22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Low pressure plasma synthesis of Pt/C catalysts for fuel cells applications
Y. Busby1, V. Stergiopoulos, N. Job and J.-J. Pireaux2
1
Research Center in the Physics of Matter and Radiation (PMR), Laboratoire Interdisciplinaire de Spectroscopie
Electronique (LISE), University of Namur, Namur, Belgium
2
Laboratoire de Génie Chimique – Génie Catalytique, University of Liège, Liège, Belgium
Abstract: We have applied low pressure plasma treatments to deposit, in a one-step
process, platinum nanoparticles on different mesoporous carbon supports starting from
organometallic and inorganic precursors. Homogeneously distributed Pt nanoparticles of
tuneable size and oxidation state are obtained by finely combining oxygen and/or argon
plasma treatments. The synthetized nanocomposite catalyst meets the basic requirements
for PEM fuel cells applications.
Keywords: Pt nanoparticles, proton exchange membranes fuel cells, low pressure plasma,
XPS
1. Introduction
Commercial catalysts employed in proton exchange
membranes (PEM) fuel cells are mainly based Platinum
nanoparticles (Pt-NPs) deposited on carbon supports
(Pt/C). The electrochemical efficiency and long term
operation of these catalysts require a high density of few
nanometre (ideally 3-4 nm) sized Pt-NPs uniformly
dispersed on the carbon matrix support. Although the
chemical or electrochemical deposition of Pt-NPs gives
good results in terms of the control of the size and density
of Pt NPs,[1] multiple steps are needed to achieve the
final morphology and chemical state of the Pt-NPs; this
makes this methodology time consuming and not
sufficiently cost-effective. On the other hand, a single step
processing of the Pt/C catalyst would be very beneficial in
reducing the fabrication time and the costs, and could
possibly open the route for larger scale commercialization
of PEM fuel cells. Based on recent results,[2] we have
developed a method based on plasma processing which
allows to deposit Pt NPs starting from a powder mixture
of the carbon support and an inorganic, or an
organometallic, precursor. The results show that metal
NPs can be successfully deposited starting from both
kinds of Pt precursors. The plasma treatment and
conditions have to be finely tuned in order to achieve the
desired morphology and chemical state of Pt NPs. Here
we present a systematic study made by alternating plasma
treatments with a chemically reactive (oxygen) and inert
(argon) gases. The results show that by finely combining
these treatments it is possible to obtain a complete
conversion of the Pt precursor to generate Pt NPs with
tuneable oxidation state.
2. Experimental
We have applied different plasma treatments to Pt/C
powder mixtures in a low pressure inductively coupled
plasma reactor. During the plasma treatment the powder
is constantly stirred by a magnetic agitator to ensure that
the mixture is homogeneously exposed to the plasma. The
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starting mixture is a carbon xerogel (CXg) characterized
by a mean pore size of about 100 nm mixed with an
organometallic precursor (OM) (Pt- acetylacetonate - P1and Pt-dimethyl 1,5-cyclooctadiene - P2-) or an inorganic
(dihydrogen hexahydroxyplatinate - P3-) precursor. The
ratio between the CXg and the Pt precursor is fixed to
obtain a Pt content of 20wt%. By varying the plasma
conditions such as the plasma environment (O 2 , Ar), and
the parameters such as the discharge power, the treatment
time and mode (continuous or pulsed discharge), we are
able to affect the distribution, size and oxidation state of
the Pt nanoparticles in a wide range. The Pt distribution
and chemical state in the nanostructured samples have
been characterized by X-ray powder diffraction (XRD) in
reflection geometry, by high resolution X-ray
photoelectron spectroscopy (XPS) and transmission
electron microscopy (TEM).
3. Results
First of all, the minimum discharge power/duration for
degrading the Pt precursor has been studied by XRD. In
Figure 1 we show as an example the diffraction patterns
after a 10min O 2 plasma treatment of the CXg/P1
(discharge power of 100W), CXg/P2 (discharge power of
100W) and CXg/P3 (discharge power at 150W). The
presence of characteristic reflections of the OM precursor
P1 indicates that this compound is not fully degraded after
10 min, while the treatment is sufficiently long for P2
which has a lower melting temperature with respect to P1
(105° instead of 250°). For the inorganic and amorphous
precursor P3, even though the melting temperature is low
(100°), a discharge power of 150W is necessary to obtain
crystalline Pt NPs.
The chemical state of Pt NPs from P1/CXg mixture
exposed to oxygen plasma for 10 to 20 min has been
studied with XPS as shown in Figure 2. For an exposition
of more than 15 min the outer shell of Pt NPs is strongly
oxidized, and the Pt4+ state becomes dominant with
respect to Pt2+. This fact limits the use of a prolonged
1
exposition to O 2 plasma in order to fully convert the Pt
precursor or eventually to obtain smaller (metallic) Pt NPs
by plasma by erosion of the formed NPs.
Since both thermal and chemical reactions take part to
the NPs formation [2], we have studied the effects of
oxygen and argon treatments. In order to fully decompose
the OM precursor but avoiding the oxidation of Pt NPs
we have applied a 100W Ar treatment for 10 min previous
to the 100W oxygen treatment for 10 min. Argon
treatment is thought to create defects and anchor sites in
the CXg matrix and thus favouring Pt NPs nucleation.
Fig. 1. XRD pattern of P1-2-3/CXg mixtures exposed to
a 10min O 2 plasma treatment. The Pt XRD reflections are
indicated in the figure by vertical lines. The other peaks
which appear in the P1 spectra are an indication of the
incomplete degradation of the OM precursor due to an
insufficient exposure time.
From the XPS spectra of the Pt4f in Figure 3 we can
clearly see that the NPs oxidation is strongly reduced by
combining Ar and O 2 plasma treatments. Moreover, the
Pt content increases from 6at% (O 2 ) to 7.5at% (Ar+O 2 )
suggesting that a more efficient conversion of the OM
precursor is obtained with this combined treatment.
Fig. 3. Spectral analysis of the Pt4f peak obtained on
P1/CXg mixture after a O 2 -100W-15min plasma
treatment (bottom panel) and after a Ar-100W-10min
treatment followed by an O 2 -100W-10min treatment
(upper panel). The comparison shows that Pt oxidation
can be significantly reduced by combining Ar and O 2
treatments.
4. Conclusions
Pt/C catalysts have been successfully deposited by low
pressure plasma processing. The XRD characterization is
used to evaluate the minimum - power/exposure time conditions which are necessary to decompose the Pt
precursor into Pt NPs. The conversion is mainly ruled by
the melting temperature of the Pt precursor. However, in
order to deposit crystalline (metal) Pt NPs starting from
an amorphous (inorganic) precursor, a relatively higher
discharge power/exposition time is needed. Our results
indicate that a prolonged O 2 plasma treatment strongly
oxidizes the Pt NPs outer shell; we described a strategy
for preserving the metallic state of the Pt NPs and fully
convert the precursor, made by a combining argon plasma
and oxygen plasma treatments.
Fig. 2. Pt4f XPS spectra obtained on P1/CXg mixture
exposed to a 100W O 2 plasma for 10, 15 and 20min. The
inset shows the relative percentage of the metallic (Pt4f0)
and the oxidized (Pt4f2+ and Pt4f4+) phases. The Pt at% is
reported as an indication for the decomposition of the OM
compound which is converted to Pt NPs. The relative
weight of the Pt4f4+ state is significantly increased for
prolonged (20min) exposition.
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5. References
[1] N. Job, S. Lambert, M. Chatenet, C.J. Gommes, F.
Maillard, S. Berthon-Fabry, J.R.Regalbuto, J.-P. Pirard,
Catalysis Today 150 119–127 (2010).
[2] M. Laurent-Brocq, N. Job, D. Eskenazi, J.-J. Pireaux,
Applied Catalysis B: Environmental, 147, 453-463
(2014).
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