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Electrospun bimetallic nanowires of PtRh and PtRu with compositional variation for methanol electrooxidation
Electrochemistry Communications 10 (2008) 1016–1019
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Electrospun bimetallic nanowires of PtRh and PtRu with compositional variation for methanol electrooxidation Yong Seok Kim a, Sang Hoon Nam a, Hee-Sang Shim a, Hyo-Jin Ahn b, Manish Anand a, Won Bae Kim a,* a b
Department of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, South Korea Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany
a r t i c l e
i n f o
Article history: Received 9 April 2008 Received in revised form 30 April 2008 Accepted 2 May 2008 Available online 6 May 2008 Keywords: Bimetallic nanowires Electrospinning Electrocatalysts PtRh PtRu Methanol electrooxidation
a b s t r a c t Binary metallic nanowires (NWs) of PtRh and PtRu were synthesized by electrospinning method with compositional variation from 1:3 to 2:1. The electrospun bimetallic NWs were highly alloyed with diameters smaller than 60 nm and lengths up to hundreds of micrometers. The PtRh and PtRu NWs with 1:1 atomic ratio resulted in the higher catalytic mass activity over the methanol electrooxidation than those with the different atomic ratios, and the mass activity of Pt1Ru1 NWs was superior to the other NWs and even better than the commercial catalyst of the highly dispersed Pt1Ru1 nanoparticles on carbon. Moreover, the bimetallic NW electrocatalysts showed the better stability than the bimetallic nanoparticles. The enhancements of electrocatalytic properties for the Pt1Rh1 and Pt1Ru1 NWs could be attributed to their one-dimensional features, which can outperform on the electro-oxidations over the fuel cell electrodes. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Direct methanol fuel cells (DMFCs) have been widely studied as an alternative power source for portable devices due to high energy density and conversion efficiency of methanol fuel [1]. However, some drawbacks like sluggish oxidation reactions even over the Pt-based anode catalysts, and high price with limited supply of Pt should be overcome to make it commercial. To solve these problems in DMFC, modifying catalyst structure into one-dimensionally nanostructured materials such as nanowires (NWs) [2], inserting various elements as active materials [3] or support materials [4] into the Pt-based electrocatalysts and changing synthesis methods of catalysts [5] have been developed until now. Among them, the nanowire electrocatalysts may provide another elegant strategy by facilitating electron transport and reducing embedded electrocatalyst on electrodes [6], increasing possibility to improve the performance of DMFC. In order to prepare nanowire electrocatalysts, template-based methods have been largely employed to synthesize uniform metallic NWs [7]. However, the hard template-based NWs have some limitations for the electrocatalyst applications. For instances, it is difficult to make template due to complex process with high cost. Moreover, although the NWs for the electrocatalysts should be produced in a large quantity enough to be used in electrode, the
* Corresponding author. Tel.: +82 62 970 2317; fax: +82 62 970 2304. E-mail address: [email protected] (W.B. Kim). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.05.003
template-based method may be inadequate to be applied for the fuel cells because of small sizes of templates. To circumvent such restrictions of hard template-based NWs synthesis, electrospinning method can be employed with its versatility and effectiveness to synthesize various metal oxides or pure metallic NWs, as demonstrated in the preparation of TiO2 or Cu nanowires [8,9]. In this work, bimetallic PtRh and PtRu NWs through the electrospinning method are synthesized with compositional variation for the first time to examine the optimum atomic ratio of the bimetallic nanowire electrocatalysts. We investigate the electrocatalytic properties of the electrospun bimetallic NWs by using cyclic voltammetry and chronoamperometric technique, and their mass activities per gram of Pt are compared with the highly dispersed PtRu nanoparticle on carbon (Pt1Ru1/C, E-TEK) that is a conventional catalyst for methanol electrooxidation. 2. Experimental For PtRh and PtRu NWs preparation, the precursor solutions in DI water that consist of H2PtCl6 6H2O (Aldrich) and RhCl3 xH2O (Aldrich) or RuCl3 xH2O (Aldrich) were mixed with poly(vinyl pyrrolidone) (PVP, Aldrich, Mw = 1,300,000 g/mol) dissolved in ethanol. The pre-calculated amounts of metal precursors according to compositional variation were controlled to be 3 wt% and the amount of PVP was kept to be 4 wt% of the total weight of electrospinning solution. The feeding rate of the precursor solution was controlled to 0.05 ml/h and a Si collector was vertically positioned
Y.S. Kim et al. / Electrochemistry Communications 10 (2008) 1016–1019
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at 6 cm apart from the needle under a fixed potential of 6 kV to collect metal precursor/PVP composite nanofibers. The PVP was selectively removed by calcination in the air at 300 °C for 3 h. To obtain the alloyed bimetallic nanowires, the reduction treatment was subsequently performed at 100 °C under H2 at a flow rate of 60 sccm. The morphology and diameter of the PtRh and PtRu NWs were investigated by field emission scanning electron microscopy (FESEM, Hitach S-4700) and by high resolution transmission electron microscopy (HRTEM, TECNAI-F20) operated at 200 kV in Korea Basic Science Institute. X-ray diffraction (XRD, Rigaku Rotalflex RU200B) equipped with a Cu Ka source (k = 1.5405 Å) was carried out to obtain diffraction patterns of the electrospun bimetallic NWs. X-ray absorption measurements were conducted on 3C1 and 7C1 beamlines of Pohang Accelerator Laboratory in Korea. Electrochemical measurements of all samples were conducted by using a three-electrode system. Pt wire and Ag/AgCl (in 3 M KCl) were used as the counter and reference electrodes and the glassy carbon (0.07 cm2) was employed as the working electrode (WE). Catalyst inks made with catalyst and Nafion in 9:1 weight ratio were loaded onto the WE. Electrochemical experiments were carried out by using Autolab PGSTAT30 Potentiostat/Galvanostat (Eco Chemi.). To investigate the electrocatalytic activities and stabilities of the PtRh and PtRu NWs for the methanol electrooxidation, cyclic voltammetry test was performed in the potential range from 0.2 to 1.0 V at a scan rate of 50 mV/s and chronoamperometry test was carried out at a constant potential of 0.6 V in electrolyte solution containing methanol for 6000 s.
3. Results and discussion Fig. 1 shows FESEM images of as-spun metal precursor/PVP composite nanofibers and bimetallic NWs. The as-spun nanofibers of metal precursors/PVP composite have average diameter of ca. 250 ± 50 nm as shown in Fig. 1a and b. After calcination at 300 °C for 3 h in air, subsequent reduction process at 100 °C for 3 h under H2 atmosphere was performed to produce the reduced bimetallic nanowires, resulting that the diameters of PtRh and PtRu NWs became smaller with the average size of ca. 50 ± 10 nm as shown in Fig. 1c and d. The SEM figures in Fig. 1 represents the nanowire morphology on a Si substrate via electrospinning. Note that the diameters of PtRh and PtRu NWs were decreased to 1/5th of the as-spun nanofibers. Fig. 1e and f present the HRTEM images of electrospun of Pt1Rh1 and Pt1Ru1 NWs, indicating that they seem to be composed of small nanoparticles with 2–3 nm size. The inset figures in Fig. 1e and f shows the crystalline (1 1 1) planes for Pt1Rh1 and Pt1Ru1 NWs with planer distances of ca. 2.17 Å, which is significantly smaller than that of the pure Pt with 2.26 Å, suggesting that they were alloyed by incorporating Rh or Ru to the Pt, as will be further investigated by XRD analysis. In Fig. 2, the electrospun bimetallic PtRh and PtRu NWs with respect to their compositional variation of Pt:2nd metal from 3:1 to 1:2 were characterized by XRD. The XRD peaks are generally shifted to the higher angles with Rh incorporation into the fcc Pt phase [10]. This feature is also observed over the electrospun bimetallic PtRh NWs in this work, as seen in the Fig. 2a by showing that the deviation of the diffraction angles for the PtRh NWs becomes greater with Rh amount. We could not find any other phases such as phase-separated Pt or Rh phase in the tested Pt:Rh ratios, except the single solution phase of alloyed PtRh, implying that the phase separation might not occur in PtRh NWs system under our preparation condition. For the bimetallic PtRu NWs, the structural evolution takes place similarly to the PtRh cases except for the appearance of phase-separated Ru metal in the highest Ru content, which can be understood by that the Ru has hexagonal close-
Fig. 1. FESEM images of as-spun (a) Pt–Rh precursor/PVP, (b) Pt–Ru precursor/PVP and bimetallic nanowires of (c) Pt1Rh1 and (d) Pt1Ru1; insets of (c) and (d) show the diameter of bimetallic NWs. HRTEM images of (e) Pt1Rh1 NWs and (f) Pt1Ru1 NWs; insets of (e) and (f) are inverse fast fourier transform image of Pt1Rh1 NWs and Pt1Ru1 NWs.
packed structure, thereby a phase separation of Ru metal might be involved in the 1:2 Pt:Ru ratio due to the structural mismatch that did not make solid solution phase with Pt in our bimetallic PtRu NWs. Fig. 3a and b shows CVs of the electrospun bimetallic PtRh and PtRu NWs for the methanol electrooxidation (CH3OH + H2O ? 6H+ + 6e + CO2). In both cases of PtRh and PtRu bimetallic NWs, the highest Pt-based mass activities were observed at the 1:1 atomic ratio, with activity order of 1:1 > 2:1 > 3:1 > 1:2. The enhancements of mass activity up to 1:1 atomic ratio could be explained with bifunctional mechanism and ligand (electronic) effect, which have been well postulated in the Pt-based nanoparticle catalysts [11– 13]. The OH species could be more readily generated over the 2nd metal phase at the low potentials according to the bifunctional mechanism in methanol electrooxidation using Pt-based bimetallic catalysts. Also, the ligand effect might contribute to the enhanced activity as the 2nd metals such as Rh, Ru and Sn modify electron density of Pt d-orbital [13–15]. Although the X-ray absorption near edge structure (XANES) data are not included in this work, Pt LIII edge of the bimetallic nanowires was obtained to investigate the electronic structure and oxidation state. Since the absorption at the Pt LIII edge corresponds to 2p ? 5d electronic transition [14,15], the intensities of the white line for the bimetallic nanowires
Y.S. Kim et al. / Electrochemistry Communications 10 (2008) 1016–1019
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60
50
i (mA/mgPt)
were reduced by at least 4–5% as compared to that of the pure Pt phase, which could be rationalized by charge transfer from Rh or Ru to Pt. In order to evaluate the electrocatalytic performance of our bimetallic NWs, their activities were compared with the highly dispersed Pt1Ru1 nanoparticles on carbon (Pt1Ru1/C, E-TEK). Interestingly, unsupported NWs have the mass activities of 355.8 mA/mgPt for the Pt1Rh1 NWs and 449.1 mA/mgPt for Pt1Ru1 NWs, which are comparable or even better by ca. 30% as compared to the commercial catalyst of Pt1Ru1/C with 347.2 mA/mgPt. The enhancements may be attributed to their one-dimensional features in the electrochemical electrodes, as demonstrated in Ref. [6] using single Pt nanowires. The stability test over the electrospun bimetallic NWs was performed and compared with the catalyst of Pt1Ru1/C through chronoamperometry measurement in 2 M CH3OH + 0.5 M H2SO4 solution at a constant potential of 0.6 V (vs. Ag/AgCl) for 6000 s as shown in Fig. 3c. After the initial transient period that shows rapid decreases in the potentiostatic currents, the currents were gradually declined for the Pt1Rh1 NWs or reached to a steady-state for the Pt1Ru1 NWs and Pt1Ru1/C. Comparing Pt1Ru1 NWs and Pt1Ru1/C, since the stabilized currents with nanowire electrocatalysts were much higher than that of nanoparticle by at least four times, it can be inferred that the nanowire morphology as electrocatalysts would be more stable than nanoparticles. Consequently, the PtRh and PtRu NWs with 1:1 atomic ratio showed the higher catalytic mass activity with the better stability
Pt1Ru1 NWs Pt1Rh1 NWs Pt1Ru1/C
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0
0
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Time (s) Fig. 3. The cyclic voltammograms of electrospun bimetallic nanowires of (a) PtRh and (b) PtRu with compositional variation in 2 M CH3OH containing 0.5 M H2SO4 solution at a scan rate of 50 mV/s and (c) chronoamperometric curves of Pt1Rh1 NWs and Pt1Ru1 NWs compared with Pt1Ru1/C (E-TEK) in 2 M CH3OH containing 0.5 M H2SO4 solution for 6000 s.
over the methanol electrooxidation than those with the different atomic ratios, and the mass activity of Pt1Ru1 NWs was superior to the other catalysts tested and even better than the conventional catalyst of the highly dispersed Pt1Ru1 nanoparticles on carbon.
Y.S. Kim et al. / Electrochemistry Communications 10 (2008) 1016–1019
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4. Conclusion
References
The bimetallic PtRh and PtRu nanowires of ca. 50 nm diameter fabricated by electrospinning method with compositional variation were used as the nanowire electrocatalysts for the methanol electrooxidation, in which the Pt1Rh1 and Pt1Ru1 nanowires resulted in the highest mass activity. The mass activity of electrospun Pt1Ru1 nanowires was significantly improved by ca. 30% with the better stability than those of Pt1Ru1/C. These enhancements could be attributed to efficient charge transport and better physical and interfacial properties arising from one-dimensional feature compared with supported nanoparticle catalysts.
[1] G.A. Olah, Angew. Chem., Int. Ed. 44 (2005) 2636. [2] G.Y. Zhao, C.L. Xu, D.J. Guo, H. Li, H.L. Li, J. Power Source 162 (2006) 492. [3] M.H. Seo, S.M. Choi, H.J. Kim, J.H. Kim, B.K. Cho, W.B. Kim, J. Power Source 179 (2007) 81. [4] M.S. Saha, R. Li, X. Sun, Electrochem. Commun. 9 (2007) 2229. [5] H. Liu, C. Song, L. Zhang, J. Zhang, H. Wang, D.P. Wilkinson, J. Power Source 155 (2006) 95. [6] S.M. Choi, J.H. Kim, J.Y. Jung, E.Y. Yoon, W.B. Kim, Electrochim. Acta 53 (2008) 5804. [7] S.V. Kuchibhattla, A.S. Karakoti, D. Bera, S. Seal, Prog. Mater. Sci. 52 (2007) 699. [8] D. Li, Y. Xia, Nano Lett. 3 (2003) 555. [9] M. Bognitzki, M. Becker, M. Graeser, W. Massa, J.H. Wendorff, A. Schaper, D. Weber, A. Beyer, A. Golzhauser, A. Greiner, Adv. Mater. 18 (2006) 2384. [10] J.-H. Choi, K.-W. Park, In.-S. Park, W.-H. Nam, Y.-E. Sung, Electrochim. Acta 50 (2004) 787. [11] B. Gurau, R. Viswanthan, R. Liu, T.J. Lafenz, E. Reddington, A. Sapienza, B.C. Chan, T.E. Mallouk, S. Sarangapani, J. Phys. Chem. B 102 (1998) 9997. [12] T. Frelink, W. Visscher, J.A.R. Veen, Surf. Sci. 335 (1995) 353. [13] J.H. Kim, S.M. Choi, S.H. Nam, M.H. Seo, S.H. Choi, W.B. Kim, Appl. Catal. B: Environ. 82 (2008) 89. [14] M.M.P. Janssen, J. Moolhuysen, Electrochim. Acta 21 (1976) 861. [15] R.R. Adzic, J.X. Wang, B.M. Ocko, J. Mcbreen, Handbook of Fuel Cells, John Wiley & Sons, England, 2003.
Acknowledgements This work was supported by Korea Research Foundation Grant funded by the Korean Government (KRF-2007-313-D00148), the Korea Energy Management Corp. through New & Renewable Energy R&D program (2005-N-FC03-P-01-0-000) and Program for Integrated Molecular System (PIMS/GIST).
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Electrospun bimetallic nanowires of PtRh and PtRu with compositional variation for methanol electrooxidation Yong Seok Kim a, Sang Hoon Nam a, Hee-Sang Shim a, Hyo-Jin Ahn b, Manish Anand a, Won Bae Kim a,* a b
Department of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, South Korea Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany
a r t i c l e
i n f o
Article history: Received 9 April 2008 Received in revised form 30 April 2008 Accepted 2 May 2008 Available online 6 May 2008 Keywords: Bimetallic nanowires Electrospinning Electrocatalysts PtRh PtRu Methanol electrooxidation
a b s t r a c t Binary metallic nanowires (NWs) of PtRh and PtRu were synthesized by electrospinning method with compositional variation from 1:3 to 2:1. The electrospun bimetallic NWs were highly alloyed with diameters smaller than 60 nm and lengths up to hundreds of micrometers. The PtRh and PtRu NWs with 1:1 atomic ratio resulted in the higher catalytic mass activity over the methanol electrooxidation than those with the different atomic ratios, and the mass activity of Pt1Ru1 NWs was superior to the other NWs and even better than the commercial catalyst of the highly dispersed Pt1Ru1 nanoparticles on carbon. Moreover, the bimetallic NW electrocatalysts showed the better stability than the bimetallic nanoparticles. The enhancements of electrocatalytic properties for the Pt1Rh1 and Pt1Ru1 NWs could be attributed to their one-dimensional features, which can outperform on the electro-oxidations over the fuel cell electrodes. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Direct methanol fuel cells (DMFCs) have been widely studied as an alternative power source for portable devices due to high energy density and conversion efficiency of methanol fuel [1]. However, some drawbacks like sluggish oxidation reactions even over the Pt-based anode catalysts, and high price with limited supply of Pt should be overcome to make it commercial. To solve these problems in DMFC, modifying catalyst structure into one-dimensionally nanostructured materials such as nanowires (NWs) [2], inserting various elements as active materials [3] or support materials [4] into the Pt-based electrocatalysts and changing synthesis methods of catalysts [5] have been developed until now. Among them, the nanowire electrocatalysts may provide another elegant strategy by facilitating electron transport and reducing embedded electrocatalyst on electrodes [6], increasing possibility to improve the performance of DMFC. In order to prepare nanowire electrocatalysts, template-based methods have been largely employed to synthesize uniform metallic NWs [7]. However, the hard template-based NWs have some limitations for the electrocatalyst applications. For instances, it is difficult to make template due to complex process with high cost. Moreover, although the NWs for the electrocatalysts should be produced in a large quantity enough to be used in electrode, the
* Corresponding author. Tel.: +82 62 970 2317; fax: +82 62 970 2304. E-mail address: [email protected] (W.B. Kim). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.05.003
template-based method may be inadequate to be applied for the fuel cells because of small sizes of templates. To circumvent such restrictions of hard template-based NWs synthesis, electrospinning method can be employed with its versatility and effectiveness to synthesize various metal oxides or pure metallic NWs, as demonstrated in the preparation of TiO2 or Cu nanowires [8,9]. In this work, bimetallic PtRh and PtRu NWs through the electrospinning method are synthesized with compositional variation for the first time to examine the optimum atomic ratio of the bimetallic nanowire electrocatalysts. We investigate the electrocatalytic properties of the electrospun bimetallic NWs by using cyclic voltammetry and chronoamperometric technique, and their mass activities per gram of Pt are compared with the highly dispersed PtRu nanoparticle on carbon (Pt1Ru1/C, E-TEK) that is a conventional catalyst for methanol electrooxidation. 2. Experimental For PtRh and PtRu NWs preparation, the precursor solutions in DI water that consist of H2PtCl6 6H2O (Aldrich) and RhCl3 xH2O (Aldrich) or RuCl3 xH2O (Aldrich) were mixed with poly(vinyl pyrrolidone) (PVP, Aldrich, Mw = 1,300,000 g/mol) dissolved in ethanol. The pre-calculated amounts of metal precursors according to compositional variation were controlled to be 3 wt% and the amount of PVP was kept to be 4 wt% of the total weight of electrospinning solution. The feeding rate of the precursor solution was controlled to 0.05 ml/h and a Si collector was vertically positioned
Y.S. Kim et al. / Electrochemistry Communications 10 (2008) 1016–1019
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at 6 cm apart from the needle under a fixed potential of 6 kV to collect metal precursor/PVP composite nanofibers. The PVP was selectively removed by calcination in the air at 300 °C for 3 h. To obtain the alloyed bimetallic nanowires, the reduction treatment was subsequently performed at 100 °C under H2 at a flow rate of 60 sccm. The morphology and diameter of the PtRh and PtRu NWs were investigated by field emission scanning electron microscopy (FESEM, Hitach S-4700) and by high resolution transmission electron microscopy (HRTEM, TECNAI-F20) operated at 200 kV in Korea Basic Science Institute. X-ray diffraction (XRD, Rigaku Rotalflex RU200B) equipped with a Cu Ka source (k = 1.5405 Å) was carried out to obtain diffraction patterns of the electrospun bimetallic NWs. X-ray absorption measurements were conducted on 3C1 and 7C1 beamlines of Pohang Accelerator Laboratory in Korea. Electrochemical measurements of all samples were conducted by using a three-electrode system. Pt wire and Ag/AgCl (in 3 M KCl) were used as the counter and reference electrodes and the glassy carbon (0.07 cm2) was employed as the working electrode (WE). Catalyst inks made with catalyst and Nafion in 9:1 weight ratio were loaded onto the WE. Electrochemical experiments were carried out by using Autolab PGSTAT30 Potentiostat/Galvanostat (Eco Chemi.). To investigate the electrocatalytic activities and stabilities of the PtRh and PtRu NWs for the methanol electrooxidation, cyclic voltammetry test was performed in the potential range from 0.2 to 1.0 V at a scan rate of 50 mV/s and chronoamperometry test was carried out at a constant potential of 0.6 V in electrolyte solution containing methanol for 6000 s.
3. Results and discussion Fig. 1 shows FESEM images of as-spun metal precursor/PVP composite nanofibers and bimetallic NWs. The as-spun nanofibers of metal precursors/PVP composite have average diameter of ca. 250 ± 50 nm as shown in Fig. 1a and b. After calcination at 300 °C for 3 h in air, subsequent reduction process at 100 °C for 3 h under H2 atmosphere was performed to produce the reduced bimetallic nanowires, resulting that the diameters of PtRh and PtRu NWs became smaller with the average size of ca. 50 ± 10 nm as shown in Fig. 1c and d. The SEM figures in Fig. 1 represents the nanowire morphology on a Si substrate via electrospinning. Note that the diameters of PtRh and PtRu NWs were decreased to 1/5th of the as-spun nanofibers. Fig. 1e and f present the HRTEM images of electrospun of Pt1Rh1 and Pt1Ru1 NWs, indicating that they seem to be composed of small nanoparticles with 2–3 nm size. The inset figures in Fig. 1e and f shows the crystalline (1 1 1) planes for Pt1Rh1 and Pt1Ru1 NWs with planer distances of ca. 2.17 Å, which is significantly smaller than that of the pure Pt with 2.26 Å, suggesting that they were alloyed by incorporating Rh or Ru to the Pt, as will be further investigated by XRD analysis. In Fig. 2, the electrospun bimetallic PtRh and PtRu NWs with respect to their compositional variation of Pt:2nd metal from 3:1 to 1:2 were characterized by XRD. The XRD peaks are generally shifted to the higher angles with Rh incorporation into the fcc Pt phase [10]. This feature is also observed over the electrospun bimetallic PtRh NWs in this work, as seen in the Fig. 2a by showing that the deviation of the diffraction angles for the PtRh NWs becomes greater with Rh amount. We could not find any other phases such as phase-separated Pt or Rh phase in the tested Pt:Rh ratios, except the single solution phase of alloyed PtRh, implying that the phase separation might not occur in PtRh NWs system under our preparation condition. For the bimetallic PtRu NWs, the structural evolution takes place similarly to the PtRh cases except for the appearance of phase-separated Ru metal in the highest Ru content, which can be understood by that the Ru has hexagonal close-
Fig. 1. FESEM images of as-spun (a) Pt–Rh precursor/PVP, (b) Pt–Ru precursor/PVP and bimetallic nanowires of (c) Pt1Rh1 and (d) Pt1Ru1; insets of (c) and (d) show the diameter of bimetallic NWs. HRTEM images of (e) Pt1Rh1 NWs and (f) Pt1Ru1 NWs; insets of (e) and (f) are inverse fast fourier transform image of Pt1Rh1 NWs and Pt1Ru1 NWs.
packed structure, thereby a phase separation of Ru metal might be involved in the 1:2 Pt:Ru ratio due to the structural mismatch that did not make solid solution phase with Pt in our bimetallic PtRu NWs. Fig. 3a and b shows CVs of the electrospun bimetallic PtRh and PtRu NWs for the methanol electrooxidation (CH3OH + H2O ? 6H+ + 6e + CO2). In both cases of PtRh and PtRu bimetallic NWs, the highest Pt-based mass activities were observed at the 1:1 atomic ratio, with activity order of 1:1 > 2:1 > 3:1 > 1:2. The enhancements of mass activity up to 1:1 atomic ratio could be explained with bifunctional mechanism and ligand (electronic) effect, which have been well postulated in the Pt-based nanoparticle catalysts [11– 13]. The OH species could be more readily generated over the 2nd metal phase at the low potentials according to the bifunctional mechanism in methanol electrooxidation using Pt-based bimetallic catalysts. Also, the ligand effect might contribute to the enhanced activity as the 2nd metals such as Rh, Ru and Sn modify electron density of Pt d-orbital [13–15]. Although the X-ray absorption near edge structure (XANES) data are not included in this work, Pt LIII edge of the bimetallic nanowires was obtained to investigate the electronic structure and oxidation state. Since the absorption at the Pt LIII edge corresponds to 2p ? 5d electronic transition [14,15], the intensities of the white line for the bimetallic nanowires
Y.S. Kim et al. / Electrochemistry Communications 10 (2008) 1016–1019
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50
i (mA/mgPt)
were reduced by at least 4–5% as compared to that of the pure Pt phase, which could be rationalized by charge transfer from Rh or Ru to Pt. In order to evaluate the electrocatalytic performance of our bimetallic NWs, their activities were compared with the highly dispersed Pt1Ru1 nanoparticles on carbon (Pt1Ru1/C, E-TEK). Interestingly, unsupported NWs have the mass activities of 355.8 mA/mgPt for the Pt1Rh1 NWs and 449.1 mA/mgPt for Pt1Ru1 NWs, which are comparable or even better by ca. 30% as compared to the commercial catalyst of Pt1Ru1/C with 347.2 mA/mgPt. The enhancements may be attributed to their one-dimensional features in the electrochemical electrodes, as demonstrated in Ref. [6] using single Pt nanowires. The stability test over the electrospun bimetallic NWs was performed and compared with the catalyst of Pt1Ru1/C through chronoamperometry measurement in 2 M CH3OH + 0.5 M H2SO4 solution at a constant potential of 0.6 V (vs. Ag/AgCl) for 6000 s as shown in Fig. 3c. After the initial transient period that shows rapid decreases in the potentiostatic currents, the currents were gradually declined for the Pt1Rh1 NWs or reached to a steady-state for the Pt1Ru1 NWs and Pt1Ru1/C. Comparing Pt1Ru1 NWs and Pt1Ru1/C, since the stabilized currents with nanowire electrocatalysts were much higher than that of nanoparticle by at least four times, it can be inferred that the nanowire morphology as electrocatalysts would be more stable than nanoparticles. Consequently, the PtRh and PtRu NWs with 1:1 atomic ratio showed the higher catalytic mass activity with the better stability
Pt1Ru1 NWs Pt1Rh1 NWs Pt1Ru1/C
40 10
0
0
1000
2000
3000
4000
5000
6000
Time (s) Fig. 3. The cyclic voltammograms of electrospun bimetallic nanowires of (a) PtRh and (b) PtRu with compositional variation in 2 M CH3OH containing 0.5 M H2SO4 solution at a scan rate of 50 mV/s and (c) chronoamperometric curves of Pt1Rh1 NWs and Pt1Ru1 NWs compared with Pt1Ru1/C (E-TEK) in 2 M CH3OH containing 0.5 M H2SO4 solution for 6000 s.
over the methanol electrooxidation than those with the different atomic ratios, and the mass activity of Pt1Ru1 NWs was superior to the other catalysts tested and even better than the conventional catalyst of the highly dispersed Pt1Ru1 nanoparticles on carbon.
Y.S. Kim et al. / Electrochemistry Communications 10 (2008) 1016–1019
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4. Conclusion
References
The bimetallic PtRh and PtRu nanowires of ca. 50 nm diameter fabricated by electrospinning method with compositional variation were used as the nanowire electrocatalysts for the methanol electrooxidation, in which the Pt1Rh1 and Pt1Ru1 nanowires resulted in the highest mass activity. The mass activity of electrospun Pt1Ru1 nanowires was significantly improved by ca. 30% with the better stability than those of Pt1Ru1/C. These enhancements could be attributed to efficient charge transport and better physical and interfacial properties arising from one-dimensional feature compared with supported nanoparticle catalysts.
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Acknowledgements This work was supported by Korea Research Foundation Grant funded by the Korean Government (KRF-2007-313-D00148), the Korea Energy Management Corp. through New & Renewable Energy R&D program (2005-N-FC03-P-01-0-000) and Program for Integrated Molecular System (PIMS/GIST).