Facile synthesis of Pd/PDDA-GN/PMo11Co composite and its enhanced catalytic performance for formic acid oxidation

Article abstract of DOI:10.1039/c6ra21497e

A novel Pd-based catalyst, Pd/poly(diallyldimethylammonium chloride) functionalized graphene (PDDA-GN)/transition-metal-substituted polyoxometalate H₇PMo₁₁CoO₄₀·xH₂O (PMo₁₁Co), for direct formic acid

Full text of DOI:10.1039/c6ra21497e

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Facile synthesis of Pd/PDDA-GN/PMo Co composite and
11
their enhanced catalytic performance for formic acid
oxidation
a, b
a
a
a
a
a, ∗
Lijuan Le , Xiaofeng Zhang , Ai Ma , Yi Zhang , Huodi Huang , Shen Lin
a
College of Chemistry & Chemical Engineering, Fujian Normal University, Fuzhou
3
50007, People’s Republic of China
b
Fujian Key Laboratory of Polymer Materials, Fuzhou 350007, Fujian, People’s
Republic of China
Abstract: A novel Pdꢀbased catalyst, Pd/poly (diallydimethylammonium chloride)
functionalized graphene (PDDAꢀGN)/transitionꢀmetalꢀsubstituted polyoxometalate
H PMo CoO ·xH O (PMo Co), for direct formic acid fuel cells (DFAFC) has been
7
11
40
2
11
prepared by layer by layer (LBL) electrostatic assembly method combined with
electroꢀdeposition of Pd particles in situ. The morphology, particles size and
composition of the asꢀprepared Pd/PDDAꢀGN/PMo Co composite were
11
characterized by scanning electron microscopy (SEM), Xꢀray diffraction(XRD),
energy dispersive spectrum (EDS). The results reveal that Pd particles are uniformly
deposited on the surface of PDDAꢀGN/PMo11Co. Electrochemical analysis shows that
the Pd/PDDAꢀGN/PMo11Co composite exhibits higher electrocatalytic activity, better
electrochemical stability, and higher resistance to CO poisoning than the
Pd/PDDAꢀGN/PMo12 and Pd/C catalysts for the formic acid oxidation
reaction(FAOR). The current density of anodic peak at Pd/PDDAꢀGN/PMo12
−
1
modified electrode in the formic acid oxidation process is up to 639.3 mA mg ,
which is 4.2 times that of the Pd/C and 1.2 times that of the Pd/PDDAꢀGN/PMo12
.
The experimental investigates indicate that PDDAꢀGN as a support preserves superior
electric conductivity of graphene sheets of the composite, and the introduction of
electroactive PMo11Co into the composite contributes to convert intermediate species
*Corresponding author. Tel: +86ꢀ591ꢀ22867399; fax: +86ꢀ591ꢀ22867399
E-mail: shenlin@fjnu.edu.cn (S. Lin).
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2
CO into CO . The synergistic effects of PDDAꢀGN and PMo11Co remarkably enhance
the electrocatalytic activity and stability of nanoꢀPd regarding formic acid oxidation.
Keywords: transitionꢀmetalꢀsubstituted polyoxometalate; layer by layer; Pd; formic
acid oxidation; electrocatalysis
1
. Introduction
Direct formic acid fuel cell (DFAFC) is a kind of electrochemical device which
can convert chemical energy into electrical energy and heat without a combustion
process as the intermediate process. Compared with direct methanol fuel cell (DMFC),
DFAFC offers a broader range of advantages, such as higher energy densities,
reduced toxicity, fast electroꢀoxidation kinetics and low fuel crossover through a
Nafion membrane[1ꢀ3].
In order to develop the high performance of DFAFCs, more interest has been
paid to fabricate highly efficient electrocatalysts towards formic acid oxidation.
Among the metalꢀbased catalysts, both Pt and Pdꢀbased materials have been widely
expected to be the most efficient catalysts for electrochemical oxidation of formic
acid. In view of the catalytic efficiency, Pdꢀbased catalysts have attracted more and
more attentions, owing to its advantages towards formic acid electrochemical
oxidation mostly through the direct pathway[4]. However, pure Pd suffers from the
catalyst poisoning during the formic acid electroꢀoxidation reaction more or less[5].
Therefore, developing new Pdꢀbased catalysts with better resistance to CO poisoning
is highly desirable[6].
Furthermore, to maximize the electrocatalytic activity of Pt/Pd nanoparticles
(NPs), a suitable carbon support is required to disperse these NPs. Graphene
nanosheets (GN), a twoꢀdimensional nanomaterial with one or several atomic layers
of carbon, has been considered to be a promise due to its superior stability, excellent
mechanical strength and unique electronic properties, especially the high specific
surface area and excellent electric conductivity[7ꢀ11]. GN used in electrochemistry
have commonly been prepared by the oxidative exfoliation of pristine graphite to get
the aqueous dispersion of graphene oxide (GO) and the subsequent chemical
reduction of graphene oxide[12ꢀ19]. However, in the process of reduction, the
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chemical reduced graphene oxide tends to agglomerate or precipitate and can not be
redispersed in water by ultrasonication. To solve this problem, modification of
graphene oxide with poly (diallydimethylammonium chloride) has to be introduced to
improve their properties and extend their functions. Nonꢀcovalent functionalization is
preferable for catalyst support applications, because it enables attachment of
molecules or charged polyelectrolytes through πꢀπ stacking or hydrophobic
interactions, and thus preserves the intrinsic electronic and structural properties of
graphene. High electronic conductivity is very important for catalyst support
applications[20].
On the other hand, polyoxometalate has been applied to various kinds of
acidꢀcatalyzed oxidation reactions due to their electrochemical oxidizing
abilities[21–23]. It is believed that Kegginꢀtype POMs in an aqueous solution could
effectively assist the electrochemical oxidation of carbon monoxide (CO) to carbon
dioxide(CO
2
)[24ꢀ30]. Especially, transition metalꢀsubstituted polyoxometalates
(TMSP) are expected to act as promoters in the formic acid electroꢀoxidation process
on fuel cell anode due to their superior redox ability. Moreover, the existence of Co
also likely facilitates the effective CO removal according to bifunctional
mechanism[31].
In this paper, Pd/PDDAꢀGN/PMo11Co composite was prepared by layer by layer
(LBL) electrostatic assembly method and were used for electroꢀdeposition of Pd
particles in situ (Scheme 1). The CV results indicate that the
Pd/{PDDAꢀGN/PMo Co} as a electrocatalyst shows the excellent electrocatalytic
1
1
2
performance for formic acid oxidation. Three advantages of the developed
Pd/PDDAꢀGN/PMo11Co required for fuel cell catalysts have been found: First,
graphene sheets can not only offer a high surface area for the deposition of noble
metal NPs, but also allow for good electrical conductivity and stability in the ternary
composite. Second, the catalyst can be used for formic acid oxidation with significant
tolerance of carbon monoxide poisoning effects due to electrochemical oxidizing
abilities of TMSPs. Third, the smallꢀsized and highlyꢀdispersed Pd NPs supported on
graphene sheets take advantage of their catalytic function, which is conducive to a
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notable enhancement of the electrocatalytic activity. It can be demonstrated that
Pd/PDDAꢀGN/PMo Co nanoparticles exhibit enhanced electrocatalytic activity and
1
1
stability for formic acid oxidation compared to Pd supported on carbon (Pd/C).
Repeat step 1 and 2
2
.TMSPs
3.PdCl2
1
.PDDAꢀGN
(
IT of electroꢀ
deposition
)
Scheme 1 The synthetic procedures of the Pd/{PDDAꢀGN/TMSPs}n catalyst.
2
. Experimental section
2
.1. Chemicals and materials
Graphite powder (−325 mesh, 99.9995%), poly(diallyldimethylammonium
chloride (PDDA,Mw 100,000ꢀ200,000,20%), Nafion(117 solution), Pd/C(10 wt.%
loading, matrix activated carbon support), were purchased from SigmaꢀAldrich. PdCl
KMnO , H (30%), hydrazine hydrate (80%), NaBH , CoSO , KCl, K , P
2
,
4
2
O
2
4
4
S
2 2
O
8
2 5
O ,
H
3
PMo12
O40·xH
2
2 4 3
O (PMo12), H SO , HCl, HNO , NaOH, acetone, formic acid and
ethanol were all purchased from Sinopharm Chemical Reagent Co.Ltd. (Shanghai,
China) and used without further purification. Indium tinoxide (ITO) glass was
purchased from CSG Holding Co.Ltd. (Shenzhen,China). Doubly distilled water is
used throughout the whole experiments.
2
.2. Preparation
Graphene oxide (GO) was synthesized from natural graphite powder by a
modified Hummer’s method[32]. The preparation of PDDAꢀGN is base on the
reference[33]. The 1:11 metalꢀsubstituted derivative of the keggin polyoxometalate
PMo11Co was prepared as follows[34]:
4 4 2
Phosphomolybdic acid (H PMo12O40) and equimolar amounts of CoSO ·7H O
were added together to 50 mL of deionized water. After the pH of the mixed solution
o
was adjusted to 4 with the NaHCO
3
, the temperature of the solution was kept at 50 C
for 5 h. The product was filtered under vacuum,washed with ethanol and water and
dried under vacuum for 24 h. Finally, the brown precipitate was obtained after
4

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recrystallization.
Prior to modification, a GCE (glass carbon electrode) was carefully polished
successively with 1, 0.3 and 0.05 mm αꢀAl slurries and sonicated in deionized
water for 5 min after each polishing step. Finally, the GCE was sonicated and washed
2
O
3
ꢀ1
with ethanol. After cleaning, it was dipped into 1 mg·mL PDDAꢀGN solution for 30
min, rinsed with distilled water, and dried with nitrogen stream. Then, it was
immersed in 2 mM PMo11Co for 30 min. The preꢀcoated substrates were alternately
immersed in PDDAꢀGN and TMSPs solutions, respectively. After each immersion,
the substrates were rinsed with distilled water and dried with nitrogen gas. Composite
films were formed on the GCE by repeating the above steps in a cyclic fashion. An
indium tin oxide(ITO) glass and a quartz slide were ultrasonically cleaned with
ethanol, isopropyl alcohol, and water subsequently and dried over N . Then, the
2
assembly of composite films on ITO followed the same procedure as that for the GCE.
Pd particles were deposited on the surface of the {PDDAꢀGN/TMSPs}n composite
film modified GCE or ITO electrode in situ {PDDAꢀGN/TMSPs}n by potentionstatic
electrodeposition in a 0.5 M H
2
SO
4
2
solution containing 2 mM PdCl at ꢀ0.2 V. For the
purpose of comparison, Pd/{PDDAꢀGN/PMo12
}
2
was also prepared under the same
conditions. Pd/C modified electrode was prepared by pipetting 10 μL of a
wellꢀdispersed mixture (2 mg Pd/C catalyst dispersed in 1 mL ethanol with 50 μL
5
2
% Nafion) on the polished GCE.
.3. Characterization
The chemical structure and elemental analysis of the synthesized Pd/PDDAꢀGN/
PMo11Co were characterized by a FTꢀIR (Nicolet MagnaꢀIR 750, America) with the
ꢀ1
scanned area from 400 to 2000 cm , UVꢀvis absorption spectra were recorded on a
quartz slide using TUꢀ1810DPC spectrophotometer(PUXI, Beijing, China). The bulk
composition of the catalysts was evaluated by energy dispersive spectra (EDS) on a
JEOL 7500F scanning electron microscope. Xꢀray diffraction (XRD) patterns were
obtained on anX’pert Pro diffractometer (Philips, USA), using Cu Kα radiation. Xꢀray
photoelectron spectroscopy (XPS) was performed at room temperature with
monochromatic Al Kα radiation (1486.6 eV) using a Quantum 2000 system (PHI,
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USA). The actual amounts of Pd, P, Mo, Co and Cu loadings of the catalysts were
determined by inductively coupled atomic emission spectroscopy (ICPꢀAES,
ICAP6300, Thermo Scientific USA). Transmission electron microscopy (TEM)
images were obtained on a TECNAI G2F20 field emission transmission electron
microanalyzer (FEI, USA).
2
.4. Electrochemical measurements
Electrochemical experiments were carried out at the CHI 660C electrochemical
work station (Chenhua, Shanghai, China). A conventional three electrode system was
used, with a composite film coated electrode as the working electrode, a platinum
wire as the counter electrode and an Ag/AgCl (3.0 M KCl) (Chenhua, Shanghai,
China).
3
.Results and discussion
The FTꢀIR spectra of asꢀprepared PMo11Co compound shows four characteristic
ꢀ1
peaks ascribed to polyoxmetalate with Keggin structure at the range of 1100ꢀ700 cm
(
Fig.1A). Namely, four characteristic vibration bands of νas(PꢀO), νas(MoꢀO),
as(MoꢀO ꢀMo) and νas(MoꢀO ꢀMo) of PMo11Co are observed at 1048.9, 944.1, 872.1
and 798.6 cm respectively. The corresponding characterisic bands of H PMo12O
40
ν
b
c
−1
4
−1
appear at 1062.1, 960.5, 865.5 and 783.6 cm . It is found by comparion that the
vibrational frequencies of PꢀO and MoꢀO are red shifted, while that of MoꢀO ꢀMo and
MoꢀO ꢀMo are blue shifted, indicating the introduction of cobalt atom[37]. In addition,
b
c
ꢀ1
the peak attributed to CoꢀO vibration is found at 732.6 cm in the FTꢀIR spectra of
asꢀprepared PMo11Co compound[38]. It further proves that the asꢀsynthesized
ꢀ1
compound is PMo11Co. The peak at 1615.9 cm arises from the flexural vibration of
HꢀOꢀH. ICP measurement indicates that the actual mass fractions of elements P, Mo,
Co in PMo11Co is 1.73%, 58.84% and 3.72%, respectively, which is close to the
theoretical values: 1.73%, 58.89% and 3.29%. Therefore, the corresponding formula
is PMo11Co. EDS spectrum of the Pd/PDDAꢀGN/PMo11Co composite (Fig.1B) also
shows the characteristic peaks assigned to Co, P, Mo, Pd, O and C elements,
indicating existence of PMo11Co in the composite.
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Fig.1 (A) FTꢀIR spectrum of PMo Co (a), PMo (b) and
1
1
12
(B) EDS spectrum of the Pd/PDDAꢀGN/PMo11Co.
The
Pd/PDDAꢀGN/PMo12 catalyst are shown in Fig.2. The characteristic peaks at 30.6°,
5.6°, 50.8°, 60.5° belong to the ITO and the diffraction peaks at 39.9°, 46.2° can be
XRD
patterns
of
Pd/PDDAꢀGN/PMo11Co,
Pd/PMo11Co,
3
indexed to (111) and (200) lattice planes of Pd faceꢀcentered cubic crystalline
according to JCPDS standard 01ꢀ1201(Pd)[35, 36].
Fig.2 XRD patterns of ITO (a), Pd/PMo11Co (b), Pd/PDDAꢀGN/PMo11Co (c),
Pd/PDDAꢀGN/PMo12 (d).
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Fig.3(A) shows the XPS full spectrum of
The signals of Pd3d, Co2p,
the
Pd/PDDAꢀGN/PMo11Co.
O1s, P2p, Mo3d, N1s
C1s,
indicate that the compound is composed of Pd, Co, C, O, P, Mo and N elements.
Fig.3(C) and (D) show the C1s XPS spectra of GO and Pd/PDDAꢀGN/PMo11Co
composites, respectively. Four types of carbon with different chemical states are
2
observed: 284.6 eV (sp C), 286.7 eV (C–O), 287.7 eV (C=O) and 288.8 eV (O–C=O).
As depicted in Fig.3(C), the C–O peak intensities significantly decrease indicating the
percentage of surface oxygen groups in PDDAꢀGN is reduced obviously after
treatment with hydrazine in the presence of PDDA. The Pd3d spectra of the
Pd/PDDAꢀGN/PMo11Co are displayed in Fig.3(B). Apparently, the two distinct peaks
located at 335.8 eV and 341.2 eV can be assigned to the Pd 3d3/2 (highꢀenergy band)
and Pd 3d5/2 (lowꢀenergy band) spinꢀorbit states of zeroꢀvalent Pd, and the weak
intensity doublets around 338.0 eV and 343.7 eV can be ascribed to Pd 3d3/2 and Pd
3d5/2 peaks of PdO[40].
Fig.3 XPS spectra of (A)full spectrum, (B) Pd 3d of Pd/PDDAꢀGN/PMo11Co,
(C) C1s of Pd/PDDAꢀGN/PMo11Co, (D) C1s of GO.
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Surface morphology of Pd/PDDAꢀGN/PMo11Co and Pd/PDDAꢀGN/PMo12 were
characterized by scanning electron microscope (SEM), as shown in Fig.4. PDDAꢀGN
shows characteristic wrinkled morphology, the Pd NPs are deposited on
PDDAꢀGN/PMo11Co (Fig.4(A)). Compared with Pd/PDDAꢀGN/PMo12, it can be
noticed that the Pd nanoparticles are well dispersed on the surface of graphene in the
Pd/PDDAꢀGN/PMo11Co.
Fig.4 SEM images of Pd/PDDAꢀGN/PMo11Co (A), Pd/PDDAꢀGN/PMo12 (B).
The morphology, structure, particle size of Pd/PMo11Co, and
Pd/PDDAꢀGN/PMo11Co were examined by TEM and HRTEM. Fig.5e and 5f display
TEM images and the corresponding size distribution histograms of the
Pd/PDDAꢀGN/PMo11Co and Pd/PMo11Co composites, respectively. The size
distributions were obtained by measuring sizes of 100 randomly selected particles in
the magnified TEM images. As shown in Fig.5e and 5f, Pd NPs on the
PDDAꢀGN/PMo11Co support are uniform and exhibit narrow size distribution with an
average particle size of ca.3.2 nm (Fig.5f). In contrast, Pd NPs in the Pd/PMo Co
1
1
composite are dispersed in a relatively wide size distribution, and the average particle
size is approximately 3.5 nm (Fig.5e). This result indicates that the introduction of
PDDAꢀGN for Pd/PDDAꢀGN/PMo11Co composite is beneficial to dispersion and
stabilization of Pd NPs. The typical high magnification TEM images of Pd/PMo11Co,
Pd/PDDAꢀGN/PMo11Co (Fig.5c and 5d) reveal that the dꢀspacing of adjacent fringes
for the Pd NPs is 0.194 nm and 0.223 nm, respectively, corresponding to the (200)
and (111) planes of faceꢀcenteredꢀcubic (fcc) Pd NPs[39]. It indicats that Pd NPs can
be effectively synthesized with uniform shape, excellent size distribution and well
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dispersed on the unique 2D PDDAꢀGN supports.
Fig.5 TEM images, HRTEM images and corresponding particle size distribution histograms of the
Pd/PMo11Co composite (a, c, e) and the Pd/PDDAꢀGN/PMo11Co composite (b, d, f).
The performance of the composites with different layers toward formic acid
1
0

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oxidation reaction in acid medium was evaluated by cyclic voltammetry. Fig.6 shows
the CVs of the electrodes coated with Pd/{PDDAꢀGN/PMo Co}n (n=1ꢀ4)
1
1
composites in a solution containing 0.5 M H
2
SO
0 mVs . The electrode modified by Pd/{PDDAꢀGN/PMo11Co}
peak current intensity, suggesting the highest catalytic activity for formic acid
4
and 1 M HCOOH at a scan rate of
ꢀ1
5
2
shows the highest
2
electroꢀoxidation. Therefore, the Pd/{PDDAꢀGN/PMo11Co} composite film was
chosen as the optimal substrate in the subsequent researches.
−
1
Fig.6 The CV of different lays modified electrodes: at scan rate 50 mV s in 0.5 M H SO containing 1
2
4
M HCOOH. (a)Pd/{PDDAꢀGN/PMo11Co}
4
3
, (b)Pd/{PDDAꢀGN/PMo11Co} ,
(
c)Pd/{PDDAꢀGN/PMo11Co} , (d)Pd/{PDDAꢀGN/PMo11Co}.
2
Fig.7 shows the linear polarization curves of the electrodes coated with
Pd/{PDDAꢀGN/PMo11Co} and Pd/PMo11Co composites in a solution containing 0.5
M H SO and different formic acid concentrations at a scan rate of 50 mVs . The
2
ꢀ1
2
4
peak potential(Ep) and the peak current density(j) are shown in table 1 below. From
Fig.7A and 7B, it can be clearly seen that formic acid concentrations have obvious
influence on electrocatalytic activity of different composite as a catalyst. When the
formic acid concentrations increased (≥2 M HCOOH) or decreased (≤0.5 M
HCOOH), the peak current density of formic acid oxidation decreased. In addition,
the peak potentials of HCOOH (1M) oxidation shifts negatively compared to those of
HCOOH (2M) and HCOOH (5M) [41]. The higher peak current density and the more
negative peak potentials of formic acid oxidation indicate the higher catalytic activity
2
of Pd/{PDDAꢀGN/PMo11Co} composite.
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−
1
Fig.7 The CV of different modified electrodes at scan rate 50 mV s in 0.5 M H SO containing
2
4
2
χ M HCOOH : (A)Pd/PMo11Co, (B)Pd/{PDDAꢀGN/PMo11Co} .
HCOOH concentrations: (a) 0.5 M, (b) 1 M, (c) 2 M, (d) 5 M.
Table1 the plot of the peak potentials and the peak current density versus formic acid concentration.
HCOOH
0.5
1
2
5
concentration
Ep /
V
j /
Ep /
V
j /
Ep /
V
j /
Ep /
V
j /
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
(M)
mA mg
106.13
153.96
mA mg
mA mg
mA mg
Pd/PMo11Co
0.25
0.29
0.30
406.59
641.36
0.35
0.35
339.45
405.75
0.43
0.45
294.15
362.98
Pd/{PDDAꢀGN/ 0.25
PMo Co}
11
2
Fig.8 presents the CVs of the Pd/{PDDAꢀGN/PMo11Co}
2
, Pd/C and
Pd/{PDDAꢀGN/PMo } as electrocatalysts for formic acid oxidation in 0.5M H SO
1
2
2
2
4
+
1 M HCOOH solution. The peak current density of Pd/{PDDAꢀGN/PMo11Co}
2
is
ꢀ1
ꢀ1
6
41.36 mA mg compared to 521.13 mA mg for Pd/{PDDAꢀGN/PMo12
2
} , whereas
ꢀ1
ꢀ1
4
05.75 mA mg for Pd/PMo Co and 155.93 mA mg for Pd/C. Obviously,
1
1
Pd/{PDDAꢀGN/PMo11Co}
2
exhibits the highest electrocatalytic activity toward
compared with Pd/C, Pd/PMo11Co and
. The better performance of Pd/{PDDAꢀGN/PMo11Co}
can be attributed to the following three major factors: (1) poly
diallydimethylammonium chloride) (PDDA) has excellent binding capability with
formic
acid
oxidation
Pd/{PDDAꢀGN/PMo12
}
2
2
(
graphene so as to increase the solubility and decrease agglomeration of graphene and
maintain the electronic structure of graphene. PDDAꢀGN with 2D flat planes exhibit
substantial advantages with regard to mass and charge transport for both electronic
and ionic transport, a higher electrode /electrolyte contact area[20, 34], (2) the well
dispersed Pd NPs with small size are beneficial to their electrocatalytic activities,
1
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stability, CO toleration and (3) most importantly, the existence of PMo11Co in the
hybrids may be helpful for the electrocatalytic activities and antiꢀpoisoning properties
of the catalysts. TMSPs are crystalline materials with very high proton conduction.
They can act as redox mediators for the electrochemical oxidation of CO[42].
Compared with PMo , electrodes modified with PMo Co catalyst showed better
12
11
effect on the electrocatalytic oxidation of formic acid, resulting from the high charge
density, and the excellent ability of electron donating of PMo11Co[43].
−
1
Fig.8 The CV of different modified electrodes at scan rate 50 mV s in 0.5 M H
containing 1 M HCOOH: (a) Pd/C, (b) Pd/PMo11Co, (c) Pd/{PDDAꢀGN/PMo12
d) Pd/{PDDAꢀGN/PMo11Co}
2 4
SO
2
} ,
(
2
.
The electrocatalytic stability of different composites was evaluated by the
chronoamperometry measurements performed in 0.5 M H SO containing 1 M
2
4
HCOOH at a fixed potential of 0.2 V as shown in Fig.9. Although the current density
of all composites decayed rapidly at the initial stage, the decrease in the oxidation
current densities on the Pd/{PDDAꢀGN/PMo Co} electrode was significantly
1
1
2
slower than that on Pd/{PDDAꢀGN/PMo12
indicating its better durability towards formic acid oxidation. At the end of 3600 s, the
residual current density of Pd/{PDDAꢀGN/PMo11Co} was also higher than that of
the other three composites. These results indicate that the Pd/{PDDAꢀGN/PMo11Co}
2
} , Pd/PMo11Co and commercial Pd/C,
2
2
composite exhibits the slowest current decay over time, confirming the excellent
catalytic stability, which is consistent with the above CV results (Fig.8).
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Fig.9 Chronoamperometric curves of different modified electrodes in 0.5 M H SO4
2
containing 1 M HCOOH at 0.2 V for 3600 s: (a) Pd/C, (b) Pd/PMo11Co,
(
2 2
c) Pd/{PDDAꢀGN/PMo12} , (d) Pd/{PDDAꢀGN /PMo11Co} .
Based on COꢀstripping voltammogram studies (Fig.10), it is found that the
electrochemically surface area of the Pd/C, Pd/{PDDAꢀGN/PMo } , Pd/PMo Co
1
2
2
2
11
ꢀ1
and Pd/{PDDAꢀGN/PMo11Co}
2
are 10.33, 16.40, 18.76 and 20.11 m g , respectively.
The oxidation peak potential of CO oxidation at the Pd/{PDDAꢀGN/PMo11Co}
negatively shift compared to that at the Pd/C, Pd/PMo11Co and
Pd/{PDDAꢀGN/PMo12 . This difference can be explained that the PMo11Co ions
2
is
}
2
facilitate the electrooxidation of intermediate species such as COads or the COꢀlike
species during formic acid electrooxidation due to its superior redox quality[4].
Therefore, PMo Co ions as an additive into the Pd/{PDDAꢀGN/PMo Co}
1
1
11
2
composite should have a positive effect on the removal of COads in the formic acid
oxidation. The above results demonstrate that the Pd/{PDDAꢀGN/PMo11Co} catalyst
exhibits better electrocatalytic properties and better resistance to poisoning than the
Pd/C, Pd/PMo11Co and Pd/{PDDAꢀGN/PMo12 catalysts.
2
}
2
1
4

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Fig.10 COꢀstripping voltammograms of different modified electrodes in 0.5 M H SO solution
2
4
−
1
at scan rate of 50 mV s : (a) Pd/C, (b) Pd/{PDDAꢀGN/PMo12
2
} , (c) Pd/PMo11Co,
(
d) Pd/{PDDAꢀGN/PMo11/Co}
2
.
Electrochemical impedance spectrum (EIS) was performed to investigate the
intrinsic role of polymer modified graphene (PDDAꢀGN) in the multilayer
composites[13, 44ꢀ46]. The impedance measurements were made with frequencies
5
ranging from 0.01 Hz to 10 Hz and an amplitude voltage of 0.1 V. The impedance
data can be fitted by an equivalent electrical circuit composed by one series circuit of
d
a resistance (Rct) and capacitor (C ) in parallel. Usually, the high frequency semicircle
diameter is equal to the charge transfer resistance (Rct), which is resulted from the
charge transfer process at the interface of electrode/electrolyte[39]. As shown in
Fig.11, R decreases remarkably with the introduction of PDDAꢀGN into multilayer
ct
films, which means that PDDAꢀGN/PMo11Co helps enhance electron transfer
obviously on the electrode interface. At the same time, with the preparation of LBL
and electrodeposition, electrons transfer faster on the smooth and thin films than on
the rough surface of Pd/C through dropping on an electrode.
2 4
Fig.11 Nyquist plots of the different composites in 0.5 M H SO under open circuit voltage with
5
frequencies ranging from 10 Hz to 0.01 Hz: (a)Pd/{PDDAꢀGN/PMo11Co}
2
,
(
2
b)Pd/{PDDAꢀGN/PMo12} , (c)Pd/{PDDAꢀGN}, (d)Pd/PMo11Co, (e)Pd/C.
3
. Conclusions
In summary, the {PDDAꢀGN/PMo11Co}n multilayer films were successfully
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fabricated by LBL method and used as support for electrodeposition of Pd
nanoparticles in situ. It is worthy of note that Pd is uniformly distributed on the
2
surface of {PDDAꢀGN/PMo11Co} film. Furthermore, due to the superior redox
property of PMo11Co, the Pd/PDDAꢀGN/PMo11Co composite using as a
electrocatalyst exhibits higher electroꢀcatalytic activity , improved CO tolerant ability
and better electrochemical stability for formic acid oxidation compared with Pd/
PDDAꢀGN/PMo12 and commercial Pd/C. Thus, the Pd/PDDAꢀGN/PMo11Co
composite is expected to be a substantial electrocatalyst in DFAFC.
Acknowledgments
This project was financially supported by the National Natural Science
Foundation of China (No. 21571034), the Natural Science Foundation of Fujian
Province (No.2014J01033) and a key item of Education Department of Fujian
Province (No. JA13085).
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Table of contents entry
The PMo11Co into the composite Pd/PDDA-GN/PMo11Co contributes to converting
intermediate species CO into CO for formic acid oxidation.
2