i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 3 5 6 6e3 5 7 3

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Co-electrospun Pd-coated porous carbon nanofibersfor hydrogen storage applications

Hongyeun Kim, Daehee Lee, Jooho Moon*

Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea

a r t i c l e i n f o

Article history:

Received 19 October 2010

Received in revised form

7 December 2010

Accepted 9 December 2010

Available online 7 January 2011

Keywords:

Carbon fiber

Electrospinning

Palladium coating

Nanoporous fibers

Hydrogen adsorption

* Corresponding author. Tel.: þ82 2 2123 285E-mail address: jmoon@yonsei.ac.kr (J. M

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2010.12.041

a b s t r a c t

Electrospinning produces sub-micron sized continuous fibers from polymer solutions or

melt by electric force. Due to its versatility and cost-effectiveness, this method has been

recently adopted for the fabrication of one-dimensional materials. Here, we fabricated

polyacrylonitrile (PAN) polymer fibers from which uniform nanoporous carbon fibers with

diameters of 100e200 nm were obtained after carbonization at 800 �C in Ar þ H2O. Water

vapor was injected during carbonization to be utilized as a nanoscale pore former. Addi-

tionally, a direct coating method using palladium nanoparticles on the carbon fibers was

developed. Palladium salt solution was electrosprayed during the electrospinning of the

polymer fibers. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were used

to confirm surface chemical composition and degree of carbonization. The specific surface

area of the palladium coated carbon fibers was 815.6 m2/g. Reversible hydrogen adsorption

capacity was determined to be 0.35 wt% at 298 K, 0.1 MPa.

Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction carbonmaterials. Bothsingle-wallCNTandmesoporouscarbon

Hydrogen is a promising energy alternative to fossil fuels be-

cause it is renewable and pollution-free. Among various tech-

nical issues, the identification of efficient hydrogen storage

mediaposesamajorhurdleto full-scaleexploitationofhydrogen

energy. Carbonmaterials such as carbonnanotubes [1], graphite

[2], and fullerene [3] have attracted significant interest due to

their overall safety, low mass density, and high reliability.

Among these, activated carbon and carbon fibers [4,5] provide

cost-effective storage media with reasonable storage capacity,

although more hydrogen can be absorbed on carbon nanotubes

(CNT).

Hydrogen adsorption on carbonmaterials occurs due toVan

der Waals interactions. The amount of adsorption is therefore

proportional to the surface area as well as pressure and

temperature. The effective surface area can be significantly

extended by introducing nanoscale pores on the surfaces of

5; fax: þ82 2 312 5375.oon).2010, Hydrogen Energy P

have extremely high surface areas of w1300 and w2000 m2/g,

respectively. These high surface area carbon materials are

usually synthesized by either vapor phase deposition or tem-

plating processes. However, their synthesis methods are

generally unsuitable for cost-effective mass production, which

prevents them from being utilized for hydrogen storage.

Electrospinning produces sub-micron sized continuous

fibers from a polymer solution or melt by electric force. Due to

its versatility and cost-effectiveness, this process has been

adopted to fabricate one-dimensional nanostructured mate-

rials. Recently, electrospun carbon fibers have been applied to

uses in fuel cells [6], supercapacitors [7], electrochemical

storage [8] and hydrogen storage media [9]. Porous carbon

fibers are capable of hydrogen storage as much as 2.5 wt% H2

at 300 K and 30 MPa [10].

The introduction of water vapor while carbonizing fibers

can assist with the formation of pores on the hydrophobic

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

Fig. 1 e Schematic diagram of the co-electrospinning

process.

60

70

80

90

100

t L

os

s (%

)

0

5

He

at F

lo

w (

42oC100%

ring reaction of PAN

a

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 3 5 6 6e3 5 7 3 3567

carbon surface by removing surface carbon atoms. The pore

formation reaction occurs as follows:

C(s) þ 2H2O / CO2 (g) þ 2H2 (1)

C(s) þ H2O / CO (g) þ H2 (2)

Molina-Sabio et al. reported that nanoporous carbon con-

taining pores less than 1 nm in diameter can be synthesized by

steam activation at high temperature (w800 �C), although the

amount of surface oxygen increases slightly [11]. These slit

pores contribute to the increase in the active surface area and,

in turn, enhance the hydrogen storage. Furthermore, surface

modification with a metal catalyst can also boost hydrogen

adsorption, either by the spillover effect [12] or metal hydride

formation [13]. The hydrogenmolecules that are preferentially

absorbed on the metal catalyst particles are dissociated, fol-

lowed by migration of hydrogen atoms to remote surface

sites on the carbon materials. In particular, Pd nanorpar-

ticles loaded on carbon materials can chemically absorb

hydrogenby forminghydride (PdHx) [14]. Particle sizealsoplays

an important role in determining hydrogen solubility. In the

present study, we prepare Pd nanoparticle-coated high surface

area carbon nanofibers by a one-step co-electrospinning

method. Pd salt containing solution was electrosprayed while

polyacrylonitrile (PAN) fibers are drawn from the core. Water

vapor was introduced during carbonization at 800 �C to obtain

nanoporous carbon fibers and, at the same time, Pd salt

attached to the PAN was reduced to Pd nanoparticles during

cooling. We performed structural and chemical analyses to

identify PAN-derived nanoporous carbon fibers and compare

the H2 adsorption abilities of carbon fibers prepared under

different synthesis conditions.

0 100 200 300 400 500 600 700 800 900

30

40

50

Temperature (o

C)

We

ig

h

31.45%-10

-5 W/g

)

0 100 200 300 400 500 600 700 800 900

30

40

50

60

70

80

90

100

Temperature (o

C)

Weig

ht L

oss (%

)

42oC100%

ring reaction of PAN

50.72%

-10

-5

0

5

He

at F

lo

w (W

/g

)

b

Fig. 2 e TG-DSC graphs of (a) as-spun PAN fibers and (b) Pd

salt coated PAN fibers.

2. Experimental procedures

To prepare the precursor solution for electrospinning, poly-

acrylonitrile (PAN, Mw ¼ 150,000, Aldrich) was dissolved in

dimethylformamide (DMF, anhydrous, 99.8%, Aldrich) at

60 �C. The viscosity of the polymer solutionwas in the range of

1600e2000 mPa s as determined by a rheometer (AR-2000EX,

TA Instruments). The polymer solution was electrospun

through a stainless steel nozzle (inner diameter¼ 0.51mm) by

applying an electric field of 1 kV/cm using a high-voltage DC

power supply unit (Nanotech) at a distance of 15 cm between

the nozzle and the drum collector. The polymer solution was

supplied by a syringe pump (KD100, KD Scientific) at a flow

rate of 1.0 ml/h. As-electrospun polymer fibers were dried at

80 �C for 24 h, followed by stabilization at 250 �C for 4 h in air

and then carbonization occurred at 800 �C for 2 h in an argon

atmosphere. During carbonization, fibers were exposed to

water vapor carried by Ar gas bubbled through a water bath.

The amount of water vapor was adjusted by controlling bath

temperature (40 �C) and exposure time to water vapor during

carbonization, which was varied from 0 h to 2 h.

Palladium solution of 1 wt% was prepared by dissolving

palladium chloride (PdCl2, 99.999% Aldrich) in a DMF solution

and the resulting solution was vigorously stirred at 60 �C to

ensure a complete dissolution. A dual concentric-type nozzle

was used to produce palladium coated carbon fibers. The PAN

solution was supplied through the inner part of the nozzle

(core), while the palladium solution was sprayed through the

outer part of the nozzle (sheath) as shown in Fig. 1. The

palladium salt coated fibers were stabilized at 250 �C for 4 h in

air, followed by carbonization at 800 �C for 2 h under a water

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 3 5 6 6e3 5 7 33568

vapor atmosphere carried by highly purified Ar gas. The

palladium ions were reduced by 4% H2 balanced Ar gas at

250 �C for 2 h during cooling.

The thermal decomposition behavior of the electrospun

polymer fibers was determined by a thermal gravimetric-

differential scanning calorimeter (TG-DSC, Q600, TA Instru-

ments) in a nitrogen atmosphere. The morphologies of the

stabilized and carbonized fibersweremonitored by a scanning

electron microscope (SEM, HITACHI S-4300, Hitachi) and

a transmission electron microscope (TEM, JEM-2000EX, Jeol).

Fig. 3 e TEM and SEM (inset) images of PAN-derived

nanoporous carbon fibers: (a) water vapor untreated fibers,

(b) 1 h water vapor treated fibers, and (c) 2 h water vapor

treated fiber.

Surface chemical structures of the carbon fibers were

analyzed by X-ray photoelectric spectroscopy (XPS, ESCALAB

22i-XL, VG Scientific Ins.) and Raman spectroscopy (LabRam

HR High Resolution, Jobin-Yvon). Specific surface area was

measured using the Brunauer-Emmett-Teller (BET) nitrogen

method (ASAP 2020, Micromeritics Ins.) and hydrogen stor-

age capacity was compared by measuring the adsorption

isotherm using high-purity dry H2 (99.999%) at both 77 K and

298 K.

3. Results and discussion

Fig. 2a shows the thermal decomposition behavior of the

electrospun polymer fiber. An abrupt weight loss (36 wt%)

occurred accompanying an exothermic peak in the vicinity of

310e340 �C, followed by a gradual weight loss. The exothermic

reaction at 310e350 �C can be ascribed to cyclization reaction

10 100 1000

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

H-K method

Diffe

re

ntia

l P

ore

V

olu

me

cm

3

/g

·

Pore size distribution (Å)

a

BJH method

10 100 1000

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

BJH method

Diffe

ren

tia

l P

ore

V

olu

me

cm

3

/g

·Å

Pore size distribution (Å)

b

H-K method

Fig. 4 e Differential plot of the BET analysis of (a) 2 h water

vapor treated carbon fibers and (b) 2 h water vapor treated

palladium coated carbon fibers. Carbon fibers are

preheated at 250 �C for 4 h in a vacuum prior to the

measurement.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 3 5 6 6e3 5 7 3 3569

of the nitrile groups [15]. Cyclization reactions cause the PAN

polymer to transform into a ladder-structured polymer by

polymer-analogue polymerization of the nitrile groups [16].

The stabilization process of the electrospun PAN fiber was

performed at 250 �C which was 50 �C lower than the temper-

ature involving a drastic weight loss based on the TG-DSC

result to maintain the fiber shape.

Electric field strength plays an important role in deter-

mining the morphologies of the electrospun PAN. At an elec-

tric field of 1.0 kV/cm, continuous and relatively uniform PAN

fibers with the diameter of 200e400 nm were produced as

confirmed by both SEM and TEM (data not shown). Under an

electric field strength condition of <0.6 kV/cm or >1.6 kV/cm,

on the other hand, we observed either bead formation or

larger-sized discontinuous fibers due to the instability of the

jet [17]. The fibers were shrunk to a diameter of 100e200 nm

after carbonization at 800 �C as shown in Fig. 3a. No other

noticeable morphological change or damage was observed.

Introduction of water vapor during carbonization did not alter

macroscopic fiber shape, but did produce nanoscale slit pores

on the carbon fiber surfaces (Fig. 3b). Extending water vapor

exposure time from 1 h to 2 h caused more pores to form, as

300 295 290 285 280 275

C=O (288.9V)

C-O (286.5eV)

Binding Energy (eV)

C-C (285eV)

a

b

300 295 290 285 280 275

Binding Energy (eV)

C=O (288.9V)

C-O (286.5eV)

C-C (285eV)

1200 800 400 0

Binding Energy (eV)

O 1s

N 1s

C 1s

1200 800 400 0

Pd 3d

O 1s

N 1s

Binding Energy (eV)

C 1s

Fig. 5 e XPS data of (a) 2 h water vapor treated carbon fibers

and (b) 2 h water vapor treated palladium coated carbon

fibers.

shown in Fig. 3c. An increased amount of adsorbed water

promotes pore formation on carbon surface as described in

Equations (1) and (2).

Water vapor exposure effectively expands the surface area

of the electrospun carbon fibers. The BET surface area for

carbon fibers not treated with water vapor was 15.1 � 6 m2/g,

whereas the surface area increased dramatically to 648.2 � 37

and 1120.8 � 27.2 m2/g as the water vapor exposure time

increased from 1 h to 2 h, respectively. This value is similar to

the theoretical value of single-wall CNT (1315 m2/g) [18]. Upon

further exposure to water vapor, the carbonized fibers were

completely burned out. Detailed pore structures of various

carbon fibers were analyzed by BET measurements. Fig. 4a

shows the pore size distribution for the carbon fibers treated

with water vapor for 2 h. Both Horvath-Kawazoe (HK) and

Barrett-Joyner-Halenda (BJH) analyses were utilized to reveal

the nanoporous (pore size 0.4e2.0 nm) and the mesoporous

(pore size 2e50 nm) characteristics. The differential pore

volume plot indicated that the water vapor treated carbon

nanofibers have a narrow pore size distribution. They con-

tained mainly sub-nm to 1 nm sized slit pores whose volume

fraction accounts for w90% of the total pore volume.

XPS analysis was performed under ultra high vacuum

(w1� 109 Torr) to study the surface chemistry of carbon fibers.

The wide scan spectrum indicated that water vapor treated

carbon fiber contained only oxygen, nitrogen, and carbon

(Fig. 5a). The C 1s spectrum can be deconvoluted into three

Gaussian distribution sub-peaks. The main peaks observed at

a binding energy of 285 eV in Fig. 5a can be attributed to carbon

single bond (CeC) [19]. The second peak at 286.5 eV originated

500 1000 1500 2000

D band

Binding Energy (eV)

a

G band

500 1000 1500 2000

b

Binding Energy (eV)

Fig. 6 e Raman spectroscopy results of (a) water vapor

untreated and (b) 2 h water vapor treated carbon fibers.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 3 5 6 6e3 5 7 33570

from the carboneoxygen single bond (CeO), whereas the third

peak observed at 288.9 eV represented the oxygen defects

caused by C]O or O]CeO bonds. These surface oxygens

likely result from adsorption during the stabilization process

in air. The surface composition calculated by the peak area

ratio was approximately 80% carbonecarbon bonds and 20%

carboneoxygen bonds. Carboneoxygen bonds reduce molec-

ular hydrogen absorption capacity on carbon surfaces because

CeO, C]O or O]CeO bonds are too chemically stable to

adsorb hydrogen molecules [20]. It is possible to reduce the

oxygen content on the fibers by raising the carbonization

temperature above 900 �C, but the specific surface area and

the synthesis yield were significantly reduced.

Fig. 6 shows Raman spectra of electrospun carbon nano-

fibers prepared under different atmospheres. The presence of

water vapor did not alter the Raman spectra. The two Raman

spectra (500e2200 cm�1) for the samples carbonized in dry

argon (Fig. 6a) and moisturized argon (Fig. 6b) were almost

identical. The peak at 1580e1600 cm�1 is called G band which

canbe assigned to the sp2 bonds. The stronger peak intensity of

theGband indicatesahigherdegreeof graphitization.Thepeak

positionedat 1350e1360 cm�1 corresponds to theD bandwhich

originated from the disordered carbon (a mixture of the sp and

sp3 bondsor thebrokenbond) [21]. For the carbonmaterials, the

relative intensity raio (R), defined as ID/IG where ID and IGrepresent the maximum peak intensity for D and G bands,

respectively, is indicative of the surface defect density. As R

shrinks, the number of sp2 bonds increases. The R value for the

carbon fiber obtained by dry carbonization was 1.24, whereas

0 2 4

c

In

te

ns

ity

(A

rb

. U

nit)

k

C

N

OPd

Fig. 7 e SEM images and the corresponding EDX spectrum of pa

0.95 for the water vapor treated carbon fibers since the peak

intensity of the D band declined. Raman analysis revealed that

the water vapor treated carbon fibers contain less surface

defects compared to those prepared by dry carbonization. The

surface defect concentration was reduced as the surface

carbon atomswere eliminated by thewatermolecules, leaving

behind the micropores on the surfaces.

Palladium coated carbon fiber was fabricated by a single-

step co-electrospinning process as shown in Fig. 7. At the stage

of stabilization at 250 �C in air, nanosized palladium salt

particles were distributed around the fibers as confirmed by

SEM images and energy dispersive X-ray (EDX) analysis in

Fig. 7. The peaks of carbon, nitrogen, oxygen and palladium

were observed for air-stabilized polymer fibers. Chlorine is

invisible since its peaks overlap with those of palladium. The

thermal decomposition behavior of the Pd salt coated polymer

fibers was similar to the PAN fibers as shown in Fig. 2b. DMF

solvent that was entrapped in the polymer fiber evapora-

ted below 200 �C and total weight loss was reduced. After

carbonization in moisturized argon and successive reduction

in a H2þAr atmosphere, Pd nanoparticles were uniformly

coated on the surface of the carbon nanoporous fibers as

shown in Fig. 8a and b. The TEM study revealed that Pd nano-

particles in a size range of 10e50 nm were attached on the

carbon surface. The macroscopic morphology of the nano-

fibers was unchanged during the co-spinning process. The

averagediameterof thefiberswere in the rangeof 100e200nm.

There was also no noticeable variation in the pore structure

of the co-electrospun carbon fibers except for the increased

6 8 10

eV

lladium salt coated PAN fibers after stabilization at 250 �C.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 3 5 6 6e3 5 7 3 3571

population of the larger pores. The EDX spectrum confirmed

that the co-electrospun fibers were composed of only carbon

and palladium, while a copper element resulted from the TEM

grid (Fig. 8c).

More detailed analyses of pore structure and surface

chemical composition were performed for the Pd-coated

nanoporous carbon fibers. The BET surface area was slightly

reduced to 815.6� 22m2/g as compared to the sample without

Pd co-electrospining. Thepresenceof Pd species on the surface

during the carbonization is thought to block the surfaces

exposed to water vapor and in turn to hinder the pore forma-

tion to some extent. A differential pore volume plot indicated

that the Pd-coated carbon nanofibers have a narrow pore size

distribution (Fig. 4b). The volume fraction of the micropores

reached 90% of the total pore volume and some large meso-

poreswere observed, as confirmedbyTEM (Fig. 8). XPS analysis

also indicated that the surface chemical structure of Pd-coated

nanoporous carbon fibers was similar to those without the Pd

coating, except for the presence of a palladium peak (Fig. 5b).

Thesurfacecomposition calculatedbasedon thedeconvoluted

peak area ratio was approximately 81% carbonecarbon bonds

and 19% carboneoxygen bonds. Pore structure and surface

chemical analyses confirmed that Pd nanoparticles were

successfully coated by co-electrospinning and the surface

coating/reduction was unable to alter the carbon structure,

except for the slightly decreased surface area.

Hydrogen sorption isotherm curves for the untreated

dense carbon fiber, water vapor treated microporous carbon

0 5 10

c

ke

C

Pd

Cu

In

te

ns

ity

(A

rb

. U

nit)

Fig. 8 e TEM images and the corresponding EDX spectr

fiber, and water vapor treated Pd-containing microporous

carbon fiber have been obtained at both 77 K (Fig. 9a) and 298 K

(Fig. 9b). At 77 K, the untreated carbon fiber and nanoporous

carbon fiber adsorbed 1.47 wt% H2 and 2.36 wt% H2, respec-

tively, at 800 mmHg (0.1 MPa). The higher surface area asso-

ciated with the water vapor treated carbon fibers lead to

increased H2 adsorption. Micropore size also contributes to

increases in hydrogen uptake since the equilibrium vapor

pressure of hydrogen is reduced for the small-sized pores

according to the Kelvin equation. For the Pd-coated nano-

porous carbon fiber, the hydrogen uptakewas slightly reduced

to 1.83 wt% H2. This can be explained by either slow adsorp-

tion kinetics of Pd at 77 K or by a decreased micropore volume

(from 0.425 to 0.3712 cm3/g) as Pd nanoparticles are attached

on the carbon surfaces [22].

Increased temperature greatly reduces the amount of

hydrogen adsorption. At 298 K, the untreated carbon fiber and

nanoporous carbon fiber adsorbed 0.06 wt% H2 and 0.10 wt%

H2, respectively, at 800 mmHg as shown in Fig. 9b. Both

samples exhibited little hysteresis behavior in adsorption and

desorption as observed in the sorption isotherm at 77 K. The

presence of Pd nanoparticles coated on the carbon fiber

significantly increased hydrogen uptake to 0.82wt%H2. Unlike

the other samples, the Pd-coated nanoporous fiber showed

hysteresis in which only 0.37 wt% of H2 was desorbed by

decreasing pressure. This observation indicated that 0.37 wt%

of H2 can be reversibly stored on the Pd-coated nanoporous

fiber while 0.45 wt% H2 remains on the fiber.

15 20

V

um of palladium coated carbon nanoporous fibers.

0 200 400 600 800

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Water vapor treated

Absolute pressure (mmHg)

Untreated

Water vapor treated with Pd coated

a

0 200 400 600 800

0.0

0.2

0.4

0.6

0.8 Water vapor treated with Pd coated Water vapor treated Untreated

Absolute pressure (mmHg)

b

Fig. 9 e Hydrogen adsorption capability determined by the

BETmethod at the temperatures of (a) 77 K and (b) 298 K for

various carbon fibers prepared at different conditions.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 3 5 6 6e3 5 7 33572

A TG ananlysis of palladium salt-coated PAN fiber was

performed to determine the Pd loading amount. The Pd-coated

fibers were pyrolzed in air at 900 �C where all of the carbon

species were eliminated, leaving behind the oxidized Pd.

Assuming that the remaining phase was PdO, we determined

the Pd loading content to be 8.7 wt% based on multiple

measurements. At room temperature and 1 atm of H2, a palla-

diumhydridewith a stoichiometry of 0.6H/Pd can form,which

corresponds to a hydrogen capacity of 0.56 wt% of bulk Pd

[23]. Accounting for the Pd loading in our carbon fiber (8.7 wt%)

as determined by TG analysis, the amount of hydrogen

adsorption in the form of palladiumhydride is calculated to be

0.05 wt% H2. The hydrogen physisorbed on the Pd-coated fiber

is proportional to the active surface area of the carbon surface,

whichwas calculated to bew0.09wt%H2. Thus, the remaining

hydrogen adsorption (w0.23wt%H2 ) out of the total reversible

adsorption occurred via a spillover effect or hydrogen accu-

mulation on the interface between Pd particle-carbon surface.

Molecular hydrogen physisorbs on carbon fiber in the

absenceofPdparticles. In thepresenceofPdparticles, however,

the hydrogenmolecules were dissociated over Pd surfaces due

to the incompletely occupied d-orbitals. Some of the chem-

isorbed hydrogen atoms dissolved to form a hydride. Dissoci-

ated hydrogen atoms can also migrate to the carbon support

near the PdeC interface. Some of these hydrogen atoms

form stable bonds with surface defects near the PdeC inter-

face, leading to irreversible adsorption (0.45 wt% H2). Other

hydrogen atoms can be reversibly desorbed by migration back

to Pd particles as well as those stored in the form of palladium

hydride [24]. This hydrogen spillover accounts for thehydrogen

adsorption of 0.23 wt% H2.

4. Conclusions

We demonstrated the synthesis of Pd nanoparticle-coated

carbon nanoporous fibers by one-step co-electrospinning.

Palladium salt solution was electrosprayed while the poly-

acrylonitrile (PAN) polymer fibers were electrospun. Uniform

nanoporous carbon fibers with Pd nanoparticles of 10e50 nm

in diameter were obtained by carbonization at 800 �C in

Ar þ H2O and reduction at 250 �C in H2 þ Ar. Water vapor was

utilized as a nanoscale pore former by injecting it during the

carbonization at 800 �C. The resulting carbon fibers had a sub-

nano scale pore structure, possessing a high surface area of

815.6e1120.8 m2/g, as determined by TEM observation and

BET surface area analysis. Raman spectroscopy and XPS

analyses also confirmed that the Pd-loaded carbon fibers are

highly carbonized (R ¼ 0.95) and are composed of 80% CeC

bonds. Hydrogen adsorption capacity of the palladium coated

carbon nanofibers was 2.36 wt% H2 at 77 K and 0.82 wt% H2 at

298 K, 0.1 MPa. The hydrogen adsorption at 298 K can be

divided into reversible uptake (0.37 wt% H2) and irreversible

uptake (0.45 wt% H2). Irreversible adsorption occurs by stable

bonding formation of the dissociated hydrogen atoms with

surface defects near the PdeC interface.

Acknowledgement

This work was supported by DAPA and ADD in Korea. It was

also partially supported by the Second Stage of the Brain Korea

21 Project.

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