Green synthesis of PdCu supported on graphene/polyoxometalate LBL films for high-performance formic acid oxidation

Article abstract of DOI:10.1039/c5ra09492e

PdCu alloy nanoparticles (with sizes of ca. 4.5 nm) have been synthesized by a one-step electrochemical process on composite films constructed from functionalized graphene (GN) and H₃PMo₁₂O₄₀ (PMo₁₂) by LBL asse

Full text of DOI:10.1039/c5ra09492e

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Green synthesis of PdCu supported on graphene/
polyoxometalate LBL films for high-performance
formic acid oxidation†
Cite this: RSC Adv., 2015, 5, 64534
a
a
a
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Received 20th May 2015
Accepted 23rd July 2015
*
Ai Ma, Xiaofeng Zhang, Xiaoying Wang, Lijuan Le and Shen Lin
DOI: 10.1039/c5ra09492e
www.rsc.org/advances
PdCu alloy nanoparticles (with sizes of ca. 4.5 nm) have been and light weight.8 On other hand, polyoxometalates possess
synthesized by a one-step electrochemical process on composite metal–oxygen frameworks and undergo reversible and stepwise,
films constructed from functionalized graphene (GN) and H3PMo12O40 multi-electron-transfer reactions. Their rich redox property has
(PMo12) by LBL assembly. The as-prepared hybrid displays enhanced made them attractive candidates for electrode modication,
eletrocatalytic activity and extended durability towards formic acid electrocatalysis, and electroanalysis.9
oxidation in an acid media.
In this study, rst, PDDA-GN/PMo12 lms were prepared by
layer-by-layer electrostatic assembly as supports: on one hand,
poly (diallydimethylammonium chloride) (PDDA) has excellent
binding capability with graphene so as to increase the solubility
and decrease agglomeration of graphene and could maintain
the electronic structure of graphene;10 on the other hand,
phosphomolybdic acid (PMo12) with strong oxidation ability
can more easily convert COads to CO2 releasing the active sites of
catalysts for further electrochemical reaction. Furthermore, LBL
assembly is an advisable method for the deposition of inor-
ganic–organic hybrid thin lms owing to the merits such as low
cost, room temperature process, and especially the controllable
thickness at a molecular scale.11 PdCu alloy is very fascinating
for its high catalytic performance and founded to exhibit strong
synergy for the formic acid electro-oxidation.12 We perform an
electrochemical synthesis experiment to allow the formation of
PdCu nanoparticles at room temperature, in a green, facile and
efficient way (see experimental details in ESI†). In addition, the
produced PdCu@PDDA-GN/PMo12 catalyst, as a continuation of
our previous research,13 is used as an anode and tested in for-
mic acid oxidation. We put forth effort to discuss the three
components of PDDA-GN, PMo12 and binary alloy PdCu play the
roles respectively, and how a synergistic effect can dramatically
improve the stability the electrocatalytic activity of the Pd-based
catalyst regarding formic acid oxidation.
Direct formic acid fuel cells (DFAFCs) which possess lots of
advantages have attracted considerable attention. Formic acid
as an energy carrier is easier to store and transport than
hydrogen. Meanwhile, formic acid has a reduced toxicity, higher
energy densities and lower fuel crossover through Naon
membranes in comparison to methanol.1 Initial catalysis
research on the DFAFCs was dominated by studies of Pt-based
catalysts as anode catalysts.2 In view of the catalytic efficiency,
recent interests focus on Pd-based catalysts which are advan-
tageous towards formic acid electrochemical oxidation mostly
through the direct pathway.3 However, pure Pd suffers from the
catalyst poisoning during the formic acid electro-oxidation
reaction more or less.4 In order to further improve the cata-
lytic activity and stability while lower the overall cost of anode
catalyst, Pd is used to combined with a transition metal (such as
Cu, Co, Ni, Fe, Ir, etc.) to form the binary alloy.5 The enhance-
ment is mainly due to the formation of bimetallic interaction
which is stronger than a single metal–oxygen (M–O) interaction
so as to weak the adsorption of inhibiting reaction intermedi-
ates (i.e. COads) on catalyst surface,6 which has been explained
by the electronic, strain, or alloying effects.7
Graphene (GN) could be a desirable support substrate for
growing and anchoring metal NPs due to its large surface area,
excellent conductivity, chemical stability, mechanical strength
Fig. 1a shows a TEM image of PdCu@PDDA-GN/PMo12
containing small homogeneous particles in nanometric size
with spherical morphology on the thin lms. In the absence of
PdCu with other conditions unchanged, the as-produced
sample is PDDA-GN/PMo12. We also observed the morphology
and thickness of PDDA-GN/PMo12 lms through FE-SEM and
AFM (ESI, Fig. S1a and S2†). The histogram (the inset of Fig. 1a)
indicates a particle size distribution with values ranging from 2
aCollege of Chemistry & Chemical Engineering, Fujian Normal University, Fuzhou
350007, China. E-mail: shenlin@nu.edu.cn; Fax: +86-591-22867399; Tel: +86-591-
22867399
bFujian Key Laboratory of Polymer Materials, Fuzhou 350007, Fujian, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra09492e
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to 5.5 nm. The distinct lattice fringes of d ¼ 0.218 nm match
with the crystallographic plane of PdCu (111) which indicate the
existence of the single-phase PdCu alloy.14 SAED pattern (the
inset of Fig. 1b) is also providing quick and easy crystal orien-
tation information of the obtained PdCu. The lattice spacing
measured from the diffraction rings of PdCu was matched well
for JCPDS: 48-1551 data, which demonstrated that the PdCu
have good face-centered crystalline structure.
EDS (Fig. 2a) indicates that the as-prepared PdCu alloy is
composed of Pd and Cu with an atomic ration of 1:1.5. The X-ray
diffraction patterns of the blank ITO (curve a) and
PdCu@PDDA/GN on ITO (curve b) are shown in Fig. 2b. The 2q
values corresponding to the (222), (400), (440) and (622) crystal
face diffraction peaks for ITO are 30.6ꢀ, 35.6ꢀ, 50.8ꢀand 60.5ꢀ,
respectively.15 The graphene and PMo12 are absent in the XRD
because their contents are low and PMo12 clusters do not exist
in the crystalline state, but in the dispersed state. As illustrated
in pattern b, the diffraction pattern shows no diffraction peaks
match to Pd or Cu reported on standard JCPDS cards (Pd, 65-
2867), (Cu, 04-0836), respectively. However, it is easy to nd one
peak at about 41ꢀ besides the peaks for ITO, which can be
assigned to the diffraction peaks from the (111) crystal face of
the face-centered cubic lattice PdCu alloy.6 This indicates the
formation of the single-phase PdCu alloy which is agree with
results obtained from data of Fig. 1b. From the full width half
maximum of the (111) peak, the volume-averaged particles size
is estimated by the Scherrer's equation: D ¼ 0.89lka/B2q
cos qmax, where D is the mean size of the PdCu particles, lka is
Fig. 2 (a) EDS spectrum of the PdCu@PDDA-GN/PMo12. (b) XRD
patterns of the blank ITO (curve a) and the PdCu@PDDA-GN/PMo12 on
an ITO electrode (curve b).
reduced obviously aer treatment with hydrazine in the pres-
ence of PDDA. These results imply that considerable deoxygen-
ation occurred by PDDA graing and hydrazine reduction. The
Cu2p peaks appeared at 933.1 eV and 952.9 eV were observed
from Fig. 3c, which were attributed to the Cu2p3/2 and Cu2p1/2
bands, respectively. Two peaks (Fig. 3d) at 335.1 eV and 340.4 eV
in the Pd3d region corresponded to Pd3d5/2 and Pd3d3/2 bands,
respectively. These peaks demonstrated the Cu and Pd on the
surface of catalyst. Compared to the standard zero-valent
copper and palladium binding energy (ESI, Table S2†),16
Cu2p3/2 and Cu2p1/2 shied to higher binding energy (shied by
0.6 eV and 0.3 eV) while Pd3d5/2 and Pd3d3/2 binding energy
shied to lower (shied by 0.3 eV and 0.4 eV), which due to
electron interaction between Cu and Pd atomic orbital.17 This
result implied further that PdCu alloy had formed.
The electrochemically active surface area (ECSA) of the
catalysts modied electrodes was determined by CO-stripping
cyclic voltammetry (Fig. 4a). The associated ECSA (ESI, Table
S1†) was obtained from the following equation:17 ECSA ¼ Q/M ꢁ
420, where Q is the charge of CO desorption-electrooxidation in
micro-coulomb (mC), M represents the total amount of Pd (mg)
on the electrode surface, and 420 mC cmꢂ2 is the charge
required to oxidize a monolayer of CO on the Pd catalyst. The
ECSA is estimated to be 23.62 m2 gꢂ1 Pd for the PdCu@PDDA-
GN/PMo12 (Fig. 4a), which is about 2.3 times of commercial Pd/
C (10.33 m2 gꢂ1 Pd). The enlarged ECSA is ascribed to the
˚
the X-ray wave-length (Cu Ka lka ¼ 1.54056 A), qmax is the
maximum angle of the (111) peak, and B2q is the half-peak width
for PdCu (111) in radians. The mean PdCu particle size of is 4.5
nm, which is consistent with the histogram obtained from TEM.
The XPS full spectrum on the PdCu@PDDA-GN/PMo12
(Fig. 3a) is mainly dominated by the signals of Pd3d, Cu2p, C1s
and O1s elements, respectively. Meanwhile, we can also nd
weak signals of P2p, Mo3d and N1s, indicating there are small
amounts of PMo12 and PDDA-GN. Besides XPS, we used CV to
detect PMo12 and the number of lm layers (ESI, Fig. S3†). The
XPS spectra of C1s in Fig. 3b correspond to PDDA-GN species.
The C–O peak intensities signicantly decrease, which indicates
that the percentage of surface oxygen groups in PDDA-GN is
Fig. 1 (a and b) TEM and HRTEM images of the PdCu@PDDA-GN/
PMo12. Insets show the corresponding size distribution in image (a) and Fig. 3 XPS spectra of full spectrum (a), C1s (b), Cu2p (c), and Pd3d (d)
SEAD pattern in image (b).
of the PdCu@PDDA-GN/PMo12 on an ITO electrode.
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homogeneous distribution of the PdCu nanoparticles on lms
In addition, the enhanced electrocatalytic performance may
of PDDA-GN/PMo12. The onset potential of CO-stripping nega- be owing to the superior electric conductivities of the polymer-
tively shis 110 mV on the PdCu@PDDA-GN/PMo12 modied modied graphene (PDDA-GN) in the multilayer lms,21 which
electrode, compared with the commercial Pd/C, indicating the can be proven by Nyquist plots of the electrochemical imped-
enhanced CO tolerance ability. For one thing, the strength of Pd ance spectrum (EIS) (Fig. 4d). The impedance measurements
and Cu interactions is great strong so as to weak the adsorption were made with frequencies ranging from 0.01 Hz to 105 Hz and
of inhibiting reaction intermediates (i.e. COads) on catalyst an amplitude voltage of 0.1 V. The impedance data can be tted
surface.18 For another, the benecial role of HPMo12 for CO by an equivalent electrical circuit composed by one series circuit
further electro-oxidation and release of surface active sites by of a resistance (Rct) and capacitor (Cd) in parallel.22 Usually, the
CO removal.19
high-frequency semicircle diameter is equal to the charge-
The electrochemical stability of the PdCu@PDDA-GN/PMo12 transfer resistance (Rct), which is resulted from the charge
was also investigated in 0.5 M H2SO4 containing formic acid at transfer process at the interface of electrode/electrolyte.23 As
an applied potential of 0.2 V for 3600 s (Fig. 4b). The polariza- shown in Fig. 4d, Rct markedly decreases with the introduction
tion currents on the PdCu@PDDA-GN/PMo12 modied elec- of PDDA-GN/PMo12 into multilayer lms. This means that
trode decrease within 1000 s and then decay quite slowly to polymer-modied graphene helps enhance electrons transfer
approach a limiting current (up to 3600 s). The initial currents obviously on the electrode interface. At the same time, with the
drop quickly for the PdCu@PDDA-GN/PMo12, probably due to preparation of LBL and electro-deposition, electrons transfer
the formation of the intermediate species during formic acid faster on the smooth and thin lms than on the rough surface
oxidation reaction.20 Moreover, the corresponding limiting of Pd/C through dropping on an electrode.
currents decrease to 31.53 mA mgꢂ1 Pd up to 3600 s, which is
In this work, a simple and facile approach was developed for
much higher than the commercial Pd/C (15.65 mA mgꢂ1 Pd) preparation of the PdCu@PDDA-GN/PMo12, with the assistance
catalysts under the same conditions. The activity results of Pd of Cu to obtain the better electrocatalytic activity and extended
with the same conditions are also lower than PdCu@PDDA-GN/ durability, using PDDA-GN as an electron carrier and PMo12 as a
PMo12 (ESI, Fig. S4†). The improved catalytic activity and better promoter to convert COads to CO2 more simply. The as-prepared
stability of the PdCu@PDDA-GN/PMo12 was further conrmed PdCu@PDDA-GN/PMo12 exhibits a signicant effect on the
by cyclic voltammetry. The catalytic current density keeps enhanced electrocatalytic activity, the improved tolerance of CO
almost constant within 100 cycles. With the peak current and better stability for formic acid oxidation. This work
density of the 5th cycle in the forward sweep as a reference, the provides a promising strategy to fabricate graphene-supported
peak current density of the PdCu@PDDA-GN/PMo12 remains and Pd-based alloy electrocatalysts for efficient fuel cell
about 92% of its original value aer 100 cycles. From Fig. 4b and applications.
c, the PdCu@PDDA-GN/PMo12 is conspicuously better than
commercial Pd/C at electrocatalytic activity and better stability
for formic acid oxidation.
Acknowledgements
This work was nancially supported by NSFC (No. 21171037).
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