PtFe/nitrogen-doped graphene for high-performance electrooxidation of formic acid with composition sensitive electrocatalytic activity

Article abstract of DOI:10.1039/c5ra05769h

With the aim to pursue novel high-performance electrocatalysts for fuel cells, a simple synthesis strategy, which consists of hydrothermal reaction followed by solid state reduction by H₂, is developed to prepare a series of Pt_xFe

Full text of DOI:10.1039/c5ra05769h

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PtFe/Nitrogen-doped graphene for high-performance
electrooxidation of formic acid with composition sensitive
electrocatalytic activity
Received 00th January 20xx,
Accepted 00th January 20xx
Yanfang Sun,a,b Ting Zhou,a Qingyan Pan,a Xiao Zhanga,* and Jinxue Guoa
DOI: 10.1039/x0xx00000x
With the aim to pursue novel high-performance electrocatalysts for fuel cell, a simple synthesis strategy, which consists of
hydrothermal reaction followed by solid state reduction by H2, is developed to prepare a series of PtxFe100-x/N-doped
graphene nanocomposites with controllable Pt:Fe compositions. The morphology, microstructure, and composition of the
samples are systematically characterized with transmission electron microscope, scanning electron microscope, energy
dispersive spectroscopy, powder X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy. The effects
of nitrogen doping and alloying with Fe, as well as their synergistic interaction, on the improvement of catalytic
performance are well revealed using the present samples as catalysts for formic acid electrooxidation. Additionally, the
composition sensitive catalytic activity and stability of these catalysts for formic acid electrooxidation are probed and the
optimum Pt:Fe ratio is presented. The optimum sample possesses both the enhanced electrochemical performances and
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the reduced dosage of noble metal, making it
a
promising candidate for fuel cell applications.
with better support performance to replace the traditional
carbon black supports due to its bulk structure and low surface
area.4
Introduction
The increasing demands for renewable and clean energy
sources with high energy density and low operating
temperature have aroused the intensive research interests in
the development of high-efficient fuel cells. To satisfy the strict
requirement of commercial fuel cells with high current density,
electrocatalysts which can high-efficiently oxide small organic
molecules are desired to achieve. Among the high-efficient
catalysts of noble metal family, Pt is the preferred catalysts for
fuel cells due to its specific catalytic ability to facilitate both
the oxidation and reduction reactions.1 At present, a state-of-
art fuel cell requires 0.5 mg Pt per cm2 electrode area.2 So the
production and application of fuel cells require relatively high
consumption of Pt, which is scarce and expensive.3 In order to
decrease the usage amount of Pt and promote the
commercialization of Pt-based fuel cells, the Pt-based catalysts
with enhanced catalytic activities are being desired to be
achieved. To meet this requirement, two major strategies can
be adopted: exploring high-efficient catalysts with lower Pt
content and developing novel alternative support materials
As for the first solution, alloying of Pt with another metal M is
believed to be an effective way to reduce the consumption of
Pt, where M is usually one of the transition metals such as Fe,
Co, Ni, Cu, Ti, Pb, Zn, Cd, Hg, Sn, Ru, and Au.5-22 Demonstrated
by the literatures, the addition of transition metals into Pt-
based alloy catalysts can not only promote its electrocatalytic
activity, but also improve the tolerance to CO poison. On the
other hand, graphene has been widely used as the alternative
and effective support materials to boost the catalytic
performance of Pt, which exhibits unique structure and
interesting physical properties such as huge surface area,
flexible 2D form, good electrical and thermal conductivities,
charge transport mobility, and good chemical stability.13-15, 25
In our previous report, FePt nanoalloys anchored graphene
composite has been presented and demonstrated excellent
electrocatalytic activity on formic acid and methanol
oxidation.9 Recently, numerous reports have predicated that,
doping graphene with heteroatoms, particularly with N atoms,
could effectively tailor its intrinsic electronic characteristics.26-
29
For instance, the N-doped graphene (NG) shows different
spin density and charge distribution compared with the
pristine graphene, due to the influence of nitrogen dopants on
their neighboring carbon atoms.28-31 It will introduce
“activation region” on the graphene surface, which can
participate in catalytic reaction directly and anchor metal
nanoparticles used in the catalytic reaction. This kind of
material has currently been studied extensively for
applications in energy conversion and storage, such as
a.Key Laboratory of Sensor Analysis of Tumor Marker (Ministry of Education), Key
Laboratory of Rubber-plastics (Ministry of Education), Laboratory of Inorganic
Synthesis and Applied Chemistry, College of Chemistry and Molecular Engineering,
Qingdao University of Science and Technology. Qingdao 266042, PR China. Tel: 86
532 84022681; E-mail: zhx1213@126.com (X. Zhang).
b.College of Science and Technology, Agricultural University of Hebei, Cangzhou
061100, China.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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supercapacitors, lithium-ion batteries, and specially oxygen homogeneous brown-black GO dispersion with
reduction reaction.32-36 So it is worthwhile exploring the concentration of 10 mg mL-1.
influence of NG on the catalytic oxidation activity of Pt-based Synthesis of catalysts
catalysts for fuel cells.
a
The synthesis of Pt79Fe21/N-doped graphene composite
(Pt79Fe21/NG) can be described as follows: Firstly, 12 mg of
K4Fe(CN)6·3H2O and 150 mg of urea are dissolved in 24 mL
deionized water. After mixed with 6 mL GO solution (5 mg mL-
1), the resultant solution is transferred into a 40 mL autoclave
For fuel cells, liquid phase of formic acid and methanol are two
more attractive energy sources than the gaseous or liquid
hydrogen due to the ease of handling, transportation and
storage.37 In particular, a great deal of research efforts have
been focused on direct formic acid fuel cells (DFAFC), because
the electrooxidation of formic acid occurs at lower positive
potentials than methanol, and the crossover of formic acid
through the polymer membrane is lower than that of
methanol.38,39 Recently, some NG supported Pt-based catalysts
have been synthesized and their electrooxidation performance
on methanol has been measured,20-22, 40, 41 but the studies on
formic acid oxidation are rare. Therefore, it would be of great
interest to evaluate the electrocatalytic performance of NG
supported Pt-based catalysts on formic acids oxidation.
o
and maintained at 180 C for 4 h. The products are collected
with centrifugation and then washed with absolute ethanol
and water in turn for vacuum drying at 60 oC. Subsequently, 60
mg of the hydrothermal products are dispersed in 60 mL
H2PtCl6·3H2O solution (0.5 mg mL-1). The dispersion is under
ultrasonic irradiation for 30 min and then evaporated at 80 oC
under vigorous stirring. Finally, the dried black powders are
o
loaded into a tube furnace and heat-treated at 120 C for 2 h
in H2/Ar (volume ratio of 5/95) atmosphere with a heating rate
of 1 oC min-1 to obtain the final products of Pt79Fe21/NG.
In this manuscript, Pt-Fe alloy nanoparticles anchored on NG
sheets are prepared as electrocatalysts for the catalytic
oxidation of formic acid. Considering that the alloy
composition is also an extremely important factor with respect
to the electrocatalytic property,11, 42, 43 PtxFe100-x/NG catalysts
with diverse composition ratios have been presented and the
effect of composition on the catalytic activity has been
revealed systematically. The results show that, the as-
The synthesis procedures of other PtxFe100-x/NG catalysts are
the same as the preparation of Pt79Fe21/NG except for the
different doses of K4Fe(CN)6·3H2O listed in Table 1. For
comparison, Pt/NG, PtFe/graphene (PtFe/G), and Pt/G
catalysts are prepared via the same synthetic procedure
mentioned above by varying the reactants listed in Table 1.
fabricated
PtxFe100-x/NG catalysts
exhibit enhanced
electrocatalytic activity on formic acid oxidation and better
catalytic stability, due to the contributions from the alloying
effect of PtFe and their synergistic interaction with NG sheets.
And Pt43Fe57/NG catalyst delivers the highest catalytic activity
for formic acid oxidation.
Experimental
Synthesis of graphene oxides
Table 1 Experimental conditions of different catalysts.
All regents are obtained from Sinopharm Chemical Reagent
Co., Ltd. They are of analytical grade, and are used as
purchased without further purification. Graphene oxides (GO)
Characterizations of catalysts
are prepared by oxidizing natural graphite according to a The crystal structure of the as-prepared samples is
modified Hummers method.44 Briefly, graphite powder (2 g) characterized with powder X-ray powder diffraction (XRD,
and NaNO3 (1 g) are added into 50 mL concentrated H2SO4 Philips X’-pert X-ray diffractometer). Scanning electron
(98%) for 30 min of vigorous stirring under 5 oC. KMnO4 (0.3 g) microscope (SEM) and energy dispersive spectroscopy (EDS)
is added into the mixture for another 30 min of stirring under are recorded with a JEOL JSM-7500F. Transmission electron
10 oC. KMnO4 (7 g) is added into the mixture gradually within 1 microscope (TEM) is observed on a JEOL, JEM-2100. Fourier
h, in order to keep the temperature not exceeding 20 oC. transform infrared (FTIR) spectra are obtained using a Nicolet
Subsequently, the mixture is stirred at 35 ± 5 oC for 2 h to Magna-IR750. Raman spectroscopy is measured on a confocal
obtain a brown dispersion solution. After 90 mL of distilled microprobe Raman system (LabRam-e010, 632 nm as the
water is poured into the dispersion, the temperature is excitation source). X-ray photoelectron spectroscopy (XPS)
o
increased quickly and kept above 90 C for 15 min. Finally, 55 measurements are carried out on a RBD upgraded PHI-5000c
mL of distilled water and 10 mL of 30% H2O2 solution are ESCA system (Perkin Elmer) with Mg Kα radiation (hν = 1253.6
poured into the mixture to terminate the reaction. The eV).
resultant light yellow dispersion is filtered, and then is washed Electrochemical measurements
with 1 M HCl aqueous solution and a huge amount of distilled
All the electrochemical tests are performed on CHI600D (CH
water in sequence. The obtained black powders are exfoliated
Instruments, Shanghai, China) using a conventional three-
with ultrasonication in water for more than 1 h to prepare
electrode cell at room temperature. Modified glassy carbon
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electrode (GCE), a saturated calomel electrode (SCE), and a
plate wire are used as the working, reference, and counter
electrode, respectively. The mixture of 2 mg catalyst, 0.5 mL
Nafion solution (0.05wt.%), and 0.5 mL ethanol is ultrasonically
dispersed to prepare homogeneous catalyst ink. One portion
of catalyst ink (3 μL) is casted on the pretreated GCE surface to
fabricate a modified GCE electrode, which is used as working
electrode for all electrochemical measurements. The catalytic
performances of the catalysts for electrooxidation of formic
acid are measured with cyclic voltammograms (CV) technique,
which is performed between -0.2 V and 1.2 V at a scan rate of
50 mV s-1 in a 0.5 M H2SO4 solution containing 1 M HCOOH.
Chronoamperometric curves are obtained at a fixed potential
of 0.3 V for 10 min. Prior to electrochemical tests, the
electrolyte solutions are deaerated by bubbling with N2 for 15
min.
Results and discussion
Characterization of samples
The as-obtained samples are characterized with TEM
techniques. Fig. 1a shows the TEM image of Pt/G composite. Pt
particles are aggregated and located unevenly on the surface
of graphene sheets with a broad particle size distribution. As
shown in Fig. 1b, PtFe alloy particles show larger particle size
and deliver worse dispersion states than Pt. On the contrast,
the alloy nanoparticles in the case of Pt79Fe21/NG are dispersed
uniformly and densely on the NG sheets surface (Fig. 1c).
Observed from the magnified TEM image (Fig. 1d), Pt79Fe21
nanoalloys are monodisperse with a narrow size distribution.
The size distribution of PtFe alloy nanoparticles on NG sheets
is evaluated statistically by measuring the diameters of two
hundred particles from the TEM images and the mean size is
around 3.7 nm (inset in Fig. 1c). It is reported that, the
nitrogen dopants in the NG sheets can lead to localized defects
and promote the graphene chemical activation for enhanced
Fig. 1. TEM images of (a) Pt/G, (b) PtFe/G, (c) and (d)
interactions with nanoparticles, thus facilitating their assembly
33, 40
Hence, it can be
Pt79Fe21/NG.
and benefiting their dispersion.20-22,
concluded that the improved dispersity of Pt79Fe21 nanoalloys
should be ascribed to the presence of the N-doped sites in the
NG sheets, which acts as bonding sites to in situ anchor the
nanoparticle precursors.
The morphology of as-obtained Pt79Fe21/NG catalyst is
measured with SEM and is shown in Fig. 2a. The representative
wrinkled layer structure of graphene sheets can be observed
clearly. EDS is an effective technique to characterize the
compositions of composite. As shown in the EDS spectrum of
Pt79Fe21/NG (Fig. 2b), Pt, Fe, C, N, and O elements are
observed. The Fe and Pt elements can be assigned to the PtFe
alloys. The
C element corresponds to graphene. The
observation of N peak suggests the existence of N element in
the composites catalyst. The small amount of O can be
ascribed to the residual oxygen-containing functional groups of
RGO.9 The inset table of Fig. 2b shows that, the weight ratio of
C:Pt:Fe:N is determined to be 53.82:26.31:2.08:3.97 with the
corresponding atomic ratio of 77.25:2.32:0.64:4.89. The EDS
mapping of C, Pt, Fe, and N are presented in Fig. 2c, d, e, and f,
respectively. The elements of C and Pt are homogeneously and
densely distributed. However, Fe and
N elements are
distributed evenly and sparsely, which is attributed to their
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lower contents. Fig. 2g shows the EDS elemental mapping
images of Pt (blue points) and Fe (green points). It can be
clearly seen that, the Pt and Fe elements are accompanied
with each other, indicating the formation of PtFe alloys. And
the NG sheets are almost fully enveloped by the uniformly and
densely dispersed PtFe nanoalloys to form a sandwich-like
composite structure, which is consistent with the TEM
measurement.
Fig. 2. (a) SEM image of Pt79Fe21/NG. (b) EDS spectrum of
Pt79Fe21/NG. Inset shows the atomic and weight ratio of Fe, Pt,
C, and N elements. The EDS mapping of C (c), Pt (d), Fe (e), and
N (f). (g) EDS elemental mapping images of Pt (blue points), Fe
(green points), and C (red points) in Pt79Fe21/NG.
To further characterize the elemental composition and
nitrogen bonding configurations in Pt79Fe21/NG, XPS
measurements are performed. As shown in Fig. 3a, the survey
spectrum of Pt79Fe21/NG indicates the existence of C, O, N, Fe,
and Pt element, which is consistent with EDS measurements.
The high-resolution N 1s XPS spectrum is shown in Fig. 3b,
which is powerful to probe the nature of N functionalities. Five
deconvoluted peaks can be observed from it, indicating the
presence of pyridinc N and Fe-Nx (298.3 eV), amino N (399.2
eV), pyrrolic N (400.2 eV), graphitic N (401.2 eV), and oxidized
N (403.0 eV).49,50 The existence of Fe-Nx reveals the connection
between the doped N and metal Fe, which explains the
overlapped elemental mapping of Fe and N, and benefits for
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the uniform distribution of alloy nanoparticles on the surface
of NG nanosheets. Among these N functionalities, graphitic N,
which represents the N atoms doped into the graphitic basal
plane, is generally believed to responsible for the enhanced
catalytic activity of N-doped carbon materials.26-29,49 Therefore,
the presence of graphitic N in Pt79Fe21/NG catalyst will
undoubtedly boost its catalytic performance.
Fig. 4. (a) XRD patterns of GO, NG, Pt/NG and Pt79Fe21/NG; (b)
the magnified XRD (111) peak of Pt/NG and Pt79Fe21/NG.
Raman spectroscopy is a very useful tool to characterize
structural defects and doping level of graphene.20, 22, 28 Fig. 5
shows the Raman spectra of the GO, NG, and Pt79Fe21/NG. Two
Fig. 3. XPS spectra of Pt79Fe21/NG: (a) survey spectrum and (b) representative bands of D band (corresponding to structural
high-resolution N 1s spectra.
defects and disorder in graphene) and G band (corresponding
to the optical mode vibration of two neighboring carbon
The XRD patterns are obtained to determine the composition atoms) are the predominant features in the spectra of all three
and crystal structure of the as-prepared GO, NG, Pt/NG and samples. Specifically, the intensity ratio of the D and G bands
Pt79Fe21/NG. As shown in Fig. 4a, the representative diffraction (ID/IG) is directly proportional to the defects and can be used to
peak of (001) for GO sheets is observed at 2
corresponding to a d-spacing of 0.95 nm. A broad (002) with GO, the NG sheets show a red-shift of D band and an
diffraction peak appears at 2 of ~25.2° in the XRD pattern of increased ID/IG value from 0.94 to 1.12, which should be
θ
of 9.3°, characterize the nitrogen doping in graphene.28 Compared
θ
NG, indicating that the GO nanosheets have been successfully assigned to the doping of nitrogen into the graphene, as well
reduced into NG sheets after the hydrothermal reaction with as the structural and edge defects in graphene. After anchored
urea.15,45 Besides the NG peak, there are three other peaks at with PtFe nanoalloys, the Pt79Fe21/NG also displays a slightly
40°, 46°, and 68° in both Pt/NG and Pt79Fe21/NG red-shifted D band compared with GO due to the nitrogen
nanocomposites, which match well with the (111), (200), and doping.20, 22, 28 And it delivers a further increased ID/IG of 1.27,
(220) planes of face-centered cubic structure of Pt or PtFe suggesting the interaction between PtFe nanoalloys and NG
nanoalloys (JCPDS card no. 04-0802). In the XRD pattern of substrates.46
Pt79Fe21/NG, no other impurity peaks according to metallic Fe
or its oxides are detected, confirming its high phase purity.
Interestingly, in case of Pt79Fe21/NG, the characteristic peaks
shift slightly to higher
2θ values with respect to the
corresponding peaks in the Pt/NG. Fig. 4b shows the magnified
(111) peak in the XRD patterns of Pt/NG and Pt79Fe21/NG. The
position of (111) peak of Pt79Fe21/NG locates at higher 2θ value
than that of Pt/NG, which can be attributed to the contraction
of the lattice constants, indicating the formation of Pt-Fe
alloy9,15
.
Fig. 5. Raman spectra of the GO, NG, and Pt79Fe21/NG.
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H2SO4 containing 1 M HCOOH. It is well known that the HCOOH
electrooxidation on Pt catalysts usually takes a dual-pathway
Electrochemical measurements
mechanism: (1) the direct pathway to CO2 via
a
Cyclic voltammograms (CV) recorded for the Pt/G, Pt79Fe21/G,
and Pt79Fe21/NG in N2-saturated 0.5 M H2SO4 at a scan rate of
50 mV s-1 are shown in Fig. 6. The pristine graphene and NG
electrodes are also tested for comparison. No obvious peak
current can be observed for graphene and NG. The other Pt-
based samples deliver similar voltammetric features. The
typical pair of broad current peaks related to the hydrogen
adsorption-desorption are observed between -0.2 and 0.0 V.
The anodic peak at about 0.4 V corresponds to the oxidation
reaction of Pt (Pt + H2O → Pt-(OH)ad + H+ + e- →Pt-Oad + H+ + e-).
And the reversible reduction reaction of Pt oxides can be
dehydrogenation process that involves a reactive intermediate
of adsorbed formate (HCOOad), and (2) the indirect pathway
through a dehydration reaction with the formation of the
poisoning intermediate COad.17,43
HCOOH → HCOOad + H+ + e- → CO2 + 2H+ + 2e- (1)
HCOOH → COad + H2O → CO2 + 2H+ + 2e-
(2)
Notably, as shown in Fig. 7, there is almost no peak current for
graphene and NG modified electrode, suggesting that they
show no catalytic activity for formic acid electrooxidation. In
contrast with this, all Pt-based catalysts deliver distinct peak
current and show similar voltammetric features. Only one peak
at around 0.7 V, which corresponds to the oxidation of CO
generated by dehydration of HCOOH, is observed during the
positive scan of the CV curve.42 The oxidation current peak for
direct oxidation of HCOOH into CO2 is indistinct. And the peak
corresponding to the oxidation of intermediate species formed
during the formic acid oxidation is observed at the negative
scan of CV curves. These voltammetric features indicate that
formic acid electrooxidation on these catalysts react mainly
through the indirect pathway. The oxidation peak current at
negative scan of Pt/G is 25 mA mg-1. In contrast with it, Pt/NG
(62 mA mg-1) and Pt79Fe21/G (111 mA mg-1) show higher peak
current due to the nitrogen doping and alloying with Fe
element, respectively. And Pt79Fe21/NG catalyst delivers the
highest value of 186 mA mg-1 among these four samples,
corresponding to the enhanced synergistic effect between the
Pt-Fe alloy and N-G. It is worth noting that, the peaks located
at negative scan are observed at ~ 0.27 and 0.29 V for Pt/NG
and Pt79Fe21/NG, respectively, while the peaks of Pt/G and
Pt79Fe21/G locate at ~ 0.37 and 0.43 V, respectively. This
obvious negative shift of the oxidation peak should be
assigned to the nitrogen doping, which is beneficial for formic
acid electrooxidation. Noting that, the oxidation peak of Pt/NG
at negative scan is located at ~ 0.27 V, showing obvious
negative shift compared with that of Pt/G, which should be
ascribed to the nitrogen doping into graphene. The
comparison of peaks position between Pt79Fe21/NG and
Pt79Fe21/G also confirms this conclusion. So it can be concluded
that, NG substrates can reduce the oxygen potential of the
composite catalyst for formic acid indirect electrooxidation,
thus improving the catalytic activities of the composite
catalyst.
observed at about 0.3
V in the cathodic scan. Such
voltammetric features have been reported for other Pt based
alloys, which can be assigned to the formation of a Pt skin on
the catalyst surface.9,
22, 42, 47
Among these three kinds of
catalysts, Pt79Fe21/NG shows the most prominent reduction
peak currents of Pt oxides and hydrogen adsorption-
desorption region, which might result from the enhanced
synergistic effect between the Pt-Fe alloy and N-G.22
Interestingly, Pt79Fe21/G catalyst shows a slightly positive-
shifted reduction peak of Pt oxides in comparison with Pt/G,
due to the presence of the alloying element that decreases the
desorption free energy of Pt-OH, Pt-O, or Pt-O2 species. It will
lower the adsorption strength of the adsorbed oxygen species
on Pt in the alloy and provide more facile oxidation sites of
intermediates.47 And a further positive shift of the reduction
peak is observed for the Pt79Fe21/NG, which should be
attributed to the N doping.
Fig. 6. CV curves of GO, NG, Pt/G, Pt79Fe21/G, and Pt79Fe21/NG
catalysts in 0.5 M H2SO4.
It is believed that, the doping of nitrogen into graphene could
introduce the atomic charge density and asymmetry in the
spin density of the graphene network, thus facilitating the
charge transfer between the graphene support and the
anchoring metals.22, 26-31 So when NG is used as the support for
Pt-based catalyst, it can evidently enhance the interaction
between catalytic metals and carbon substrate, thus improving
the catalytic activity for formic acid electrooxidation. With the
aim to reveal the enhanced catalytic activity of the presented
PtxFe100-x/NG catalysts, the electrochemical formic acid
oxidation reaction on GO, NG, Pt/G, Pt/NG, Pt79Fe21/G, and
Pt79Fe21/NG catalysts is evaluated by recording CV in 0.5 M
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Fig. 7. CV curves of GO, NG, Pt/G, Pt/NG, Pt79Fe21/G, and
Pt79Fe21/NG catalysts in 0.5 M H2SO4 containing 1 M HCOOH.
Fig. 8 compares the catalytic activity of PtxFe100-x/NG catalysts
to elucidate the composition sensitive of the formic acid
oxidation. Initially, with the increasing Fe composition in the
alloy, the oxidation peak current at negative scan increases
from 186 mA mg-1 of Pt79Fe21/NG to 421 mA mg-1 of
Pt60Fe40/NG and reaches up to the highest value of 603 mA mg-
1
for Pt43Fe57/NG, which is ~24 times that of Pt/G. After that,
the catalytic activity of PtxFe100-x/NG decreases with the
further increase of Fe component. The oxidation peak current
values are 281 and 97 mA mg-1 for Pt27Fe73/NG and
Pt20Fe80/NG, respectively. When the Fe composition increases
up to 84%, the as-obtained Pt16Fe84/NG nanocomposites
almost show no detectable catalytic activity for formic acid
oxidation.
Fig. 8. CV curves of the PtxFe100-x/NG catalysts in 0.5 M H2SO4
solution containing 1 M HCOOH at a scan rate of 50 mV s-1.
Fig. 9 presents the TEM images of Pt60Fe40/NG, Pt43Fe57/NG,
Pt27Fe73/NG, Pt20Fe80/NG, and Pt16Fe84/NG. Except for
Pt16Fe84/NG, the alloy nanoparticles in the other four kinds of
catalysts are dispersed on the NG sheets with a narrow size
distribution, which is similar to that of Pt79Fe21/NG. As for
Pt16Fe84/NG, a few large particles are observed, due to the
high ratio of Fe element in alloy. So it can be concluded that,
the presented PtxFe100-x/NG catalysts show obvious
composition
sensitive
activities
on
formic
acid
electrooxidation. And the optimum composition of Pt:Fe
atomic ratio is obtained at about 1:1 in our presented study. It
is believed that, under certain optimum composition, the
surface area of Pt skin is maximized to absorb HCOOH and the
second metal supplies enough surface sites to promote the
effective oxidative removal of poisonous intermediates in Pt-
based bimetallic alloy electrocatalysts. This optimum
composition ratio is consistent with the previous results for
FePt and PtRu catalysts.42, 48
Fig. 9. TEM images of (a) Pt60Fe40/NG, (b) Pt43Fe57/NG, (c)
Pt27Fe73/NG, (d) Pt20Fe80/NG, and (e) Pt16Fe84/NG.
To further compare the catalytic activity and durability of
PtxFe100-x/NG catalysts, Chronoamperometry tests are
conducted and shown in Fig. 10. The catalytic performance of
graphene and NG are also conducted for comparison. In
accordance with the CV results in Fig. 7, graphene and NG
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modified electrodes show no catalytic activity for formic acid activity and stability with the reduced dosage of noble metal,
oxidation. Observed from Fig. 10, the catalytic stability of making it a promising candidate for fuel cells applications.
Pt16Fe84/NG is very poor and only can maintain several Moreover, the solutions developed in this manuscript will
seconds. Pt20Fe80/NG delivers similar curve as the results greatly benefit the rational design and preparation of the high-
reported in most literatures, which drops rapidly at the performance metal catalysts, including Pt or Pd-based
primary stage and then decays slowly to a limiting value. As for catalysts, for small organic molecules oxidation reaction and
Pt79Fe21/NG, Pt60Fe40/NG, Pt43Fe57/NG and Pt27Fe73/NG oxygen reduction reaction.
catalysts, the Chronoamperometric curves show an interesting
increase at the first stage, and then show analogous features
Acknowledgements
like Pt20Fe80/NG. This exceptional increase should be
attributed to the indirect oxidation pathway of formic acid on
these catalysts. In the primary oxidation process, the
intermediate species have not formed. So the oxidation
current density is low. With the reaction of formic acid
oxidation going on, the amount of intermediate species rises,
thus inducing the increase of current density at the first stage.
This result further confirms the indirect oxidation pathway of
formic acid on PtxFe100-x/NG catalysts. One can see from these
curves, Pt43Fe57/NG delivers the highest start current density,
which is consistent with the CV tests presented in Fig. 8. But
after ~ 200 s, the curve of Pt43Fe57/NG is overlapping with that
of Pt60Fe40/NG, suggesting that Pt:Fe atomic ratio of ~ 1:1
seems to be the optimum composition. And the PtxFe100-x/NG
catalysts show composition sensitive catalytic stability and
decrease in the sequence of Pt43Fe57/NG ≈ Pt60Fe40/NG >
Pt27Fe73/NG > Pt79Fe21/NG > Pt20Fe80/NG > Pt16Fe84/NG ≈ 0. It is
in accordance with the CV results.
We gratefully acknowledge for the National Natural Science
Foundation of China (21003079) and Shandong Provincial
Natural
Science
Foundation,
China
(ZR2014JL015,
ZR2014EMM004).
Notes and references
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