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Core–shell nanostructure supported Pt catalyst with improved electrocatalytic stability in oxygen reduction reaction
Materials Chemistry and Physics 137 (2013) 704e708
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Coreeshell nanostructure supported Pt catalyst with improved electrocatalytic stability in oxygen reduction reaction Do-Young Kim, Sang-Beom Han, Young-Woo Lee, Kyung-Won Park* Department of Chemical Engineering, Soongsil University, Seoul 156-743, South Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< We synthesize coreeshell nanostructure support (TiO2@C) for oxygen reduction reaction. < The Pt/TiO2@C shows an excellent ORR activity in comparison with Pt/ C. < The Pt catalyst on TiO2@C exhibits a superior ORR stability to Pt/C.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 July 2012 Received in revised form 29 October 2012 Accepted 1 November 2012
Coreeshell nanostructure electrode (TiO2@C) for oxygen reduction reaction is prepared with TiO2 nanoparticles at 900 C in a methane atmosphere. The TiO2@C supported Pt catalyst (Pt/TiO2@C) contains Pt nanoparticles on TiO2@C nanostructure electrodes consisting of TiO2 as a core and carbon as a shell. In the accelerated stability test, the Pt/TiO2@C exhibits a superior ORR stability to conventional carbon supported Pt catalyst. It is likely that the enhanced catalytic properties of the nanostructure supported Pt catalyst may be due to graphite-like carbon and an improved electronic conductivity of the coreeshell nanostructure. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Composite materials Nanostructures Electrochemical properties Electrochemical techniques
1. Introduction Oxygen reduction reaction (ORR) for polymer electrolyte membrane fuel cells has been of considerable interest due to critical problems such as kinetic limitation for oxygen diffusion rate and long-term stability of oxygen reduction [1e4]. In general, in order to enhance ORR activity and stability of Pt-based catalysts, there have been many efforts to manipulate structure (such as
* Corresponding author. Tel.: þ82 2 820 0613; fax: þ82 2 812 5378. E-mail address: [email protected] (K.-W. Park). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.11.006
coreeshell, alloy, and mixed structure) and shape of the catalysts during the synthesis process of metallic nanoparticles [5e7]. Furthermore, it has been reported that the addition of transition metal oxides to carbon support is an effective way to promote the electrocatalytic properties of Pt catalysts for ORR [8,9]. Transition metal oxides such as SnO2, WO3, RuO2, and TiO2 have been well-known as supporting materials to enhance the electrocatalytic activity and stability of the catalyst [10e14]. In particular, TiO2 has been proposed as an excellent candidate for the supporting material because of its chemical stability, cheapness, and nontoxicity [15e19]. However, many efforts have been made to improve the conductivity of TiO2 by adding 2nd materials as dopants or electronically conductive materials [20].
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Fig. 1. (a) TEM image of TiO2@C NPs [The inset indicates the HR-TEM image of TiO2@C.]. (b) TGA curves of TiO2@C and carbon black (Vulcan XC-72R). (c) XRD patterns of TiO2, TiO2@C, and Pt/TiO2@C.
Herein, we prepared the nanostructure support consisting of TiO2 as a core and carbon as a shell (TiO2@C) for ORR. The particle size and crystal structure of the supported catalysts were characterized using X-ray diffraction (XRD) method and transmission electron microscopy (TEM). The electrocatalytic properties of the TiO2@C supported Pt catalyst (Pt/TiO2@C) were characterized using a potentiostat and compared with those of conventional carbon supported Pt catalyst (Pt/C).
2. Experimental The TiO2@C nanostructure support was prepared by means of heat treatment under methane gas atmosphere with TiO2 as a starting material. The TiO2 powders (Degussa, P-25, 0.2 g) were put into a quartz tube system under the flow of methane gas. Firstly, the flow rate of N2 gas (99.99%) was kept for 15 min to get rid of O2 inside the tube. Under CH4 flow rate of 50 mL min1, the furnace
Fig. 2. (a) TEM image, (b) HR-TEM image, and (c) size histogram of Pt nanoparticles of as-prepared Pt/TiO2@C. (d) TEM image, (e) HR-TEM image, and (f) size histogram of Pt nanoparticles of Pt/C.
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D.-Y. Kim et al. / Materials Chemistry and Physics 137 (2013) 704e708
was heated at 900 C for 3 h and then cooled down to 25 C under methane atmosphere. The Pt (20 wt%)/TiO2@C was prepared by means of polyol process using ethylene glycol (EG) as both reductant and solvent. The 3 mM H2PtCl6 (Sigma Aldrich) and 5 mM NaOH (Sigma Aldrich) were dissolved in 50 mL of EG (Sigma Aldrich) containing TiO2@C powder (117 mg). The solution was kept at 150 C for 1 h until Pt salt was completely reduced by EG. The resulting precipitate was cooled at room temperature and washed with water and ethanol several times. The Pt/TiO2@C was dried at 80 C oven for 12 h. Structural analysis of the samples was carried out using a XRD (Rigaku Co.) equipped with a Cu Ka radiation source of l ¼ 0.15,418 nm with a Ni filter. The tube current was 100 mA with a tube voltage of 40 kV. The 2q between 20 and 80 was explored at a scan rate of 4 min1. The supported catalysts were characterized by TEM using a Philips CM20T/STEM system at 200 kV. TEM samples were prepared by placing a drop of the catalyst suspension with ethanol on a carbon-coated copper grid. Thermogravimetric analysis (TGA) curves were obtained in a thermal analyzer (SDT Q-600, TA Instruments) in the range of 25e900 C at a heating rate of
10 C min1 in air flow of 60 cm3 min1. The electrical resistance of the compressed pellets was measured in current vs. voltage curves using potentiaostat (CH Instrument, CHI 700C) at ambient temperature. Also, the conductivity was calculated by the follow equation:
s¼
1
r
¼
l RA
where s is electrical conductivity (Siemens, S cm1), r is specific resistance, l is thickness of pellet, R is resistance, and A is area of pellet. Electrochemical properties were obtained using a typical three electrode cell. A Pt wire and Ag/AgCl (in saturated KCl) were used as a counter and reference electrode, respectively. The glassy carbon electrode as a working electrode was polished with 1, 0.3, and 0.05 mm Al2O3 paste and then washed in deionized water. The catalyst inks were prepared by ultrasonically dispersing catalyst powders in an appropriate amount of Millipore water. The catalyst was coated onto a glassy carbon working electrode with 0.7 mL of the ink. After drying in 50 C oven, the total loading of Pt catalyst
Fig. 3. CVs of (a) Pt/C and (b) Pt/TiO2@C in Ar-saturated 0.1 M HClO4 at 25 C with a scan rate of 50 mV s1. ORR polarization curves of (c) Pt/C and (d) Pt/TiO2@C before and after the accelerated stability test with a scan rate of 5 mV s1 and rotating speed of 1600 rpm in O2-saturated 0.1 M HClO4 at 25 C. (e) Normalized electrochemical active surface area versus acceleration time of supported catalysts. (f) Normalized ORR current density versus acceleration time of supported catalysts.
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Fig. 4. (a) TEM image, (b) HR-TEM image, and (c) size histogram of Pt nanoparticles of as-prepared Pt/TiO2@C after the accelerated stability test. (d) TEM image, (e) HR-TEM image, and (f) size histogram of Pt nanoparticles of Pt/C after the accelerated stability test.
was 40 mgmetal cm2. To characterize ORR properties of the catalysts, voltammetry was carried out using a potentiostat (Eco Chemie, AUTOLAB). The accelerated stability test was performed by applying the positive potential of 1.2 V in O2-saturated 0.1 M HClO4 at 25 C [21]. All potentials were reported with respect to Ag/AgCl. 3. Results and discussion As shown in HR-TEM image of Fig. 1(a), the d-spacings of the core and shell correspond to 0.327 nm of rutile TiO2 and 0.342 nm of graphitic layers, respectively. It is evident that TiO2@C is a coree shell nanostructure consisting of TiO2 as a core coated by carbon as a shell. The normalized mass ratio of TiO2 to C in TiO2@C is 60:40 as indicated in TGA data (Fig. 1(b)). The measured thickness of the shell in TiO2@C is w11 nm. Furthermore, the electrical conductivity of TiO2@C is 1.23 S cm1 in comparison with TiO2 (2.89 108 S cm1) and carbon black (0.98 S cm1). Fig. 1(c) shows XRD patterns of TiO2 as a starting material, TiO2@C support prepared at 900 C in CH4 atmosphere for 3 h, and Pt/TiO2@C. The TiO2 as a starting material consists of dominant anatase and rutile phase. In contrast, the TiO2@C support displays dominant rutile phase of TiO2, which means phase transformation of anatase into rutile phase. Furthermore, the XRD peak at 2q ¼ 25.5 of the TiO2@C support represents the characteristic peak of graphite-like carbon, which is similar to the (002) in typical graphite. The XRD pattern of Pt/TiO2@C contains diffraction peaks of both TiO2@C support and Pt catalyst corresponding to a typical face-centered-cubic crystal structure. The Pt nanoparticles with average particle size of 3.97 nm were well dispersed on TiO2@C (Fig. 2(a)e(c)) with the average size of w140 nm. In the case of Pt/TiO2@C, the Pt nanoparticles with dominant {111} facets are well deposited on the coreeshell nanostructure supports in comparison with Pt(20 wt%)/C (ETEK Co.) having Pt nanoparticles with an average size of 3.57 nm on the carbon black (Fig. 2(d)e(f)). To identify electrochemical properties of the as-prepared catalysts, cyclic voltammograms (CVs) were obtained in 0.1 M HClO4 between 0.2 and þ1.0 V with a scan rate of 50 mV s1 (Fig. 3(a) and
(b)). The enhanced electrochemical properties for the coreeshell nanostructure supported catalyst may be due to an increased electronic conductivity and stability in acid solution of the nanostructure support and its interaction with catalysts. The ORR activity of Pt/TiO2@C was measured using linear sweep voltammetry in O2saturated HClO4 solution in comparison with Pt/C. The initial current densities of Pt/TiO2@C and Pt/C at 0.55 V are 0.68 and 1.99 mA cm2, respectively (Fig. 3(c) and (d)). The accelerated stability test of the ORR was performed by applying positive potential of 1.2 V for 30 or 60 min in O2-saturated 0.1 M HClO4 at 25 C [22,23]. After the stability test for 60 min, the ORR current densities of Pt/TiO2@C and Pt/C at 0.55 V are 0.49 and 0.96 mA cm2, respectively. As shown in Fig. 3(e), Pt/TiO2@C exhibits losses of 1.9 and 4.3% in electrochemical surface area (EASA) after the stability test for 30 and 60 min, respectively, whereas Pt/C exhibits considerable losses of 4.4 and 7.1% in EASA after the stability test for 30 and 60 min, respectively. Furthermore, Pt/TiO2@C exhibits losses of 16 and 28% in ORR current density at 0.55 V after the stability test for 30 and 60 min, respectively, whereas Pt/C exhibits considerable losses of 41and 52% in ORR current density at 0.55 V after the stability test for 30 and 60 min, respectively (Fig. 3(f)). As indicated in Fig. 4, Pt/TiO2@C displays an average size of 4.83 nm with increase of 22% after the stability test representing an improved electrocatalytic activity. In contrast, after the stability test, Pt/C exhibits an average size of 4.72 nm with increase of 32% resulting in deteriorated electrocatalytic activity. Thus, the electrochemical stability of the Pt/TiO2@C may be attributed to corrosion resistance of the TiO2@C compared to the Vulcan XC-72R. 4. Conclusions The TiO2@C nanostructure support with TiO2 core and carbon shell has been synthesized for improved ORR properties. The Pt/ TiO2@C exhibits slight losses in ORR current after the accelerated stability test as compared to the Pt/C. This represents that the ORR stability of Pt/TiO2@C is superior ORR stability to conventional carbon supported Pt catalyst. The enhanced catalytic properties of
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the nanostructure supported Pt catalyst may be due to an improved electronic conductivity by graphite-like carbon in the coreeshell nanostructure. Acknowledgments This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF-20110030335). References [1] T. Yajima, H. Uchida, M. Watanabe, J. Phys. Chem. B 108 (2004) 2654. [2] D. Wang, H.L. Xin, Y. Yu, H. Wang, E. Rus, D.A. Muller, H.D. Abruña, J. Am. Chem. Soc. 132 (2010) 17664. [3] A.R. Malheiro, J. Perez, H.M. Villullas, J. Power Sources 195 (2010) 7255. [4] Q. He, S. Mukerjee, R. Zeis, S. Parres-Esclapez, M.J. Illán-Gómez, A. BuenoLópez, Appl. Catal. A Gen. 381 (2010) 54. [5] G.E. Ramirez-Caballero, P. Hirunsit, P.B. Balbuena, J. Chem. Phys. 133 (2010) 134705. [6] N.V. Long, M. Ohtaki, T.D. Hien, J. Randy, M. Nogami, Electrochim. Acta 56 (2011) 9133.
[7] N.V. Long, T.D. Hien, T. Asaka, M. Ohtaki, M. Nogami, Int. J. Hydrog. Energy 36 (2011) 8478. [8] H. Chhina, S. Campbell, O. Kesler, J. Power Sources 179 (2008) 50. [9] R. Kou, Y. Shao, D. Mei, Z. Nie, D. Wang, C. Wang, V.V. Viswanathan, S. Park, I.A. Aksay, Y. Lin, Y. Wang, J. Liu, J. Am. Chem. Soc. 133 (2011) 2541. [10] H.L. Pang, X.H. Zhang, X.X. Zhong, B. Liu, X.G. Wei, Y.F. Kuang, J.H. Chen, J. Colloid Interface Sci. 319 (2008) 193. [11] S. Jayaraman, T.F. Jaramillo, S.-.H. Baeck, E.W. McFarland, J. Phys. Chem. B 109 (2005) 22958. [12] C.L. Campos, C. Roldán, M. Aponte, Y. Ishikawa, C.R. Cabrera, J. Electroanal. Chem. 581 (2005) 206. [13] K.-W. Park, B.-K. Kwon, J.-H. Choi, I.-S. Park, Y.-M. Kim, Y.-E. Sung, J. Power Sources 109 (2002) 439. [14] S.V. Kraemer, K. Wikander, G. Lindbergh, A. Lundblad, A.E.C. Palmqvist, J. Power Sources 180 (2008) 185. [15] L. Xiong, A. Manthiram, Electrochim. Acta 49 (2004) 4163. [16] D.-S. Kim, E.F.A. Zeid, Y.-T. Kim, Electrochim. Acta 55 (2010) 3628. [17] K. Drew, G. Girishkumar, K. Vinodgopal, P.V. Kamat, J. Phys. Chem. B 109 (2005) 11851. [18] M. Hepel, I. Kumarihamy, C.J. Zhong, Electrochem. Commun. 8 (2006) 1439. [19] S. Beak, D. Jung, K.S. Nahm, P. Kim, Catal. Lett. 134 (2010) 288. [20] M.D. Koninck, P. Manseau, B. Marsan, J. Electroanal. Chem. 611 (2007) 67. [21] J.-M. Lee, S.-B. Han, Y.-J. Song, J.-Y. Kim, K.-W. Park, J. Alloys Compd. 506 (2010) 57. [22] S. Zhang, Y. Shao, G. Yin, Y. Lin, Appl. Catal. B Environ. 102 (2011) 372. [23] S.-H. Liu, F.-S. Zheng, J.-R. Wu, Appl. Catal. B Environ. 108e109 (2011) 81.
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Coreeshell nanostructure supported Pt catalyst with improved electrocatalytic stability in oxygen reduction reaction Do-Young Kim, Sang-Beom Han, Young-Woo Lee, Kyung-Won Park* Department of Chemical Engineering, Soongsil University, Seoul 156-743, South Korea
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< We synthesize coreeshell nanostructure support (TiO2@C) for oxygen reduction reaction. < The Pt/TiO2@C shows an excellent ORR activity in comparison with Pt/ C. < The Pt catalyst on TiO2@C exhibits a superior ORR stability to Pt/C.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 July 2012 Received in revised form 29 October 2012 Accepted 1 November 2012
Coreeshell nanostructure electrode (TiO2@C) for oxygen reduction reaction is prepared with TiO2 nanoparticles at 900 C in a methane atmosphere. The TiO2@C supported Pt catalyst (Pt/TiO2@C) contains Pt nanoparticles on TiO2@C nanostructure electrodes consisting of TiO2 as a core and carbon as a shell. In the accelerated stability test, the Pt/TiO2@C exhibits a superior ORR stability to conventional carbon supported Pt catalyst. It is likely that the enhanced catalytic properties of the nanostructure supported Pt catalyst may be due to graphite-like carbon and an improved electronic conductivity of the coreeshell nanostructure. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Composite materials Nanostructures Electrochemical properties Electrochemical techniques
1. Introduction Oxygen reduction reaction (ORR) for polymer electrolyte membrane fuel cells has been of considerable interest due to critical problems such as kinetic limitation for oxygen diffusion rate and long-term stability of oxygen reduction [1e4]. In general, in order to enhance ORR activity and stability of Pt-based catalysts, there have been many efforts to manipulate structure (such as
* Corresponding author. Tel.: þ82 2 820 0613; fax: þ82 2 812 5378. E-mail address: [email protected] (K.-W. Park). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.11.006
coreeshell, alloy, and mixed structure) and shape of the catalysts during the synthesis process of metallic nanoparticles [5e7]. Furthermore, it has been reported that the addition of transition metal oxides to carbon support is an effective way to promote the electrocatalytic properties of Pt catalysts for ORR [8,9]. Transition metal oxides such as SnO2, WO3, RuO2, and TiO2 have been well-known as supporting materials to enhance the electrocatalytic activity and stability of the catalyst [10e14]. In particular, TiO2 has been proposed as an excellent candidate for the supporting material because of its chemical stability, cheapness, and nontoxicity [15e19]. However, many efforts have been made to improve the conductivity of TiO2 by adding 2nd materials as dopants or electronically conductive materials [20].
D.-Y. Kim et al. / Materials Chemistry and Physics 137 (2013) 704e708
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Fig. 1. (a) TEM image of TiO2@C NPs [The inset indicates the HR-TEM image of TiO2@C.]. (b) TGA curves of TiO2@C and carbon black (Vulcan XC-72R). (c) XRD patterns of TiO2, TiO2@C, and Pt/TiO2@C.
Herein, we prepared the nanostructure support consisting of TiO2 as a core and carbon as a shell (TiO2@C) for ORR. The particle size and crystal structure of the supported catalysts were characterized using X-ray diffraction (XRD) method and transmission electron microscopy (TEM). The electrocatalytic properties of the TiO2@C supported Pt catalyst (Pt/TiO2@C) were characterized using a potentiostat and compared with those of conventional carbon supported Pt catalyst (Pt/C).
2. Experimental The TiO2@C nanostructure support was prepared by means of heat treatment under methane gas atmosphere with TiO2 as a starting material. The TiO2 powders (Degussa, P-25, 0.2 g) were put into a quartz tube system under the flow of methane gas. Firstly, the flow rate of N2 gas (99.99%) was kept for 15 min to get rid of O2 inside the tube. Under CH4 flow rate of 50 mL min1, the furnace
Fig. 2. (a) TEM image, (b) HR-TEM image, and (c) size histogram of Pt nanoparticles of as-prepared Pt/TiO2@C. (d) TEM image, (e) HR-TEM image, and (f) size histogram of Pt nanoparticles of Pt/C.
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D.-Y. Kim et al. / Materials Chemistry and Physics 137 (2013) 704e708
was heated at 900 C for 3 h and then cooled down to 25 C under methane atmosphere. The Pt (20 wt%)/TiO2@C was prepared by means of polyol process using ethylene glycol (EG) as both reductant and solvent. The 3 mM H2PtCl6 (Sigma Aldrich) and 5 mM NaOH (Sigma Aldrich) were dissolved in 50 mL of EG (Sigma Aldrich) containing TiO2@C powder (117 mg). The solution was kept at 150 C for 1 h until Pt salt was completely reduced by EG. The resulting precipitate was cooled at room temperature and washed with water and ethanol several times. The Pt/TiO2@C was dried at 80 C oven for 12 h. Structural analysis of the samples was carried out using a XRD (Rigaku Co.) equipped with a Cu Ka radiation source of l ¼ 0.15,418 nm with a Ni filter. The tube current was 100 mA with a tube voltage of 40 kV. The 2q between 20 and 80 was explored at a scan rate of 4 min1. The supported catalysts were characterized by TEM using a Philips CM20T/STEM system at 200 kV. TEM samples were prepared by placing a drop of the catalyst suspension with ethanol on a carbon-coated copper grid. Thermogravimetric analysis (TGA) curves were obtained in a thermal analyzer (SDT Q-600, TA Instruments) in the range of 25e900 C at a heating rate of
10 C min1 in air flow of 60 cm3 min1. The electrical resistance of the compressed pellets was measured in current vs. voltage curves using potentiaostat (CH Instrument, CHI 700C) at ambient temperature. Also, the conductivity was calculated by the follow equation:
s¼
1
r
¼
l RA
where s is electrical conductivity (Siemens, S cm1), r is specific resistance, l is thickness of pellet, R is resistance, and A is area of pellet. Electrochemical properties were obtained using a typical three electrode cell. A Pt wire and Ag/AgCl (in saturated KCl) were used as a counter and reference electrode, respectively. The glassy carbon electrode as a working electrode was polished with 1, 0.3, and 0.05 mm Al2O3 paste and then washed in deionized water. The catalyst inks were prepared by ultrasonically dispersing catalyst powders in an appropriate amount of Millipore water. The catalyst was coated onto a glassy carbon working electrode with 0.7 mL of the ink. After drying in 50 C oven, the total loading of Pt catalyst
Fig. 3. CVs of (a) Pt/C and (b) Pt/TiO2@C in Ar-saturated 0.1 M HClO4 at 25 C with a scan rate of 50 mV s1. ORR polarization curves of (c) Pt/C and (d) Pt/TiO2@C before and after the accelerated stability test with a scan rate of 5 mV s1 and rotating speed of 1600 rpm in O2-saturated 0.1 M HClO4 at 25 C. (e) Normalized electrochemical active surface area versus acceleration time of supported catalysts. (f) Normalized ORR current density versus acceleration time of supported catalysts.
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Fig. 4. (a) TEM image, (b) HR-TEM image, and (c) size histogram of Pt nanoparticles of as-prepared Pt/TiO2@C after the accelerated stability test. (d) TEM image, (e) HR-TEM image, and (f) size histogram of Pt nanoparticles of Pt/C after the accelerated stability test.
was 40 mgmetal cm2. To characterize ORR properties of the catalysts, voltammetry was carried out using a potentiostat (Eco Chemie, AUTOLAB). The accelerated stability test was performed by applying the positive potential of 1.2 V in O2-saturated 0.1 M HClO4 at 25 C [21]. All potentials were reported with respect to Ag/AgCl. 3. Results and discussion As shown in HR-TEM image of Fig. 1(a), the d-spacings of the core and shell correspond to 0.327 nm of rutile TiO2 and 0.342 nm of graphitic layers, respectively. It is evident that TiO2@C is a coree shell nanostructure consisting of TiO2 as a core coated by carbon as a shell. The normalized mass ratio of TiO2 to C in TiO2@C is 60:40 as indicated in TGA data (Fig. 1(b)). The measured thickness of the shell in TiO2@C is w11 nm. Furthermore, the electrical conductivity of TiO2@C is 1.23 S cm1 in comparison with TiO2 (2.89 108 S cm1) and carbon black (0.98 S cm1). Fig. 1(c) shows XRD patterns of TiO2 as a starting material, TiO2@C support prepared at 900 C in CH4 atmosphere for 3 h, and Pt/TiO2@C. The TiO2 as a starting material consists of dominant anatase and rutile phase. In contrast, the TiO2@C support displays dominant rutile phase of TiO2, which means phase transformation of anatase into rutile phase. Furthermore, the XRD peak at 2q ¼ 25.5 of the TiO2@C support represents the characteristic peak of graphite-like carbon, which is similar to the (002) in typical graphite. The XRD pattern of Pt/TiO2@C contains diffraction peaks of both TiO2@C support and Pt catalyst corresponding to a typical face-centered-cubic crystal structure. The Pt nanoparticles with average particle size of 3.97 nm were well dispersed on TiO2@C (Fig. 2(a)e(c)) with the average size of w140 nm. In the case of Pt/TiO2@C, the Pt nanoparticles with dominant {111} facets are well deposited on the coreeshell nanostructure supports in comparison with Pt(20 wt%)/C (ETEK Co.) having Pt nanoparticles with an average size of 3.57 nm on the carbon black (Fig. 2(d)e(f)). To identify electrochemical properties of the as-prepared catalysts, cyclic voltammograms (CVs) were obtained in 0.1 M HClO4 between 0.2 and þ1.0 V with a scan rate of 50 mV s1 (Fig. 3(a) and
(b)). The enhanced electrochemical properties for the coreeshell nanostructure supported catalyst may be due to an increased electronic conductivity and stability in acid solution of the nanostructure support and its interaction with catalysts. The ORR activity of Pt/TiO2@C was measured using linear sweep voltammetry in O2saturated HClO4 solution in comparison with Pt/C. The initial current densities of Pt/TiO2@C and Pt/C at 0.55 V are 0.68 and 1.99 mA cm2, respectively (Fig. 3(c) and (d)). The accelerated stability test of the ORR was performed by applying positive potential of 1.2 V for 30 or 60 min in O2-saturated 0.1 M HClO4 at 25 C [22,23]. After the stability test for 60 min, the ORR current densities of Pt/TiO2@C and Pt/C at 0.55 V are 0.49 and 0.96 mA cm2, respectively. As shown in Fig. 3(e), Pt/TiO2@C exhibits losses of 1.9 and 4.3% in electrochemical surface area (EASA) after the stability test for 30 and 60 min, respectively, whereas Pt/C exhibits considerable losses of 4.4 and 7.1% in EASA after the stability test for 30 and 60 min, respectively. Furthermore, Pt/TiO2@C exhibits losses of 16 and 28% in ORR current density at 0.55 V after the stability test for 30 and 60 min, respectively, whereas Pt/C exhibits considerable losses of 41and 52% in ORR current density at 0.55 V after the stability test for 30 and 60 min, respectively (Fig. 3(f)). As indicated in Fig. 4, Pt/TiO2@C displays an average size of 4.83 nm with increase of 22% after the stability test representing an improved electrocatalytic activity. In contrast, after the stability test, Pt/C exhibits an average size of 4.72 nm with increase of 32% resulting in deteriorated electrocatalytic activity. Thus, the electrochemical stability of the Pt/TiO2@C may be attributed to corrosion resistance of the TiO2@C compared to the Vulcan XC-72R. 4. Conclusions The TiO2@C nanostructure support with TiO2 core and carbon shell has been synthesized for improved ORR properties. The Pt/ TiO2@C exhibits slight losses in ORR current after the accelerated stability test as compared to the Pt/C. This represents that the ORR stability of Pt/TiO2@C is superior ORR stability to conventional carbon supported Pt catalyst. The enhanced catalytic properties of
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the nanostructure supported Pt catalyst may be due to an improved electronic conductivity by graphite-like carbon in the coreeshell nanostructure. Acknowledgments This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF-20110030335). References [1] T. Yajima, H. Uchida, M. Watanabe, J. Phys. Chem. B 108 (2004) 2654. [2] D. Wang, H.L. Xin, Y. Yu, H. Wang, E. Rus, D.A. Muller, H.D. Abruña, J. Am. Chem. Soc. 132 (2010) 17664. [3] A.R. Malheiro, J. Perez, H.M. Villullas, J. Power Sources 195 (2010) 7255. [4] Q. He, S. Mukerjee, R. Zeis, S. Parres-Esclapez, M.J. Illán-Gómez, A. BuenoLópez, Appl. Catal. A Gen. 381 (2010) 54. [5] G.E. Ramirez-Caballero, P. Hirunsit, P.B. Balbuena, J. Chem. Phys. 133 (2010) 134705. [6] N.V. Long, M. Ohtaki, T.D. Hien, J. Randy, M. Nogami, Electrochim. Acta 56 (2011) 9133.
[7] N.V. Long, T.D. Hien, T. Asaka, M. Ohtaki, M. Nogami, Int. J. Hydrog. Energy 36 (2011) 8478. [8] H. Chhina, S. Campbell, O. Kesler, J. Power Sources 179 (2008) 50. [9] R. Kou, Y. Shao, D. Mei, Z. Nie, D. Wang, C. Wang, V.V. Viswanathan, S. Park, I.A. Aksay, Y. Lin, Y. Wang, J. Liu, J. Am. Chem. Soc. 133 (2011) 2541. [10] H.L. Pang, X.H. Zhang, X.X. Zhong, B. Liu, X.G. Wei, Y.F. Kuang, J.H. Chen, J. Colloid Interface Sci. 319 (2008) 193. [11] S. Jayaraman, T.F. Jaramillo, S.-.H. Baeck, E.W. McFarland, J. Phys. Chem. B 109 (2005) 22958. [12] C.L. Campos, C. Roldán, M. Aponte, Y. Ishikawa, C.R. Cabrera, J. Electroanal. Chem. 581 (2005) 206. [13] K.-W. Park, B.-K. Kwon, J.-H. Choi, I.-S. Park, Y.-M. Kim, Y.-E. Sung, J. Power Sources 109 (2002) 439. [14] S.V. Kraemer, K. Wikander, G. Lindbergh, A. Lundblad, A.E.C. Palmqvist, J. Power Sources 180 (2008) 185. [15] L. Xiong, A. Manthiram, Electrochim. Acta 49 (2004) 4163. [16] D.-S. Kim, E.F.A. Zeid, Y.-T. Kim, Electrochim. Acta 55 (2010) 3628. [17] K. Drew, G. Girishkumar, K. Vinodgopal, P.V. Kamat, J. Phys. Chem. B 109 (2005) 11851. [18] M. Hepel, I. Kumarihamy, C.J. Zhong, Electrochem. Commun. 8 (2006) 1439. [19] S. Beak, D. Jung, K.S. Nahm, P. Kim, Catal. Lett. 134 (2010) 288. [20] M.D. Koninck, P. Manseau, B. Marsan, J. Electroanal. Chem. 611 (2007) 67. [21] J.-M. Lee, S.-B. Han, Y.-J. Song, J.-Y. Kim, K.-W. Park, J. Alloys Compd. 506 (2010) 57. [22] S. Zhang, Y. Shao, G. Yin, Y. Lin, Appl. Catal. B Environ. 102 (2011) 372. [23] S.-H. Liu, F.-S. Zheng, J.-R. Wu, Appl. Catal. B Environ. 108e109 (2011) 81.