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Hydrogen Adsorption of PAN-based Porous Carbon Nanofibers using MgO as the Substrate
Hydrogen Adsorption of PAN-based Porous Carbon Nanofibers using MgO as the Substrate
Carbon letters. 2009. Sep, 10(3): 217-220
Copyright ©2009, Korean Carbon Society
  • Received : June 06, 2009
  • Accepted : August 08, 2009
  • Published : September 30, 2009
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About the Authors
Min-Jung Jung
Dept. of Applied Chemistry and Biological Engineering, Chungnam National University, Daejeon 305-764, Korea
Ji Sun Im
Dept. of Applied Chemistry and Biological Engineering, Chungnam National University, Daejeon 305-764, Korea
Euigyung Jeong
Dept. of Applied Chemistry and Biological Engineering, Chungnam National University, Daejeon 305-764, Korea
Hangkyo Jin
Korea Research Institute of Chemical Technology, P.O. Box 107, Yuseong, Daejeon 305-600, Korea
Young-Seak Lee
Dept. of Applied Chemistry and Biological Engineering, Chungnam National University, Daejeon 305-764, Korea
youngslee@cnu.ac.kr
Abstract
In this study, porous electrospun carbon fibers were prepared by electrospinning with PAN and MgCl 2 , as a MgO precursor. MgO was selected as a substrate because of its chemical and thermal stability, no reaction with carbon, and ease of removal after carbonization by dissolving out in acidic solutions. MgCl 2 was mixed with polyacrylonitrile (PAN) solution as a precursor of MgO with various weight ratios of MgCl 2 /PAN. The average diameter of porous electrospun carbon fibers increased from 1.3 to 3 ㎛, as the MgCl 2 to PAN weight ratio increased. During the stabilization step, MgCl 2 was hydrolyzed to MgOHCl by heat treatment. At elevated temperature of 823 K for carbonization step, MgOHCl was decomposed to MgO. Specific surface area and pore structure of prepared electrospun carbon fibers were decided by weight ratio of MgCl 2 /PAN. The amount of hydrogen storage increased with increase of specific surface area and micropore volume of prepared electrospun carbon fibers.
Keywords
1. Introduction
Diversity of porous carbon materials produced so far is remarkable, due to their variety of applications, such as adsorbents, catalytic supports, and etc. [1] and activated carbons are one of the important materials for porous carbon materials. Activated carbons can be produced from various carbonaceous materials. Polymer precursors are especially preferred when carbon with low inorganic impurities is needed. The most well studied polymer is PAN (polyacrylonitrile). PAN-based activated carbons have drawn increasing attention because of their excellent surface properties and adsorption capacity [2] .
Porous carbon fibers with smaller fiber diameter have advantages, such as narrower pore size distribution and better adsorption capacity at low concentration of adsorbates compared than conventional activated carbons [3] . The preparation of porous carbon materials needs one more step activation of carbon materials with KOH [4 , 5] , NaOH [6 , 7] or oxidizing gas [8] . However, there have been reported various precursors and processes to prepare porous carbons without any activation process. Porous carbons were prepared by using various templates, such as zeolites and silicas [9 - 11] . The template had to be dissolved out by strong acids after carbonization, so its mass production was not easy. A simple process was developed to prepare porous carbon materials from PVA (polyvinyl alcohol) or PVC (polyvinyl chloride) using MgO as the substrate [12 , 13] . MgO was selected as a substrate because of its chemical and thermal stability, no structural and compositional changes, no reaction with carbon, and convenience of dissolving out in acidic solutions [14] . In this experiment, MgCl 2 was newly used to prepare porous carbon fibers as a MgO precursor, because using MgCl 2 as a MgO precursor was easier to control pore size distribution than using MgO directly [14] . This suggested a new preparation process of porous carbons without activation processes.
Therefore, in this study, porous electrospun carbon fibers were prepared to apply as a hydrogen carrier by electrospinning with a PAN and MgCl 2 solution. The capacity of hydrogen storage was compared by using these electrospun fibers based on the investigation of their pore structure and shape.
2. Experimental
- 2.1. Materials and Sample Preparation
As a MgO precursors, MgCl 2 (magnesium chloride anhydrous,Junsei, assay: 97.0%) was used and PAN (polyacrylonitrile, Aldrich) was used as a carbon precursor. PAN was dissolved in DMF (N,N-dimethyl formamide, Acros) at 353 K, with 10 wt% concentration. MgCl 2 was added to the PAN solution with various mixing ratios of MgCl 2 /PAN; 1/3, 2/3, and 3/3, respectively. And the mixed solution was used for electrospinning (voltage: 17 kV, syringe rate: 1.5 cc/h). Then, electrospun fibers were stabilized at a temperature of 523 K for 8 hr in air. Stabilized fibers were
Lager Image
The schematic diagram of procedure for PAN-based porous carbon nanofibers.
heated at 1323 K for 1 hr under nitrogen atmosphere for carbonization. After carbonization, the substrate MgO was dissolved out of electrospun fibers with 1mol/L H 2 SO 4 . The schematic diagram of this procedure is illustrated in Fig. 1 . The final electrospun fiber products were called EM1, EM2, and EM3, based on the mixing ratios of MgCl 2 /PAN.
- 2.2. Characterization
The morphologies of samples were observed with Scanning Electron Microscope (SEM, JSM-6300, JEOL LTD, Japan). The Samples were subjected to X-ray diffraction (XRD, Model D/MAX-2200 Ultima/PC, CuKα Rigaku, Japan) and thermo gravimetric analysis (TGA, Mettler?Toledo, TGA/SDTA851, Switzerland) in N 2 from room temperature to 1073 K, with heating rate of 10 K/min. And Pore structure of carbons was assessed by N 2 adsorption with weight ratio of MgCl 2 . To test hydrogen adsorption capacities, samples were degassed at 473 K for 2 hr and hydrogen adsorption was conducted at 303 K from 0 to 50 atm by using volumetric method with PCT apparatus (Mirae SI Co., Korea).
3. Results and Discussion
Fig. 2 shows the morphologies of porous electrospun carbon fibers prepared in this study. Average diameter of porous electrospun carbon fibers increases from 1.3 to 3 ㎛ with increasing the amount of MgCl 2 added in polymer solution. It is believed that surface tensions, viscosities, and conductivities of polymer solution are changed with the polymer solution composition change [3] , and those changes of polymer solution also increase or decrease average diameter of electrospun fibers.
Fig. 3 presents the XRD spectra of MgCl 2 in electrospun carbon fibers at each preparation step. Other work by the
Lager Image
SEM images of porous electrospun carbon fibers various mixing ratios in MgCl2/PAN: (a) 1/3; (b) 2/3 and (c) 3/3.
authors [15 - 17] describes the procedure to produce MgOHCl from chemical grade MgCl 2 . This is in good agreement with our results as shown in Fig. 3 . During the stabilization step ( Fig. 3 (b)), hydrolysis of MgCl 2 is enacted by heat treatment around 458~513 K. Cl produced HCl and then removed from electrospun fiber as a gas state [15] .
Lager Image
At the elevated temperature for carbonization step,remained MgOHCl in electrospun fiber is decomposed to MgO and HCl [16] .
Lager Image
The decomposition temperature of MgOHCl is calculated to be 828 K [17] . As we can see (c) in this Fig. 3 , the MgOHCl mostly decomposed to MgO. The characteristic peaks of MgOHCl at Bragg angles (2 theta) of roughly 15, 32, and 55 degrees diminished after carbonization and eventually completely
Lager Image
XRD patterns: (a) pure MgCl2; (b) stabilized electrospun fiber (EM2); (c) carbonized electrospun carbon fiber (EM2) and (d) electrospun carbon fiber (EM2) after dissolving out MgO using 1 mol H2SO4.
Lager Image
Thermogravimetric curves of (a) stabilized electrospun fiber with MgCl2 (EM2) and (b) stabilized electrospun PAN fiber.
disappeared when it was dissolved out by using H 2 SO 4 .
Thermal gravimetric analysis (TGA) was used to study the decomposition of MgOHCl. Fig. 4 is thermogravimetric curves of stabilized electrospun fiber prepared from MgCl 2 /PAN (2/3) solution ((a); EM2) and stabilized electrospun PAN fiber ((b); without MgCl 2 ). Serious weight loss of the samples EM2 is observed at around 823 K in TGA results, even though the weight of stabilized electrospun PAN fiber (b) decreased linearly with the temperature. This weight loss caused by decomposition of MgOHCl, which occurred around this temperature due to the change from MgOHCl to MgO. This is also supported by XRD results shown in Fig. 3 as mentioned earlier.
Fig. 5 , 6 and Table 1 show the textual properties of porous electrospun carbon fiber prepared. MgO in electrospun carbon fiber was dissolved out completely by using H 2 SO 4 (1 mol/L) and ended up pores in the carbon fiber. The surface area of electrospun fiber increased dramatically, as
Lager Image
Nitrogen adsorption-desorption isotherms various mixing ratios in MgCl2/PAN.
Lager Image
DFT pore size cumulative volume various mixing ratios in MgCl2/PAN.
the weight ratio of MgCl 2 /PAN increased. It is believed that weight ratio of MgCl 2 /PAN determines specific surface area and pore structure. According to the adsorption isotherm by IUPAC classification [18] , the sample EM1 is showing Type II, whereas the other samples are showing Type I. It is shown that carbon fiber has almost mesopores, when weight ratio of MgCl 2 /PAN is 1/3. Whereas on the other weight ratios of MgCl 2 /PAN is 2/3 and 3/3, dissolving out MgO decomposed from MgCl 2 mainly create micropore on electrospun carbon fiber surface. Both EM2 and EM3 samples have the steep change of isotherm curves in low relative pressure (less than 0.01 bar), showing they have the developed micropore structure [19] . They also have the slow change of isotherm curves over 0.1 of relative pressure, indicating the mesopore also was developed through dissolving MgO.
Fig. 7 shows the hydrogen adsorption capacity with changes of the weight ratio of MgCl 2 /PAN. It is observed that hydrogen adsorption capacity is a linear function of pressure. As the specific surface area and micropore volume increase,
Pore Parameters Determined by BET and t-plot on the Electrospun Carbon Fibe1)SBET: BET specific surface area,2)VT: Total pore volume,3)VM: t-plot micro pore volume
Lager Image
Pore Parameters Determined by BET and t-plot on the Electrospun Carbon Fibe 1)SBET: BET specific surface area, 2) VT: Total pore volume, 3)VM: t-plot micro pore volume
Lager Image
H2 adsorption isotherms of various mixing ratios in MgCl2/PAN.
the hydrogen adsorption of the materials also increases. This result indicates that hydrogen adsorption capacity is positively related to the pore volume in the range of micropore [3 , 20] . Eventually, the use of electrospinning method from PAN and MgCl 2 as the precursor of MgO can be surely one of the way to apply for the hydrogen storage.
4. Conclusions
Porous electrospun carbon fibers were prepared by using PAN/MgCl 2 mixture solution with various weight ratios. Average diameter of porous electrospun carbon fibers increased from 1.3 to 3 ㎛ with increasing the amount of MgCl 2 added. During the stabilization step, MgCl 2 was hydrolyzed to MgOHCl by heat treatment. This MgOHCl was decomposed to MgO at elevated temperature of 823 K for carbonization step. Specific surface area and pore structure of prepared electrospun carbon fiber were decided by weight ratio of MgOHCl/PAN. When weight ratio of MgCl 2 /PAN is 1/3, carbon fiber has almost mesopore. While on the other weight ratios of MgCl 2 /PAN are 2/3 and 3/3, carbon fibers have micropores. It is surely suggested that these developed pore structure developed from removing MgO electrospun fiber. The amount of hydrogen storage increased with increase of specific surface area and micropore volume of prepared electrospun carbon fibers. Therefore, this preparation method is expected to be prepared porous materials that it can adsorb hydrogen gas.
Acknowledgements
The authors gratefully acknowledge the 21C Hydrogen Frontier R&D Program of the Ministry of Education, Science and Technology of Korea.
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