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C ternary alloy catalysts for methanol electrooxidation
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Electrochemistry Communications 10 (2008) 443–446 www.elsevier.com/locate/elecom
Investigation of the Pt–Ni–Pb/C ternary alloy catalysts for methanol electrooxidation Meng Chen a, Zhen-Bo Wang b,*, Yu Ding a, Ge-Ping Yin b a
College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China b Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, China Received 17 December 2007; received in revised form 22 December 2007; accepted 9 January 2008 Available online 17 January 2008
Abstract This research is aimed to increase the activity of anodic catalysts and thus to lower noble metal loading in anodes for methanol electrooxidation. The Pt–Ni–Pb/C catalysts with different molar compositions were prepared. Their performance were tested by using a glassy carbon disk electrode through cyclic voltammetric curves in a solution of 0.5 mol L 1 CH3OH and 0.5 mol L 1 H2SO4. The performances of Pt–Ni–Pb/C catalyst with optimum composition (the molar ratio of Pt/Ni/Pb is 5:4:1) and Pt/C (E-Tek) were also compared. Their particle sizes and structures were determined by means of X-ray diffraction (XRD). The XRD results show, compared with that of Pt/C, the lattice parameter of Pt–Ni–Pb (5:4:1)/C catalyst decreases, its diffraction peaks are shifted slightly to a higher 2h values. This indicates the formation of an alloy involving the incorporation of Ni and Pb atoms into the fcc structure of Pt. The electrochemical measurement shows the activity of Pt–Ni–Pb/C catalyst with an atomic ratio of 5:4:1 for methanol electrooxidation is the best among all different compositions. The activity of Pt–Ni–Pb (5:4:1)/C catalyst is much higher than that of Pt/C (E-Tek). Ó 2008 Elsevier B.V. All rights reserved. Keywords: Direct methanol fuel cell; Pt–Ni–Pb/C catalyst; Methanol electrooxidation; Composition investigation
1. Introduction Although a lot of progress has been made in the development of direct methanol fuel cell (DMFC) in the past decades, its performance still can not meet the demand of the commercialization [1–3]. One of the challenges of DMFC is the effective catalysts for methanol oxidation reaction [4,5]. Although, of the pure metals, platinum has shown the highest activity for methanol oxidation at low temperatures, it is readily poisoned by the intermediates generated in the process. To overcome the blocking effect of CO-like species on platinum, it can be alloyed with other metals to prepare alloy materials. Many investigations indicate the alloying of Pt with the other metals enhances significantly the catalytic activities and poison *
Corresponding author. Tel.: +86 451 86417853; fax: +86 451 86413707. E-mail address: [email protected] (Z.-B. Wang). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.01.012
tolerance of Pt on the basis of a bifunctional mechanism and/or an electronic effect [2,3,5]. At present, Pt–Ru is excellent anodic catalyst for DMFC, because it gives significant activity for methanol electrooxidation as well as dehydrogenation of water [6,7], but still is not good enough for commercial applications due to its high cost and limited supply. As a result, the development and characterization of a better poison-tolerant catalyst is of tremendous interest to this technology. Theoretical calculations have shown that the segregation processes that generally lead to Pt surface enrichment are unlikely to occur in the Pt–Ni system [8]. Furthermore, in the potential range useful for methanol electrooxidation, the Ni from the Pt–Ni alloy would not be dissolved in the electrolyte. The resistance to dissolution in the electrolyte is attributed to a nickel-hydroxide-passivated surface and the enhanced stability of Ni in the Pt lattice. Pt–Ni catalysts have been investigated extensively for oxygen reduction reaction [9–13]. The methanol electrooxidation on
444
M. Chen et al. / Electrochemistry Communications 10 (2008) 443–446
Pt–Ni thin film and alloy Pt–Ru–Ni catalysts has been reported [14–16]. The Pt–Ru–Ni/C catalysts for methanol electrooxidation have been investigated in detail in our previous work [17,18]. Recently, investigation found the ordered intermetallic phases, such as Pt–Pb, showed a better catalytic activity, a less positive onset potential, and a higher CO-tolerance than those observed for Pt in the case of the electrooxidation of methanol and the other organic small molecules [19]. However, the performance of Pt–Ni–Pb/C catalyst has not been reported. This research is aimed to improve the activity of alloy catalysts, thus to lower noble metals loading in anode for methanol electrooxidation. Pt–Ni–Pb/C catalysts reported here were used for methanol electrooxidation in low temperature fuel cells.
hydrogen electrode (RHE) with its solution connected to the working electrode by a Luggin capillary. All potentials are reported versus RHE. The solution of 0.5 mol L 1 CH3OH and 0.5 mol L 1 H2SO4 was kept on constantly stirring and purging with ultra-pure argon gas. The cyclic voltammograms (CV) were generated within a potential range from 0.05 to 1.2 V with a scanning rate of 0.02 V s 1. To get rid of the possible contamination caused by Nafion film, the working electrodes were treated by continuously cycling at 0.05 V s 1 until a stable response was obtained before the measurement curves were recorded. XRD analysis of nanoparticle catalyst was carried out with the D/max-rB diffractometer (made in Japan) using a Cu Ka X-ray source operating at 45 kV and 100 mA, scanning at the rate of 4 ° min 1 with an angular resolution of 0.05° of the 2h scan to get the XRD patterns.
2. Experimental 3. Results and discussion The preparation of Pt–Ni–Pb/C catalyst was based on the procedure described by Wang et al. [17,18]. All the catalysts consisted of 20% metal in weight, with the carbon black powder (Vulcan XC-72, Cabot) served as the support. The sodium borohydride was used to chemically reduce the precursors of H2PtCl6, RuCl3, and Pb(NO3)2 at 80 °C, with the target products of 0.25 g of Pt–Ni–Pb catalysts with different atomic ratios supported on carbon. At first, the carbon black was dispersed into a mixture of ultra-pure water and isopropyl alcohol through 20 min of ultrasonication, resulting in the uniform carbon ink, into which the precursors were then added with the subsequent mixing process for 15 min. The pH value of the mixture was adjusted by NaOH solution to 8 and then its temperature was raised to 80 °C, immediately before the addition of 25 ml of 0.2 mol L 1 solution of NaBH4, which was followed by stirring the bath for 1 h. The product was cooled, dried, and washed repeatedly with ultra-pure water (18.2 MX cm) until no Cl ions were detected. The formed powder of catalyst was dried for 3 h at 120 °C and then stored in a vacuum vessel. The glassy carbon working electrode with 3 mm of diameter (0.0706 cm2) was polished with 0.05 lm alumina to a mirror-finish before being used as the substrate for carbon-supported catalyst. The ultrasonically re-dispersed catalyst suspension was spread by pipette onto the glassy carbon substrate. The subsequent evaporation of solvent led to the formation of the deposited catalyst layer (28 lgmetal cm 2), onto which 5 lL of a dilute aqueous Nafion solution (DuPont) was applied. The resulting Nafion film with a thickness less than 0.2 lm had the sufficient strength to keep carbon particles permanently to the glassy carbon electrode without producing significant film diffusion resistances [20]. Electrochemical measurements were carried out in the conventional three-electrode cell at 25 °C, with the glassy carbon electrode made in the above mentioned procedure as the working electrode and a piece of Pt foil (1 cm2) as the counter one. The reference electrode was a reversible
The combinatorial method is used to study Pt–Ni–Pb composition. The performance of methanol electrooxidation on Pt–Ni–Pb/C, in which composition (molar ratio) is different, is summarized in Table 1. The data were obtained from cyclic voltammograms during positive potential scanning with a solution of 0.5 mol L 1 H2SO4 containing 0.5 mol L 1 CH3OH at 25 °C. Table 1 presents the peak current density against the content of metal in the catalysts. It can be seen that the peak potential, about 0.85 V, of the ten catalysts is almost the same. The peak current density is the highest on Pt–Ni–Pb (5:4:1 by molar ratio)/C, i.e. its performance is the best. Fig. 1 shows the cyclic voltammograms of methanol electrooxidation on Pt/C (E-Tek) and Pt–Ni–Pb (5:4:1 by molar ratio)/C electrodes after the optimum preparation conditions. It is clear that involvement of Ni and Pb significantly increases the electrode activity. The onset potential of a current rise on Pt–Ni–Pb/C catalyst corresponds to that on Pt/C, i.e. about 0.55 V. The peak potential, about 0.85 V, at which the peak currents occur on the two catalysts are the same. The peak current density occurred on Pt–Ni–Pb/C catalyst of 35.1 mA mgPt1 is twice higher than
Table 1 Comparison of the peak current density and the peak potential on CVs of methanol electrooxidation on Pt–Ni–Pb/C catalysts with different molar compositions Composition (molar ratio)
EP (V)
i (mA cm 2)
4:2:4 4:3:3 4:4:2 4:5:1 5:2:3 5:3:2 5:4:1 6:2:2 6:3:1 7:2:1
0.872 0.873 0.844 0.858 0.829 0.833 0.848 0.862 0.849 0.863
0.22 0.40 1.17 0.42 0.44 0.61 2.91 0.69 2.24 0.63
M. Chen et al. / Electrochemistry Communications 10 (2008) 443–446 40
Pt/C(E-Tek) Pt-Ni-Pb/C
-1 i / mA mgPt
30
If Ib
20 10 0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
E / V (vs. RHE) Fig. 1. Cyclic voltammograms of methanol electrooxidation on Pt/C (ETek) and Pt–Ni–Pb (5:4:1)/C catalysts in a solution of 0.5 mol L 1 CH3OH and 0.5 mol L 1 H2SO4 at 25 °C. Scan rate: 0.02 V s 1.
that on Pt/C of 16.1 mA mgPt1 . The anodic peak in the backward scan, which indicates the removal of carbonaceous species not completely oxidized in the anodic scan [21], can be used to form the ratio of anodic current density, If/Ib. The If/Ib ratio of Pt–Ni–Pb/C catalyst (1.36) is evident higher than that of Pt/C (0.86), which indicates that Pt–Ni–Pb/C catalyst has less carbonaceous accumulation and higher CO tolerance in the anodic scan [22]. This can be attributed to the presence of Ni and Pb sites on the catalysts surface. The ‘electron effect’ of Ni is known to improve complete oxidation of intermediates from methanol electrooxidation [17,18]. The function of Pb element for methanol electrooxidation is in progress. Fig. 2 shows the XRD patterns of Pt–Ni–Pb/C and Pt/C catalysts. The first broad peak located at the 2h value of
about 24.8° in the XRD pattern is attributed to the carbon support, whereas the four other peaks, characteristic of the face centered cubic crystalline Pt, correspond to the planes (1 1 1), (2 0 0), (2 2 0), and (3 1 1), respectively. It is worth while to note the lack of diffraction peaks characteristic of Ni, Pb, or their oxides/hydroxides, which suggests that Ni and Pb atoms either form an alloy with Pt or exist as oxides in amorphous phases. The data in Table 2 based on the Pt (2 2 0) crystal face not only reflect the formation of a solid solution but also demonstrate the much smaller lattice parameter for Pt–Ni–Pb/C catalyst than that for Pt/C. Significant difference between Pt–Ni–Pb/C and Pt/C catalyst has been found in Table 2 with respect to the average particle sizes and specific surface areas, which are estimated from full width at half maximum according to Debye–Scherrer formula [23]. Comparing with those of Pt/C, diffraction peaks of Pt–Ni–Pb/C catalyst are shifted to the slightly higher 2h values, its lattice parameter evidently decreases. It indicates the progressive increase in the incorporation of Ni and Pb into the alloy state. The particle size of Pt–Ni–Pb/C of 3.7 nm is bigger than that of Pt/C of 2.5 nm. Both specific areas are 91.4 and 112.1 m2/g, respectively. The former area is about 20 m2/g smaller than the latter. However, the activity of Pt–Ni–Pb/C is evident higher than of Pt/C, which indicates the addition of Ni and Pb into Pt catalyst significantly improve the electrode performance for methanol electrooxidation. The mechanism study of Ni and Pb on Pt–Ni–Pb/C is in progress. 4. Conclusions The addition of Ni and Pb into Pt catalysts can significantly improve the electrode performance for methanol electrooxidation. The activity of Pt–Ni–Pb/C for methanol electrooxidation in acid medium is much higher than that of Pt/C (E-Tek) catalyst. Its performance with an atomic ratio of 5:4:1 is the best.
Pt(220)
Pt(200)
Pt(311)
Pt(111)
Intensity / a.u.
445
Acknowledgments Pt/C
Pt-Ni-Pb/C 0
20
40
60
80
100
Fig. 2. XRD patterns of Pt/C (E-Tek) and Pt–Ni–Pb (5:4:1)/C catalysts.
This work was financially supported by the National Natural Science Foundation of China (No.20606007), Postdoctoral Scientific Research Foundation of Heilongjiang Province of China and Harbin Innovation Science Foundation for Youths. References
Table 2 The lattice parameter, particle size, and specific area of Pt/C (E-Tek) and Pt–Ni–Pb (5:4:1)/C catalysts Catalysts
2h (°)
d-value (nm)
Lattice parameter (nm)
Particle size (nm)
Specific area (m2/g)
Pt/C Pt–Ni– Pb/C
67.64 68.48
0.13839 0.13690
0.3914 0.3872
2.5 3.7
112.1 91.4
[1] T. Okada, Y. Suzuki, T. Hirose, T. Ozawa, Electrochim. Acta 49 (2004) 385. [2] J.S. Wang, J.Y. Xi, Y.X. Bai, Y. Shen, J. Sun, L.Q. Chen, W.T. Zhu, X.P. Qiu, J. Power Sources 164 (2007) 555. [3] J. Tian, G.Q. Sun, L.H. Jiang, S.Y. Yan, Q. Mao, Q. Xin, Electrochem. Commun. 9 (2007) 563. [4] V. Selvaraj, M. Alagar, Electrochem. Commun. 9 (2007) 1145. [5] I.-S. Park, K.-S. Lee, D.-S. Jung, H.-Y. Park, Y.-E. Sung, Electrochim. Acta 52 (2007) 5599.
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[6] D. Kardash, C. Korzeniewski, N. Markovic, J. Electroanal. Chem. 500 (2001) 518. [7] H.N. Dinh, X.M. Ren, F.H. Garzon, P. Zelenay, S. Gottesfeld, J. Electroanal. Chem. 491 (2000) 222. [8] T.C. Deivaraj, W.X. Chen, J.Y. Lee, J. Mater. Chem. 13 (2003) 2555. [9] H. Yang, C. Coutanceau, J.M. Leger, N. Alonso-Vante, C. Lamy, J. Electroanal. Chem. 576 (2005) 305. [10] H. Yang, W. Vogel, C. Lamy, N. Alonso-Vante, J. Phys. Chem. B 108 (2004) 11024. [11] V. Stamenkovic, T.J. Schmidt, P.N. Ross, N.M. Markovic, J. Phys. Chem. B 106 (2002) 11970. [12] U.A. Paulus, A. Wokaun, G.G. Scherer, T.J. Schmidt, V. Stamenkovic, V. Radmilovic, N.M. Markovic, P.N. Ross, J. Phys. Chem. B 106 (2002) 4181. [13] J.F. Drillet, A. Ee, J. Friedemann, R. Kotz, B. Schnyder, V.M. Schmidt, Electrochim. Acta 47 (2002) 1983. [14] T. Toda, H. Igarashi, H. Uchida, M. Watanabe, J. Electrochem. Soc. 146 (1999) 3750.
[15] K.W. Park, J.H. Choi, Y.E. Sung, J. Phys. Chem. B 107 (2003) 5851. [16] K.W. Park, J.H. Choi, B.K. Kwon, S.A. Lee, Y.E. Sung, H.Y. Ha, S.A. Hong, H. Kim, A. Wieckowski, J. Phys. Chem. B 106 (2002) 1869. [17] Z.B. Wang, G.P. Yin, J. Zhang, Y.C. Sun, P.F. Shi, Electrochim. Acta 51 (2006) 5691. [18] Z.B. Wang, G.P. Yin, P.F. Shi, Y.C. Sun, Electrochem. Solid-State Lett. 9 (2006) A13. [19] E. Casado-Rivera, D.J. Volpe, L. Alden, C. Lind, C. Downie, T. Vazquez-Alvarez, A.C.D. Angelo, F.J. DiSalvo, H.D. Abruna, J. Am. Chem. Soc. 126 (2004) 4043. [20] T.J. Schmidt, H.A. Gasteiger, G.D. Stab, P.M. Urban, D.M. Kolb, R.J. Behm, J. Electrochem. Soc. 145 (1998) 2354. [21] R. Manohara, J.B. Goodenough, J. Mater. Chem. 2 (1992) 875. [22] Z.L. Liu, X.Y. Ling, X.D. Su, J. Yang, J. Phys. Chem. B 108 (2004) 8234. [23] L. Giorgi, A. Pozio, C. Bracchini, R. Giorgi, S. Turtu, J. Appl. Electrochem. 31 (2001) 325.
Electrochemistry Communications 10 (2008) 443–446 www.elsevier.com/locate/elecom
Investigation of the Pt–Ni–Pb/C ternary alloy catalysts for methanol electrooxidation Meng Chen a, Zhen-Bo Wang b,*, Yu Ding a, Ge-Ping Yin b a
College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China b Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, China Received 17 December 2007; received in revised form 22 December 2007; accepted 9 January 2008 Available online 17 January 2008
Abstract This research is aimed to increase the activity of anodic catalysts and thus to lower noble metal loading in anodes for methanol electrooxidation. The Pt–Ni–Pb/C catalysts with different molar compositions were prepared. Their performance were tested by using a glassy carbon disk electrode through cyclic voltammetric curves in a solution of 0.5 mol L 1 CH3OH and 0.5 mol L 1 H2SO4. The performances of Pt–Ni–Pb/C catalyst with optimum composition (the molar ratio of Pt/Ni/Pb is 5:4:1) and Pt/C (E-Tek) were also compared. Their particle sizes and structures were determined by means of X-ray diffraction (XRD). The XRD results show, compared with that of Pt/C, the lattice parameter of Pt–Ni–Pb (5:4:1)/C catalyst decreases, its diffraction peaks are shifted slightly to a higher 2h values. This indicates the formation of an alloy involving the incorporation of Ni and Pb atoms into the fcc structure of Pt. The electrochemical measurement shows the activity of Pt–Ni–Pb/C catalyst with an atomic ratio of 5:4:1 for methanol electrooxidation is the best among all different compositions. The activity of Pt–Ni–Pb (5:4:1)/C catalyst is much higher than that of Pt/C (E-Tek). Ó 2008 Elsevier B.V. All rights reserved. Keywords: Direct methanol fuel cell; Pt–Ni–Pb/C catalyst; Methanol electrooxidation; Composition investigation
1. Introduction Although a lot of progress has been made in the development of direct methanol fuel cell (DMFC) in the past decades, its performance still can not meet the demand of the commercialization [1–3]. One of the challenges of DMFC is the effective catalysts for methanol oxidation reaction [4,5]. Although, of the pure metals, platinum has shown the highest activity for methanol oxidation at low temperatures, it is readily poisoned by the intermediates generated in the process. To overcome the blocking effect of CO-like species on platinum, it can be alloyed with other metals to prepare alloy materials. Many investigations indicate the alloying of Pt with the other metals enhances significantly the catalytic activities and poison *
Corresponding author. Tel.: +86 451 86417853; fax: +86 451 86413707. E-mail address: [email protected] (Z.-B. Wang). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.01.012
tolerance of Pt on the basis of a bifunctional mechanism and/or an electronic effect [2,3,5]. At present, Pt–Ru is excellent anodic catalyst for DMFC, because it gives significant activity for methanol electrooxidation as well as dehydrogenation of water [6,7], but still is not good enough for commercial applications due to its high cost and limited supply. As a result, the development and characterization of a better poison-tolerant catalyst is of tremendous interest to this technology. Theoretical calculations have shown that the segregation processes that generally lead to Pt surface enrichment are unlikely to occur in the Pt–Ni system [8]. Furthermore, in the potential range useful for methanol electrooxidation, the Ni from the Pt–Ni alloy would not be dissolved in the electrolyte. The resistance to dissolution in the electrolyte is attributed to a nickel-hydroxide-passivated surface and the enhanced stability of Ni in the Pt lattice. Pt–Ni catalysts have been investigated extensively for oxygen reduction reaction [9–13]. The methanol electrooxidation on
444
M. Chen et al. / Electrochemistry Communications 10 (2008) 443–446
Pt–Ni thin film and alloy Pt–Ru–Ni catalysts has been reported [14–16]. The Pt–Ru–Ni/C catalysts for methanol electrooxidation have been investigated in detail in our previous work [17,18]. Recently, investigation found the ordered intermetallic phases, such as Pt–Pb, showed a better catalytic activity, a less positive onset potential, and a higher CO-tolerance than those observed for Pt in the case of the electrooxidation of methanol and the other organic small molecules [19]. However, the performance of Pt–Ni–Pb/C catalyst has not been reported. This research is aimed to improve the activity of alloy catalysts, thus to lower noble metals loading in anode for methanol electrooxidation. Pt–Ni–Pb/C catalysts reported here were used for methanol electrooxidation in low temperature fuel cells.
hydrogen electrode (RHE) with its solution connected to the working electrode by a Luggin capillary. All potentials are reported versus RHE. The solution of 0.5 mol L 1 CH3OH and 0.5 mol L 1 H2SO4 was kept on constantly stirring and purging with ultra-pure argon gas. The cyclic voltammograms (CV) were generated within a potential range from 0.05 to 1.2 V with a scanning rate of 0.02 V s 1. To get rid of the possible contamination caused by Nafion film, the working electrodes were treated by continuously cycling at 0.05 V s 1 until a stable response was obtained before the measurement curves were recorded. XRD analysis of nanoparticle catalyst was carried out with the D/max-rB diffractometer (made in Japan) using a Cu Ka X-ray source operating at 45 kV and 100 mA, scanning at the rate of 4 ° min 1 with an angular resolution of 0.05° of the 2h scan to get the XRD patterns.
2. Experimental 3. Results and discussion The preparation of Pt–Ni–Pb/C catalyst was based on the procedure described by Wang et al. [17,18]. All the catalysts consisted of 20% metal in weight, with the carbon black powder (Vulcan XC-72, Cabot) served as the support. The sodium borohydride was used to chemically reduce the precursors of H2PtCl6, RuCl3, and Pb(NO3)2 at 80 °C, with the target products of 0.25 g of Pt–Ni–Pb catalysts with different atomic ratios supported on carbon. At first, the carbon black was dispersed into a mixture of ultra-pure water and isopropyl alcohol through 20 min of ultrasonication, resulting in the uniform carbon ink, into which the precursors were then added with the subsequent mixing process for 15 min. The pH value of the mixture was adjusted by NaOH solution to 8 and then its temperature was raised to 80 °C, immediately before the addition of 25 ml of 0.2 mol L 1 solution of NaBH4, which was followed by stirring the bath for 1 h. The product was cooled, dried, and washed repeatedly with ultra-pure water (18.2 MX cm) until no Cl ions were detected. The formed powder of catalyst was dried for 3 h at 120 °C and then stored in a vacuum vessel. The glassy carbon working electrode with 3 mm of diameter (0.0706 cm2) was polished with 0.05 lm alumina to a mirror-finish before being used as the substrate for carbon-supported catalyst. The ultrasonically re-dispersed catalyst suspension was spread by pipette onto the glassy carbon substrate. The subsequent evaporation of solvent led to the formation of the deposited catalyst layer (28 lgmetal cm 2), onto which 5 lL of a dilute aqueous Nafion solution (DuPont) was applied. The resulting Nafion film with a thickness less than 0.2 lm had the sufficient strength to keep carbon particles permanently to the glassy carbon electrode without producing significant film diffusion resistances [20]. Electrochemical measurements were carried out in the conventional three-electrode cell at 25 °C, with the glassy carbon electrode made in the above mentioned procedure as the working electrode and a piece of Pt foil (1 cm2) as the counter one. The reference electrode was a reversible
The combinatorial method is used to study Pt–Ni–Pb composition. The performance of methanol electrooxidation on Pt–Ni–Pb/C, in which composition (molar ratio) is different, is summarized in Table 1. The data were obtained from cyclic voltammograms during positive potential scanning with a solution of 0.5 mol L 1 H2SO4 containing 0.5 mol L 1 CH3OH at 25 °C. Table 1 presents the peak current density against the content of metal in the catalysts. It can be seen that the peak potential, about 0.85 V, of the ten catalysts is almost the same. The peak current density is the highest on Pt–Ni–Pb (5:4:1 by molar ratio)/C, i.e. its performance is the best. Fig. 1 shows the cyclic voltammograms of methanol electrooxidation on Pt/C (E-Tek) and Pt–Ni–Pb (5:4:1 by molar ratio)/C electrodes after the optimum preparation conditions. It is clear that involvement of Ni and Pb significantly increases the electrode activity. The onset potential of a current rise on Pt–Ni–Pb/C catalyst corresponds to that on Pt/C, i.e. about 0.55 V. The peak potential, about 0.85 V, at which the peak currents occur on the two catalysts are the same. The peak current density occurred on Pt–Ni–Pb/C catalyst of 35.1 mA mgPt1 is twice higher than
Table 1 Comparison of the peak current density and the peak potential on CVs of methanol electrooxidation on Pt–Ni–Pb/C catalysts with different molar compositions Composition (molar ratio)
EP (V)
i (mA cm 2)
4:2:4 4:3:3 4:4:2 4:5:1 5:2:3 5:3:2 5:4:1 6:2:2 6:3:1 7:2:1
0.872 0.873 0.844 0.858 0.829 0.833 0.848 0.862 0.849 0.863
0.22 0.40 1.17 0.42 0.44 0.61 2.91 0.69 2.24 0.63
M. Chen et al. / Electrochemistry Communications 10 (2008) 443–446 40
Pt/C(E-Tek) Pt-Ni-Pb/C
-1 i / mA mgPt
30
If Ib
20 10 0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
E / V (vs. RHE) Fig. 1. Cyclic voltammograms of methanol electrooxidation on Pt/C (ETek) and Pt–Ni–Pb (5:4:1)/C catalysts in a solution of 0.5 mol L 1 CH3OH and 0.5 mol L 1 H2SO4 at 25 °C. Scan rate: 0.02 V s 1.
that on Pt/C of 16.1 mA mgPt1 . The anodic peak in the backward scan, which indicates the removal of carbonaceous species not completely oxidized in the anodic scan [21], can be used to form the ratio of anodic current density, If/Ib. The If/Ib ratio of Pt–Ni–Pb/C catalyst (1.36) is evident higher than that of Pt/C (0.86), which indicates that Pt–Ni–Pb/C catalyst has less carbonaceous accumulation and higher CO tolerance in the anodic scan [22]. This can be attributed to the presence of Ni and Pb sites on the catalysts surface. The ‘electron effect’ of Ni is known to improve complete oxidation of intermediates from methanol electrooxidation [17,18]. The function of Pb element for methanol electrooxidation is in progress. Fig. 2 shows the XRD patterns of Pt–Ni–Pb/C and Pt/C catalysts. The first broad peak located at the 2h value of
about 24.8° in the XRD pattern is attributed to the carbon support, whereas the four other peaks, characteristic of the face centered cubic crystalline Pt, correspond to the planes (1 1 1), (2 0 0), (2 2 0), and (3 1 1), respectively. It is worth while to note the lack of diffraction peaks characteristic of Ni, Pb, or their oxides/hydroxides, which suggests that Ni and Pb atoms either form an alloy with Pt or exist as oxides in amorphous phases. The data in Table 2 based on the Pt (2 2 0) crystal face not only reflect the formation of a solid solution but also demonstrate the much smaller lattice parameter for Pt–Ni–Pb/C catalyst than that for Pt/C. Significant difference between Pt–Ni–Pb/C and Pt/C catalyst has been found in Table 2 with respect to the average particle sizes and specific surface areas, which are estimated from full width at half maximum according to Debye–Scherrer formula [23]. Comparing with those of Pt/C, diffraction peaks of Pt–Ni–Pb/C catalyst are shifted to the slightly higher 2h values, its lattice parameter evidently decreases. It indicates the progressive increase in the incorporation of Ni and Pb into the alloy state. The particle size of Pt–Ni–Pb/C of 3.7 nm is bigger than that of Pt/C of 2.5 nm. Both specific areas are 91.4 and 112.1 m2/g, respectively. The former area is about 20 m2/g smaller than the latter. However, the activity of Pt–Ni–Pb/C is evident higher than of Pt/C, which indicates the addition of Ni and Pb into Pt catalyst significantly improve the electrode performance for methanol electrooxidation. The mechanism study of Ni and Pb on Pt–Ni–Pb/C is in progress. 4. Conclusions The addition of Ni and Pb into Pt catalysts can significantly improve the electrode performance for methanol electrooxidation. The activity of Pt–Ni–Pb/C for methanol electrooxidation in acid medium is much higher than that of Pt/C (E-Tek) catalyst. Its performance with an atomic ratio of 5:4:1 is the best.
Pt(220)
Pt(200)
Pt(311)
Pt(111)
Intensity / a.u.
445
Acknowledgments Pt/C
Pt-Ni-Pb/C 0
20
40
60
80
100
Fig. 2. XRD patterns of Pt/C (E-Tek) and Pt–Ni–Pb (5:4:1)/C catalysts.
This work was financially supported by the National Natural Science Foundation of China (No.20606007), Postdoctoral Scientific Research Foundation of Heilongjiang Province of China and Harbin Innovation Science Foundation for Youths. References
Table 2 The lattice parameter, particle size, and specific area of Pt/C (E-Tek) and Pt–Ni–Pb (5:4:1)/C catalysts Catalysts
2h (°)
d-value (nm)
Lattice parameter (nm)
Particle size (nm)
Specific area (m2/g)
Pt/C Pt–Ni– Pb/C
67.64 68.48
0.13839 0.13690
0.3914 0.3872
2.5 3.7
112.1 91.4
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