Copper-enriched palladium-copper alloy nanoparticles for effective electrochemical formic acid oxidation

Article abstract of DOI:10.1016/j.elecom.2016.05.021

In this work, Pd-Cu alloy nanoparticles (NPs) with different atomic ratios are prepared on functionalized carbon nanotubes (CNTs) and applied as electrocatalysts for formic acid oxidation. The Cu-enriched Pd-Cu alloy NPs exhibit improved electrocatalytic

Full text of DOI:10.1016/j.elecom.2016.05.021

Electrochemistry Communications 69 (2016) 55–58
Contents lists available at ScienceDirect
Electrochemistry Communications
journal homepage: www.elsevier.com/locate/elecom
Copper-enriched palladium-copper alloy nanoparticles for effective
electrochemical formic acid oxidation
a
a
b,
a
c
a,
Qi Zhao , Juan Wang , Xing Huang ⁎, Yuanying Yao , Wei Zhang , Lidong Shao ⁎
a
Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power, Shanghai University of Electric Power, Shanghai 200090, China
Department of Inorganic Chemistry, Fritz-Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany
CIC Energigune, Parque Tecnológico de Álava, Miñano 01510, Bilbao 48011, Spain
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 April 2016
Received in revised form 31 May 2016
Accepted 31 May 2016
Available online 01 June 2016
In this work, Pd-Cu alloy nanoparticles (NPs) with different atomic ratios are prepared on functionalized carbon
nanotubes (CNTs) and applied as electrocatalysts for formic acid oxidation. The Cu-enriched Pd-Cu alloy NPs ex-
hibit improved electrocatalytic activity and stability. Functionalized carbon supports are applied as substrates to
tune the nanoscale morphologies of the obtained bimetallic phases under appropriate calcination and hydroge-
nation treatments. Spill-over effect aids a reduction of a high weight loading of Cu in its metallic phase, in turn,
these Cu atoms integrate into Pd lattice and isolate Pd neighbouring atoms. Surface analyses show that a certain
amount of the isolated Pd remains on the surfaces of Pd-Cu alloy NPs, which is responsible for the enhanced
electrocatalytic performance.
Keywords:
Formic acid
Copper-enriched
Palladium-copper alloy
Carbon nanotube
© 2016 Elsevier B.V. All rights reserved.
1
. Introduction
In recent years, formic acid, which represents a new feedstock for
increase the stability of Pd-based catalysts. However, the practical appli-
cation of Pd-based alloys as electrocatalysts for DFAFCs remains chal-
lenging. First, Pd and other added metals have been reported to
exceed 20–30 wt% [9,10]. Using this amount of metals prohibits the
metal dispersion in nanoscale and limits the accessible surface areas
during catalysis. Second, the effects of the surface electronic properties
and structural stabilities of bimetallic compounds (controlled by bond-
ing patterns) on HCOOH oxidation remain unclear.
fuel cells, has been widely investigated because it is non-toxic and ex-
hibits a low crossover rate through Nafion membranes. Direct formic
acid fuel cells (DFAFCs) have favourable oxidation kinetics, low operat-
ing temperatures, and high theoretical open-circuit potentials, and they
2
effectively overcome the H -storage and transport problems associated
with proton-exchange membrane fuel cells (PEMFCs) and the toxicity
of the methanol used in direct methanol fuel cells (DMFCs). Thus, they
open up a pathway for the development of a new type of portable
power [1–3].
Herein, Pd-Cu alloys with various Pd:Cu molar ratios are fabricated
on carbon nanotubes (Pd
Cu /CNTs) for catalytic studies in HCOOH ox-
x y
idation. Functionalized CNTs embed nanocrystals of the formed alloy
particles and facilitate their nanoscale dispersion during calcination
and reduction treatments. The HCOOH oxidation results indicate that
Cu-enriched Pd-Cu alloy NPs exhibit enhanced activity and stability rel-
ative to the other Pd Cu alloys with lower Cu contents. Furthermore,
x y
surface and structural analyses reveal that the surface electronic proper-
ties of Cu-enriched Pd-Cu alloy NPs may govern the catalytic pathways.
Currently, the catalytic materials that are most commonly used as
fuel cell anodes are Pt-based alloys [4,5]. However, Pt resources are lim-
ited, expensive, and prone to carbon monoxide (CO) poisoning, and
thus, the application of Pt catalysts is limited [3,6,7]. Pd and Pt are ele-
ments of the same group and have similar characteristics, including
face-centred cubic (fcc) crystal structures and similar atomic sizes. Pd-
based catalysts are considered to be better electrocatalysts than Pt-
based ones because the reaction on the Pd surface follows a direct path-
way. Nevertheless, the inferior electrocatalytic stability of Pd catalysts
has limited their development [8]. Introducing structural modifications
by adding foreign atoms to generate Pd-based alloys has been investi-
gated to enhance the catalytic activity at low overpotentials and
2. Materials and methods
PR24 low-heat-treated (LHT) CNTs (1 g) were functionalized by ox-
idation in concentrated HNO
CNTs (where x:y = 1:0, 3:1, 1:1, 1:3, or 0:1) were synthesized as follow-
ing: in the case of Pd Cu /CNTs, 31.8 μL of Pd(NO (10 wt%, Sigma–
Aldrich) solution, 11.2 mg Cu(NO (Sigma-Aldrich) were mixed into
0 mL ethanol, followed by adding 50 mg of CNTs to allow coordination
3 x y
(300 mL) for 1 h at 120 °C [11]. Pd Cu /
1
3
3 2
)
3 2
)
2
⁎
Corresponding authors.
with functionalities as anchoring sites. Stirring was maintained at 45 °C
throughout the impregnation process until the solution had dried. The
E-mail addresses: xinghuang@fhi-berlin.mpg.de (X. Huang), lidong.shao@shiep.edu.cn
(L. Shao).
http://dx.doi.org/10.1016/j.elecom.2016.05.021
388-2481/© 2016 Elsevier B.V. All rights reserved.
1

56
Q. Zhao et al. / Electrochemistry Communications 69 (2016) 55–58
collected powder was calcined in a muffle for 2 h at 250 °C and then
annealed in a tube furnace for 2 h at 250 °C under a He/H atmosphere
He, 100 mL/min; H , 25 mL/min). The preparation of other samples
used a similar method. The atom ratios in terms of real metal weight
percentages in the alloy NPs were measured by the inductively coupled
plasma atomic emission spectroscopy (ICP): Pd/CNTs (3.25 wt%),
x y
Fig. 2 shows the XRD patterns of the Pd/CNTs, Pd Cu /CNTs, and Cu/
2
CNTs samples. The Pd/CNTs sample shows peaks at 40.1° (Pd(111)),
46.7° (Pd(200)), and 68.1° (Pd(220)), suggesting the synthesis of Pd
NPs (JCPDS standard Pd 46-1043). The Cu/CNTs sample shows peaks
at 43.3° (Cu(111)) and 50.4° (Cu(200)), indicating the synthesis of Cu
NPs (JCPDS standard 04-0836(Cu)). The peaks located at ~42.8° for
(
2
1 3 1 1 3 1
Pd Cu (Pd 3 wt%, Cu 5.25 wt%), Pd Cu (Pd 2.8 wt%, 1.6 wt%), Pd Cu (Pd
1 1 3 1
the Pd Cu /CNTs, Pd Cu /CNTs, and Pd/CNTs samples correspond to
2
.8 wt%, Cu 0.7 wt%). The weight percent of Cu in Cu/CNTs was 3 wt%. X-
the C(101) reflection planes of carbon (JCPDS standard C 26-1077).
Fig. 2b–d shows that the Pd peaks broaden and the 2θ angles increase
from 40.1 to 41.4° as the Cu content increases, indicating that the Cu
atoms have entered the Pd crystal to generate a fcc Pd-Cu alloy structure
[12,13]. It should be noted that systems containing two or more metallic
ray diffraction (XRD) was performed using a Bruker D8 Advance XRD
instrument with Cu-Kα radiation. An aberration-corrected JEOL JEM-
ARM200CF transmission electron microscope was employed to investi-
gate structural and chemical properties in STEM mode. X-ray photoelec-
tron spectroscopy (XPS) was conducted with a Thermo ESCALAB 250
XPS instrument using a monochromatic Al Kα source (1486.6 eV).
1 3
species may exist in several phases. The Pd Cu /CNTs show a perturba-
tion peak at 41.3°, indicating that a PdCu intermetallic phase (JCPDS
standard 48-1551 (CuPd)) may have formed within the Pd
1 3
Cu /CNTs
3
. Results and discussion
sample.
XPS was further utilized to examine the surface compositions and
valence electronic structures of Pd Cu /CNTs catalysts. Upon addition
of Cu, the original Pd peak profile is gradually weakened (Fig. 3A). How-
ever, the preserved Pd 3d spectra of all the Pd Cu /CNTs indicate that
Fig. 1a shows the dispersion of Pd-Cu NPs on the Pd
1
Cu
3
/CNT. Fig. 1b
x
y
presents the high-resolution scanning TEM (HR-STEM) image of a Pd-
Cu alloy NP on the Pd Cu /CNTs. The HR-STEM image shows that the
1
3
x
y
interplanar distances were 0.220 nm and presents two sets of (111)
planes with an acute angle 70.5°. Fig. 1c is the STEM image of dispersed
some Pd in monometallic phase remains dispersed on the catalyst sur-
faces. On the other hand, Fig. 3B shows a higher Cu0 peak on the
Pd-Cu alloy NP on the Pd
1
Cu
3
/CNT, and Fig. 1d–e presents the STEM-
Pd
1
Cu
3
/CNTs than on the Cu/CNTs. Owing to the fact that more Cu
Cu /CNTs than in Cu/CNTs (3 wt%), we pro-
energy-dispersive X-ray spectroscopy (EDX) element mapping distri-
butions of Pd and Cu, respectively. Based on the STEM-EDX elemental
mapping, Cu and Pd are homogeneously distributed within the NPs.
Fig. 1 indicates the alloying states of the nanocatalysts. The lattice pa-
rameters of fcc Pd and Cu metals are 0.226 and 0.208 nm, respectively,
and the lattice parameters of the alloying NPs fall between these values.
(5.3 wt%) is present in Pd
pose that the enhanced Cu peak on Pd
spillover effect. In this process, the H adsorbed on the noble metal sur-
faces decomposes to H atoms, and these dissociative H atoms migrate to
alloyed metals or supporting oxides [14,15]. Accordingly, spill-over ef-
fects is known to exert significant influences in the field of
1
3
0
1 3
Cu /CNTs was caused by the
2
1 3 1 3 1 3
Fig. 1. (a) STEM image of Pd Cu /CNT. (b) HRTEM image of a Pd-Cu alloy NP on the Pd Cu /CNT. (c) STEM image of a dispersed Pd-Cu alloy NPs on the Pd Cu /CNT. (d) and (e) STEM-EDX
element mapping distributions of Pd and Cu, respectively.

Q. Zhao et al. / Electrochemistry Communications 69 (2016) 55–58
57
1 3 1 1 3 1
Fig. 2. XRD patterns of the synthesized (a) Cu, (b) Pd Cu , (c) Pd Cu , (d) Pd Cu , and
(e) Pd NPs. The black and red lines represent Cu and PdCu standard in JCPDS, respectively.
heterogeneous catalysis by shaping and affecting the supported active
phases [16].
The catalytic activities of these Pd
gated by cyclic voltammetry (CV). Fig. 4A shows the mass activities of
Pd Cu /CNTs catalysts for formic acid electrooxidation. The potentials
of both Pd Cu /CNTs and Pd/CNTs are 0.15 V, whereas those of both
/CNTs and Pd Cu /CNTs are 0.14 V.
No obvious difference was noticed in the anodic peak potential. For
x y
Cu /CNTs catalysts were investi-
x
y
1
3
Pd Cu
3 1
1
1
−
1
comparison, the mass activity of Pd
that of Pd/CNTs is 487 A g . Thus, the Pd
1
Cu
3
/CNTs is 560 A g , whereas
Cu /CNTs catalyst is more ac-
tive than the Pd/CNTs catalyst for formic acid oxidation. The mass activ-
−
1
1
3
and 0.5-M HCOOH at a scan rate of 50 mVs−1
.
Fig. 4. (A) CVs of catalysts in 0.5-M H
SO
2 4
−
1
−1
ity of Pd
The activities of these two samples were lower than those of Pd/CNTs.
To further confirm the activities of the Pd Cu /CNTs and Pd/CNTs cata-
lysts, chronoamperometry tests were conducted in 0.5-M H SO and
.5-M HCOOH at 0.15 V for 3600 s. As shown in Fig.4B, during the first
000 s, the mass activities of Pd Cu /CNTs, Pd/CNTs, Pd Cu /CNTs, and
/CNTs were 17.7 A g , 11.0 A g , 6.9 A g , and 2.1 A g , re-
3
Cu
1
/CNTs is 238 A g , and that of Pd
Cu
1 1
/CNTs is 270 A g
.
2 4
(B) Chronoamperometry curves of catalyst samples in 0.5-M H SO and 0.5-M HCOOH.
(a) Pd Cu /CNTs, (b) Pd/CNTs, (c) Pd Cu /CNTs, and (d) Pd Cu /CNTs.
1
3
3
1
1
1
1
3
2
4
0
where Q is the charge for CO-desorption electrooxidation, m is the Pd
loading amount, and C is the charge needed for the adsorption of a CO
1
Pd
1
3
3
1
−
1
−1
−1
−1
−2
1 1
Cu
monolayer (C = 420 μC cm ).
spectively. After 1000 s, the mass activities of the four samples stabi-
lized, and that of Pd Cu /CNTs remained higher than those of the others.
The EASA values are 38.4 m g−1 for Pd/CNTs, 32.5 m2
2
g
−1
for
for
1
3
Pd
Pd
1
Cu
Cu
3
/CNTs, 31.7 m2
g
−1
1 1
for Pd Cu g
/CNTs, and 29.8 m2
−1
The electrochemically active surface area (EASA) is one of the impor-
tant standards for catalysts. Typically, CO-stripping measurements are
used to estimate the EASA of Pd-based catalysts. The specific EASA
was calculated according to the following equation:
3
1
/CNTs. The specific area values were calculated in terms of the
ratio of the mass activity and EASA. The specific activities of Pd/CNTs,
2
Pd
17.2 A m , 8.5 A m , and 8.0 A m , respectively. These values confirm
the enhanced electrocatalytic activity of Pd Cu /CNTs.
1 3 1 1 3 1
Cu /CNTs, Pd Cu /CNTs, and Pd Cu /CNTs are 12.7 A m ,
2
2
2
1
3
EASA ¼ Q=ðmCÞ
ð1Þ
1 3 1 1 3 1 1 3 1 1 3 1
Fig. 3. XPS spectra of (A) Pd3d: (a) Pd Cu , (b) Pd Cu , (c) Pd Cu , and (d) Pd; and (B) Cu2p: (a) Cu, (b) Pd Cu , (c) Pd Cu , and (d) Pd Cu .

58
Q. Zhao et al. / Electrochemistry Communications 69 (2016) 55–58
Previous reports have addressed the preparation of Pd-Cu bimetallic
Acknowledgements
catalysts. Cu has been used to modify the Pd crystalline structure and
surface electronic properties by forming alloy phases [12,17]. For exam-
ple, Park [17] reported the generation of Pd-Cu/C catalysts with high
electrocatalytic activity via chemical dealloying, which resulted in a po-
rous dealloyed structure. Lu and co-workers [12] described a prepara-
tion method similar to that presented here, involving impregnation-
This work is supported by the National Natural Science Foundation
of China (21403137).
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4
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