Volume 4 Issue 7 - May 30, 2008
Enhanced electrocatalytic performance for methanol oxidation of a novel Pt particles dispersed poly(3,4-ethylenedioxythiophene)- poly(styrene sulfonic acid) electrode
Chung-Wen Kuo, Li-Ming Huang, Ten-Chin Wen* and A. Gopalan

Department of Chemical Engineering,  National Cheng Kung University

Paper published in Journal of Power Sources, Vol. 160, pp. 65-72 (2006)

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Professor Ten-Chin Wen(left) and Dr. Li-Ming Huang(right).
n the context of fast depletion of fossil fuel resources, research activities on batteries and full cells have been triggered. Among several types of fuel cells, polymer electrolyte direct methanol fuel cells (DMFCs) are being projected for a variety of applications ranging from micro-power to mega-power devices. The attractive features of DMFCs are also due to a high theoretical energy density expected from methanol among the several small organic molecules. Furthermore, the fabrication regenerative DMFCs, which is based on the concept of electrochemical reduction of CO2 to CH3OH and employing the latter as the fuel, seems to be interesting as this is also related to address the global warming problem. Two possible approaches were tried to circumvent the problem of using Pt in DMFCs and related applications. In a straightforward approach, bi- or tri-metallic catalysts involving transition metals were attempted. In the another approach, the electrodes that exhibits enhanced electrocatalytic activities as compared with the bulk-form of metal electrodes toward oxidation of small organic molecules have been formed through incorporation of transition metal particles into conducting polymers.

In the present study, we have employed a composite form of conducting polymer, poly(3,4-ethylenedioxythiophene) (PEDOT) -poly(styrene sulfonic acid) (PSS) as 3D-random matrix for loading Pt particles. PEDOT has been attracting interests due to its high compatibility with other materials, very good film forming properties, high stability, high conductivity, and high degree of doping. PEDOT doped with excess of PSS, designated as PEDOT-PSS, is commercially available as stable aqueous dispersion. Furthermore, the network structure of PEDOT-PSS has resemblance to Nafion. PEDOT-PSS provides three dimensional reaction zones and increase the active surface area of Pt particles while in contact with electrolyte. The presence of anionic dopants in PEDOT-PSS generate pathway for protonic species and hence possesses the characteristics of using in DMFCs applications. Motivated by the above characteristics for PEDOT-PSS, we have developed a new type of electrode material incorporating Pt particles into PEDOT-PSS matrix for the application in DMFC.
Fig. 1. SEM images for (a) ITO/Pt, (b) ITO/PEDOT-PSS-Pt (growth of Pt for 30 cycles) and (c) ITO/PEDOT-PSS-Pt (growth of Pt for 90 cycles) electrodes.

Fig. 1 shows the difference in morphology between ITO/Pt and ITO/PEDOT-PSS-Pt, at different stages of loading Pt particles (30 and 90 cycles). Pt particles deposited onto ITO substrate shows more aggregation than Pt particles loaded into ITO/ PEDOT-PSS, under identical conditions of experiments. Fig. 1(b) reveals that Pt particles are homogenously distributed into PEDOT-PSS matrix. Pt particles incorporated with in PEDOT-PSS for higher number of cycles have larger sizes (Fig. 1c). However, the larger Pt particles are homogeneously distributed into PEDOT-PSS matrix.

Film of Pt particles deposited on simple ITO was not stable due to the poor adherence with the surface of ITO. Hence, instead of ITO/Pt, we have used bulk Pt electrode for the comparative evaluation of ITO/PEDOT-PSS-Pt with ITO/Pt electrode toward voltammetric behavior in 0.5 M H2SO4. In further electrochemical experiment, we have used bulk Pt electrode instead of ITO/Pt electrode. Cyclic voltammograms (CVs) of ITO/PEDOT-PSS, ITO/PEDOT-PSS-Pt and bulk Pt electrodes recorded with a scan rate of 50 mV/sec in 0.5 M H2SO4, are presented in Fig. 2. In the potential region between -0.2 and +0.2 V/SCE, the responses corresponding to hydrogen adsorption/desorption were noticed with an accompanying bisulfate adsorption/desorption. In contrast, a large background current without hydrogen adsorption/desorption peak was witnessed at ITO/PEDOT-PSS electrode (Fig. 2, insert). There is a difference in current density of hydrogen desorption between bulk Pt and PEDOT-PSS-Pt electrodes. It is known that the integrated intensity of hydrogen desorption represents the number of sites of Pt available for hydrogen adsorption and desorption. Charges for hydrogen desorption on the electrode surfaces were calculated by assuming constant double-layer charging current over the whole potential range. The charge for hydrogen desorption on the PEDOT-PSS-Pt surface was 2.99 mC/cm2, which is 5 timers larger than that on the bulk Pt surface (0.587 mC/cm2). This is reminiscence of much higher active surface area for PEDOT-PSS-Pt electrodes and consistent with the well-dispersed morphological environment for Pt particles in the PEDOT-PSS matrix (as shown in Fig. 1(a)). Besides that, in the cathodic sweep, a peak at 0.4 V was observed for ITO/PEDOT-PSS-Pt, is assigned to the reduction of PtO to metallic Pt. Indeed, PEDOT-PSS film behaves as a good probe bed for the deposition of Pt particles and increases the density of the active sites on the electrode surface.
Fig. 2. Cyclic voltammograms of (a) ITO/PEDOT-PSS-Pt, (b) bulk Pt and (c) PEDOT-PSS electrodes in 0.5 M H2SO4 at the potential range from -0.2 to +1.2 V with a scan rate = 50 mV/s. The insert is bare PEDOT-PSS.

The catalytic activities of bulk Pt, ITO/PEDOT-PSS, and ITO/PEDOT-PSS-Pt composite electrodes were evaluated by monitoring the performance toward electrooxidation of methanol. (Fig. 3). It can be clearly observed that there is no electrocatalytic property for PEDOT-PSS (Fig. 3c). Comparing the CV results of ITO/PEDOT-PSS-Pt (Fig. 3a) and bulk Pt (Fig. 3b), a significantly higher oxidation current for methanol oxidation was observed for ITO/PEDOT-PSS-Pt (Fig. 3a). For example, the current at 0.60 V on forward sweep is 2.51 mA/cm2 for PEDOT-PSS-Pt in comparison to 0.45 mA/cm2 at bulk Pt electrode. The reason of the improvement of catalytic activity of Pt embedded in PEDOT-PSS are presented here.
Fig. 3. Cyclic voltammograms of (a) ITO/PEDOT-PSS-Pt, (b) bulk Pt and (c) PEDOT-PSS electrodes in 0.1 M CH3OH + 0.5 M H2SO4 at the potential range from -0.2 to +1.2 V with a scan rate = 50 mV/s. The insert is bulk Pt electrode.

Firstly, a high surface area for Pt particles is anticipated due to the uniform distribution of Pt particles into the three-dimensional conducting PEDOT-PSS matrix. Results on methanol oxidation, as observed by us, suggest that the enhanced catalytic activities for Pt particles loaded into PEDOT-PSS may be due to the amount and the structure peculiarities of the microdeposits of Pt and their distribution in the polymer matrix (PEDOT-PSS). We also anticipate that PEDOT-PSS may act as stabilizer for Pt particles and prevent aggregation of Pt particles. Also, PEDOT-PSS contributes to the enhanced stability due to the presence of both steric and electrostatic stabilization mechanisms resulting in small particle size.

ITO/PEDOT-PSS-Pt electrode also exhibited a minimum catalyst poisoning. Chronoamperometric responses at ITO/PEDOT-PSS-Pt and Pt electrodes for a solution of 0.1 M CH3OH in 0.5 M H2SO4 recorded at 0.6 V and 0.8 V were recorded (Fig. 4(a) and (b)) and compared to evaluate the catalyst poisoning effect by these electrodes. In each of the potential step experiment, an initial potential (E1) of 0.0 V was set for 30 sec. At this potential, formation of CO on the electrode surface is more probable. The potential was then increased and kept at E2 (0.6 V) for 5 min to reach a constant current value at the potential. At 0.6 V, ITO/PEDOT-PSS-Pt shows its higher catalytic properties toward the methanol oxidation in comparison to bulk Pt electrode. For example, the current at 150 s is 1.25 mA/cm2 for ITO/PEDOT-PSS-Pt and 0.62 mA/cm2 at the bulk Pt electrode. Upon setting E2 as 0.8 V, ITO/PEDOT-PSS-Pt still showed a higher current density (1.63 mA/cm2) in comparison with bulk Pt electrode (0.19 mA/cm2). However, a decrease in steady-state current density was noticed at both the electrodes. This might be due to the formation of oxide and consequent diminishing of the number of sites available for methanol oxidation.
Fig. 4. Chronoamperometric response of (I) bulk Pt and (II) PEDOT-PSS-Pt at (a) 0.6 V and (b) 0.8 V (vs. Ag/AgCl) in 0.1 M CH3OH + 0.5 M H2SO4 solution.

In summary, Pt particles were successfully embedded into PEDOT-PSS matrix to form PEDOT-PSS-Pt film. The composite, PEDOT-PSS-Pt, based electrode is proved to be a promising material as catalysts for methanol oxidation. A greater increase in the anodic current of methanol oxidation is observed for the electrode having Pt particles embedded in PEDOT-PSS. PEDOT-PSS matrix provides an environment for dispersing Pt particles without aggregation, which is evident from SEM results. Also, PEDOT-PSS matrix may provide pathway for proton migration in DMFCs application. The enhanced electrocatalytic activity of Pt in PEDOT-PSS opens up the possibility to use lesser amount of Pt and make way to decrease in the use of Pt content in fuel cell applications.
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