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Ultrasmall PtSn alloy catalyst for ethanol electro-oxidation reaction
Journal of Power Sources 275 (2015) 557e562
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Ultrasmall PtSn alloy catalyst for ethanol electro-oxidation reaction Da-Hee Kwak, Young-Woo Lee, Sang-Beom Han, Eui-Tak Hwang, Han-Chul Park, Min-Cheol Kim, Kyung-Won Park* Department of Chemical Engineering, Soongsil University, Seoul 156-743, Republic of Korea
h i g h l i g h t s Pt3Sn nanoparticles with an ultrasmall size of 2.5 nm were synthesized. Pt3Sn nanoparticles showed improved electrocatalytic activity and stability. The improved activity may be attributed to a well-defined alloy formation.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 September 2014 Received in revised form 25 October 2014 Accepted 11 November 2014 Available online 12 November 2014
To improve the electrocatalytic properties for an ethanol electro-oxidation reaction, modifications of Pt nanocrystallites have been used by alloying with other elements such as Ru, Sn, and Au. Here we demonstrate carbon supported Pt3Sn alloy electrocatalyst (Pt3Sn/C) synthesized using a thermaldecomposition method. The PtSn/C prepared by the present synthetic process shows a homogeneous distribution of ultrasmall alloy nanoparticles (~2.5 nm) in the presence of Pt and Sn metallic states. At 0.45 V, the Pt3Sn/C (0.35 mA cm2) exhibits much higher current density as compared with Pt/C (0.13 mA cm2). In an electrochemical stability test, the Pt3Sn/C supported quite high current density and thus showed 3% current reduction after the stability test. © 2014 Published by Elsevier B.V.
Keywords: PtSn Alloy Nanoparticles Ethanol Electro-oxidation
1. Introduction Direct ethanol fuel cells (DEFCs) have attracted much attention for power generation because ethanol as a fuel is less toxic and offers higher theoretical mass density (8 kWh kg1) than methanol (6 kWh kg1) [1e3]. Also, ethanol is an attractive fuel for low temperature fuel cells because it can be produced in large quantities form agricultural products or biomass [4e7]. However, complete electro-oxidation of ethanol to CO2 is more complicated than methanol electro-oxidation due to the strong CeC bonds and COintermediates that can poison Pt-based anode catalysts [8,9]. As a result, since the anode performance of ethanol electro-oxidation (EOR) in DEFCs still remains poor, exploitation of an effective catalyst with higher EOR activity will be a key research objective in the development of DEFCs [10,11].
* Corresponding author. E-mail address: [email protected] (K.-W. Park). http://dx.doi.org/10.1016/j.jpowsour.2014.11.050 0378-7753/© 2014 Published by Elsevier B.V.
Pt-based nanomaterials, as one kind of important noble-metal catalysts, are of great interest because of their many applications on electrocatalysis and fuel cells [12e14]. Bimetallic alloy nanocrystals can effectively reduce the consumption of noble-metals [12,15]. Although Pt is generally known to be one of the best electrocatalysts, it is high priced, has limited capability for CeC bond scission, and is vulnerable to poisoning by carbonaceous intermediates generated during the EOR process. These intermediate species can be easily adsorbed on the surface of Pt, which blocks the subsequent adsorption and oxidation of ethanol [16,17]. To improve the electrocatalytic activity, modifications of Pt nanocrystallites have been made by alloying with other elements such as Sn [18e21], Ru [22], and Au [23]. Among them, Sn was suggested to modify the electronic structure of Pt by forming an alloy with it, which improved the EOR activity on the catalyst [10,24e31]. In this study, we report the synthesis of Pt3Sn alloy nanoparticles (NPs) with an average size of ~2.5 nm deposited on Vulcan XC-72R (Pt3Sn/ C) for enhanced EOR using thermal-decomposition with oleylamine as a reducing agent and 1-octadecene/oleic acid as a surfactant. The catalysts were examined for EOR catalysis in acidic electrolyte and
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oxidative conditions. The Pt3Sn/C showed high activity and stability for EOR, serving as a promising candidate for electro-oxidation of small organic molecules. 2. Experimental Carbon supported 20 wt% Pt3Sn alloy catalysts were synthesized using thermal-decomposition. The carbon support used in this study was Vulcan XC-72R treated with nitric acid and hydrochloric acid. In a typical synthesis of Pt3Sn alloy NPs, carbon support (0.106 g) was added into the solution with 20 mL 1-octadecene, 5 mL oleylamine, and 1 mL oleic acid with vigorous stirring. Then, Pt(acac)2 (Platinum(II) acetylacetonate, 4.5 mM, 4.4 mg), and Sn(acacCl)4 (Tin(IV) bis(acetylacetonate), 1.5 mM, 1.5 mg) were added into the as-prepared solution. After stirring and sonication for dispersion, the sample was heated to 70 C for 1 h and the
temperature was subsequently raised to 250 C for 3 h under N2 atmosphere. The resulting colloid solution with Pt3Sn alloy NPs was cooled down to 25 C. To remove the reducing agent, the obtained powder was mixed with 30 mL acetic acid and heated at 70 C for overnight. The products were washed with ethanol, acetone, and de-ionized water several times and dried in oven at 50 C. To characterize the structure of the as-prepared catalysts, X-ray diffraction (XRD) patterns were obtained using a D2 PHASE SYSTEM, BRUKER with Cu Ka source (l ¼ 0.15406 nm) at 30 kV and 10 mA in 2q ¼ 20 e80 . The size distribution and morphology of catalysts were analyzed by transmission electron microscopy (TEM) using a Tecnai G2 F30 system microscope operating at 300 kV. TEM samples were prepared by placing drops of catalyst suspension dispersed in ethanol on a carbon-coated copper grid. Furthermore, energy dispersive X-ray (EDX) analysis was performed on a TEM equipment to confirm the chemical composition of the samples.
Fig. 1. (A,B) TEM images, (C) size histogram, (D) high-resolution TEM image, and (E) EDX spectrum of Pt3Sn1/C.
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Fig. 2. XRD patterns of Pt3Sn1/C and Pt/C compared to reference patterns.
The electrochemical properties were measured in a threeelectrode cell using a potentiostat (Eco Chemie, AUTOLAB) as previously reported [32,33]. The catalyst inks were prepared by mixing catalyst, de-ionized water, isopropyl alcohol, and 5 wt% Nafion solution. The glassy carbon working electrode was coated with 0.5 mL of catalyst ink and dried in 50 C oven. The total loading of catalyst in all the samples was 40 mg cm2. In addition, Pt wire and Ag/AgCl (in saturated 3 M KCl) were used as a counter and reference electrode, respectively. Cyclic voltammograms (CVs) of the catalysts were obtained in Ar-purged 0.1 M HClO4 and 0.1 M HClO4 þ 2.0 M C2H5OH with a scan rate of 50 mV s1 at 25 C. To evaluate an electrocatalytic stability, the catalysts were kept at 0.5 V for 7200 s in 0.1 M HClO4 þ 2.0 M C2H5OH and then CVs were obtained in 0.1 M HClO4 and 0.1 M HClO4 þ 2.0 M C2H5OH after the stability test.
3. Results and discussion Structural analysis of the as-synthesized Pt3Sn alloy nanoparticles (NPs) deposited on carbon black (Pt3Sn/C) was carried out by FE-TEM as shown Fig. 1A and B. The PtSn/C exhibited an average size of ~2.5 nm (Fig. 1C). As shown in Fig. 1D, the Pt3Sn NPs represented the (111) interplanar spacing of 0.214 nm with a facecentered cubic (fcc) crystal structure, indicating that the NPs were highly crystallized. As confirmed by the EDX in Fig. 1E, an alloy composition of the Pt3Sn NPs was measured to be 73.63 at% Pt and 26.36 at% Sn. In the present synthesis process, 1-octadecene, oleylamine, and oleic acid, as a reducing agent or capping agent [34], can lead to a facile fast reduction reaction from metal ions to metal atoms, forming ultrasmall alloy NPs [35e37].
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Fig. 2 shows XRD patterns of Pt3Sn/C and Pt/C (E-TEK, Co.) in comparison with reference data of Pt and Pt3Sn. In the case of Pt3Sn/C, the XRD peaks at 38.83 , 44.61, and 66.02 correspond to the (111), (200) and (220) planes, respectively, of an fcc structure. The first broad peak around 25.8 is associated with (002) plane of the hexagonal structure of Vulcan XC-72R carbon black support [38]. The broader XRD peaks of Pt3Sn/C indicate the formation of smaller Pt3Sn particles compared to Pt/C [16]. Furthermore, no peaks relating to the formation of crystalline tin metals or oxides were apparent, suggesting a well-defined alloy formation between Pt and Sn via the present synthetic process. The XRD peaks showed a good match with the fcc Pt3Sn standard (JCPDS# 35e1360) and a low angle shift compared to the Pt standard peaks (JCPDS# 04e0802), showing that alloying Pt with Sn leads to a lattice expansion [39,40]. On the basis of the DebyeeScherrer equation [41], an average particle size of the PtSn NPs was determined to be ~2.6 nm as compared to that of ~3.6 nm of commercial Pt NPs deposited on carbon black (E-TEK, Co.). This shows that the Pt3Sn/C prepared by the present synthetic process how a homogeneous distribution of alloy NPs consisting of Pt and Sn atoms. The wide scan and fine XPS spectra of Pt 4f and Sn 3d for the Pt3Sn/C are shown in Fig. 3. A charge correction was applied to the C 1s (284.5 eV) signal, and all other XPS peaks in the Pt 4f and Sn 3d regions. The Pt 4f7/2 and 4f5/2 lines appeared at 71 and 74 eV, respectively, with a theoretical ratio of peak areas of 4:3. Small portions of platinum oxides were present in PtSn/C as indicated by the peaks for Pt2þ at 73.8 eV. The surface metallic state of platinum (Pto) would provide more suitable sites for methanol electrooxidation than PtIIO [42e44]. Hence, the surface metallic state of Pt on a Pt-based catalyst is essential for high activity with respect to ethanol electrooxidation. The Sn 3d5/2 XPS spectrum exhibited signals consistent with the presence of metallic (485.0 eV) and oxidized (486.5 eV) Sn states. The Sn metallic state was further confirmed by XRD analysis; however, no peaks relating to the formation of crystalline tin metals were apparent (Fig. 2). In the case of tin oxide, the XRD peaks indicated that 2q ¼ 26.6, 33.9, and 51.7 (JCPDS 41-1445, not shown here). Accordingly, the oxidized Sn species in the PtSn/C are not present as crystalline, but as surface and subsurface oxidized states. To identify electrochemical properties of the as-prepared catalysts, CVs were obtained in 0.1 M HClO4 with a scan rate of 50 mV s1 at 25 C in Fig. 4A. The electrochemically active surface areas (EASAs) of the catalysts were measured by integrating the charges on the Hupd desorption region assuming a value of 210 mC cm2 after double layer correction [45]. The EASAs of Pt3Sn/ C and Pt/C were 15.56 and 45.68 m2 g1, respectively. As shown in Fig. 4B, the CVs of the as-prepared catalysts were obtained in 0.1 M HClO4 þ 2.0 M C2H5OH at a scan rate of 50 mV s1. In general, among noble metals, Pt has been well known as an excellent
Fig. 3. Wide scan and fine XPS spectra of Pt 4f and Sn 3d for Pt3Sn1/C.
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Fig. 4. CVs of Pt3Sn1/C and Pt/C in (A) 0.1 M HClO4 and (B) 0.1 M HClO4 þ 2 M C2H5OH with a scan rate of 50 mV s1 at room temperature. (C) Specific and mass activity of the catalysts at 0.45 V. (D) Chronoamperometric curves of the catalysts at 0.5 V for 7200 s in 0.1 M HClO4 þ 2 M C2H5OH.
catalyst for electrooxidation especially in acid electrolytes [46]. However, since Pt has limited capability to break CeC bonds and is vulnerable toward poisoning by carbonaceous intermediates generated during the EOR, poor electrocatalytic activity of Pt/C was observed (Fig. 4B). Thus, a modification of Pt nanocrystallites, such as by alloying with Sn, could result in improved EOR activity on
Pt3Sn1/C [16]. Furthermore, the forward anodic peak potential of Pt3Sn/C (0.75 V) showed a relatively negative shift compared to Pt/C (0.83 V). According to Kim et al., the negative shift of forward anodic peak implies that an electrooxidation on the Pt-based catalyst is easier than that on pure Pt [16,47]. It has been reported that the anodic peak in the reverse scan during alcohol
Fig. 5. CVs of (A) Pt3Sn1/C and (B) Pt/C in 0.1 M HClO4 with a scan rate of 50 mV s1 at room temperature before and after stability test. CVs of (C) Pt3Sn1/C and (D) Pt/C in 0.1 M HClO4 þ 2 M C2H5OH with a scan rate of 50 mV s1 at room temperature before and after stability test. The stability test of the catalysts was carried out by applying the oxidation potential of 0.5 V for 2 h in 0.1 M HClO4 þ 2.0 M C2H5OH.
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Fig. 6. TEM images and size histograms of (AeC) Pt3Sn1/C and (DeF) Pt/C after the stability test.
electrooxidation might be attributed to the removal of the incompletely oxidized carbonaceous species [48,49]. Hence, the ratio of the forward anodic peak current density (If) to the reverse anodic peak current density (Ib), i.e., If/Ib, can be used to describe the tolerance of the catalyst to accumulation of carbonaceous species. Accordingly, the high If/Ib ratio (0.74) of Pt3Sn1/C indicates less accumulated residues on the catalyst during ethanol electrooxidation and thus excellent catalytic activity. The Pt3Sn/C indicates much higher specific activity and mass activity at 0.45 V, as compared to Pt/C (Fig. 4C). This demonstrates that the Pt3Sn/C exhibits much improved electrocatalytic activity toward ethanol electrooxidation due to particular elemental composition between Pt and Sn. Furthermore, the stability of the catalysts under an electrooxidation conditions is essential for practical applications of Pt-based alloy NPs. To evaluate the electrochemical stability for ethanol electrooxidation, the as-prepared catalysts were maintained at 0.5 V for 7200 s in 0.1 M HClO4 þ 2.0 M C2H5OH. The Pt3Sn/C showed less degradation in current density and higher current density compared to Pt/C (Fig. 4D). After the stability test, the CVs of all of the catalysts were obtained in 0.1 M HClO4 (Fig. 5). The EASAs of the catalysts after the stability test were measured by the charges on the hydrogen desorption region. In the case of the Pt3Sn/C, the EASAs after the stability test show 2% reduction of from the initial value in Fig. 5A. In contrast, Pt/C show a serious EASA reduction of 11% after the stability test (Fig. 5B). The Pt3Sn/C supports high current density and thus shows current reduction of 3% after the stability test (Fig. 5C). In contrast, the current density of the Pt/C at 0.5 V after the stability test seriously decreases by 25% from the initial value (Fig. 5D). In the plot of polarization current versus time measured at 0.5 V for each catalyst, the Pt3Sn/C exhibited both a slower deterioration rate and higher current than Pt/C. Since the change of size and morphology for the catalysts before and after the stability test can affect the whole activity, their shape and size distribution should be noted and compared as shown in Fig. 6. As indicated in
Fig. 6, the Pt3Sn/C displayed an average size of 2.9 nm with an increase of 15% after the stability test, exhibiting improved electrocatalytic activity. In contrast, after the stability test, Pt/C exhibits an average size of 4.3 nm with an increase of 40% resulting in deteriorated electrocatalytic activity. Therefore, it can be concluded that the Pt3Sn/C is a promising candidate to be an excellent electrocatalyst for methanol electrooxidation. 4. Conclusions In summary, carbon supported Pt3Sn alloy NPs have been synthesized using thermal-decomposition for an ethanol electrooxidation reaction. The Pt3Sn/C prepared by the present synthetic process showed a homogeneous distribution of ultrasmall alloy nanoparticles of ~2.5 nm. The Pt3Sn/C exhibited much higher current density and mass activity as compared to Pt/C. Furthermore, the Pt3Sn/C supported high current density and thus showed small current reduction after the stability test showing electrochemical stability for ethanol electrooxidation. In contrast, in the case of the Pt/C, the current density after the stability test seriously decreased. The Pt3Sn/C can be a promising candidate for ethanol electrooxidation due to a well-defined alloy formation between Pt and Sn in Pt3Sn NPs. Acknowledgments This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF2012M1A2A2671689). References [1] F. Delime, J.M. Leger, C. Lamy, J. Appl. Electrochem. 38 (1998) 27e35. [2] R.F.B. De Souza, L.S. Parreira, D.C. Rascio, J.C.M. Silva, E. Teixeira-Neto, M.L. Calegaro, E.V. Spinace, A.O. Neto, M.C. Santos, J. Power Sources 195 (2010) 1589e1593.
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Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Ultrasmall PtSn alloy catalyst for ethanol electro-oxidation reaction Da-Hee Kwak, Young-Woo Lee, Sang-Beom Han, Eui-Tak Hwang, Han-Chul Park, Min-Cheol Kim, Kyung-Won Park* Department of Chemical Engineering, Soongsil University, Seoul 156-743, Republic of Korea
h i g h l i g h t s Pt3Sn nanoparticles with an ultrasmall size of 2.5 nm were synthesized. Pt3Sn nanoparticles showed improved electrocatalytic activity and stability. The improved activity may be attributed to a well-defined alloy formation.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 September 2014 Received in revised form 25 October 2014 Accepted 11 November 2014 Available online 12 November 2014
To improve the electrocatalytic properties for an ethanol electro-oxidation reaction, modifications of Pt nanocrystallites have been used by alloying with other elements such as Ru, Sn, and Au. Here we demonstrate carbon supported Pt3Sn alloy electrocatalyst (Pt3Sn/C) synthesized using a thermaldecomposition method. The PtSn/C prepared by the present synthetic process shows a homogeneous distribution of ultrasmall alloy nanoparticles (~2.5 nm) in the presence of Pt and Sn metallic states. At 0.45 V, the Pt3Sn/C (0.35 mA cm2) exhibits much higher current density as compared with Pt/C (0.13 mA cm2). In an electrochemical stability test, the Pt3Sn/C supported quite high current density and thus showed 3% current reduction after the stability test. © 2014 Published by Elsevier B.V.
Keywords: PtSn Alloy Nanoparticles Ethanol Electro-oxidation
1. Introduction Direct ethanol fuel cells (DEFCs) have attracted much attention for power generation because ethanol as a fuel is less toxic and offers higher theoretical mass density (8 kWh kg1) than methanol (6 kWh kg1) [1e3]. Also, ethanol is an attractive fuel for low temperature fuel cells because it can be produced in large quantities form agricultural products or biomass [4e7]. However, complete electro-oxidation of ethanol to CO2 is more complicated than methanol electro-oxidation due to the strong CeC bonds and COintermediates that can poison Pt-based anode catalysts [8,9]. As a result, since the anode performance of ethanol electro-oxidation (EOR) in DEFCs still remains poor, exploitation of an effective catalyst with higher EOR activity will be a key research objective in the development of DEFCs [10,11].
* Corresponding author. E-mail address: [email protected] (K.-W. Park). http://dx.doi.org/10.1016/j.jpowsour.2014.11.050 0378-7753/© 2014 Published by Elsevier B.V.
Pt-based nanomaterials, as one kind of important noble-metal catalysts, are of great interest because of their many applications on electrocatalysis and fuel cells [12e14]. Bimetallic alloy nanocrystals can effectively reduce the consumption of noble-metals [12,15]. Although Pt is generally known to be one of the best electrocatalysts, it is high priced, has limited capability for CeC bond scission, and is vulnerable to poisoning by carbonaceous intermediates generated during the EOR process. These intermediate species can be easily adsorbed on the surface of Pt, which blocks the subsequent adsorption and oxidation of ethanol [16,17]. To improve the electrocatalytic activity, modifications of Pt nanocrystallites have been made by alloying with other elements such as Sn [18e21], Ru [22], and Au [23]. Among them, Sn was suggested to modify the electronic structure of Pt by forming an alloy with it, which improved the EOR activity on the catalyst [10,24e31]. In this study, we report the synthesis of Pt3Sn alloy nanoparticles (NPs) with an average size of ~2.5 nm deposited on Vulcan XC-72R (Pt3Sn/ C) for enhanced EOR using thermal-decomposition with oleylamine as a reducing agent and 1-octadecene/oleic acid as a surfactant. The catalysts were examined for EOR catalysis in acidic electrolyte and
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oxidative conditions. The Pt3Sn/C showed high activity and stability for EOR, serving as a promising candidate for electro-oxidation of small organic molecules. 2. Experimental Carbon supported 20 wt% Pt3Sn alloy catalysts were synthesized using thermal-decomposition. The carbon support used in this study was Vulcan XC-72R treated with nitric acid and hydrochloric acid. In a typical synthesis of Pt3Sn alloy NPs, carbon support (0.106 g) was added into the solution with 20 mL 1-octadecene, 5 mL oleylamine, and 1 mL oleic acid with vigorous stirring. Then, Pt(acac)2 (Platinum(II) acetylacetonate, 4.5 mM, 4.4 mg), and Sn(acacCl)4 (Tin(IV) bis(acetylacetonate), 1.5 mM, 1.5 mg) were added into the as-prepared solution. After stirring and sonication for dispersion, the sample was heated to 70 C for 1 h and the
temperature was subsequently raised to 250 C for 3 h under N2 atmosphere. The resulting colloid solution with Pt3Sn alloy NPs was cooled down to 25 C. To remove the reducing agent, the obtained powder was mixed with 30 mL acetic acid and heated at 70 C for overnight. The products were washed with ethanol, acetone, and de-ionized water several times and dried in oven at 50 C. To characterize the structure of the as-prepared catalysts, X-ray diffraction (XRD) patterns were obtained using a D2 PHASE SYSTEM, BRUKER with Cu Ka source (l ¼ 0.15406 nm) at 30 kV and 10 mA in 2q ¼ 20 e80 . The size distribution and morphology of catalysts were analyzed by transmission electron microscopy (TEM) using a Tecnai G2 F30 system microscope operating at 300 kV. TEM samples were prepared by placing drops of catalyst suspension dispersed in ethanol on a carbon-coated copper grid. Furthermore, energy dispersive X-ray (EDX) analysis was performed on a TEM equipment to confirm the chemical composition of the samples.
Fig. 1. (A,B) TEM images, (C) size histogram, (D) high-resolution TEM image, and (E) EDX spectrum of Pt3Sn1/C.
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Fig. 2. XRD patterns of Pt3Sn1/C and Pt/C compared to reference patterns.
The electrochemical properties were measured in a threeelectrode cell using a potentiostat (Eco Chemie, AUTOLAB) as previously reported [32,33]. The catalyst inks were prepared by mixing catalyst, de-ionized water, isopropyl alcohol, and 5 wt% Nafion solution. The glassy carbon working electrode was coated with 0.5 mL of catalyst ink and dried in 50 C oven. The total loading of catalyst in all the samples was 40 mg cm2. In addition, Pt wire and Ag/AgCl (in saturated 3 M KCl) were used as a counter and reference electrode, respectively. Cyclic voltammograms (CVs) of the catalysts were obtained in Ar-purged 0.1 M HClO4 and 0.1 M HClO4 þ 2.0 M C2H5OH with a scan rate of 50 mV s1 at 25 C. To evaluate an electrocatalytic stability, the catalysts were kept at 0.5 V for 7200 s in 0.1 M HClO4 þ 2.0 M C2H5OH and then CVs were obtained in 0.1 M HClO4 and 0.1 M HClO4 þ 2.0 M C2H5OH after the stability test.
3. Results and discussion Structural analysis of the as-synthesized Pt3Sn alloy nanoparticles (NPs) deposited on carbon black (Pt3Sn/C) was carried out by FE-TEM as shown Fig. 1A and B. The PtSn/C exhibited an average size of ~2.5 nm (Fig. 1C). As shown in Fig. 1D, the Pt3Sn NPs represented the (111) interplanar spacing of 0.214 nm with a facecentered cubic (fcc) crystal structure, indicating that the NPs were highly crystallized. As confirmed by the EDX in Fig. 1E, an alloy composition of the Pt3Sn NPs was measured to be 73.63 at% Pt and 26.36 at% Sn. In the present synthesis process, 1-octadecene, oleylamine, and oleic acid, as a reducing agent or capping agent [34], can lead to a facile fast reduction reaction from metal ions to metal atoms, forming ultrasmall alloy NPs [35e37].
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Fig. 2 shows XRD patterns of Pt3Sn/C and Pt/C (E-TEK, Co.) in comparison with reference data of Pt and Pt3Sn. In the case of Pt3Sn/C, the XRD peaks at 38.83 , 44.61, and 66.02 correspond to the (111), (200) and (220) planes, respectively, of an fcc structure. The first broad peak around 25.8 is associated with (002) plane of the hexagonal structure of Vulcan XC-72R carbon black support [38]. The broader XRD peaks of Pt3Sn/C indicate the formation of smaller Pt3Sn particles compared to Pt/C [16]. Furthermore, no peaks relating to the formation of crystalline tin metals or oxides were apparent, suggesting a well-defined alloy formation between Pt and Sn via the present synthetic process. The XRD peaks showed a good match with the fcc Pt3Sn standard (JCPDS# 35e1360) and a low angle shift compared to the Pt standard peaks (JCPDS# 04e0802), showing that alloying Pt with Sn leads to a lattice expansion [39,40]. On the basis of the DebyeeScherrer equation [41], an average particle size of the PtSn NPs was determined to be ~2.6 nm as compared to that of ~3.6 nm of commercial Pt NPs deposited on carbon black (E-TEK, Co.). This shows that the Pt3Sn/C prepared by the present synthetic process how a homogeneous distribution of alloy NPs consisting of Pt and Sn atoms. The wide scan and fine XPS spectra of Pt 4f and Sn 3d for the Pt3Sn/C are shown in Fig. 3. A charge correction was applied to the C 1s (284.5 eV) signal, and all other XPS peaks in the Pt 4f and Sn 3d regions. The Pt 4f7/2 and 4f5/2 lines appeared at 71 and 74 eV, respectively, with a theoretical ratio of peak areas of 4:3. Small portions of platinum oxides were present in PtSn/C as indicated by the peaks for Pt2þ at 73.8 eV. The surface metallic state of platinum (Pto) would provide more suitable sites for methanol electrooxidation than PtIIO [42e44]. Hence, the surface metallic state of Pt on a Pt-based catalyst is essential for high activity with respect to ethanol electrooxidation. The Sn 3d5/2 XPS spectrum exhibited signals consistent with the presence of metallic (485.0 eV) and oxidized (486.5 eV) Sn states. The Sn metallic state was further confirmed by XRD analysis; however, no peaks relating to the formation of crystalline tin metals were apparent (Fig. 2). In the case of tin oxide, the XRD peaks indicated that 2q ¼ 26.6, 33.9, and 51.7 (JCPDS 41-1445, not shown here). Accordingly, the oxidized Sn species in the PtSn/C are not present as crystalline, but as surface and subsurface oxidized states. To identify electrochemical properties of the as-prepared catalysts, CVs were obtained in 0.1 M HClO4 with a scan rate of 50 mV s1 at 25 C in Fig. 4A. The electrochemically active surface areas (EASAs) of the catalysts were measured by integrating the charges on the Hupd desorption region assuming a value of 210 mC cm2 after double layer correction [45]. The EASAs of Pt3Sn/ C and Pt/C were 15.56 and 45.68 m2 g1, respectively. As shown in Fig. 4B, the CVs of the as-prepared catalysts were obtained in 0.1 M HClO4 þ 2.0 M C2H5OH at a scan rate of 50 mV s1. In general, among noble metals, Pt has been well known as an excellent
Fig. 3. Wide scan and fine XPS spectra of Pt 4f and Sn 3d for Pt3Sn1/C.
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Fig. 4. CVs of Pt3Sn1/C and Pt/C in (A) 0.1 M HClO4 and (B) 0.1 M HClO4 þ 2 M C2H5OH with a scan rate of 50 mV s1 at room temperature. (C) Specific and mass activity of the catalysts at 0.45 V. (D) Chronoamperometric curves of the catalysts at 0.5 V for 7200 s in 0.1 M HClO4 þ 2 M C2H5OH.
catalyst for electrooxidation especially in acid electrolytes [46]. However, since Pt has limited capability to break CeC bonds and is vulnerable toward poisoning by carbonaceous intermediates generated during the EOR, poor electrocatalytic activity of Pt/C was observed (Fig. 4B). Thus, a modification of Pt nanocrystallites, such as by alloying with Sn, could result in improved EOR activity on
Pt3Sn1/C [16]. Furthermore, the forward anodic peak potential of Pt3Sn/C (0.75 V) showed a relatively negative shift compared to Pt/C (0.83 V). According to Kim et al., the negative shift of forward anodic peak implies that an electrooxidation on the Pt-based catalyst is easier than that on pure Pt [16,47]. It has been reported that the anodic peak in the reverse scan during alcohol
Fig. 5. CVs of (A) Pt3Sn1/C and (B) Pt/C in 0.1 M HClO4 with a scan rate of 50 mV s1 at room temperature before and after stability test. CVs of (C) Pt3Sn1/C and (D) Pt/C in 0.1 M HClO4 þ 2 M C2H5OH with a scan rate of 50 mV s1 at room temperature before and after stability test. The stability test of the catalysts was carried out by applying the oxidation potential of 0.5 V for 2 h in 0.1 M HClO4 þ 2.0 M C2H5OH.
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Fig. 6. TEM images and size histograms of (AeC) Pt3Sn1/C and (DeF) Pt/C after the stability test.
electrooxidation might be attributed to the removal of the incompletely oxidized carbonaceous species [48,49]. Hence, the ratio of the forward anodic peak current density (If) to the reverse anodic peak current density (Ib), i.e., If/Ib, can be used to describe the tolerance of the catalyst to accumulation of carbonaceous species. Accordingly, the high If/Ib ratio (0.74) of Pt3Sn1/C indicates less accumulated residues on the catalyst during ethanol electrooxidation and thus excellent catalytic activity. The Pt3Sn/C indicates much higher specific activity and mass activity at 0.45 V, as compared to Pt/C (Fig. 4C). This demonstrates that the Pt3Sn/C exhibits much improved electrocatalytic activity toward ethanol electrooxidation due to particular elemental composition between Pt and Sn. Furthermore, the stability of the catalysts under an electrooxidation conditions is essential for practical applications of Pt-based alloy NPs. To evaluate the electrochemical stability for ethanol electrooxidation, the as-prepared catalysts were maintained at 0.5 V for 7200 s in 0.1 M HClO4 þ 2.0 M C2H5OH. The Pt3Sn/C showed less degradation in current density and higher current density compared to Pt/C (Fig. 4D). After the stability test, the CVs of all of the catalysts were obtained in 0.1 M HClO4 (Fig. 5). The EASAs of the catalysts after the stability test were measured by the charges on the hydrogen desorption region. In the case of the Pt3Sn/C, the EASAs after the stability test show 2% reduction of from the initial value in Fig. 5A. In contrast, Pt/C show a serious EASA reduction of 11% after the stability test (Fig. 5B). The Pt3Sn/C supports high current density and thus shows current reduction of 3% after the stability test (Fig. 5C). In contrast, the current density of the Pt/C at 0.5 V after the stability test seriously decreases by 25% from the initial value (Fig. 5D). In the plot of polarization current versus time measured at 0.5 V for each catalyst, the Pt3Sn/C exhibited both a slower deterioration rate and higher current than Pt/C. Since the change of size and morphology for the catalysts before and after the stability test can affect the whole activity, their shape and size distribution should be noted and compared as shown in Fig. 6. As indicated in
Fig. 6, the Pt3Sn/C displayed an average size of 2.9 nm with an increase of 15% after the stability test, exhibiting improved electrocatalytic activity. In contrast, after the stability test, Pt/C exhibits an average size of 4.3 nm with an increase of 40% resulting in deteriorated electrocatalytic activity. Therefore, it can be concluded that the Pt3Sn/C is a promising candidate to be an excellent electrocatalyst for methanol electrooxidation. 4. Conclusions In summary, carbon supported Pt3Sn alloy NPs have been synthesized using thermal-decomposition for an ethanol electrooxidation reaction. The Pt3Sn/C prepared by the present synthetic process showed a homogeneous distribution of ultrasmall alloy nanoparticles of ~2.5 nm. The Pt3Sn/C exhibited much higher current density and mass activity as compared to Pt/C. Furthermore, the Pt3Sn/C supported high current density and thus showed small current reduction after the stability test showing electrochemical stability for ethanol electrooxidation. In contrast, in the case of the Pt/C, the current density after the stability test seriously decreased. The Pt3Sn/C can be a promising candidate for ethanol electrooxidation due to a well-defined alloy formation between Pt and Sn in Pt3Sn NPs. Acknowledgments This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF2012M1A2A2671689). References [1] F. Delime, J.M. Leger, C. Lamy, J. Appl. Electrochem. 38 (1998) 27e35. [2] R.F.B. De Souza, L.S. Parreira, D.C. Rascio, J.C.M. Silva, E. Teixeira-Neto, M.L. Calegaro, E.V. Spinace, A.O. Neto, M.C. Santos, J. Power Sources 195 (2010) 1589e1593.
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