Electrospun polyacrylonitrile-based carbon nanofibers and their hydrogen storages

*Corresponding Author. E-mail: [email protected]

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Macromolecular Research, Vol. 13, No. 6, pp 521-528 (2005)

Electrospun Polyacrylonitrile-Based Carbon Nanofibers and Their Hydrogen Storages

Dong-Kyu Kim, Sun Ho Park, and Byung Chul KimDivision of Applied Chemical Engineering, Hanyang University, Seoul 133-791, Korea

Byung Doo Chin, Seong Mu Jo*, and Dong Young KimOptoelectronic Materials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Korea

Received September 14, 2005; Revised November 21, 2005

Abstract: Electrospun polyacrylonitrile (PAN) nanofibers were carbonized with or without iron (III) acetylaceto-nate to induce catalytic graphitization within the range of 900-1,500 oC, resulting in ultrafine carbon fibers with adiameter of about 90-300 nm. Their structural properties and morphologies were investigated. The carbon nanofi-bers (CNF) prepared without a catalyst showed amorphous structures and very low surface areas of 22-31 m2/g. Thecarbonization in the presence of the catalyst produced graphite nanofibers (GNF). The hydrogen storage capacitiesof these CNF and GNF materials were evaluated through the gravimetric method using magnetic suspension balance(MSB) at room temperature and 100 bar. The CNFs showed hydrogen storage capacities which increased in therange of 0.16-0.50 wt% with increasing carbonization temperature. The hydrogen storage capacities of the GNFswith low surface areas of 60-253 m2/g were 0.14-1.01 wt%. Micropore and mesopore, as calculated using the nitro-gen gas adsorption-desorption isotherms, were not the effective pore for hydrogen storage.

Keywords: electrospun PAN nanofiber, carbon nanofiber, graphite nanofiber, hydrogen storage.

Introduction

Since hydrogen storage using carbon nano-materials wasfirst reported by Dillon et al.,1 several studies on hydrogenstorage using single wall nanotube (SWNT), multi wallnanotube (MWNT), graphite nanofiber (GNF), etc., havebeen conducted.2-4 The recent hydrogen storage capacity ofSWNTs measured using volumetric method, however,showed scattered capacities within the range of 0.03-4 wt%at room temperature because of introduction of some errorduring the measurement.5,6 The SWNTs in more accuratevolumetric measurement showed low capacity within therange of 0.14-0.43 wt%. Those of MWNT and graphitepowder were less than 0.04 wt%.7 Typical carbon materials,such as active carbon, active carbon fiber, and graphitepowder, were also investigated as potent materials forhydrogen storage.8,9 They have very low capacities at roomtemperature because their pores that are effective for hydro-gen storage are too few, although they generally have veryhigh surface areas.

There is a growing interest in the electrospinning processof polymer solution or melt because of their several potent

applications such as sensors, polymer electrolytes forlithium ion polymer battery, dye-sentized solar cell etc..10-14

Electrospinning is a powerful method of producing poly-meric ultrafine fibers within the range of a few nanometersto a few hundred nanometers in diameter, which cannot beeasily obtained using the traditional methods for makingfibers.15 Nano-sized fibers may be more helpful in obtainingcarbon materials with well-defined pore structure comparedto the carbonization of micro-sized polymer fibers. Carbonfibers have been used in a variety of fields as high-perfor-mance and functional materials. Woven or nonwoven car-bon fibers are used as absorbed materials because of betteradsorption capacity than conventional activated granularand powder carbon materials. They are being applied to gasseparation and liquid adsorption. Because thinner fibers aremore desirable for those separation and adsorption applica-tions, there has been growing interest in electrospinning toproduce ultrafine fibers. Thus, such electrospun polymericnanofibers may be used as effective precursors of carbonnanofibers. In addition, a transition metal would promotecarbonization and graphitization of polymeric precursor,which was verified for Kapton films by various researchgroups.16-18 Recently, we reported the effects of a transitionmetal, iron (III) acetylacetonate (IAA) on the carbonization

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522 Macromol. Res., Vol. 13, No. 6, 2005

behavior of the electrospun polyimide and polyacrlyonitrilenanofibers,19,20 resulting in graphite nanofiber (GNF).

In this study, carbon nanofibers (CNF) and graphite nano-fiber (GNF) were prepared through the carbonization ofelectrospun PAN-based nanofibers. Their structural proper-ties, morphologies, and hydrogen storage capacities wereinvestigated.

Experimental

A polyacrylonitrile (PAN) solution was obtained by dis-solving PAN (Mw 150,000, polyscience) in N,N-dimethyl-acetamide. The PAN solution contained 2, 5, and 7.5 wt%of iron (III) acetylacetonate (IAA) on the weight of thepolymer as a catalyst for graphitization. PAN-based nanofi-bers were prepared through the typical electrospinning pro-cess of the above PAN solutions. The resulted fibers werecoded as PAN-0, PAN-2, PAN-5, and PAN-7.5 according toIAA content of polymer solutions.

The electrospun PAN-based nanofibers were stabilized byheating them at 1 oC/min up to 260 oC, and by holding themfor 2 hrs under air atmosphere. Carbonization was performedat a given temperature within the range of 900 to 1,500 oC,under nitrogen atmosphere. The samples were heated at arate of 3 oC/min, and kept for 1 hr sequentially at 400 and600 oC. The resulted carbon fibers were coded as CNF,GNF-2, GNF-5, and GNF-7.5 according to IAA content.

The surface morphologies of carbon nanofibers wereobserved using an SEM. The WAXS patterns were scannedusing an X-ray diffractometer with CuKα within the rangeof 2θ=10-80 o. Raman spectra were obtained using MicroRaman with argon. The nitrogen gas adsorption-desorptionisotherms of the carbon nanofibers were measured using anautomatic gas adsorption apparatus (Themo, Sorptomatic1990). The BET surface areas were calculated from theadsorption isotherms. The pore size distribution was obtainedby applying the Horvath Kawazoe Pore sizes for microporous samples and the B.J.H. Pore sizes for meso poroussamples.

The hydrogen storage capacity of carbon nanofibers wasevaluated through the gravimetric method using magneticsuspension balance (MSB, Rubotherm) as shown in Figure1. First, the blank test chamber containing samples wasevacuated, to remove the impurities and water, at 150 oC/10-6

torr for 6 hrs. The weights of the sample basket and sampleswere then measured at 10-6 torr/25 oC (±0.5 oC) and at a Hegas atmosphere of 10 bars, respectively. It was assumed thatHe gas was not adsorbed by the carbon nanofiber samples inthis condition. The weight difference between the vacuumand the 10 bars He gas, which indicates buoyancy due to theHe gas, was used to determine the volume of the carbonnanofiber samples, as follows:

Vs= ΔW1/dHe (1)

where Vs is the volume of the samples, ΔW1 is the weightdifference of the samples between in the vacuum and at the10-bar He gas atmosphere, and dHe is the density of He at aspecific pressure and temperature.

The weights of the samples were measured under differentH2 pressures (10-100 bars) at 25 oC (±0.5 oC). The weights ofthe absorbed hydrogen were determined after the correctionof the buoyancy due to the hydrogen gas atmosphere, usingthe sample volume (Vs), as follows:

The weight of the adsorbed H2 = ΔW2 + Vs dH2 (2)

where ΔW2 is the weight difference of the samplesbetween in the vacuum and at the specific H2 pressure, Vs isthe volume of the samples, and dH2 is the density of H2 at aspecific pressure and temperature.

The densities of He and H2 gas for buoyancy correctionwere calculated from a real gas equation using the Thermo-dynamic and Transport Properties of Pure Fluid Program(NIST-supported).

Results and Discussion

Figure 2(a) shows electrospun PAN-based nanofibers witha diameter of about 90 nm, prepared using 6 wt% PAN in aDMAc solution, and carbon nanofibers (CNF) prepared at atemperature of 1,000-1,500 oC after stabilization under airatmosphere. PAN nanofibers with a diameter of 90 nm aftercarbonization at 1,300 oC hardly changed, while those witha diameter of about 240 nm remarkably shrank to 110 nmand showed roughened surfaces. Solvent evaporation duringthe electrospinning process greatly affected the internalstructure of the resulting PAN nanofibers. Thin fiber isdenser and has a higher orientation than thick fiber becauseit is formed at a much higher draw ratio and with a muchfaster solvent evaporation during the electrospinning pro-

Figure 1. Procedure for evaluation of hydrogen storage byGravimetric method using Magnetic Suspension Balance.


Macromol. Res., Vol. 13, No. 6, 2005 523

cess. The oxidation and carbonization of PAN nanofiberswere carried out under a tensionless condition or with aslight tension. Therefore, the dimensions of dense, highlyoriented ultra thin fibers are thought to hardly changeduring carbonization. The PAN-based carbon nanofibers inFigures 2 and 3(a) show very smooth surfaces. As shown inFigure 3(b), all PAN-based carbon nanofibers (CNFs)showed disordered, amorphous carbon structures withd002>0.37 nm, and had broad peaks structures in the XRDregardless of the carbonization temperature.

PAN nanofibers were also carbonized with IAA, to inducecatalytic graphitization, within the range of 900-1,500 oC,under nitrogen atmosphere. Figure 4(a)-(c) shows SEMimages of PAN-based graphite nanofibers (GNFs) preparedby heat treatment at 900-1,500 oC after the oxidation of PANnanofibers containing 2, 5, and 7.5 wt% of IAA, respectively.The catalytic graphitization of the PAN nanofibers producedGNFs with a diameter of 150-300 nm. At 1,100 oC, the GNFswere shown to have white spots on the surface of the fibers,indicating the development of graphite crystal structurescentered on the Fe catalyst. The TEM image around thewhite spot on the surface of the GNF showed a well-orderedgraphite structure similar to natural graphite.20 Since theGNFs, however, did not entirely have this graphite structure,its d002 was almost 0.34 nm, indicating a turbostratic carbonstructure.20

Figure 2. SEM images of the electrospun PAN-based carbon nanofibers.

Figure 3. (a) AFM image and (b) XRD of the electrospun PAN-based carbon nanofiber.

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Figure 4(a). SEM images of graphite nanofiber (GNFs) prepared through the catalytic graphatization (GNF-2).

Figure 4(b). SEM images of graphite nanofibers (GNFs) prepared through the catalytic graphatization (GNF-5).



The catalytic graphitization of PAN nanofibers containingIAA intensively proceeded from 900 oC. The GNF preparedat 1,100 oC showed sharp peaks at around 26 o (002) and 42-46 o (100), respectively. New peaks also appeared at 35 and50 o, corresponding to Fe3O4. It was assumed that IAA wasconverted into Fe3O4 via α-FeO (OH) during carbonization,and that the reduction of Fe3O4 at above 900 oC resulted inthe production of the α-Fe catalyst to be able to induce thegraphitization reaction of the PAN nanofibers. In the case ofthe GNF prepared at above 1,300 oC, most of the Fe3O4

transformed to the α-Fe. As shown in Figure 5, the GNFsusing higher contents of IAA obviously showed α-Fe peakat around 42-44 o (110) and 65 o (200). The Raman spectra ofthem showed that the relative intensity of the G band (1,580cm-1) over the D band (1,360 cm-1) increased with the increaseof the carbonization temperature. La (nm) greatly increased to4.1 nm (900 oC), 4.75 nm (1,100 oC), and 6.54 nm (1,300 oC).20

The catalytic graphitization was accelerated at above 1,300 oC.The CNFs and the GNFs were prepared without further

activation. Therefore, the CNFs showed the typical ad-sorption curves very similar to that of nonporous carbon inthe nitrogen gas adsorption-desorption isotherms. Those ofthe GNFs showed the typical curve of microporous carbon

in addition to a hysteresis loop that indicates existence ofthe mesopore, as shown in Figure 6. The CNFs and theGNFs had very low surface areas within the range of 22-31and 60-253 m2/g, respectively, as listed in Table I. TheGNFs showed much higher surface areas than the CNFs did.Their surface areas decreased with increase of carbonizationtemperature and increased with increase of IAA content.Although this could not be fully explained at present, it maybe due to the surface roughness and inhomogeneous struc-ture of the GNFs, which resulted from the induction of themetal catalyst in the GNFs. But they still had low surfacearea of 60-253 m2/g compared to common active carbon.

The pores in carbon materials are classified by their size

Figure 4(c). SEM and TEM images of graphite nanofibers (GNFs) prepared through the catalytic graphatization (GNF-7.5).

Figure 5. XRD of PAN based graphite nanofibers (GNF). Figure 6. The typical nitrogen gas adsorption-desorption iso-therms of (a) the CNF and (b) the GNF.

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into macropores (>50 nm), mesopores (>2-50 nm), andmicropores (<2 nm) according to IUPAC. Micropores arefurther divided into supermicropores with a size of 0.7-2 nmand ultramicropores of less than 0.7 nm.21 Unfortunately, itis difficult to exactly analyze the ultramicropore sizedistribution and volume in porous carbon through thenitrogen gas adsorption-desorption isotherms measurement.In the case of direct observations of pores on the surface ofcarbon materials by scanning tunneling microscopy (STM/AFM), the problem is how to differentiate the pores fromother surface defects, such as depressions and trenches. Thenet ultramicropore volume of carbon material cannot obtainfrom the pore analysis of small area on the surface bySTM.21 The GNFs had micropores unlike those in the CNFsin the nitrogen gas adsorption-desorption isotherms. In thecase of the CNFs the change of the pore volumes withincrease of carbonization temperature did not showedbecause of their very low surface area. The microporevolumes and mesopore volumes of the GNFs, however,decreased with increase of carbonization temperature, asshown in Table I. The oxidation and carbonization of PANprecursor fiber for making carbon fiber usually accompanywith the release of NH3, HCN, N2 gases, etc., resulting inthe formation of pores within the carbon fiber structure.21

And so carbon fiber with high tensile strength requires theremoval of pore structure by heat treatment to high tempera-ture. Therefore, we assume the generation of ultramicroporesand supermicropore during the carbonization of the CNFsand the GNFs precursors. The pore structure of the GNFsbecame dense with the increase of carbonization tempera-ture. Therefore, increase of carbonization temperature mightbring out the increase of ultra- and supermicropore volume

instead of loss of large pores.In the case of monolayer condensation of hydrogen on

carbon absorbents, theoretical quantity of absorption is 1.3� 10-5 mol/m2, and the quantity of reversible hydrogenabsorption is known to proportional to specific surface areaof absorbents. Commercial active carbons and active carbonfibers generally have very high surface areas of above1,000 m2/g, and SWNT also has a surface area of a fewhundred m2/g. They showed low storage capacities, however,within the range of 0.35-0.41 wt%, at room temperature.7Low storage capacity of carbon materials at ambienttemperature is due to low absorption potential betweencarbon and hydrogen. If the tendency that hydrogen is goingto escape from carbon absorbent is smaller than absorptionpotential, hydrogen will be absorbed as condensed phase bywhole micropore. It is predicted there are the optimum poresize and pore geometry for hydrogen absorption. Therefore,when the kinetic diameter of hydrogen (0.29 nm) is consi-dered, ultramicropores (<0.7 nm) are expected by doingimportant contribution for a hydrogen storage by means ofnanocapillary mechanism and superposing of potential onthe pore wall substantially. Hydrogen adsorption of theCNFs with very low surface area may not be expectedbecause of their low surface area. If they have ultramicro-pores, however, which may be effective for hydrogen storage,they will show hydrogen adsorption.

Figure 7 shows the hydrogen storage results of the CNFsand GNFs under several hydrogen pressures and at roomtemperature. Their hydrogen storages were measured after 2hrs under a specific hydrogen pressure. Their hydrogenadsorptions continuously increased even after 2 hrs, andalso increased under hydrogen pressure. The blank circle

Table I. Surface Areas and Pore Analysis for PAN-Based Carbon Nanofibers

Temperature (oC) IAA (wt%) Surface Area (m2/g)

Pore Size Distribution (cm3/g)

<1 nma 1-2 nma 2-4 nmb 4-10 nmb

CNF

1,000 0 31 - - 0.011 0.012

1,300 0 22 - - 0.012 0.012

1,500 0 22 - - 0.008 0.007

GNF

900 2 198 0.048 0.027 0.048 0.039

1,100 2 198 0.032 0.024 0.050 0.059

1,300 2 60 - - 0.021 0.046

1,500 2 65 0.008 0.007 0.023 0.039

900 5 253 0.044 0.027 0.047 0.060

1,100 5 247 0.065 0.030 0.050 0.084

1,300 5 116 0.025 0.017 0.040 0.066

1,500 5 109 0.030 0.012 0.034 0.056

1,300 7.5 163 0.047 0.019 0.041 0.062aDetermined by applying the Horvath Kawazoe Poresizes for microporous samples. bDetermined by applying the B.J.H. poresizes for mesoporous samples.



symbols in Figure 7(a) indicates the increased hydrogenstorage capacities after 16 hrs. The hydrogen storagescapacities of the CNFs were shown to be 0.37 and 0.50 wt%at 1,100 and 1,300 oC although the CNFs were nonporouscarbons with very low surface areas in the nitrogen gasadsorption-desorption isotherms. Therefore, the hydrogen

adsorption of the CNFs may be due to the presence ofultramicropores that cannot differentiate by using nitrogengas adsorption-desorption isotherms. The reduction ofhydrogen storage of the CNF(1,500 oC) is thought to be dueto the disappearance of pore structure including ultramicro-pore at higher carbonization temperature of above 1,300 oC.

In the case of the GNFs the hydrogen storage capacitiesincreased with increase of carbonization temperature andthe content of IAA catalyst, as shown in Figure 7(b)-(c).The hydrogen storages of GNF-5 showed highest capacityof 1.01 wt% at 1,300 oC and lowest capacity of 0.14 wt% at1,500 oC. The hydrogen storages showed highest at 1,300 oCand lowest at 1,500 oC similar to those of the CNF samplesthough the micro- and the mesopore volumes of them at1,300 oC were lowest, as shown in Table I. Increase of thecontent of IAA resulted in increase of surface areas and thehydrogen storage. The micro- and mesopore volume alsoincreased with increase of the content of IAA. The hydro-gen storage of GNF-7.5 at 1,300 oC, however, showed verylow storage of 0.32 wt% though it had higher surface area,micro- and mesopore volume than those of GNF-2 andGNF-5 at 1,300 oC. The hydrogen storage of CNF and GNFdid not show the correlation with surface area, micro- andmesopore volume in Table I. Therefore, we thought thatmicro- and mesopores that were calculated using the nitro-gen gas adsorption-desorption isotherms were not theeffective pore for hydrogen storage. The effective pore forhydrogen storage may require more small size, when com-pared to the kinetic diameter of hydrogen molecule sizes(about 0.29 nm). So, the hydrogen storage capacities of theGNFs and CNFs may be due to the presence of ultrami-cropores rather than micro- and mesopore even though theyhave very low surface areas compared to commercial activecarbons and active carbon fibers.

Fe metal catalyst in the GNFs may contribute to thehydrogen adsorption. Figure 8 showed the cycle property

Figure 7. (a) The hydrogen storages of PAN-based carbonnanofibers(CNF); (b) The hydrogen storages of PAN-basedgraphite nanofibers(GNF-2 and GNF-7.5); (c) The hydrogenstorages of PAN-based graphite nanofibers(GNF-5).

Figure 8. The cycle property about the hydrogen adsorption ofthe GNF-5 (1,300 oC) which were measured after 1 hr at 100 barof hydrogen pressure.

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about the hydrogen adsorption of the GNF-5 (1,300 oC), whichshowed highest storage capacity. The GNF-5 (1,300 oC) stillretained initial hydrogen capacity storages, indicating physi-sorption of hydrogen. However, about 0.078 wt% of hydro-gen did not desorpted under atmosphere and vacuum of 10-6

torr at room temperature. This is thought to be chemisorp-tions by Fe metal catalyst.

Conclusions

Carbon nanofibers (CNF) and graphite nanofibers (GNF)were prepared through the carbonization of the electrospunPAN-based nanofibers without further activation. All PAN-based carbon nanofibers showed disordered carbon structuresin the XRD regardless of the carbonization temperature.The catalytic graphitization of PAN nanofibers containingIAA was intensively accelerated at above 1,300 oC and withincrease of IAA content. The resulting GNF showed a well-ordered graphite structure with d002�0.34 nm, indicating aturbostratic carbon structure.

The CNFs showed the typical nonporous carbon in thenitrogen gas adsorption-desorption isotherms, while theGNFs were microporous carbon in addition to a hysteresisloop that indicates existence of the mesopore.

The CNFs showed hydrogen storage capacities of 0.16-0.50 wt% in spite of their very low surface areas of 22-31 m2/g. The GNFs had much higher surface areas of 60-253 m2/g and showed higher hydrogen storage capacities of0.14-1.01 wt% than did the CNFs. The hydrogen storagecapacities increased with increase of carbonization tempera-ture and the content of IAA catalyst. We thought that micro-and mesopores, which were calculated using the nitrogengas adsorption-desorption isotherms, were not the effectivepore for hydrogen storage. So, the hydrogen storage capaci-ties of the GNFs and CNFs may be due to the presence ofultramicropores rather than micro- and mesopore eventhough they have very low surface areas compared to com-mercial active carbons and active carbon fibers. The GNF-5(1,300 oC) still retained initial hydrogen capacity storages,indicating physisorption of hydrogen. But chemisorptionsof 0.078 wt% by Fe metal catalyst was also observed.

Acknowledgements. The Hydrogen Energy R & D Cen-ter in Republic of Korea financially supported this work.

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Electrospun polyacrylonitrile-based carbon nanofibers and their hydrogen storages