The surface decoration and electrochemical hydrogen storage of carbon nanofibers

Article abstract of DOI:10.1016/S0009-2614(03)00427-5

The tube-like carbon nanofibers (CNF) with cone-shaped structure were synthesized by catalytic pyrolysis of methane over Ni/MgO catalyst at 550°C for 30 min. The outer diameter of CNF with a rough surface increased to ～ 50-80 nm due to the deposition of Ni-P alloy particles in a ball shape. In the dark field under the selected electron diffraction, the Ni-P alloy particles with fine crystallites existed along the outer surface of CNF, like chain morphology. The heat treatment of CNF decorated with Ni-P alloy particles led to higher crystallization of surface alloy particles. The maximum discharge capacities of the composites with Ni-P content of 76.3 wt % before and after heat treatment were 101 and 149 mA h/g, respectively.

Full text of DOI:10.1016/S0009-2614(03)00427-5

Chemical Physics Letters 372 (2003) 336–341
www.elsevier.com/locate/cplett
The surface decoration and electrochemical hydrogen
storage of carbon nanofibers
a
a,
a
b
a,c
Xiaoqi Yan , Xueping Gao *, Ying Li , Zhanquan Liu , Feng Wu
,
d
Yutian Shen , Deying Song
a
a
Institute of New Energy Material Chemistry, Nankai University, Weijin Road 94, Tianjin 300071, China
b
Institute of Applied Chemistry, Nankai University, Tianjin 300071, China
National Development Center of Hi-Tech Green Materials, Beijing 100081, China
c
d
School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China
Received 29 December 2002; in final form 9 March 2003
Abstract
The tube-like CNFs with cone-shaped structure were synthesized by catalytic pyrolysis of methane. The outer
surface of purified CNFs was decorated with Ni–P alloy particles having polycrystalline or nanocrystalline structure
instead of amorphous structure. The low Ni–P content appeared to be more efficient to cover the outer surface of
CNFs. The electrochemical discharge capacity increased with increasing the Ni–P content on the outer surface of CNFs
owing to the synergistic effect between metal and carbon in the electrochemical reaction. The heat treatment contributed
to the higher crystallization of surface alloy and improvement of the electrochemical capacity of the composite.
Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction
reported to be very attractive as candidates for
hydrogen storage in electrochemical reactions
[4–7]. The electrochemical method for hydrogen
storage on these materials in alkaline solution is
more reasonable and feasible at ambient pressure
even though the capacity is smaller than that in
gas–solid reaction at high pressure. The acid
treatment and purification of carbon nanoscale
materials are shown to increase their hydrogen
storage capacities [6]. However, it is well known
that the electrocatalytic activity of carbon nano-
scale materials in alkaline solution is relatively
poor as compared with that of hydrogen storage
alloy. Our previous work showed that the deco-
ration of CNTs with metal particles can improve
AB5, AB2 and Mg-based hydrogen storage al-
loys have been investigated extensively as electrode
materials of Ni/MH batteries. The surface modi-
fication with metallic Pd, Ni and Cu has been
demonstrated to be effective in improving electro-
chemical properties of the hydrogen storage alloys
[1–3]. Currently, carbon nanoscale materials in-
cluding carbon nanotubes (CNTs), carbon nano-
fibers (CNFs) and activated carbons have been
* Corresponding author. Fax: +86-22-2350-2604.
E-mail address: xpgao@nankai.edu.cn (X. Gao).
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0009-2614(03)00427-5

X. Yan et al. / Chemical Physics Letters 372 (2003) 336–341
337
the electrochemical activity of its electrode [8]. The
discharge capacity of CNTs was measured and
calculated after eliminating the capacity of Ni–P
alloy.
In this work, the content of the Ni–P alloy on
the surface of CNFs was reduced and CNFs with
surface Ni–P alloy were subjected to a heat treat-
ment. In the meantime, both CNFs and Ni–P alloy
were considered as active materials to measure and
calculate their discharge capacities for eliminating
the disturbance of atomic hydrogen adsorbed on
the surface of Ni–P alloy layer.
the electrode were performed using a sintered
nickel electrode with a large capacity as a counter
electrode and an Hg/HgO electrode as a reference
electrode in 6 M KOH electrolyte at room tem-
perature in normal atmosphere. These electrodes
were fully charged at 1000 mA/g and discharged at
the same current density after a rest of 10 min. The
discharge cutoff potential was set to be )0.4 V vs.
Hg/HgO.
3. Results and discussion
The TEM images of purified tube-like CNFs
are displayed in Fig. 1. The long CNFs were
curved with an outer diameter of 30–50 nm and an
inner hollow core of 6–10 nm. The open tips were
observed clearly after eliminating the Ni catalyst.
The HRTEM image showed that the graphite
layers of CNFs were with an angle respected to the
fiber axis, which were different from the ideal
CNTs with a concentric cylinder structure. The
structure of the cone-shaped CNFs was similar to
that reported by several groups [11,12]. The open-
ended graphite sheets can be expected as the major
channel for the hydrogen storage [13]. In addition,
poor graphitization of CNFs was found, similar to
the multiwalled CNTs obtained from the thermal
decomposition of organic compounds.
TEM images of CNFs decorated with Ni–P
alloy (50.3 wt% carbon) are shown in Fig. 1. It
revealed in the bright field that most of CNF
surface was coated with smaller metallic nickel
particles. The deposited electroless nickel is a
metastable, supersaturated alloy consisting of P
atoms trapped between the nickel atoms as dis-
cussed by Ang et al. [10]. The outer diameter of
CNFs with a rough surface increased to about
50–80 nm due to the deposition of Ni–P alloy
particles in a ball shape. However, the hollow core
of some CNFs with Ni–P alloy particles was also
observed. The mean value of carbon content was
calculated to be 50.3 wt% through element analy-
sis. The ring patterns were observed using a se-
lected area electron diffraction (SAED) as inserted
in Fig. 2b, demonstrating that the smaller Ni–P
alloy particles have polycrystalline or nanocrys-
talline structure instead of amorphous structure.
2. Experimental details
CNFs were prepared via catalytic pyrolysis of
methane over Ni/MgO catalyst at 550 °C for 30
min. The as-prepared samples were soaked in 37%
HCl acid for 1 h to remove catalyst and then
rinsed with de-ionized water. The samples were
then dried at 110 °C for 10 h. The carbon nanof-
ibers were decorated chemically with metallic
nickel particles using SnCl2–HCl sensitizing solu-
tion, PdCl2–HCl activating solution and NiSO4–
NaH2PO2 plating solution. The detailed method
was presented previously [9,10]. The composite
of CNFs with Ni–P alloy particles was treated at
400 °C for 1 h in H2 atmosphere. A transmission
electron microscopy (TEM, TECNAI 20) was
employed to characterize CNFs with surface Ni–P
alloy. X-ray diffraction (XRD) measurements of
composites were performed using a Rigaku-D/
max-2500 with Cu Ka radiation. The carbon
content was measured and calculated according to
elementary analysis (Uario EL).
Twenty mg Ni-CNFs and 200 mg carbonyl
nickel powder (INCO 255) were mixed to fill the
substrate of porous nickel for preparing the elec-
trode to measure the electrochemical capacity. The
carbonyl nickel powder added here is to avoid
dropping of CNFs from the electrode and increase
the capability of current collecting. The electrodes
of carbonyl nickel powder and pure CNFs were
also prepared for blank measurements, respec-
tively. Both CNFs and Ni–P alloy were considered
as active materials in calculating the electrochem-
ical capacity. Electrochemical measurements of

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X. Yan et al. / Chemical Physics Letters 372 (2003) 336–341
Fig. 1. TEM (a) and HRTEM (b) images of the purified CNFs.
Fig. 2. TEM images of CNFs decorated with Ni–P alloy par-
ticles (Ni–P content is 49.7 wt%) in the bright field (a) and the
dark field (b). The selected area electron diffraction is inserted
in (b).
Moreover, it was observed in the dark field under
the selected electron diffraction that the Ni–P alloy
particles with fine crystallites existed along the
outer surface of CNFs, like chain morphology.
When the deposited Ni–P content increased to
76.3 wt% (as indicated in Fig. 3), the small Ni–P
alloy particles aggregated and grew up into a big
one. Some alloy particles existed off the outer
surface of CNFs. It was further confirmed in the
dark field that the Ni–P alloy particles with fine
crystallites were distributed disorderly and existed
separately, different from the chain-like morphol-
ogy along the outer surface of CNFs as observed

X. Yan et al. / Chemical Physics Letters 372 (2003) 336–341
339
have largely enhanced the hydrogen storage [14].
The Ni–P alloy particles could increase the elec-
trochemical activity of hydrogen storage for both
hydrogen storage alloy and CNTs [8,15,16]. Both
CNFs and Ni–P alloy were considered as active
composite materials in calculating the electro-
chemical capacity in order to eliminate the dis-
turbance of atomic hydrogen adsorbed on the
surface of Ni–P alloy layer. The pure Ni–P alloy
was also deposited on the surface of porous nickel
under the same process of electroless plating to
obtain the capacity of Ni–P alloy particles. It was
observed that the maximum discharge capacity of
the composites of CNFs and Ni–P alloy was
measured to be 85.5 and 101 mA h/g for the Ni–P
alloy content of 49.7 and 76.3, respectively, al-
though the low content of Ni–P alloy was more
efficient for covering the outer surface of CNFs.
The capacities of the pure CNFs and Ni–P alloy
were only 33 and 70 mA h/g, respectively, lower
than that of the composites as mentioned above. It
means that the synergistic effect between metal and
carbon may contribute to the higher hydrogen
storage in electrochemical reaction, similar to the
hydrogen storage in gas–solid reaction of nano-
sized Ni/ graphite composite [14].
The CNFs decorated with Ni–P alloy (23.7 wt%
carbon) were subjected to heat treatment in H2
atmosphere for 1 h. It was found from the XRD
patterns (not shown here) that the diffraction peak
at 26.2°, which corresponds to the diffraction
characteristics of graphite phase, is detected for
both samples before and after heat treatment.
There was no dramatic change in the diffraction
intensity for carbon diffraction peak. However, the
crystallization of Ni–P alloy particles on the outer
surface of CNFs before and after heat treatment
was obviously different. The Ni–P alloy particles
before heat treatment seemed to have the amor-
phous structure from only X-ray diffraction pat-
tern. The selected area electron diffraction with a
higher sensitivity confirmed that the Ni–P alloy
particles had polycrystalline or nanocrystalline
structure even though their crystallization was
relatively poor. After heat treatment, the crystal-
lization of Ni–P alloy increased and the main
phase and coexisted phase with sharp peaks were
assigned to the metallic Ni and Ni3P alloy phases,
Fig. 3. TEM images of CNFs decorated with Ni–P alloy par-
ticles (Ni–P content is 76.3 wt%) in the bright field (a) and the
dark field (b). The selected area electron diffraction is inserted
in (b).
above. Therefore, the low content of Ni–P alloy
appeared to be more efficient to cover the outer
surface of CNFs.
The nano-sized metallic Ni/graphite composites
prepared by arc-discharge method were found to

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X. Yan et al. / Chemical Physics Letters 372 (2003) 336–341
respectively. The TEM images of CNFs with Ni–P
alloy after heat treatment as illustrated in Fig. 4
demonstrated that coating layer particles were
adhered to the outer surface of CNFs and grown
up to 50–70 nm in diameter. The large crystal
grains shown in dark field using the selected elec-
tron diffraction were formed, further supporting
the higher crystallization as measured by XRD
after heat treatment. In addition, it was found that
the carbon content of CNFs decorated with Ni–P
alloy after heat treatment was reduced from 23.7
to 20.6 wt%.
The electrochemical discharge curves of the
composite of CNFs and Ni–P alloy before and
after heat treatment, as well as pure CNFs are
indicated in Fig. 5. The maximum discharge ca-
pacity obtained was 149 mA h/g of the composite
of CNFs and Ni–P alloy after heat treatment,
larger than that of the sample before heat treat-
ment. The heat treatment in H2 atmosphere could
enhance the attachment ability to the outer surface
of CNFs, which was of significance to the electric
conductivity and the electrocatalytic activity of the
composite electrode materials. Moreover, atomic
hydrogen generated by electrolysis at metallic sites
of the surface could diffuse easily through the
interface into the graphite layer of CNFs. Our
previous work using the cyclic voltammetry dem-
onstrated that the produced atomic hydrogen was
adsorbed preferentially at metallic sites of the
electrode surface in electrochemical reaction [8,16].
The programmed temperature desorption (TPD)
in gas–solid reaction at high temperature con-
firmed that the increased hydrogen storage indeed
partly comes from the highly dispersed metal for
Fig. 4. TEM images of CNFs decorated with Ni–P alloy par-
ticles after heat treatment (Ni–P content is 76.3 wt%) in the
bright field (a) and the dark field (b). The selected area electron
diffraction is inserted in (b).
Fig. 5. The electrochemical discharge curves of different ma-
terials under the discharge current density of 1000 mA h/g.

X. Yan et al. / Chemical Physics Letters 372 (2003) 336–341
341
nanosized Ni/graphite composite [14]. Therefore,
heat treatment in H2 atmosphere can improve the
electrochemical hydrogen storage of CNFs deco-
rated with Ni–P alloy particles.
Universiry and Tianjin University Joint Academy,
China.
References
4. Conclusions
[1] C.N. Park, R. Kirchheim, J. Alloys Compd. 182 (1992)
321.
The CNFs were synthesized by catalytic pyro-
lysis of methane over Ni/MgO catalyst. The
graphite layers of CNFs were with an angle re-
spected to the fiber axis, different from the ideal
CNTs with a concentric cylinder structure. The
ring patterns in a selected area electron diffraction
demonstrated that the smaller Ni–P alloy particles
decorated on the outer surface of CNFs have
polycrystalline or nanocrystalline structure instead
of amorphous structure. The electrochemical dis-
charge capacity increased with increasing the Ni–P
content on the outer surface of CNFs owing to the
synergistic effect between metal and carbon in the
electrochemical reaction. The heat treatment of
CNFs decorated with Ni–P alloy particles led to
higher crystallization of surface alloy particles.
The maximum discharge capacities of the com-
posites with Ni–P content of 76.3 wt% before and
after heat treatment were measured to be 101 and
149 mA h/g, respectively.
[2] X.P. Gao, Y.M. Sun, E. Higuchi, E. Toyoda, S. Suda,
J. Alloys Compd. 293–295 (1999) 707.
[3] D.B. Willey, I.R. Harris, A.S. Pratt, J. Alloys Compd.
293–295 (1999) 613.
€
€
[4] C. Nutzenadel, A. Nuttel, D. Chartouni, L. Schlapbach,
Electrochem. Solid-State Lett. 2 (1999) 30.
[5] X. Qin, X.P. Gao, H. Liu, H.T. Yuan, D.Y. Yan, W.L.
Gong, D.Y. Song, Electrochem. Solid-State Lett. 3 (2000)
532.
[6] H.S. Youn, H. Ryu, T.H. Cho, W.K. Choi, Int. J.
Hydrogen Energy 27 (2002) 937.
ꢀ
[7] K. Jurewicz, E. Frackowiak, F. Beguin, Electrochem.
Solid-State Lett. 4 (2001) A27.
[8] X.P. Gao, Y. Lan, G.L. Pan, F. Wu, S.S. Fan, D.Y. Song,
P.W. Shen, Electrochem. Solid-State Lett.
A173.
4 (2001)
[9] F.Z. Kong, X.B. Zhang, W.Q. Xiong, F. Liu, W.Z. Huang,
Y.L. Sun, J.P. Tu, Surf. Coat. Technol. 155 (2002) 33.
[10] L.M. Ang, T.S.A. Hor, G.Q. Xu, C.H. Tung, S.P. Zhao,
J.L.S. Wang, Carbon 38 (2000) 363.
[11] A. Chambers, C. Park, R.T.K. Baker, N.M. Rodriguez,
J. Phys. Chem. B 102 (1998) 4253.
[12] J.Y. Hwang, S.H. Lee, K.S. Sim, J.W. Kim, Synth. Met.
126 (2002) 81.
[13] Y. Chen, D.T. Shaw, X.D. Bai, E.G. Wang, C. Lund,
W.M. Lu, D.D.L. Chung, Appl. Phys. Lett. 78 (2001)
2128.
Acknowledgements
[14] Z.Y. Zhong, Z.T. Xiong, L.F. Sun, J.Z. Luo, P. Chen,
X. Wu, J. Lin, K.L. Tan, J. Phys. Chem. B 106 (2002)
9507.
This work was subsidized by 973 Programs
(2002CB211800), the National Key Program for
Basic Research (2001CCA05000), the Excellent
Young Teachers Program of MOE and the Chi-
nese Education Ministry Foundation for Nankai
[15] H.J. Chuang, S.L.I. Chan, J. Alloys Compd. 314 (2001)
224.
[16] X.P. Gao, J. Liu, S.H. Ye, D.Y. Song, Y.S. Zhang,
J. Alloys Compd. 253–254 (1997) 515.