- Email: [email protected]
graphene catalysts for the methanol oxidation reaction
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 4 2 8 8 e1 4 2 9 5
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
The effect of Sn on platinum dispersion in Pt/graphene catalysts for the methanol oxidation reaction Xiaomin Wang*, Jie Lian, Yong Wang College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China
article info
abstract
Article history:
Sn-modified platinum catalysts are presently one of the most active catalysts for the room
Available online 21 June 2014
temperature electrooxidation of ethanol at low potentials. In this study, PteSn/graphene catalysts containing different ratios of Pt and Sn were prepared by the solution-phase
Keywords:
reduction. Microstructural characterization shows that metallic Pt, PteSn alloy and tin
PteSn/graphene
dioxide (SnO2) nanoparticles are distributed on the graphene sheets in the synthetic pro-
Equal ratios
cess. In terms of the electrocatalytic properties, graphene-supported PteSn catalysts
Methanol electrooxidation
exhibit much higher current densities with increasing Sn proportions. It's proved that the
Microstructrual characterization
addition of Sn not only decreases catalyst particles growth and agglomeration, but also promotes methanol electrooxidation by geometric effects on expanding Pt's lattice spacing, causing a synergistic effect between Pt and Sn nanoparticles. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Direct methanol fuel cells (DMFC) are considered to be the most promising energy devices for mobile and stationary applications such as cellular phones, laptop computers and secondary electricity generators [1]. Nowadays, the main problem is the deficient activity and selectivity of anode electrocatalysts at temperatures compatible with available membranes [2]. Platinum is the best catalyst for alcohol oxidation, but its surface is rapidly poisoned by strongly irreversibly adsorbed CO-like intermediates coming from the dissociative adsorption of alcohols. The manufacture of Pt-based nanostructured bimetallic catalysts, such as Sn, by electrochemical techniques promotes the high catalytic activity of the catalysts, since it is a simple operation and low cost, which also makes it possible to obtain
deposits of high purity and uniform deposition [3e5]. The presence of Sn alters the electronic structure of Pt, weakening CO adsorption on the Pt surface, thus reducing catalyst poisoning. In addition, the geometric environment could be readily changed with Sn additions to the fcc Pt crystallites by forming solid solution of PteSn alloy phase, showing the larger lattice parameter. This elongation of crystal structure may affect catalytic reactions that require specific geometric arrangements of the surface atoms, thus leading to the improvement of catalytic properties [6]. Moreover, Sn also provides OHads species by dissociating water at a lower potential than that on Pt, and these OHads species are helpful for the subsequent oxidation of adsorbed intermediates to acetaldehyde, acetic acid, and CO2 [7e9]. Besides the active metals, support is another key factor for a highly efficient catalyst, which requires low cost, good
* Corresponding author. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.ijhydene.2014.05.158 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 4 2 8 8 e1 4 2 9 5
chemical stability, and large surface area, as well as high conductivity. To date, some research has been developed to the preparation of graphene-supported PteSn. As a simple method, the one-step synthesis procedure of graphenesupported PteSn to increase the electrocatalytic ability seems very attractive. The strategies of single step preparation of graphene supported PteSn can be roughly classified as solution-phase reduction [10], thermal decomposition [11], microwave-assisted method [12], and electrochemical deposition [13]. Based on our previous exploratory works [14], graphene-supported PteSn catalysts have been prepared by solution-phase reduction, and their enhanced electrocatalytic activity have been investigated as anode catalysts for a DMFC. Thus, in order to improve the performance of PteSn anode catalysts, it is of great importance to hunt for the roles of Sn for the Pt/graphene catalysts. In the present work, the role of the structural features of PteSn deposited graphene catalysts in the electro-oxidation of ethanol was analyzed. PteSn nanostructured materials of different compositions were prepared by multiple cycles of potentiostatic pulses onto graphene. Physical property characterizations were carried out to investigate the microstructure, dispersion and degree of Sn in the bimetallic catalysts. In addition, the electrochemical activity of the catalysts for ethanol and ethylene glycol electro-oxidation was evaluated by cyclic voltammetry and chronoamperometry.
Experimental Synthesis of PteSn/graphene catalysts As the raw material, graphene oxide was produced by the modified Hummers method [12,15]. For the synthesis of graphene-supported PteSn (PteSn/graphene) catalysts, 30 mg of graphene oxide powder was dispersed in 40 ml of ethylene glycol solution. Subsequently, a certain amount of hexachloroplatinic acid-EG (H2PtCl6-EG) solution (0.5 mol/L) and tin dichlorideeEG (SnCl2eEG) solution (0.5 mol/L) were added to the graphene oxide solution and sonicated for 3 h. The pH of the solution was adjusted to 9-10 using sodium hydroxideeEG (NaOHeEG) solution, and then the solution was stirred under flowing argon at 130 C for 3 h. The solid material produced was then washed with water and then ethanol and finally dried in a vacuum oven at 50 C for 24 h. The catalysts prepared as such were designated as PteSn/GNs. Similar procedures were used to prepare graphene supported-Pt catalysts (Pt/GNs).
Electrochemical measurements An electrochemical instrument (Autolab GSTAT302) and a conventional three-electrode test cell with a saturated calomel electrode (SCE) as the reference electrode and platinum wire as the counter electrode were used for the electrochemical measurements. To examine the electrocatalytic properties, cyclic voltammetry was performed using a conventional three electrode electrochemical system in 0.5 mol/L H2SO4 aqueous solution and 0.5 mol/L H2SO4 þ 0.5 mol/L CH3OH aqueous solution at room temperature. The modified glassy carbon electrode with catalysts, Pt wire and saturated calomel electrode were used as the working electrode, counter electrode and reference electrode, respectively. Catalyst inks were prepared as follows: catalyst powder (5 mg) was dispersed in 5% Nafion (100 mL) and isopropyl alcohol (900 mL) and the mixture was sonicated for 30 min. Twenty-five micro liter of the catalyst inks from each kind of catalyst was then coated onto the surface of three glassy carbon electrodes.
Results and discussion Physical property Table 1 shows XPS results of the samples. 1# and 2# are Pt/GNs samples, 3# and 4# are PteSn/GNs samples. Pt:Sn indicates the atomic ratio of Pt and Sn, Pt wt% and Sn wt% indicate the mass fractions of Pt and Sn, respectively. Modified carbon content indicates the ratio of carbon atoms with the functional groups containing Oxygen in the samples. Pt0 indicates the ratio of the zero-valence Pt in all the Pt atoms. Sn2þ/4þ represents the ratios of divalent and quadrivalent Sn atoms in all the Sn atoms. In XPS spectra, Fig. 1 shows C1s spectra of the samples, and the peaks at 288.4 , 285.9 , 286.8 and 288.9 are corresponding to sp2 hybrid CeC and the functional groups containing Oxygen, such as CeOH, CeOeC, HOeC]O [4,12,13]. Fig. 1(a) shows large quantities of functional groups containing Oxygen in GO. Most of the functional groups containing Oxygen are removed and only a small part of the functional groups containing Oxygen still exist after GO is reduced by ethylene glycol(Fig. 1ceh). From Table 1 we find that carbon atoms in the functional groups containing Oxygen account for 62.73% of the total number of carbon atoms, and it is between 35% and 46% when the reduction reaction occurs. The functional groups containing Oxygen and GNs surface defects play a role in anchoring Pt and Sn precursor [14,16,17]. The functional
Characterization The structural characteristics of the synthesized catalysts were investigated by X-ray diffraction (XRD) (Y-2000) using CuKa as the radiation source, and the working voltage and current were maintained at 30 kV and 20 mA. High resolution transmission electron microscopy (HRTEM) were performed on a JEM-2010 equipped with a LaB6 electron gun operating at 200 kV. X-ray photoelectron spectra (XPS) were acquired using an ESCALAB250 spectrometer fitted with an Al X-ray source.
Table 1 e XPS analysis: elemental atomic concentrations of as-prepared samples. Number Pt:Sn Pt wt% Sn wt% Modified carbon content GO 1# 2# 3# 4#
e e e 1.17 1.03
e 10.07% 16.33% 13.37% 22.38%
e 0 0 6.95% 13.22%
62.73% 45.82% 43.35% 35.08% 40.12%
Pt (0) e 54.36% 56.80% 59.91% 63.17%
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Fig. 1 e X-ray photoelectron spectra of C1s for (a)GO, (b)1#, (c)2#, (d)3#, (e)4#.
groups containing Oxygen can prevent Pt nanoparticles from reuniting, so dispersive and small nanoparticles form [18e20]. From Pt4f spectra(Fig. 2) we can find that the double peak of Pt 4f7/2 and Pt 4f5/2 is at 70 eV ~ 75 eV, and Pt 4f7/2 is composed of Pt0 and Pt2þ, which means that the quadrivalent Pt in chloroplatinic acid is reduced to zero valent Pt and divalent Pt. The Sn 3d5/2 and Sn 3d3/2 double peak of Sn3d appears at 484 eV ~ 495 eV. Fig. 3 shows that Sn mainly exists in the oxidation state Sn2þ/4þ, which is in correspondance with the fact that Sn is easily oxidized at room temperature, and that it is difficult to distinguish Sn2þ and Sn4þ from XPS spectrum. Also from Table 1 we can find that Pt in Pt/GNs and PteSn/GNs catalysts mainly exists in zero valent Pt, and Sn in PteSn/GNs catalyst mainly exists in the oxidation state Sn2þ/4þ.
The interplanar distance of the samples is calculated by Bragg equation (Formula (1))
2d sinq ¼ l
(1)
Among them, d is the interplanar distance, l is electron wavelength, and q is grazing angle. In XRD spectrum, C(002) peak of graphite is at 26 . Fig. 4 shows C(002) peak of graphene oxide appears at 11.8 , and C(002) peaks of 1#~4# samples, reduced by ethanediol, appear at 22.8 . According to Bragg equation, the interlayer spacing of graphene obtained by reduction reaction is between the interlayer spacing of graphene oxide and the interlayer
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 4 2 8 8 e1 4 2 9 5
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Fig. 2 e X-ray photoelectron spectra of Pt4f for (a)1#, (b)2#, (c)3#, (d)4#.
spacing of graphite, because a lot of functional groups containing oxygen appear at the interlayers with the oxidization of graphite, which makes the interlayer spacing of graphite increase, and then some functional groups containing oxygen are removed with the reduction reaction of ethanediol, the interlayer spacing decreases. This is consistent with XPS results. Furthermore, Pt(111), (200), (220), (311) peaks of other samples appear at 39.8 , 46.3 , 67.5 , 81.3 , which indicates that Pt is successfully loaded on the carrier. From the Pt peak intensities of the above samples we can discover that Pt peak of Pt/GNs sample with large Pt loading is sharp. According to Debye-Scherrer formula (Formula (2)),
L ¼ 0.94lKa1/( B(2q) cosqB)
(2)
Where lKa1 is the X ray wavelength, B(2q) is the full-width at half maximum of corresponding peak (FWHM, radian), and qB is diffraction angle of corresponding peak. The average particle sizes were calculated as 2.9, 4.3, 3.1, and 3.2 nm for 1# ~4# samples, respectively. From the above results we can conclude that the grain size of 2# sample with higher Pt loading is larger. This can be observed from TEM image. It is because of Pt particle reuniting in Pt/GNs when Pt loading is large, whereas Pt particles of PteSn/GNs with the same Pt loading don't reunite.
Fig. 3 e X-ray photoelectron spectra of Sn3d for (a)3#, (b)4#.
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Fig. 4 e XRD patterns of specimens, GO, 1#, 2#, 3# and 4#.
Fig. 5 shows TEM images of the samples, (a), (b) belong to 2# sample (Pt/GNs), and (b) is the local magnification of (a). (c) and (d) are TEM images of 4# sample (PteSn/GNs), The insert in (d) is the cartogram of the grain sizes. The wrinkles on the graphene sheets are observed, revealing the flake-like shapes of
graphene sheets, and the grain size of 2# is mainly between 1 and 2 nm. Some large particles (about 10 nm grain size) in 2# sample are due to partial particle reuniting. As shown in Fig 6, the Pt and Sn ions would interact with and attach to these surface functional groups on the graphene via a coordination reaction or an ioneexchange reaction [21]. Subsequently, because the standard reduction potential of the Pt4þ/Pt redox pair (0.755 V vs the standard hydrogen electrode (SHE)) is higher than that of Sn4þ/Sn2þ (0.151 V vs SHE), the Sn ions adsorbed on the surface of the graphene can act as reducing agents to reduce the Pt ions in-situ. The resulting Pt atoms by in-situ reduction form clusters attach to the graphene, which can also act as seeds for further particle growth, and the dispersion of Pt particles on the support would be improved [22,23]. In 4# sample, the metal particles are uniformly on the surface of graphene, and there is almost no reuniting in 4# sample, and the grain size is uniformly between 2.5 nm and 3.5 nm.
Electrochemical measurements The cyclic voltammetry (CV) curves for different electrocatalysts in 0.5 M H2SO4 solution are shown in Fig. 7(a), in which H2 adsorption/desorption peaks (0.2 to 0.1 V vs. SCE),
Fig. 5 e TEM images of specimens, (a,b)2# and (c,d)4#. Insets are the corresponding statistical datas.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 4 2 8 8 e1 4 2 9 5
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Fig. 6 e Schematic process for loading Pt and Sn nanoparticles onto the surface of graphene support.
electrochemical double-layer region (0.1 to 0.5 V vs. SCE) and peaks for the formation and reduction of Pt oxide (0.4 to 1 V vs. SCE) can be observed [14,24]. The ECSA of catalyst can be calculated according to Eq. (3): ECSA ¼
QH G 210
(3)
Where QH (mC) represents the charges exchanged during the desorption of H2 on the Pt surface, G (mg) is the Pt loading in the electrode and 210 (mC/cm2) is the charge required to oxidize a monolayer of hydrogen on the Pt surface. The ECSA of different catalyst were calculated based on Eq. (3) and listed in Table 2. Considering the large theoretical specific surface of graphene, such graphene-based Pt catalysts should have higher ECSA. However, with increased Pt loading, the ECSA of graphene-based Pt catalysts decreases due to the aggregation of particles as shown in TEM. It is worth noting that the high loading PteSn/graphene remains small dispersed and small particle as shown in TEM, which might be caused by structural modifications of Pt due to the addition of Sn. As a result, its ECSA is the highest in the samples. Electrocatalytic activities of catalysts for methanol oxidation were also measured by CV. As displayed in Fig. 7(b), all CV
curves show similar methanol oxidation current peak in the forward scan and an oxidation peak in the backward scan corresponding to the removal of the residual carbonaceous species formed in the forward scan [14,25]. The ratio of the forward anodic peak current (If) to the backward anodic peak current (Ib) can be used to evaluate the catalyst tolerance to the intermediate carbonaceous species accumulated on electrode surface [14,24,25]. The higher If/Ib value indicates higher tolerance to intermediate carbon species. The dates obtained from Fig. 7(b) are also listed in Table 2. There is a positive correlation between peak current densities and ECSA. The MOR activity can be improved by the increase in the Pt lattice parameter due to the addition of Sn. It attributes to that the increase in the Pt lattice parameter has facilitated CeH bond cleavage and lower oxidation potential of adsorbed methanol [26,27].
Conclusions (1) Pt/GNs and PteSn/GNs are synthesized using a simple one-step chemical reduction method with ethylene glycol. The wrinkles on the graphene sheets are
Fig. 7 e Cyclic voltammograms of specimens in (a) 0.5 M H2SO4 aqueous solution using a scan rate of 50 mV s¡1 and (b) 0.5 mol/L H2SO4 containing 2 mol/L CH3OH aqueous solutions, using a scan rate of 20 mV s¡1.
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Table 2 e Comparison of electrocatalytic activity of methanol oxidation on Pt/GNs and PteSn/GNs. Samples
ECSA (m2/gPt)
1# 2# 3# 4#
34.18 13.33 45.13 57.89
Forward peak
IF/Ib
Backward peak
IF (mA)
E (V)
Ib (mA)
E (V)
0.563 0.369 0.854 1.703
0.635 0.651 0.652 0.647
0.357 0.123 0.784 1.270
0.438 0.461 0.410 0.447
observed, revealing the flake-like shapes of graphene sheets. Highly dispersed metal nanoparticles are successfully deposited on the graphene supports. Pt is shown mainly to be present in the zero-valence state, and Sn is the form of Sn2þ/4þ species. (2) As the Pt wt.% is increased, the aggregation of particles for the Pt/GNs catalysts begin to appear accordingly. This results in a decrease in the ECSA and MOR mass activity of catalysts. With the same Pt:Sn atomic ratio, Sn is add to the Pt/GNs catalysts which has different Pt wt.%, and the MOR mass activity of catalysts is improved. In particularly, the PteSn/GNs with high Pt loadings show Pt mass activity in methanol electrooxidation that is at least thrice as large as conventional Pt/GNs catalysts. The addition of Sn not only decrease particle growth and particle agglomeration, but also promotes methanol electrooxidation by geometric effects that increases the lattice spacing of Pt. As a result, PteSn synthesized onto graphene displays enhanced catalytic activity.
Acknowledgments This work was supported by the National Natural Science Foundation of China (51372160, 51172152 and 51242007).
references
[1] Stankovich S, Dikin DA, Dommett GHB, Kohlhaas KM, Zimney EJ, Stach EA, et al. Graphene-based composite materials. Nature 2006;442:282e6. [2] Geim AK, Novoselov KS. The rise of graphene. Nat Mater 2007;6:183e91. [3] Sharma S, Ganguly A, Papakonstantinou P, Miao X, Li M, Hutchison JL, et al. Rapid microwave synthesis of CO tolerant reduced graphene oxide-supported platinum electrocatalysts for oxidation of methanol. J Phys Chem C 2010;114:19459e66. [4] Xu C, Wang X, Zhu J. Graphene metal particle nanocomposites. J Phys Chem C 2008;112:19841e5. [5] Si Y, Samulski ET. Exfoliated graphene separated by platinum nanoparticles. Chem Mater 2008;20:6792e7. [6] Kim HJ, Choi SM, Green S, Tompsett GA, Lee SH, Huber GW, et al. Highly active and stable PtRuSn/C catalyst for electrooxidations of ethylene glycol and glycerol. Appl Catal B Environ 2011;101:366e75. [7] Li Y, Tang L, Li J. Preparation and electrochemical performance for methanol oxidation of pt/graphene nanocomposites. Electrochem Commun 2009;11:846e9.
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[8] Xin Y, J-g Liu, Zhou Y, Liu W, Gao J, Xie Y, et al. Preparation and characterization of Pt supported on graphene with enhanced electrocatalytic activity in fuel cell. J Power Sources 2011;196:1012e8. [9] Hu Y, Wu P, Yin Y, Zhang H, Cai C. Effects of structure, composition, and carbon support properties on the electrocatalytic activity of Pt-Ni-graphene nanocatalysts for the methanol oxidation. Appl Catal B Environ 2012;111e112:208e17. [10] He W, Jiang H, Zhou Y, Yang S, Xue X, Zou Z, et al. An efficient reduction route for the production of PdePt nanoparticles anchored on graphene nanosheets for use as durable oxygen reduction electrocatalysts. Carbon 2012;50:265e74. [11] Seo MH, Choi SM, Kim HJ, Kim WB. The graphene-supported Pd and Pt catalysts for highly active oxygen reduction reaction in an alkaline condition. Electrochem Commun 2011;13:182e5. [12] Stankovich S, Dikin DA, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, et al. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007;45:1558e65. [13] Puziy AM, Poddubnaya OI, Socha RP, Gurgul J, Wisniewski M. XPS and NMR studies of phosphoric acid activated carbons. Carbon 2008;46:2113e23. [14] Li Y, Gao W, Ci L, Wang C, Ajayan PM. Catalytic performance of Pt nanoparticles on reduced graphene oxide for methanol electro-oxidation. Carbon 2010;48:1124e30. [15] Wang Y, Liu J, Liu L, Sun DD. High-quality reduced graphene oxide-nanocrystalline platinum hybrid materials prepared by simultaneous co-reduction of graphene oxide and chloroplatinic acid. Nanoscale Res Lett 2011;6:241e8. [16] Kou R, Shao Y, Wang D, Engelhard MH, Kwak JH, Wang J, et al. Enhanced activity and stability of Pt catalysts on functionalized graphene sheets for electrocatalytic oxygen reduction. Electrochem Commun 2009;11:954e7. [17] Chien CC, Jeng KT. Effective preparation of carbon nanotubesupported PteRu electrocatalysts. Mater Chem Phys 2006;99:80e7. [18] Choi SM, Seo MH, Kim HJ, Kim WB. Synthesis of surfacefunctionalized graphene nanosheets with high Pt-loadings and their applications to methanol electrooxidation. Carbon 2011;49:904e9. [19] Singh V, Joung D, Zhai L, Das S, Khondaker SI, Seal S. Graphene based materials: past, present and future. Prog Mater Sci 2011;56:1178e271. [20] Wang H, Robinson JT, Diankov G, Dai H. Nanocrystal growth on graphene with various degrees of oxidation. J Am Chem Soc 2010;132:3270e1. [21] Li W, Liang C, Zhou W, Qiu J, Zhou Z, Sun G, et al. Preparation and characterization of multiwalled carbon nanotubesupported platinum for cathode catalysts of direct methanol fuel cells. J Phys Chem B 2003;107:6292e9. [22] Zhong L-S, Hu J-S, Cui Z-M, Wan L-J, Song W-G. In-situ loading of noble metal nanoparticles on hydroxyl-group-rich titania precursor and their catalytic applications. Chem Mater 2007;19:4557e62.
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[23] Hsin YL, Hwang KC, Yeh CT. Poly (vinylpyrrolidone)-modified graphite carbon nanofibers as promising supports for PtRu catalysts in direct methanol fuel cells. J Am Chem Soc 2007;129:9999e10010. [24] Giorgi L, Makris TD, Giorgi R, Lisi N, Salernitano E. Electrochemical properties of carbon nanowalls synthesized by HF-CVD. Sens Actuators B Chem 2007;126:144e52. [25] Lin Y, Cui X, Yen CH, Wai CM. PtRu/Carbon nanotube nanocomposite synthesized in supercritical fluid: a novel electrocatalyst for direct methanol fuel cells. Langmuir 2005;21:11474e9.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
The effect of Sn on platinum dispersion in Pt/graphene catalysts for the methanol oxidation reaction Xiaomin Wang*, Jie Lian, Yong Wang College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China
article info
abstract
Article history:
Sn-modified platinum catalysts are presently one of the most active catalysts for the room
Available online 21 June 2014
temperature electrooxidation of ethanol at low potentials. In this study, PteSn/graphene catalysts containing different ratios of Pt and Sn were prepared by the solution-phase
Keywords:
reduction. Microstructural characterization shows that metallic Pt, PteSn alloy and tin
PteSn/graphene
dioxide (SnO2) nanoparticles are distributed on the graphene sheets in the synthetic pro-
Equal ratios
cess. In terms of the electrocatalytic properties, graphene-supported PteSn catalysts
Methanol electrooxidation
exhibit much higher current densities with increasing Sn proportions. It's proved that the
Microstructrual characterization
addition of Sn not only decreases catalyst particles growth and agglomeration, but also promotes methanol electrooxidation by geometric effects on expanding Pt's lattice spacing, causing a synergistic effect between Pt and Sn nanoparticles. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Direct methanol fuel cells (DMFC) are considered to be the most promising energy devices for mobile and stationary applications such as cellular phones, laptop computers and secondary electricity generators [1]. Nowadays, the main problem is the deficient activity and selectivity of anode electrocatalysts at temperatures compatible with available membranes [2]. Platinum is the best catalyst for alcohol oxidation, but its surface is rapidly poisoned by strongly irreversibly adsorbed CO-like intermediates coming from the dissociative adsorption of alcohols. The manufacture of Pt-based nanostructured bimetallic catalysts, such as Sn, by electrochemical techniques promotes the high catalytic activity of the catalysts, since it is a simple operation and low cost, which also makes it possible to obtain
deposits of high purity and uniform deposition [3e5]. The presence of Sn alters the electronic structure of Pt, weakening CO adsorption on the Pt surface, thus reducing catalyst poisoning. In addition, the geometric environment could be readily changed with Sn additions to the fcc Pt crystallites by forming solid solution of PteSn alloy phase, showing the larger lattice parameter. This elongation of crystal structure may affect catalytic reactions that require specific geometric arrangements of the surface atoms, thus leading to the improvement of catalytic properties [6]. Moreover, Sn also provides OHads species by dissociating water at a lower potential than that on Pt, and these OHads species are helpful for the subsequent oxidation of adsorbed intermediates to acetaldehyde, acetic acid, and CO2 [7e9]. Besides the active metals, support is another key factor for a highly efficient catalyst, which requires low cost, good
* Corresponding author. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.ijhydene.2014.05.158 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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chemical stability, and large surface area, as well as high conductivity. To date, some research has been developed to the preparation of graphene-supported PteSn. As a simple method, the one-step synthesis procedure of graphenesupported PteSn to increase the electrocatalytic ability seems very attractive. The strategies of single step preparation of graphene supported PteSn can be roughly classified as solution-phase reduction [10], thermal decomposition [11], microwave-assisted method [12], and electrochemical deposition [13]. Based on our previous exploratory works [14], graphene-supported PteSn catalysts have been prepared by solution-phase reduction, and their enhanced electrocatalytic activity have been investigated as anode catalysts for a DMFC. Thus, in order to improve the performance of PteSn anode catalysts, it is of great importance to hunt for the roles of Sn for the Pt/graphene catalysts. In the present work, the role of the structural features of PteSn deposited graphene catalysts in the electro-oxidation of ethanol was analyzed. PteSn nanostructured materials of different compositions were prepared by multiple cycles of potentiostatic pulses onto graphene. Physical property characterizations were carried out to investigate the microstructure, dispersion and degree of Sn in the bimetallic catalysts. In addition, the electrochemical activity of the catalysts for ethanol and ethylene glycol electro-oxidation was evaluated by cyclic voltammetry and chronoamperometry.
Experimental Synthesis of PteSn/graphene catalysts As the raw material, graphene oxide was produced by the modified Hummers method [12,15]. For the synthesis of graphene-supported PteSn (PteSn/graphene) catalysts, 30 mg of graphene oxide powder was dispersed in 40 ml of ethylene glycol solution. Subsequently, a certain amount of hexachloroplatinic acid-EG (H2PtCl6-EG) solution (0.5 mol/L) and tin dichlorideeEG (SnCl2eEG) solution (0.5 mol/L) were added to the graphene oxide solution and sonicated for 3 h. The pH of the solution was adjusted to 9-10 using sodium hydroxideeEG (NaOHeEG) solution, and then the solution was stirred under flowing argon at 130 C for 3 h. The solid material produced was then washed with water and then ethanol and finally dried in a vacuum oven at 50 C for 24 h. The catalysts prepared as such were designated as PteSn/GNs. Similar procedures were used to prepare graphene supported-Pt catalysts (Pt/GNs).
Electrochemical measurements An electrochemical instrument (Autolab GSTAT302) and a conventional three-electrode test cell with a saturated calomel electrode (SCE) as the reference electrode and platinum wire as the counter electrode were used for the electrochemical measurements. To examine the electrocatalytic properties, cyclic voltammetry was performed using a conventional three electrode electrochemical system in 0.5 mol/L H2SO4 aqueous solution and 0.5 mol/L H2SO4 þ 0.5 mol/L CH3OH aqueous solution at room temperature. The modified glassy carbon electrode with catalysts, Pt wire and saturated calomel electrode were used as the working electrode, counter electrode and reference electrode, respectively. Catalyst inks were prepared as follows: catalyst powder (5 mg) was dispersed in 5% Nafion (100 mL) and isopropyl alcohol (900 mL) and the mixture was sonicated for 30 min. Twenty-five micro liter of the catalyst inks from each kind of catalyst was then coated onto the surface of three glassy carbon electrodes.
Results and discussion Physical property Table 1 shows XPS results of the samples. 1# and 2# are Pt/GNs samples, 3# and 4# are PteSn/GNs samples. Pt:Sn indicates the atomic ratio of Pt and Sn, Pt wt% and Sn wt% indicate the mass fractions of Pt and Sn, respectively. Modified carbon content indicates the ratio of carbon atoms with the functional groups containing Oxygen in the samples. Pt0 indicates the ratio of the zero-valence Pt in all the Pt atoms. Sn2þ/4þ represents the ratios of divalent and quadrivalent Sn atoms in all the Sn atoms. In XPS spectra, Fig. 1 shows C1s spectra of the samples, and the peaks at 288.4 , 285.9 , 286.8 and 288.9 are corresponding to sp2 hybrid CeC and the functional groups containing Oxygen, such as CeOH, CeOeC, HOeC]O [4,12,13]. Fig. 1(a) shows large quantities of functional groups containing Oxygen in GO. Most of the functional groups containing Oxygen are removed and only a small part of the functional groups containing Oxygen still exist after GO is reduced by ethylene glycol(Fig. 1ceh). From Table 1 we find that carbon atoms in the functional groups containing Oxygen account for 62.73% of the total number of carbon atoms, and it is between 35% and 46% when the reduction reaction occurs. The functional groups containing Oxygen and GNs surface defects play a role in anchoring Pt and Sn precursor [14,16,17]. The functional
Characterization The structural characteristics of the synthesized catalysts were investigated by X-ray diffraction (XRD) (Y-2000) using CuKa as the radiation source, and the working voltage and current were maintained at 30 kV and 20 mA. High resolution transmission electron microscopy (HRTEM) were performed on a JEM-2010 equipped with a LaB6 electron gun operating at 200 kV. X-ray photoelectron spectra (XPS) were acquired using an ESCALAB250 spectrometer fitted with an Al X-ray source.
Table 1 e XPS analysis: elemental atomic concentrations of as-prepared samples. Number Pt:Sn Pt wt% Sn wt% Modified carbon content GO 1# 2# 3# 4#
e e e 1.17 1.03
e 10.07% 16.33% 13.37% 22.38%
e 0 0 6.95% 13.22%
62.73% 45.82% 43.35% 35.08% 40.12%
Pt (0) e 54.36% 56.80% 59.91% 63.17%
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Fig. 1 e X-ray photoelectron spectra of C1s for (a)GO, (b)1#, (c)2#, (d)3#, (e)4#.
groups containing Oxygen can prevent Pt nanoparticles from reuniting, so dispersive and small nanoparticles form [18e20]. From Pt4f spectra(Fig. 2) we can find that the double peak of Pt 4f7/2 and Pt 4f5/2 is at 70 eV ~ 75 eV, and Pt 4f7/2 is composed of Pt0 and Pt2þ, which means that the quadrivalent Pt in chloroplatinic acid is reduced to zero valent Pt and divalent Pt. The Sn 3d5/2 and Sn 3d3/2 double peak of Sn3d appears at 484 eV ~ 495 eV. Fig. 3 shows that Sn mainly exists in the oxidation state Sn2þ/4þ, which is in correspondance with the fact that Sn is easily oxidized at room temperature, and that it is difficult to distinguish Sn2þ and Sn4þ from XPS spectrum. Also from Table 1 we can find that Pt in Pt/GNs and PteSn/GNs catalysts mainly exists in zero valent Pt, and Sn in PteSn/GNs catalyst mainly exists in the oxidation state Sn2þ/4þ.
The interplanar distance of the samples is calculated by Bragg equation (Formula (1))
2d sinq ¼ l
(1)
Among them, d is the interplanar distance, l is electron wavelength, and q is grazing angle. In XRD spectrum, C(002) peak of graphite is at 26 . Fig. 4 shows C(002) peak of graphene oxide appears at 11.8 , and C(002) peaks of 1#~4# samples, reduced by ethanediol, appear at 22.8 . According to Bragg equation, the interlayer spacing of graphene obtained by reduction reaction is between the interlayer spacing of graphene oxide and the interlayer
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Fig. 2 e X-ray photoelectron spectra of Pt4f for (a)1#, (b)2#, (c)3#, (d)4#.
spacing of graphite, because a lot of functional groups containing oxygen appear at the interlayers with the oxidization of graphite, which makes the interlayer spacing of graphite increase, and then some functional groups containing oxygen are removed with the reduction reaction of ethanediol, the interlayer spacing decreases. This is consistent with XPS results. Furthermore, Pt(111), (200), (220), (311) peaks of other samples appear at 39.8 , 46.3 , 67.5 , 81.3 , which indicates that Pt is successfully loaded on the carrier. From the Pt peak intensities of the above samples we can discover that Pt peak of Pt/GNs sample with large Pt loading is sharp. According to Debye-Scherrer formula (Formula (2)),
L ¼ 0.94lKa1/( B(2q) cosqB)
(2)
Where lKa1 is the X ray wavelength, B(2q) is the full-width at half maximum of corresponding peak (FWHM, radian), and qB is diffraction angle of corresponding peak. The average particle sizes were calculated as 2.9, 4.3, 3.1, and 3.2 nm for 1# ~4# samples, respectively. From the above results we can conclude that the grain size of 2# sample with higher Pt loading is larger. This can be observed from TEM image. It is because of Pt particle reuniting in Pt/GNs when Pt loading is large, whereas Pt particles of PteSn/GNs with the same Pt loading don't reunite.
Fig. 3 e X-ray photoelectron spectra of Sn3d for (a)3#, (b)4#.
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Fig. 4 e XRD patterns of specimens, GO, 1#, 2#, 3# and 4#.
Fig. 5 shows TEM images of the samples, (a), (b) belong to 2# sample (Pt/GNs), and (b) is the local magnification of (a). (c) and (d) are TEM images of 4# sample (PteSn/GNs), The insert in (d) is the cartogram of the grain sizes. The wrinkles on the graphene sheets are observed, revealing the flake-like shapes of
graphene sheets, and the grain size of 2# is mainly between 1 and 2 nm. Some large particles (about 10 nm grain size) in 2# sample are due to partial particle reuniting. As shown in Fig 6, the Pt and Sn ions would interact with and attach to these surface functional groups on the graphene via a coordination reaction or an ioneexchange reaction [21]. Subsequently, because the standard reduction potential of the Pt4þ/Pt redox pair (0.755 V vs the standard hydrogen electrode (SHE)) is higher than that of Sn4þ/Sn2þ (0.151 V vs SHE), the Sn ions adsorbed on the surface of the graphene can act as reducing agents to reduce the Pt ions in-situ. The resulting Pt atoms by in-situ reduction form clusters attach to the graphene, which can also act as seeds for further particle growth, and the dispersion of Pt particles on the support would be improved [22,23]. In 4# sample, the metal particles are uniformly on the surface of graphene, and there is almost no reuniting in 4# sample, and the grain size is uniformly between 2.5 nm and 3.5 nm.
Electrochemical measurements The cyclic voltammetry (CV) curves for different electrocatalysts in 0.5 M H2SO4 solution are shown in Fig. 7(a), in which H2 adsorption/desorption peaks (0.2 to 0.1 V vs. SCE),
Fig. 5 e TEM images of specimens, (a,b)2# and (c,d)4#. Insets are the corresponding statistical datas.
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Fig. 6 e Schematic process for loading Pt and Sn nanoparticles onto the surface of graphene support.
electrochemical double-layer region (0.1 to 0.5 V vs. SCE) and peaks for the formation and reduction of Pt oxide (0.4 to 1 V vs. SCE) can be observed [14,24]. The ECSA of catalyst can be calculated according to Eq. (3): ECSA ¼
QH G 210
(3)
Where QH (mC) represents the charges exchanged during the desorption of H2 on the Pt surface, G (mg) is the Pt loading in the electrode and 210 (mC/cm2) is the charge required to oxidize a monolayer of hydrogen on the Pt surface. The ECSA of different catalyst were calculated based on Eq. (3) and listed in Table 2. Considering the large theoretical specific surface of graphene, such graphene-based Pt catalysts should have higher ECSA. However, with increased Pt loading, the ECSA of graphene-based Pt catalysts decreases due to the aggregation of particles as shown in TEM. It is worth noting that the high loading PteSn/graphene remains small dispersed and small particle as shown in TEM, which might be caused by structural modifications of Pt due to the addition of Sn. As a result, its ECSA is the highest in the samples. Electrocatalytic activities of catalysts for methanol oxidation were also measured by CV. As displayed in Fig. 7(b), all CV
curves show similar methanol oxidation current peak in the forward scan and an oxidation peak in the backward scan corresponding to the removal of the residual carbonaceous species formed in the forward scan [14,25]. The ratio of the forward anodic peak current (If) to the backward anodic peak current (Ib) can be used to evaluate the catalyst tolerance to the intermediate carbonaceous species accumulated on electrode surface [14,24,25]. The higher If/Ib value indicates higher tolerance to intermediate carbon species. The dates obtained from Fig. 7(b) are also listed in Table 2. There is a positive correlation between peak current densities and ECSA. The MOR activity can be improved by the increase in the Pt lattice parameter due to the addition of Sn. It attributes to that the increase in the Pt lattice parameter has facilitated CeH bond cleavage and lower oxidation potential of adsorbed methanol [26,27].
Conclusions (1) Pt/GNs and PteSn/GNs are synthesized using a simple one-step chemical reduction method with ethylene glycol. The wrinkles on the graphene sheets are
Fig. 7 e Cyclic voltammograms of specimens in (a) 0.5 M H2SO4 aqueous solution using a scan rate of 50 mV s¡1 and (b) 0.5 mol/L H2SO4 containing 2 mol/L CH3OH aqueous solutions, using a scan rate of 20 mV s¡1.
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Table 2 e Comparison of electrocatalytic activity of methanol oxidation on Pt/GNs and PteSn/GNs. Samples
ECSA (m2/gPt)
1# 2# 3# 4#
34.18 13.33 45.13 57.89
Forward peak
IF/Ib
Backward peak
IF (mA)
E (V)
Ib (mA)
E (V)
0.563 0.369 0.854 1.703
0.635 0.651 0.652 0.647
0.357 0.123 0.784 1.270
0.438 0.461 0.410 0.447
observed, revealing the flake-like shapes of graphene sheets. Highly dispersed metal nanoparticles are successfully deposited on the graphene supports. Pt is shown mainly to be present in the zero-valence state, and Sn is the form of Sn2þ/4þ species. (2) As the Pt wt.% is increased, the aggregation of particles for the Pt/GNs catalysts begin to appear accordingly. This results in a decrease in the ECSA and MOR mass activity of catalysts. With the same Pt:Sn atomic ratio, Sn is add to the Pt/GNs catalysts which has different Pt wt.%, and the MOR mass activity of catalysts is improved. In particularly, the PteSn/GNs with high Pt loadings show Pt mass activity in methanol electrooxidation that is at least thrice as large as conventional Pt/GNs catalysts. The addition of Sn not only decrease particle growth and particle agglomeration, but also promotes methanol electrooxidation by geometric effects that increases the lattice spacing of Pt. As a result, PteSn synthesized onto graphene displays enhanced catalytic activity.
Acknowledgments This work was supported by the National Natural Science Foundation of China (51372160, 51172152 and 51242007).
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