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Synthesis of hydrous ruthenium oxide supported platinum catalysts for direct methanol fuel cells
Electrochemistry Communications 7 (2005) 593–596 www.elsevier.com/locate/elecom
Synthesis of hydrous ruthenium oxide supported platinum catalysts for direct methanol fuel cells Zhenguo Chen a, Xinping Qiu a,*, Bin Lu b, Shichao Zhang b, Wentao Zhu a, Liquan Chen a b
a Department of Chemistry, Tsinghua University, Beijing 100084, P.R. China Department of Material Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, P.R. China
Received 21 February 2005; received in revised form 4 April 2005; accepted 5 April 2005
Abstract Hydrous ruthenium oxide supported platinum catalysts, i.e., Pt/RuO2 Æ xH2O, were synthesized by a feasible solution-phase method. X-ray diffraction (XRD) and transmission electron microscope (TEM) investigations showed that the uniform Pt nanoparticles were highly dispersed on the support materials. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analyses revealed that RuO2 Æ xH2O has the structure of RuO2 Æ 0.25H2O. This support material is believed to be more active than Ru0 as part of Pt–Ru alloy catalysts. As proved by the cyclic voltammetry (CV) results, the Pt/RuO2 Æ xH2O electrode shows higher reactivity towards methanol than that of E-TEK Pt–Ru black electrode. This can be interpreted as that the remarkable boundaries of Pt nanoparticles to RuO2 Æ xH2O support provide active sites for complete methanol electrooxidation. Ó 2005 Published by Elsevier B.V. Keywords: Solution-phase synthesis method; Pt/RuO2 Æ xH2O catalysts; Methanol oxidation; Direct methanol fuel cells
1. Introduction Direct methanol fuel cells (DMFCs) show great promise as future portable power sources. One of the key problems inhibiting its development is the low methanol oxidation reactivity at low temperature. At present, a Pt–Ru binary alloy is the most widely used catalyst. And the state-of-the-art catalyst is provided by ETEK, Inc. However, the most active Pt:Ru ratio and redox state of the Ru component are still under investigation. Early research revealed that hydrous ruthenium oxide, i.e., RuO2 Æ xH2O was a more active catalyst for methanol oxidation than did Ru0 as part of bimetallic *
Corresponding author. Tel.: +86 10 627 94235; fax: +86 10 627 94234. E-mail address: [email protected] (X. Qiu). 1388-2481/$ - see front matter Ó 2005 Published by Elsevier B.V. doi:10.1016/j.elecom.2005.04.002
Pt–Ru alloy [1]. Additionally, the Pt–RuO2 catalyst was synthesized by the sol–gel method [2], in which only a mixture of anhydrous RuO2 and Pt nanoparticles could be achieved. However, enhanced effects for the oxidation of methanol were observed, which was attributed to the presence of Ru–OH bonds on the RuO2 surface. Since the methanol oxidation on a Pt–Ru catalyst follows the so-called bifunctional mechanism [3,4], Ru–OHad facilitates the Pt–COad oxidation at low potential by providing adsorbed hydroxy to adjacent active Pt atoms. To fully exploit the fascinating properties of a Pt–RuO2 catalyst, it is apparent that new strategies for synthesizing novel RuO2 Æ xH2O supported Pt catalyst, i.e., Pt/RuO2 Æ xH2O with adequate PtiRuO2 Æ xH2O boundaries need to be developed. In this report, the solution-phase method was demonstrated as a facile and practical method to prepare this catalyst.
Z. Chen et al. / Electrochemistry Communications 7 (2005) 593–596
2. Experiment
0.4 TGA
1.00
0.2
Weight (%)
0.99
DSC
0.98
0.0
0.97
-0.2
0.96 -0.4 0
100
200
300
400
500
600
700
800
900
T / oC Fig. 1. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of the as-prepared Pt/RuO2 Æ xH2O catalyst with a Pt:Ru molar ratio of 1:2. The TGA result was examined by 2050 TGA equipment, TA Inc., 4 °C/min; and DSC by 2910 MDSC, TA Inc.
scribed to the decomposition of PtOxHy to Pt metal [1]. The curve is flat between 380 and 480 °C. From the DSC curve, we can see that the RuO2 Æ xH2O support material has no phase changes below 270 °C. Then, the three prepared PRC catalysts are heat-treated in flowing N2 at 250 °C for 1 h. Fig. 2 shows the XRD structure studies of the as-prepared and heat-treated Pt/RuO2 Æ xH2O catalysts. From the patterns, we can see that the as-prepared Pt/RuO2 Æ xH2O catalysts all have only four characteristic diffraction peaks of a Pt crystal (PDF card 4-0802#). The RuO2 Æ xH2O support materials are amorphous and cause the obvious inclination of the base line between 20° and 36°, especially in the pattern of PRC 1:2. After the heat treatment, the diffraction peaks of RuO2 can be found (PDF card 40–1290#), while the Pt diffraction peaks remain
Intensity
The Pt/RuO2 Æ xH2O catalyst was prepared by a two-step process, the synthesis of RuO2 Æ xH2O nanoparticles and subsequent loading of ultrafine platinum nanoparticles onto this support material. Magnetic stirring was employed all through. The typical synthesis of RuO2 Æ xH2O involved the dissolution of ruthenium trichloride and oxidation in weakly acidic condition. Firstly, RuCl3 was dissolved into deionized water and the pH of the solution is about 2.0. Then, the pH was adjusted to 4.00 by adding sodium hydroxide, approximate amounts of hydrogen peroxide solution were added to the reaction solution by constant-flow pump in 1 h as the pH value of the solution was stabilized to 4.00. Thereby, a suspension of spherical RuO2 Æ xH2O nanoparticles was prepared. After the pH of the solution was increased to 5.50 by adding sodium hydroxide, amounts of hexachloroplatinate acid (H2PtCl6 Æ 6H2O) solution was added drop by drop. The pH of the reaction solution was stabilized at 5.50 in order to facilitate the formation of Pt clusters. At the same time, the suspension of RuO2 Æ xH2O nanoparticles remains stable favoring the adsorption of formed Pt clusters on their surface for half an hour. This behavior makes the solution-phase method unique, achieving the high dispersion of Pt clusters adsorbed on the RuO2 Æ xH2O nanoparticles before their reduction to metal Pt nanoparticles. The amount of hexachloroplatinate acid was controlled by the molar ratio of Pt to Ru in the final catalyst. The three samples we produced had Pt:Ru molar ratios of 1:2, 1:1 and 2:1, which were named as PRC 1:2, PRC 1:1 and PRC 2:1, respectively. Then 1.5 times of required sodium tetrahydroborate solution was added drop by drop in 30 min. Another 30 min later, the Pt/ RuO2 Æ xH2O catalyst nanoparticles were prepared. This power was separated by centrifuge, washed with deionized water three times and then dried in air for 12 h at 80 °C. For electrochemical investigations, these as-prepared catalysts and E-TEK Pt–Ru black catalyst are, respectively, mixed with Nafion solution and glycol in ultrasonic sound bath for 5 min. Then, this slurry was spreaded on a glassy carbon disk of ca. 2.0 cm2 and dry at 70 °C for test.
Heat Flow (mW)
594
3. Results and discussion
6
#
5
#
4
#
3
#
#
2
3.1. Structure of the support materials and the catalysts
1
Fig. 1 shows the TGA and DSC investigations over the PRC 1:2 catalyst. From the TGA result, the loss of 2.98 wt% between room temperature and 380 °C under flowing N2 is caused by the water losing of the RuO2 Æ xH2O support material [5] and a further weight loss of 1.13 wt% between 480 and 800 °C can be de-
20
30
40
50
60
70
#
80
90
2θ Fig. 2. X-ray diffraction (XRD) patterns of the Pt/RuO2 Æ xH2O catalysts. The nos. 1–6 patterns represent the as-prepared PRC 1:2, PRC 1:1, PRC 2:1, and the heat-treated PRC 1:2, PRC 1:1, PRC 2:1, respectively.
Z. Chen et al. / Electrochemistry Communications 7 (2005) 593–596
unchanged. This reveals that the Pt nanoparticles are highly dispersed on the hydrous RuO2 support material. And the absence of any superimposed sharp diffraction Pt peaks, both for the as-prepared and heattreated PRC catalysts, indicates a uniform Pt particle distribution [6].
595
to the RuO2 Æ xH2O support. As mentioned above, it is believed that the boundaries provide the active site towards complete methanol electro-oxidation. Therefore, the different electrocatalytic reactivity of these catalysts towards methanol oxidation can be attributed to the differences among their boundaries of Pt to the RuO2 Æ xH2O support.
3.2. Morphologies of the support and catalysts materials Morphologies of the RuO2 Æ xH2O support material and the catalysts are shown by TEM images in Fig. 3. From Fig. 3(a), it can be seen that the particle size of RuO2 Æ xH2O is ca. 20 nm, and the size distribution is remarkably uniform. The dispersion of Pt nanoparticles on the RuO2 Æ xH2O support materials are shown in Figs. 3(b)–(d) representing the catalysts of PRC 1:2, 1:1 and 2:1, respectively. From these images, it can be seen that at low Pt:Ru molar ratios as of 1:2 and 1:1, the Pt nanoparticles are highly dispersed on the RuO2 Æ xH2O support without any obvious aggregation. However, as the Pt:Ru molar ratio increases to 2:1, remarkable aggregations of Pt nanoparticles are observed as marked by arrow mark in Fig. 3(d). It is said that in the as-prepared catalysts, the Pt nanoparticles reveal no clear difference in terms of structure and size distribution. However, the dispersion of the Pt nanoparticles is extremely influenced by the Pt:Ru molar ratio. High molar ratio of Pt to Ru results in notable aggregation of Pt nanoparticles decreasing the boundaries of Pt
3.3. Electrochemical characters of the Pt/RuOx Æ nH2O electrodes Electrochemical properties of PRC catalysts 1:2, 1:1 and 2:1 have been investigated by cyclic voltammetry as shown in Fig. 4. This figure also shows the response of the E-TEK PtRu black catalyst (1:1 atomic ratio) for comparison. From Fig. 4, it can be obtained that the general features of CV curves at PRC catalysts are very similar to that of E-TEK PtRu. However, it is obvious that the electrode composition has a significant influence on the behavior towards methanol electro-oxidation, as summarized in Table 1. The PRC 1:2 catalyst shows the highest reactivity towards methanol electro-oxidation, as the peak current density (ip) at a peak potential of Ep = 0.79 V is as high as 300 mA cm 2 mgPt 1, which is much higher than that of an E-TEK PtRu black electrode, which has an ip at Ep = 0.76 V of only 246 mA cm 2 mgPt 1. For the PRC 1:1 electrode, the peak current density ip at Ep = 0.79 V is decreased to 242 mA cm 2 mgPt 1. The PRC 2:1 electrode shows
Fig. 3. TEM images of the Pt/RuO2 Æ xH2O catalysts, showing highly dispersed Pt nanoparticles on the RuO2 Æ xH2O support with unimodal particle distribution. (a) the as-produced RuO2 Æ xH2O support material, and the as produced Pt/RuO2 Æ xH2O catalysts with Pt:Ru molar ratio of (b) 1:2, (c) 1:1 and (d) 2:1.
Z. Chen et al. / Electrochemistry Communications 7 (2005) 593–596
4. Conclusion
300
-1
Current Density / (mA cm mgPt )
596
-2
250
200
150
100
PRC 1:2 PRC 1:1 PRC 2:1 E-TEK
50
0 -0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Potential / V vs. SCE Fig. 4. Cyclic voltammograms of Pt/RuO2 Æ xH2O and E-TEK PtRu Black electrodes in the potential range between 0.20 and 1.20 V vs. SCE in 1.0 M CH3OH + 1.0 M HClO4 aqueous solution. The curves were obtained at a sweep rate of 10 mV s 1. The catalyst loading is 1 mg on the glassy carbon disk of ca. 2.0 cm2. Table 1 Electrochemical properties of the catalyst electrodes Electrodes
E-TEK PtRu
PRC 1:2
PRC 1:1
PRC 2:1
ip Ep
246 0.76
300 0.79
242 0.79
82 0.80
the lowest catalytic reactivity, as ip at Ep = 0.80 V is only 82 mA cm 2 mgPt 1. It is believed that the abundance of Pt to RuO2 Æ xH2O boundaries determine the electrochemical characters of different PRC electrodes. In other words, in the PRC 1:2 electrode there are adequate boundaries between Pt0 and RuO2 Æ xH2O available to be active for methanol oxidation, thus the electrode shows most active reactivity towards methanol oxidation reaching the highest peak current density.
In summary, the solution-phase method has been successfully used to synthesize RuO2 Æ xH2O supported Pt catalysts. This method involves much simpler operations, if compared to other methods employed to prepare Pt–Ru fuel cells catalysts. Additionally, the as-produced Pt/RuO2 Æ xH2O catalysts show high electrocatalytic activity towards methanol oxidation with respect to the state-of-art E-TEK Pt–Ru black catalysts. The enhanced performance can be contributed to the presence of the amorphous ruthenium oxide rather than Ru0, a high dispersion of uniform Pt nanoparticles on the RuO2 Æ xH2O support materials, and the abundance of boundaries between Pt and RuO2 Æ xH2O promoting the methanol oxidation.
Acknowledgments The authors appreciate the financial support of the NSFC project (90410002) and State Key Basic Research Program of PRC (2002CB211803).
References [1] J.W. Long, R.M. Stroud, K.E. Swider-Lyons, D.R. Rolison, Journal of Physical Chemistry B 104 (2000) 9772. [2] H.M. Villullas, F.I. Mattos-Costa, L.O.S. Bulhoes, Journal of Physical Chemistry B 108 (2004) 12898. [3] M. Watanabe, S. Motoo, Journal of Electroanalytical Chemistry 60 (1975) 267. [4] H.A. Gasteiger, N. Markovic, P.N. Ross, E.J. Cairns, Electrochimica Acta 39 (1994) 1825. [5] K.E. Swider, C.I. Merzbacher, P.L. Hagans, D.R. Rolison, Journal of Non-Crystalline Solids 225 (1998) 348. [6] V. Radmilovic, H.A. Gasteiger, P.N. Ross, Journal of Catalysis 154 (1995) 98.
Synthesis of hydrous ruthenium oxide supported platinum catalysts for direct methanol fuel cells Zhenguo Chen a, Xinping Qiu a,*, Bin Lu b, Shichao Zhang b, Wentao Zhu a, Liquan Chen a b
a Department of Chemistry, Tsinghua University, Beijing 100084, P.R. China Department of Material Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, P.R. China
Received 21 February 2005; received in revised form 4 April 2005; accepted 5 April 2005
Abstract Hydrous ruthenium oxide supported platinum catalysts, i.e., Pt/RuO2 Æ xH2O, were synthesized by a feasible solution-phase method. X-ray diffraction (XRD) and transmission electron microscope (TEM) investigations showed that the uniform Pt nanoparticles were highly dispersed on the support materials. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analyses revealed that RuO2 Æ xH2O has the structure of RuO2 Æ 0.25H2O. This support material is believed to be more active than Ru0 as part of Pt–Ru alloy catalysts. As proved by the cyclic voltammetry (CV) results, the Pt/RuO2 Æ xH2O electrode shows higher reactivity towards methanol than that of E-TEK Pt–Ru black electrode. This can be interpreted as that the remarkable boundaries of Pt nanoparticles to RuO2 Æ xH2O support provide active sites for complete methanol electrooxidation. Ó 2005 Published by Elsevier B.V. Keywords: Solution-phase synthesis method; Pt/RuO2 Æ xH2O catalysts; Methanol oxidation; Direct methanol fuel cells
1. Introduction Direct methanol fuel cells (DMFCs) show great promise as future portable power sources. One of the key problems inhibiting its development is the low methanol oxidation reactivity at low temperature. At present, a Pt–Ru binary alloy is the most widely used catalyst. And the state-of-the-art catalyst is provided by ETEK, Inc. However, the most active Pt:Ru ratio and redox state of the Ru component are still under investigation. Early research revealed that hydrous ruthenium oxide, i.e., RuO2 Æ xH2O was a more active catalyst for methanol oxidation than did Ru0 as part of bimetallic *
Corresponding author. Tel.: +86 10 627 94235; fax: +86 10 627 94234. E-mail address: [email protected] (X. Qiu). 1388-2481/$ - see front matter Ó 2005 Published by Elsevier B.V. doi:10.1016/j.elecom.2005.04.002
Pt–Ru alloy [1]. Additionally, the Pt–RuO2 catalyst was synthesized by the sol–gel method [2], in which only a mixture of anhydrous RuO2 and Pt nanoparticles could be achieved. However, enhanced effects for the oxidation of methanol were observed, which was attributed to the presence of Ru–OH bonds on the RuO2 surface. Since the methanol oxidation on a Pt–Ru catalyst follows the so-called bifunctional mechanism [3,4], Ru–OHad facilitates the Pt–COad oxidation at low potential by providing adsorbed hydroxy to adjacent active Pt atoms. To fully exploit the fascinating properties of a Pt–RuO2 catalyst, it is apparent that new strategies for synthesizing novel RuO2 Æ xH2O supported Pt catalyst, i.e., Pt/RuO2 Æ xH2O with adequate PtiRuO2 Æ xH2O boundaries need to be developed. In this report, the solution-phase method was demonstrated as a facile and practical method to prepare this catalyst.
Z. Chen et al. / Electrochemistry Communications 7 (2005) 593–596
2. Experiment
0.4 TGA
1.00
0.2
Weight (%)
0.99
DSC
0.98
0.0
0.97
-0.2
0.96 -0.4 0
100
200
300
400
500
600
700
800
900
T / oC Fig. 1. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of the as-prepared Pt/RuO2 Æ xH2O catalyst with a Pt:Ru molar ratio of 1:2. The TGA result was examined by 2050 TGA equipment, TA Inc., 4 °C/min; and DSC by 2910 MDSC, TA Inc.
scribed to the decomposition of PtOxHy to Pt metal [1]. The curve is flat between 380 and 480 °C. From the DSC curve, we can see that the RuO2 Æ xH2O support material has no phase changes below 270 °C. Then, the three prepared PRC catalysts are heat-treated in flowing N2 at 250 °C for 1 h. Fig. 2 shows the XRD structure studies of the as-prepared and heat-treated Pt/RuO2 Æ xH2O catalysts. From the patterns, we can see that the as-prepared Pt/RuO2 Æ xH2O catalysts all have only four characteristic diffraction peaks of a Pt crystal (PDF card 4-0802#). The RuO2 Æ xH2O support materials are amorphous and cause the obvious inclination of the base line between 20° and 36°, especially in the pattern of PRC 1:2. After the heat treatment, the diffraction peaks of RuO2 can be found (PDF card 40–1290#), while the Pt diffraction peaks remain
Intensity
The Pt/RuO2 Æ xH2O catalyst was prepared by a two-step process, the synthesis of RuO2 Æ xH2O nanoparticles and subsequent loading of ultrafine platinum nanoparticles onto this support material. Magnetic stirring was employed all through. The typical synthesis of RuO2 Æ xH2O involved the dissolution of ruthenium trichloride and oxidation in weakly acidic condition. Firstly, RuCl3 was dissolved into deionized water and the pH of the solution is about 2.0. Then, the pH was adjusted to 4.00 by adding sodium hydroxide, approximate amounts of hydrogen peroxide solution were added to the reaction solution by constant-flow pump in 1 h as the pH value of the solution was stabilized to 4.00. Thereby, a suspension of spherical RuO2 Æ xH2O nanoparticles was prepared. After the pH of the solution was increased to 5.50 by adding sodium hydroxide, amounts of hexachloroplatinate acid (H2PtCl6 Æ 6H2O) solution was added drop by drop. The pH of the reaction solution was stabilized at 5.50 in order to facilitate the formation of Pt clusters. At the same time, the suspension of RuO2 Æ xH2O nanoparticles remains stable favoring the adsorption of formed Pt clusters on their surface for half an hour. This behavior makes the solution-phase method unique, achieving the high dispersion of Pt clusters adsorbed on the RuO2 Æ xH2O nanoparticles before their reduction to metal Pt nanoparticles. The amount of hexachloroplatinate acid was controlled by the molar ratio of Pt to Ru in the final catalyst. The three samples we produced had Pt:Ru molar ratios of 1:2, 1:1 and 2:1, which were named as PRC 1:2, PRC 1:1 and PRC 2:1, respectively. Then 1.5 times of required sodium tetrahydroborate solution was added drop by drop in 30 min. Another 30 min later, the Pt/ RuO2 Æ xH2O catalyst nanoparticles were prepared. This power was separated by centrifuge, washed with deionized water three times and then dried in air for 12 h at 80 °C. For electrochemical investigations, these as-prepared catalysts and E-TEK Pt–Ru black catalyst are, respectively, mixed with Nafion solution and glycol in ultrasonic sound bath for 5 min. Then, this slurry was spreaded on a glassy carbon disk of ca. 2.0 cm2 and dry at 70 °C for test.
Heat Flow (mW)
594
3. Results and discussion
6
#
5
#
4
#
3
#
#
2
3.1. Structure of the support materials and the catalysts
1
Fig. 1 shows the TGA and DSC investigations over the PRC 1:2 catalyst. From the TGA result, the loss of 2.98 wt% between room temperature and 380 °C under flowing N2 is caused by the water losing of the RuO2 Æ xH2O support material [5] and a further weight loss of 1.13 wt% between 480 and 800 °C can be de-
20
30
40
50
60
70
#
80
90
2θ Fig. 2. X-ray diffraction (XRD) patterns of the Pt/RuO2 Æ xH2O catalysts. The nos. 1–6 patterns represent the as-prepared PRC 1:2, PRC 1:1, PRC 2:1, and the heat-treated PRC 1:2, PRC 1:1, PRC 2:1, respectively.
Z. Chen et al. / Electrochemistry Communications 7 (2005) 593–596
unchanged. This reveals that the Pt nanoparticles are highly dispersed on the hydrous RuO2 support material. And the absence of any superimposed sharp diffraction Pt peaks, both for the as-prepared and heattreated PRC catalysts, indicates a uniform Pt particle distribution [6].
595
to the RuO2 Æ xH2O support. As mentioned above, it is believed that the boundaries provide the active site towards complete methanol electro-oxidation. Therefore, the different electrocatalytic reactivity of these catalysts towards methanol oxidation can be attributed to the differences among their boundaries of Pt to the RuO2 Æ xH2O support.
3.2. Morphologies of the support and catalysts materials Morphologies of the RuO2 Æ xH2O support material and the catalysts are shown by TEM images in Fig. 3. From Fig. 3(a), it can be seen that the particle size of RuO2 Æ xH2O is ca. 20 nm, and the size distribution is remarkably uniform. The dispersion of Pt nanoparticles on the RuO2 Æ xH2O support materials are shown in Figs. 3(b)–(d) representing the catalysts of PRC 1:2, 1:1 and 2:1, respectively. From these images, it can be seen that at low Pt:Ru molar ratios as of 1:2 and 1:1, the Pt nanoparticles are highly dispersed on the RuO2 Æ xH2O support without any obvious aggregation. However, as the Pt:Ru molar ratio increases to 2:1, remarkable aggregations of Pt nanoparticles are observed as marked by arrow mark in Fig. 3(d). It is said that in the as-prepared catalysts, the Pt nanoparticles reveal no clear difference in terms of structure and size distribution. However, the dispersion of the Pt nanoparticles is extremely influenced by the Pt:Ru molar ratio. High molar ratio of Pt to Ru results in notable aggregation of Pt nanoparticles decreasing the boundaries of Pt
3.3. Electrochemical characters of the Pt/RuOx Æ nH2O electrodes Electrochemical properties of PRC catalysts 1:2, 1:1 and 2:1 have been investigated by cyclic voltammetry as shown in Fig. 4. This figure also shows the response of the E-TEK PtRu black catalyst (1:1 atomic ratio) for comparison. From Fig. 4, it can be obtained that the general features of CV curves at PRC catalysts are very similar to that of E-TEK PtRu. However, it is obvious that the electrode composition has a significant influence on the behavior towards methanol electro-oxidation, as summarized in Table 1. The PRC 1:2 catalyst shows the highest reactivity towards methanol electro-oxidation, as the peak current density (ip) at a peak potential of Ep = 0.79 V is as high as 300 mA cm 2 mgPt 1, which is much higher than that of an E-TEK PtRu black electrode, which has an ip at Ep = 0.76 V of only 246 mA cm 2 mgPt 1. For the PRC 1:1 electrode, the peak current density ip at Ep = 0.79 V is decreased to 242 mA cm 2 mgPt 1. The PRC 2:1 electrode shows
Fig. 3. TEM images of the Pt/RuO2 Æ xH2O catalysts, showing highly dispersed Pt nanoparticles on the RuO2 Æ xH2O support with unimodal particle distribution. (a) the as-produced RuO2 Æ xH2O support material, and the as produced Pt/RuO2 Æ xH2O catalysts with Pt:Ru molar ratio of (b) 1:2, (c) 1:1 and (d) 2:1.
Z. Chen et al. / Electrochemistry Communications 7 (2005) 593–596
4. Conclusion
300
-1
Current Density / (mA cm mgPt )
596
-2
250
200
150
100
PRC 1:2 PRC 1:1 PRC 2:1 E-TEK
50
0 -0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Potential / V vs. SCE Fig. 4. Cyclic voltammograms of Pt/RuO2 Æ xH2O and E-TEK PtRu Black electrodes in the potential range between 0.20 and 1.20 V vs. SCE in 1.0 M CH3OH + 1.0 M HClO4 aqueous solution. The curves were obtained at a sweep rate of 10 mV s 1. The catalyst loading is 1 mg on the glassy carbon disk of ca. 2.0 cm2. Table 1 Electrochemical properties of the catalyst electrodes Electrodes
E-TEK PtRu
PRC 1:2
PRC 1:1
PRC 2:1
ip Ep
246 0.76
300 0.79
242 0.79
82 0.80
the lowest catalytic reactivity, as ip at Ep = 0.80 V is only 82 mA cm 2 mgPt 1. It is believed that the abundance of Pt to RuO2 Æ xH2O boundaries determine the electrochemical characters of different PRC electrodes. In other words, in the PRC 1:2 electrode there are adequate boundaries between Pt0 and RuO2 Æ xH2O available to be active for methanol oxidation, thus the electrode shows most active reactivity towards methanol oxidation reaching the highest peak current density.
In summary, the solution-phase method has been successfully used to synthesize RuO2 Æ xH2O supported Pt catalysts. This method involves much simpler operations, if compared to other methods employed to prepare Pt–Ru fuel cells catalysts. Additionally, the as-produced Pt/RuO2 Æ xH2O catalysts show high electrocatalytic activity towards methanol oxidation with respect to the state-of-art E-TEK Pt–Ru black catalysts. The enhanced performance can be contributed to the presence of the amorphous ruthenium oxide rather than Ru0, a high dispersion of uniform Pt nanoparticles on the RuO2 Æ xH2O support materials, and the abundance of boundaries between Pt and RuO2 Æ xH2O promoting the methanol oxidation.
Acknowledgments The authors appreciate the financial support of the NSFC project (90410002) and State Key Basic Research Program of PRC (2002CB211803).
References [1] J.W. Long, R.M. Stroud, K.E. Swider-Lyons, D.R. Rolison, Journal of Physical Chemistry B 104 (2000) 9772. [2] H.M. Villullas, F.I. Mattos-Costa, L.O.S. Bulhoes, Journal of Physical Chemistry B 108 (2004) 12898. [3] M. Watanabe, S. Motoo, Journal of Electroanalytical Chemistry 60 (1975) 267. [4] H.A. Gasteiger, N. Markovic, P.N. Ross, E.J. Cairns, Electrochimica Acta 39 (1994) 1825. [5] K.E. Swider, C.I. Merzbacher, P.L. Hagans, D.R. Rolison, Journal of Non-Crystalline Solids 225 (1998) 348. [6] V. Radmilovic, H.A. Gasteiger, P.N. Ross, Journal of Catalysis 154 (1995) 98.