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Yung-Huang Chang , Cheng-Te Lin , Tzu-Yin C
Wenjing Zhang , Kung-Hwa Wei , and Lain-Jon
Highly Effi cient Electrocatalyti
by MoS x Grown on Graphene-
Academia Sinica, Taipei 10617, Taiwan, ROC
Hydrogen energy is clean and serves as one of the most prom-
ising candidates for replacing petroleum fuels in the future.
Although the rare metals, such as platinum, have high effi ciency
The MoS x layer was then formed after subsequent annealing at
E-mail: lanceli@gate.sinica.edu.tw
C.-L. Hsu, Dr. K.-H. Wei
Department of Materials Science & Engineering
National Chiao Tung University
HsinChu 300, Taiwan, ROC
Dr. L.-J. Li
Department of Physics
National Tsing Hua University
Taiwan, ROC
DOI: 10.1002/adma.201202920
various temperatures (100, 120, 170, 200, 250, 300 ° C) in a H 2 /
Ar environment (500 torr; H 2 :Ar = 20:80) for 20 min. Figure 1 c
shows SEM images of the graphene-protected Ni foam grown
with the MoS x annealed at 120 ° C. These images suggest that
the MoS x layer exhibits almost full coverage over the graphene
surfaces, demonstrating that effi cient loading of MoS x on 3D
Ni foam is achievable. The high resolution SEM image in
Figure 1 d reveals that the surface of the deposited MoS x mate-
rials is very rough, and the MoS x materials are composed of
nanometer-scaled structures with large amounts of edges. The
Dr. Y.-H. Chang, Dr. C.-T. Lin, T.-Y. Chen, Dr. Y.-H. Lee,
Dr. W. Zhang, Dr. L.-J. Li
Institute of Atomic and Molecular Sciences
in the hydrogen evolution reaction (HER), their scarcity and
high cost inhibit large scale applications. [ 1–6 ] Recently, inorganic
catalysts such as nanometer-scaled MoS 2 and WS 2 have drawn
great attention due to their low cost, high chemical stability, and
excellent photocatalytic [ 7–24 ] and electrocatalytic properties in
HERs. They are potentially useful if they can be tailored for the
development of hydrogen energy devices. In order to enhance
the effi ciency of inorganic catalysts, many research efforts have
been made toward the modifi cation of material properties, [ 25 ]
the formation of composite catalysts, [ 26–31 ] and the fabrication
of the electrodes with nano-architecture. [ 30–33 ] Recently, MoS 2 /
reduced graphene oxide catalyst composites have been success-
fully made for enhancing the electrocatalytic HER effi ciency,
where the reduced graphene oxide sheets serve the function of
hosting MoS 2 as well as enhancing the conductance of the com-
posites. [ 31 , 34 ] However, most of the reported electrode materials
were still based on two-dimensional (2D) planar structures.
To improve the electrocatalytic HER effi ciency, it is crucial to
effectively increase the surface area for catalyst loading. Hence,
the research into three-dimensional (3D) electrode structures is
emergent. A three-dimensional graphene foam synthesized on
the Ni foam skeleton by chemical vapor deposition (CVD) has
been reported. [ 36 , 37 ] The graphene foam without the support of
an Ni skeleton is brittle and is not able to serve as a 3D elec-
trode for hosting catalysts. The 3D Ni foam is a low cost and
conductive metal with a high surface area, which is ideal for
use as a template to host catalysts for increasing the number of
reaction sites. [ 38–40 ] However, it suffers from instability in acidic
solutions, and thus is not suitable for the electrocatalytic HER.
Here, we report that the graphene sheets grown on Ni foams
© 2012 WILEY-VCH Verlag GAdv. Mater. 2012,
DOI: 10.1002/adma.201202920
hen , Chang-Lung Hsu , Yi-Hsien Lee ,
g Li *
c Hydrogen Production
Protected 3D Ni Foams
provide robust protection and effi ciently increase their stability
in acid. The highly conductive 3D graphene/Ni foam structure
also effectively increases the catalyst loading, leading to the
enhancement in electrocatalytic HER effi ciency. Meanwhile,
we formulated MoS x ( x ≥ 2) catalytic materials on graphene-
protected Ni foam to form a rigid 3D electrocatalytic architec-
ture, where the MoS x materials are grown by the thermolysis of
ammonium thiomolybdates at different temperatures in a CVD
chamber. The electrocatalytic HER of the MoS x /graphene/3D
Ni foam was performed in a 0.5 M H 2 SO 4 solution. The HER
current density for the MoS x /graphene/3D Ni foam, either nor-
malized by geometrical area or electrochemical surface area
(ESA), is higher compared with the MoS x on various planar
carbon electrodes including carbon paper, carbon cloth, and
graphene mats. X-ray photoelectron spectroscopy (XPS) anal-
ysis of the materials reveals that the higher HER effi ciency is
related to the presence of bridging S 2 2 − or apical S 2 − in amor-
phous states.
The three-dimensional Ni foam (110 ppi; thickness =
1.6 mm) was obtained from Nexcell battery Co. (Taiwan). The
growth of a few layers of graphene on the Ni-foam by CVD has
been reported elsewhere. [ 35 ] In brief, the Ni foams are reduced
with H 2 fl ow (100 sccm) at 1050 ° C for half an hour before the
CVD growth (gas ratio CH 4 :H 2 = 15:100; growth temperature
1050 ° C for 1 h; pressure 500 mtorr). Figure 1 a shows the scan-
ning electron microscopy (SEM) images for the as-obtained Ni
foam, where submillimeter pores can be clearly seen and the Ni
grains of the skeletons are observable at a higher magnifi cation.
Figure 1 b displays the SEM images after graphene layers are
grown on the surfaces of Ni foam. The Ni surfaces are fully cov-
ered with graphene layers and the graphene wrinkles are clearly
identifi ed. The Raman spectrum in Figure S1, Supporting
Information, proves that these graphene sheets are of a few
layers. To grow MoS x catalysts on the graphene surfaces, the
graphene-protected Ni foam was immersed in an ammonium
thiomolybdate solution (5 wt% of (NH 4 ) 2 MoS 4 in DMF). The
Ni foam was then backed on a hot plate at 100 ° C for 10 min.
mbH & Co. KGaA, Weinheim 1wileyonlinelibrary.com
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electrocatalytic HER is normally performed in acidic solu-
tions, where the Ni foam is not stable in acid. The dashed line
in Figure 1 d displays the polarization curve of the as-obtained
Ni foam electrode in a 0.5 M H 2 SO 4 solution, where the oxida-
tive potential (positive potential vs reactive hydrogen electrode,
RHE) induces a high current which indicates the dissolution
Figure 1 . a) SEM images for the as-obtained Ni foam, where sub-mil-
limeter pores can be clearly observed. b) SEM images of the Ni foam
surfaces after graphene layers are grown. c) SEM images of the graphene-
protected Ni foam grown with the MoS x annealed at 120 ° C. d) High
resolution SEM image of the sample in shown in (c). e) The polarization
curves of the as-obtained Ni foam electrode (dotted line), the graphene-
protected Ni foam electrode (solid line), and the graphene-protected Ni
foam electrode with MoS x grown at 120 ° C (dotted line). The measure-
ments were performed in a 0.5 M H 2 SO 4 solution. The current was nor-
malized by the geometrical area of the Ni foam.
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of Ni in acidic solutions. After CVD graphene layers are depos-
ited on Ni foam surfaces, the oxidative current is signifi cantly
suppressed (solid line). This observation proves that the CVD
graphene is able to protect the Ni foam from oxidative corro-
sion in acidic environments. The dotted line in Figure 1 d dem-
onstrates the polarization curve of the graphene-protected Ni
foam electrode grown with MoS x (annealed at 120 ° C), where
the negative current at a negative potential is the current associ-
ated with the HER. The inset is a photograph taken during the
electrocatalytic hydrogen reduction (potential: –0.2 V), where
the large (millimeter size) H 2 bubbles are formed due to ease of
merging of the evolved tiny H 2 gas bubbles in the pores of the
3D electrodes.
Figure 2 a shows the polarization curves (measured current
normalized by the geometrical area of the Ni foam) for the MoS x
prepared at different annealing temperatures. It is observed
that the HER effi ciency exhibits a maxima at T = 120 ° C and it
decreases with the further increase in annealing temperature.
The inset plots the current density at the applied potential of
0.2 V as a function of annealing temperature. It is noted that
the geometrical size of the Ni foam and the loading amount
of the MoS x are similar for each annealing temperature (see
Table S1, Supporting Information, for details). Figure 2 b replots
the polarization curves using the measured current normalized
by loading weight of MoS x and the electrochemical surface area
(ESA) (see Table S1 for details). The ESA was determined by the
reported method. [ 41 , 42 ] The HER effi ciency still shows a maxima
at T = 120 ° C, consistent with Figure 2 a.
A Tafel plot is normally used to evaluate the effi ciency of
the catalytic reaction. Table S2 and Figure S2, Supporting
Information, show the HER activity of the graphene-protected
Ni foam electrodes decorated with MoS x prepared at different
temperatures. These values were derived from the polarization
measurements. Figure 2 c shows that the Tafel slope for the
electrodes decorated with the MoS x formed at 120 ° C is the
smallest ( ≈ 42.8 meV dec − 1 ). The classical theory of hydrogen
generation [ 34 , 43 , 44 ] suggests that a Tafel slope of ≈ 40 meV dec − 1
indicates a low surface coverage of adsorbed hydrogen and
the reaction is as shown in Equation (1) and (2). The MoS x
prepared at a higher annealing temperature, e.g., 300 ° C,
results in a lower HER effi ciency and the reaction mechanism
moves to a larger surface coverage and the reaction follows
Equations (1) and (3). [ 34 , 35 ] Also, these results suggest that the
different current density values actually originate from the
various catalytic activities of MoS x catalysts formed at different
temperatures.
H3O
+ + e− + catalyst −→ catalyst-H + H2O (1)
H3O
+ + e− + catalyst-H −→ catalyst + H2 + H2O (2)
catalyst-H + catalyst-H −→ 2 catalyst + H2 (3)
To understand the differences between the MoS x catalysts
prepared at various temperatures, XPS was adopted to char-
acterize the chemical bonding structures. Figure 3 displays
the detailed XPS scans for the Mo and S binding energies for
these MoS x catalysts. The MoS x prepared at 300 ° C exhibits
two characteristic peaks at 232.1 and 228.9 eV, attributed to the
bH & Co. KGaA, Weinheim Adv. Mater. 2012,
DOI: 10.1002/adma.201202920
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Mo 3d 3/2 and 3d 5/2 binding energies for Mo 4 + . [ 45–47 ] The peaks,
corresponding to the S 2p 1/2 and 2p 3/2 orbitals of divalent
sulfi de ions (S 2 − ) are observed at 162.9 and 161.8 eV. [ 45–47 ] The
stoichiometric ratio (S:Mo) estimated from the respective inte-
grated peak area of XPS spectra is close to ≈ 2.09, suggesting
Figure 2 . Polarization curves for the MoS x prepared at different annealing
temperatures, where the current density is normalized by a) geometrical
area of the Ni foam, and b) both the loading weight of MoS x and the
electrochemical surface area (ESA). c) Tafel plot for the MoS x grown
(at 120 ° C) on a graphene-protected Ni foam.
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Adv. Mater. 2012,
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that the structure is close to MoS 2 . [ 48 , 49 ] When the annealing
temperature is lowered, in addition to the XPS peaks for the
MoS 2 structure, other sets of peaks are also observed. The
observation of Mo 3d 3/2 and 3d 5/2 binding energies at 233.1 and
230 eV suggests the presence of Mo 5 + ions. [ 45–47 ] Meanwhile,
the S 2p 1/2 and 2p 3/2 energies at 164.3 and 163.2 eV suggest
the existence of bridging S 2 2 − or apical S 2 − . [ 45 , 46 ] Although it
is not possible to exclusively identify the ratio between these
sulfur species due to their similar binding energies, the pres-
ence of these higher energy peaks indicate that the active HER
species are likely related to these species. It is noted that the
HER effi ciency for highly crystalline MoS 2 obtained at 1000 ° C
is very low (Figure S3, Supporting Information), indicating that
MoS 2 is less active for electrocatalytic HER. The S:Mo atomic
ratios for these samples are labeled in Figure 3 , from which
we conclude that the structure of the MoS x obtained at lower
temperatures such as 100, 120, and 170 ° C is stoichiometrically
close to Mo 2 S 5 . The transmission electron microscopy (TEM)
and X-ray diffraction analyses reveal that all the MoS x materials
obtained in the temperature range of 100 to 300 ° C are basi-
cally amorphous (data not shown). This evidence implies that
the higher HER effi ciency is related to the presence of bridging
S 2 2 − or apical S 2 − in amorphous states. We have also examined
the XPS spectra for the MoS x sample prepared at 120 ° C after
electrocatalytic hydrogen generation reaction (Figure S4, Sup-
porting Information). Interestingly, the the content of Mo 6 + and
Mo 5 + in a MoS x material is increased after electrocatalytic reac-
tion even though the Mo 6 + oxidation state does not exist before
the electrocatalytic reaction, in good agreement with the groups
of Tang [ 30 ] and Vrubel. [ 45 ] However, the binding energies of S 2 − ,
located at 161.7 and 162.8 eV, and energies for bridging S 2 2 − or
apical S 2 − , located at 163.2 and 164.3 eV, are still present after
electrocatalytic reactions, not showing any apparent change in
XPS peak profi le.
Figure 4 shows the measured hydrogen gas evolution rate
(mmol of H 2 normalized by the weight of the catalyst and
the geometrical area of the graphene-protected Ni foam) for
the MoS x prepared at various temperatures. The H 2 evolu-
tion rate normalized by catalyst weight and ESA is shown
in Figure S5, Supporting Information, for comparison. The
highest hydrogen production rate we have achieved so far
is around 13.47 mmol g − 1 cm − 2 h − 1 (302 mL g − 1 cm − 2 h − 1 ) at a
potential of V = 0.2 V for the MoS x obtained by annealing at
120 ° C. Note that the current density for the sample operated
at 0.2 V is around 45 mA cm − 2 . Moreover, Figure S6, Sup-
porting Information, shows the current density as a function of
hydrogen evolution time. The hydrogen production effi ciency
is superior to several recent HER reports based on MoS 3 parti-
cles, [ 45 ] amorphous MoS x prepared by electro-polymerization, [ 50 ]
and MoS 2 /reduced graphene oxides. [ 34 ]
The advantage of using a 3D Ni foam as an electrode is that
the loading weight of the MoS x catalyst is larger than other
carbon-based electrodes such as carbon paper, carbon cloth, and
graphite mats as detailed in Table S3, Supporting Information.
Figure 5 compares the polarization current density (normal-
ized by ESA) for the MoS x grown on various carbon electrodes,
where the graphene-protected 3D Ni foam exhibits the highest
current density. The superior performance is attributed to the
relatively high catalyst loading weight as well as the relatively
3wileyonlinelibrary.combH & Co. KGaA, Weinheim
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N in graphene-based 3D electrodes may fur-
ther advance the effi ciency of various elec-
trocatalytic reactions, which warrants more
investigations.
Experimental Section
Growth of Graphene on Ni Foam : Three-dimensional
Ni foam (110 ppi; thickness = 1.6 mm) was obtained
from Nexcell battery Co. (Taiwan). The Ni foams were
reduced with H 2 fl ow (100 sccm) at 1050 ° C for half
an hour in a CVD furnace before the CVD growth.
For the growth of graphene layers, a mixture of CH 4
and H 2 gases (ratio 15:100; pressure 500 mtorr) was
introduced to the furnace at 1050 ° C for 1 h.
Thermolysis to Form MoS x Catalysts : The graphene-
protected Ni foam was immersed in an ammonium
thiomolybdate solution (5 wt% of (NH 4 ) 2 MoS 4 in
DMF). The Ni foam was then backed on a hot-plate at
100 ° C for 10 min. The MoS x layer was then formed after
subsequent annealing at various temperatures (100,
protected Ni foam, an
a
lower resistance of the electrodes: the sheet resistance of these
carbon electrodes is listed in Table S3.
In summary, a 3D Ni foam deposited with graphene layers on
its surface was used as a conducting solid support to load MoS x
Figure 3 . XPS scans for the Mo and S binding energies of various MoS x cat
wileyonlinelibrary.com © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Wein
emission scanning e
were recorded by an
rate of 5 mV s − 1 in a
using an Ag/AgCl (
a graphite rod as th
foam samples as th
electrolysis measurem
reversible hydrogen e
catalysts for electrocatalytic hydrogen evolution. The graphene
sheets grown on Ni foams provide robust protection and effi -
ciently increase its stability in acid. The hydrogen evolution rate
reaches 302 mL g − 1 cm − 2 h − 1 (13.47 mmol g − 1 cm − 2 h − 1 ) at an
overpotential of V = 0.2 V and the catalytic species were likely
related to the bridging S 2 2 − or apical S 2 − . The developments
Figure 4 . The measured hydrogen gas evolution rate normalized by the
weight of the catalysts and the geometrical area of the graphene-pro-
tected Ni foam.
0 5 10 15 20 25 30 35
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Figure 5 . The polar
MoS x (annealed at 1
graphene-protected
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d 3D MoS x /graphene/Ni foam was examined by a fi eld-
lectron microscope (JSM-6500F). Polarization curves
AUTOLAB pontentiostat (PGSTAT 302N) with a scan
0.5 M H 2 SO 4 solution. A three-electrode confi guration
KCl saturated) electrode as the reference electrode,
e counter electrode, and the 3D MoS x /graphene/Ni
e working electrode was adopted for polarization and
ents. In 0.5 M H 2 SO 4 , potentials were referenced to a
lectrode (RHE) by adding a value of 0.21 V.
ization current density (normalized by ESA) for the
20 ° C) grown on various carbon electrodes, where the
3D Ni foam exhibits the highest current density.
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otential (V) vs