NLDFT GCMC CO2 uptake, mmol/g (1 bar, 273 K)
Ssp, 2 -1 m2 ■ g 1 Vsp, 3 -1 cm3 ■ g 1 Ssp, 2 -1 m2 ■ g 1 Vsp, 3 -1 cm3 ■ g 1 Ssp, 2 -1 m2 ■ g 1 Vsp, 3 -1 cm3 ■ g 1
1 308 0.103 305 0.090 282 0.086 1.31
2 424 0.141 312 0.098 340 0.126 1.35
3 392 0.131 291 0.095 276 0.094 1.21
4 367 0.122 243 0.082 252 0.093 1.04
An important difference between the samples, in addition to their specific surface area and volume, is their pore size distribution. Considering the CO2 adsorption method, the NLDFT and GCMC methods are among the most common for obtaining a pore size distribution. The NLDFT and GCMC pore size distributions are shown on Fig. 3 as the specific surface area derivative dS/dD dependence on the pore diameter D.
Discussion of the results obtained. As can be seen from Table 2, the specific surface area and specific volume of micropores increase after pyrolysis of the initial polymer nanofiber mat at 900 °C (from 308 to 424 m2 • g1 and from 0.103 to 0.141 cm3 • g1 respectively) according to the DR method. Similar changes are observed by the NLDFT and GCMC methods. It is accompanied by an increase in carbon content from 54.7 to 83.9 % (see Table 1) and a decrease in nitrogen content from 18.2 to 3.8 % according to the elemental analysis. It reflects the processes of carbonization and graphitization occurring during the pyrolysis of PAN. As a result, the sample becomes electrically conductive and the in-plane electrical conductivity of the mat reaches 14.2 S • cm1 which makes it suitable for use in electrochemical power sources, particularly, as electrodes in fuel cell. An increase of the pyrolysis temperature to 1000 °C leads to a further increase of the electrical conductivity to 16.2 S • cm1 and a further decrease of nitrogen content (2.3 %). At the same time, the specific volume of micropores decreases to 0.131 cm3 • g1 according to the DR method and also decreases according to the NLDFT and GCMC methods. For the pyrolysis temperature of 1200 °C, the electrical conductivity drastically increases to 29.3 S • cm1,
dS/dD, m2 • mm-1 • g_1
4000 3000 2000 1000 0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 D, nm
a
dS/dD, m2 • mnT1 •
3000 2000 1000 0
0
Fig. 3. NLDFT (a) and GCMC (b) pore size distribution for 1 at 300 °C, air, 1 atm (•), 2 at 300 °C, air, 1 atm; 900 °C, vacuum, 2-10-5 mbar (▲), 3 at 300 °C, air, 1 atm; 1000 °C, vacuum, 2-10-5 mbar (■), 4 at 300 °C, air, 1 atm; 1200 °C, vacuum, 2-10-5 mbar (♦)
and nitrogen content drops to 0.7 %. The specific surface area and specific volume of micropores continue to decrease. Carbon dioxide uptake follows almost the same trends. It increases slightly when the sample was pyrolyzed at 900 °C (2) compared with the initial stabilized sample (1) from 1.31 to 1.35 mmol ■ g1. Further increase of the pyrolysis temperature (1000 and 1200 °C) leads to lower CO2 uptake values (1.21 and 1.04 mmol ■ g1 respectively). The Ssp and Vsp values for 3 and 4 are closer to the ones of the initial sample 1 according to the DR method or even lower according to the NLDFT and GCMC methods. However, it is unlikely that the pore structure after pyrolysis at 1000 and 1200 °C returns to the one of the original stabilized sample. To verify this point, the pore size distributions were obtained using the
NLDFT and GCMC methods (see Fig. 3). As can be seen from Fig. 3, the NLDFT and GCMC pore size distributions are quite similar, and most of the specific surface area of the samples belong to ultramicropores (< 0.7 nm). After pyrolysis, the specific surface area of pores with a size of ~ 0.5 nm drastically decreases compared with the initial stabilized CNF. However, the specific surface area related with pores less than 0.45 nm and more than 0.55 nm increases, it explains the observed increase in the total micropore surface area for the sample pyrolyzed at 900 °C (2) compared with the initial stabilized CNF sample (1). At higher pyrolysis temperatures (1000 and 1200 °C), the specific surface area decreases in almost the entire range of micropore sizes compared with the sample pyrolyzed at 900 °C. It explains the observed decrease in the total specific surface area of micropores compared with 2, which becomes closer to the specific surface area of the initial stabilized CNF (1). However, as can be seen from the pore size distributions, the pore structure for pyrolyzed samples is different compared with the one of the initial stabilized CNF (1).
Summary. Microporosity of the carbonized materials for electrochemical applications (including electrospun CNF) is extremely important for their practical use. Generally known that such critical issues as phosphoric acid distribution, platinum electrocatalyst deposition, triple point optimization, gas (hydrogen, oxygen or air) diffusion, etc. strongly affect HT-PEM fuel cell performance. Obviously, it is also highly important for electrospun CNF mats when Pt/CNF are used as gas diffusion electrodes (GDE). Here, for the first time, we apply the Dubinin — Radushkevich, NLDFT and GCMC methods to CO2 adsorption isotherms of PAN-based composite Zr- and Ni-containing electrospun CNF mats to study their microporosity. It was shown that pyrolysis (carbonization) of the polyacrylonitrile composite nanofibers makes them electrically conductive; and, depending on pyrolysis temperature, microporosity can be tuned accordingly. The obtained porosimetric, elemental analysis, microscopy and electrical conductivity data may help to tailor the porosimetric parameters and pore size distribution in order to make CNF mats more suitable for use in different electrochemical power source applications, particularly, as GDEs in the HT-PEM fuel cells. Further development of these materials will allow to improve performance of the HT-PEM fuel cells and will help to produce new materials with high carbon dioxide uptake.
Acknowledgements
Elemental analysis was supported by the Ministry of Science and Higher Education of the Russian Federation employing the equipment of Center for Molecular Composition Studies of A.N. Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences.
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Vtyurina Elizaveta S. — Master's Degree (Mater. Sc.), Junior Researcher, A.N. Nesmeya-nov Institute of Organoelement Compounds of Russian Academy of Sciences (Vavi-lova ul. 28, str. 1, Moscow, 119334 Russian Federation).
Ponomarev Igor I. — Dr. Sc. (Chem.), Professor; Principal Researcher and Head of Laboratory, A.N. Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences (Vavilova ul. 28, str. 1, Moscow, 119334 Russian Federation).
Buyanovskaya Anastasia G. — Cand. Sc. (Chem.), Principal Researcher and Head of Laboratory, A.N. Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences (Vavilova ul. 28, str. 1, Moscow, 119334 Russian Federation).
Ponomarev Ivan I. — Cand. Sc. (Chem.), Senior Researcher, A.N. Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences (Vavilova ul. 28, str. 1, Moscow, 119334 Russian Federation).
Skupov Kirill M. — Ph.D. (Chem.), Senior Researcher, A.N. Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences (Vavilova ul. 28, str. 1, Moscow, 119334 Russian Federation).
Please cite this article as:
Vtyurina E.S., Ponomarev Ig.I., Buyanovskaya A.G., et al. Polyacrylonitrile-based composite carbon nanofibers with tailored microporosity. Herald of the Bauman Moscow State Technical University, Series Natural Sciences, 2023, no. 2 (107), pp. 160-172. DOI: https://doi.org/10.18698/1812-3368-2023-2-160-172
