Pt, PtNi and PtCoNi film electrocatalysts prepared by chemical vapor deposition for the oxygen reduction reaction in 0.5 M KOH

Article abstract of DOI:10.1016/j.jallcom.2010.03.206

Pt, PtNi and PtCoNi films were deposited by metal-organic chemical vapor deposition (MOCVD) upon glassy carbon rods. Physical characterization was performed by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. It was found that films consisted of crystalline and nanometric particles with more than one phase as a result of the interactions among reactants, intermediate and product species during the chemical vapor deposition process. The electrochemical set-up to test the films for the oxygen reduction reaction was a three-electrode cell where the reference electrode was Hg/HgO/0.5 M KOH, the working electrodes were the films upon glassy carbon rods. The films were tested in 0.5 M KOH electrolyte; polarization curves were obtained by using the rotating disk electrode technique to evaluate the electrocatalytic activity of the prepared films. The kinetic parameters of Pt, PtNi and PtCoNi electrocatalysts were calculated. PtCoNi showed the best performance for the oxygen reduction reaction due to a synergetic effect of alloying Pt with transition metals.

Full text of DOI:10.1016/j.jallcom.2010.03.206

Journal of Alloys and Compounds 504S (2010) S425–S428
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Pt, PtNi and PtCoNi film electrocatalysts prepared by chemical vapor deposition
for the oxygen reduction reaction in 0.5 M KOH
M.A. García-Contrerasa,∗, S.M. Fernández-Valverdea, J.R. Vargas-Garcíab
a Instituto Nacional de Investigaciones Nucleares, Depto. de Química, Apdo. Postal 18-1027, Col. Escandón, C.P. 11801 México D.F., Mexico
b Instituto Politécnico Nacional, Depto. de Ing. Metalúrgica, 07300 México D.F., Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 July 2009
Received in revised form 20 March 2010
Accepted 29 March 2010
Available online 2 April 2010
Pt, PtNi and PtCoNi films were deposited by metal–organic chemical vapor deposition (MOCVD) upon
glassy carbon rods. Physical characterization was performed by X-ray diffraction, scanning electron
microscopy and transmission electron microscopy. It was found that films consisted of crystalline and
nanometric particles with more than one phase as a result of the interactions among reactants, interme-
diate and product species during the chemical vapor deposition process. The electrochemical set-up to
test the films for the oxygen reduction reaction was a three-electrode cell where the reference electrode
was Hg/HgO/0.5 M KOH, the working electrodes were the films upon glassy carbon rods. The films were
tested in 0.5 M KOH electrolyte; polarization curves were obtained by using the rotating disk electrode
technique to evaluate the electrocatalytic activity of the prepared films. The kinetic parameters of Pt,
PtNi and PtCoNi electrocatalysts were calculated. PtCoNi showed the best performance for the oxygen
reduction reaction due to a synergetic effect of alloying Pt with transition metals.
Keywords:
Electrode materials
Fuel cells
Nanostructured materials
Thin films
Vapor deposition
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Chemical vapor deposition (CVD) is a versatile process, suitable
for the manufacturing of coatings, powders, fibers and monolithic
Oxygen reduction reaction (ORR) is a key step in cathodes of
fuel cells due to its relatively high overpotential, around four times
of that one for hydrogen oxidation, the anodic reaction [1,2]. The
majority of research in oxygen electrocatalysis on supported plat-
inum electrodes has focused on the study of oxygen reduction in
acid media. This is due to the fact that acid electrolytes are mainly
used in fuel cell systems. Platinum is also the material of choice
for technical application in alkaline fuel cells because of its high
electrocatalytic activity and chemical stability. It is known that the
oxygen reduction overpotential is lower in alkaline than that in
acid solutions [3]. Concentrated KOH solution is used in alkaline
fuel cells and in metal-air batteries. It has been found from practi-
cal applications that the oxygen reduction kinetics is more rapid in
alkaline electrolytes than in acids and a wide range of electrocata-
lysts can be used in alkali, including Ag, Ni, metal oxides and spinels
as well as noble metals [4]. In search of electrocatalysts with lower
cost, to replace platinum as much as possible with metals of higher
abundance, a variety of proposals has arisen for this application
including mainly Pt alloys with transition metals prepared by sev-
eral techniques that showed interesting results of electrocatalytic
activity for the ORR [5–10].
components. With CVD, it is possible to produce most metals, many
nonmetallic elements such as carbon and silicon as well as a large
number of compounds including carbides, nitrides, oxides, inter-
metallics and many others. This technology is now an essential
factor in the manufacture of semiconductors and other electronic
components, in the coating of tools, bearings and other wear-
resistant parts and in many optical, optoelectronic and corrosion
applications [11]. In spite of CVD versatility, there are not many pro-
posals of electrocatalysts prepared by this process [12–14]. CVD is
by definition a chemical process; various process parameters, such
as reaction temperature, reactor pressure and precursor chemistry
(which can be metal–organic) as well as the nature of the substrate,
affect film growth and microstructure [15].
The aim of this work was to prepare electrocatalysts of Pt, Co
and Ni by using the CVD process and to evaluate their performance
for the ORR in alkaline media.
2. Experimental
2.1. Films deposition
The precursors for CVD experiments were cobalt (II), nickel and platinum acety-
lacetonates as supplied by Aldrich. CVD of all films was carried out in a low pressure,
horizontal hot-wall quartz reactor. Prior to film deposition, the reactor was purged
with argon gas (Praxair 99.999%) at a flow rate of 100 cm3 min−1 for 30 min. The
precursors, which are subliming solids, were vaporized at 423–473 K. Argon was
used as carrier gas with a flow rate of 180 cm3 min−1, which was maintained
steady by using an electronic mass-flow controller. Reactor pressure was 1 mmHg.
∗
Corresponding author. Tel.: +52 5553297200x2280; fax: +52 5553297301.
E-mail address: miguel.garcia@inin.gob.mx (M.A. García-Contreras).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.03.206

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M.A. García-Contreras et al. / Journal of Alloys and Compounds 504S (2010) S425–S428
Fig. 1. DRX patterns of (a) Pt, (b) PtNi and (c) PtCoNi films prepared by CVD.
Substrates were glassy carbon (GC) rods, 5 mm in diameter, 10 mm length, polished
with emery paper No. 1000 in the transversal surface which was the film receptor.
The substrate (growth) temperatures were in the 673–723 K range. The deposition
time for all films was 20 min.
2.2. Structural and morphological characterization
The as-deposited films were characterized by X-ray diffraction (XRD), scanning
electron microscopy (SEM) and transmission electron microscopy (TEM). XRD was
carried out in a Siemens D-5000 diffractometer (Cu K␣ radiation, ꢀ = 0.15418 nm).
Particle size was calculated by using X-Powder software (trial version). Surface mor-
phology was observed and thickness film was estimated with a JEOL JSM-6300
scanning electron microscope, equipped to perform elemental chemical analysis
by energy dispersive analysis by X-rays (EDAX). Transmission electron microscopy
images were obtained by using a JEOL JEM-2000 FXII microscope to have a deeper
sight of the films, to support particle size calculation and to confirm crystallinity
analysis.
2.3. Electrochemical experiments
To prepare the working electrodes, the GC rods with the as-deposited films were
mounted into concentric Teflon cylinders holders. Insulated epoxy resin was used
to seal the gap between the Teflon holder and the GC rods. Ohmic contact was made
with silver epoxy paste (Spi supplies).
Fig. 2. SEM images of (a) Pt and (b) PtCoNi films. Insets are EDAX spectra of chemical
analysis.
To perform the electrochemical measurements, a typical three compartments
electrochemical cell was used. A pre-calibrated house-made Hg/HgO electrode
(0.194 V vs. NHE) was set as reference electrode and a platinum mesh was the
counter electrode. A 0.5 M KOH solution was used as the electrolyte which was
prepared by dissolving KOH pellets (Aldrich) in bidistilled water. Before electro-
chemical experiments, the electrolyte was deoxygenated by bubbling N2 (Praxair
99.999%) during 30 min. Cyclic voltammetry was performed to working electrodes
with a scanning rate of 50 mV s−1 until voltammograms were reproducible. To per-
form ORR measurements, the electrolyte was saturated with O2 (AGA 99.999%).
Linear voltammetry was carried out by using the rotating disk electrode (RDE) tech-
nique to evaluate the electrocatalytic performance of CVD films for the ORR. The
electrochemically active surface area of the catalysts was calculated by using the
hydrogen adsorption method [16,17], from the cyclic voltammogram of each cat-
alyst, the charge associated with the cathodic deposition of hydrogen atoms was
calculated by integration of the area under this region and subtracting the double
layer contribution. A hydrogen monolayer adsorption charge of 210 ␮C cm−2 was
assumed. A Potensiostat/Galvanostat EG&G Model 273A and a unit for rotation speed
control (EG&G Model 636) were employed for the electrochemical experiments.
◦
∼
2ꢁ 48.5 corre◦sponds to (1 1 1) plane for PtCo. Two more reflec-
=
tions at 2ꢁ 29 and 61◦, probably for PtCoNi alloy but PDF card
was not available to confirm this observation.
∼
=
The presence of more than one phase in XRD patterns of as-
deposited films could be explained as it has been pointed out by
several authors; chemical vapor co-deposition of multi-component
films with separate source precursors is the most common practice
for depositing multi-component films [18–22], however, this route
can be complex because of interactions among reactant, interme-
diate and product species on the surface substrate and in the gas
phase [18,21,23]. The films oxygen content (as confirmed later by
EDAX) is due to the fact that film top surface is oxidized by air.
Oxygen content can vary through film thickness [24,25].
Fig. 2 shows the Pt and PtCoNi films morphology in micrographs
obtained by SEM. Surface of these films is rather rough. This obser-
vation suggests that film growth involves a high nucleation density
and proceeds by coalescence of small grains, leading to rather sub-
stantial roughness. Insets in Fig. 2 are EDAX spectra of each film.
Particle size and EDAX analysis of film catalysts are given in
Table 1. It can be observed that all films have nanometric particle
size. Chemical composition indicates more than one phase for each
film, except Pt, supporting what was observed by XRD analysis.
3. Results and discussion
3.1. XRD, SEM and TEM analysis
Fig. 1 shows XRD patterns of as-deposited Pt, PtNi and PtCoNi
films. XRD reflections were assigned according to Joint Commit-
tee of Powder Diffraction Standards (JCPDS). A broad reflection at
◦
∼
2ꢁ 26.5 observed in all diffraction patterns corresponds to (0 0 3)
=
Table 1
plane of GC hexagonal structure. XRD patterns in Fig. 1 show that for
PtNi and PtCoNi films more than one phase can be observed. XRD
pattern of PtNi film shows reflections for GC sup◦port, a◦lso, those
Particle size and chemical analysis by EDAX of Pt, PtNi and PtCoNi film
electrocatalysts.
for planes of fcc Pt and three peaks at 2ꢁ 44.5 , 46.5 and 80◦
Electrocatalyst
Particle size/nm
Co
Ni
Pt
O
∼
=
Pt
PtNi
PtCoNi
30
15
16
94.78
70.32
33.95
5.22
14.56
25.95
corresponding very likely to Pt3Ni phase (PDF card not available).
15.12
22.15 18.15
Finally, XRD pattern of PtCoNi film shows reflections for (1 0 0) and
◦
(2 0 0) planes of Pt Co at 2ꢁ 23 and 47◦, respectively. A peak at
∼
=
3

M.A. García-Contreras et al. / Journal of Alloys and Compounds 504S (2010) S425–S428
S427
Table 2
Kinetic parameters for the ORR in KOH 0.5 M @ 298 K on Pt, PtNi and PtCoNi film electrocatalysts.
Electrocatalyst
Eoc /V
−b/V dec−1
Io/mA cm−2
Potential/V @ i = −0.4 mA cm−2
Pt
PtNi
PtCoNi
0.244
0.251
0.262
0.108
0.115
0.127
5.34 × 10−4
4.61 × 10−4
2.69 × 10−3
-0.078
0.062
0.094
Films thickness was estimated by cross-sectional SEM (micro-
graphs not shown) and was about 180–250 nm, in accordance with
other films reported elsewhere [24,25].
tial range. This could have been caused by bubbling of oxygen gas.
In Fig. 4, it can be observed an enhanced electrocatalytic activity
for Pt < PtNi < PtCoNi.
Fig. 3 shows a TEM image of PtCoNi film obtained at 200 kV
and 400,000×. Particles of nanometric size can be observed,
consistently with results obtained by XRD analysis. In Fig. 3,
agglomerated particles are distinguishable, confirming what was
observed through SEM analysis. Inset in Fig. 3 is an electron diffrac-
tion pattern with concentric rings which confirms that this film is
polycrystalline.
To analyze ORR kinetics, mass-transfer corrected Tafel plots
were constructed. Two regions with different slopes are seen in
Fig. 5 for Pt, PtNi and PtCoNi films. Table 2 shows kinetic parame-
ters for the ORR upon films prepared in this study. Exchange current
densities Io (mA cm−2) were obtained by the extrapolation of Tafel
curves at low current density to thermodynamic potential of the
O2/OH—couple in 0.5 M KOH (0.401 V vs. NHE).
According to Table 2, PtCoNi shows the best performance with
the highest current density and the lowest overpotential (last col-
umn in Table 2). It is well known that alloying Pt with transition
metals causes an enhanced activity for the ORR. Several investiga-
tions have been carried out to determine the role of alloying for the
ORR. Alloying is believed to retard the formation of the OH (ads)
electrode surface poison [26–30].
3.2. Electrochemical characterization
Cyclic voltammetry was performed for electrode activation and
to calculate normalized surface area of catalysts. Active area was
higher than geometric surface area, 0.52, 0.41 and 0.44 cm2 for Pt,
PtNi and PtCoNi, respectively. Oxygen reduction current–potential
curves were obtained at six rotation frequencies (between 100
and 1600 rpm) with a 20 mV s−1 scan rate. Fig. 4 shows typical
RDE curves for the ORR on Pt, PtNi and PtCoNi film electrocat-
alysts at 1600 rpm in 0.5 M KOH at 298 K. ORR density currents
commenced at ca 0.2 V and tend to diffusion limits around −0.2 V.
Density currents of PtNi and PtCoNi begin to increase at a more
positive potential than that for Pt film by ca. 20 and 30 mV,
respectively, indicating an enhanced ORR activity for these elec-
trocatalysts. In Pt film voltammogram there is a small decay of
density current at a potential value prior to the hydrogen evolu-
tion reaction. Evidently, the number of surface sites that are needed
for O2 molecules to undergo reduction is decreasing due to hydro-
gen electrosorption. For PtCoNi film polarization curve, it can be
observed some instability of density current in almost all the poten-
Fig. 4. Polarization curves for the ORR on Pt, PtNi and PtCoNi films in 0.5 M KOH at
1600 rpm.
Fig. 5. Tafel plots for the ORR on Pt, PtNi and PtCoNi films in 0.5 M KOH.
Fig. 3. TEM image of PtCoNi film. Inset presents the electron diffraction pattern.

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4. Conclusions
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Electrocatalysis for the ORR in 0.5 M KOH was studied upon
films of Pt, PtNi and PtCoNi prepared by metal–organic chemical
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in order to obtain only the desired phases the deposition process
is being redesigned accordingly for the next experimental stage.
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projects ININ-CB-906 and IPN-SPI-20070781.
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