Preparation of FePd-RGO Bimetallic Composites with High Catalytic Activity for Formic Acid Electro-Oxidation

Article abstract of DOI:10.1002/cjoc.201600427

FePd-RGO composites through the growth of uniformly dispersed iron-palladium bimetallic nanoparticles (NPs) on reduced graphene oxide (RGO) nanosheets were prepared by a two-step method. The firstly formed Fe is used as the seed for the subsequent Pd grow

Full text of DOI:10.1002/cjoc.201600427

FULL PAPER
DOI: 10.1002/cjoc.201600427
Preparation of FePd-RGO Bimetallic Composites with High
Catalytic Activity for Formic Acid Electro-Oxidation
Nanting Li*
College of Materials Science and Engineering, Anhui University of Science & Technology, Huainan, Anhui 232001,
China
FePd-RGO composites through the growth of uniformly dispersed iron-palladium bimetallic nanoparticles (NPs)
on reduced graphene oxide (RGO) nanosheets were prepared by a two-step method. The firstly formed Fe is used as
the seed for the subsequent Pd growth. The formation of Fe NPs on RGO in the first step is performed by an in-situ
reduction reaction with the reducer ethylene glycol under oil bath at 180 ℃. NPs in the as-prepared FePd-RGO
have an average particle size of 6.5 nm, and Pd is added to one side of Fe which leads to the formation of Fe-Pd bi-
metallic interfaces. As compared with the commercial Pd black at the same loading, the composites have higher
electro-catalytic activity, better electrochemical stability and higher resistance to CO poisoning for formic acid elec-
tro-oxidation.
Keywords graphene, bimetallic nanoparticles, microstrcture, formic acid electro-oxidation
Introduction
lacetonate [Pd(acac) ] and poly(vinylpyrrolidone) (PVP)
in the N-methylpyrrolidone (NMP) solution, and the
cubic Pd/RGO catalyst shows a significantly high elec-
tro-catalytic ability in the FAOR. However, fabricating
2
[
1-4]
Direct formic acid fuel cells (DFAFCs)
can di-
rectly convert the chemical energy of formic acid into
electricity, and they are expected to be the first energy
conversion devices for commercial applications of
powering portable electronics due to the high energy
active electro-catalysts with lower noble metal loading
[21]
is essential for the DFAFCs’ development,
and re-
searchers have developed techniques for designing spe-
[5-9]
density.
Formic acid oxidation reaction (FAOR) cat-
cial nanostructures with a second element in order to
alyzed by palladium (Pd) at the anode is one of the most
important reactions in DFAFCs. However, Pd is a kind
of noble metal, making that the cost of fuel cells is very
high, and the electro-catalytic activity and stability of
metallic Pd still can not meet the requirement of the
commercialization.
aggregation/growth of Pd nanoparticles (NPs) can rap-
idly degrade the performance of DFAFCs. It has been
reported that a conductive support material can play a
role in altering the geometry and electronic characteris-
[22,23]
achieve the high catalytic activity,
high utilization
stability and
[
24]
of Pd-based catalysts. For example,
recently we have synthesized bimetallic core-shell
structured Au@Pd NPs on graphene nanosheets by a
[25]
simple one-step thermal reducing method.
In com-
[
10]
Especially, CO poisoning and
parison, Pd-based bimetallic catalysts (Pd－M, M＝Fe,
[26-31]
Co, Cu, Ni, Cr, etc.)
with a noble metal and a tran-
[
11]
sition base metal have more merits for the full use of the
noble metal.
Herein, we report that reduced graphene oxide
nanosheets supported uniformly dispersed Fe-Pd bime-
tallic NPs (FePd-RGO composites) are prepared by a
two-step method. The composite’s morphology, micro-
structure, size and composition are characterized by
transmission electron microscopy (TEM), selected area
electron diffraction (SAED), X-ray photoelectron spec-
troscopy (XPS) and X-ray diffraction (XRD), and the
involved formation mechanism is proposed. Based on
cyclic voltammetry, chronoamperometry, etc., we have
also investigated the electro-catalytic performance and
possible origin of the high activity and stability for the
FAOR in alkaline electrolyte.
[
12,13]
tic of catalyst particles dispersed in the system.
Thus, researchers have a strong motivation of dispersing
Pd catalysts as small particles on a support material to
increase their utilization.
Graphene is a novel two-dimensional one-atom-thick
graphitic carbon system, which has attracted great atten-
tion as an electro-catalyst support due to its extremely
2
1 [14]
high specific surface area (2600 m •g ), good elec-
[
15]
[16-19]
trical conductivity and chemical stability.
Yang
have synthesized Pd NPs with controllable
morphology on the surface of reduced graphene oxide
RGO) by a novel procedure with palladium acety-
[20]
et al.
(
*
E-mail: lamlinanting@163.com
Received July 13, 2016; accepted September 12, 2016; published online October 24, 2016.
Chin. J. Chem. 2016, 34, 1129—1134
© 2016 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1129

Li
FULL PAPER
Experimental
Nafion (Sigma-Aldrich, USA) under ultrasonic agitation
to form the electro-catalyst ink, respectively. The elec-
tro-catalyst ink (15 μL) was then deposited on the sur-
face of a glassy carbon electrode (GCE, geometric area
Materials and synthesis
Natural graphite powder (spectrum pure), H SO (98
wt%, A.R.), KMnO (A.R.), H O (30 wt%, A.R.), HCl
2
4
2
4
2
2
＝0.1256 cm ) and dried at room temperature.
(
36 － 38 wt%, A.R.), NaOH (A.R.), BaCl2 (C.P.),
Electrochemical measurements were performed by a
CHI 660D electrochemical analyzer (CH Instruments,
Inc., Shanghai) with a conventional three-electrode cell
including GCE, saturated calomel electrode (SCE) and
platinum foil, and the potentials were measured with
SCE as the reference.
HCOOH (A.R.) and anhydrous ethanol were all pur-
chased from Sinopharm (Group) Shanghai Chemical
Reagent Co. FeSO •7H O was obtained from Sino-
4
2
pharm Chemical Reagent Co. Ltd. Pd(NO ) was pur-
3
2
chased from Nanjing Ning-Shi Chemical Reagent Co.
Ltd, and ethylene glycol (EG) was obtained from
Shanghai Jiu-Yi Chemical Reagent Co. Ltd. Commer-
cial Pd black catalyst was purchased from Sigma-
Aldrich Co. LLC. All reagents were used as received
without further purification. Deionized water with a
resistivity of exceeding 18.0 MΩ•cm from a JL-RO 100
Millipore-Q Plus water purifier was used throughout the
experiments.
Results and Discussion
TEM observation
Figure 1a shows the TEM image of Fe-RGO compo-
sites, and iron gathers together to form unevenly dis-
persed aggregates on the RGO surface due to the cause
of the preparation process and the nature of Fe NPs.
Inset 1a is the corresponding SAED pattern, which con-
firms the completely amorphous nature of aggregates.
Figure 1b is the TEM image of as-prepared FePd-RGO
composites, showing that the composites have highly
dispersed Fe-Pd NPs with the narrow size distribution
on the RGO surface, and a continuous sonication for 2
min before TEM characterization does not remove NPs
from the RGO surface, indicating a strong interaction
between them. Inset 1b is the corresponding local mag-
nified image, which shows that the supported NPs have
the irregular shape with less smooth edges. The HRTEM
image of Figure 1c indicates that Fe and Pd elements
co-exist on the RGO surface and NPs with an average
Graphite oxide was synthesized from natural graph-
[32]
ite powder using the modified Hummers’ method,[
28]
and a typical synthesis refers to the reference.
Fe-RGO and FePd-RGO composites can also be synthe-
sized using our previously reported two-step method.
In the first step, using EG as a reducing, stabilizing and
dispersing agent, 31.4 mg of graphite oxide dispersed in
5
of the entire solution was adjusted to 10 by adding
NaOH (1.0 mol/L) followed by adding 81.4 mg
FeSO •7H O under vigorous stirring in the oil bath at
[28]
0 mL of EG was sonicated for 20 min, and then the pH
4
2
1
80 ℃ for 0.5 h. After the solution was cooled to room
temperature, the product (Fe-RGO) was washed and
centrifuged several times by deionized water to remove
the excess EG, then dried at 40 ℃ in a vacuum oven.
In the second step, typically, 15 mL of 12.7 mmol/L
Pd(NO ) and 7 mg Fe-RGO were dispersed in a beaker
with 50 mL deionized water. The resulting solution was
uniformly dispersed by sonication for 10 min, then vig-
orously stirred for 24 h at room temperature. The black
product (FePd-RGO) was washed and centrifuged sev-
eral times with deionized water, and finally dried in a
vacuum oven at 40 ℃.
6
.5 nm size have Fe-Pd bimetallic interfaces with multi-
ple orientations of lattice fringes. Due to that the (111)
plane of face-centered cubic (fcc) Pd and the (110)
plane of cubic (bcc) α-Fe have a d-spacing of 0.22 nm
3
2
[33]
(
0
JCPDS No. 46-1043)
6-0696),
and 0.20 nm (JCPDS No.
[34]
respectively, the measured d-spacing be-
tween the two nearest atom rows is 0.19 nm, and this
Characterizations
TEM, HRTEM images and SAED patterns were ob-
tained on a JEOL JEM-2100 TEM operating at 200 kV.
XPS was measured on a Thermofish K-alpha X-ray
photoelectron spectrometer. Binding energies were de-
termined with reference to carbon’s C1s＝284.6 eV. The
products’ composition and crystallographic properties
were analyzed by XRD on a Rigaku Ultima III diffrac-
tometer (using Cu Kα＝1.5418 Å radiation) at a scan-
1
ning rate of 5 (°)•min in the range of 20°－90°.
Electrode preparation and electrochemical meas-
urements
Figure 1 (a) TEM image of Fe-RGO; (b) TEM image of
FePd-RGO with the corresponding local magnified image, and (c)
HRTEM image of (b). Insets (a) and (c) are the corresponding
SAED patterns, respectively.
5
.7 mg commercial Pd black and 5.0 mg FePd-RGO
were dispersed in 250 μL anhydrous ethanol and 250 μL
1130
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© 2016 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2016, 34, 1129—1134

Preparation of FePd-RGO Bimetallic Composites with High Catalytic Activity
[
35]
result indicates that Fe and Pd elements have a much
larger mismatch degree, leading to the existence of the
lattice contraction effect in bimetallic NPs which have a
much larger strain. Lattice contraction effect illustrates
that lattices of metal Fe and Pd do not exist inde-
pendently but are interlaced with each other, which also
evidences that Fe in the first step can act as the seed for
the formation and subsequent growth of Pd. Inset 1c is
the corresponding SAED pattern of Fe-Pd NPs, and the
diffraction rings are indexed to the (111), (200), (220)
and (311) crystal planes for fcc Pd on the outer layer of
NPs, confirming the polycrystalline nature.
Fe2p3/2 and Fe2p1/2, respectively, and one strong peak
(Figure 2c) at 334.9 eV corresponds to the Pd3d. These
results indicate that Fe and Pd are present as elements in
FePd-RGO rather than oxygen-containing compounds.
2
The PdO peak at 337.3 eV in Figure 2c may be caused
by the surface oxidation and/or chemisorption of envi-
ronmental oxygen during the preparation process. By
calculating the peak area, the Pd and Fe contents are
2
.19 wt% and 0.22 wt%, respectively.
XRD patterns
Crystal structures of FePd-RGO composites and
graphite oxide are characterized by XRD, as shown in
Figure 3. The FePd-RGO shows a wide peak of the C
XPS patterns
(
002) plane at 2θ＝~24° (red curve), confirming that the
Figure 2a shows XPS patterns of GO and FePd-RGO
before and after the reduction. The bands at 284.8 and
2
conjugated graphene network (sp C) of RGO nano-
sheets has been re-established after the partial reduction.
The XRD pattern also reveals that Fe-Pd NPs have two
weak Fe peaks at 2θ＝44.7° and 65.0° in good agree-
ment with cubic (bcc) α-Fe (110) and (200) planes
5
31.0 eV are associated with C1s and O1s, respectively,
and as compared with GO, oxygen-containing function-
al groups of RGO are partially removed after the reduc-
tion. Two weak peaks (Figure 2b) at binding energies of
[
34]
(
JCPDS No. 06-0696),
addition to Fe diffraction peaks, five peaks at 2θ＝40.1°,
6.7°, 68.1°, 82.1° and 86.6° correspond to Pd (111),
200), (220), (311) and (222) crystal planes, respective-
respectively. Moreover, in
7
11.1 and 724.8 eV can be assigned to the metallic
4
(
[
33]
ly. The particle size of bimetallic Fe-Pd NPs is esti-
mated by Scherrer’s equation from Pd (111), (200) and
(
6
220) primary crystal plane parameters, and averaged as
.5 nm which is very close to the TEM result.
Figure 3
XRD patterns of graphite oxide (black) and
FePd-RGO (red) with the standard peaks of Fe and Pd added.
The formation mechanism for Fe-Pd NPs supported
on RGO can refer to the related content of our previous
[
28]
paper, which is demonstrated as follows: in the first
step, as compared with the bare graphene surface, re-
sidual functional groups such as COOH, OH, etc., on
the RGO surface have stronger attraction toward metal
ions, thus acting as metal-anchoring sites to facilitate
the formation of Fe nuclei with good stability by chem-
isorption and physisorption. The formed Fe is used as
the seed for the subsequent Pd growth, and preferential-
2
＋
Figure 2 XPS patterns of GO and FePd–RGO (a), Fe2p (b) and
ly starts to reduce Pd to obtain Pd added to one side
Pd3d (c).
of Fe on the RGO surface, thus forming Fe-Pd NPs with
Chin. J. Chem. 2016, 34, 1129—1134
© 2016 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
1131

Li
FULL PAPER
bimetallic interfaces. Fe-Pd NPs with bimetallic inter-
faces have the formation of the modified electronic
structure of Fe decorated Pd NPs, and these electrons in
metals can interact with existing free electrons on the
surface of graphene, leading to the existence of the
electric synergistic effect between metals and graphene.
The overall result is the enhancement of the electro-
chemical activity and stability of FePd-RGO.
Electrochemical studies on FePd-RGO for HCOOH
oxidation
Figure 4 shows CVs of commercial Pd black (red)
and FePd-RGO (blue) modified electrodes in 200 mL
mixed solution containing 0.5 mol/L NaOH and 0.5
1
mol/L HCOOH at a scan rate of 50 mV•s . Weak oxi-
dation current is observed on the commercial Pd black
modified electrode, while the FePd-RGO modified elec-
trode has much stronger oxidation currents in the for-
ward and reverse scan. As can be seen from Figure 4,
Figure 4 CVs of commercial Pd black (red) and FePd-RGO
(
0
blue) modified electrodes in 200 mL mixed solution containing
.5 mol/L NaOH and 0.5 mol/L HCOOH at a scan rate of 50

1
mV•s .
there is a weak wide formic acid oxidation peak at about
.13 V, and the oxidation current is 4×10 A. The CV
5
0
area of FePd-RGO is apparently much larger than that
of commercial Pd black, indicating that the FePd-RGO
composites have better catalytic activity for HCOOH
electro-oxidation. Firstly, the Pd sites in bimetallic in-
terfaces are responsible for formic acid electro-oxida-
tion to form adsorbed carbon monoxide (COads), which
poisons the catalyst surface against the further oxidation,
while the Fe sites provide adsorbed hydroxyl groups
(OHads), which are in favor of the oxidant for the re-
moval of COads. Secondly, due to the interaction be-
tween Fe and Pd, such as the electronic effect, the pres-
ence of Fe changes the electronic structure of Pd in a
[30]
way that it lowers the CO adsorption energy, and Pd
can simultaneously improve the catalytic performance
by establishing a good conductive network on the RGO
surface, thus facilitating charge-transfer and mass-
transfer processes. These results reveal that Fe has a
synergistic effect on the Pd catalyst and uniformly dis-
persed Fe-Pd NPs with bimetallic interfaces provide
more active atoms, making Fe-Pd NPs electrochemical-
ly more accessible for reactants and improving the ac-
tivity of FePd-RGO and the output when used in fuel
cells.
Figure 5 Current-time curves of commercial Pd black (red) and
FePd-RGO (blue) modified electrodes in 200 mL mixed solution
containing 0.5 mol/L NaOH and 0.5 mol/L HCOOH.
maintains a higher steady state during the whole meas-
urement. This can be attributed to that the intrinsically
electronic structure of RGO and the stable nanostructure
with bimetallic interfaces have been preserved in
FePd-RGO. The slower decay of the steady-state current
in FePd-RGO suggests that the electrode surface does
not experience deactivation during the electro-oxidation
process and the FePd-RGO modified electrode has a
better tolerance towards the CO-like poisoning.
Researchers have widely applied chronoamperome-
try to explore high durability, long-term electro-catalytic
[
36-38]
activity and stability of fuel cells.
Figure 5 shows
current-time curves of commercial Pd black (red) and
FePd-RGO (blue) modified electrodes in 200 mL mixed
solution containing 0.5 mol/L NaOH and 0.5 mol/L
HCOOH. Under a fixed potential of 1.2 V, an initial fast
current decay has been displayed, followed by a slower
attenuation in both electrodes that gradually reaches a
pseudo-steady state, which is due to the initially ad-
sorbed intermediate species being depleted to an equi-
librium level. As compared with commercial Pd black,
the FePd-RGO modified electrode exhibits 5.5 times
higher initial current and the current of FePd-RGO
Figure 6 shows open circuit voltage (OCV)-time
curves of commercial Pd black (red) and FePd-RGO
(blue) modified electrodes for 200 s. The value of OCV
illustrates the extent of CO-like poisoning, and the larg-
er the value, the more serious the poisoning. Obviously,
the FePd-RGO electrode exhibits more stable and lower
voltage with a low polarization loss than the commercial
Pd black electrode, which is attributed to that a good
conductive network with lower internal resistance has
formed between Fe and Pd, which facilitates more com-
2
plete oxidation of HCOOH to CO with less carbona-
1132
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© 2016 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2016, 34, 1129—1134

Preparation of FePd-RGO Bimetallic Composites with High Catalytic Activity
[39-43]
ceous accumulation.
Figure 7 Nyquist plots of commercial Pd black (red) and
FePd-RGO (blue) modified electrodes in 200 mL mixed solution
containing 0.5 mol/L NaOH and 0.5 mol/L HCOOH. Insets show
the magnified high-frequency region and the equivalent electric
circuit.
Figure 6 OCV-time curves of commercial Pd black (red) and
FePd-RGO (blue) modified electrodes in 200 mL mixed solution
containing 0.5 mol/L NaOH and 0.5 mol/L HCOOH.
Electrochemical impedance spectroscopy (EIS) is a
powerful tool which characterizes electrode surface in-
Conclusions
[44,45]
teractions and ion diffusion kinetic features.
Figure
7
shows Nyquist plots for commercial Pd black (red)
In summary, RGO supported uniformly dispersed
Fe-Pd NPs with an average particle size of 6.5 nm are
prepared by a two-step method, where the firstly formed
Fe is used as the seed for the subsequent Pd growth and
Pd is added to one side of Fe, thus forming Fe-Pd bime-
tallic interfaces. The composites’ morphology, micro-
structure, size and composition have been characterized,
and the involved formation mechanism is proposed. The
FePd-RGO composites are used as an electro-catalyst
for the HCOOH oxidation in alkaline electrolyte and the
catalytic performance has been investigated. As com-
pared with the commercial Pd black at the same loading,
the FePd-RGO composites have higher electro-catalytic
activity, better electrochemical stability and higher re-
sistance to CO poisoning. The advantage and innovation
of our method are fast reaction speed, short required
time and good products’ reproducibility, and it will be
used to synthesize other metals/RGO composites for
energy-related field applications, i.e., supercapacitors,
photocatalysts, solar cells, etc.
and FePd-RGO (blue) modified electrodes in 200 mL
mixed solution containing 0.5 mol/L NaOH and 0.5
mol/L HCOOH. Based on the equivalent electric circuit
as shown in the inset, the charge transfer resistance (Rct)
can be estimated from the analysis of EIS, where R is
s
the uncompensated solution resistance between the ref-
erence electrode and the working electrode and Q is
used for constant phase elements instead of capacitances.
The values of the charge transfer resistance R (derived
ct
from the amplitude of the plot semicircles and account-
ing for the kinetics of the electronic transfer) can be
calculated by fitting the experimental data using
ZsimpWin software based on the equivalent circuit
shown in the inset of Figure 7. The R values for com-
mercial Pd black and FePd-RGO are 104.1 and 52.6
ct
2
ꢀ
•cm , respectively. It is clear from Figure 7 that the
curve of the commercial Pd black electrode is more ver-
tical than that of the FePd-RGO electrode in the low
frequency region, and the value order of R estimated
ct
from the equivalent electric circuit is R (FePd-RGO/
ct
GCE)＜Rct(commercial Pd black/GCE), indicating that
the commercial Pd black electrode with a more vertical
Acknowledgement
This work was supported by the PAPD, the Funda-
mental Research Funds for the Central Universities, the
Innovation Fund of Jiangsu Province, the Natural Sci-
ence Foundation of Jiangsu Province, the National Nat-
ural Science Foundation of China, and the State Key
Program for Basic Research of China.
curve has a larger R value. This result also demon-
ct
strates that the electron transfer rate on FePd-RGO is
faster than that on commercial Pd black, which is con-
sistent with the higher catalytic activity of FePd-RGO.
While the commercial Pd black electrode has a larger
semicircle than the FePd-RGO electrode (with an
oblique line) in the high frequency region (inset), indi-
cating that the commercial Pd black electrode has a
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