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Spatially Electrodeposited Platinum in
Polyaniline Doped with Poly(styrene sulfonic acid) for Methanol
Oxidation
Li-Ming Huang, Wang-Rung
Tang and Ten-Chin Wen*
Department of Chemical Engineering, National Cheng
Kung University,
Tainan, Taiwan
70101
*Corresponding Author: E-mail: tcwen@mail.ncku.edu.tw
Paper
published in Journal of Power Sources, Vol. 164, pp. 519-526
(2006)
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Professor Ten-Chin Wen(left) and Dr. Li-Ming Huang(right).
Fuel
cells are attractive sources of electrical power since they realize
the direct conversion of chemical to electrical energy. One
promising system is the direct methanol fuel cell (DMFC).
Unfortunately, the expensive catalytic materials, such as platinum,
and relatively low electrocatalytic efficiency for electrochemical
reactions of the fuel are drawbacks. To date, efforts have focused
on the development if techniques of produce Pt catalysts with a high
surface area. It has been proved that the support materials in
electrocatalysts play an important role to the electrochemical
performance. Through improvement has been made in the catalytic
activity and stability of the electrocatalyst by effectively
dispersing Pt particles onto the electrically conducting supports,
many efforts are still continuing. Modification of electrode
surfaces provides an attractive way of confining catalytic species
to the effective spatial region and combines the experimental
advantages of heterogeneous catalysts with the benefits of a
three-dimensional distribution of active centers typically
characteristic of homogeneous catalysts. Hence, the introduced
sulfonic acid groups (SO3H) might affect the properties
of supporting materials and performance for its application in DMFC.
This motivated us to investigate the feasibility of incorporating
SO3H groups into conducting polymer as support for Pt. We
employed a simple “doping-dedoping-redoping” method to introduce the
SO3H groups (poly(styrene sulfonic acid), PSS) into PANI
in the present study. PANI-PSS composite behaves as a good probe for
the deposition Pt particles and increased the density of the active
sites in the polymer film. The well-dispersed Pt particles inside
such conducting composite support can lead to good Pt utilization
and an improvement of the catalytic activity for methanol
oxidation.
Figure 1 UV-Vis spectra of (a)
PANI-H2SO4 (emeraldine salt), (b) emeraldine
base (EB) and (c) redoped by PSS (PANI-PSS, emeraldine
salt). The incorporation of PSS into PANI has been achieved
by “doping-dedoping-redoping” process, being evidenced by UV-Vis
spectroscopy. Fig. 1 shows UV-Vis spectra for PANI doped form
(electrochemical polymerization of PANI in
H2SO4 medium), dedoped form and redoped form
with PSS. The doped form of PANI (doped with
H2SO4, curve a) possesses three peaks at ca.
356 nm, 434 nm and 780 nm (free carrier tail). These peaks
correspond to electron transition from valance band to polaronic
band characteristic of the doped emeraldine oxidation state of PANI.
Dipping the emeraldine salt of PANI with ammonium hydroxide yields
the dedoped form of PANI. The corresponding UV-Vis spectrum (curve
b) exhibits two absorption maxima at 326 nm and 598 nm, representing
the π-π* transition and charge transfer excitation-like transition
bands of the emeraldine base form of PANI. After immersing the
dedoped form of PANI in PSS solution, three characteristic peaks can
be observed in UV-Vis spectrum (curve c) and similar to that of the
PANI doped form. It indicates that PANI can be doped with PSS
(PANI-PSS) by the simple “doping-dedoping-redoping” technique.
Figure 2 SEM images of (a) PANI and (b) PANI-PSS.
Figure 3 SEM images of (a) PANI-Pt and (b) PANI-PSS-Pt.
Inset: x-ray maps show bright spots for Pt.
As revealed
by the SEM photographs in Fig. 2, PANI (Fig. 2a) has grains with
porous structure, whereas PANI-PSS (Fig. 2b) shows better cohesion
with compact layer structure. The change in morphology of PANI-PSS
in contrast to PANI arises from the influence of PSS molecules on
the orientation of PANI to the ITO substrate. PSS acts as not only
the dopant but also the bridge to connect the interchain of PANI to
form the spatial network structure. This spatial network structure
of PANI-PSS might be valuable for the electrodeposition of Pt. In
this study, platinum was electrodeposited into PANI and PANI-PSS at
constant potential (-0.2 V vs. Ag/AgCl) with the same charge of 0.1
C. It can be seen from SEM photographs, morphologies of PANI and
PANI-PSS did not change much with the electrodeposited Pt. An
examination of Fig. 3a and 3b reveals that there are particles in
PANI surface (Fig. 3a) while the same spatial structure in PANI-PSS
surface (Fig. 3b). We anticipated that Pt particles are embedded in
PANI-PSS spatial network structure. The insets in Fig. 3 show EDS
results of Pt in PANI and PANI-PSS. The bright spots of Pt represent
the existence of platinum in PANI and PANI-PSS (denoted as PANI-Pt
and PANI-PSS-Pt).
Figure 4 AES depth profiles of (a) PANI-Pt and (b)
PANI-PSS-Pt. (▲) sulfur and (◆) platinum signals.
The
Pt and S depth profiles of PANI-Pt and PANI-PSS-Pt are shown in Fig.
4. Pt reaches the maximum at 440 s and then decreases to a shoulder
between 1240 and 1940 s for PANI-Pt (Fig. 4a). Afterwards, it
decreased sharply between 1940 and 2340 s. From this curve, Pt is
enriched on the surface of PANI matrix although Pt particles are
deposited in PANI matrix. In contrast to PANI-Pt, Pt reaches the
maximum at 100 s, and then a broad peak is located between 200 and
1500 s for PANI-PSS-Pt (Fig. 4b). It implies that Pt is dispersed
uniformly into PANI-PSS spatial network structure. To deposit Pt
into pre-synthesized PANI matrix, proper electrodeposition method
should be used. The theoretical potential of
[PtCl6]2- reduction (Pt4+ →
Pt0) is approximately 0.5 V, whereas the cathodic peak
current of Pt deposition appears at more negative potentials around
-0.2 V (vs. Ag/AgCl). This may be ascribed to a kinetic hindrance of
the [PtCl6]2- reduction in the interior of
PANI film. Under this negative potential applied, the structural
characteristic of PANI is transformed into insulating state
(leucoemeraldine) with increasing the charge transfer resistance.
Meanwhile, SO42- and
PtCl62- will be driven out from the polymer
matrix to the bulk electrolyte. Consequently, the nucleation of Pt
spontaneously occurred at PANI surface. The above discussion and
hypothesis can be validated by Fig. 4a, showing the enrichment of Pt
near the surface of PANI and decreasing trend in sulfur
(SO42-). The enrichment of Pt near the surface
of PANI is consistent with the spherical Pt particles on PANI
surface from the morphology of PANI-Pt (Fig. 3a). In contrast to
PANI-Pt, PANI doped with PSS will twist up to form a spatial network
structure. Under the applied negative potential, SO3H
ions would not be driven out from PANI-PSS matrix due to
SO3H groups pending on PSS polymer chain. Fig. 4b
exhibits a plateau of sulfur content between 200 and 1500 s in
PANI-PSS-Pt, indicating that a spatial network structure of PANI-PSS
can hold SO3H groups in PANI at the negative potential
(-0.2 V). The existence of SO3H groups in PANI-PSS
assists holding [PtCl6]2- ions in polymer
matrix by the interaction of Pt4+ ions with
SO3H groups even though under the negative potential. As
the negative potential applied, the nuclei of Pt are immediately
generated around the SO3H groups, resulting the
homogenous distribution of Pt in PANI-PSS spatial network structure
evidenced by Fig. 4b. The homogenous distribution of Pt in PANI-PSS
spatial network structure might increase the utilization of Pt for
methanol oxidation.
Figure 5 Cyclic voltammograms of (a) PANI-Pt and (b) PANI-PSS-Pt in 0.1 M CH3OH + 0.5 M H2SO4 solution, scan rate = 10 mV/s. Inset: cyclic voltammograms of (c) PANI-Pt and (d) PANI-PSS-Pt in 0.5M H2SO4 solution, scan rate = 10 mV/s. To evaluate the performance of PANI-Pt
and PANI-PSS-Pt in the electrooxidation of methanol, cyclic
voltammograms (CVs) were recorded in 0.1 M CH3OH + 0.5 M
H2SO4 solution. For comparative purpose, CVs
were also collected in 0.5 M H2SO4 (Fig. 5,
inset) for corresponding electrochemical characteristics, before
methanol oxidation was undertaken. The CVs for both PANI-Pt and
PANI-PSS-Pt are similar to those for the PANI and PANI-PSS in 0.5 M
H2SO4 solution. Three well-defined redox pairs
can be seen due to the conversion of leucoemeraldine to emeraldine
(0.3 V), emeraldine to pernigraniline (0.8 V) and hydroquinone to
quinone (0.5 V). It is worth to note that the response corresponding
to hydrogen adsorption/desorption appears in the potential range
between -0.2 V and 0.0 V. It is known that the charge passed for
H-adsorption (QH) represents the number of sites of Pt
available for hydrogen adsorption and desorption. The charge for
hydrogen adsorption for PANI-PSS-Pt was 2.13 mC/cm2,
which is 1.60 times larger than that for PANI-Pt (1.33
mC/cm2). It implies that PANI-PSS-Pt owns higher surface
area of Pt than PANI-Pt, being attributable to the homogeneous
dispersion of Pt in PANI-PSS spatial network structure. The
electrocatalytic activity of PANI-Pt and PANI-PSS-Pt toward methanol
oxidation is shown in Fig. 5. The onset potential for methanol
oxidation is 0.44 and 0.38 V for PANI-Pt and PANI-PSS-Pt,
respectively. The lowing onset potential of PANI-PSS-Pt is
attributed to SO3H in PANI-PSS that can shuttle the
electrons easily (high positive charge nitrogen content) and provide
the pathway for proton migration (homogenous distributed PSS, Fig.
4b) toward methanol oxidation. Based on the peak current at ca. 0.75
V, the oxidation current densities for methanol oxidation are about
0.6 and 1.81 mA/cm2 for PANI-Pt and PANI-PSS-Pt,
respectively. The reasons for the improvement of catalytic activity
of Pt embedded in PANI-PSS are as follow. PANI doped with PSS can
twist up to form a spatial network structure. The uniform dispersion
of Pt in PANI-PSS can be achieved and enhance Pt utilization
efficiency for methanol oxidation. Secondly, we anticipated that
SO3H in PANI-PSS may act as a stabilizer for Pt particles
and prevent aggregation of Pt particles. The steric and
electrostatic stabilization of Pt particles might be achieved by
PANI-PSS.
Figure 6 Potential-time of (a) PANI-Pt and (b)
PANI-PSS-Pt in 0.1 M CH3OH + 0.5 M
H2SO4 solution at 0.02
mA/cm2.
The utilization efficiency of Pt
toward methanol oxidation is limited by CO poison on Pt surface. The
stability and CO poison effect on PANI-Pt and PANI-PSS-Pt can be
evaluated from the chronopotentiometric response of PANI-Pt and
PANI-PSS-Pt in 0.1 M ch3oh + 0.5 M
H2SO4 as shown in Fig. 6. Generally, the
driving voltage will increase when CO adsorbed on Pt surface in
order to maintain the same electrocatalytic properties of Pt for
methanol oxidation. It is evident form Fig. 6 that PANI-PSS-Pt can
be operated at a longer time than PANI-Pt at the same current
density. This demonstrates that the activity and the stability of
PANI-PSS-Pt was higher than PANI-Pt. The better stability for
PANI-PSS-Pt might be related to the more homogeneous dispersion of
Pt than PANI-Pt. Besides that, the incorporation of PSS into PANI
might hinder the formation of strongly absorbed poisonous species on
the Pt surface and lower the poison effect of CO species.
In
summary, the spatial network structure of PANI-PSS can be prepared
via “doping-dedoping-redoping” process. The existence of
SO3H in PANI-PSS spatial structure assists holding
Pt4+ ions in the polymer matrix, resulting the homogenous
distribution of Pt in PANI-PSS even thought under negative applied
potential. PANI-PSS-Pt revealed the better electrocatalytic activity
and stability for methanol oxidation than PANI-Pt. |
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