Volume 3 Issue 1 - January 11, 2008
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)

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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