Introduction

Much attention has been given to fuel cells in recent years because they can directly convert chemical energy to electrical energy with higher efficiency than other sources of electrical power. Among fuel cells, the direct methanol fuel cell (DMFC) is one of the most attractive ones because it requires no methanol reforming processes needing high-pressure vessels and because methanol is effectively converted into electrical energy from its liquid state, which is advantageous in methanol transfer [1–4]. Unfortunately, however, the DMFC has at least two problems to be solved for practical use: (1) it requires expensive electrocatalysts such as Pt for the methanol oxidation and the electrocatalytic efficiency is relatively low; (2) the active sites of the electrocatalysts are poisoned by the adsorption of reaction intermediates such as CO [5–8].

A promising strategy for alleviating these problems is the introduction of conductive polymers. Polymers have been widely employed as porous supports on which to disperse the electrocatalysts. The dispersion can decrease the poisoning effect described previously as a problem (2) [9, 10]. Among the polymers, polyaniline (PANI) is one of the best candidates because it can easily be prepared on the electrode substrate as a homogeneous and strong adherent film with a high surface area and good stability in acid media [11–14]. Although some promising results have been reported and the highest electrocatalytic activity is obtained for dispersed Pt particles on PANI film [15–24], the electrocatalytic activity has to be enhanced further for practical use. In addition, every needed effort should be made to decrease the amount of the expensive Pt particles.

In this communication, in order to decrease the amount of the Pt particles, Pt and other metal particles (Cr, Ni, In, Co, Sb, Bi, Pb, and Mn) were simultaneously dispersed on the PANI films by cathodic electrodeposition. The electrocatalytic activity was examined using the dispersed metal and Pt particles on PANI films. The highest activity is obtained for the dispersed Sn and Pt particle on PANI films, and the activity is almost the same as that of the dispersed Pt particle on PANI films without the Sn particles.

Experimental details

A standard three-electrode cell was employed, comprising of glassy carbon (GC) disc electrodes with an electrode area of 0.196 cm2, a commercial Ag/AgCl electrode, and a Pt plate counter electrode. The GC disk electrodes were polished with alumina powder prior to use. Electrochemical experiments were carried out with a Hokuto Denko HZ-3000 potentiostat connected to a Rikadenki RY-101 x–y recorder.

The monomer, aniline (Katayama Chemicals Co.), was of reagent grade and was distilled under reduced pressure prior to use. Other chemicals were of analytical grade and were used as received. Aqueous solutions were prepared using distilled and deionized water. The PANI films were electrochemically deposited by constant-potential electropolymerization at +0.8 V onto the GC electrodes in aqueous acidic solution containing 0.20 M (1 M = 1 mol dm−3) aniline ([H+] = 1.0 M) under N2 atmosphere at 25 °C. The deposited amount of the PANI films was controlled by the charge passed during the electropolymerization, and the amount was approximately 170 μg cm−2. The PANI film-covered electrodes were reduced electrochemically in 0.5-M H2SO4 aqueous acidic solution ([H+] = 1.0 M) at –0.2 V until the current reached background level.

Platinum particles were dispersed on the PANI films by electrodeposition from 1.0-M HCl solution containing 0.02 wt.% H2[PtCl6] at a constant current of 50 μA cm−2. Likewise, the other metal particles were dispersed from protonic acid solutions containing 0.02 wt.% of the corresponding metal ions at a constant current of 50 μA cm−2. The deposited amount of the metal particles was determined by the charge passed during the electrodeposition.

The morphology of the obtained PANI film was observed using a Hitachi S-2300 scanning electron microscope (SEM).

Results and discussion

Figure 1 shows the cyclic voltammogram of the dispersed Pt particle PANI film measured in aqueous 0.5-M (1 M = 1 mol dm-3) H2SO4 solution and in H2SO4 solution containing CH3OH. The redox current of PANI appears around 0.3 and 0.6 V. The anodic peak current at 0.82 V is ascribable to the oxidation of CH3OH. The electrocatalytic activity was evaluated by the anodic current (i MeOH), which was calculated by subtracting the background current in the absence of CH3OH.

Fig. 1
figure 1

Cyclic voltammograms at 20 mV s−1 of the dispersed Pt particle on PANI films measured in 0.5-M H2SO4 solution containing 10-M CH3OH (────) and in 0.5-M H2SO4 solution (——). The deposited amount of the PANI film, 170 μg cm−2; the dispersed amount of the Pt particles; 101 μg cm−2

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We have shown that i MeOH for the dispersed Pt particle on PANI film electrode was about three times as large as the i MeOH for the dispersed Pt particle electrode without the PANI film [25]. There is no doubt that i MeOH is influenced by the conductivity and morphology of the PANI film. Both the conductivity and morphology could be changed by the type of dopant anion, in other words, the type of acid for the polymerizing solution [14, 26–29]. To determine the best acid for the largest i MeOH value, by electropolymerizing aniline in five polymerizing solutions containing different acids, we prepared five electrodes covered with PANI films having different dopant anions: \({\text{SO}}_{{\text{4}}} ^{{2 - }} \), \({\text{NO}}_{{\text{3}}} ^{ - } \), Cl−, \({\text{BF}}_{{\text{4}}} ^{ - } \), and \({\text{ClO}}_{{\text{4}}} ^{ - } \). The Pt particles were then dispersed on the PANI films. Figure 2 shows how i MeOH depends on the dopant anions. For comparison, i MeOH for the PANI films without Pt particles is also shown. In any case, i MeOH is enhanced by the presence of the PANI film. This enhancement is probably due to a substantial decrease in the poisoning species for Pt such as CO, which are intermediate products [9]. The largest i MeOH is observed for the \({\text{SO}}_{{\text{4}}} ^{{2 - }} \)-doped PANI film. In addition, little difference in i MeOH is seen between the five PANI films without dispersed Pt particles. Although we have no evidence to clarify why the \({\text{SO}}_{{\text{4}}} ^{{2 - }} \)-doped PANI film is the most effective, we speculate that the highest electrocatalytic activity is strongly related to morphology and conductivity. The morphology and conductivity depends on the type of the dopant anion and the dependence varies in the electropolymerization conditions. For example, some reports say that the \({\text{ClO}}_{{\text{4}}} ^{ - } \)- or \({\text{BF}}_{{\text{4}}} ^{ - } \)-doped PANI film has a fibril structure, while the \({\text{SO}}_{{\text{4}}} ^{{2 - }} \)- or \({\text{NO}}_{{\text{3}}} ^{ - } \)-doped one is granular [26–29]. Others say that the granular structure is seen for the \({\text{BF}}_{{\text{4}}} ^{ - } \)-doped one, while the fibril structure is for the \({\text{SO}}_{{\text{4}}} ^{{2 - }} \)- or \({\text{NO}}_{{\text{3}}} ^{ - } \)-doped one [14, 30]. To examine the morphology dependence in our electropolymerization conditions, the surface of the PANI films were observed by the SEM. The SEM observation revealed that the surface of the Cl–-, \({\text{NO}}_{{\text{3}}} ^{ - } \)-, \({\text{ClO}}_{{\text{4}}} ^{ - } \)-, or \({\text{BF}}_{{\text{4}}} ^{ - } \)-doped PANI film has the fibril structure, while that of the \({\text{SO}}_{{\text{4}}} ^{{2 - }} \)-doped one was not granular but a unique leafage structure. The Pt particles are effectively dispersed on the PANI film with this leafage structure. In addition, we are examining why the effective dispersion is achieved on the PANI film with the leafage structure and the reason will be published elsewhere (Yano et al., manuscript in preparation).

Fig. 2
figure 2

Effect of the type of the PANI dopant anion on i MeOH. The deposited amount of the PANI film, 170 μg cm−2; the dispersed amount of the Pt particles: 101 μg cm−2

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In order to relatively reduce the dispersed amount of expensive Pt particles, we dispersed other metal particles on the \({\text{SO}}_{{\text{4}}} ^{{2 - }} \)-doped PANI film prior to the dispersion of the Pt particles. Ten metals (Mn, Pb, Bi, Sb, Co, In, Ni, Cr, Cu, and Sn) were employed and dispersed by constant-current electrodeposition at 50 μA cm−2 up to a passed charge of 50 mC cm−2. Figure 3 illustrates which metal is effective as dispersed particles. As can readily be seen in any case, the PANI film significantly enhances the i MeOH value and the enhancement differs with the kind of the metals. The largest i MeOH value was obtained for Sn, although we have not clarified the reason.

Fig. 3
figure 3

Effect of the type of metal particles dispersed with Pt particles on i MeOH. The deposited amount of the PANI film, 170 μg cm−2

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Because Sn turned out to show the highest electrocatalytic activity as the particles dispersed with the Pt particles, we next examined the effect of the relative ratio of the dispersed Sn to Pt on i MeOH. The ratio was intentionally changed by the charge passed during the electrodeposition of Sn and Pt, and, in any case, the charge was 100 mC cm−2, which corresponded to 61.5 μg cm−2 for Sn and 101 μg cm−2 for Pt. The relationship between the ratio and i MeOH is shown in Fig. 4. As can readily be seen in Fig. 4, the positive effect of the dispersed Sn is evident except for the case where the dispersed amount of Pt is 25 mC cm−2 and that of Sn is 75 mC cm−2. In any case, without the PANI film (see the two curves at the bottom), i MeOH is enhanced when the Sn particles are dispersed with the Pt particles. The magnitude of the enhancement is further increased by the presence of the PANI film. Furthermore, it is interesting to find that i MeOH is observable for the dispersed Sn particle on PANI film without the dispersed Pt particles, while no i MeOH is seen for the other cases without the dispersed Pt particles. These results are practically useful because Sn could replace the expensive Pt.

Fig. 4
figure 4

Relationship between the relative ratio of the dispersed Sn to Pt particles and i MeOH. Filled triangle, dispersed Pt particles on the carbon electrode. Open triangle, dispersed Pt and Sn particles on the carbon electrode. Filled circle, dispersed Pt particles on the PANI film. Open circle, dispersed Pt and Sn particles on the PANI film. The deposited amount of the PANI film: 170 μg cm−2

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There is no doubt that the PANI film plays a significant role in the electrocatalytic reaction. This role is probably based on the following two facts: adsorption of the intermediate species and high dispersion of the Pt particles, in other words, a large surface area for the Pt particles. According to a study using electrochemically modulated infrared reflectance spectroscopy [9], the PANI film adsorbs the intermediate species such as CO, which act as poisoning species for Pt. The adsorption prevents the dispersed Pt particles from becoming deactivated. On the other hand, in order to confirm whether the PANI film promotes effective Pt dispersion, the size of the Pt particles was calculated based on the charge required for the hydrogen adsorption and desorption. The average diameter of the dispersed Pt particles directly on the carbon electrode was 0.12 μm, while that on the PANI film was 0.23 μm.

Little is known about why the dispersed Sn particles are effective. It was recently reported that adsorbed CO coming from the ethanol dissociative chemisorption is greatly reduced by modifying the composition of the Pt anode by adding Sn during the electrooxidation of ethanol [30]. Likewise, the Sn particles may reduce the adsorbed CO during the electrooxidation of methanol. Anyway, the dispersed Sn and Pt particles on PANI film are expected to be useful for the anode of the DMFC because these reduce the cost.