Electrochemical characterization of platinum nanoparticles stabilized by amines

Article abstract of DOI:10.1016/j.jallcom.2008.08.141

In this work we present the synthesis by Chaudret approach of Pt nanoparticles stabilized by primary amine (-NH₂) compounds. Their electrochemical performance as cathodes in low temperature polymer electrolite fuel cells on the oxygen reduction

Full text of DOI:10.1016/j.jallcom.2008.08.141

Journal of Alloys and Compounds 483 (2009) 573–577
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Electrochemical characterization of platinum nanoparticles stabilized by amines
E. Ramírez-Menesesa,∗, M.A. Domínguez-Crespoa, V. Montiel-Palmab, V.H. Chávez-Herreraa,
E. Gómezc, G. Hernández-Tapiad
a Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada, CICATA-IPN Unidad Altamira, Km 14.5 Carretera Tampico-Puerto Industrial,
C.P. 89600 Altamira, Tamaulipas, Mexico
b Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, Colonia Chamilpa, C.P. 62201 Cuernavaca, Morelos, Mexico
c Instituto de Química-Universidad Nacional Autónoma de México, Ciudad Universitaria, C.P. 04510 México, D.F., Mexico
d Gerencia de Catalizadores y Proceso, Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas norte 152, 07730 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:
In this work we present the synthesis by Chaudret approach of Pt nanoparticles stabilized by primary
amine (–NH2) compounds. Their electrochemical performance as cathodes in low temperature polymer
electrolite fuel cells on the oxygen reduction reaction (ORR) is also presented. Transmission electron
microscopy (TEM) images of the samples show Pt nanostructures with particle size varying from 10
to 100 nm depending on the kind of the stabilizer used during the catalyst preparation. In some cases
well-dispersed isolated platinum nanoparticles were observed. The activity of the dispersed catalysts
(Pt/C) with respect to the ORR was investigated using steady state polarization measurements. The kinetic
parameters showed that although no significant differences between the Tafel slopes of the Pt catalysts
exist, transfer coefficients and exchange current densities show higher activities when the Pt nanoparticles
were stabilized by tert-butylamine (TBA). The performance with respect to the ORR of the Pt/C catalyst
on vulcan carbon substrate is active and comparable to that reported in the literature for state-of-art
electrocatalysts.
Received 30 August 2007
Received in revised form 27 July 2008
Accepted 5 August 2008
Available online 17 November 2008
Keywords:
Nanostructured materials
Chemical synthesis
Electrochemical reactions
Transmission electron microscopy
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
in size and dispersion. However, the contamination on the sur-
face of the obtained nanoparticles due to some of the employed
Platinum supported on high surface area carbon substrates is
still the most widely used electrocatalyst in low temperature poly-
mer electrolyte fuel cells (PEMFCs), particularly for the cathode
reaction [1]. While the overpotential for the oxidation of pure
hydrogen (anode reaction) is negligible even at high current den-
sities due to very facile H2 oxidation kinetics on platinum, the
strong kinetic inhibition of the cathodic oxygen reduction reaction
(ORR) leads to high overpotentials, amounting to several hun-
dreds of millivolts under typical PEMFC operating conditions [1].
Therefore, optimization of the Pt catalysts performance through-
out new synthesis routes is needed. For this a large catalytic active
surface area is required, which is typically achieved by having
the noble metal nanoparticles small and well dispersed on an
electron conducting support material with a large surface area.
Several methods have been developed to synthesized platinum
nanoparticles including salt reduction technique [2], where a wide
range of reducing agents (such as alcohols [3], borohydrides [4,5],
hydrazine [6] or dihydrogen [7]) have been used with good results
reducing agents is an important disadvantage of this method. An
interesting alternative to obtain “clean surface” nanostructured
materials has been proposed by Chaudret and co-workers [8]; who
have studied the coordination properties of different ligands on
metal nanoparticles. They established from a series of experiments
the predominant role of chemical equilibrium between all poten-
tially coordinating agents (ligands resulting from the precursor,
solvent and stabilizers) and concluded on their important effect
over the size, shape and dispersion of the generated nanoparti-
cles. They also mentioned that these materials can be obtained
free of possible surface contamination. Recently, the same authors
have reported interesting results on the obtention of Pt nanopar-
ticles from Pt2(dba)3 as metallic precursor and hexadecylamine as
stabilizer [9]. However, up to now, synthesis of Pt nanoparticles
stabilized by amines from organometallic precursors for electro-
catalysts is not found to best of scientist knowledge. In the present
work we aim to investigate how the nature of different stabi-
lizers containing amine group (–NH2), such as tert-butylamine
(TBA), C(CH3)3NH2; 1,3-diaminopropane (DAP), H2N(CH2)3NH2,
and anthranilic acid (AA), 2-(H2N)C6H4CO2H, modifies the size,
shape and dispersion of Pt nanoparticles and their behaviour as
cathode catalysts in the PEMFC. The ORR activities are measured
∗
Corresponding author.
E-mail address: esthervincent@yahoo.com (E. Ramírez-Meneses).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.08.141

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E. Ramírez-Meneses et al. / Journal of Alloys and Compounds 483 (2009) 573–577
Fig. 1. Transmission electron micrographs and corresponding electron diffraction pattern of Pt-TBA (a and b), Pt-DAP (c and d) and Pt-AA (e and f) after the purification
process.
and compared with those reported in the literature. Characteri-
zation by transmission electron microscopy (TEM) before fuel cell
experiments was also performer in order to determine the nanosize
distribution of the Pt particles.
2.3. Electrochemical measurements
The electrochemical measurements were conducted via cyclic voltammetry (CV)
and rotating disk electrode using a thermo stated standard three compartment elec-
trochemical cell by means of an interchangeable ring-disk electrode setup with a
bi-potentiostat and rotation control. 0.5 M H2SO4 was used as electrolyte. Before
the electrochemical tests, the electrolyte was saturated with nitrogen gas for CV or
oxygen gas for oxygen reduction. The counter electrode was a Pt grid and the refer-
ence electrode a reversible saturated calomel electrode (0.2415 V vs. RHE) connected
to the working electrode compartment via Lugging capillary. The cyclic voltammo-
grams were measured in the range from −0.50 to 1.3 V (vs. RHE) and the scan rate
was 50 mV s−1. All the specimens were analyzed until 50 cyclic potential scans in
order to produce clean surfaces. The steady state polarization measurements were
recorded in the range from −0.1 to 1.3 V vs. RHE at different rotation rates 400, 800,
2. Experimental
2.1. Synthesis of metallic nanoparticles
The platinum precursor, Pt2(dba)3, was prepared according to literature pro-
cedures [10] from K2PtCl4 (99.9%). The synthesis of nanoparticles was carried out
as a standard procedure in a Fischer–Porter bottle at room temperature to obtain
platinum colloids. Three different compounds were employed to stabilize plat-
inum nanostructures; tert-butylamine, 1,3-diaminopropane and anthranilic acid
according to the following experimental procedure: Pt2(dba)3 (0.036 mmol) was
introduced in a Fisher–Porter bottle under argon atmosphere and 40 ml of THF
distilled and degassed by freeze-pump cycles, was added. Thereafter, 0.6 mmol of
tert-butylamine (98%) or 1,3-diaminopropane (99%) or anthranilic acid (98%) was
added to the resulting violet solution. Then, the obtained dark brown solutions
were pressurized under hydrogen atmosphere (3 bar) and after 20 h; a homogeneous
black colloidal solution was obtained. During this time, the colloidal solutions were
purified by hexane washings (to eliminate the dibenzylideneacetone). Finally, the
resulting black solution was evaporated in vacuum until the residue was completely
dry.
1200, 1500 and 2000 rpm at a scan rate of 5 mV s−1
.
2.4. Characterization
Specimens for TEM analysis were prepared by slow evaporation of a drop of
each colloidal solution after purification process deposited onto a holey carbon cov-
ered copper grid. TEM experiments were performed on a JEOL-1200 EX electron
microscope, operating at 120 KV.
3. Results and discussion
TEM analysis of Pt-TBA systems display agglomerate parti-
cles with an elongated shape, mean size between 80 and 100 nm
(Fig. 1a). The corresponding selected area electron diffraction pat-
tern (SAED) exhibits the diffraction peaks from (1 1 1), (2 0 0),
(2 2 0), (3 1 1) and (3 3 1) planes of (FCC) Pt (Fig. 1b). On the other
hand, TEM analysis of the Pt-DAP system shows the presence
of well-dispersed small particles of spherical shape, mean size
5–8 nm (Fig. 1c). The corresponding SAED pattern also exhibits
(1 1 1), (2 0 0), (2 2 0), (3 1 1) and (3 3 1) planes corresponding to
(FCC) Pt (Fig. 1d). Finally, TEM image of the Pt-AA specimen depicts
sponge-like particles, mean size 30–50 nm (Fig. 1e). These particles
2.2. Catalysts preparation
The support of the electrode was a glassy carbon rod (5.0 mm in diameter). The
base of the rod was polished with a cloth and alumina powder (ca. 0.3 ␮m). The
working electrodes were prepared by attaching ultrasonically re-dispersed catalyst
suspension containing a 4:1 ratio vulcan carbon black/total metal (20 wt.%) pow-
ders in deionized water onto the glassy carbon. After drying in flowing high purity
argon at room temperature, the deposited catalyst layer was then covered with≈4 ␮l
of a diluted aqueous Nafion solution and finally, the electrode was immersed in
nitrogen purged electrolyte to record the electrochemical measurements. The elec-
trodes were carefully prepared in order to obtain reproducible electrode surfaces
and comparable electrocatalytic results.

E. Ramírez-Meneses et al. / Journal of Alloys and Compounds 483 (2009) 573–577
575
Fig. 2. Cyclic voltammograms after 50 potential cycles of different Pt catalysts
obtained by organometallic method using different stabilizers at a sweep rate
50 mV s−1
.
result from the agglomeration of nanocrystallites. The correspond-
ing SAED pattern confirms the presence of (FCC) Pt (Fig. 1f). It seems
to be that Pt-DAP and Pt-AA nanoparticles are better dispersed than
those of the Pt-TBA system. However, the particle sizes suggest
that the synthesized system could be a suitable material for use
in electrocatalysis.
Electrochemical properties of the ORR on the electrode electro-
catalysts have been studied by analyzing rotating disk electrode
(RDE) electrochemical data. The activation process of each sample
was performed previously to the measurement, by scanning the
cyclic voltammetry in the range of −0.1 to 1.3 V vs. RHE. Although
the cyclic voltammograms of all catalysts reached a steady state
after a few potential cycles, test were recorded up to 50 potential
cycles to evaluate electrochemical stability of the samples, which
were characterized by small pseudocapacitances between 0.4 and
1.3 V (Fig. 2). However, the shapes of the cyclic voltammograms
were different based on the kind of stabilizer used during cata-
lysts preparation. Fig. 2 also shows typical features of Pt and carbon
surfaces after 50 potential cycles, with slightly less well-resolved H-
adsorption–desorption peaks for the samples than those reported
in the literature for Pt-ETEK [11,12]. Pt–OH peaks in the adsorption
and desorption regions are distinct for each Pt catalysts, e.g., two
anodic (desorption) and cathodic (adsorption) peaks H1a, H2a, H1c and
H2c , respectively, can be distinguished for Pt-TBA and Pt-DAP, while
in Pt-AA only one anodic peak is observed. In the hydrogen des-
orption region, the peak present at more negative potentials (H2a)
is considered to be result of hydrogen desorption from the bulk of
the metal together with the desorption of hydrogen adsorbed on
the surface [13]. The peak obtained at more positive potentials (H1a)
is attributed either exclusively to adsorbed hydrogen [14]. Changes
in the amount of hydrogen desorbed manifested by the peaks H1a
and H2a are proportional to the changes of the real surface area of
the Pt-catalysts [15]. As it is observed, Pt-TBA shows the highest
real surface area for this reaction. These curves also show a reduc-
tion peak (Pr) around 0.6 V; the peak is normally assigned to the
electro-reduction of oxygen in Pt. It can be seen that the position
of the current peak is shifted to a more positive value (i.e., lower
overvoltage) when TBA and AA are used as stabilizers. The fact that
the shape of the curves and displacement of the peaks (reduction of
oxides/hydroxides) are modified by the stabilizer type indicates a
modification in the form, size and dispersion of the Pt nanoparticles
as was observed by TEM studies, which affects the electrochemical
behaviour. In addition, it is difficult for the catalysts to be evaluated
Fig. 3. Polarization curves for oxygen reduction reaction on different nanoparticles
of Pt obtained from organometallic method using different stabilizers. (a–c) The
Pt particles were dispersed into Nafion film on a glassy carbon electrode, in a O2
saturated 0.5 M H2SO4 with a rotating geometric electrode area = 0.7 cm2 and sweep
rate = 5 mV s−1at 298 K.
for their electrochemical stability based on the cyclic voltammo-
grams shape. However, because the catalysts reached a steady state
soon (only after 5 potential cycles), it can be considered that the
catalysts are electrochemically stable.
Current density potential curves at four different rotation rates
produced by the molecular oxygen reduction at the Pt-catalysts
incorporated into a Nafion film are shown in Fig. 3a–c. In all graph-

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E. Ramírez-Meneses et al. / Journal of Alloys and Compounds 483 (2009) 573–577
Table 1
Kinetic parameters obtained from the mass-corrected Tafel plots of the ORR in 0.5 M H2SO4.
Sample
Tafel slope (−b) (mV dec−1
)
Transfer coefficient (␣)
Log i0 (mA cm−2
)
i1 E = 0.9 V (mA cm−2
)
i2 E = 0.8 V (mA cm−2
)
i3 E = 0.75 V (mA cm−2
)
Pt-DAP
Pt-AA
Pt-TBA
100
95
84
4
2
3
0.56 0.02
0.59 0.01
0.72 0.01
−4.1 0.2
−4.4 0.1
−5.3 0.2
0.11 0.01
0.12 0.02
0.26 0.03
0.70 0.3
0.98 0.2
1.35 0.1
0.89 0.2
1.30 0.3
1.86 0.2
ics and results presented, the current density corresponds to the
geometric area and was normalized to the Pt-loading in the elec-
trodes. It can be noted on all catalysts that the ORR is diffusion
controlled, when the potential is lower than 0.6 V vs. RHE and is
under mixed diffusion-kinetic control between 0.6 and 0.8 V. Also,
it is seen that the half-wave potentials are very similar for the three
Pt-catalysts. The abrupt change in curvature observed at potential
of 0.6 V corresponds to the limitations imposed by mass transfer.
Differences between current densities can be associated with the
kind of stabilizer used during catalysts preparation or an obstruc-
tion of active sites and the influence of the reduction area due
to the liquid hydrocarbon used to prepare the carbon paste as it
has been reported by other authors [16]. In the carbon paste elec-
trodes, the resistance of the Nafion film which covers the supported
catalysts is sufficiently small and the experimental catalytic cur-
rent densities could be adjusted to the simple Koutechy´–Levich
first order reaction equation, without further need additional
terms:
mass transport corrected Tafel plots, as presented in Fig. 4. The
catalytic activity towards the oxygen reduction reaction is higher
when TBA is used as stabilizer. Pt catalysts obtained from DAP and
AA displayed in the lower overpotential region similar behaviour
with a slope close to −0.100 V dec−1 while in the high overpotential
region (E < 0.6 V) a difference is observed in the current densities.
The favourable effect obtained with TBA could be attributed to the
elongated shape of the Pt nanoparticles observed by TEM stud-
ies, which have better electronic properties of the active centre
and facilities the electron transfer. We believe that the presence
of two NH2 groups contained in DAP can give better stabiliza-
tion to platinum nanoparticles because of the coordination of the
amine groups on the surface but, at the same time, DAP could
present less mobility of NH2 group than that for TBA stabilizer.
Therefore, the electroactivity in this reaction is influence by the
mobility of NH2 group. Finally, although Pt particles with AA as sta-
bilizer showed agglomeration, its electroactivity is better than the
Pt-DAP system. We suggest that the presence of AA, could possi-
bly enhance the charge transfer phenomena due to the presence
of the phenyl group in its structure and the likely coordination
of the NH2 moiety at the surface of the metallic particles. How-
ever, further analyses have to be carried out to confirm these
hypotheses.
The kinetic parameters deduced for the ORR on Pt-catalysts are
showninTable1. Thesevaluesarecomparablewiththosepublished
elsewhere [12,13]. It can be seen that although no significant dif-
ferences exist between the Tafel slopes of the Pt catalysts, transfer
coefficients and exchange current densities show higher activities
when Pt nanoparticles were stabilized with TBA. This could indi-
cate that the effect of the stabilizer is observed only in the effective
area exposed. The electrochemical results suggest that Pt-catalysts
synthesized from organometallic approach using a properly stabi-
lizer could be considered as a good alternative in the fabrication of
membrane electrode assemblies and for testing as a cathode for the
ORR in a polymer electrolyte fuel cell.
1
i
1
ik
1
id
=
+
(1)
where ik is the kinetic current density due to the charge-transfer
at the electrode surface and id represents the diffusion limited cur-
rent density, id = Bω0.5; ω is the angular frequency of rotation and B
parameter is defined as B = 0.2nFDO2/3
ꢀ
1/6CO2 ; n is the number of
2
electrons transferred in the overall reduction process, F is the Fara-
day constant, D is the diffusion coefficient of the molecular oxygen,
ꢀ is the kinematic viscosity and CO is the concentration of molec-
ular oxygen [17–19]. Typical Kout2echy´–Levich plots (not shown)
of the experimental values indicated that at high polarizations,
slopes over a wide frequency interval can be fitted when the num-
ber of electrons transferred in the overall reduction process is four.
Therefore, the limiting current densities observed for these materi-
als follows a four-electron mechanism, i.e., O2 + 4H+ + 4e → 2H2O,
which has also seen in other works [16,20]. The electrocatalytic
actives for the ORR on the Pt-catalysts were compared through
4. Conclusions
The present study has shown that the nature of stabilizers influ-
ences shape, sized, dispersion and the electrocatalytic activity of
the Pt nanoparticles when the organometallic approach is used.
Therefore, thecorrectchoiceofthesematerialsproducesasynergis-
tic interaction enhancing the electron transfer rate for the ORR. The
electrochemical performance showed that Pt-catalysts prepared
from TBA displayed higher activity than that showed when AA and
DAP are used as stabilizers. Pt-TBA catalyst could be considered as a
good alternative for its use as cathode in low temperature polymer
electrolyte fuel cells.
Acknowledgements
The authors wish to acknowledge financial support and a M.Phil.
grant (V.H.C.-H.) from IPN-Mexico (project SIP 20071254) and
CONACyT (project 59921). We are also indebted to Mr. C. Flores-
Morales (I.I. Materiales-UNAM) for TEM analysis.
Fig. 4. Mass transfer corrected Tafel plots for Oxygen reduction on different
nanoparticles of Pt dispersed into Nafion membrane in an O2 saturated 0.5 M H2SO4.

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577
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