The role of reducing agent in perylene tetracarboxylic acid coating on graphene sheets enhances Pd nanoparticles-electrocalytic ethanol oxidation

Article abstract of DOI:10.1039/c3cy00275f

New catalysts, consisting of perylene tetracarboxylic acid functionalized graphene sheets support-enhanced electrocatalytic Pd nanoparticles (Pd/PTCA-GS), were fabricated using different reducing agents, including H₂, NaBH₄ and ethylene glycol (EG). The graphene sheets (GS) were functionalized via π-π stacking and hydrophobic forces. The information of the morphologies, sizes, and dispersion of Pd nanoparticles (NPs) for the as-prepared catalysts was verified by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Raman spectra and X-ray diffraction (XRD). As the ethanol electro-oxidation anode catalysts, the new catalysts exhibited better kinetics, higher electrocatalytic activity, better tolerance and better electrochemical stability than the Pd/GS and Pd/C, which illustrated that the new catalysts had potential applications in direct ethanol alkaline fuel cells (DEAFCs). Most attractively, the role of the chemical reduction methods (the NaBH₄, EG and H₂ as reducing agents) were studied systematically for the ethanol electro-oxidation anode catalysts in DEAFCs. As expected, the chemical reduction method remarkably affected the electrochemical behavior. Among all the Pd/PTCA-GS catalysts tested, Pd/PTCA-GS(NaBH₄) exhibited the highest catalytic activity and stability, which may be due to the Pd NPs for Pd/PTCA-GS(NaBH₄) having a narrow size distribution, uniform distribution and more perfect crystal structure than that of other as-prepared nanocomposites. These Pd/PTCA-GS are promising catalysts for developing a highly efficient direct ethanol alkaline fuel cells system for power applications. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cy00275f

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The role of reducing agent in perylene tetracarboxylic
acid coating on graphene sheets enhances Pd
nanoparticles-electrocalytic ethanol oxidation†
Cite this: DOI: 10.1039/c3cy00275f
Shuwen Li, Honglei Yang, Zhengping Dong, Shujing Guo, Jiahui Zhao, Galian Gou,
Ren Ren, Jingwei Huang, Jun Jin* and Jiantai Ma*
New catalysts, consisting of perylene tetracarboxylic acid functionalized graphene sheets support-enhanced
electrocatalytic Pd nanoparticles (Pd/PTCA–GS), were fabricated using different reducing agents, including
H2, NaBH4 and ethylene glycol (EG). The graphene sheets (GS) were functionalized via p–p stacking and
hydrophobic forces. The information of the morphologies, sizes, and dispersion of Pd nanoparticles (NPs)
for the as-prepared catalysts was verified by transmission electron microscopy (TEM), high-resolution
transmission electron microscopy (HRTEM), Raman spectra and X-ray diffraction (XRD). As the ethanol
electro-oxidation anode catalysts, the new catalysts exhibited better kinetics, higher electrocatalytic
activity, better tolerance and better electrochemical stability than the Pd/GS and Pd/C, which illustrated that
the new catalysts had potential applications in direct ethanol alkaline fuel cells (DEAFCs). Most attractively,
the role of the chemical reduction methods (the NaBH4, EG and H2 as reducing agents) were studied
systematically for the ethanol electro-oxidation anode catalysts in DEAFCs. As expected, the chemical
reduction method remarkably affected the electrochemical behavior. Among all the Pd/PTCA–GS catalysts
tested, Pd/PTCA–GS(NaBH4) exhibited the highest catalytic activity and stability, which may be due to the
Pd NPs for Pd/PTCA–GS(NaBH4) having a narrow size distribution, uniform distribution and more perfect
crystal structure than that of other as-prepared nanocomposites. These Pd/PTCA–GS are promising catalysts
for developing a highly efficient direct ethanol alkaline fuel cells system for power applications.
Received 21st April 2013,
Accepted 21st May 2013
DOI: 10.1039/c3cy00275f
www.rsc.org/catalysis
agricultural products or biomass, but also for its unique pro-
perties including high energy density, low toxicity, ease in
Introduction
Considering the ever-increasing environmental problems and
the up-coming depletion of fossil fuels, a multitude of efforts
have been devoted to developing the biomass-derived alcohols
and their clean energy conversion devices over the past
decades.1 As important biomass-derived power sources, direct
alcohol fuel cells (DAFCs) based on liquid fuels have received a
growing interest in recent years as promising candidates for
portable power sources, electric vehicles and transportation
applications.2,3 Among various fuel cell types, direct ethanol
fuel cell (DEFCs) have drawn special attention not only because
ethanol is a carbon-neutral and sustainable fuel, which can be
produced in great quantities through the fermentation of
handling and transportation. Therefore, designing and
developing efficient ethanol electro-oxidation anode catalysts
for DEFCs has become a hot topic research. As is well known, Pt
and Pt based catalysts are the most effective and commonly
used as electrocatalysts for the anode catalyst in fuel cells,
which have been extensively investigated.4–6 However, a great
deal of research has already been carried out to develop low
cost non-platinum electrocatalysts on account of the intrinsic
drawbacks of Pt and Pt based catalysts, such as high cost,
limited resources, susceptibility to deactivation and vulner-
ability toward poisoning by intermediate products.7 Pd-based
nanomaterials have been widely used as catalysts for a variety of
research fields ranging from organic synthesis and fuel cells to
environmental protection.8,9 In view of the higher abundance,
lower price, high electrocatalytic activity and greater resistance
to intermediate products for ethanol oxidation in alkaline
medium compared with Pt-based catalysts, Pd-based nano-
State Key Laboratory of Applied Organic Chemistry, Gansu Provincial Engineering
Laboratory for Chemical Catalysis and College of Chemistry and
Chemical Engineering, Lanzhou University, Lanzhou, 730000, P. R. China.
E-mail: jinjun@lzu.edu.cn, majiantai@lzu.edu.cn; Fax: +86 931 8912577
† Electronic supplementary information (ESI) available: See DOI: 10.1039/c3cy00275f electrocatalysts are emerging as an excellent substitute to
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Pt-based catalysts for direct ethanol alkaline fuel cells (DEAFCs).10–14 applications in DEAFCs. As expected, the chemical reduction
Nevertheless, a number of challenges and obstacles for method remarkably affected the electrochemical behavior. The
Pd-based nano-electrocatalysts, such as poor catalytic activity Pd/PTCA–GS(NaBH4) catalyst exhibited better kinetics, significantly
and low utilization efficiency of Pd, are a barrier to the develop- enhanced activity, better tolerance and better electrochemical
ment of DEAFCs and hinder its practical application in com- stability than the product using other reducing agents. The
mercialization of portable fuel cell technology.
results of the effects of chemical reduction methods are
Another alternative effective approach to treat the above beneficial to designing and developing efficient catalysts.
problems is to develop novel catalyst supports such as ordered
mesoporous carbons (OMCs),15 Vulcan XC-72,16 carbon nano-
Experiment
Materials
fibers (CNFs),17 and carbon nanotubes (CNTs).18 It is due to the
size and morphology of Pd nanostructures (NPs) being effec-
tively controlled by highly dispersing the Pd particles on a high Graphene sheets (GS, 95%) were purchased from Nanjing
surface area support. Graphene, as an atom-layer-thick two- XFNano Materials Tech Co., Ltd, China. PdCl2 (99%), sodium
dimensional material, is a good supporting substrate for active borohydride (NaBH4), and 3,4,9,10-perylene tetracarboxylic
species because of its good conductivity, high chemical stability dianhydride (PTCDA, 98%) were purchased from Alfa Aesar.
and large surface area.19,20 For DEAFCs, the combination of All other reagents were purchased from the Tianjin Guangfu
graphene and Pd NPs may open up a new avenue for designing Chemical Co. Ltd. Water used in the experiments was deionized
the next generation catalysts. Unfortunately, the pristine and doubly distilled.
graphene sheets (GS) are hard to disperse in aqueous solution
and have a lack of binding sites for metal ions or metal NPs to
Synthetic and preparation procedures
anchor with them, usually resulting in poor dispersion and 3,4,9,10-perylene tetracarboxylic acid (PTCA) was obtained by
large metal particles. In order to overcome its poor processi- hydrolyzing PTCDA in a minimal volume of 1.0 M sodium
bility, the functionalization of the GS by covalent or non- hydroxide. Red deposits appeared in the yellow-green solution
covalent method is considerably investigated.21–23 The covalent and were collected by centrifugation and dried under vacuum
functionalization of the GS (including graphene oxide) cannot at room temperature.
avoid the structural damage of graphene, which adversely
PTCA–GS. The GS (50 mg) were ultrasonicated in 40 mL of
affects the electrical, mechanical, and stability properties of the ethanol containing PTCA (10 mg) for 2 h. Then, the pH value of
material because of the transformation of carbon atoms from a the mixture solution was adjusted to about 9 with 1.0 M KOH
planar sp2-hybridized geometry to a distorted sp3-hybridized aqueous solution. The solution was stirred continuously for 5 h
geometry.24 A noncovalent method through supermolecular at ambient temperature and held still overnight. After that, the
interactions such as p–p stacking has advantages of operating mixture was filtered through a membrane, and washed repeatedly
and preserving the intact electronic structure of GS compared with absolute ethyl alcohol, 1.0 M HCl and distilled water several
with covalent functionalization.25,26 Moreover, GS will be imbued times. The final products were dried in a vacuum oven at 60 1C
with other properties when it is combined with functionalized overnight.
molecules through noncovalent methods.27 On the other hand,
Pd/PTCA–GS(H2) and Pd/GS(H2). An aqueous solution of
it is well known that the chemical reduction method for preparing PTCA–GS (20 mg) or GS (20 mg) in deionized water (20 mL)
catalysts is a critical factor to enhance the catalytic activity. For was ultrasonicated for 1 h to form a uniform suspension. Then,
these reasons, the investigation into the effects of chemical an aqueous solution of PdCl2 (0.01 M, 3.4 mL) was added and
reduction methods in electrocatalysis is highly desirable.
the mixture was stirred for 1 h under 1 atm of H2 (balloon) at
Herein, Pd NPs were combined with the noncovalent func- room temperature. Subsequently, the black solid material was
tionalization of GS and used in DEAFCs. We reported an separated by centrifugation, and washed successively with
alternative strategy to disperse Pd NPs on 3,4,9,10-perylene deionized water and ethanol several times. The filtered black
tetracarboxylic acid (PTCA) functionalized GS (Pd/PTCA–GS). solid was dried under vacuum at 60 1C for 12 h.
As a bifunctional molecule with an aromatic pyrenyl group and
Pd/PTCA–GS(NaBH4) and Pd/GS(NaBH4). The PTCA–GS (20 mg)
four carboxyl groups, PTCA is anchored on the GS via p–p or GS (20 mg) was dissolved in H2O (20 mL) with ultrasonic
stacking and this introduced a large number of highly dis- treatment to form a uniform suspension. Then, PdCl2 solution
persed carboxyl groups and pyrenyl groups, which is vital to (0.01 M, 3.4 mL) was added into the suspension under stirring and
enhance the electrocatalytic activity. In addition, the electro- the mixture was stirred for 30 min at room temperature. Sub-
catalysts were fabricated by different chemical reduction sequently, the NaBH4 solution (0.1 M, 2.5 mL) was delivered by
methods. As common reducing agents, H2, NaBH4 and Ethylene drops into the above suspension under continuous stirring. After
Glycol (EG) and their corresponding reducing conditions reacting for 4 h at room temperature, the black solid material was
were used to prepare the electrocatalysts. Significantly, the separated by centrifugation, washed repeatedly with water and
Pd/PTCA–GS exhibited better kinetics, significantly enhanced ethanol several times, and dried in a vacuum at 60 1C for 12 h.
activity, better tolerance and better electrochemical stability
Pd/PTCA–GS(EG) and Pd/GS(EG). The PTCA–GS (20 mg) or
towards alcohol oxidation than Pd NPs on GS (Pd/GS) in GS (20 mg) was dissolved in ethylene glycol (EG, 20 mL) solution
DEAFCs, which demonstrated the Pd/PTCA–GS had potential and sonicated for 1 h. Subsequently, PdCl2 –EG (0.01 M, 3.4 mL)
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Scheme 1 The Schematic diagram of synthesis of the Pd/PTCA–GS.
solution was added to the suspension and sonicated for 2 h. The
pH of the solution was adjusted to 10 using sodium hydroxide–
EG (NaOH–EG) solution (0.5 M), and then the solution was
stirred under flowing argon at 130 1C for 3 h. The solid material
produced was then centrifuged, washed several times with
deionized water and finally dried in a vacuum at 60 1C for 12 h.
The fabricated route of the Pd/PTCA–GS was summarized in
Scheme 1.
Sample characterization
The morphology and microstructure of the nanohybrids were
characterized by TEM and HRTEM images were obtained by
Tecnai G2 F30 electron microscope operating at 300 kV. Raman
spectra were recorded on an inVia Reinishaw confocal spectro-
scope with 633 nm laser excitation. XRD measurements were
carried out at room temperature and performed on a Rigaku
D/max-2400 diffractometer using Cu-Ka radiation as the X-ray
source in the 2y range of 10–901. X-ray photoelectron spectro-
scopy (XPS) analysis was carried on a PHI-5702 X-ray photo-
electron spectrometer.
Fig. 1 (A) Raman spectra of (a) GS and (b) PTCA–GS and (B) XRD patterns of
(a) GS and (b) PTCA–GS.
Electrochemical measurements were carried out in a standard
three-electrode cell with a CHI model 660C electrochemical
workstation (Shanghai Chenhua Instrument Factory, China) at
room temperature. A platinum wire was used as the counter
electrode and a saturated calomel electrode (SCE) as the
reference. The working electrode was a glassy carbon electrode
(GCE, diameter: 3.0 mm) polished with Al2O3 powders (Aldrich,
0.05 mm). The working electrodes were fabricated as follows:
2 mg of the sample was dispersed in a 2 mL solution of double
distilled water, ethanol and Nafion solution (10 : 9 : 1) with
ultrasonic treatment to produce a homogeneous suspension.
Then, the well-dispersed suspension (5 mL) was drop-cast onto
the GCE, which was then dried in air at room temperature.
assigned to the E2g structure mode of graphite, which reflects
the structure of the sp2 hybridized carbon atoms.28 Therefore,
the level of chemical modification of the graphitic carbon
sample is commonly quantified by the intensity ratio of the
D band and G bands (ID/IG).28,29 It was noteworthy that the ID/IG
ratio of the PTCA–GS (1.46) was smaller than that of the pristine
GS (1.54), which indicated that the functionalization of GS by
the PTCA molecules via p–p stacking has no detrimental effect
on the structure of GS. Furthermore, the original defect sites on
the surface of the pristine GS were coated by the PTCA mole-
cules, leading to the decrease of the ID/IG ratio. The results of
Raman spectroscopy confirmed that the functionalization was
efficient and the PTCA–GS should be a promising catalyst
support for noble metal NPs.
Fig. 1(B) displays the XRD patterns of the GS and PTCA–GS
composites. The diffraction peak centered at around 251 was
Result and discussion
Raman spectroscopy is a powerful tool and is frequently used to evidently attributed to the (002) plane.27,30 The broadening of
investigate the structural and electronic properties of graphene diffraction peak (002) indicated the formation of graphene
to obtain disorder, defect density, and functional levels for GS. nanosheets.31 In addition, the diffraction peaks corresponding
Fig. 1(A) showed the Raman spectra of the GS and the PTCA–GS to the (002) plane of GS were clearly observed with no shift for
with 633 nm laser excitation. As expected, two characteristic the PTCA–GS, which demonstrated that the functionalization of
peaks (D band, B1311 cmÀ1 and G band, B1587 cmÀ1) were GS by PTCA did not destroy or alter the original structure of GS.
clearly observed for the GS and the PTCA–GS. The D band
The morphologies, the sizes, and the dispersion of Pd NPs
should be assigned to the A1g breathing mode of the disordered deposited on GS and PTCA–GS supports have been investigated
graphite structure, due to the defect sites in the hexagonal by TEM (Fig. 2). As exhibited in Fig. 2A, C and E, the Pd NPs
framework of graphite materials, and the G band should be were randomly dispersed on GS and some of them appeared to
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NPs for Pd/PTCA–GS(NaBH4) had a narrow size distribution,
uniform distribution on the carbonaceous carrier and more
perfect crystal structure than that of the other nanocomposites,
which implied that controlling the reducing agent and reducing
conditions was important for the design of the catalyst.
Typical XRD patterns of the as-prepared nanocomposites
were shown in Fig. 3(A). The broad diffraction peak centered at
around 251 was evidently attributed to the (002) plane reflection
of the graphene nanoplates, implying all the nanocomposites
contained few-layer graphene sheets. The other four peaks in
each nanocomposite at 40.01, 46.61, 68.01 and 81.21 were
characteristic of face centered cubic (fcc)-structured Pd lattices,
which were assigned to the (111), (200), (220) and (311) crystal-
line planes of the fcc-structured palladium (JCPDS no. 46-1043).
Furthermore, the diffraction (111) peak width gradually
widened with the following order of Pd/GS(H2), Pd/GS(EG),
Pd/GS(NaBH4), Pd/PTCA–GS(H2), Pd/PTCA–GS(EG), Pd/PTCA–
GS(NaBH4), indicating that the dimensions of the crystal
domains for Pd NPs on the nanocomposites gradually reduced
with the same order. The results of the XRD patterns confirmed
that the Pd NPs were efficiently immobilized on GS and PTCA–GS
and possessed a highly crystalline fcc phase.
To further obtain the information about the surface chemical
composition of the as-prepared catalysts, the XPS of Pd/PTCA–
GS(NaBH4) was performed as a representative. Fig. 3(B) shows a
Fig. 2 TEM images of (A) Pd/GS(EG), (B) Pd/PTCA–GS(EG), (C) Pd/GS(H2),
(D) Pd/PTCA–GS(H2), (E) Pd/GS(NaBH4) and (F) Pd/PTCA–GS(NaBH4). Inset: size
distribution histograms and HRTEM images of the Pd NPs crystal structure in
detail on GS and PTCA–GS.
aggregate with a wide size distribution. However, the quasi-
spherical shape of Pd NPs, which were observed with a narrow
size distribution were equably attached on the surface of
PTCA–GS (Fig. 2B, D and F) and possessed a more perfect
crystal structure than that of the Pd/GS (HRTEM). The better
dispersion of Pd NPs on PTCA–GS should be attributed to the
following two reasons. (1) By comparison with the pristine GS,
the PTCA layer on the GS produced a large number of carboxyl
groups with uniform distribution that serve as the functional
groups for the immobilization of Pd precursors on the PTCA–GS
surface through electrostatic interaction and coordination.
(2) The amount of the unsaturated C–C bonds (Cp sites) species
is higher in the PTCA–GS composite than GS (Fig. 1A). As is
known, the Pd precursors are prone to form stable p-complexes
with unsaturated C–C bonds of organic compounds when Pd
NPs are deposited on the carbon surface.31,32 Therefore, a much
more uniform distribution of Pd NPs was observed on the
surface of the PTCA–GS. Moreover, the morphologies, the sizes,
and the dispersion of Pd NPs were distinctly influenced by the
Fig. 3 (A) XRD patterns of (a) Pd/PTCA–GS(NaBH4), (b) Pd/PTCA–GS(EG),
(c) Pd/PTCA–GS(H2), (d) Pd/GS(NaBH4), (e) Pd/GS(EG) and (f) Pd/GS(H2). (B) XPS
reducing agent and reducing conditions. From Fig. 2, the Pd wide-scan and Pd 3d high-resolution spectra (the inset) of Pd/PTCA–GS(NaBH4).
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typical survey XPS spectra of the Pd/PTCA–GS(NaBH4) and the
high resolution XPS spectra for Pd. Successful functionalization
of the GS by the PTCA molecules via p–p stacking was further
confirmed by the presence of the O 1s. The two sets of intense
peaks at 335.1 eV and 340.4 eV, as shown in the inset of
Fig. 3(B), were for 3d5/2 and 3d3/2 of the Pd NPs deposited
onto the surface of the PTCA–GS, which correspond to a
metallic state of Pd. These values were in good agreement with
previous reports.31 Based on the XRD and XPS analysis, it could
be concluded that crystalline Pd existed in the as-prepared
catalysts.
Cyclic voltammetry (CV) is a highly useful technique to
investigate the heterogeneous electron transfer ability occurring
on the electrode surface and the nature of interactions between
molecules and the electrode surface.33,34 A ferro/ferricyanide
redox probe was used to establish the electron transfer ability
of the studied chemically modified GS. Fig. 4 shows a typical CV
scan of K3Fe(CN)6 (1 mM) at the GS modified electrode and the
PTCA–GS modified electrode in a solution of KCl (0.1 M) with a
scan rate of 50 mV sÀ1. It can be seen that the PTCA–GS showed
a lower peak-to-peak separation (DEp, 65 mV) than the GS
(89 mV). Moreover, the peak current (Ip) obtained at the
PTCA–GS modified electrode was higher than that attained at
the GS modified electrode. The facts of lower DEp and higher Ip
indicated that the PTCA facilitated electron transfer,26,33,34
which is attributed to the original defect sites on the surface
of the GS being covered by the PTCA molecules.35,36
Fig. 5 CV scans of (a) Pd/PTCA–GS(NaBH4), (b) Pd/PTCA–GS(EG), (c) Pd/PTCA–
GS(H2), (d) Pd/GS(NaBH4), (e) Pd/GS(EG), (f) Pd/GS(H2) and (g) Pd/C in (A) 1 M
H2SO4 solution and (B) 1 M KOH solution at a scan rate of 50 mV sÀ1
.
As an important parameter, the electrochemically active surface
area (ECSA) can not only determine the number of catalytically
active sites available for an electrochemical reaction, but also the amount of hydrogen desorbed and provided an estimation
account for the access of a conductive path available to transfer of the ECSA for the electrocatalyst.39 Obviously, the integrated
electrons to and from the electrode surface.31,37 Fig. 5(A) sum- area of the hydrogen absorption and desorption peaks decay
marizes the typical CV scans of the GCE coated with the rates followed the order of Pd/PTCA–GS(NaBH4), Pd/PTCA–GS(EG),
different electrocatalyst films in a N2-saturated solution of Pd/PTCA–GS(H2), Pd/GS(NaBH4), Pd/GS(EG), Pd/GS(H2), Pd/C.
0.5 M H2SO4 at a scan rate of 50 mV sÀ1. In Fig. 5(A), the Moreover, the electrocatalytic activities of the as obtained
current peaks between À0.10 and 0.03 V were attributed to catalysts were also studied in alkaline medium. Fig. 5(B) shows
hydrogen adsorption and desorption and the cathodic peak at the CV scans of electrodes modified with the different electro-
0.5 V was caused by the reduction of metal oxide.38 The catalysts obtained in an N2-saturated solution of KOH (0.1 M) at
integrated peak area under the curve is commonly used to give a scan rate of 50 mV sÀ1. From Fig. 5(B), the anodic current
increased during the forward sweep and a peak current could
be observed on the electrodes except for Pd/C. An obvious
cathodic current peak at around 0.37 V during the backward
sweep was caused by the reduction of the platinum oxide
formed during the positive-direction sweep.40 The hump from
À0.6 V to À1.0 V was associated with hydrogen adsorption and
desorption.3,40 The ECSA of the catalyst for ethanol oxidation in
alkaline medium can be measured from the coulombic charge
for hydrogen desorption, which is generated from integrated
peak area under the hump.3,41 Similarly, the ECSA decay rates
also followed the order of Pd/PTCA–GS(NaBH4), Pd/PTCA–GS(EG),
Pd/PTCA–GS(H2), Pd/GS(NaBH4), Pd/GS(EG), Pd/GS(H2), Pd/C,
which was in agreement with the results of the H2SO4 solution.
Furthermore, the broader CV curves obtained from the
Pd/PTCA–GS(NaBH4, EG or H2) indicated that the Pd/PTCA–GS
composites had a larger double layer capacitance than the Pd/GS
composites and Pd/C. These results further proved that the
Fig. 4 CV scans of 1 mM K3Fe(CN)6 (a) GS and (b) PTCA–GS in 0.1 M KCl solution
with a scan rate of 50 mV sÀ1
.
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PTCA–GS support had a larger surface area and therefore
attracted more electroactive species onto its surface than the
GS and C supports, which is important for catalyst activity
enhancement.42 It was reasonable to expect that the PTCA not
only hindered the aggregation of GS but also introduced a large
number of highly dispersed carboxyl groups on the GS surface
without structural damage of GS. The carboxyl groups could
interact with the precursors of Pd NPs (Pd2+) during the process
of preparing the Pd/PTCA–GS(NaBH4, EG or H2), which was
beneficial to the uniform dispersion of small Pd NPs on the
surface of PTCA–GS. From Fig. 5(B), the onset oxidation
potentials and the reduction peaks of the oxidized Pd for
Pd/PTCA–GS(NaBH4, EG or H2) were all shifted compared to
those of the Pd/GS(NaBH4, EG or H2) and Pd/C, which indicated
that PTCA–GS as a support could promote the electron transfer
from Pd to PTCA–GS across the PTCA–GS–Pd interface, making
it more difficult to oxidize metallic Pd NPs but easier to reduce
the oxidized Pd NPs.42 The results indicated that the function-
alization of GS through the PTCA could increase the ECSA and
promote the electron transfer in alkaline media, which demon-
strated that Pd/PTCA–GS(NaBH4, EG or H2) possessed excellent
catalytic activity in alkaline media. Contemporaneously, the
ECSA also could be affected by the reducing agent and reducing
condition. Compared with the other electrocatalysts, the
Pd/PTCA–GS(NaBH4) was possessed of a larger ECSA, which
may be ascribed to the Pd NPs in Pd/PTCA–GS(NaBH4) having a
narrower size distribution, more uniform distribution and
more perfect crystal structure than those of Pd NPs in the other
as-prepared nanocomposites (Fig. 2).
Fig. 6 (A) CV scans of (a) Pd/PTCA–GS(NaBH4), (b) Pd/PTCA–GS(EG),
(c) Pd/PTCA–GS(H2), (d) Pd/GS(NaBH4), (e) Pd/GS(EG), (f) Pd/GS(H2) and (g) Pd/C
and (B) magnified curves between À1.0 and À0.5 V in nitrogen-saturated 1 M
KOH + 1 M CH3CH2OH solution at a scan rate of 50 mV sÀ1
.
As electrocatalysts, the as-prepared nanocomposites were
applied in DEAFCs and their electrocatalytic activities were
evaluated by CV tests. Fig. 6(A) shows the CV of the as-prepared
catalyst-modified electrodes for ethanol oxidation in nitrogen-
saturated aqueous solution of KOH (1 M) containing ethanol
(1 M) at a scan rate of 50 mV sÀ1. For clear observation, the
magnified curves between À1.0 and À0.5 V are displayed in
Fig. 6(B). As can be seen, all the CV scans generated for the
as-prepared catalysts were reminiscent of those observed in the
ethanol electro-oxidation process on Pt-based electrocatalysts
in acidic media, featuring two well-defined anodic current
Table 1 Comparison of electrocatalytic activity with different catalysts
Catalysts
ES (V vs. SCE)
If (mA mgÀ1
)
Is (mA mgÀ1
)
Pd/PTCA–GS(NaBH4)
Pd/PTCA–GS(EG)
Pd/PTCA–GS(H2)
Pd/GS(NaBH4)
Pd/GS(EG)
Pd/GS(H2)
Pd/C
À0.681
À0.671
À0.665
À0.651
À0.648
À0.640
À0.635
1226.6
1104.0
1009.3
707.3
573.3
511.5
389.7
184.0
170.3
154.9
139.6
129.7
115.6
77.3
peaks in the forward and reverse sweeps.43–45 The onset Moreover, the order of If was Pd/PTCA–GS(NaBH4) 4 Pd/PTCA–
potential of the faradaic current (Es) and the forward peak GS(EG) 4 Pd/PTCA–GS(H2) 4 Pd/GS(NaBH4) 4 Pd/GS(EG) 4
current (If) for ethanol oxidation on the as-prepared catalysts Pd/GS(H2) 4 Pd/C. The results of the CV tests for ethanol
are shown in Table 1. It is worth noting that the (Es) for ethanol oxidation revealed that the functionalization of GS through
oxidation on the Pd/PTCA–GS(NaBH4, EG or H2) electrodes PTCA could remarkably improve the electrocatalytic activity for
were much more negative than those on the Pd/GS(NaBH4, ethanol electro-oxidation and the reducing agent and reducing
EG or H2) electrodes. In the process of ethanol electro-oxidation, conditions could affect the electrocatalytic activity for electro-
the dissociative adsorption of ethanol to adsorbed COads and oxidation in terms of the onset potential and peak current in
CHx species usually occurs at lower potentials, which leads to alkaline media. Thus, the improved electrochemical response
the complete oxidation of ethanol to CO2 when in the presence obtained at the as-prepared catalysts could be ascribed to the
of the oxygen-containing species.46,47 This was evidence that following three aspects. (1) From an electron-transfer perspec-
the functionalization of GS through PTCA could tremendously tive, the PTCA–GS had a stronger ability for electron transfer
improve the kinetics of the ethanol oxidation.16,48 Additionally, compared with the GS, due to the functionalization of GS by
the Es for reducing agents and reducing conditions were PTCA (Fig. 3). (2) As known, the electrocatalyst possessing a
different, which indicated that the reducing agent and reducing larger ECSA would have a higher elec-trocatalytic activity.
conditions could affect the kinetics of the ethanol oxidation. Obviously, the Pd/PTCA–GS(NaBH4, EG or H2) had a larger
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ECSA than Pd/GS(NaBH4, EG or H2) (Fig. 5). (3) In contrast with measurement results for all catalysts could be ascribed as
the Pd/GS, the Pd/PTCA–GS(NaBH4, EG or H2) could be better follows. (1) The small size and high dispersion of the Pd NPs
dispersed in the solvent and had greater processing perfor- on GS or PTCA–GS, which were affected by the reducing agent,
mance, owing to a large number of highly dispersed carboxyl reducing condition and carbonaceous carrier, determined the
groups on the PTCA–GS. Therefore, Pd/PTCA–GS(NaBH4, EG or electrocatalytic activities and stabilities of fuel cells. (2) The
H2) could be uniformly drop-cast onto the working-electrode PTCA–GS were endowed with a lower density of graphene lattice
surface, which was conducive to ethanol oxidation. On the defects compared with the pristine GS. The high density of
other hand, the reducing agent and reducing conditions could graphene lattice defects in the composite could make the
impact the size, distribution and lattice constants of Pd NPs, catalysts corrode easily and thus they became less stable under
which was correlated with enhanced performance for ethanol harsh electrochemical conditions.50 The results further
oxidation.11 Due to these reasons, the Pd NPs that were confirmed that the functionalization of GS by PTCA enhanced
generated from NaBH4 as the reducing agent possessed smaller both the electro-catalytic activity and stability of the catalysts.
sizes, more uniform distribution and more suitable lattice Consequently, the Pd/PTCA–GS(NaBH4) showed better toler-
constants than those generated from the other reducing agents. ance and electrochemical stability than the other as-prepared
Therefore, the Pd/PTCA–GS(NaBH4) exhibited better kinetics electrocatalysts.
and higher electrocatalytic activity than the other as-prepared
electrocatalysts.
Conclusion
It is well known that improving the durability and stability of
electrocatalysts is extremely important for the practical applica-
tions in fuel cells. The chronoamperometric technique has
been carried out to appraise the long-term electrocatalytic
In summary, the PTCA-functionalized GS was successfully
synthesized through the p–p stacking and hydrophobic forces.
Then, the enhanced catalytic Pd NPs were successfully
anchored on the PTCA–GS using different reducing agents,
activities and stabilities of the as-prepared catalysts under a
constant potential of À0.35 V in nitrogen-saturated aqueous
including NaBH4, EG and H2, and corresponding reducing
solution of KOH (1 M) containing ethanol (1 M) (Fig. 7). As
conditions. Using the PTCA efficiently functionalized GS facili-
shown in Fig. 7, the current intensity for all catalysts decreased
tated electron transfer in DEAFCs. Furthermore, the effects of
rapidly at the initial stage due to the poisoning of the formation
the chemical reduction methods (using NaBH4, EG and H2 as
of the intermediate carbonaceous species during the ethanol
reducing agents) were studied systematically on the ethanol
oxidation reaction.49 It was worth noting that the current decay
electro-oxidation anode catalysts in DEAFCs. The present
rates followed the order of Pd/PTCA–GS(NaBH4), Pd/PTCA–GS(EG),
catalyst exhibited excellent electrocatalytic activity for use as
Pd/PTCA–GS(H2), Pd/GS(NaBH4), Pd/GS(EG), Pd/GS(H2), Pd/C,
the ethanol electro-oxidation anode catalysts in DEAFCs due to
which was also in perfect agreement with the results of the CV
two reasons. Firstly, this was attributed to the affect of PTCA.
tests. More importantly, the current intensity during the duration
of the scan and the pseudo-steady current value (Is, shown in
Table 1) at a time of 7200 s were in accordance (the same order)
On the other hand, the chemical reduction method remarkably
affected the electrochemical behavior. Among all the as-prepared
electrocatalysts, the Pd/PTCA–GS(NaBH4) displayed better
with the current decay rates, which indicated that the electro-
kinetics, higher electrocatalytic activity, better tolerance and
catalytic performance and stability of the as-prepared catalysts
electrochemical stability than the others. It may be due to the
for ethanol oxidation were consistent with the same order
Pd NPs for Pd/PTCA–GS(NaBH4) having a narrower size distri-
shown above. The reasons for the chronoamperometric
bution, more uniform distribution and more perfect crystal
structure than those of the other as-prepared nanocomposites.
According to the results of the CV behavior the new catalyst, as
an efficient anode catalyst, has potential applications in
DEAFCs. What’s more, the strategy of PTCA functionalization
is available to prepare other carbonaceous carrier supported
metal NPs for enhanced dispersivity, stability and catalytic
performance, as well for being used in fuel-cell technologies,
and the effects for chemical reduction methods could be used
to design and develop more efficient catalysts.
Acknowledgements
This work was supported by the Fundamental Research Funds
for the Central Universities (lzujbky-2013-234 and lzujbky-2013-67).
The authors are grateful to the Scholarship Award for
Fig. 7 Chronoamperometric curves of (a) Pd/PTCA–GS(NaBH4), (b) Pd/PTCA–
GS(EG), (c) Pd/PTCA–GS(H2), (d) Pd/GS(NaBH4), (e) Pd/GS(EG), (f) Pd/GS(H2) and
Excellent Doctoral Student granted by Lanzhou University and
Dr Xinghua Li for TEM and HRTEM characterization.
(g) Pd/C for ethanol electrooxidation at À0.35 V in nitrogen-saturated 1 M KOH +
1 M CH3CH2OH solution.
c
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