In situ preparation of multi-wall carbon nanotubes/Au composites for oxygen electroreduction

Article abstract of DOI:10.1039/c6ra16533h

Multi-wall carbon nanotubes (CNTs)/Au nanocomposites have been prepared by the in situ reduction approach. The as-prepared hybrid materials were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The composition of the nanocomposites was fine tuned by varying the mass ratio of Au-to-CNTs. Among a series of samples tested, hybrid materials with a Au/carbon nanotubes (Au/CNTs) ratio = 1:2 demonstrated the best activity towards oxygen reduction reaction (ORR), as most positive onset potential, largest kinetic current density, highest amount of electron transfer and lowest H₂O₂ yields were obtained. Notably, the Au/CNTs also exhibited remarkable long-term durability higher than commercial Pt/C.

Full text of DOI:10.1039/c6ra16533h

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In situ preparation of multi-wall carbon nanotubes/
Au composites for oxygen electroreduction†
Cite this: RSC Adv., 2016, 6, 91209
ac
Na Li,ab Zhenghua Tang,
Likai Wang,a Qiannan Wang,a Wei Yan,a Hongyu Yang,a
*
Shaowei Chenad and Changhong Wang
b
*
Multi-wall carbon nanotubes (CNTs)/Au nanocomposites have been prepared by the in situ reduction
approach. The as-prepared hybrid materials were characterized by X-ray diffraction (XRD), X-ray
photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The composition of the
nanocomposites was fine tuned by varying the mass ratio of Au-to-CNTs. Among a series of samples
tested, hybrid materials with a Au/carbon nanotubes (Au/CNTs) ratio ¼ 1 : 2 demonstrated the best
activity towards oxygen reduction reaction (ORR), as most positive onset potential, largest kinetic current
density, highest amount of electron transfer and lowest H2O2 yields were obtained. Notably, the Au/
CNTs also exhibited remarkable long-term durability higher than commercial Pt/C.
Received 27th June 2016
Accepted 16th September 2016
DOI: 10.1039/c6ra16533h
www.rsc.org/advances
catalytic activity in multiple organic reactions16 and electro-
chemical reaction. Particularly, for those ultrasmall nanoclusters
Introduction
with core diameter less than 2 nm, excellent activity toward
oxygen electroreduction was observed by Chen's group.17 Inter-
estingly, strong size effects were observed and the activity
increased with the decreasing of the core size. Smaller sized
particles possess higher fraction of low-coordinated surface
atoms,18,19 which can facilitate the oxygen adsorption on the
surface and making them easily activated.
A proton exchange membrane fuel cell (PEMFC) has been
considered one of the most promising technologies to tackle the
global energy crisis and environmental problems.1,2 Oxygen
reduction reaction is a key process for energy conversion in fuel
cells, unfortunately, the slow reaction kinetics of ORR as well as
the high price of the Pt based catalysts signicantly hinder the
widespread commercialization of fuel cells.3,4 Moreover, the Pt
based catalysts also suffer from the low abundance of Pt on
earth as well as the poor stability of such catalysts. Therefore,
a great deal of research efforts have been continuously devoted
to developing new strategies to lower the amount of Pt or
substituting Pt with other non-precious metals or even using
metal-free materials.5–15
However, when employing gold nanoparticles or clusters
alone for catalyzing electrochemical process, the surface ligands
may block some active sites hence detrimental to the mass
transport behaviors and electron transfer kinetics,20 meanwhile,
gold nanoparticles tend to aggregate, dissolve, sinter or decom-
pose during the electrochemical reaction.21 To conquer these
issues, a variety of substrates including porous carbon,22 carbon
Bulk gold has been considered to be catalytically inert previ-
ously for a long time. However, when the dimension reaches
nanometer scale, gold nanoparticles demonstrated excellent
25
nanosheets,23 carbon nanotubes,24 TiO2
as well as other
materials26 have been employed as supports to stabilize or entrap
the gold nanoparticles. For instances, Alexeyeva et al. found that
glassy carbon electrode modied by gold nanoparticle (AuNP)/
multi-walled carbon nanotubes (MWCTs) possessed effective
ORR activity in acidic media, and a two-electron reduction
pathway was taken.27 By employing layer-by-layer deposition
technique, nanocomposite catalysts of AuNP/PDDA-MWCTs
were synthesized and enhanced activity was acquired.28 Inter-
estingly, the hybrid materials of AuNP/MWCTs can also be
prepared by sputter deposition of gold on MWCTs followed by
heat-treatment.29
aNew Energy Research Institute, School of Environment and Energy, South China
University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou,
510006, P. R. China. E-mail: zhht@scut.edu.cn
bSchool of Materials and Energy, Guangdong University of Technology, Guangzhou,
510006, P. R. China. E-mail: wangchh@gdut.edu.cn
cGuangdong Provincial Key Laboratory of Atmospheric Environment and Pollution
Control, Guangdong Provincial Engineering and Technology Research Center for
Environmental Risk Prevention and Emergency Disposal, School of Environment and
Energy, South China University of Technology, Guangzhou Higher Education Mega
Centre, Guangzhou, 510006, China
To enhance the activity, these hybrid materials were gener-
ally subjected to pyrolysis or elevated temperature calcination to
remove the surface ligands. Note that, these processes normally
require high energy input and sophisticated sample prepara-
tion procedures. Surfactant-free Au nanoclusters in graphene
dDepartment of Chemistry and Biochemistry, University of California, 1156 High
Street, Santa Cruz, California 95064, USA
† Electronic supplementary information (ESI) available: TEM images of CNTs,
HR-TEM images of the Au/CNTs ¼ 1 : 2 composite, additional linear scanning
voltammetric (LSV) curves, and repeated LSV measurements of the Au/CNTs ¼
1 : 2 composite. See DOI: 10.1039/c6ra16533h
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were prepared by Tang and coworkers, and the nanoclusters
exhibited remarkable catalytic activity towards ORR.30 Although
much progress has been achieved on supported gold nano-
particles for ORR, to date, few examples have been reported
regarding ligand-free gold nanoparticles prepared without high-
temperature treatment for ORR. This is the main motivation of
our current investigation.
Herein, we report the in situ formation of Au nanoparticles
on the surface of multi-wall carbon nanotubes. The as-prepared
hybrid materials were fully characterized by XRD, XPS and TEM.
The composition of the nanocomposites was tuned by varying
the mass ratio of Au-to-CNTs. Among a series of samples tested,
hybrid materials with Au/CNTs ¼ 1 : 2 demonstrated the best
activity for ORR, in terms of onset potential, kinetic current
density as well as long-term durability.
Electrochemical measurements
The electrochemical tests were performed on a electrochemical
workstation with CHI 750E (CH instruments Inc.) in 0.1 M KOH
solution at room temperature. The platinum wire electrode and
a Ag/AgCl electrode worked as the counter electrode and the
reference electrode, respectively. The working electrode was
a glassy carbon rotating ring disk electrode (37% collection
efficiency) from the PINE instruments. Prior to use, the working
electrode was cleaned with 0.3 mm alumina powder on a pol-
ishing mica cloth (CH instruments Inc.).
Typically, 1 mg catalyst was dispersed in 0.5 mL of anhydrous
ethanol solution, and 5 mL 5 wt% Naon was added into the
mixture and sonicated for at least half an hour. 10 mL mixed
liquor was dropcast onto the glassy carbon electrode and dried at
room temperature. The loading capacity of all catalyst samples
on the electrode surface was 80.8 mg cmꢀ2. In all tests, the Ag/
AgCl reference electrode was calibrated with respect to a revers-
ible hydrogen electrode (RHE). The cyclic voltammograms (CV)
Experimental section
Materials
were conducted at a scan rate of 10 mV sꢀ1. ERHE ¼ EAg/AgCl
+
Hydrogen tetrachloroauric acid (III) trihydrate (HAuCl4$3H2O, 0.966 V. Chronoamperometric responses were recorded in an
98%, Energy Chemicals, Shanghai), sodium borohydride O2-saturated 0.1 M KOH solution at +0.5 V for 30 000 s.
(NaBH4, 98%, Aladdin industrial Corporation, Shanghai), Pt/C
(20 wt%, Alfa Aesar), multiwall carbon nanotubes solution
Results and discussion
(9–10 wt%, Aladdin industrial Corporation, Shanghai), Naon
(99%, Alfa Aesar), water was supplied by a Barnstead nanopure
water system (18.3 MU cm). All the chemicals and solvents were
used as received.
The morphology and surface structure of the as-prepared Au/
CNTs composites were rst examined by TEM measurements.
The typical TEM images of carbon nanotubes can be found in
Fig. S1.† Fig. 1 presents the representative TEM images of the
In situ preparation of Au/CNTs composites
nanocomposites and their corresponding size distribution
histograms. One can see that, with lower gold mass loadings
such as the samples of Au/CNTs (1 : 4 and 1 : 2), all the particles
were dispersed onto CNTs without aggregation. The represen-
tative HR-TEM images of Au/CNTs (1 : 2) sample can be found
in Fig. S2,† and well-dened particles with high crystalline were
observed. However, with higher gold mass loading such as the
samples of Au/CNTs (1 : 1 and 2 : 1), apparent aggregation
phenomenon can be observed, and large bulky materials
formed especially for the sample of Au/CNTs (2 : 1). Based on
more than 100 counts of individual particles, the average
diameter was calculated as 9.2 ꢁ 2.3 nm, 9.7 ꢁ 4.5 nm, 23.2 ꢁ
9.6 nm and 60.8 ꢁ 9.8 nm for the sample of 1 : 4, 1 : 2, 1 : 1 and
2 : 1, respectively.
The Au/CNTs nanocomposites were prepared by a in situ
reduction method. Briey, 2 mL of 0.5 M HAuCl4$3H2O and
4 mL of 1 mg mLꢀ1 aqueous dispersion of carbon nanotubes
were mixed in a 25 mL round-bottom ask. The mass ratio of
Au-to-CNTs was 1 : 2. The mixture was stirred for half an hour at
room temperature. Then, 2 mL of 0.026 M NaBH4 aqueous
solution was added rapidly under vigorously stirring. The
reaction was proceeded for half an hour, and the composite
solution was obtained. The impurities in the solution were
removed by centrifugation with de-ionized water. The
composite catalyst solid powder was obtained by freezing drying
for 24 h. Samples with other mass ratios were also prepared in
a similar manner, of which the mass ratios of Au-to-CNTs were
1 : 4, 1 : 1 and 2 : 1, respectively.
Further structural insights regarding this series of nano-
composites were obtained by XRD and XPS measurements. As
depicted in Fig. 2a, two peaks at 2q ¼ 25.9ꢂ and 42.9ꢂ can be
found, which can be ascribed to (002) and (101) hexagonal
Characterizations
The composition and microstructure of the composite catalysts carbon crystal face (jcpds 75-1621), respectively. For Au/CNTs
were examined by XPS, XRD and TEM tests. For high-resolution composites, four additional diffraction peaks can be easily
(HR) TEM tests, the samples were dispersed in absolute identied at 2q ¼ 38.3, 44.5, 64.6, 77.6, which correspond well
ethanol, and dropcast directly onto a copper grid coated with with (111), (200), (220), (311) diffraction of fcc gold.31 Such
a holey carbon lm. The XRD patterns were obtained with the results suggest that the Au nanoparticles were well incorporated
Bragg angle (2q) changes in the scope of 10–90 degrees at room into the CNTs. Consistent results can be found in the XPS tests
temperature by using Bruker D8 diffraction and Cu K alpha in Fig. 2b. Besides the C1s peak at 284.3 eV and O1s peak at
radiation (l ¼ 0.1541 nm). The XPS analysis was conducted with 531.4 eV from CNTs, additional peaks from Au (Au4d, 353.1 eV
a VG MultiLab 2000 instrument with a monochromatic Al K and 334.8 eV, Au4f, 83.2 eV) with strong signals can be easily
X-ray source (Thermo VG Scientic).
identied. For all the nanocomposites, two peaks with strong
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Fig. 1 Representative TEM images and corresponding size distribution histograms of (a, e) Au/CNTs ¼ 1 : 4, (b, f) Au/CNTs ¼ 1 : 2, (c, g) Au/CNTs
¼ 1 : 1, and (d, h) Au/CNTs ¼ 2 : 1.
Fig. 2 (a) XRD patterns and (b) XPS survey spectra and (c) XPS Au4f core-level spectra of the Au/CNTs nanocomposites.
signal at ꢃ87.5 eV and ꢃ83.8 eV can be easily recognized, which easily recognized, while in nitrogen-saturated 0.1 M KOH
are attributed to the binding energy of Au4f5/2 and Au4f7/2 solution, only featureless charging currents were observed in
electrons, respectively. The Au4f XPS spectra of the samples the potential range from ꢀ0.03 V to +1.17 V, which can be seen
with different Au loadings were presented in Fig. 2c. It can be in Fig. S4.† Among the series of samples, the nanocomposites
seen that, the binding energy of the Au4f5/2 electrons decreased with Au/CNTs (1 : 2) possessed the most positive onset poten-
from 84.0 eV of Au/CNTs ¼ 1 : 4 sample to 83.7 eV of Au/CNTs ¼ tial, cathodic peak potential and largest kinetic current density,
2 : 1 sample, in proportion with the gold mass loading superior than carbon nanotubes and other samples as well. The
decreasing. The blue shi of the binding energy indicates that onset potential and kinetic current densities (at +0.55 V and
covalent hybrid materials of Au/CNTs were obtained.23–26 It is 2500 rpm) can be estimated to 0.79 V and 1.4 mA cmꢀ2 for Au/
worth noting that such hybridization caused electronic inter- CNTs (1 : 4), 0.86 V and 2.6 mA cmꢀ2 for Au/CNTs (1 : 2), 0.71 V
action has been documented to facilitate the electron transfer and 1.7 mA cmꢀ2 for Au/CNTs (1 : 1), 0.72 V and 1.85 mA cmꢀ2
kinetics and mass transport behaviors during the ORR for Au/CNTs (2 : 1) and 0.69 V and 1.39 mA cmꢀ2 for CNTs,
process.23–26,30 In addition, as shown in Fig. S3† of the Raman respectively. The Au/CNTs (1 : 2) sample possessed the best
spectra, the intensity ratio of D/G band (Id/IG) of Au/CNTs ¼ 1 : 2 ORR activity among the series, which were further conrmed by
(0.99) is much larger than that of CNTs alone (0.63). The higher the following RRDE measurements shown in Fig. 3b. It can be
Id/IG value of the hybrid materials further conrmed the strong observed that for all the samples, when the electrode potential
interaction between Au elements and the CNTs support.30
was scanned to ꢃ0.7 V, non-zero cathodic currents started to
Next, the ORR activities of these nanocomposites were appear and reached an plateau at ꢃ0.5 V. In addition, the vol-
examined by CV and rotating ring disk electrode (RRDE) tests. tammetric current of the ring electrode was about one order of
As seen in Fig. 3a, in oxygen-saturated 0.1 M KOH solution, magnitude lower than that of the disk electrode, suggesting that
a sharp cathodic peak attributed to oxygen reduction can be a considerably small amount of hydrogen peroxide product was
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Fig. 4 (a) RRDE voltammograms at the 2500 rpm, (b) plots of H2O2
yield and number of electron transfer of a glassy carbon electrode
modified with nanocomposite of Au/CNTs (1 : 2) in O2-saturated 0.1 M
KOH solution. Statistic results were based on data of three repeated
measurements. (c) The corresponding KꢀL plots for nanocomposite of
Au/CNTs (1 : 2) at different potentials. (d) The corresponding Tafel
plots for nanocomposite of Au/CNTs (1 : 2) and commercial Pt/C
catalyst. All measurements were conducted at a catalyst loading of
80.8 mg cmꢀ2 in an O2 saturated 0.1 M KOH aqueous solution with
Fig. 3 CV (a) and RRDE (b) voltammograms of catalysts with different
Au loadings in O2-saturated 0.1 M KOH at 2500 rpm.
a sweep rate of 10 mV sꢀ1
.
produced during the ORR. The catalyst performance varied with
the change of Au-to-CNTs mass ratio. The huge variation
between the catalytic performance with different Au loadings
indicated a subtle balance between the gold content and the
effective surface area. In principle, the number of electro-
catalytic active sites increased with the increasing of the Au
loading, and this is why the performance of Au/CNTs ¼ 1 : 2 was
better than that of Au/CNTs ¼ 1 : 4. However, if HAuCl4 was
overloaded at the initial stage of synthesis (e.g. the sample of
Au/CNTs ¼ 2 : 1), aggregation as-prepared gold nanoparticles
occurred and bulky materials were formed, evidenced by TEM
images observed in Fig. 1d. The aggregation block some active
sites hence signicantly lower the ORR activity.
The electrocatalytic activities toward the ORR of the nano-
composites were then compared with Pt/C by the RRDE vol-
tammetric method. The RRDE voltammetric measurements in
Fig. 4a shows that the onset potential of Au/CNTs (1 : 2) and Pt/
C were 0.85 V and 0.96 V, while the diffusion limited current
density of Au/CNTs (1 : 2) and Pt/C was 2.64 and 4.80 mA cmꢀ2
at 0.45 V and 2500 rpm, respectively.
in which, Id is the disk current, Ir is the ring current, N is the
collection efficiency of RRDE with a value of 0.37. As can be seen
from Fig. 4b, for the nanocomposite of Au/CNTs (1 : 2), in the
potential range from +0.2 V to +0.8 V, the n value varied from
3.50 to 3.69, while the H2O2 yield changed from 16.6% to 25.0%,
both of which were close to the n (n ¼ 3.70–3.86) and H2O2 yield
of Pt/C (5.0% to 8.4%). It indicated that the reaction was a four
electron process, with a small amount of intermediate product
H2O2.
Fig. S5† presents the RRDE result of oxygen reduction for
nanocomposite of Au/CNTs (1 : 2) collected with different
rotation rates (from 100 to 2500 rpm) in the oxygen-saturated
0.1 M KOH solution. Clearly, as the rotation rates increased,
the voltammetric current also increased. Fig. S4† presents the
RRDE result of oxygen reduction for nanocomposite of Au/CNTs
(1 : 2) collected with different rotation rates (from 100 to 2500
rpm) in the oxygen-saturated 0.1 M KOH solution. Clearly, as
the rotation rates increased, the voltammetric current also
increased. Under steady state conditions, the Koutecky–Levich
(K–L) eqn (3) can express the relative contribution of kinetics
and mass transport to the current generated in a RRDE
measurement:32
According to the results of RRDE tests, the electron transfer
number (n) and the yield of H2O2 in oxygen reduction process
can be calculated by eqn (1) and (2):
4Id
n ¼
(1)
Id þ Ir=N
1
1
1
1
1
¼
þ
¼
þ
(3)
2=3
0:62nFADO u1=2
v
ꢀ1=6CO*
i
ik il;c
ik
200Ir=N
Ir
H2O2 ¼
(2)
where i is the measured current density, iK and il,c are the
kinetic and diffusion limited current density, respectively, u is
the angular velocity of the disk (rad sꢀ1), F is the Faraday
þ Id
N
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constant, A is the cross sectional area of GC electrode (0.196 desirable onset potential and kinetic current density, however,
cm2), CO is the bulk concentration of dissolved O2 (1.21 ꢄ 10ꢀ6 the control of the thickness of Au/C-Naon layer is rather
mol cmꢀ3), DO is the diffusion coefficient of O2 in 0.1 M KOH sophiscated and knotty.41 Carbon nanotube-gold hybrid has
(1.93 ꢄ 10ꢀ5 cm2 sꢀ1), and v is the kinematic viscosity of the been employed as potent catalyst for ORR, however, such hybrid
electrolyte solution (0.01 cm2 sꢀ1). Therefore, at potentials was prepared through layer-by-layer assembly of small gold
where mass transport and kinetics effects are dominant, a plot nanoparticles anchored onto the modied carbon nanotubes.24
of Iꢀ1 versus uꢀ1/2 should be linear. Indeed, the corresponding The assembly requires relatively complicated surface function-
K–L plots in Fig. 4c exhibited a good linearity within the alization of gold nanoparticles and tedious synthetic proce-
potential range from 0.6 V to 0.78 V with rotation rate from 400 dures while the modication of carbon nanotubes needs
to 2500 rpm.
cautious chemical conjugation without high yield.24
Furthermore, based on the K–L plots, the electron transfer
To disclose the physical origin of the higher ORR activity of
number n can also be obtained from eqn (3). The n value of the Au/CNTs ¼ 1 : 2 sample, the electrochemically active surface
Au/CNTs ¼ 1 : 2 was determined as 3.61, which is in good area (ECSA) test was conducted.42,43 Based on the cyclic volta-
accordance with the n value range of 3.50 to 3.69 that obtained mograms in Fig. S6,† the ECSA value was calculated as 9.45 m2
from RRDE measurements, suggesting a rst order reaction gꢀ1 for Au/CNTs ¼ 1 : 4, 11.6 m2 gꢀ1 for Au/CNTs ¼ 1 : 2, 2.22
kinetics of ORR with regard of the oxygen concentration. Fig. 4d m2 gꢀ1 for Au/CNTs ¼ 1 : 1 and 2.18 m2 gꢀ1 for Au/CNTs ¼ 2 : 1.
shows the corresponding Tafel plots for nanocomposite of Au/ The sample of Au/CNTs ¼ 1 : 2 possessed the highest ECSA
CNTs (1 : 2) (red curve) (57.8 mV decꢀ1) and Pt/C (black curve) value among the series. Table 1 summarizes the specic activ-
(58 mV decꢀ1). The two slopes were almost identical, which ities and mass activities of nanocomposite catalysts as well as
implied that they probably exhibited a similar reaction mech- commercial Pt/C. One can see that, for Au/CNTs ¼ 1 : 2, even if
anism on the catalyst surface. Note that for Pt/C catalyst, the rst the mass activity is lower than Pt/C, the specic activity is
electron transfer to molecular oxygen has been well recognized slightly higher, so a close or comparable activity with Pt/C was
as the rate determining step in the ORR process.33,34 Interest- acquired.
ingly, such Tafel slope value of Au/CNTs (1 : 2) also agrees well
Finally, the durability of the nanocomposite of Au/CNTs ¼
with recently reported Au/C catalysts prepared by Erikson et al.,35 1 : 2 was examined by the stability test and compared with
in which the Tafel slope value of Au nanospheres, Au octahedra
and Au nanocubes ranged from 43 mV decꢀ1 to 60 mV decꢀ1
.
One can see that, the ORR activity of the nanocomposite was
not only remarkably higher than CNTs alone, but also superior
than the direct carbon nanotubes supported samples or similar
samples prepared by other approaches. In a recent report by
employing bi-metallic Pt–Au nanoparticles supported on multi-
wall carbon nanotubes for ORR, the onset potential achieved
was only about +0.80 V.36 The carbon nanotube-gold hybrid
materials prepared by Morozan et al. demonstrated effective
Table 1 The specific activities and mass activities at 0.6 V of Au/CNTs
nanocomposites and Pt/C
Specic activity
Mass activity
Sample
at 0.6 V (mA cmꢀ2
)
at 0.6 V (mA mgꢀ1
)
Au/CNTs ¼ 1 : 4
Au/CNTs ¼ 1 : 2
Au/CNTs ¼ 1 : 1
0.29
0.67
0.32
0.3
49.1
72.5
16.3
7.2
ORR activity in both acidic and basic electrolytes, even if low Au/CNTs ¼ 2 : 1
Pt/C
0.66
101
overpotential requirement was achieved, the number of elec-
tron transfer was as low as 3.2.24
There are several factors that may contribute to the
enhanced activity. First of all, as we employed in situ synthetic
approach, no surface capping agents of gold nanoparticles were
present. The absence of surface ligands and appropriate gold
loading in the composite can promote the interfacial charge
transfer during the electrochemical reaction.20,37 Secondly, the
integration of carbon nanotubes and gold can provide a syner-
getic effect on the enhanced ORR activity. Such interaction was
evidenced by the Au4f electron binding energy shi observed in
XPS measurements. Note that, carbon nanotubes probably not
merely serve as a support but also play a critical role in the
metal–carbon interactions.23,38–40 It has postulated that in the
gold/carbon hybrid, the defective carbon nanotubes probably
can lower the oxygen dissociation energy through accelerating
the charge transfer from gold atoms to oxygen molecules.6
In addition, the sample preparation was quite simple and
straightforward, rather more advantageous than other conven-
Fig. 5 Chronoamperometric responses for ORR at Au/CNTs ¼ 1 : 2
tional approaches. For instances, Erikson et al. demonstrated
electrode in an oxygen-saturated 0.1 M KOH solution at +0.5 V for
the ORR activity of carbon supported gold catalysts with 30 000 s.
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commercial Pt/C. As shown in Fig. 5, aer more than 8 h's 11 P. Zhang, F. Sun, Z. Xiang, Z. Shen, J. Yun and D. Cao, Energy
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Acknowledgements
This work was supported by the National Natural Science 26 W. Chen, D. Ny and S. Chen, J. Power Sources, 2010, 195, 412–
Foundation of China (No. 21501059 from Z. H. T and No. 418.
51306040 from C. H. W). Z. H. T also acknowledges nancial 27 N. Alexeyeva, T. Laaksonen, K. Kontturi, F. Mirkhalaf,
support from Project of Public Interest Research and Capacity
Building of Guangdong Province (2015A010105009), Guang-
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