Formation and evaluation of electrochemically-active ultra-thin palladium-Nafion nanocomposite films

Article abstract of DOI:10.1039/b702537h

A simple method for producing electrochemically-active palladium nanoparticles within ultra-thin Nafion films is described. The Royal Society of Chemistry.

Full text of DOI:10.1039/b702537h

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Formation and evaluation of electrochemically-active ultra-thin
palladium–Nafion nanocomposite films{
Paolo Bertoncello,{* Massimo Peruffo{ and Patrick R. Unwin*
Received (in Cambridge, UK) 19th February 2007, Accepted 12th March 2007
First published as an Advance Article on the web 19th March 2007
DOI: 10.1039/b702537h
A
simple method for producing electrochemically-active
A schematic of the procedure used is illustrated in Fig. 1. Nafion
LS films were deposited on indium tin oxide (ITO) electrodes using
palladium nanoparticles within ultra-thin Nafion films is
described.
9,10c
a well established procedure.
Films were formed by depositing
between 10 and 50 layers, corresponding to film thicknesses
ranging from 15 to 75 nm, (a) in Fig. 1. Nafion LS films produced
in this way were subsequently dipped for 30 min in a solution
containing palladium nitrate (b). After removing the electrode
The synthesis of nanometre-sized particles is of tremendous
1
interest in different fields of chemistry and allied disciplines. For
catalytic applications, metal particles usually exhibit size-dependent
2
reactivity, with smaller particles typically more active. However,
2+
from this loading solution, Pd ions incorporated in the films were
reduced to Pd using NaBH as the reducing agent (c).
the synthesis of naked metal nanoparticles remains challenging,
with the aggregation and sintering providing an obstacle in
3
practical applications. To overcome such problems, metal
4
The morphology of the Pd produced in this way was
investigated by transmission electron microscopy (TEM), with
typical results for a 20-layer Nafion LS film shown in Fig. 2. This
image shows clearly that Pd nanoparticles are formed and that
although there is some clustering, the particles are discrete. The
diameter of Pd nanoparticles measured by TEM was in the range
nanoparticles are often synthesized or immobilized on solid
4,5
supports (zeolites, polymers) or stabilized by capping with small
organic ligands or polymers, in order to reduce agglomeration
6
processes. The dispersion of nanoparticles in polymer films is
attractive, as it may enhance the processibility and allows a wider
variety of surfaces and interfaces to be functionalised with
nanoparticles.
5
¡ 1 nm.
Fig. 3 shows typical tapping mode AFM images of a 10-layer
2+
Nafion LS film before (a) and after (b) reduction of Pd by
Nafion is a perfluorinated ionomeric cation exchanger, well
7
known for its thermal, chemical and electrochemical stability. As
NaBH
4
: a relatively smooth and uniform film is evident for the
pristine Nafion film, while features with a characteristic size of 20–
a membrane, Nafion is used extensively as an electrolyte in
8
methanol and hydrogen fuel cells. Traditionally, Nafion films
3
0 nm are visible after treatment with NaBH , which is reasonably
4
consistent with the scale of the nanoparticle clustering observed by
have been rather thick (. 1 mm), but recent developments have
9
seen the use of the Langmuir–Schaefer (LS) technique to produce
TEM. This characteristic topography was further confirmed by
analysis of the phase images of the AFM data (figure not shown).
Cyclic voltammetry (CV) was used to investigate whether the Pd
nanoparticles formed were in electrical contact with the underlying
ITO electrode. Fig. 4 reports a typical CV of a 40-layer
Nafion films on solid supports on the nanometre and mesoporous
scale. Such films have proved useful for incorporating different
1
0a
10b–d
species such as metal oxides and redox mediators.
Pd nanoparticles are attracting considerable interest for
hydrogen-related applications, such as hydrogenation, hydrogen
3
nanocomposite film in 0.1 M KNO at pH 3.
The CV shows a small broad oxidation wave at about 0.3 V and
a sharp reduction peak at about 20.05 V (vs. Ag quasi-reference
electrode, QRE). These peaks are attributed to the formation of
palladium oxide (forward anodic scan), while on the return
cathodic sweep, the surface oxide is reduced giving rise to a well-
1
1
evolution and hydrogen oxidation. Several strategies have
recently emerged for the preparation of nanostructured Pd films,
including template electrodeposition at solid/liquid and liquid/
1
2
liquid interfaces. In this paper, we describe a simple strategy for
fabricating relatively monodisperse palladium nanoparticles
embedded in a Nafion ultra-thin polymeric film. The method
takes advantage of the fact that Nafion films have a structure in
which the hydrophilic sulfonated end groups aggregate in domains
located within the hydrophobic matrix constituted by the
12a
defined cathodic stripping peak.
These results suggest that
although Pd is embedded in the Nafion film, the surface activity is
similar to that of naked Pd.
The electrochemically active surface area of Pd nanoparticles
was estimated by integrating the charge passed in the Pd oxide
7
b,13
fluorocarbon backbone.
The hydrophilic domains can be
2
1
2+
stripping reaction at low scan rate (10 mV s , figure not shown),
loaded with Pd which, upon reduction, facilitates the formation
of small particles in these nanocompartments.
Department of Chemistry, University of Warwick, Coventry, UK
CV4 7AL. E-mail: p.r.unwin@warwick.ac.uk;
p.bertoncello@warwick.ac.uk; Fax: +44 (0)2474 524112;
Tel: +44 (0)2476 523264
{
Electronic supplementary information (ESI) available: Materials, film
fabrication procedure and methods. See DOI: 10.1039/b702537h
These authors contributed equally to the work.
Fig. 1 Schematic of the fabrication of palladium nanoparticles within a
{
Nafion LS film.
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ultra-thin composite films was further considered through studies
of hydrogen oxidation. Palladium is receiving considerable
attention in fuel cell technologies due to low cost, greater
availability compared to platinum and its ability to absorb large
1
1
quantities of hydrogen. Fig. 5 reports typical CVs for various
modified ITO electrodes, specifically functionalised with a pristine
4
0-layer Nafion film (dotted line) and a 40-layer Pd–Nafion film,
before (dashed line) and after (solid line) purging an aqueous 0.1 M
KNO solution with H
The CV for Nafion alone on ITO after purging with H
that the electrode is essentially inert with respect to the oxidation of
over the potential range shown. The CV of the 40-layer Pd–
Nafion film before purging with H shows the presence of the
typical peak for the cathodic stripping of Pd oxide at about
0.22 V (vs. Ag QRE). Note that the potential of the cathodic
3
2
.
2
reveals
H
2
2
Fig. 2 TEM image of palladium nanoparticles in a 20-layer Nafion LS
film.
2
peak is shifted cathodically by about 0.2 V compared to the peak
evidenced in Fig. 4, because the pH of the solution was ca. 6,
causing the stripping of palladium oxide to occur at a more
negative potential. A dramatic change in the response was
2
observed when the solution was purged with H : the CV reveals
an oxidation peak at about 0.1 V (vs. Ag QRE), characteristic of
1
5
the oxidation of hydrogen. Note that the peak current density
22
value of 30 mA cm is about half that expected for a diffusion-
23
23
limited reaction, assuming 10 mol dm as the concentration of
2
H after saturation in aqueous solution and a diffusion coefficient
Fig. 3 AFM topography images of a 10-layer Nafion LS film before (a)
and after (b) formation of Pd nanoparticles. The scale is 1 mm 6 1 mm.
25
2 21 16
.
of 5 6 10 cm s
At potentials beyond the peak value, the
current magnitude decreases significantly and then, surprisingly,
increases during the reverse scan. This behaviour can be attributed
to the fact that during the oxidation scan there is the concomitant
formation of Pd oxide with hydrogen oxidation. Oxide formation
leads to passivation of the Pd nanoparticles, thus resulting in a
decrease in the rate of hydrogen oxidation. Thus, during the
reverse (reduction) scan, the electrode is bathed in hydrogen and,
as oxide is removed from the surface, the hydrogen oxidation
reaction is able to occur at driving potentials.
The results in Fig. 5 suggested that it might be beneficial to
precondition the electrode by forming Pd oxide at anodic
potentials, quickly stripping it from the surface, followed by an
anodic sweep for hydrogen evolution. The beneficial effect is
clearly visible in Fig. 6 which reports the forward anodic
3
Fig. 4 CV of a 40-layer Pd–Nafion LS film in 0.1 M KNO , pH 3
21
(
HNO
3
), scan rate 50 mV s .
following the procedure of Rand and using a conversion factor of
22 14
4
24 mC cm . For the data in Fig. 4, the surface area was 6.1 6
22
2
cm Pd/cm ITO. However, even though the effective surface
2
10
area of Pd is only 6% of the geometric area of the ITO electrode,
the modified electrode shows regular macroscopic behaviour in
voltammetric experiments (vide infra). Considering 5 nm as the
average diameter of the Pd nanoparticles, as estimated by TEM,
the number density of nanoparticles in Nafion is approximately
1
6
23
1
.3 6 10 cm . Since each particle, on average, comprises 4.4 6
3
0 atoms, the Pd concentration in the film that is converted to
2+
1
23
Pd is y 0.1 mol dm . This value is typical of the concentration of
suggesting that much of the Pd is
10c,d
cationic redox mediators,
2+
Fig. 5 CVs of a 40-layer Pd–Nafion LS film before (dashed line) and
converted to Pd nanoparticles that are electroactive. Studies on
other thickness films suggested a close correlation of the magnitude
of the oxide stripping peak with film thickness. The activity of the
after saturation with H
2
(solid line) in 0.1 M KNO
scan rate 20 mV s . The dotted line shows a CV of a 40-layer Nafion LS
film on ITO after saturation of the solution with H
3
supporting electrolyte,
21
2
.
1
598 | Chem. Commun., 2007, 1597–1599
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from a Marie Curie Intra-European Fellowship (MEIF-CT-2005-
515356).
Notes and references
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21
supporting electrolyte 0.1 M KNO
with H : without preconditioning (dashed line) and with preconditioning
at 1 V for 30 s (solid line). Inset: LSV peak current magnitude, I , vs.
3
, scan rate 100 mV s , after saturation
2
p
preconditioning potential; other conditions as indicated above for the solid
line.
6
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voltammograms for hydrogen oxidation related to the same 40-
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application of a conditioning anodic potential (+1.0 V, 30 s
duration). The magnitude of the anodic peak current under these
conditions increased with increasingly anodic preconditioning
potentials, reaching a maximum value at 1 V and beyond (see Inset
of Fig. 6). Coupled with our earlier observations, these results
suggest that the procedure herein produces electrochemically active
Pd nanoparticles, without any protecting group, which are
characterised by the usual oxide formation/stripping of naked Pd
nanoparticles.
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9
In summary, we have developed a very simple procedure for
functionalising ultra-thin Nafion films with relatively monodis-
perse Pd nanoparticles. The nanoparticles have characteristics
typical of naked Pd and are electrochemically active towards the
catalytic oxidation of hydrogen. Further work is underway to
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and applications of these films.
1
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We are grateful to Mr S. York and Dr N. R. Wilson
(Department of Physics, University of Warwick) for TEM and
AFM images. This work is supported by the European Union
under a Marie Curie Research Training Network Grant ‘UNI-
NANOCUPS’ (MRTN-CT-2003-504233), COST D31/0001/04
and COST D36/005/06 under the 6th European Community
Framework Programme. P. B. acknowledges financial support
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