Enhanced electrocatalytic activities of pulse-mode potentiostatic electrodeposited single-crystal, fern-like Pt nanorods

Article abstract of DOI:10.1039/c4ra04153d

Single-crystal and fern-like Pt nanorods (Pt-NR) were electrodeposited on carbon paper. The electrocatalytic activity of prepared Pt-NR electrodes, as determined by the catalyst mass activity (MA) and specific activity (SA) for the methanol oxidation reaction, are respectively 4.59 times and 10.54 times better than that of a commercial Pt-black catalyst. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra04153d

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Enhanced electrocatalytic activities of pulse-mode
potentiostatic electrodeposited single-crystal,
fern-like Pt nanorods†
Cite this: RSC Adv., 2014, 4, 31424
Received 5th May 2014
Accepted 4th July 2014
a
a
b
ad
a
Huei-Yu Chou, Ming-Chi Tsai, Sung-Yen Wei, Yu-Hsuan Wei, Tsung-Kuang Yeh,
a
c
a
d
Fu-Rong Chen, Chen-Chi M. Ma, Chuen-Horng Tsai* and Chien-Kuo Hsieh*
DOI: 10.1039/c4ra04153d
www.rsc.org/advances
Single-crystal and fern-like Pt nanorods (Pt-NR) were electro- known to correlate strongly with its morphology and
4,5
deposited on carbon paper. The electrocatalytic activity of prepared crystallinity.
Pt-NR electrodes, as determined by the catalyst mass activity (MA) and For the electrocatalyst in a fuel cell to exhibit high efficiency,
specific activity (SA) for the methanol oxidation reaction, are respec- three qualities are required: (1) large specic area, (2) three
tively 4.59 times and 10.54 times better than that of a commercial Pt- phase contact (including electronic conductivity through the
black catalyst.
catalyst support), and (3) high durability during and aer MEA
assembling. In our previous study, we demonstrated that
6
carbon nanotubes (CNT) grown directly on carbon cloth could
possess high specic surface area and display uniform catalyst
Platinum (Pt) is a very important material for many applica-
tions, and syntheses of size- and shape-controlled Pt or Pt-
embedded nanostructures are among today's most important
research topics. Because of their excellent physical and chem-
ical characteristics, they have been widely used as catalysts in
many areas such as ne chemical synthesis, hydrogen produc-
(Pt or its alloy) dispersion, and therefore exhibit high catalytic
efficiency. However, damage to (and indeed, collapse of) the
catalytic CNT layer is likely to occur during the MEA assembly,
7
suggesting the importance of an alternative assembly method.
It has been found that Pt has distinct electrocatalytic activity
1,2
tion, gas sensors and electrocatalysis.
8
on different crystalline planes, suggesting the maximum elec-
Among the important applications, Pt is the key catalyst in
trocatalytic activity may be achieved by controlling single crys-
tallinity in fabricating the catalysts. To date, most relevant
research has focused on syntheses of Pt nanocrystals with
3
proton exchange membrane (PEM) fuel cells. The core
component in such a fuel cell is the membrane electrode
assembly (MEA), which generally comprises a thin, at proton
exchange membrane with catalyzed electrodes bonded to both
sides. Conventionally, the electrodes are made of electro-
nconducting carbon cloth or carbon paper (CP) plated with a
mixture of carbon-black-supported catalyst particles and a
proton-conducting ionomer. For a PEM fuel cell to operate
efficiently, the catalyst must be both highly dispersed and
nanoscale in size so as to optimize the rate of hydrogen or
methanol oxidation reaction at the anode. Supporting evidence
for these requirements is that the activity of a nanocatalyst is
9
1,10
specic crystallinities, mainly polycrystalline structure. For
example, tetrahexahedral Pt nanocrystals with high-index facets
9
have been synthesized by electrochemical approaches. Very
recently, however, some single-crystal Pt structures with unique
anisotropic structures, such as one-dimensional nanowires and
nanorods, have been reported and have attracted considerable
attention because of their attractive surface properties and
11–13
superior catalytic efficiencies.
For example, Xia and
coworkers generated single-crystal Pt nanowires by a polyol
process combined with the introduction of a trace amount of
11
ionic iron species with PVP (used as a surfactant). Song and
coworkers produced Pt nanowires networks by chemical
a
Department of Engineering and System Science, National Tsing Hua University,
12
reduction using a so template. Sun and coworkers synthe-
Hsingchu, 30013, Taiwan, ROC
sized Pt nanowires and/or nanoowers by aging hexa-
b
Department of Advanced Optoelectronic Materials and Devices, Industrial Technology
chloroplatinic acid in a formic acid aqueous solution for several
Research Institute, Chutung 31040, Taiwan, ROC
13,14
c
hours to 3 days under ambient conditions.
A challenge to synthesize single-crystal Pt nanowires or
Department of Chemical Engineering, National Tsing Hua University, Hsingchu
3
0013, Taiwan, ROC
d
Department of Materials Engineering, Ming Chi University of Technology, New Taipei nanorods by surfactant- and template-free approaches under
City 24301, Taiwan, ROC. E-mail: jack_hsieh@mail.mcut.edu.tw
mild conditions still remains. Another key challenge is to
increase catalyst dispersion and electronic conductivity
†
Electronic supplementary information (ESI) available: Experimental details and
supplementary reactions. See DOI: 10.1039/c4ra04153d
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between the gas diffusion layer and the catalysts themselves. In The insets in Fig. 1(c) and (d) show respectively the selected-area
this communication, we address both these challenges by electron diffraction (SAED) patterns at corresponding region,
proposing a novel pulse-mode potentiostatic electrodeposition both these SAED from [001] electron beam zone axis show the
(PPE) technique using a simple aqueous electrolyte to directly typical F.C.C. diffraction, not only the end of stem and branch
synthesize single-crystal, fern-like Pt nanorods (Pt-NR) on a CP but also whole Pt-NR show the same diffraction pattern, which
surface. Then, we examine the electrocatalytic activity of such a indicates that the single crystallinity of the fern-like Pt-NR.
system for the oxidation of formic acid (CH
HCHO), and methanol (CH OH).
2
O
2
), formaldehyde
Thus, the PPE technique can be used to synthesize the
single-crystal Pt-NR. We herein suggest the associated mecha-
(
3
To control the Pt-NR crystal structure during synthesis, we nism is as follows. During the application of À0.24 VSCE, reac-
investigated the effect of non-hydrogen-bubble formation tion S1 and S2 occur (ESI†). Moreover, during the application of
9
during electrodeposition. An applied potential of less than 1.2 VSCE, other reactions may occur, as previously suggested.
À0.241 V
during electrodeposition is known to cause the Specically, reduced Pt may react with H O to form the
2
SCE
generation of hydrogen bubbles by the standard hydrogen adsorption product Pt(O)ad (reaction S4 in ESI†), which in turn,
15
17–19
2 2
reduction reaction (SHRR). Such hydrogen-bubble formation again reacts with H O to form PtO (reaction S5 in ESI†).
apparently alters the morphologies of Pt that has been reduced During the next application of À0.24 VSCE, these resultants then
on the CP surface, in our case, causing the reduced Pt to assume form pure Pt by reactions (S1), (S2), and (S6)–(S8) in ESI.† In
a petal-like structure.
other words, during PPE, the nucleation/growth of Pt atoms and
We used the PPE technique by applying periodic cycles of subsequent structural modications on the Pt surface occur
high positive potential (1.2 VSCE), followed by the negative continuously. It has been shown that the PPE technique without
potential (À0.24 VSCE), slightly higher than the value known to the inuence of hydrogen bubbles enables the synthesis of fern-
induce the SHRR. As expected, hydrogen bubbles were not like single crystals in an aqueous solution containing sulphuric
generated during electrodeposition, and Pt-NR was electro- acid and Pt precursor salt.
deposited on the CP surface. Obviously, then, as has previously
To determine the catalytic efficiency of the as-prepared Pt-NR
been reported, hydrogen-bubble adsorption on the electrode catalyst and compare it with that of a commercial Pt-black
surface in an aqueous sulphuric electrolyte during deposition catalyst for the reaction of methanol oxidation, we performed
16
can affect the nal morphology of the reduced Pt metal.
cyclic voltammetry (CV) measurements in an Ar-saturated 0.5 M
Fig. 1(a) shows a typical SEM micrograph of the fern-like Pt- H SO + (a) 1 M CH OH, (b) 0.1 M CH O , and (c) 0.1 M HCHO
2
4
3
2 2
NR so created. Each Pt-NR is approximately 300 nm to 1 mm in aqueous solution, respectively.
long. Fig. 1(b) shows a TEM micrograph of the Pt-NR. Their Fig. 2(a) shows the voltammograms from h cycle for each
length is approximately 600 nm, consistent with that deter- catalyst. Table 1 lists the following electrochemical character-
mined by SEM observation. Fig. 1(c) and (d) show HR-TEM istics derived for each catalyst: forward peak potential (V ),
micrographs of the regions indicated by circles in Fig. 1(b). The forward peak current density (i ), ratio of i to reverse peak
Pt-NR end and branch both possess a single-crystal structure. density (i /i ), and mass activity (MA, the peak current density of
P
f
f
f
b
methanol oxidation obtained from cyclic voltammogram per
unit of Pt loading mass). Compared with Pt black, Pt-NR
exhibits a distinctly higher i value for the methanol oxidation
f
reaction. The peaks that appeared during the reverse scans
signify the desorption of intermediate products generated by
methanol oxidation during the forward scans. It has been sug-
gested that these intermediates might be associated with the
adsorption of CO (an intermediate during methanol oxidation)
on the Pt surfaces, a process that has been termed “CO
4,20
poisoning”. Consequently, the i /i value can be considered to
f
b
represent the CO tolerance of the catalysts. From Table 1, for
Pt-NR, we see that the i /i value is signicantly better (23%)
than that for Pt black, suggesting more efficient CO desorption
on Pt-NR. From Fig. 2(a) and Table 1, we see that the V value of
f
b
P
Pt-NR (0.63 VSCE) is lower than that of Pt black (0.64 VSCE),
indicating that Pt-NR exhibits superior electrocatalytic activity.
In addition, at a given potential of 0.5 V , i of Pt-NR (76.1 mA
SCE
P
À2
À2
cm ) is 3.59 times better than that of Pt black (21.2 mA cm
)
of Pt black catalysts. Similarly, at a given current density of 40
À2
mA cm , the potential of Pt-NR (0.45 VSCE) is 120 mV lower
than that of Pt black (0.57 VSCE), resulting in lower overpotential
during methanol oxidation.
Fig. 1 (a) SEM micrograph of Pt-NR electrodeposited on carbon
paper; (b) typical TEM micrograph of prepared Pt-NR; (c) HR-TEM
micrograph of the tip region circled in (b) and the related SAED pattern;
Thus, compared with Pt black, Pt-NR exhibits not only a
(
d) HR-TEM image of the branch region circled in (b) and the related
SAED pattern.
f b f
higher i /i value, but also a higher i value, indicating its
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Fig. 3 ECSA of cyclic voltammograms of the Pt-NR catalyst (red
curve) and Pt black catalyst (black curve) tested in Ar-saturated 0.5 M
À1
H
2
SO
4
. The scan rate was 20 mV s . The test temperature was
ꢀ
controlled at 30 C.
HCHO oxidation. From Fig. 2(b) for CH O , we see the
2
2
following: (1) at 0.2 VSCE, the oxidation reaction begins, result-
4
ing in the decomposition of CH O to CHOO or COOH. (2) At
2
2
0
.6 VSCE, forward peaks appear, caused by the formation of CO
2
À2
with i
f
of 40 mA cm on the Pt surface. (3) During the reverse
scan from 0.65 to near 0 VSCE, peaks appear, caused by the
desorption of CO gas from the Pt surfaces (desorption bubbles
can be clear seen during the experiments). The i value of Pt-NR
is higher than that of Pt black, suggesting a higher rate of
2
f
21,22
catalytic oxidation of formic acid and intermediates.
Similar
results have been found that Pt-NR again has a higher i value
f
(decomposition of HCHO forms HCH or HCO and further
formation of CO on Pt surfaces at about 0.58 VSCE) and both the
reverse peak current densities are caused by CO desorption
from the Pt surfaces. For Pt black, a shoulder appears at 0.5 VSCE
during the forward scan, caused perhaps by indirect HCHO
decomposition and CO formation, which may be a rate-deter-
mining-step for methanol oxidation. In contrast, for Pt-NR, no
such reaction occurs, suggesting that single-crystal Pt-NR
Fig. 2 Cyclic voltammograms of the Pt-NR catalyst (red curve) and Pt-
2 4
black catalyst (black curve) tested in Ar-saturated 0.5 M H SO
for the exhibits better electrocatalytic activity for methanol oxidation
oxidation reactions of (a) 1 M CH
HCHO in aqueous solutions. The scan rate was 20 mV s . The test
temperature was controlled at 30 C.
3
OH, (b) 0.1 M CH
2
O
2
, and (c) 0.1 M
than a commercial Pt black.
À1
Fig. 3 shows the electrochemical surface area (ECSA)
ꢀ
measurements of both Pt-NR and Pt black in 0.5 M H
2
SO
4
at
ꢀ
3
0 C. As shown in Fig. 3, the signicantly strong peak appeared
at À0.15 V
indicated that our synthesized Pt-NR showed
SCE
considerably superior catalytic efficiency for methanol oxida-
tion reaction. To verify this superior catalytic efficiency, we also
23,24
ECSA features characteristic of Pt(110) surfaces rich (ESI†).
A
lower poisoning effect of CO(ad) will rarely occur on the Pt(110)
2 2
examined the performance of both catalysts for CH O and
Table 1 Estimated Pt loadings and electrochemical characteristics of specimens during CV analyses in Fig. 2(a) and Fig. 3
a
À2
b
b
f
À2
b
b
c
PtÀ1
d
PtÀ2
)
Specimen
Loadings (mg cm
)
V
P
(VSCE
)
i
(mA cm
)
i
f
/i
MA (mA mg
)
SA (mA cm
Pt-NR
Pt black
0.55
0.65
0.63
0.64
195
54
1.23
1
354.5
77.1
9.036
0.857
a
b
ICP-MS analysis of Pt catalyst prepared sample. Forward peak current density obtained from the voltammograms (Fig. 2(a)) per unit Pt loading
c
f
mass. The mass and specic activities are obtained by the normalizing the peak current (i ) to the Pt loadings and electrochemical surface active
area (ECSA), respectively (ESI†). The mass and specic activities are obtained by the normalizing the peak current (i
d
f
) to the Pt loadings and
electrochemical surface active area (ECSA), respectively (ESI†).
31426 | RSC Adv., 2014, 4, 31424–31427
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25
due to the Pt catalyst surface atomic conguration (ESI†). By
comparison, the current density of Pt black without peak
probably because the carbon may wrap around Pt to reduce the
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used as the support for Pt black, a wider double layer (D.L.)
region can be seem of Pt black in Fig. 3. The electrochemical
characteristics of specic activity (SA) derived from the Had area
À2
À2
of ECSA listed in Table 1, are 9.036 mA cm and 0.857 mA cm
for Pt-NR and Pt black, respectively. The catalyst specic activity
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Conclusions
8
35.
We synthesized single-crystal, fern-like Pt nanorods on the
surface of carbon paper by a pulse-mode potentiostatic elec-
trodeposition technique without use of either surfactant or
template. Compared with a commercial Pt-black catalyst, the
prepared Pt nanorods exhibit denitely improved electro-
catalytic activity (4.59 times better mass activity, and 10.54
times better specic activity) for methanol oxidation. More
detailed electrochemical and physical characterizations of the
prepared nanorods are in progress, with the purpose of opti-
mizing their use as electrodes in PEM-based fuel cells and other
electrochemical applications.
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This study was supported nancially by the National Science
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