Pt-CeO2/reduced graphene oxide nanocomposite for the electrooxidation of formic acid and formaldehyde

Article abstract of DOI:10.1039/c5ra09770c

The Pt-CeO₂/reduced graphene oxide nanocomposite on the carbon-ceramic electrode (Pt-CeO₂/RGO/CCE) was prepared by a two-step electrodeposition process. RGO was deposited on the CCE through a simple electrochemical method using graphene oxide and CCE as the precursor and electrode substrate, respectively. Then, RGO/CCE was used as a support for the Pt-CeO₂ nanoparticle deposition by another electrochemical process to form the nanocomposite based electrocatalyst; Pt-CeO₂/RGO/CCE. The obtained electrocatalyst was characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and cyclic voltammetric techniques. The electrocatalytic activity of the Pt-CeO₂/RGO/CCE toward the oxidation of formic acid and formaldehyde was investigated by cyclic voltammetry and chronoamperometry in 0.1 M H₂SO₄ solution. It was found that the Pt-CeO₂/RGO/CCE is electrocatalytically more active than the Pt-CeO₂/CCE, Pt/RGO/CCE and Pt/CCE electrocatalysts. The increased electrocatalytic efficiency of Pt in Pt-CeO₂/RGO/CCE is likely to result from its combination with CeO₂ to form Pt-CeO₂ nanoparticles and their deposition on/in the RGO layers at the CCE surface. From the obtained results, it could be concluded that the Pt-CeO₂/RGO/CCE can be used as a potential and effective electrocatalyst for direct liquid fuel cell applications.

Full text of DOI:10.1039/c5ra09770c

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Page 1 of 36
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Pt-CeO2/reduced graphene oxide nanocomposite for the electrooxidation of
formic acid and formaldehyde
Biuck Habibi*, Nasrin Delnavaz
Electroanalytical Chemistry Laboratory, Department of Chemistry, Faculty of Sciences,
Azarbaijan Shahid Madani University, Tabriz, Iran
*Corresponding
author
(Biuck
Habibi).
Eꢀmail:
B.Habibi@azaruniv.edu,
biuckhabibi_a@yahoo.com and Tel & Fax: +98ꢀ41 34327541

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Abstract
The PtꢀCeO2/reduced graphene oxide nanocomposite on the carbonꢀceramic electrode
(PtꢀCeO2/RGO/CCE) was prepared by a twoꢀsteps electrodeposition process. RGO was
deposited on the CCE through a simple electrochemical method using graphene oxide and CCE
as the precursor and electrode substrate, respectively. Then, RGO/CCE was used as a support for
the PtꢀCeO2 nanoparticles deposition by another electrochemical process to form the
nanocomposite based electrocatalyst; PtꢀCeO2/RGO/CCE. The obtained electrocatalyst was
characterized by scanning electron microscopy, energy dispersive Xꢀray spectroscopy, Xꢀray
diffraction and cyclic voltammetric techniques. The electrocatalytic activity of the Ptꢀ
CeO2/RGO/CCE toward the oxidation of formic acid and formaldehyde was investigated by
cyclic voltammetry and chronoamperometry in 0.1 M H2SO4 solution. It was found that the Ptꢀ
CeO2/RGO/CCE is electrocatalytically more active than the PtꢀCeO2/CCE, Pt/RGO/CCE and
Pt/CCE electrocatalysts. The increased electrocatalytic efficiency of Pt in PtꢀCeO2/RGO/CCE is
likely to result from its combination with CeO2 to form PtꢀCeO2 nanoparticles and their
deposition on/in the RGO layers at the CCE surface. From the obtained results, it could be
concluded that the PtꢀCeO2/RGO/CCE can be used as a potential and effective electrocatalyst for
direct liquid fuel cell applications.
Keywords: PtꢀCeO2 nanoparticles, Reduced graphene oxide, Nanocomposite, Carbonꢀceramic
electrode, Formic acid, Formaldehyde.

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1. Introduction
Tailoring of electrode surface with nanomaterials or nanocomposites is one of the
attractive approaches in designing electrochemical interfaces for electrocatalysis of fuels
oxidation in direct liquid fuel cells (DLFCs) [1, 2]. The nanostructured noble metals are of
special interest in developing such interfaces owing to their unique electronic and electrocatalytic
properties [3, 4]. At the same time, one of the main obstacles facing the development of the
DLFCs via application of nanostructured noble metals as an anodic material is its sluggish
kinetics caused by carbonaceous species poisoning effect. The intermediate carbonaceous
products, such as carbon monoxide (CO), from the fuels oxidation over the noble metals surface
are strongly adsorbed on the electrocatalyst surface and caused catalyst poisoning. On the other
hand, noble metal nanostructures due to the high surface energy have a tendency to aggregate,
which will decrease their surface area and result in a remarkable reduction in their performance.
Meanwhile, the increasing request for noble metals and their finite availability in the nature
cause them to be expensive. Consequently, it is desirable to explore simple and effective
methods to prepare wellꢀdispersed noble metal nanostructures with good size controllability to
enhance the performance, tolerance against carbonaceous species and minimize the used
amounts of noble metals. In practice, to solve the problem of poisoning effect, agglomeration
and lower the usage of noble metals, an efficient strategy is to immobilize them along with
different promoters in the form of alloy or conducting metal oxide/noble metal composite [5ꢀ7]
at the nanosize particles in the inside or on the surface of solid supports with low cost, high
surface area and superior chemical stability [8]. In fact, the electrocatalytic support retains the
structure and morphology of the noble metalꢀbased alloys or noble metal/metal oxide composites
and exhibits a vital role in their swift performance and functionality. As mentioned above, to
prevent the noble metal electrocatalysts such as Ptꢀbased catalysts from being poisoned by
carbonaceous species especially CO, besides the efforts to developing the noble metalꢀbased
alloys [9, 10], the recently developed strategy is synthesize the conducting metal oxideꢀbased
noble metal electrocatalysts via utilization of transition metal oxides [5]. Among the high surface
area transition metal oxides, CeO2, a material with fluorite structure in which the cation valence
state can be switch between Ce+3 and Ce+4, is of particular interest due to the high oxygen
transfer ability, high efficiency for CO oxidation and much lower price, which might
significantly promotes fuels oxidation and reduces the preparation cost of the electrocatalysts [6,

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7, 11, 12]. Several researchers have reported different kinds of interaction between noble metals
and CeO2 and their effects on electrocatalytic activities [5ꢀ7, 11ꢀ14]. These studies have been
shown that the nature and level of interaction depend on particular noble metal, electrocatalyst
pretreatment, preparation technique, size of CeO2 crystallite, lattice oxygen, and so on [13, 14].
On the other hand, practically in the electrocatalytic oxidation of fuels in DLFCs, the Ptꢀ
based electrocatalysts were supported on a carbonic substrate surface and always the supported
electrocatalysts have shown higher electrocatalytic activities and utilization efficiencies than
unsupported ones [13ꢀ15]. Nano dimensional carbonic materials such as carbon nanotubes,
carbon nanofibers, carbon nanopowder, carbon dots and graphene not only have promising
applications in many technological fields such as sensors [16], nanocomposites [17], batteries
[18] and supercapacitors [19], because of their excellent physical and chemical properties, but
also they have high potential applications as a heterogeneous catalyst support to dispersion and
stabilization of the noble metal nanoparticles [20ꢀ23]. Graphene and its famous family, reduced
graphene oxide (RGO), which can be considered as the unrolled carbon nanotubes, owing to
their unique sheet structure, high mechanical stiffness, ultrahigh electron conductivity and
extremely large surface area as well as superior thermal/chemical stability, are novel carbonic
nanomaterials that have emerged as rapidly rising stars in the field of material science [23ꢀ33].
RGO which was produced through the reduction of graphene oxide displays a wrinkled structure
due to the presence of lattice defects and provides a good platform to utilizes it [34ꢀ37] and its
composites, hybrids and doped forms, which integrate the RGO with polymers, metal and metal
oxides nanoparticles, nanotubes, fullerenes and etc., in various applications [30, 31, 38ꢀ42]. In
this way, different nanocomposites and nanohybrids have been integrated with RGO or
synthesized by using RGO as a template and were applied in different applications especially in
electrocatalytic utilizations [36] such as sensors [34, 35, 43ꢀ46], biosensors [34, 35, 45, 47, 48],
cathodic and anodic reactions in the fuel cells [38, 41, 45, 47, 49ꢀ54] and so on [45, 46, 53, 54].
In this work for the first time, we have prepared the PtꢀCeO2/RGO/CCE by a twoꢀsteps
electrochemical procedure: (I) In the first step, the carbonꢀceramic electrode (CCE), including
interesting properties such as high porosity, conductivity, relative chemical inertness, good
mechanical properties, physical rigidity and stability in various solvents [55ꢀ58], was coated by
RGO via an electrochemical process. (II) In the second step, PtꢀCeO2 nanoparticles were
deposited electrochemically on the RGO/CCE. After physiochemical characterization of the Ptꢀ

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CeO2/RGO/CCE, its electrocatalytic activity toward the oxidation of formic acid and
formaldehyde was evaluated by cyclic voltammetric and chronoamperometric techniques in 0.1
M H2SO4 solution and the obtained results were compared with those obtained at the PtꢀCeO2
and Pt modified carbonꢀceramic electrode and also Pt/RGO/CCE. It was found that the Ptꢀ
CeO2/RGO/CCE was electrocatalytically more active than them and had satisfactory stability
and reproducibility when stored in ambient conditions or continues cycling. Consequently, the
combination of the PtꢀCeO2 nanoparticles, RGO and carbonꢀceramic substrate may lead to a
nanocomposite material with improved electrocatalytic activities for application in DLFCs.
2. Experimental
2.1. Chemicals
Methyl trimethoxy silane (MTMOS), formic acid, formaldehyde, methanol,
H2PtCl6.6H2O, HCl, NaOH, H2SO4, Ce(NO3)3.6H2O, graphite powder, Na2HPO4 and NaH2PO4
were purchased from Merck or Fluka and graphene oxide was synthesized from natural flake
graphite by the Hummers method [59]. All solutions were prepared with double distilled water.
Also all the experiments were carried out at room temperature.
2.2. Apparatus
The electrochemical experiments were carried out using an AUTOLABPGSTATꢀ30
(potentiostat/galvanostat) equipped with a USB electrochemical interface and a driven GEPS
software was used for electrochemical experiments. A conventional three electrodes cell was
used at room temperature. The Pt/CCE, PtꢀCeO2/CCE, Pt/RGO/CCE and PtꢀCeO2/RGO/CCE (3
mm diameter) were used as working electrodes. A saturated calomel electrode (SCE) and a
platinum wire were used as the reference and auxiliary electrodes, respectively. JULABO
thermostat was used to control cell temperature at 25°C. Scanning electron microscopy (SEM)
and energy dispersive Xꢀray spectroscopy (EDX) were performed on a LEO 440i Oxford
instrument. Xꢀray diffraction (XRD) of the nanocomposite was studied using a Brucker AXF
(D8 Advance) Xꢀray power diffractometer with a Cu Kα radiation source (λ= 0.154056 nm)
generated at 40 kV and 35 mA.

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2.3. Preparation of the electrocatalysts
2.3.1. Preparation of the Pt-CeO2/CCE
We have prepared the PtꢀCeO2 nanoparticles supported on the CCE by a twoꢀsteps
procedure:
Step I: The solꢀgel processing method was used for the fabricating of the CCE substrate
according to the following procedure [57, 58]: The amount of 0.9 ml MTMOS was mixed with
0.6 ml methanol. After addition of 0.6 ml HCl 0.1 M as the catalyst, the mixture was
magnetically stirred (for about 15 min) until producing a clear and homogeneous solution. Then,
0.3 g graphite powder was added and the mixture was stirred for other 5 minutes. Subsequently,
the homogenized mixture was firmly packed into a Teflon tube (with 3 mm inner diameter and
10 mm length) and dried for at least 24 h at room temperature. A copper wire was inserted
through the other end to set up electric contact. The electrode surface was polished with emery
paper grade 1500 and rinsed with double distilled water.
Step II: 1g cerium (III) nitrate hexahydrate as a starting material was dissolved in 10 ml
double distillated water [60, 61]. A solution of 0.15 M NaOH as the precipitating agent was
added into the reaction mixture. The reaction solution was magnetically stirred to obtain CeO2
nanoparticles by the homogeneous precipitation. Then, the solid product was filtered and washed
with double distillated water and ethanol. Finally, the obtained precipitate was dried for at least
24 h at room temperature. The PtꢀCeO2 coꢀelectrodeposition process was done in a 0.1 M H2SO4
continuous stirring solution containing an excess amount of CeO2 particles as a suspension and
1×10ꢀ3 M of H2PtCl6.6H2O. The PtꢀCeO2 nanoparticles were electrodeposited on the CCE at a
constant potential of ꢀ0.3 V vs. SCE. In this process, which is known as the ‘‘occlusion
electrodeposition’’ method for the deposition of metal together with an oxide on the surface of a
ꢀ2
substrate [61, 62], the Pt nanoparticles (PtCl6 + 4eꢀ → Pt + 6Clꢀ) assimilate the CeO2 particles
during film growth to obtain the final modified electrode, PtꢀCeO2/CCE. The electrodeposited
mass of Pt in PtꢀCeO2/CCE can be calculated by integration of the electrical charge consumed
during the electrodeposition process: QPt= 0.0277 C and assuming, 100% current efficiency, the
quantity of electrodeposited Pt (WPt) was obtained (WPt = Qdep M/ zF) to be 0.02 mg/cm2. It
should be noted that, by passing the same electrical charge in preparation process of other
electrocatalysts; Pt/RGO/CCE, Pt/CCE and PtꢀCeO2/RGO/CCE, the amount of Pt in all
electrocatalysts will be the same: 0.02 mg/cm2.

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2.3.2. Preparation of the Pt-CeO2/RGO/CCE
The CCE was prepared according to the aboveꢀmentioned process. Graphene oxide was
synthesized from natural flake graphite by the Hummers method [59]. The obtained graphene
oxide powder was exfoliated in a 0.07 M phosphate buffer solution (pH 8.5) by ultrasonication to
form a colloidal dispersion of 1.0 mg mlꢀ1 graphene oxide. Then, the polished CCE was
immersed into the graphene oxide colloid and its potential was swept from (ꢀ1.5 to 0.5 V) for
about 8ꢀ10 cycles at scan rate of 20 mV sꢀ1. Fig. 1 shows the eight continues cyclic
voltammograms (CVs) during the electrolysis process of graphene oxide at the CCE, where one
anodic peak (a1 at about ꢀ0.3 V) and two cathodic peaks [c1 at about ꢀ0.5 V and c2 at about ꢀ1.0
V] are observed during the electrodeposition process [inset of Fig.1 shows the CV of CCE in the
phosphate buffer solution (0.07 M and pH 8.5) without graphene oxide at scan rate of 20 mV sꢀ1].
In the first CV, the reduction peak (c2) at around to ꢀ1.0 V is due to the electrochemical reduction
of graphene oxide [63] on the CCE. By increasing the potential cycling, the current of this
cathodic peak, c2, increases firstly (until 10 cycles), then decreases and finally disappears as the
potential cycling proceeds (›15 cycles). When the c2 cathodic peak disappears, the
electrochemical reduction of graphene oxide is considered to be completed (however there are
still some oxygenꢀcontaining groups such as carboxylic and hydroxide groups on RGO which are
too stable under electrochemical conditions) [63, 64]. On the other hand, continues increase of
the peak currents (a1 and c1) with successive potential cycling indicates that the electrodeposition
of RGO film on the CCE has been progressed and completed after 10 cycles [63ꢀ65]: the
requested substrate, RGO/CCE, was achieved. It should be noted that, the CV of the RGO/CCE
in the phosphate buffer solution shows only peaks a1 and c1 (not shown). Therefore, the cathodic
peak, c2, is undoubtedly attributed to the irreversible electrochemical reduction of graphene
oxide [63ꢀ65] and the anodic peak, a1, and its cathodic pair, c1, are attributed to quasiꢀreversible
redox couple of RGO on the CCE. The obtained RGO/CCE was washed with double distilled
water and then dried it in air. The PtꢀCeO2 nanoparticles were electrodeposited on the RGO/CCE
from a 0.1 M H2SO4 continuous stirring solution containing an excess amount of CeO2 particles
as a suspension and 1×10ꢀ3 M of H2PtCl6.6H2O. During the electrodeposition, the CeO2 particles
ꢀ2
in a suspension promoted by magnetic stirring and PtCl6 ions together with its particles are
dragged toward the RGO/CCE surface. As mentioned above, the idea is that the metal ions

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assimilate the CeO2 particles during film growth to obtain the requested PtꢀCeO2 nanocomposite
modified RGO/CCE: PtꢀCeO2/RGO/CCE [61, 62].
3. Results and discussion
3.1. Physical characteristics of the Pt-CeO2/RGO/CCE electrocatalyst
In order to perform surface characterization of the PtꢀCeO2/RGO/CCE electrocatalyst and
comparison studies, the surface morphology of the bare CCE, RGO/CCE, CeO2/RGO/CCE,
Pt/RGO/CCE, PtꢀCeO2/CCE and PtꢀCeO2/RGO/CCE was investigated by SEM and
corresponding results are shown in Fig. 2. Fig. 2A shows the structure of the bare CCE surface
immediately after polishing with emery paper, grade 1500. As seen in this image, the surface of
bare CCE is dense and scaly. Fig. 2B shows the SEM image of the RGO/CCE surface. As can be
seen, the surface of CCE is completely protected with RGO layers and layer structure of the
RGO is favorable and ready for immobilization of compounds or dispersion and
electrodeposition of metal nanoparticles. After coꢀdeposition of the PtꢀCeO2 nanoparticles on the
RGO/CCE surface (Fig. 2C), flower like of PtꢀCeO2 nanoparticles are appeared on the
RGO/CCE. It seems, the flower like structures of PtꢀCeO2 nanoparticles are not the individual Ptꢀ
CeO2 crystallites. Probably, they are collections, like as a flower, consisting of crystallite
aggregates. Fig. 2, images D, and E show the surface of the PtꢀCeO2/CCE and Pt/RGO/CCE,
respectively. As can be seen, the surface morphology of these modified electrodes is completely
different in comparison with PtꢀCeO2/RGO/CCE surface. The PtꢀCeO2 nanoparticles in the Ptꢀ
CeO2/RGO/CCE are smaller and have distinct shapes than those in the PtꢀCeO2/CCE and
Pt/RGO/CCE, which may suggest that the former have more active sites and should have better
electrocatalytic activity. On the other hand, as reported in the literature [42], among the various
shapes, nanostructures with branched morphology such as flower shaped (like as image C) and
dendritic structures are of particular interest. These materials not only show maximum
electrocatalytic performance by their high specific surface area but also utilize fully their
intrinsic capability because of the presence of highly reactive edges, corners and stepped atoms
on their branches. Finally, Fig. 2F shows the surface morphology of the CeO2/RGO/CCE. As can
be seen, the surface of the CeO2/RGO/CCE is very similar to the surface of RGO/CCE; just like
its previous surface. Indeed it seems that any additional particles aren’t electrodeposited on its
surface.

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Furthermore, the EDX analysis of the PtꢀCeO2/RGO/CCE in Fig. 3 and Pt/RGO/CCE,
CeO2/RGO/CCE and RGO/CCE (inset a, b and c, respectively) detected Pt, Ce, O, Si (from
MTMOS) and C atoms. Combining the facts of the crystal plane formation (XRD results) and a
large amount of Pt and Ce detected in Fig. 3, it can be inferred that the nanoparticles mainly
consist of Pt and CeO2. The spectrum in inset a, corresponds to the Pt/RGO/CCE, shows only Pt
along with other common atoms. The EDX spectra of the CeO2/RGO/CCE (inset b) and
similarly the RGO/CCE (inset c) contain only strong peaks for C, Si and O elements, suggesting
that the CeO2 particles aren’t present on them. These results are in agreement with the obtained
SEM images and indicate that in the absence of the PtCl6ꢀ2 ions at the occlusion electrodeposition
process, the CeO2 particles couldn’t deposit on the surface of the RGO/CCE.
The crystallographic structure and chemical phase composition of the electrocatalysts
were determined by XRD patterns. Fig. 4 shows the XRD patterns of the Pt/RGO/CCE (A) and
PtꢀCeO2/RGO/CCE (B). The peaks at 2θ values of 26.96° and 55° in both XRD patterns
correspond to the supporting carbonic material (graphite powder) (002) and (006), respectively.
Also, a broad peak at 2θ value of about 23.32° in both XRD patterns corresponds to (002)
graphitic peak of RGO [63, 66]. Appearance of the broad diffraction peak at 2θ values of 23.32°
in the XRD patterns of both electrocatalysts gives evidence that the graphite oxide was reduced
to RGO at the surface of CCE [66]. XRD pattern of the Pt/RGO/CCE exhibits the characteristic
diffraction peaks of Pt (111), (200), (220), (311) and (222) at 2θ values of 40.08°, 46.8°, 67.88°,
83.76° and 87.24°, respectively [67]. For the PtꢀCeO2/RGO/CCE the main characteristic peaks at
40.4° (111), 46.6° (200), 68.27° (220), 83.84° (311) 87.28° (222) for Pt and at 57.04° (331), 60.24°
(222), 77.92° (420) for CeO2 are observed [13, 68]. As seen, no shift at the position of Pt
diffraction peaks was observed which indicate that adding of the CeO2, has no effect on the
crystallographic structure of the Pt nanoparticles. Using the Scherrer equation [69]; Dc= 0.9 λ/β
cos θ (where λ = 0.154056 nm, β is the full width at halfꢀmaximum in radians and θ is the peak
position in degrees), the average crystallite size (Dc) of Pt and CeO2 nanoparticles is estimated to
be 25 nm and 95 nm, respectively [60, 70].
3.2. Electrochemical characteristics of the Pt-CeO2 /RGO/CCE electrocatalyst
The electrochemical behavior of the PtꢀCeO2/RGO/CCE and also PtꢀCeO2/CCE for
comparison was investigated using cyclic voltammetry at a scan rate of 50 mV sꢀ1 in 0.1 M

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H2SO4 aqueous solution within the potential range of ꢀ0.3 and 1.3 V. Fig. 5 shows the CVs of the
PtꢀCeO2/RGO/CCE (black line) and PtꢀCeO2/CCE (dashed line). The Ptꢀpeaks for the hydrogen
under potential deposition (Hupd), the oxidation of hydrogen (Hoh), formation of Pt oxides (a1)
and their reduction (a2) are present in the CVs of the PtꢀCeO2/RGO/CCE and PtꢀCeO2/CCE
electrocatalysts but they become illꢀshaped compared with Pt/CCE [57].
The actual active surface area (Ar) of the Ptꢀbased electrocatalyst is equivalent to the
number of Pt sites available for hydrogen adsorption/desorption [71]. The Ar of coꢀ
electrodeposited PtꢀCeO2 nanoparticles was estimated from the hydrogen adsorption peaks
[hydrogen under potential deposition (Hupd)] on CV according to Ar = QH/Q0, where QH is the
electric charge related to hydrogen adsorption on the PtꢀCeO2/RGO/CCE and Q0 is the
theoretical charge required to reduce a monolayer of hydrogen on the Pt atoms. Integration of
charge in the potential ranges of ꢀ300 to ꢀ30 mV in Fig. 5, black line, allows to estimates the QH
(ꢁC), due to the hydrogen adsorption. For evaluation of the Ar, an assumption must be made
about the atoms on the electrocatalyst surface which are accessible to hydrogen adsorption;
assuming one Had per Pt surface atom, the theoretical charge associated with a monolayer of
hydrogen formed on the bases of: H+ + eꢀ → Had, Q0, was estimated to be 210 ꢁC/real cm2 as
follows: Q0(ꢁC)/real cm2= F×n/N, where n is the number of platinum atoms (1.3 × 1015/cm2) and
N is Avogadro constant [55, 56]. Form the obtained QH and the theoretical charge, Q0, Ar of the
PtꢀCeO2/RGO/CCE was calculated to be 1.41 cm2. The specific surface area, S (in m2 gꢀ1), of the
PtꢀCeO2/RGO/CCE electrocatalyst was also estimated as follows [30]: S=100Ar/(WAg)=10.07
[Ag: geometric surface area=0.07 cm2, W (in µg cmꢀ2): amount of loaded platinum]. On the other
hand, assuming spherical particles with similar radius, the average particle size of the Pt
nanoparticles in the PtꢀCeO2/RGO/CCE electrocatalyst, d, can be calculated from the following
equation using the Gloaguen and et al. method [72]: d=6000/(ρS), where d is the average particle
size of the Pt nanoparticles in nm, ρ is the density of Pt (ρ=21.4 g cmꢀ3) and S is the specific
surface area (10.07 m2 gꢀ1) of the Pt nanoparticles. The average particle size of Pt nanoparticles
in the PtꢀCeO2/RGO/CCE was calculated to be d=27.84 nm which is in good agreement with the
average particle size obtained for XRD result using Scherrer equation.

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3.3. Electrocatalytic activity of the Pt-CeO2/RGO/CCE toward the oxidation of formic acid and
formaldehyde
The electrocatalytic activity of the PtꢀCeO2/RGO/CCE toward the oxidation of formic
acid and formaldehyde were evaluated by cyclic voltammetry. Fig. 6 shows the CV of the Ptꢀ
CeO2/RGO/CCE in a 0.1 M H2SO4 solution + 0.1 M formic acid. For comparison, the CVs of the
PtꢀCeO2/CCE (inset A), Pt/RGO/CCE (inset B), Pt/CCE (inset C) [55], CeO2/RGO/CCE and
RGO/CCE (inset D, curve1 and 2, respectively) in the same conditions are shown in Fig. 6. It
can be seen from the CV of formic acid on the PtꢀCeO2/RGO/CCE that the reaction commences
in the hydrogen adsorption/desorption region, proceeds slowly in the positive direction and then
reaches a plateau. At potentials more than ca. 0.5 V, the reaction becomes accelerated and
maximum rate occurs at ca. 0.86 V. An increase in the current at potentials more than ca. 1.1 V is
assigned to oxygen evolution. Upon reversing the potential sweep, a very steep increase courses
in the current at ca. 0.48 V and a maximum current is observed at ca. 0.36 V. After that, the
current gradually decreases but the reaction rate is still faster than in the forward scan. This
anodic peak in the reverse scan is attributed to the removal of the incompletely oxidized
carbonaceous species formed in the forward scan [73]. As evidenced in Fig. 6 and its insets, the
PtꢀCeO2/RGO/CCE electrocatalyst exhibits a much higher anodic peak current density for formic
acid oxidation than the PtꢀCeO2/CCE (inset A), Pt/RGO/CCE (inset B) and Pt/CCE (inset C) in
the forward scan. The anodic peak current density at the PtꢀCeO2/RGO/CCE is about 60.1 mA
cmꢀ2, while it is 51.8 mA cmꢀ2 at the PtꢀCeO2/CCE, 46.2 mA cmꢀ2 at the Pt/RGO/CCE and 6.6
mA cmꢀ2 at the Pt/CCE electrocatalysts. This indicates that the incorporation of Pt nanoparticles
along with CeO2, PtꢀCeO2, supported on the RGO greatly enhanced the utilization of Pt
nanoparticles in the electrooxidation of formic acid. Moreover, the ratio of the anodic peak
current density in the forward scan (Jf) to the anodic peak current density in the backward scan
(Jb) can be used to evaluate the electrocatalyst tolerance to the intermediate carbonaceous species
which formed in the electrooxidation reaction and accumulated on electrocatalyst surface [55].
The higher Jf/Jb value indicates higher tolerance to intermediate carbonaceous species which
means formic acid can be oxidized to final product much more efficiently. The ratio of Jf/Jb for
formic acid electrooxidation on the PtꢀCeO2/RGO/CCE is calculated to be 1.14, while for the Ptꢀ
CeO2/CCE, Pt/RGO/CCE and Pt/CCE electrocatalysts are 1.13, 1.31 and 0.49, respectively,
which suggests that PtꢀCeO2/RGO/CCE has less carbonaceous accumulation and hence is more

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tolerant toward CO poisoning especially in comparison with Pt/CCE (Jf/Jb =0.49). The results of
the electrochemical parameters of formic acid oxidation at the present electrocatalysts were
summarized in Table 1. As can be seen in Table 1, the onset potential for the oxidation of formic
acid at the PtꢀCeO2/RGO/CCE electrocatalyst is about ꢀ0.20 V, which shifts negatively about
100ꢀ200 mV compared to that at the PtꢀCeO2/CCE, Pt/RGO/CCE and Pt/CCE electrocatalysts.
The fact that the electrooxidation of formic acid taking place at lower potential with higher Jf/Jb
value and lower onset potential discloses that the PtꢀCeO2/RGO/CCE electrocatalyst exhibits a
higher electrocatalytic activity for the electrooxidation of formic acid than that of the others. The
significant enhancement in the electrocatalytic activity of the PtꢀCeO2/RGO/CCE as compared
with PtꢀCeO2/CCE, Pt/RGO/CCEB and Pt/CCE electrocatalysts can be attributed to two factors:
First: the role of the RGO in the improvement of the catalytic performances of the Ptꢀ
CeO2/RGO/CCE electrocatalyst. The role of RGO can also be explained by two facts: (I) the role
of the RGO as a support in well, uniform and smaller distribution of the PtꢀCeO2 nanoparticles
and (II) possible bifunctional effect between the remaining oxygenated groups on the ORG and
PtꢀCeO2 nanoparticles similar with the commonly accepted bifunctional effect between Pt and
Ru [26, 74]. As can be found in the literature [74, 75], oxygenꢀcontaining groups on the RGO
can improve the electrocatalytic activity of Pt nanoparticles slightly by playing the role of
ruthenium in the case of PtꢀRu electrocatalyst [75] to remove the intermediate carbonaceous
species and contribute to the low poisoning. On the other hand, the remaining functional groups
like as carboxylic and hydroxide groups on the RGO (incomplete reduction) may be responsible
for improved tolerance to CO observed in the PtꢀCeO2/RGO/CCE electrocatalyst [26].
Second: the synergistic effects between the Pt nanoparticles and CeO2 [7, 68, 70]. The
improvement of the electrooxidation performances as assisted by CeO2 can be described by two
probabilities [13]: (I) Chemical enhancement of the electrooxidation of the adsorbed CO as
supported by CeO2 as an oxygen donor, or by the effect of CeO2’s altering the electrocatalyst’s
surface morphology (as can be seen in the SEM images), physically preventing the CO
adsorption. The probable mechanism for this proposes can be described by the following
reactions [13, 76]:
PtꢀCOads + 4CeO2 + 2H+ + 2eꢀ ↔ 2Ce2O3 + Pt + CO2 + H2O
4CeO2 + 4H+ + 4eꢀ ↔ 2Ce2O3 + 2H2O
(1)
(2)

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which assist on the weakening of the PtꢀCO bond, thus reducing the CO poisoning effect and
promoting the electrooxidation of the formic acid. (II) A second possible mechanism for CeO2’s
proposed improvement of the electrooxidation of formic acid can be explained by the
bifunctional mechanism; the OHads species on metal oxide can transform COads on the Pt
nanoparticles to CO2, releasing the active sites on Pt nanoparticles for further electrochemical
reaction [70, 77, 78].
CeO2 + H2O ↔ CeO2–OHads + H+ + eꢀ
Pt–COads + CeO2–OHads ↔ Pt + CeO2 + CO2 + H+ + eꢀ
(3)
(4)
On the other hand, the combination of Pt and CeO2 could assists the oxidation of the adsorbed
intermediated, COads, according to the bifunctional mechanism, where the OHad species could
form on the surface of the CeO2 and could transform COads on the surface of Pt nanoparticles to
CO2, which release the active sites of the Pt nanoparticles for contribution in main
electrooxidation reactions. All these results demonstrate that the PtꢀCeO2/RGO/CCE in the
electrooxidation of formic acid is more active than the PtꢀCeO2/CCE, Pt/RGO/CCE, Pt/CCE [57,
70, 79ꢀ81].
Moreover, comparison of the obtained results in the electrooxidation of formic acid on
the PtꢀCeO2/CCE and Pt/RGO/CCE show that the effects of CeO2 in the improvement of the
electrocatalytic activity of Pt nanoparticles is much higher than the RGO effects. As reported in
the literature [68], CeO2 has been generally used as a kind of oxygen reservoir to regulate the
oxygen species on the electrocatalysts surface [82]. CeO2 has a fluorite structure whose cations
can switch between oxidation states (Ce+3/Ce+4) (Eq.s 2, 3), and consequently has the ability to
act as an oxygen species buffer [68]. Therefore, introduction of CeO2 into the electrocatalyst
improves the dispersion of the Pt nanoparticles and consequently increasing the proportion of Pt
exposed to oxygen. On the other hand, CeO2 provides a good situation for oxygen transport
through valence variation, so that the CeO2 synergism with Pt effectively promotes the oxygen
reduction reaction [68, 83, 84]. These results indicate that the oxygenated Pt species have a
weaker adsorption energy on the surface of the PtꢀCeO2/CCE electrocatalyst than on the surface
of the Pt/RGO/CCE electrocatalyst, which means that the desorption of oxygenated Pt species is
easier on the PtꢀCeO2/CCE than on the Pt/RGO/CCE [5].

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Inset D in Fig. 6 shows the CVs of the CeO2/RGO/CCE and RGO/CCE in a 0.1 M H2SO4
solution + 0.1 M formic acid. It can be seen, there is no anodic peak corresponds to the formic
acid oxidation on the CeO2/RGO/CCE (curve 1) and RGO/CCE (curve 2) [68], indicating that
the RGO and CeO2/RGO have no electrocatalytic activity toward formic acid oxidation.
Fig. 7 shows the CV of the PtꢀCeO2/RGO/CCE electrocatalyst in a solution of 0.1 M
H2SO4 + 0.1 M formaldehyde. For comparison, the CVs of the PtꢀCeO2/CCE (inset A),
Pt/RGO/CCE (inset B), Pt/CCE (inset C) [57] and CeO2/RGO/CCE and RGO/CCE (inset D,
curve1 and 2, respectively) were also shown in that. As can be seen in Fig. 7, the electrochemical
behavior of formaldehyde is similar to formic acid electrooxidation at the same electrocatalysts:
One main oxidation peak in the forward scan at all electrocatalysts which is located at potentials
between 0.7 V ꢀ 0.9 V with peak current density corresponds to the type and performance of the
studied electrocatalysts and one main oxidation peak in the backward scan. CV responses reveal
that the PtꢀCeO2/RGO/CCE has higher electrocatalytic activity in the electrooxidation of
formaldehyde than the others electrocatalysts. The electrochemical parameters of formaldehyde
electrooxidation at the present electrocatalysts were summarized in Table 1. As can be seen in
Table 1, the PtꢀCeO2/RGO/CCE exhibits a higher electrocatalytic activity in the electrooxidation
of formaldehyde than that of the others.
Inset D in Fig. 7 shows that the CeO2/RGO/CCE (curve 1) and RGO/CCE (curve 2) have
no electrocatalytic activity toward formaldehyde electrooxidation [68].
Finally, to compare the electrocatalytic performance of PtꢀCeO2/RGO/CCE toward
formic acid and formaldehyde oxidation with other electrocatalysts in same conditions, the
electrochemical parameters in oxidation of formic acid and formaldehyde (Eonset, Epf, Epb, Ipf, Ipb,
Ipf/Ipb and also the stability of the electrocatalysts) obtained in this work and others studies were
listed in Table 2. Table 2 shows that the PtꢀCeO2/RGO/CCE exhibits better or comparable
electrocatalytic performance than other reported electrocatalysts toward formic acid and
formaldehyde oxidation. These results confirmed that the PtꢀCeO2/RGO/CCE exhibits a
promising catalytic behavior toward formic acid and formaldehyde oxidation and demonstrate
that PtꢀCeO2 nanoparticles by providing a flower like nanostructure on the RGO/CCE and
reducing the loading of expensive noble metals improve Pt utilization efficiency.
A further investigation was done to find out the transport characteristics of formic acid
and formaldehyde on the PtꢀCeO2/RGO/CCE electrocatalyst. The influence of the scan rate (v)

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on the electrooxidation of formic acid and formaldehyde at the PtꢀCeO2/RGO/CCE was
investigated and shown in Fig. 8 (A) and (B), respectively. The results show that the currents
associated to the formic acid and formaldehyde electrooxidation increase with scan rate. The
anodic peak current densities in the forward scan are linearly proportional to v1/2, as shown in
inset of (І) in Fig. 8 A and (І') in Fig. 8 B, suggest that the electrocatalytic oxidation reactions of
formic acid and formaldehyde on the PtꢀCeO2/RGO/CCE are diffusionꢀcontrolled processes.
Also, as can be seen in insets (ІІ) and (ІІ') of Fig. 8 (A) and (B), respectively, the potentials of
anodic peaks in the forward scan (curve a1 and a'1) shift to high potentials with increasing of the
scan rate [99]. Whereas, the potentials of cathodic peaks (c1 and c'1 for formic acid and
formaldehyde, respectively) and anodic peak (a2 and a'2 for formic acid and formaldehyde,
respectively) in the backward scan shift to higher positive potentials with increasing of the scan
rate. These results indicate that the electrooxidation of formic acid and formaldehyde and also
the process in the cathodic reaction at the PtꢀCeO2/RGO/CCE are irreversible electrode
processes.
In order to evaluate the capacity of the PtꢀCeO2/RGO/CCE in the electrooxidation of
formic acid and formaldehyde, the effect of fuels concentrations on the corresponding anodic
peaks currents was investigated by CV. According to experimental data, the anodic peak current
density of formic acid and formaldehyde increased by their concentrations, and reached to the
nearly constant values for concentrations higher than 1.5 M and 1.2 M for formic acid and
formaldehyde, respectively. We assume this effect caused by saturation of active sites at the
surface of PtꢀCeO2/RGO/CCE electrocatalyst [100].
3.4. Long-term stability of the Pt-CeO2/RGO/CCE electrocatalyst
Practically, longꢀterm stability of the anodic material in the fuel cells is important. The
longꢀterm activity and durability of the PtꢀCeO2/RGO/CCE and PtꢀCeO2/CCE electrocatalysts
were assessed by chronoamperometry tests. Fig. 9 shows the typical chronoamperometric
response curves of 0.1 M formic acid (A) and 0.1 M formaldehyde (B) in 0.1 M H2SO4 on the Ptꢀ
CeO2/RGO/CCE and PtꢀCeO2/CCE electrocatalysts under a constant potential of 0.85 V for
formic acid and 0.9 V for formaldehyde for 800 s. It was found that the currents observed from
chronoamperograms were in good agreement with the currents observed from CVs and show that
the both electrocatalysts have significantly stability toward formic acid and formaldehyde

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electrooxidation. The longꢀterm stability of the PtꢀCeO2/RGO/CCE and PtꢀCeO2/CCE
electrocatalysts was further examined in 0.1 M H2SO4 solution containing 0.1 M formic acid
(Fig. 10A) and 0.1 M formaldehyde (Fig. 10B) by CV in consecutive scans. It can be observed
from Fig. 10A that the anodic peak current density remains constant with an increase in the scan
number at the initial stage. By increasing in the number of scans the anodic peak current
densities gradually decrease. In general, the loss of the catalytic activity after successive number
of scans may result from the consumption of formic acid during the CV scan. It may also be due
to poisoning and the structure change of the PtꢀCeO2 nanoparticles as a result of the perturbation
of the potentials during the scanning in aqueous solutions. After the longꢀterm cyclic
voltammetry experiments, the PtꢀCeO2/RGO/CCE was stored in water for a week and then the
formic acid oxidation was carried out again by the CV. This process revealed that the excellent
electrocatalytic activity the formic acid oxidation was still observable. Similar behavior was
obtained for formaldehyde oxidation in continues cycling (Fig. 10B) and stability study [57, 79].
4. Conclusions
PtꢀCeO2/RGO/CCE electrocatalyst was successfully fabricated by a simple twoꢀsteps
electrochemical deposition process consisting: step I; electrodeposition of the RGO on the CCE
through a simple electrochemical process using graphene oxide and CCE as the precursor and
electrode substrate, respectively and step II; coꢀelectrodeposition of the PtꢀCeO2 nanoparticles on
the RGO/CCE by using another electrochemical process. The obtained PtꢀCeO2/RGO/CCE
electrocatalyst was characterized by SEM, EDX, XRD and electrochemical methods. Then, the
electrocatalytic performances of the PtꢀCeO2/RGO/CCE toward the oxidation of formic acid and
formaldehyde were investigated by cyclic voltammetric and chronoamperometric methods.
Compared with Pt/RGO/CCE, PtꢀCeO2/CCE and Pt/CCE, PtꢀCeO2/RGO/CCE electrocatalyst
exhibited higher catalytic activity for formic acid and formaldehyde oxidation. The significant
enhancement in the catalytic activity of the PtꢀCeO2/RGO/CCE electrocatalyst as compared with
PtꢀCeO2/CCE, Pt/RGO/CCE and Pt/CCE electrocatalysts may be attributed to: the role of the
RGO in the increased performances of PtꢀCeO2 nanoparticles and the synergistic effects between
the Pt nanoparticles and CeO2 nanoparticles in the enhanced catalytic performances of the Ptꢀ
CeO2/RGO/CCE electrocatalyst.

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Acknowledgment
The authors are grateful to the research office of Azarbaijan Shahid Madani University for its
financial support.
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Legend of Figures:
Fig. 1. CVs (2ꢀ8th cycles) showing the electrochemical reduction of graphene oxide (1.0 mg mLꢀ
1) in phosphate buffer solution (0.07 M, pH 8.5) on the CCE surface at 20 mV sꢀ1. Inset is the CV
of CCE in the same solution without graphene oxide.
Fig. 2. SEM images of CCE (A), RGO/CCE (B), PtꢀCeO2/RGO/CCE (C), PtꢀCeO2/CCE (D),
Pt/RGO/CCE (E) and CeO2/RGO/CCE (F).
Fig. 3. EDX data for PtꢀCeO2/RGO/CCE and EDX data for Pt/RGO/CCE (inset a),
CeO2/RGO/CCE (inset b) and RGO/CCE (inset c).
Fig. 4. XRD patterns of the Pt/RGO/CCE (A) and PtꢀCeO2/RGO/CCE (B).
Fig. 5. CVs of PtꢀCeO2/RGO/CCE (black line) and PtꢀCeO2/CCE (dashed line) in 0.1 M H2SO4
at a scan rate of 50 mV sꢀ1.
Fig. 6. CV of 0.1 M formic acid on the PtꢀCeO2/RGO/CCE in 0.1 M H2SO4 at a scan rate of 50
mV sꢀ1. Insets are the CVs of PtꢀCeO2/CCE (A), Pt/RGO/CCE (B), Pt/CCE (C), CeO2/RGO/CCE
and RGO/CCE (D, curve1 and 2 respectively) in the same conditions.
Fig. 7. CV of 0.1 M formaldehyde on the PtꢀCeO2/RGO/CCE in 0.1 M H2SO4 at a scan rate of
50 mV sꢀ1. Insets are the CVs of PtꢀCeO2/CCE (A), Pt/RGO/CCE (B), Pt/CCE (C),
CeO2/RGO/CCE and RGO/CCE (D, curve1 and 2 respectively) in the same conditions.
Fig. 8. Effect of scan rate on the electrooxidation of 0.1 M formic acid (A) and 0.1 M
formaldehyde (B) obtained in 0.1 M H2SO4 using PtꢀCeO2/RGO/CCE. Insets (І and І') are the
dependence of the anodic peak current densities in the forward scan on the square root of scan
rates, (ІІ and ІІ') Variation of anodic peak potentials in forward scan (a1, a'1), backward scan (a2,
a'2) and reduction peak (c1, c'1) vs. the square root of scan rates.
Fig. 9. Chronoamperometric curves of 0.1 M formic acid (A) and 0.1 M formaldehyde (B)
electrooxidation in 0.1 M H2SO4 at the PtꢀCeO2/CCE and PtꢀCeO2/RGO/CCE, respectively.

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Fig. 10. Longꢀterm stability of PtꢀCeO2/RGO/CCE in 0.1 M H2SO4 and 0.1 M formic acid (A)
and 0.1 M formaldehyde (B) vs. number of scans (scan rate 50 mV sꢀ1).
Table captions:
Table 1: Electrochemical parameters of formic acid and formaldehyde electrooxidation on the
present electrocatalysts.
Table 2. Comparison of the electrocatalytic performances of the different electrocatalysts toward
formic acid and formaldehyde oxidation.

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Fig. 1. CVs (2-8th cycles) showing the electrochemical reduction of graphene oxide (1.0 mg mL-1) in
phosphate buffer solution (0.07 M, pH 8.5) on the CCE surface at 20 mV s-1. Inset is the CV of CCE in the
same solution without graphene oxide.
133x117mm (96 x 96 DPI)

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Fig. 2. SEM images of CCE (A), RGO/CCE (B), Pt-CeO2/RGO/CCE (C), Pt-CeO2/CCE (D), Pt/RGO/CCE (E)
and CeO2/RGO/CCE (F).
326x173mm (96 x 96 DPI)

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Fig. 3. EDX data for Pt-CeO2/RGO/CCE and EDX data for Pt/RGO/CCE (inset a), CeO2/RGO/CCE (inset b)
and RGO/CCE (inset c).
223x158mm (96 x 96 DPI)

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Fig. 4. XRD patterns of the Pt/RGO/CCE (A) and Pt-CeO2/RGO/CCE (B
411x346mm (96 x 96 DPI)

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Fig. 5. CVs of Pt-CeO2/RGO/CCE (black line) and Pt-CeO2/CCE (dashed line) in 0.1 M H2SO4 at a scan rate
of 50 mV s-1.
144x133mm (96 x 96 DPI)

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Fig. 6. CV of 0.1 M formic acid on the Pt-CeO2/RGO/CCE in 0.1 M H2SO4 at a scan rate of 50 mV s-1. The
insets are CVs of Pt-CeO2/CCE (A), Pt/RGO/CCE (B), Pt/CCE (C), CeO2/RGO/CCE and RGO/CCE (D, curve1
and 2 respectively) in the same conditions.
268x166mm (96 x 96 DPI)