FePt nanoalloys anchored reduced graphene oxide as high-performance electrocatalysts for formic acid and methanol oxidation

Article abstract of DOI:10.1016/j.jallcom.2014.03.077

Reduced graphene oxide (RGO) supported FePt alloy nanoparticles are synthesized as high-performance electrocatalysts for methanol and formic acid (FA) oxidation. The microstructure, composition and morphology of the sample are systematically characterized

Full text of DOI:10.1016/j.jallcom.2014.03.077

Accepted Manuscript
FePt nanoalloys anchored reduced graphene oxide as high-performance elec-
trocatalysts for formic acid and methanol oxidation
Jinxue Guo, Yanfang Sun, Xiao Zhang, Lin Tang, Hongtian Liu
PII:
S0925-8388(14)00665-3
DOI:
http://dx.doi.org/10.1016/j.jallcom.2014.03.077
JALCOM 30848
Reference:
To appear in:
Received Date:
Revised Date:
Accepted Date:
6 January 2014
10 March 2014
12 March 2014
Please cite this article as: J. Guo, Y. Sun, X. Zhang, L. Tang, H. Liu, FePt nanoalloys anchored reduced graphene
oxide as high-performance electrocatalysts for formic acid and methanol oxidation, (2014), doi: http://dx.doi.org/
1
0.1016/j.jallcom.2014.03.077
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FePt nanoalloys anchored reduced graphene oxide as
high-performance electrocatalysts for formic acid and
methanol oxidation
∗
Jinxue Guo, Yanfang Sun, Xiao Zhang , Lin Tang, Hongtian Liu
Key Laboratory of Sensor Analysis of Tumor Marker (Ministry of Education), Lab of
Advanced Energy Materials Chemistry, College of Chemistry and Molecular
Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
ABSTRACT
Reduced graphene oxide (RGO) supported FePt alloy nanoparticles are synthesized as
high-performance electrocatalysts for methanol and formic acid (FA) oxidation. The
microstructure, composition and morphology of the sample are systematically
characterized with X-ray diffraction (XRD), fourier transform infrared spectroscopy
(
FT-IR), scanning electron microscope (SEM), transmission electron microscope (TEM),
energy dispersive spectroscopy (EDS) and Raman spectroscopy. The electrocatalytic
tests reveal that, in comparison with pure Pt nanoparticles supported on RGO sheets, the
FePt nanoalloys anchored RGO sheets deliver remarkably enhanced electrocatalytic
performance on FA/methanol oxidation, improved tolerance to CO poisoning, and
superior catalytic durability.
Keywords: nanostructured materials; catalyst; composite materials; fuel cells
1. Introduction
∗
Corresponding author. Tel.: +86 532 84022681; Fax: +86 532 84023927.
E-mail address: zhx1213@126.com (X. Zhang).
1

In recent decades, fuel cells have been investigated intensively as the sustainable
energy source because they can electrochemically convert hydrogen into water or
methanol/formic acid (FA)/ethanol into water and carbon dioxide to generate electricity
[
1]. Among different types of electrocatalysts, platinum (Pt) has been demonstrated as
the most efficient catalyst for low-temperature fuel cells [2-4]. Unfortunately, the
prohibitive cost of Pt catalyst as well as the finite utilization efficiency has hampered its
widespread commercialization. To overcome these obstacles, recent efforts have been
focused on decreasing the Pt content in fuel cell catalysts through the effective strategy
of alloying Pt with other inexpensive metals, such as Fe, Co, Ni, Cu, Ti, Pb, Zn, Cd, Hg,
and Sn [5-19]. Xu et al. have prepared Pt-Cu nanocube catalysts with good catalytic
stability for FA oxidation through a colloidal approach [11]. Habibi and co-workers
have prepared Pt-Sn bimetallic electrocatalyst which shows good catalytic activity for
FA oxidation via the electrochemical deposition process [16]. Pt-Pb hollow sphere has
been synthesized by Zhao et al. for FA electrooxidation [19]. Specially, the preparation
and application of FePt alloys are on the focus. For instance, Guo et al. have deposited
FePt nanoparticles on reduced graphene oxide (RGO) through solution-phase
self-assembly for oxygen reduction reaction [13]. Wei and co-workers have prepared
4
FePt-graphene nanohybrids for ethanol and methanol oxidation using NaBH as
reducing agent [14]. Ji et al. have prepared RGO supported FePt alloy for methanol
oxidation using hydrazine hydrate as reducing agent [18]. Nevertheless, it is still a great
challenge to synthesize high-quality FePt bimetallic nanoalloys as high-performance
electrocatalyst through a simple approach.
2

In this paper, we report a facile and efficient two-step approach to prepare FePt alloy
nanoparticles anchored on RGO composites (FePt/RGO) as a cost sustainable
electrocatalyst for fuel cell applications. RGO sheets are used as the substrate due to its
high electrical conductivity, 2-dimensional mechanical flexibility, huge surface area,
and good chemical stability [20,21]. The electrochemical tests reveal that, the obtained
FePt/RGO catalysts exhibit enhanced electrocatalytic activity on FA/methanol oxidation,
improved tolerance to CO poison, and better catalytic stability than that of Pt
nanoparticles supported on RGO composites (Pt/RGO).
2. Experimental
2.1 Materials preparation
Natural graphite is oxidized according to a modified Hummers method [22], and the
detailed process can be presented as follows: Graphite powder (2 g) and NaNO
3
(1 g)
2 4
are added into 50 mL of concentrated H SO (98%) with vigorous stirring. The mixture
o
is kept below 5 C in an ice bath and stirred for 30 min. Then 0.3 g KMnO
4
is added
o
into the above mixture and stirred for 30 min, the temperature is kept below 10 C.
Subsequently, 7 g of KMnO
4
is added gradually within 1 h under stirring, in order to
o
prevent the temperature of the mixture from exceeding 20 C. After the ice bath is
o
removed, the mixture is stirred at 35 ± 3 C for 2 h and a brown dispersion solution is
obtained. Then 90 mL of distilled water is poured into the above dispersion, and the
o
temperature is kept above 90 C for 15 min. The reaction is terminated by adding 55 mL
2 2
of distilled water and 10 mL of 30% H O solution. The resultant light yellow
3

dispersion is filtered, washed with 1 M HCl aqueous solution two times, and then
washed with distilled water. The obtained products are exfoliated with ultrasonication in
water for more than 1 h. The resultant homogeneous brown-black GO dispersion is
-1
about 10 mg ml and remains stable for months.
4 6 2
The synthesis of FePt/RGO consists of two steps: Firstly, 24 mg of K Fe(CN) ·3H O
is dissolved in 24 mL deionized water. The solution is poured into 6 mL GO solution
-
1
(
10 mg mL ) under vigorous stirring. After ultrasonic for 30 min, the mixed solution is
o
transferred into a 40 mL autoclave and maintained at 180 C for 24 h. The products are
harvested and then washed with absolute ethanol and water in turn before they are dried
o
at 60 C for 12 h in vacuum. In the second step, 60 mg of the hydrothermal products are
-
O solution (0.5 mg mL ). The dispersion is under
1
dispersed into 60 mL H
2
PtCl
6
·3H
2
o
ultrasonic irradiation for 30 min and then evaporated at 80 C under vigorous stirring.
o
After dried, the black powders are loaded into a tube furnace and sintered at 120 C for
o
-1
2
h in H
2
/Ar (volume ratio of 5/95) atmosphere with a heating rate of 1 C min to
prepare the final products of FePt/RGO. For comparison, 6 mL GO solution (10 mg
-
1
-1
2 6 2
mL ) is directly mixed with 60 mL H PtCl ·3H O solution (0.5 mg mL ). After
o
ultrasonic for 30 min and vigorous stirring at 80 C, Pt/RGO is obtained by calcination
o
at 120 C in H
2
/Ar atmosphere.
2.2 Characterizations
The crystal structure and composition of the sample are determined with powder
X-ray powder diffraction (XRD, Philips X’-pert X-ray diffractometer). Scanning
4

electron microscope (SEM) and energy dispersive spectroscopy (EDS) are obtained
with a JEOL JSM-7500F. Transmission electron microscope (TEM) is recorded on a
JEOL, JEM-2100. Fourier transform infrared (FTIR) spectra are performed using a
Nicolet Magna-IR750. Raman spectroscopy is measured on a confocal microprobe
Raman system (LabRam-010, 632 nm as the excitation source).
2.3 Electrochemical measurements
All the electrochemical measurements are carried out on CHI600D (CH Instruments,
Shanghai, China) with a conventional three-electrode cell at 25 ˚C. Modified glassy
carbon electrode (GCE), a plate wire, and a saturated calomel electrode (SCE) are used
as the working, counter, and reference electrode, respectively. To fabricate a working
electrode, homogeneous catalyst ink is prepared by ultrasonically dispersing the mixture
of 2 mg catalyst, 0.5 mL Nafion solution (0.05wt.%), and 0.5 mL ethanol. The catalyst
ink (3 µL) is casted on the pretreated GCE surface, leading to a catalyst loading of 6 µg.
After the solvent evaporates, the modified electrode is obtained as the working electrode
for all the electrochemical tests. The electrocatalytic activities of the Pt/RGO and
FePt/RGO catalysts for oxidation of FA/methanol are measured in 0.5 M H
solution containing 1 M HCOOH/ 0.5 M CH OH by cyclic voltammograms (CV) from
0.2 V to 1.2 V at a scan rate of 50 mV s . Choronoamperometrc curves are obtained at
a fixed potential of 0.3 V (FA oxidation) and 0.6 V (methanol oxidation) for 10 min.
2 4
SO
3
-
1
-
The electrolyte solutions are deaerated by bubbling with N
electrochemical tests.
2
for 15 min prior to
5

3. Results and discussion
3.1 Formation of FePt-RGO
The XRD patterns are used to determine the composition and crystal structure of the
as-prepared GO, Pt/RGO nanocomposites and FePt/RGO nanocomposites. As is shown
in Fig. 1a, the sharp diffraction peak at 2θ of ~9.3°corresponding to the (001) reflection
is observed in the XRD pattern of GO, indicating the successful transformation of
natural graphite into GO. As for the Pt/RGO and FePt/RGO nanocomposites, the (001)
peak is absent, while a broad peak located at 2θ of ~25° appears, which can be assigned
to the characteristic short-range order of stacked graphene sheets [18,23]. This result
suggests that GO has been reduced into RGO in the composites. Moreover, three other
peaks at 40°, 46°, and 68° are observed in both Pt/RGO and FePt/RGO nanocomposites,
which can be well indexed to the (111), (200), and (220) planes of face-centered cubic
structure of Pt or FePt nanoalloys (JCPDS card no. 04-0802). In the XRD pattern of
FePt/RGO nanocomposites, no other impurity peaks according to metallic Fe or its
oxides are detected, indicating its high phase purity. It is well accepted that, the
nanoparticle size can be approximately calculated from the widths of peaks using
Debye-Scherrer equation: d = Kλ/βcosθ, where d is the average particle size, K is
Debye-Scherrer constant (0.89), λ is the X-ray wavelength, β is the full-width half
maximum (FWHM) and θ is the angle of diffraction. The mean particle size of FePt
alloy nanoparticles is evaluated to be ~5.9 nm from the strongest (111) peak, which is
smaller than the Pt nanoparticles of ~7.3 nm. Interestingly, the characteristic peaks
6

corresponding to FePt nanoalloys shift slightly to higher 2θ values compared with that
of Pt nanoparticles in Pt/RGO. In order to observe more clearly, the magnified
diffraction intensity of the strongest peak of (111) plane is presented in Fig. 1b. It is
shown that the 2θ value of (111) plane for FePt nanoalloys is obviously higher than that
of Pt nanocrystals, which can be attributed to the contraction of the lattice constants,
confirming the successfully alloy formation between Fe and Pt.
The FTIR spectra of GO and FePt/RGO nanocomposites are measured. As shown in
Fig. 2, the FTIR spectrum of GO reveals the successful oxidation of graphite [18,24].
-1
The absorption bands centered at 3400 and 1400 cm are corresponding to O-H
-
1
(
(
carboxyl). The bands of 1720 cm can be assigned to the stretching vibration of C=O
-
carboxyl). The bands located at 1225 and 1050 cm can be attributed to C-O (epoxy)
1
and C-O (carboxyl or alkoxy), respectively. The vibration of C-OH (hydroxyl) is
-
1
-1
observed at 1625 cm . The peak at 1580 cm is corresponding to the characteristic
skeletal ring vibrations of graphite. However, in the case of FePt/RGO, most of the
peaks related to the oxygen functional groups including C=O, C-O, and O-H bands are
absent, suggesting the reduction of GO into RGO. This result is consistent with XRD
-1
measurement. The band corresponding to the C-O groups at 1190 cm still exists,
because the C-O groups can’t be totally removed by chemical reduction method [25].
-1
The intense band located at 1565 cm corresponds to the C=C stretching vibration,
indicating the healing of the basal structure of the graphite domains.
Raman spectroscopy is a powerful technique to investigate order and disorder in the
crystal structure of carbon [26-28]. Fig. 3 shows the typical Raman spectra of GO and
7

-1
FePt/RGO. Two characteristic peaks at about 1330 cm (D band, corresponding to
-1
κ-point phonons of A1g symmetry) and 1590 cm (G band, corresponding to an E2g
2
phonon of sp -bonded carbon atoms) are observed in the Raman spectra of GO and
D G
FePt/RGO. The intensity ratio of D band to G band (I /I ), which is an indication of the
extent π–conjugation and concentration of defects, are 0.95 and 1.10 for GO and
FePt/RGO, respectively. This result suggests the electronic interactions between FePt
nanoalloys and RGO sheets, thus benefiting electron transfer between them [29].
Fig. 4a and b show the representative TEM images of Pt/RGO and FePt/RGO
nanocomposites, respectively. It is clearly shown that, Pt nanoparticles or FePt alloy
nanoparticles are uniformly deposited on the RGO sheets. Most of the FePt and Pt
nanoparticles is smaller than 10 nm, matching well with the results calculated with the
Debye-Scherrer equation from (111) peak of the XRD pattern. The size distribution of
FePt nanoparticles is more uniform than Pt. Most of the surface of RGO sheets is fully
loaded with the FePt alloy nanoparticles. No free particles are observed outside the
RGO sheets for both the two kinds of nanocomposites, suggesting the tight interaction
between the RGO sheets and Pt/FePt nanoparticles, which are beneficial for rapid
electron transfer between them.
Fig. 5a shows the SEM image of FePt/RGO nanocomposites. The nanocomposites
exhibit typical wrinkled layer morphology of RGO sheets. The highly coarse surface of
RGO sheets is densely covered with tiny FePt alloy nanoparticles. The corresponding
EDS elemental mapping images of Pt (Fig. 5b) and Fe (Fig. 5c) further confirm the
uniform and dense dispersion of FePt alloy nanoparticles on the surface of RGO sheets.
8

It can be clearly observed that, the RGO sheets are almost fully enveloped with the FePt
alloy nanoparticles to form a sandwich-like composite structure. It is consistent with the
TEM result. Moreover, the Pt and Fe elements are accompanied with each other in the
elemental mapping images, indicating the successfully formation of FePt nanoalloys.
The chemical composition of FePt/RGO nanocomposites is measured using EDS
technique (Fig. 5d), which reveals the presence of respective elements of Fe, Pt, C, and
O. The elements of Fe, Pt, and C should be ascribed to FePt/RGO nanocomposites. The
small amount of O can be ascribed to the residual oxygen-containing functional groups
-
1
of RGO, which agrees well with the existing C-O groups at 1190 cm in the FTIR
spectrum of FePt/RGO. As shown in the inset table of Fig. 5d, the weight ratio of Fe :
Pt : C is measured to be 2.50 : 27.32 : 55.83 with the corresponding atomic ratio of 0.78 :
2.44 : 81.13 according to the EDS results. And the content of Pt in Pt/RGO
nanocomposites is measured to be ~29% according to EDS analysis.
3.2 Electrochemical properties
The electrochemical behavior of the Pt/RGO nanocomposites and FePt/RGO
nanocomposites is investigated using cyclic voltammograms (CV) at a scan rate of 50
-
1
2 2 4
mV s in N -saturated 0.5 M H SO solution. From Fig. 6, the voltammetric features of
Pt/RGO and FePt/RGO are similar and show characteristic curves of polycrystalline Pt
electrode, indicating the formation of the Pt skin on the alloy nanoparticles surface after
2 4
electrochemical activation in 0.5 M H SO [30]. For instance, for FePt/RGO electrode,
a typical pair of broad current peaks related to the hydrogen adsorption-desorption can
9

be observed between +0.01 and -0.2 V. During the anodic scan, the Pt oxides are formed
at the potential of above 0.5 V. And the reduction peak is located at 0.24 V in the
cathodic sweep, which can be attributed to the reduction of Pt oxides. It is worth noting
that, the peak currents of FePt/RGO electrode, including the reduction of Pt oxides and
hydrogen adsorption-desorption, become more prominent compared with those of
Pt/RGO electrode, suggesting a larger Pt skin layer on the particle surface.
The FePt/RGO catalyst is further employed in the subsequent electrooxidation of FA
and methanol, revealing its enhanced electrocatalytic activity than Pt/RGO. Fig. 7a
shows the CV curves of Pt/RGO and FePt/RGO in FA. It is well known that, the
electrooxidation of FA on Pt could follow the so-called dual pathways mechanism
[
31-33]. One is a direct pathway involving the oxidation of FA into CO
The other is an indirect pathway involving the production of poisoning CO intermediate
and the further oxidation of CO to CO at higher potential. As shown in Fig. 7a, only
2
at low potential.
2
one peak at ~0.71 V (Ja2), which corresponds to the oxidation of CO generated by
dehydration of HCOOH, is observed for Pt/RGO catalyst during the positive scan of the
2
CV curve [33]. The oxidation current peak for direct oxidation of FA into CO (Ja1) is
absent, presenting a substantial proof of indirect electrocatalytic oxidation of FA on
Pt/RGO catalyst. During the reversal scan, a peak corresponding to the direct oxidation
2 c
of FA into CO (J ) with the catalyst surface free of CO poisoning is observed at ~0.41
V [30,33-35]. In comparison with it, the CV curve of FePt/RGO catalyst shows different
features. It can be clearly seen in the inset of Fig. 7a, a new peak (Ja1) at ~0.3 V
2
corresponding to the direct oxidation of FA into CO is observed, indicating that the
1
0

FePt alloy nanoparticles exhibit enhanced catalytic activity than pure Pt nanoparticles.
The current densities at Ja2 and J of FePt/RGO increase dramatically compared with
c
Pt/RGO. And during the negative scan, the onset and peak potential of FePt/RGO is
positive than Pt/RGO, which reveals that FePt alloy nanoparticles possess higher
catalytic activity on FA oxidation than Pt nanoparticles. Moreover, the Ja1 is attributed to
c
the direct FA oxidation and the J is assigned to the FA oxidation on the catalyst surface
free of CO poisoning. So the Ja1/J
c
ratio can essentially reflect the fraction of the
catalysts surface that is not poisoned by CO adsorption, and can be used to measure the
catalyst tolerance ability to CO poisoning [30]. For Pt/RGO, the absence of Ja1 peak
implies the serious poisoning of the catalyst surface by absorbed CO. On the other hand,
c
the Ja1/J ratio for FePt/RGO is 0.26, which means that initially about 74% of the
catalyst surface is poisoned by CO absorption, indicating the enhanced catalytic
tolerance to CO. It is due to the fact that, the addition of Fe in FePt bimetallic alloy
electroctalysts can maximize the absorption of FA on the Pt active sites as well as
supply enough surface sites of the second metals to promote the effective oxidative
removal of poisonous CO with adsorbed hydroxyl species [30,36].
In case of methanol oxidation, FePt/RGO catalyst also shows higher catalytic activity
than Pt/RGO (Fig. 7b). It is generally accepted that, there are two peaks on the CV
curve of methanol oxidation on Pt. As shown in Fig. 7b, the peak at the positive scan (I
corresponds to the electrooxidation of methanol, forming Pt adsorbed carbonaceous
intermediates (mainly as CO). The peak located at the negative scan (I ) is assigned to
the oxidation of the absorbed intermediates generated via incomplete oxidation of
f
)
b
11

methanol during the positive scan. It can be clearly seen that, the peak currents of I
for FePt/RGO are higher than that of Pt/RGO, and the onset potential of I peak for
FePt/RGO is more negative, implying the superior catalytic activity of FePt/RGO on
methanol oxidation. The I /I ratio is often employed to estimate the catalyst tolerance to
f
and
I
b
f
f
b
CO [14,18]. FePt/RGO catalyst delivers higher value of 2.26 than 1.75 of Pt/RGO,
indicating its better tolerance to CO poisoning, which is consistent with the result
obtained from FA oxidation.
The catalytic activity and durability of Pt/RGO and FePt/RGO nanocomposites for
FA/methanol oxidation are evaluated by choronoamperometrc measurements,
respectively. Fig. 8a shows the i-t current curves of Pt/RGO and FePt/RGO on oxidation
of FA. The currents of the two catalysts exhibit a fade and then reach steady values after
several seconds, but the current of FePt/RGO fades slowly. The steady current density
-1
-1
of FePt/RGO is 8.9 mA mg , which is much higher than that of Pt/RGO (3.5 mA mg ),
suggesting its superior catalytic stability. In Fig. 8b, the current of FePt/RGO on
oxidation of methanol shows slighter decay than that of Pt/RGO. At 600 s, FePt/RGO
-
delivers a higher current density of 8.1 mA mg than 3.1 mA mg for Pt/RGO, further
1
-1
confirming its enhanced catalytic activity and durability.
4. Conclusions
In summary, a novel two-step approach for the construction of FePt/RGO
nanocomposites has been presented. The FePt nanoalloys are successfully formed and
anchored on the surface of RGO sheets densely and uniformly. And the tight interaction
1
2

between the Pt/FePt nanoalloys and RGO sheets is beneficial for the electron transport.
Aided with the introduction of Fe, the FePt/RGO catalyst exhibits higher
electrocatalytic activity on FA/methanol oxidation, better tolerance to CO, and superior
catalytic stability than Pt/RGO, thus making it a promising candidate as an efficient
electrocatalyst in fuel cells. Furthermore, the presented FePt/RGO may find its potential
applications in cell imaging, sensors, and drug delivery.
Acknowledgements
This work is financially supported by the National Natural Science Foundation of
China (21003079), Research Award Fund for Outstanding Middle-Aged and Young
Scientist of Shandong Province (BS2011CL020), Natural Science Foundation of
Shandong Province (ZR2011BM018) and Qingdao Project of Science
(
12-1-4-3-(20)-jch).
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8

Figure captions:
Fig. 1. (a) XRD patterns of GO, Pt/RGO nanocomposites and FePt/RGO
nanocomposites; (b) the magnified XRD (111) peaks of Pt/RGO nanocomposites and
FePt/RGO nanocomposites.
Fig. 2. FTIR spectra of GO and FePt/RGO nanocomposites.
Fig. 3. Raman spectra of GO and FePt/RGO nanocomposites.
Fig. 4. TEM images of (a) Pt/RGO nanocomposites and (b) FePt/RGO nanocomposites.
Fig. 5. SEM image (a) and the corresponding EDS elemental mapping images of (b) Pt,
(
c) Fe. (d) EDS spectrum of FePt/RGO nanocomposites. The inset shows the atomic and
weight ratio of Fe, Pt, and C elements in the FePt/RGO nanocomposites.
Fig. 6. CV curves of Pt/RGO and FePt/RGO nanocomposites in 0.5 M H
Fig. 7. CV curves of Pt/RGO and FePt/RGO nanocomposites in 0.5 M H
containing (a) 1 M HCOOH and (b) 0.5 M CH OH at a scan rate of 50 mV s . Inset in
a) shows the enlarged CV curves of electrooxidation of HCOOH between -0.2~0.6 V
during the positive scan.
Fig. 8. Choronoamperometrc curves of Pt/RGO and FePt/RGO nanocomposites in 0.5
M H SO solution containing (a) 1 M HCOOH and (b) 0.5 M CH OH.
2
4
SO solution.
2
4
SO solution
-1
3
(
2
4
3
1
9

2
0

2
1

2
2

2
3

2
4

2
5

2
6

2
7

FePt alloy nanoparticles anchored RGO composites are successfully synthesized.
The alloying of Fe with Pt is demonstrated with XRD and elemental maps.
FePt/RGO show enhanced catalytic activity on FA/methanol oxidation than Pt/RGO.
FePt/RGO also exhibit improved tolerance to CO poisoning than Pt/RGO.
2
8