Electrodeposition preparation of Nafion-Pt-H_xMoO₃ and its activity toward methanol oxidation

LI Wei(李  伟), HUANG Qing-dan(黄青丹), ZHANG Qing-long(张庆龙),

HUANG You-ju(黄幼菊), ZHAO Ling-zhi(赵灵智), LI Wei-shan(李伟善)

Department of Chemistry, South China Normal University, Guangzhou 510006, China

Received 15 July 2007; accepted 10 September 2007

Abstract: A composite catalyst Nafion-Pt-H_xMoO₃ was prepared on a glassy carbon by cyclic voltammetry methods in sulfuric acid solution containing Na₂MoO₄, H₂PtCl₆ and Nafion, and its activity for the methanol oxidation was studied with Pt-H_xMoO₃ as comparison. It is found that the electrocatalytic activity and the stability of Pt-H_xMoO₃ are improved by the Nafion. The activity of Pt-H_xMoO₃ reaches its maximum when the content of Nafion in the preparation solution is 0.012%(mass fraction). In this case the oxidation peak current of methanol on Nafion-Pt-H_xMoO₃ is 1.76 times larger than that on Pt-H_xMoO₃ and keeps unchanged when the potential is set at 0.3 V (vs Hg-Hg₂SO₄) for 5 000 s in 1 mol/LCH₃OH+0.5 mol/L H₂SO₄ solution. The results from energy dispersive spectroscopy show that the content of Nafion in the composite is 0.57%. The results from scanning electron spectroscopy show that the composite of Nafion-Pt-H_xMoO has a gain size of about 90 nm and distributes more uniformly than Pt-H_xMoO₃.

Key words: platinum-hydrogen molybdenum bronze; Nafion; electrodeposition preparation; methanol oxidation

1 Introduction

During the operation of a direct methanol fuel cell at low temperature, the intermediates of methanol oxidation poison the platinum catalyst surface. Among different approaches to solving this problem, the composite of platinum with hydrogen molybdenum bronze (Pt-H_xMoO₃) seems effective[1-6]. However, hydrogen molybdenum bronze is unstable in acid solution. It was reported that the stability of hydrogen molybdenum bronze could be improved to some extent by polymer, such as polyanine[1, 4] and polyethylene[7]. It is well accepted that Nafion is a good material for proton exchange membrane used in direct methanol fuel cell[8]. In this paper, the composite of Nafion with Pt-H_xMoO₃ (Nafion-Pt-H_xMoO₃) was prepared by electrodeposition and its activity and stability for methanol oxidation were considered.

2 Experimental

All the electrochemical experiments were performed on a potentiostat/galvanostat (PGSTAT 30, Autolab, ECO Echemie B.V. Company) controlled by a GPES program. A conventional three-electrode cell was used. A rotating glassy carbon disk with a diameter of 3 mm was used as the working electrode. The rotating rate of the working electrode in catalyst preparation and performance determination was 600 r/min. A platinum foil was used as the counter electrode and a mercurous sulfate electrode (Hg/Hg₂SO₄, +0.64 V(vs RHE)) was used as the reference electrode. All the potentials mentioned in this paper referred to this reference.

All the chemicals used were of analytical grade reagents, the Nafion solution with 5%(mass fraction) was purchased from the Aldrich Chemical Company. Solutions were prepared with de-ionized and bidistilled water. All the experiments were carried out at room temperature. The surface morphology and the element composition were obtained on a scanning electron microscope (SEM) (JSM-6380(LA), JEOL Japan) operated at 15 kV and energy-dispersive X-ray spectroscopy (EDS, JED-2200, Japan), respectively. The Nafion in catalyst was detected by FTIR investigation (IRPrestige-21, Japan).

Pt/C and Pt-H_xMoO₃/C were prepared by cycling the rotating electrode between -0.65 and 0.2 V for 5 cycles at a scanning rate of 50 mV/s in x mmol/L Na₂MoO₄ +5 mmol/L H₂PtCl₆+0.5 mol/L H₂SO₄(x=0 and 2.5) under nitrogen atmosphere.

Nafion-Pt-H_xMoO₃/C was prepared in the similar ways as the preparation of Pt/C and Pt-H_xMoO₃/C. Four composite catalysts with different contents of Nafion in Nafion-Pt-H_xMoO₃ were obtained in 5 mmol/L H₂PtCl₆ +2.5 mmol/L Na₂MoO₄+0.5 mol/L H₂SO₄ solutions containing 0.08%, 0.12%, 0.16%, and 0.20% Nafion, respectively.

3.1 Electrocatalytic activity for methanol oxidation

Fig.1 presents the cyclic voltammograms of different catalysts in 1 mol/L CH₃OH+0.5 mol/L H₂SO₄ solution. It can be seen from Fig.1 that the current of methanol oxidation on the catalysts containing Nafion is larger than that without Nafion. This indicates that the activity of Pt-H_xMoO₃ toward methanol can be improved by the composite of Nafion in it.

Fig.1 Cyclic voltammograms of catalysts in 1 mol/L CH₃OH+0.5 mol/L H₂SO₄ solution(scanning rate: 50 mV/s)

It is noted that the improvement in catalytic activity is related to the content of Nafion in preparation solution. The current of methanol oxidation is largest for the catalyst prepared in the solution containing 0.12％Nafion (curve b in Fig.1. The electrode b is compared in the following experiment), with an increase of 75.8% in peak current compared with the catalyst without Nafion.

Fig.2 shows the cyclic voltammograms of Nafion-Pt-H_xMoO₃/C (b) and Pt-H_xMoO₃ in 0.5 mol/L H₂SO₄ solution. It can be seen that the current for the hydrogen adsorption and desorption with Nafion is larger that that on the catalysts without Nafion. The real platinum surface area in the catalyst with Nafion, estimated by electric charge for the desorption of hydrogen from Fig.2, has an increase of 70.1% compared with the catalyst without Nafion, which is in the same order of the increase in peak current of methanol oxidation. This suggests that the improvement in catalytic activity of the Pt-H_xMoO₃ is ascribed to platinum dispersion due to the composite of Nafion in it.

Fig.2 Cyclic voltammograms of catalysts in 0.5 mol/L H₂SO₄ solution(scanning rate: 50 mV/s)

3.2 Stability of catalyst

In order to understand the stability of the catalysts, potentiostat experiments were conducted. Fig.3 shows the chronoamperometric plots of catalysts Pt, Pt-H_xMoO₃ and Nafion-Pt-H_xMoO₃.

Fig.3 Chronoamperometric plots of Pt/C, Pt-H_xMoO₃/C and Nafion-Pt-H_xMoO₃/C in 1 mol/L CH₃OH+0.5 mol/L H₂SO₄ solution at 0.3 V

It can be seen from Fig.3 that the oxidation current of methanol on Pt-H_xMoO₃ is higher than that on Pt/C before 2 000 s, indicating that the activity of platinum is improved by hydrogen molybdenum bronze. However the current on Pt-H_xMoO₃ becomes smaller than that on Pt with the time increasing further. This is caused by the instability of hydrogen molybdenum. It should be noted that the current for the methanol oxidation on Nafion-Pt-H_xMoO₃ is far larger than that on Pt-H_xMoO₃ and remains almost unchanged with time. This indicates that the Nafion in the composite catalyst improves not only the activity but also the stability of Pt-H_xMoO₃.

The stability improvement of Pt-H_xMoO₃ by Nafion can be confirmed by cycling the catalysts in 1 mol/L CH₃OH+0.5 mol/L H₂SO₄ solution after the experiment of Fig.3. The results are shown in Fig.4. After the potentiostat experiment, the peak current of methanol oxidation on Pt-H_xMoO₃ decreases by 45.2% while that on Nafion-Pt-H_xMoO₃/C almost keeps its previous voltammogram.

Fig.4 Cyclic voltammograms of Pt-H_xMoO₃ (a) and Nafion-Pt-H_xMoO₃ (b) in 1 mol/L CH₃OH+0.5 mol/L H₂SO₄ solution(scanning rate: 50 mV/s)

3.3 Surface morphology and composition of catalysts

Fig.5 shows the SEM images of the surface of Pt-H_xMoO₃/C and Nafion-Pt-H_xMoO₃/C. It can be seen from Fig.5 that the distribution of Nafion-Pt-H_xMoO₃ is more uniform than that of Pt-H_xMoO₃ and has a spherical size with average diameters of about 90 nm.

Fig.6 and Table 1 show the results obtained from SEM and EDS. It can be seen from Fig.6 and Table 1 that element F exists in Nafion-Pt-H_xMoO₃ but not in Pt-H_xMoO₃, indicating that Nafion is present in Nafion-Pt-H_xMoO₃ because Nafion contains element F, as shown in Fig.7. The calculated molar ratio of Pt to Mo ratio is approximately 82?18 for both Nafion-Pt-H_xMoO₃and Pt-H_xMoO₃, indicating that the composite of Nafion with Pt-H_xMoO₃ does not change the ratio of Pt to Mo in the catalyst. The content of Nafion in the catalyst, estimated with Table 1 and Fig.7 is 0.57%(mass fraction).

Fig.5 SEM images of Pt-H_xMoO₃/C (a) and Nafion-Pt- H_xMoO₃/C (b)

Fig.6 EDS of Pt-H_xMoO₃/C (a) and Nafion-Pt-H_xMoO₃/C (b)

Table 1 Composition of surface of electrode Pt-H_xMoO₃/C (a) and Nafion-Pt-H_xMoO₃/C (b)

Fig.7 Structure of Nafion (x=6-10, y=1)

3.4 FTIR analysis of catalyst

The existence of Nafion in the catalyst can be confirmed by FTIR. Fig.8 shows the FTIR spectra of Nafion, Pt-H_xMoO₃ and Nafion-Pt-H_xMoO₃. For Nafion, the peak at 1 200 cm^-1 is assigned to symmetric stretching vibration mode of S=O; the 1 147 cm^-1 peak to the stretching vibration of C—F bonds in CF₂, the   1 056 cm^-1 peak to the stretching vibration of S—OH single bonds in sulfonate; the 981 cm^-1 peak to the stretching vibration of C—F bonds of side chain group; the 969 cm^-1 peak to the stretching vibration of C—O—C bonds mold[9]. It can be seen from Fig.8 that there are no obvious absorption peaks for Pt-H_xMoO₃ while the absorption peaks of Nafion-Pt-H_xMoO₃ are similar to that of Nafion. The displacement of absorption peaks is likely due to the change of chemical environment when Nafion is codeposited with Pt and hydrogen molybdenum bronze.

Fig.8 FTIR spectra of Nafion, Pt-H_xMoO₃ and Nafion- Pt-H_xMoO₃/C

4 Conclusions

1) Nafion can be co-deposited with platinum and hydrogen molybdenum bronze in sulfuric acid solution containing Nafion, H₂PtCl₆ and Na₂MoO₄ by cyclic voltametry.

2) The activity and the stability of Pt-H_xMoO₃ toward methanol oxidation can be improved by the codeposited Nafion.

3) The activity of Pt-H_xMoO₃ is related to the content of Nafion in it. The activity of Pt-H_xMoO₃ reaches its maximum when the content of Nafion in preparation solution is 0.12%.

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(Edited by YUAN Sai-qian)

Foundation item: Project(20573039) supported by the National Natural Science Foundation of China; project(2005DFA60580) supported by CISTC; project (2005B50101003) supported by Key Project of Guangdong Province

Corresponding author: LI Wei-shan; Tel: +86-20-39310256; E-mail: liwsh@scnu.edu.cn