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Superior prospect of chemically activated electrospun carbon fibers for hydrogen storage
Materials Research Bulletin 44 (2009) 1871–1878
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Superior prospect of chemically activated electrospun carbon fibers for hydrogen storage Ji Sun Im a, Soo-Jin Park b, Young-Seak Lee a,* a b
Department of Fine Chemical Engineering and Applied Chemistry, Chungnam National University, Yuseong-Gu, Daejeon 305-764, Republic of Korea Department of Chemistry, Inha University, Nam-Gu, Incheon 402-751, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 January 2009 Received in revised form 14 April 2009 Accepted 21 May 2009 Available online 29 May 2009
In this study, the capacity of hydrogen storage was evaluated by using electrospun activated carbon fibers prepared by electrospinning and chemical activation based on the comparison with other carbon materials such as active carbon, single walled carbon nanotube, and graphite. For an improved hydrogen storage system, the optimized conditions of carbon materials were investigated with studying their specific surface area, pore volume, size, and shape. The hydrogen adsorption capacity of chemically activated electrospun carbon fiber itself is better than that of other porous carbon materials. This is attributed to the optimized pore structure of electrospun activated carbon fibers that might provide better sites for hydrogen adsorption than other carbon materials. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Microporous materials A. Polymers B. Energy storage A. Structural materials
1. Introduction Due to the exhaustion of gasoline or diesel fuel, new energy sources are necessary to be developed as an assistant or alternative energy. Among them, hydrogen gas is an attractive possibility to provide new solutions for ecological and power problems [1–5]. The efficient usage of hydrogen as a fuel is mainly hampered by the lack of proper media for its storage. Storage media have to be light, industrial, and in compliance with national and international safety laws [2–7]. Additionally, hydrogen adsorption and extraction have to be totally reversible for easy usage. The search for suitable media for hydrogen storage has been hampered by the above requirements [2,8,9]. The most promising materials for hydrogen storage are carbonaceous materials because of the high specific surface area and pore volume; in particular, activated carbon, activated carbon fibers, and carbon nanotubes have been discussed as candidates for hydrogen storage media. These materials have quite different characteristics, such as specific surface area, pore structure, surface morphology, and shape [10–14]. In this study, the capacity of hydrogen storage was investigated using activated electrospun fibers based on the investigation of their pore structure and shape. And the comparison of hydrogen adsorption was carried out comparing with other carbon materials. An electrospinning method was adapted due to the ability to produce ultra-fine fibers with diameters ranging from nanometers
* Corresponding author. Tel.: +82 42 821 7007; fax: +82 42 822 6637. E-mail addresses: [email protected], [email protected] (Y.-S. Lee). 0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2009.05.010
to less than microns [15–17]. Electrospun nanofibers produced by this technique have an optimized shape and a pore structure which can offer many more sites for hydrogen adsorption by chemical activation because of the large surface area and controlled pore size, compared with other carbon materials such as activated carbon or meltspun fibers [18,19]. In order to control the pore structure with high specific surface area, chemical activation has been used widely because it is a simple method and has excellent effects [20,21]. Sodium hydroxide and potassium carbonate activations were used to obtain developed pore structures with high specific surface areas. 2. Experimental 2.1. Reagents The proper solvent has to be selected for dissolving a polymer source completely before carrying out the electrospinning. N,N-dimethyl formamide (DMF) is considered to be the best solvent among various organic solvents due to its proper boiling point (426 K) and enough electrical conductivity (electrical conductivity = 10.90 mS/cm, dipole moment = 3.82 Debye) for electrospinning [14,15,17,19]. In this study, DMF (d = 133, 766137, Fisher) was used as a solvent and PAN (polyacrylonitrile, d = 1.184, 181315, Aldrich) was adopted as a polymer source. In general, various agents have been used to activate carbon materials with high specific surface area and porosity using KOH, NaOH, Na2CO3, K2CO3, ZnCl2, H3PO4, and SiO2. [20–24]. In this study, NaOH (sodium hydroxide, 480878, Aldrich) and K2CO3
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(potassium carbonate, 367877, Aldrich) were adopted as chemical activation agents. 2.2. Preparing PAN-based carbon fibers Polymer solution (PAN/DMF, 10 wt.%) was prepared for electrospinning. The prepared polymer solution was ejected from a syringe tip onto an aluminum foil-covered collector using an electrospinning apparatus. A schematic diagram of the electrospinning apparatus was depicted in our previous paper [15]. There are several factors that determine the conditions of electrospinning [15]. With increasing ejection voltage, the average diameter of CNFs (carbon nanofibers) is decreased, because the electric strength which can pull the polymer solution from the syringe tip to the collector is increased. Increasing the revolutions per minute (rpm) of the collector has the same effect as increasing the voltage [15–17]. Eventually, electrospinning was accomplished with following electrospinning conditions [feeding rate of polymer solution: 1 ml/h, supplied voltage: 15 kV, tip to collector distance: 10 cm, and collector rpm: 100]. The stabilization of electrospun materials was carried out under air by heating up to 523 K with heating rate of 1 K/min, and finally samples were treated at 523 K for 8 h. Heat treatment of stabilized electrospun materials was carried out under nitrogen atmosphere with the following conditions [heating rate: 10 K/min; reaction temperature: 1323 K; holding time: 1 h; nitrogen feeding rate: 100 ml/h] to carbonize oxidized fibers. The final product was named CF-R in this study. 2.3. Chemical activation using sodium hydroxide NaOH solutions were prepared with various concentrations (2, 4, 6, and 8 M, 300 ml). The CF-R sample (3 g) was immersed in each NaOH solution. To immerse the CF-R sample uniformly, a shaker apparatus (SK-300, JEIO TECH, Korea) was used at 70 rpm for 10 h. Wet CF-R was placed in an alumina boat in a steel pipe to carry out chemical activation. Activation was conducted at 1023 K for 3 h in a nitrogen atmosphere. The heating rate was 5 K/min and the nitrogen gas feeding rate was 50 ml/min. After chemical activation, ACFs (activated carbon fibers) were washed with distilled water several times to remove non-reacted agents in the ACFs. These samples were dried at 383 K over night. The final products were called ACF-SH2, ACF-SH4, ACF-SH6, and ACF-SH8, as shown in Table 1. 2.4. Chemical activation using potassium carbonate K2CO3 solutions with two concentrations (2 and 4 M, 300 ml) were used for chemical activation. The CF-R sample (3 g) was immersed in each K2CO3 solution using a shaker apparatus for 10 h, and chemical activation was conducted with the same procedure as the NaOH activation. These samples were called ACF-PC2 and ACFPC4 for K2CO3 solutions of 2 and 4 M, respectively. The procedure of electrospinning and chemical activation is illustrated in Fig. 1. 2.5. Characterization of samples In order to investigate the surface morphology of the CNFs, two kinds of apparatus were used. In the case of images with low
Fig. 1. Procedure diagram of preparation of ACF.
magnification (10K), SEM (scanning-electron-microscope) images were obtained by an SEM apparatus (VEGAII LMU, TESCAN Co.). Every sample was sputter coated with gold–palladium and placed in a vacuum room to take the SEM images. In the case of images with high magnification (250K), a UHR FE-SEM (ultra high resolution field emission–scanning electron microscope, Hitachi, S-5500) was used. Every sample was imaged without any treatment such as coating. The average diameters of the CNFs were calculated by a soft program (VEGAII LMU, TESCAN Co.). Each sample was degassed at 423 K for 3 h and nitrogen adsorption was carried out at 77 K using a BET apparatus (Micromeritics ASAP 2020) to study the textural properties such as specific surface area, total pore volume, and micropore volume. To study the hydrogen adsorption capacities of ACFs, samples were degassed at 473 K for 2 h and hydrogen adsorption and desorption of ACFs was conducted at 303 K from 1.33 103 torr to 5 MPa using a PCT (Pressure-Composition-Temperature) apparatus (miraeSI co., KOREA). The capacity of hydrogen uptake was calculated by following Eq. (1). The amount of hydrogen storage weight ofH2 ¼ 100 weight of sample and H2
(1)
3. Results 3.1. Surface morphology analysis Fig. 2 shows SEM images of resultant CNFs. The surface morphology is uniform and smooth. The average diameter of the CF-R samples was about 275 nm. After K2CO3 and NaOH activation, the average diameter of ACFs is considerably decreased. In the case of K2CO3 activation, the average diameter of ACFs has a range from 190 to 160 nm, and in case of NaOH activation, the diameters decreased from 140 to 50 nm. These results indicate that the carbon surface was burned off by generating carbon monoxide and
Table 1 Constituent mass ratios of the mixtures used in this study. Sample name
CF-R
ACF-PC2
ACF-PC4
ACF-SH2
ACF-SH4
ACF-SH6
ACF-SH8
PAN (g) DMF (g) Immersing K2CO3 solution (M) Immersing NaOH solution (M)
3 27 – –
3 27 2 –
3 27 4 –
3 27 – 2
3 27 – 4
3 27 – 6
3 27 – 8
J.S. Im et al. / Materials Research Bulletin 44 (2009) 1871–1878
carbon dioxide from outside to inside by the following reactions (2)–(6) [25] (here, M = Na or K): 6MOH þ C $ 2 M þ 3H2 þ 2M2 CO3
(2)
M2 CO3 þ C $ M2 O þ 2CO
(3)
M2 CO3 $ M2 O þ CO2
(4)
2M þ CO2 $ M2 O þ CO
(5)
M2 O þ C $ 2M þ CO
(6)
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In images with high magnification (250K), the uniform diameter was observed in the same condition. 3.2. Textural properties analysis Isotherms of all samples are presented in Fig. 3. The CF-R sample does not have any significant changes, indicating nonporous material (Fig. 3a). According to the Zsigmondy model, the initial part of the isotherm adsorption is restricted to a thin layer on the walls; capillary condensation commences in the finest pores and wider pores are filled as the pressure
Fig. 2. SEM images of carbon fibers; (a, b): CF-R, (c, d): ACF-PC2, (e, f): ACF-PC4, (g, h): ACF-SH2, (i, j): ACF-SH4, (k, l): ACF-SH6, (m, n): ACF-SH8; (a, c, e, g, i, m: 10K), (b, d, f, h, j, l, n: 250K).
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Fig. 2. (Continued ).
is progressively increased [26]. And, the first inflection point, which is called the ‘knee’, indicates that monolayer adsorption and filling the micropores are completed [15]. Isotherm curves of ACF-PC2 and ACF-PC4 show steep increases at less than 0.05 P/P0. This indicates that ACF-PC2 and ACF-PC4 are highly microporous. Isotherm curves have very small changes above 0.9 P/P0, which indicates that potassium carbonate activated carbon materials have some mesopores. This kind of isotherm is characterized as type I according to the IUPAC classification. In Fig. 3(b), isotherms of NaOH-activated carbon materials are presented. The isotherms of four NaOH-activated CNFs have inflection points at around 0.02 P/P0. Each sample has different characteristics with increasing concentration of NaOH solution. When the 2 M NaOH solution was used for activation, there was no clear change over inflection point; thus, sample ACF-SH2 is a microporous material [15]. However, very significant changes are observed above 0.1 P/P0 in the other cases, such as ACF-SH4, ACF-SH6, and ACF-SH8, which indicates that they have mesopores and macropores. Thus, it is confirmed that sodium hydroxide activation makes micropores initially with low concentration, and then makes mesopores with high concentration later. It is believed that mesopores are gradually formed by merged micropores using a high concentration of the activation agent.
In order to investigate the micropore structure of the samples, the HK (Horvath-Kawazoe) equation has been applied widely [27–30]. The HK cumulative pore size distribution is presented in Fig. 4. Sample CF-R does not have any significant curve, confirming its nonporous nature. Samples ACF-PC2 and ACF-PC4 have steep slopes in the pore width range between 0.5 and 0.8 nm, which reveals that many pores exist with pore widths from 0.5 to 0.8 nm. This result agrees with the capacity of hydrogen adsorption from predictions of Monte-Carlo simulations [12–14]. The cumulative micropore volume of sample ACF-PC4 reaches about 0.3 cm3/g, and in the case of ACF-PC2 it approaches around 0.2 cm3/g. NaOH-activated carbon materials show similar trends with K2CO3-activated CNFs in Fig. 4(b). The clear changes of the curves are confirmed in the pore width range between 0.5 and 0.8 nm. The cumulative micropore volumes increased with increasing concentrations of the chemical agent solution. A summary of the textural properties is presented in Table 2. Both NaOH and K2CO3 activation have effects in increasing specific surface area, total pore volume, and micropore volume with increasing concentration of the used chemical agent solution. In the case of potassium carbonate activated carbon materials, the fractions of micropore volume are over 80%. This data is fit well with isotherm data from Fig. 3(a) [15].
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Fig. 3. Isotherms of samples; (a): CF-R, ACF-PC2, and ACF-PC4, (b): ACF-SH2, ACFSH4, ACF-SH6, and ACF-SH8, Po: saturation vapor pressure of the adsorptive.
The micropore volume fractions of NaOH-activated carbon materials are in the range of 50–90%. The micropore volume fraction decreased with increasing concentration of NaOH solution. This result agrees with the isotherm from Fig. 3(b) [15]. Thus, it is confirmed again that the fraction of micropore structure developed with low concentration of NaOH solution and regressed at high concentration of NaOH solution. 3.3. Investigation of hydrogen adsorption on ACFs The capacity of hydrogen storage of our samples is presented in Fig. 5. All of adsorption lines seem to be linear. The reason of that was explained in our previous work simply due to the low amount
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Fig. 4. HK cumulative micropore volume; (a): CF-R, ACF-PC2, and ACF-PC4, (b): ACFSH2, ACF-SH4, ACF-SH6, and ACF-SH8.
of hydrogen uptake [31]. There is no hysteresis in all samples indicating that adsorbed hydrogen can be desorbed reversibly [32]. NaOH-activated samples showed the higher hydrogen adsorption than K2CO3-activated samples. This is believed to be due to the higher specific surface area and micropore volume of NaOHactivated CNFs than in K2CO3-activated CNFs. Comparing ACF-PC4 with ACF-SH8, even though ACF-SH8 has 3.3 times more specific surface area and 4.1 times more total pore volume, the hydrogen storage capacity of ACF-SH8 is just 2.8 times higher than that of ACF-PC4. This may be due to the fraction of micropore volume (ACF-PC4: 83%, ACF-SH8: 51%). This clearly suggests that micropore volume favors the capacity of hydrogen adsorption better than specific surface area and total pore volume.
Table 2 The textual properties and hydrogen adsorption capacity of ACFs.
SBET (m2/g)a VT (cc/g)b VM (cc/g)c FM/V (%)d P AH ( wt.%)e a b c d e
CF-R
ACF-PC2
ACF-PC4
ACF-SH2
ACF-SH4
ACF-SH6
ACF-SH8
17.0 0.030 0.020 67 0.02
527.9 0.272 0.227 83 0.34
587.2 0.357 0.309 87 0.39
868.3 0.397 0.356 90 0.43
1804.4 0.991 0.575 58 0.68
1867.8 1.119 0.705 63 0.91
1933.2 1.459 0.751 51 1.05
SBET: BET specific surface area. VT: Total pore volume. VM: HK micropore volume. FM/V: Fraction of micropore. AH: Amount of hydrogen adsorption.
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Fig. 5. Hydrogen storage of samples; (a): CF-R, ACF-PC2, and ACF-PC4, (b): ACF-SH2, ACF-SH4, ACF-SH6, and ACF-SH8.
4. Discussion In Fig. 6, a comparison of hydrogen storage data from this study and other studies is presented. The samples studied in this paper were not modified using any catalyst for increasing the capacity of hydrogen storage, because the purpose of our study is to compare the capacity of hydrogen storage using various kinds of carbon materials. Other groups have used single walled carbon nanohorn (SWNH), single walled carbon nanotube (SWNT), graphic carbon nanofiber (GNF), and activated carbon (AC) as a hydrogen storage media [33,34]. In Fig. 6(a), three regressions are predicted from the results of our and other groups. Even though they show the same trend, where the hydrogen adsorption capacities increase with increasing specific surface area, the slopes of the regression lines have quite different values based on their unique shapes and pore structures. The slope of the regression from our study is around 1.5–2.0 times those from the other studies. In Fig. 6(b), the relation between the capacity of hydrogen storage and micropore volume is investigated. Although some of the data is scattered, a clear trend is observed. The capacity of hydrogen storage is proportional to micropore volume. Even though the capacity of hydrogen adsorption was measured at lower pressure than in the other studies, the slope of the regression line from our experiment is steeper than the other studies. Finally, it is clear that the samples made using electrospinning and chemical
Fig. 6. Hydrogen storage against (a) specific surface area and (b) micropore volume measured at 303 K and 5 MPa. Data of Ref. [33] was obtained at 303 K and 10 MPa; data of Ref. [34] was measured at 298 K and 6.5 MPa.
activation are more effective than other carbon materials as a hydrogen storage media. It seems that electrospun ACFs with thin diameters provide an optimized pore structure which can offer sites for hydrogen adsorption by chemical activation much more efficiently. The mechanism of hydrogen adsorption using various carbon materials is suggested in Fig. 7. In the case of using AC, macro and meso pores would not work for hydrogen adsorption because it is difficult to trap hydrogen molecules due to the large pore diameter [11]. In addition, there are large spaces which do not contain pores. When SWNT was used for hydrogen adsorption in Fig. 7(b), they can keep hydrogen molecules inside, outside, and between them, but there is a space restriction inside SWNT because of aspect ratios that are too high. It is difficult to create uniform and inner diameters and control the distance between the walls of SWNTs. Using graphite as a hydrogen storage media has space restrictions because intercalation and extraction of hydrogen molecules contained deep inside with a high aspect ratio are difficult, as shown in Fig. 7(c). Thus, adsorption and desorption reactions are easy only outside of graphite layers. In Fig. 7(d), electrospun ACFs might be expected to have an optimized pore structure with controlled pores size for effective hydrogen adsorption. This result may come from that the diameter of electrospun fiber can be controlled easily and optimized pore size can be obtained with highly developed pore structure.
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Fig. 7. The mechanism of hydrogen adsorption using various carbon materials; (a): AC, (b): SWNT, (c): Graphite, (d): Electrospun ACF.
5. Conclusions The electrospinning method and chemical activation were used to produce microporous carbon materials with a high specific surface area and total pore volume for use as a hydrogen storage media. The developed pore structure of the samples was obtained by chemical activation with decreasing fiber diameters. Even
though the hydrogen storage capacities increased with specific surface area and total pore volume, micropore volume mainly favors the amount of hydrogen adsorption. Comparing the hydrogen adsorption of electrospun activated carbon fibers with other carbon materials such as single walled carbon nanohorn, single walled carbon nanotube, graphite carbon nanofiber, and activated carbon, electrospun activated carbon fibers show better
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results for hydrogen storage, because they have an optimized shape that can produce sites with controlled pore size and high surface area for hydrogen adsorption. Acknowledgements This research was performed for the Hydrogen Energy R&D Center, one of the 21st Century Frontier R&D Programs, funded by the Ministry of Science and Technology of Korea. The authors would like to thank Mr. Tae-Sung Bae at KBSI (Korea Basic Science Institute) in Jeonju Center for providing FE-SEM images. References [1] L.L. Vasiliev, L.E. Kanonchik, A.G. Kulakov, V.A. Babenko, Int. J. Therm. Sci. 46 (2007) 914–925. [2] Y.S. Lee, Y.H. Kim, J.S. Hong, J.K. Suh, G.J. Cho, Catal. Today 120 (2007) 420–425. [3] H.K. Jin, Y.S. Lee, I.P. Hong, Catal. Today 120 (2007) 399–406. [4] H. Sakaguchi, K. Hatakeyama, S. Kobayashi, T. Esaka, Mater. Res. Bull. 37 (2002) 1547–1556. [5] T.K. Mandal, L. Sebastian, J. Gopalakrishnan, L. Abrams, J.B. Goodenough, Mater. Res. Bull. 39 (2004) 2257–2264. [6] F. Dolci, M.D. Chio, M. Baricco, E. Giamello, Mater. Res. Bull. 44 (2009) 194–197. [7] A. Anso´n, E. Lafuente, E. Urriolabeitia, R. Navarro, A.M. Benito, W.K. Maser, M.T. Martı´nez, J. Alloys Compd. 436 (2007) 294–297. [8] P. Be´nard, R. Chahine, Scripta Mater. 56 (2007) 803–808. [9] X. Ye, X. Gu, X.G. Gong, T.K.M. Shing, Z.F. Liu, Carbon 45 (2007) 315–320. [10] Ma Aili, Wang Xiaomin, Li Tianbao, Liu Xuguang, Xu Bingshe, Mater. Sci. Eng. A 443 (2007) 54–59. [11] B.J. Kim, Y.S. Lee, S.J. Park, Int. J. Hydrogen Energy 33 (2008) 2254–2259.
[12] N.V. Petrova, I.N. Yakovkin, Y.G. Ptushinskii, Surf. Sci. 497 (2002) 349–355. [13] K.A. Williams, P.C. Eklund, Chem. Phys. Lett. 320 (2000) 352–358. [14] A.W.C. van den Berg, S.T. Bromley, J.C. Wojdel, J.C. Jansen, Microporous Mesoporous Mater. 87 (2006) 235–242. [15] J.S. Im, S.J. Park, Y.S. Lee, J. Colloid Interface Sci. 314 (2007) 32–37. [16] M.V. Jose, B.W. Steinert, V. Thomas, D.R. Dean, M.A. Abdalla, G. Price, G.M. Janowski, Polymer 48 (2007) 1096–1104. [17] G. Eda, J. Liu, S. Shivkumar, Eur. Polym. J. 43 (2007) 1154–1167. [18] J.S. Jeong, J.S. Moon, S.Y. Jeon, J.H. Park, P.S. Alegaonkar, J.B. Yoo, Thin Solid Films 515 (2007) 5136–5141. [19] O. Ayodeji, E. Graham, D. Kniss, J. Lannutti, D. Tomasko, J. Supercrit. Fluid 41 (2007) 173–178. [20] S. Mitani, S.I. Lee, K. Saito, S.H. Yoon, Y. Korai, I. Mochida, Carbon 43 (2005) 2960– 2967. [21] B.J. Kim, Y.S. Lee, S.J. Park, J. Colloid Interface Sci. 306 (2007) 454–458. [22] Q. Jiang, M.Z. Qu, G.M. Zhou, B.L. Zhang, Z.L. Yu, Mater. Lett. 57 (2002) 988–991. [23] G. Gryglewicz, J. Machnikowski, E. Lorenc-Grabowska, G. Lota, E. Frackowiak, Electrochim. Acta 50 (2005) 1197–1206. [24] J.S. Im, S.J. Park, T.J. Kim, Y.S. Lee, Int. J. Hydrogen Energy 34 (2009) 3382–3388. [25] M.A. Lillo–Rodenas, J. Juan–Juan, D. Cazorla–Amoros, A. Linares–Solano, Carbon 42 (2004) 1371–1375. [26] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press Inc, 1982. [27] P.A. Gauden, A.P. Terzyk, S. Furmaniak, R.P. Wesołowski, P. Kowalczyk, J.K. Garbacz, Carbon 42 (2004) 573–583. [28] M. Bastos-Neto, D.V. Canabrava, A.E.B. Torres, E. Rodriguez-Castello´n, A. Jime´nezLo´pez, D.C.S. Azevedo, C.L. Cavalcante Jr., Appl. Surf. Sci. 253 (2007) 5721–5725. [29] J. Kurdi, A.Y. Tremblay, Desalination 148 (2002) 341–346. [30] A.H. Lu, J.T. Zheng, Carbon 40 (2002) 1353–1361. [31] J.S. Im, S.J. Park, Y.S. Lee, Int. J. Hydrogen Energy 34 (2009) 1423–1428. [32] Y. Yu¨ru¨m, A. Taralp, T.N. Veziroglu, Int. J. Hydrogen Energy 34 (2009) 3784–3798. [33] W.-C. Xu, K. Takahashi, Y. Matsuo, Y. Hattori, M. Kumagai, S. Ishiyama, K. Kaneko, S. Iijima, Int. J. Hydrogen Energy 32 (2007) 2504–2512. [34] B. Panella, M. Hirscher, S. Rose, Carbon 43 (2005) 2209–2214.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Superior prospect of chemically activated electrospun carbon fibers for hydrogen storage Ji Sun Im a, Soo-Jin Park b, Young-Seak Lee a,* a b
Department of Fine Chemical Engineering and Applied Chemistry, Chungnam National University, Yuseong-Gu, Daejeon 305-764, Republic of Korea Department of Chemistry, Inha University, Nam-Gu, Incheon 402-751, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 January 2009 Received in revised form 14 April 2009 Accepted 21 May 2009 Available online 29 May 2009
In this study, the capacity of hydrogen storage was evaluated by using electrospun activated carbon fibers prepared by electrospinning and chemical activation based on the comparison with other carbon materials such as active carbon, single walled carbon nanotube, and graphite. For an improved hydrogen storage system, the optimized conditions of carbon materials were investigated with studying their specific surface area, pore volume, size, and shape. The hydrogen adsorption capacity of chemically activated electrospun carbon fiber itself is better than that of other porous carbon materials. This is attributed to the optimized pore structure of electrospun activated carbon fibers that might provide better sites for hydrogen adsorption than other carbon materials. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Microporous materials A. Polymers B. Energy storage A. Structural materials
1. Introduction Due to the exhaustion of gasoline or diesel fuel, new energy sources are necessary to be developed as an assistant or alternative energy. Among them, hydrogen gas is an attractive possibility to provide new solutions for ecological and power problems [1–5]. The efficient usage of hydrogen as a fuel is mainly hampered by the lack of proper media for its storage. Storage media have to be light, industrial, and in compliance with national and international safety laws [2–7]. Additionally, hydrogen adsorption and extraction have to be totally reversible for easy usage. The search for suitable media for hydrogen storage has been hampered by the above requirements [2,8,9]. The most promising materials for hydrogen storage are carbonaceous materials because of the high specific surface area and pore volume; in particular, activated carbon, activated carbon fibers, and carbon nanotubes have been discussed as candidates for hydrogen storage media. These materials have quite different characteristics, such as specific surface area, pore structure, surface morphology, and shape [10–14]. In this study, the capacity of hydrogen storage was investigated using activated electrospun fibers based on the investigation of their pore structure and shape. And the comparison of hydrogen adsorption was carried out comparing with other carbon materials. An electrospinning method was adapted due to the ability to produce ultra-fine fibers with diameters ranging from nanometers
* Corresponding author. Tel.: +82 42 821 7007; fax: +82 42 822 6637. E-mail addresses: [email protected], [email protected] (Y.-S. Lee). 0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2009.05.010
to less than microns [15–17]. Electrospun nanofibers produced by this technique have an optimized shape and a pore structure which can offer many more sites for hydrogen adsorption by chemical activation because of the large surface area and controlled pore size, compared with other carbon materials such as activated carbon or meltspun fibers [18,19]. In order to control the pore structure with high specific surface area, chemical activation has been used widely because it is a simple method and has excellent effects [20,21]. Sodium hydroxide and potassium carbonate activations were used to obtain developed pore structures with high specific surface areas. 2. Experimental 2.1. Reagents The proper solvent has to be selected for dissolving a polymer source completely before carrying out the electrospinning. N,N-dimethyl formamide (DMF) is considered to be the best solvent among various organic solvents due to its proper boiling point (426 K) and enough electrical conductivity (electrical conductivity = 10.90 mS/cm, dipole moment = 3.82 Debye) for electrospinning [14,15,17,19]. In this study, DMF (d = 133, 766137, Fisher) was used as a solvent and PAN (polyacrylonitrile, d = 1.184, 181315, Aldrich) was adopted as a polymer source. In general, various agents have been used to activate carbon materials with high specific surface area and porosity using KOH, NaOH, Na2CO3, K2CO3, ZnCl2, H3PO4, and SiO2. [20–24]. In this study, NaOH (sodium hydroxide, 480878, Aldrich) and K2CO3
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(potassium carbonate, 367877, Aldrich) were adopted as chemical activation agents. 2.2. Preparing PAN-based carbon fibers Polymer solution (PAN/DMF, 10 wt.%) was prepared for electrospinning. The prepared polymer solution was ejected from a syringe tip onto an aluminum foil-covered collector using an electrospinning apparatus. A schematic diagram of the electrospinning apparatus was depicted in our previous paper [15]. There are several factors that determine the conditions of electrospinning [15]. With increasing ejection voltage, the average diameter of CNFs (carbon nanofibers) is decreased, because the electric strength which can pull the polymer solution from the syringe tip to the collector is increased. Increasing the revolutions per minute (rpm) of the collector has the same effect as increasing the voltage [15–17]. Eventually, electrospinning was accomplished with following electrospinning conditions [feeding rate of polymer solution: 1 ml/h, supplied voltage: 15 kV, tip to collector distance: 10 cm, and collector rpm: 100]. The stabilization of electrospun materials was carried out under air by heating up to 523 K with heating rate of 1 K/min, and finally samples were treated at 523 K for 8 h. Heat treatment of stabilized electrospun materials was carried out under nitrogen atmosphere with the following conditions [heating rate: 10 K/min; reaction temperature: 1323 K; holding time: 1 h; nitrogen feeding rate: 100 ml/h] to carbonize oxidized fibers. The final product was named CF-R in this study. 2.3. Chemical activation using sodium hydroxide NaOH solutions were prepared with various concentrations (2, 4, 6, and 8 M, 300 ml). The CF-R sample (3 g) was immersed in each NaOH solution. To immerse the CF-R sample uniformly, a shaker apparatus (SK-300, JEIO TECH, Korea) was used at 70 rpm for 10 h. Wet CF-R was placed in an alumina boat in a steel pipe to carry out chemical activation. Activation was conducted at 1023 K for 3 h in a nitrogen atmosphere. The heating rate was 5 K/min and the nitrogen gas feeding rate was 50 ml/min. After chemical activation, ACFs (activated carbon fibers) were washed with distilled water several times to remove non-reacted agents in the ACFs. These samples were dried at 383 K over night. The final products were called ACF-SH2, ACF-SH4, ACF-SH6, and ACF-SH8, as shown in Table 1. 2.4. Chemical activation using potassium carbonate K2CO3 solutions with two concentrations (2 and 4 M, 300 ml) were used for chemical activation. The CF-R sample (3 g) was immersed in each K2CO3 solution using a shaker apparatus for 10 h, and chemical activation was conducted with the same procedure as the NaOH activation. These samples were called ACF-PC2 and ACFPC4 for K2CO3 solutions of 2 and 4 M, respectively. The procedure of electrospinning and chemical activation is illustrated in Fig. 1. 2.5. Characterization of samples In order to investigate the surface morphology of the CNFs, two kinds of apparatus were used. In the case of images with low
Fig. 1. Procedure diagram of preparation of ACF.
magnification (10K), SEM (scanning-electron-microscope) images were obtained by an SEM apparatus (VEGAII LMU, TESCAN Co.). Every sample was sputter coated with gold–palladium and placed in a vacuum room to take the SEM images. In the case of images with high magnification (250K), a UHR FE-SEM (ultra high resolution field emission–scanning electron microscope, Hitachi, S-5500) was used. Every sample was imaged without any treatment such as coating. The average diameters of the CNFs were calculated by a soft program (VEGAII LMU, TESCAN Co.). Each sample was degassed at 423 K for 3 h and nitrogen adsorption was carried out at 77 K using a BET apparatus (Micromeritics ASAP 2020) to study the textural properties such as specific surface area, total pore volume, and micropore volume. To study the hydrogen adsorption capacities of ACFs, samples were degassed at 473 K for 2 h and hydrogen adsorption and desorption of ACFs was conducted at 303 K from 1.33 103 torr to 5 MPa using a PCT (Pressure-Composition-Temperature) apparatus (miraeSI co., KOREA). The capacity of hydrogen uptake was calculated by following Eq. (1). The amount of hydrogen storage weight ofH2 ¼ 100 weight of sample and H2
(1)
3. Results 3.1. Surface morphology analysis Fig. 2 shows SEM images of resultant CNFs. The surface morphology is uniform and smooth. The average diameter of the CF-R samples was about 275 nm. After K2CO3 and NaOH activation, the average diameter of ACFs is considerably decreased. In the case of K2CO3 activation, the average diameter of ACFs has a range from 190 to 160 nm, and in case of NaOH activation, the diameters decreased from 140 to 50 nm. These results indicate that the carbon surface was burned off by generating carbon monoxide and
Table 1 Constituent mass ratios of the mixtures used in this study. Sample name
CF-R
ACF-PC2
ACF-PC4
ACF-SH2
ACF-SH4
ACF-SH6
ACF-SH8
PAN (g) DMF (g) Immersing K2CO3 solution (M) Immersing NaOH solution (M)
3 27 – –
3 27 2 –
3 27 4 –
3 27 – 2
3 27 – 4
3 27 – 6
3 27 – 8
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carbon dioxide from outside to inside by the following reactions (2)–(6) [25] (here, M = Na or K): 6MOH þ C $ 2 M þ 3H2 þ 2M2 CO3
(2)
M2 CO3 þ C $ M2 O þ 2CO
(3)
M2 CO3 $ M2 O þ CO2
(4)
2M þ CO2 $ M2 O þ CO
(5)
M2 O þ C $ 2M þ CO
(6)
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In images with high magnification (250K), the uniform diameter was observed in the same condition. 3.2. Textural properties analysis Isotherms of all samples are presented in Fig. 3. The CF-R sample does not have any significant changes, indicating nonporous material (Fig. 3a). According to the Zsigmondy model, the initial part of the isotherm adsorption is restricted to a thin layer on the walls; capillary condensation commences in the finest pores and wider pores are filled as the pressure
Fig. 2. SEM images of carbon fibers; (a, b): CF-R, (c, d): ACF-PC2, (e, f): ACF-PC4, (g, h): ACF-SH2, (i, j): ACF-SH4, (k, l): ACF-SH6, (m, n): ACF-SH8; (a, c, e, g, i, m: 10K), (b, d, f, h, j, l, n: 250K).
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Fig. 2. (Continued ).
is progressively increased [26]. And, the first inflection point, which is called the ‘knee’, indicates that monolayer adsorption and filling the micropores are completed [15]. Isotherm curves of ACF-PC2 and ACF-PC4 show steep increases at less than 0.05 P/P0. This indicates that ACF-PC2 and ACF-PC4 are highly microporous. Isotherm curves have very small changes above 0.9 P/P0, which indicates that potassium carbonate activated carbon materials have some mesopores. This kind of isotherm is characterized as type I according to the IUPAC classification. In Fig. 3(b), isotherms of NaOH-activated carbon materials are presented. The isotherms of four NaOH-activated CNFs have inflection points at around 0.02 P/P0. Each sample has different characteristics with increasing concentration of NaOH solution. When the 2 M NaOH solution was used for activation, there was no clear change over inflection point; thus, sample ACF-SH2 is a microporous material [15]. However, very significant changes are observed above 0.1 P/P0 in the other cases, such as ACF-SH4, ACF-SH6, and ACF-SH8, which indicates that they have mesopores and macropores. Thus, it is confirmed that sodium hydroxide activation makes micropores initially with low concentration, and then makes mesopores with high concentration later. It is believed that mesopores are gradually formed by merged micropores using a high concentration of the activation agent.
In order to investigate the micropore structure of the samples, the HK (Horvath-Kawazoe) equation has been applied widely [27–30]. The HK cumulative pore size distribution is presented in Fig. 4. Sample CF-R does not have any significant curve, confirming its nonporous nature. Samples ACF-PC2 and ACF-PC4 have steep slopes in the pore width range between 0.5 and 0.8 nm, which reveals that many pores exist with pore widths from 0.5 to 0.8 nm. This result agrees with the capacity of hydrogen adsorption from predictions of Monte-Carlo simulations [12–14]. The cumulative micropore volume of sample ACF-PC4 reaches about 0.3 cm3/g, and in the case of ACF-PC2 it approaches around 0.2 cm3/g. NaOH-activated carbon materials show similar trends with K2CO3-activated CNFs in Fig. 4(b). The clear changes of the curves are confirmed in the pore width range between 0.5 and 0.8 nm. The cumulative micropore volumes increased with increasing concentrations of the chemical agent solution. A summary of the textural properties is presented in Table 2. Both NaOH and K2CO3 activation have effects in increasing specific surface area, total pore volume, and micropore volume with increasing concentration of the used chemical agent solution. In the case of potassium carbonate activated carbon materials, the fractions of micropore volume are over 80%. This data is fit well with isotherm data from Fig. 3(a) [15].
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Fig. 3. Isotherms of samples; (a): CF-R, ACF-PC2, and ACF-PC4, (b): ACF-SH2, ACFSH4, ACF-SH6, and ACF-SH8, Po: saturation vapor pressure of the adsorptive.
The micropore volume fractions of NaOH-activated carbon materials are in the range of 50–90%. The micropore volume fraction decreased with increasing concentration of NaOH solution. This result agrees with the isotherm from Fig. 3(b) [15]. Thus, it is confirmed again that the fraction of micropore structure developed with low concentration of NaOH solution and regressed at high concentration of NaOH solution. 3.3. Investigation of hydrogen adsorption on ACFs The capacity of hydrogen storage of our samples is presented in Fig. 5. All of adsorption lines seem to be linear. The reason of that was explained in our previous work simply due to the low amount
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Fig. 4. HK cumulative micropore volume; (a): CF-R, ACF-PC2, and ACF-PC4, (b): ACFSH2, ACF-SH4, ACF-SH6, and ACF-SH8.
of hydrogen uptake [31]. There is no hysteresis in all samples indicating that adsorbed hydrogen can be desorbed reversibly [32]. NaOH-activated samples showed the higher hydrogen adsorption than K2CO3-activated samples. This is believed to be due to the higher specific surface area and micropore volume of NaOHactivated CNFs than in K2CO3-activated CNFs. Comparing ACF-PC4 with ACF-SH8, even though ACF-SH8 has 3.3 times more specific surface area and 4.1 times more total pore volume, the hydrogen storage capacity of ACF-SH8 is just 2.8 times higher than that of ACF-PC4. This may be due to the fraction of micropore volume (ACF-PC4: 83%, ACF-SH8: 51%). This clearly suggests that micropore volume favors the capacity of hydrogen adsorption better than specific surface area and total pore volume.
Table 2 The textual properties and hydrogen adsorption capacity of ACFs.
SBET (m2/g)a VT (cc/g)b VM (cc/g)c FM/V (%)d P AH ( wt.%)e a b c d e
CF-R
ACF-PC2
ACF-PC4
ACF-SH2
ACF-SH4
ACF-SH6
ACF-SH8
17.0 0.030 0.020 67 0.02
527.9 0.272 0.227 83 0.34
587.2 0.357 0.309 87 0.39
868.3 0.397 0.356 90 0.43
1804.4 0.991 0.575 58 0.68
1867.8 1.119 0.705 63 0.91
1933.2 1.459 0.751 51 1.05
SBET: BET specific surface area. VT: Total pore volume. VM: HK micropore volume. FM/V: Fraction of micropore. AH: Amount of hydrogen adsorption.
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Fig. 5. Hydrogen storage of samples; (a): CF-R, ACF-PC2, and ACF-PC4, (b): ACF-SH2, ACF-SH4, ACF-SH6, and ACF-SH8.
4. Discussion In Fig. 6, a comparison of hydrogen storage data from this study and other studies is presented. The samples studied in this paper were not modified using any catalyst for increasing the capacity of hydrogen storage, because the purpose of our study is to compare the capacity of hydrogen storage using various kinds of carbon materials. Other groups have used single walled carbon nanohorn (SWNH), single walled carbon nanotube (SWNT), graphic carbon nanofiber (GNF), and activated carbon (AC) as a hydrogen storage media [33,34]. In Fig. 6(a), three regressions are predicted from the results of our and other groups. Even though they show the same trend, where the hydrogen adsorption capacities increase with increasing specific surface area, the slopes of the regression lines have quite different values based on their unique shapes and pore structures. The slope of the regression from our study is around 1.5–2.0 times those from the other studies. In Fig. 6(b), the relation between the capacity of hydrogen storage and micropore volume is investigated. Although some of the data is scattered, a clear trend is observed. The capacity of hydrogen storage is proportional to micropore volume. Even though the capacity of hydrogen adsorption was measured at lower pressure than in the other studies, the slope of the regression line from our experiment is steeper than the other studies. Finally, it is clear that the samples made using electrospinning and chemical
Fig. 6. Hydrogen storage against (a) specific surface area and (b) micropore volume measured at 303 K and 5 MPa. Data of Ref. [33] was obtained at 303 K and 10 MPa; data of Ref. [34] was measured at 298 K and 6.5 MPa.
activation are more effective than other carbon materials as a hydrogen storage media. It seems that electrospun ACFs with thin diameters provide an optimized pore structure which can offer sites for hydrogen adsorption by chemical activation much more efficiently. The mechanism of hydrogen adsorption using various carbon materials is suggested in Fig. 7. In the case of using AC, macro and meso pores would not work for hydrogen adsorption because it is difficult to trap hydrogen molecules due to the large pore diameter [11]. In addition, there are large spaces which do not contain pores. When SWNT was used for hydrogen adsorption in Fig. 7(b), they can keep hydrogen molecules inside, outside, and between them, but there is a space restriction inside SWNT because of aspect ratios that are too high. It is difficult to create uniform and inner diameters and control the distance between the walls of SWNTs. Using graphite as a hydrogen storage media has space restrictions because intercalation and extraction of hydrogen molecules contained deep inside with a high aspect ratio are difficult, as shown in Fig. 7(c). Thus, adsorption and desorption reactions are easy only outside of graphite layers. In Fig. 7(d), electrospun ACFs might be expected to have an optimized pore structure with controlled pores size for effective hydrogen adsorption. This result may come from that the diameter of electrospun fiber can be controlled easily and optimized pore size can be obtained with highly developed pore structure.
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Fig. 7. The mechanism of hydrogen adsorption using various carbon materials; (a): AC, (b): SWNT, (c): Graphite, (d): Electrospun ACF.
5. Conclusions The electrospinning method and chemical activation were used to produce microporous carbon materials with a high specific surface area and total pore volume for use as a hydrogen storage media. The developed pore structure of the samples was obtained by chemical activation with decreasing fiber diameters. Even
though the hydrogen storage capacities increased with specific surface area and total pore volume, micropore volume mainly favors the amount of hydrogen adsorption. Comparing the hydrogen adsorption of electrospun activated carbon fibers with other carbon materials such as single walled carbon nanohorn, single walled carbon nanotube, graphite carbon nanofiber, and activated carbon, electrospun activated carbon fibers show better
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results for hydrogen storage, because they have an optimized shape that can produce sites with controlled pore size and high surface area for hydrogen adsorption. Acknowledgements This research was performed for the Hydrogen Energy R&D Center, one of the 21st Century Frontier R&D Programs, funded by the Ministry of Science and Technology of Korea. The authors would like to thank Mr. Tae-Sung Bae at KBSI (Korea Basic Science Institute) in Jeonju Center for providing FE-SEM images. References [1] L.L. Vasiliev, L.E. Kanonchik, A.G. Kulakov, V.A. Babenko, Int. J. Therm. Sci. 46 (2007) 914–925. [2] Y.S. Lee, Y.H. Kim, J.S. Hong, J.K. Suh, G.J. Cho, Catal. Today 120 (2007) 420–425. [3] H.K. Jin, Y.S. Lee, I.P. Hong, Catal. Today 120 (2007) 399–406. [4] H. Sakaguchi, K. Hatakeyama, S. Kobayashi, T. Esaka, Mater. Res. Bull. 37 (2002) 1547–1556. [5] T.K. Mandal, L. Sebastian, J. Gopalakrishnan, L. Abrams, J.B. Goodenough, Mater. Res. Bull. 39 (2004) 2257–2264. [6] F. Dolci, M.D. Chio, M. Baricco, E. Giamello, Mater. Res. Bull. 44 (2009) 194–197. [7] A. Anso´n, E. Lafuente, E. Urriolabeitia, R. Navarro, A.M. Benito, W.K. Maser, M.T. Martı´nez, J. Alloys Compd. 436 (2007) 294–297. [8] P. Be´nard, R. Chahine, Scripta Mater. 56 (2007) 803–808. [9] X. Ye, X. Gu, X.G. Gong, T.K.M. Shing, Z.F. Liu, Carbon 45 (2007) 315–320. [10] Ma Aili, Wang Xiaomin, Li Tianbao, Liu Xuguang, Xu Bingshe, Mater. Sci. Eng. A 443 (2007) 54–59. [11] B.J. Kim, Y.S. Lee, S.J. Park, Int. J. Hydrogen Energy 33 (2008) 2254–2259.
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