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Preparation and characterization of carbon-supported Pt, PtSnO2 and PtRu nanoparticles for direct methanol fuel cells
Materials Chemistry and Physics 105 (2007) 222–228
Preparation and characterization of carbon-supported Pt, PtSnO2 and PtRu nanoparticles for direct methanol fuel cells Zhaolin Liu ∗ , Liang Hong, Siok Wei Tay Institute of Materials Research & Engineering, 3 Research Link, Singapore 117602, Singapore Received 5 September 2006; received in revised form 11 April 2007; accepted 13 April 2007
Abstract Nanosized Pt, PtSnO2 and PtRu particles were prepared by a microwave-assisted polyol process. Vulcan XC-72 was selected as catalyst support for Pt, PtSnO2 and PtRu particles. The catalysts were characterized by TEM, XRD and XPS. TEM investigations showed nanoscale particles and narrow size distribution for Pt, PtSnO2 and PtRu catalysts. All Pt, PtSnO2 and PtRu catalysts showed the X-ray diffraction pattern of a face-centered cubic (fcc) crystal structure. As evidenced by XPS, most Pt, Sn and Ru atoms in the nanoparticles were Pt(0), Sn(IV) and Ru(0). Preliminary tests on a single cell of a direct methanol fuel cell (DMFC) indicate that PtSnO2 /C catalyst gave the best electrocatalytic performance among three catalysts. The cell performance was analyzed based on the measurement of the current–voltage characteristics and impedance spectroscopy in the single fuel cell. © 2007 Elsevier B.V. All rights reserved. Keywords: PtSnO2 nanoparticles; PtRu nanoparticles; Direct methanol fuel cell; Catalyst loading; Impedance spectroscopy
1. Introduction In the past 20 years the direct methanol fuel cells (DMFCs) have been widely studied and considered as possible power sources for the portable electric apparatus and electric vehicles. DMFCs has a variety of benefits such as high energy density of methanol, availability and portability, but methanol ‘crossover’ from anode to cathode through membrane leads to low system efficiency. The carbon supported Pt catalyst has proven unfavourable for electro-oxidation of methanol. In the literature, both PtRu and PtSn systems have been reported to be promising catalysts for electro-oxidation of methanol or ethanol in direct methanol or ethanol fuel cells [1–9]. In terms of the electrode structure, up to now, the electrodes of the DMFC are generally based on the gas diffusion electrodes employed in proton exchange membranes fuel cells. Typically, the structure of such electrodes comprises a catalyst layer and a diffusion layer, the latter being carbon paper or carbon cloth. To optimize the electrode structure, many parameters such as the composition and loading of the catalyst, ionomer content in the catalyst layer and porosity of the electrode should be evaluated.
∗
Corresponding author. Tel.: +65 68727532; fax: +65 68720785. E-mail address: [email protected] (Z. Liu).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.04.045
In the present work, pure Pt, Pt65 Sn35 and Pt50 Ru50 (subscript denotes the atomic percentage of the alloying metal) anode catalysts were synthesized by a simple microwave-assisted polyol procedure. These catalysts were tested as anode in single direct methanol fuel cells. In spite of many factors affecting its performance, the single fuel cell is still considered as a very effective reactor to evaluate electrocatalysts when other operation parameters are well defined. Nafion 115 membrane was used as solid electrolyte and commercial Pt/C as cathode catalyst for the reaction of oxygen reduction in all single fuel cell tests. Other parameters, such as electrode preparation and membraneelectrode assembly (MEA) preparation procedure were kept consistent. The effect of anode catalyst loading on the performance of the DMFC was investigated. The cell performance was evaluated and analyzed by measuring the current–voltage characteristics and electrode impedance. 2. Experimental The Pt/C, PtSnO2 /C (Cabot Vulcan XC-72; Pt65 Sn35 , subscript denotes the atomic percentage of the alloying metal) and PtRu/C (Cabot Vulcan XC-72; Pt50 Ru50 , subscript denotes the atomic percentage of the alloying metal) catalysts were prepared by microwave heating of ethylene glycol (EG) solutions of Pt and Sn salts [10]. A typical preparation would consist of the following steps: 2.6 ml of 0.02 M H2 PtCl6 ·6H2 O (Aldrich, A.C.S. Reagent) and 1.4 ml of 0.02 M SnCl2 ·2H2 O (Aldrich, A.C.S. Reagent) was mixed with 20 ml of ethy-
Z. Liu et al. / Materials Chemistry and Physics 105 (2007) 222–228 lene glycol (Mallinckrodt, AR). 0.5 ml of 0.8 M NaOH was added dropwise. 0.04 g of Vulcan XC-72 carbon with a specific BET surface area of 250 m2 g−1 and an average particle size of 40 nm was added to the mixture and sonicated. The solution was placed in a CEM “Discover” microwave reactor (CEM Corporation) with the maximum temperature set at 170 ◦ C at atmospheric conditions for 60 s. The flask in volume of 50 ml and condenser for refluxing were used. The resulting suspension was filtered; and the residue was washed with acetone and dried at 100 ◦ C over night in a vacuum oven. An EG&G Model 263A potentiostat/galvanostat, and a conventional threeelectrode test cell were used for electrochemical measurements. The working electrode was a thin layer of Nafion-impregnated catalyst cast on a vitreous
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carbon disk held in a Teflon cylinder. The catalyst layer was obtained in the following way: (i) a slurry was first prepared by sonicating for 1 h a mixture of 0.5 ml of deionized water, 0.013 g of Pt/C, PtRu/C or PtSnO2 /C catalyst, and 0.5 ml of Nafion solution (Aldrich: 5 w/o Nafion); (ii) 4 l of the slurry was pipetted and spread on the carbon disk; (iii) the electrode was then dried at 90 ◦ C for 1 h and mounted on a stainless steel support. The surface area of the vitreous carbon disk was 0.25 cm2 . Pt gauze and Ag/AgCl electrode were used as the counter and reference electrodes respectively. All potentials quoted in this report were referred to the Ag/AgCl. All electrolyte solutions were deaerated by high-purity argon for 2 h prior to any measurement. For cyclic voltammetry of methanol oxidation, the electrolyte solution was 2 M CH3 OH in 1 M H2 SO4 ,
Fig. 1. TEM images of the PtSnO2 /C (a), PtRu/C (b) and Pt/C (c) catalysts. Histograms of particle size distribution for PtSnO2 /C (d), PtRu/C (e) and Pt/C (f) catalysts.
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Fig. 2. XRD patterns of microwave-synthesized Pt/C, PtSnO2 /C and PtRu/C catalysts.
then deposited on 3 mm Cu grids covered with a continuous carbon film. X-ray diffraction (XRD) patterns were recorded by a Bruker GADDS diffractometer ˚ operating at 40 kV and with area detector using a Cu K␣ source (λ = 1.54056 A) 40 mA. The samples were prepared by depositing carbon-supported nanoparticles on a glass slide, and drying the later in vacuum overnight. The samples were also analyzed by X-ray photoelectron spectroscopy (XPS) on a VG ESCALAB MKII spectrometer. Narrow scan photoelectron spectra were recorded for Pt(4f), Sn(3d) and Ru(3p) and a vendor supplied curve-fitting program (VGX900) was used for spectral deconvolution. The MEA (membrane electrode assembly) for the DMFC test cell was made by hot-pressing pretreated Nafion 115 together with an anode sheet and a cathode sheet. The anode sheet was a carbon paper (Toray TGPH-090) with carbon-supported Pt, PtSn and PtRu catalyst layer. The cathode sheet was a carbon paper with a carbon-supported 20 wt.% Pt catalyst layer supplied by Alfa Aesar. The Pt loadings at the anode and cathode were 0.8–6.8 mg cm−2 and 0.8 mg cm−2 , respectively, and the effective electrode area was 6 cm2 . The fuel was 2 M CH3 OH delivered at 2 ml min−1 by a micropump and oxygen flow was regulated by a flowmeter at 500 cm3 min−1 . Polarization curves were obtained by using a Hewlett Packard 6051A DC electronic load system. The measurement of anode impedance was performed by connecting the EG&G model 263A to a FRD-100 lock-in amplifier over a frequency range of 100 kHz to 1 mHz. Impedances were measured under galvanostatic or potentiostatic control of the cell. The anode was supplied with a 2 M aqueous solution of methanol at 2 ml min−1 or 4 ml min−1 by a micropump. The cath-
which was prepared from high-purity sulphuric acid, high-purity grade methanol and distilled water. The catalysts were examined by TEM on a JEOL JEM 2010. For microscopic examinations the samples were first ultrasonicated in acetone for 1 h and
Fig. 3. X-ray photoelectron spectra of the PtSnO2 /C catalyst.
Fig. 4. X-ray photoelectron spectra of the PtRu/C catalyst.
Z. Liu et al. / Materials Chemistry and Physics 105 (2007) 222–228 ode was operated on oxygen; it served as a reference and counter electrode [11].
3. Results and discussion In our approach, Pt, PtSnO2 and PtRu nanoparticles are prepared and directly deposited on the carbon surface by microwave heating of ethylene glycol (EG) solutions of Pt, Sn and Ru salts. The supported PtSnO2 and PtRu catalysts prepared as such are expected to maintain good electrocatalytic activity and CO tolerance in the direct methanol oxidation reaction at room temperature. Fig. 1(a)–(c) is the typical TEM images of PtSnO2 /C, PtRu/C and Pt/C catalysts, showing a remarkably uniform and high dispersion of metal particles on the carbon surface. The particle size distribution of the metal in the supported catalyst was obtained by directly measuring the size of 150 randomly chosen particles in the magnified TEM image. The average diameters of 4.6 nm (for PtSnO2 ), 4.1 nm (for PtRu) and 3.8 nm (for Pt) were accompanied by relatively narrow particle size distributions as shown in Fig. 1(c)–(e) (standard deviations are 0.4 nm, 0.4 nm and 0.3 nm, respectively). The microwave assisted heating of H2 PtCl6 /SnCl2 or RuCl3 /NaOH/H2 O in ethylene glycol had evidently facilitated the formation of smaller and more uniform Pt, PtSnO2 and PtRu particles and their dispersion on the Vulcan carbon support. It is generally agreed that the size of metal nanoparticles is determined by the rate of reduction of the metal precursor. The dielectric constant (41.4 at 25 ◦ C) and the dielectric loss of ethylene glycol are high, and hence rapid heating occurs easily under microwave irradiation [12]. In ethylene glycol mediated reactions (the ‘polyol’ process), ethylene glycol also acts as a reducing agent to reduce the metal ion to metal powders [13]. The fast heating by microwave accelerates the reduction of the metal precursor and the nucleation of the metal clusters. The easing of the nucleation limited process greatly assists in small particle formation. Additionally the homogeneous microwave heating of liquid samples reduces the
Fig. 5. Cyclic voltammograms of PtSnO2 /C, PtRu/C and Pt/C catalysts in 1 M H2 SO4 , 2 M CH3 OH with a scan rate of 10 mV s−1 at room temperature.
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temperature and concentration gradients in the reaction medium, thus providing a more uniform environment for the nucleation and growth of metal particles. The carbon surface may contain sites suitable for heterogeneous nucleation and the presence of a carbon surface interrupts particle growth. The smaller and nearly single dispersed Pt, PtSnO2 and PtRu nanoparticles on carbon XC-72 prepared by microwave irradiation can be rationalized in terms of these general principles. The power XRD pattern for Pt/C, PtSnO2 /C and PtRu/C are shown in Fig. 2. The diffraction peak between 20◦ and 30◦ observed in all the XRD patterns of the carbon-supported catalysts is due to the (0 0 2) reflection of the hexagonal structure of Vulcan XC-72 carbon. The diffraction peaks at about 39, 46, 68 and 81 are due to the Pt(1 1 1), (2 0 0), (2 2 0), and (3 1 1) reflections, respectively, which represents the typical character of a crystalline Pt face-centered cubic (fcc) phase. There are
Fig. 6. Polarization curves (a) and output power (b) of single cell at operating temperature of 80 ◦ C. Anode: PtSnO2 /C, PtRu/C and Pt/C with a Pt loading (3.4 mg cm−2 ), 2 M CH3 OH 2 ml min−1 . Cathode: Pt/C with a Pt loading (0.8 mg cm−2 ), O2 500 cm3 min−1 .
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not other distinct reflection peaks in all spectra than those of the four peaks mentioned above, indicating that these electrocatalysts have prevailed Pt (fcc) crystal structure. The 2θ values for PtRu/C and PtSnO2 /C were shifted to slightly higher values (40.5◦ ) and lower values (39.5◦ ), respectively, whereas the value was 39.9◦ for pure Pt. The same trend was replicated for the Pt(2 2 0) diffraction, which was 68.1◦ for Pt/C, 69.1◦ for PtRu/C, and 67.0◦ for PtSnO2 /C. The shift for PtRu/C is an indication of the reduction in lattice constant. The reduction of the lattice constant arose primarily from the substitution of platinum atoms by Ru atoms, which led to the contraction of the fcc lattice, an indication of the formation of PtRu alloys. The addition of Sn to the Pt/C increases the lattice constant of fcc crystal inducing the (2 2 0) reflection peak shift to lower position. The differences of the lattice constants of PtRu and PtSnO2 samples indicate that the interaction between Pt and Sn is contrary to that between Pt and Ru [4,14]. The surface oxidation states of the PtSnO2 and PtRu catalysts were investigated by X-ray photoelectron spectroscopy (XPS). As the binding energy (BE) for the Ru 3d line of zero-valent ruthenium at 284.3 eV [15] is very close to the C 1s line resulting from adventitious carbonaceous species, the Ru 3p spectrum was used instead for the analysis of Ru oxidation state. Fig. 3 shows the Pt 4f and Ru 3p regions of the XPS spectrum of the PtRu/C catalyst. The Pt 4f signal consisted of three pairs of doublets. The most intense doublet (71.07 eV and 74.4 eV) was due to metallic Pt. The second set of doublets (72.4 eV and 75.7 eV), which was observed at BE 1.4 eV higher than Pt(0), could be assigned to the Pt(II) chemical state as in PtO and Pt(OH)2 [16]. The third doublet of Pt was the weakest in intensity, and occurred at even higher BEs (74.2 eV and 77.7 eV). These are the indications that they were most likely caused by a small amount of Pt(IV) species on the surface. The slight shift in the Pt(0) peak to higher binding energies is a known effect for small particles, as has been reported by Roth et al. [17]. The Ru 3p3/2 signal could be deconvoluted into two distinguishable pairs of peaks of different intensities located at BE = 461.1 eV and 462.7 eV which corresponded well with Ru(0) and RuO2 [15], respectively. Fig. 4 shows the Pt 4f and Sn 3d regions of the XPS spectrum of the PtSnO2 /C catalyst. As stated previously, the Pt 4f signal also consisted of three pairs of doublets. The Sn 3d spectra have been deconvoluted into two components and are ascribed to elemental Sn and Sn(IV) oxide. The binding energies and relative intensities of the elemental Sn and Sn(IV) oxide obtained from their XPS spectra are 485.4 eV and 17%, 486.7 eV and 83%, respectively. From these data a substantial amount of SnO2 was observed to be present in the PtSnO2 catalyst. The PtSnO2 /C, PtRu/C and Pt/C catalysts are characterized by cyclic voltammetry in an electrolyte of 1 M H2 SO4 and 2 M methanol, and the resulting voltammograms are shown in Fig. 5. The comparison of onset potentials for methanol oxidation shows that the onset potentials for PtSnO2 /C (0.18 V) and PtRu/C (0.20 V) are lower than that for Pt/C (0.30 V), which indicates a high electrocatalytic activity with respect to methanol oxidation for PtSnO2 /C and PtRu/C catalysts compared to the Pt/C catalyst. Ru and Sn or SnO2 can supply surface oxygencontaining species for the oxidative removal of CO-like species
Fig. 7. Effect of Pt loading in the PtSnO2 /C anode on the polarization curves (a) and output power (b) of single cell at operating temperature of 80 ◦ C. Anode: PtSnO2 /C with different Pt loading, 2 M CH3 OH 2 ml min−1 . Cathode: Pt/C with a Pt loading (0.8 mg cm−2 ), O2 500 cm3 min−1 . Effect of Pt loading in the PtSnO2 /C anode on the polarization curves (c) of single cell at operating temperature of 80 ◦ C, 2 M CH3 OH 4 ml min−1 , other conditions unchanged.
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strongly adsorbed on adjacent Pt active sites, which is the socalled bi-functional mechanism and activate the chemisorbed CO-like intermediates. PtSnO2 and PtRu catalysts improve the removal of COads species formed on the platinum surface during methanol electro-oxidation. Therefore, the addition of Sn or to Pt enhances the electro-oxidation activity of methanol. Fig. 6(a) shows the performances of single cells with Pt/C, PtRu/C and PtSnO2 /C as anode catalysts. Each data point represents typical steady state voltages which were taken after continuous operation for 60 s at the indicated current density. The single methanol fuel cell using pure Pt, PtRu and PtSnO2 as anode catalysts exhibits a very low open circuit voltage of 0.50 V, 0.54 V and 0.55 V, which are far from the standard electromotive force of 1.213 V [18]. The great difference between the open circuit potential and the standard electromotive force is mainly attributed to the lower anode catalytic activity and the methanol crossover. PtSnO2 /C catalyst shows higher activity with respect to Pt/C and PtRu/C catalysts in the single fuel cell tests. As seen from Fig. 6(b), the maximum power density of this single fuel cell is 27 mW cm−2 , 42 mW cm−2 and 50 mW cm−2 at 80 ◦ C for Pt/C, PtRu/C and PtSnO2 /C as anode. Fig. 7(a) shows the performance of the single cell for various amounts of Pt varying from 0.8 mg cm−2 to 6.8 mg cm−2 in the PtSnO2 /C anode. The DMFC was operated at 80 ◦ C with 2 M methanol at anode and oxygen at cathode. The cell performance increases throughout the whole range of current densities when the Pt loading was increased from 0.8 mg cm−2 to 5.0 mg cm−2 . DMFC performance is largely limited by charge transfer kinetics, the activation overvoltage is major portion in the total overvoltage. When the Pt loading was further increased, the performance started to decline at high current densities in the case where the loading was 6.8 mg cm−2 . The thickness of the electrode increases with increasing catalyst loading, and a steeper concentration gradient for methanol appears. The concentration overvoltage resulted from the mass transfer resistance
of methanol through the catalyst layer. Fig. 7(c) shows the effect of flow rate of methanol at high Pt loading sample. When the flow rate of methanol was increased from 2 ml min−1 to 4 ml min−1 , the declination of cell performance at high current densities in the Pt loading of 6.8 mg cm−2 disappeared. As presented in Fig. 7(b), the maximum power density of 84 mW cm−2 was obtained from the cell with a Pt loading of 5.0 mg cm−2 in the PtSnO2 /C anode. The impedance plots for the DMFCs with different Pt loading in the PtSnO2 /C anode are shown in Fig. 8. The distorted semicircles occur at higher frequencies, and the size decreased with increasing Pt loading from 0.8 mg cm−2 to 6.8 mg cm−2 . The semicircles correspond to the impedance behavior of methanol electro-oxidation kinetics. The inductance loops at very low frequencies are due to the reaction mechanism with a series of single steps of adsorbed intermediate species as reported by M¨uller et al. and Nakagawa and Xiu [11,19]. In this
Fig. 8. Nyquist plots of the full-cell impedance with different Pt loading in the PtSnO2 /C anode at the cell voltage of 0.3 V and operating temperature of 80 ◦ C. Anode: PtSn/C with different Pt loading, 2 M CH3 OH 2 ml min−1 . Cathode: Pt/C with a Pt loading (0.8 mg cm−2 ), O2 500 cm3 min−1 .
Fig. 9. Effect of different temperatures on the polarization curves (a) and output power (b) of single cell. Anode: PtSnO2 /C with a Pt loading (3.4 mg cm−2 ), 2 M CH3 OH 2 ml min−1 . Cathode: Pt/C with a Pt loading (0.8 mg cm−2 ), O2 500 cm3 min−1 .
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Fig. 10. Nyquist plots of the full-cell impedance with different temperatures at the cell voltage of 0.3 V. Anode: PtSnO2 /C with a Pt loading (3.4 mg cm−2 ), 2 M CH3 OH 2 ml min−1 . Cathode: Pt/C with a Pt loading (0.8 mg cm−2 ), O2 500 cm3 min−1 .
reactions. The PtSnO2 and PtRu nanoparticles, which were uniformly dispersed on carbon, were 2–7 nm in diameters. The PtSnO2 /C and PtRu/C catalysts prepared as such displayed the characteristic diffraction peaks of a Pt face-centered cubic (fcc), excepting that the 2θ values were shifted to slightly lower value for PtSnO2 /C and higher value for PtRu/C. XPS analysis revealed that the PtSnO2 /C contained mostly Pt(0) and Sn(IV), with traces of Pt(II), Pt(IV) and Sn(0), and the PtRu/C contained mostly Pt(0) and Ru(0), with traces of Pt(II), Pt(IV) and Ru(IV). Preliminary data from a DMFC single stack test cell using the PtSnO2 /C catalyst as anode showed higher power density with respect to Pt/C and PtRu/C catalysts. The cell performance increased with the increasing Pt loading in the PtSnO2 /C anode. When the Pt loading was further increased, the performance started to decline at high current densities. The electrode impedance plots showed the distorted semicircles occurring at higher frequencies correspond to the impedance behavior of methanol electro-oxidation kinetics and the inductance loops at very low frequencies are due to the adsorbed intermediate species. References
case, mass transport limitations have been eliminated by using higher fuel flow rates. Fig. 9 shows the effect of different temperatures on the polarization curves (a) and output power (b) of single cell for the PtSnO2 /C anode. The open circuit voltage, performance and power density increased with temperature. These behaviors of DMFC can be explained by the increase in the temperature reduced the activation overvoltage according to the Arrhenius relation. The power-current curve forms a peak at the region of medium discharge current. The highest power density of 48 mW cm−2 was obtained from the cell with a Pt loading of 3.4 mg cm−2 in the PtSnO2 /C anode at 80 ◦ C. The impedance plots for the DMFCs with different temperature are shown in Fig. 10. The size of the plot decreased with increasing temperature, and the shape of the plot remained similar in each one. As shown in Fig. 10, the anode kinetics was improved with temperature. The oxidation of methanol and intermediates are faster at elevated temperatures. Therefore, inductance loops corresponding to the adsorbed intermediate species at very low frequencies is also smaller. 4. Conclusion A microwave assisted rapid heating method was used to prepare carbon supported Pt, PtSnO2 and PtRu nanoparticles with high electrocatalytic activities in direct methanol fuel cell
[1] B.D. McNicol, J. Electroanal. Chem. 118 (1981) 71. [2] R. Parsons, T. VanderNoot, J. Electroanal. Chem. 257 (1988) 9. [3] A.K. Shukla, M.K. Ravikumar, K.S. Gandhi, J. Solid State Electrochem. 2 (1998) 117. [4] A.K. Shukla, A.S. Aric`o, K.M. El-Khatib, H. Kim, P.L. Antonucci, V. Antonucci, Appl. Surf. Sci. 137 (1999) 20. [5] L. Jiang, Z. Zhou, W. Li, W. Zhou, S. Song, H. Li, G. Sun, Q. Xin, Energy Fuels 18 (2004) 866. [6] F. Vigier, C. Coutanceau, F. Hahn, E.M. Belgsir, C. Lamy, J. Electroanal. Chem. 563 (2004) 81. [7] L. Jiang, G. Sun, Z. Zhou, W. Zhou, Q. Xin, Catal. Today 93–95 (2004) 665. [8] W. Zhou, S. Song, W. Li, Z. Zhou, G. Sun, Q. Xin, S. Douvartzides, P. Tsiakaras, J. Power Sources 140 (2005) 50. [9] Z. Liu, B. Guo, L. Hong, T.H. Lim, Electrochem. Commun. 8 (2006) 83. [10] Z. Liu, J.Y. Lee, W. Chen, M. Han, L.M. Gan, Langmuir 20 (2004) 181. [11] J.T. M¨uller, P.M. Urban, W.F. H¨olderich, J. Power Sources 84 (1999) 157. [12] R.C. Weast, Handbook of Chemistry and Physics, 47th ed., The Chemical Rubber Co., Cleveland, Ohio, 1966. [13] F. Fievet, J.P. Lagier, B. Blin, B. Beaudoin, M. Figlarz, Solid State Ionics 32/33 (1989) 198. [14] W.J. Zhou, S.Q. Song, W.Z. Li, G.Q. Sun, Q. Xin, S. Kontou, K. Poulianitis, P. Tsiakaras, Solid State Ionics 175 (2004) 797. [15] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, MN, 1992. [16] R. Woods, J. Electroanal. Chem. 9 (1976) 1. [17] C. Roth, M. Goetz, H. Fuess, J. Appl. Electrochem. 31 (2001) 793. [18] C. Lamy, E.M. Belgsir, J.-M. Leger, J. Appl. Electrochem. 31 (2001) 799. [19] N. Nakagawa, Y. Xiu, J. Power Sources 118 (2003) 248.
Preparation and characterization of carbon-supported Pt, PtSnO2 and PtRu nanoparticles for direct methanol fuel cells Zhaolin Liu ∗ , Liang Hong, Siok Wei Tay Institute of Materials Research & Engineering, 3 Research Link, Singapore 117602, Singapore Received 5 September 2006; received in revised form 11 April 2007; accepted 13 April 2007
Abstract Nanosized Pt, PtSnO2 and PtRu particles were prepared by a microwave-assisted polyol process. Vulcan XC-72 was selected as catalyst support for Pt, PtSnO2 and PtRu particles. The catalysts were characterized by TEM, XRD and XPS. TEM investigations showed nanoscale particles and narrow size distribution for Pt, PtSnO2 and PtRu catalysts. All Pt, PtSnO2 and PtRu catalysts showed the X-ray diffraction pattern of a face-centered cubic (fcc) crystal structure. As evidenced by XPS, most Pt, Sn and Ru atoms in the nanoparticles were Pt(0), Sn(IV) and Ru(0). Preliminary tests on a single cell of a direct methanol fuel cell (DMFC) indicate that PtSnO2 /C catalyst gave the best electrocatalytic performance among three catalysts. The cell performance was analyzed based on the measurement of the current–voltage characteristics and impedance spectroscopy in the single fuel cell. © 2007 Elsevier B.V. All rights reserved. Keywords: PtSnO2 nanoparticles; PtRu nanoparticles; Direct methanol fuel cell; Catalyst loading; Impedance spectroscopy
1. Introduction In the past 20 years the direct methanol fuel cells (DMFCs) have been widely studied and considered as possible power sources for the portable electric apparatus and electric vehicles. DMFCs has a variety of benefits such as high energy density of methanol, availability and portability, but methanol ‘crossover’ from anode to cathode through membrane leads to low system efficiency. The carbon supported Pt catalyst has proven unfavourable for electro-oxidation of methanol. In the literature, both PtRu and PtSn systems have been reported to be promising catalysts for electro-oxidation of methanol or ethanol in direct methanol or ethanol fuel cells [1–9]. In terms of the electrode structure, up to now, the electrodes of the DMFC are generally based on the gas diffusion electrodes employed in proton exchange membranes fuel cells. Typically, the structure of such electrodes comprises a catalyst layer and a diffusion layer, the latter being carbon paper or carbon cloth. To optimize the electrode structure, many parameters such as the composition and loading of the catalyst, ionomer content in the catalyst layer and porosity of the electrode should be evaluated.
∗
Corresponding author. Tel.: +65 68727532; fax: +65 68720785. E-mail address: [email protected] (Z. Liu).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.04.045
In the present work, pure Pt, Pt65 Sn35 and Pt50 Ru50 (subscript denotes the atomic percentage of the alloying metal) anode catalysts were synthesized by a simple microwave-assisted polyol procedure. These catalysts were tested as anode in single direct methanol fuel cells. In spite of many factors affecting its performance, the single fuel cell is still considered as a very effective reactor to evaluate electrocatalysts when other operation parameters are well defined. Nafion 115 membrane was used as solid electrolyte and commercial Pt/C as cathode catalyst for the reaction of oxygen reduction in all single fuel cell tests. Other parameters, such as electrode preparation and membraneelectrode assembly (MEA) preparation procedure were kept consistent. The effect of anode catalyst loading on the performance of the DMFC was investigated. The cell performance was evaluated and analyzed by measuring the current–voltage characteristics and electrode impedance. 2. Experimental The Pt/C, PtSnO2 /C (Cabot Vulcan XC-72; Pt65 Sn35 , subscript denotes the atomic percentage of the alloying metal) and PtRu/C (Cabot Vulcan XC-72; Pt50 Ru50 , subscript denotes the atomic percentage of the alloying metal) catalysts were prepared by microwave heating of ethylene glycol (EG) solutions of Pt and Sn salts [10]. A typical preparation would consist of the following steps: 2.6 ml of 0.02 M H2 PtCl6 ·6H2 O (Aldrich, A.C.S. Reagent) and 1.4 ml of 0.02 M SnCl2 ·2H2 O (Aldrich, A.C.S. Reagent) was mixed with 20 ml of ethy-
Z. Liu et al. / Materials Chemistry and Physics 105 (2007) 222–228 lene glycol (Mallinckrodt, AR). 0.5 ml of 0.8 M NaOH was added dropwise. 0.04 g of Vulcan XC-72 carbon with a specific BET surface area of 250 m2 g−1 and an average particle size of 40 nm was added to the mixture and sonicated. The solution was placed in a CEM “Discover” microwave reactor (CEM Corporation) with the maximum temperature set at 170 ◦ C at atmospheric conditions for 60 s. The flask in volume of 50 ml and condenser for refluxing were used. The resulting suspension was filtered; and the residue was washed with acetone and dried at 100 ◦ C over night in a vacuum oven. An EG&G Model 263A potentiostat/galvanostat, and a conventional threeelectrode test cell were used for electrochemical measurements. The working electrode was a thin layer of Nafion-impregnated catalyst cast on a vitreous
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carbon disk held in a Teflon cylinder. The catalyst layer was obtained in the following way: (i) a slurry was first prepared by sonicating for 1 h a mixture of 0.5 ml of deionized water, 0.013 g of Pt/C, PtRu/C or PtSnO2 /C catalyst, and 0.5 ml of Nafion solution (Aldrich: 5 w/o Nafion); (ii) 4 l of the slurry was pipetted and spread on the carbon disk; (iii) the electrode was then dried at 90 ◦ C for 1 h and mounted on a stainless steel support. The surface area of the vitreous carbon disk was 0.25 cm2 . Pt gauze and Ag/AgCl electrode were used as the counter and reference electrodes respectively. All potentials quoted in this report were referred to the Ag/AgCl. All electrolyte solutions were deaerated by high-purity argon for 2 h prior to any measurement. For cyclic voltammetry of methanol oxidation, the electrolyte solution was 2 M CH3 OH in 1 M H2 SO4 ,
Fig. 1. TEM images of the PtSnO2 /C (a), PtRu/C (b) and Pt/C (c) catalysts. Histograms of particle size distribution for PtSnO2 /C (d), PtRu/C (e) and Pt/C (f) catalysts.
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Fig. 2. XRD patterns of microwave-synthesized Pt/C, PtSnO2 /C and PtRu/C catalysts.
then deposited on 3 mm Cu grids covered with a continuous carbon film. X-ray diffraction (XRD) patterns were recorded by a Bruker GADDS diffractometer ˚ operating at 40 kV and with area detector using a Cu K␣ source (λ = 1.54056 A) 40 mA. The samples were prepared by depositing carbon-supported nanoparticles on a glass slide, and drying the later in vacuum overnight. The samples were also analyzed by X-ray photoelectron spectroscopy (XPS) on a VG ESCALAB MKII spectrometer. Narrow scan photoelectron spectra were recorded for Pt(4f), Sn(3d) and Ru(3p) and a vendor supplied curve-fitting program (VGX900) was used for spectral deconvolution. The MEA (membrane electrode assembly) for the DMFC test cell was made by hot-pressing pretreated Nafion 115 together with an anode sheet and a cathode sheet. The anode sheet was a carbon paper (Toray TGPH-090) with carbon-supported Pt, PtSn and PtRu catalyst layer. The cathode sheet was a carbon paper with a carbon-supported 20 wt.% Pt catalyst layer supplied by Alfa Aesar. The Pt loadings at the anode and cathode were 0.8–6.8 mg cm−2 and 0.8 mg cm−2 , respectively, and the effective electrode area was 6 cm2 . The fuel was 2 M CH3 OH delivered at 2 ml min−1 by a micropump and oxygen flow was regulated by a flowmeter at 500 cm3 min−1 . Polarization curves were obtained by using a Hewlett Packard 6051A DC electronic load system. The measurement of anode impedance was performed by connecting the EG&G model 263A to a FRD-100 lock-in amplifier over a frequency range of 100 kHz to 1 mHz. Impedances were measured under galvanostatic or potentiostatic control of the cell. The anode was supplied with a 2 M aqueous solution of methanol at 2 ml min−1 or 4 ml min−1 by a micropump. The cath-
which was prepared from high-purity sulphuric acid, high-purity grade methanol and distilled water. The catalysts were examined by TEM on a JEOL JEM 2010. For microscopic examinations the samples were first ultrasonicated in acetone for 1 h and
Fig. 3. X-ray photoelectron spectra of the PtSnO2 /C catalyst.
Fig. 4. X-ray photoelectron spectra of the PtRu/C catalyst.
Z. Liu et al. / Materials Chemistry and Physics 105 (2007) 222–228 ode was operated on oxygen; it served as a reference and counter electrode [11].
3. Results and discussion In our approach, Pt, PtSnO2 and PtRu nanoparticles are prepared and directly deposited on the carbon surface by microwave heating of ethylene glycol (EG) solutions of Pt, Sn and Ru salts. The supported PtSnO2 and PtRu catalysts prepared as such are expected to maintain good electrocatalytic activity and CO tolerance in the direct methanol oxidation reaction at room temperature. Fig. 1(a)–(c) is the typical TEM images of PtSnO2 /C, PtRu/C and Pt/C catalysts, showing a remarkably uniform and high dispersion of metal particles on the carbon surface. The particle size distribution of the metal in the supported catalyst was obtained by directly measuring the size of 150 randomly chosen particles in the magnified TEM image. The average diameters of 4.6 nm (for PtSnO2 ), 4.1 nm (for PtRu) and 3.8 nm (for Pt) were accompanied by relatively narrow particle size distributions as shown in Fig. 1(c)–(e) (standard deviations are 0.4 nm, 0.4 nm and 0.3 nm, respectively). The microwave assisted heating of H2 PtCl6 /SnCl2 or RuCl3 /NaOH/H2 O in ethylene glycol had evidently facilitated the formation of smaller and more uniform Pt, PtSnO2 and PtRu particles and their dispersion on the Vulcan carbon support. It is generally agreed that the size of metal nanoparticles is determined by the rate of reduction of the metal precursor. The dielectric constant (41.4 at 25 ◦ C) and the dielectric loss of ethylene glycol are high, and hence rapid heating occurs easily under microwave irradiation [12]. In ethylene glycol mediated reactions (the ‘polyol’ process), ethylene glycol also acts as a reducing agent to reduce the metal ion to metal powders [13]. The fast heating by microwave accelerates the reduction of the metal precursor and the nucleation of the metal clusters. The easing of the nucleation limited process greatly assists in small particle formation. Additionally the homogeneous microwave heating of liquid samples reduces the
Fig. 5. Cyclic voltammograms of PtSnO2 /C, PtRu/C and Pt/C catalysts in 1 M H2 SO4 , 2 M CH3 OH with a scan rate of 10 mV s−1 at room temperature.
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temperature and concentration gradients in the reaction medium, thus providing a more uniform environment for the nucleation and growth of metal particles. The carbon surface may contain sites suitable for heterogeneous nucleation and the presence of a carbon surface interrupts particle growth. The smaller and nearly single dispersed Pt, PtSnO2 and PtRu nanoparticles on carbon XC-72 prepared by microwave irradiation can be rationalized in terms of these general principles. The power XRD pattern for Pt/C, PtSnO2 /C and PtRu/C are shown in Fig. 2. The diffraction peak between 20◦ and 30◦ observed in all the XRD patterns of the carbon-supported catalysts is due to the (0 0 2) reflection of the hexagonal structure of Vulcan XC-72 carbon. The diffraction peaks at about 39, 46, 68 and 81 are due to the Pt(1 1 1), (2 0 0), (2 2 0), and (3 1 1) reflections, respectively, which represents the typical character of a crystalline Pt face-centered cubic (fcc) phase. There are
Fig. 6. Polarization curves (a) and output power (b) of single cell at operating temperature of 80 ◦ C. Anode: PtSnO2 /C, PtRu/C and Pt/C with a Pt loading (3.4 mg cm−2 ), 2 M CH3 OH 2 ml min−1 . Cathode: Pt/C with a Pt loading (0.8 mg cm−2 ), O2 500 cm3 min−1 .
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not other distinct reflection peaks in all spectra than those of the four peaks mentioned above, indicating that these electrocatalysts have prevailed Pt (fcc) crystal structure. The 2θ values for PtRu/C and PtSnO2 /C were shifted to slightly higher values (40.5◦ ) and lower values (39.5◦ ), respectively, whereas the value was 39.9◦ for pure Pt. The same trend was replicated for the Pt(2 2 0) diffraction, which was 68.1◦ for Pt/C, 69.1◦ for PtRu/C, and 67.0◦ for PtSnO2 /C. The shift for PtRu/C is an indication of the reduction in lattice constant. The reduction of the lattice constant arose primarily from the substitution of platinum atoms by Ru atoms, which led to the contraction of the fcc lattice, an indication of the formation of PtRu alloys. The addition of Sn to the Pt/C increases the lattice constant of fcc crystal inducing the (2 2 0) reflection peak shift to lower position. The differences of the lattice constants of PtRu and PtSnO2 samples indicate that the interaction between Pt and Sn is contrary to that between Pt and Ru [4,14]. The surface oxidation states of the PtSnO2 and PtRu catalysts were investigated by X-ray photoelectron spectroscopy (XPS). As the binding energy (BE) for the Ru 3d line of zero-valent ruthenium at 284.3 eV [15] is very close to the C 1s line resulting from adventitious carbonaceous species, the Ru 3p spectrum was used instead for the analysis of Ru oxidation state. Fig. 3 shows the Pt 4f and Ru 3p regions of the XPS spectrum of the PtRu/C catalyst. The Pt 4f signal consisted of three pairs of doublets. The most intense doublet (71.07 eV and 74.4 eV) was due to metallic Pt. The second set of doublets (72.4 eV and 75.7 eV), which was observed at BE 1.4 eV higher than Pt(0), could be assigned to the Pt(II) chemical state as in PtO and Pt(OH)2 [16]. The third doublet of Pt was the weakest in intensity, and occurred at even higher BEs (74.2 eV and 77.7 eV). These are the indications that they were most likely caused by a small amount of Pt(IV) species on the surface. The slight shift in the Pt(0) peak to higher binding energies is a known effect for small particles, as has been reported by Roth et al. [17]. The Ru 3p3/2 signal could be deconvoluted into two distinguishable pairs of peaks of different intensities located at BE = 461.1 eV and 462.7 eV which corresponded well with Ru(0) and RuO2 [15], respectively. Fig. 4 shows the Pt 4f and Sn 3d regions of the XPS spectrum of the PtSnO2 /C catalyst. As stated previously, the Pt 4f signal also consisted of three pairs of doublets. The Sn 3d spectra have been deconvoluted into two components and are ascribed to elemental Sn and Sn(IV) oxide. The binding energies and relative intensities of the elemental Sn and Sn(IV) oxide obtained from their XPS spectra are 485.4 eV and 17%, 486.7 eV and 83%, respectively. From these data a substantial amount of SnO2 was observed to be present in the PtSnO2 catalyst. The PtSnO2 /C, PtRu/C and Pt/C catalysts are characterized by cyclic voltammetry in an electrolyte of 1 M H2 SO4 and 2 M methanol, and the resulting voltammograms are shown in Fig. 5. The comparison of onset potentials for methanol oxidation shows that the onset potentials for PtSnO2 /C (0.18 V) and PtRu/C (0.20 V) are lower than that for Pt/C (0.30 V), which indicates a high electrocatalytic activity with respect to methanol oxidation for PtSnO2 /C and PtRu/C catalysts compared to the Pt/C catalyst. Ru and Sn or SnO2 can supply surface oxygencontaining species for the oxidative removal of CO-like species
Fig. 7. Effect of Pt loading in the PtSnO2 /C anode on the polarization curves (a) and output power (b) of single cell at operating temperature of 80 ◦ C. Anode: PtSnO2 /C with different Pt loading, 2 M CH3 OH 2 ml min−1 . Cathode: Pt/C with a Pt loading (0.8 mg cm−2 ), O2 500 cm3 min−1 . Effect of Pt loading in the PtSnO2 /C anode on the polarization curves (c) of single cell at operating temperature of 80 ◦ C, 2 M CH3 OH 4 ml min−1 , other conditions unchanged.
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strongly adsorbed on adjacent Pt active sites, which is the socalled bi-functional mechanism and activate the chemisorbed CO-like intermediates. PtSnO2 and PtRu catalysts improve the removal of COads species formed on the platinum surface during methanol electro-oxidation. Therefore, the addition of Sn or to Pt enhances the electro-oxidation activity of methanol. Fig. 6(a) shows the performances of single cells with Pt/C, PtRu/C and PtSnO2 /C as anode catalysts. Each data point represents typical steady state voltages which were taken after continuous operation for 60 s at the indicated current density. The single methanol fuel cell using pure Pt, PtRu and PtSnO2 as anode catalysts exhibits a very low open circuit voltage of 0.50 V, 0.54 V and 0.55 V, which are far from the standard electromotive force of 1.213 V [18]. The great difference between the open circuit potential and the standard electromotive force is mainly attributed to the lower anode catalytic activity and the methanol crossover. PtSnO2 /C catalyst shows higher activity with respect to Pt/C and PtRu/C catalysts in the single fuel cell tests. As seen from Fig. 6(b), the maximum power density of this single fuel cell is 27 mW cm−2 , 42 mW cm−2 and 50 mW cm−2 at 80 ◦ C for Pt/C, PtRu/C and PtSnO2 /C as anode. Fig. 7(a) shows the performance of the single cell for various amounts of Pt varying from 0.8 mg cm−2 to 6.8 mg cm−2 in the PtSnO2 /C anode. The DMFC was operated at 80 ◦ C with 2 M methanol at anode and oxygen at cathode. The cell performance increases throughout the whole range of current densities when the Pt loading was increased from 0.8 mg cm−2 to 5.0 mg cm−2 . DMFC performance is largely limited by charge transfer kinetics, the activation overvoltage is major portion in the total overvoltage. When the Pt loading was further increased, the performance started to decline at high current densities in the case where the loading was 6.8 mg cm−2 . The thickness of the electrode increases with increasing catalyst loading, and a steeper concentration gradient for methanol appears. The concentration overvoltage resulted from the mass transfer resistance
of methanol through the catalyst layer. Fig. 7(c) shows the effect of flow rate of methanol at high Pt loading sample. When the flow rate of methanol was increased from 2 ml min−1 to 4 ml min−1 , the declination of cell performance at high current densities in the Pt loading of 6.8 mg cm−2 disappeared. As presented in Fig. 7(b), the maximum power density of 84 mW cm−2 was obtained from the cell with a Pt loading of 5.0 mg cm−2 in the PtSnO2 /C anode. The impedance plots for the DMFCs with different Pt loading in the PtSnO2 /C anode are shown in Fig. 8. The distorted semicircles occur at higher frequencies, and the size decreased with increasing Pt loading from 0.8 mg cm−2 to 6.8 mg cm−2 . The semicircles correspond to the impedance behavior of methanol electro-oxidation kinetics. The inductance loops at very low frequencies are due to the reaction mechanism with a series of single steps of adsorbed intermediate species as reported by M¨uller et al. and Nakagawa and Xiu [11,19]. In this
Fig. 8. Nyquist plots of the full-cell impedance with different Pt loading in the PtSnO2 /C anode at the cell voltage of 0.3 V and operating temperature of 80 ◦ C. Anode: PtSn/C with different Pt loading, 2 M CH3 OH 2 ml min−1 . Cathode: Pt/C with a Pt loading (0.8 mg cm−2 ), O2 500 cm3 min−1 .
Fig. 9. Effect of different temperatures on the polarization curves (a) and output power (b) of single cell. Anode: PtSnO2 /C with a Pt loading (3.4 mg cm−2 ), 2 M CH3 OH 2 ml min−1 . Cathode: Pt/C with a Pt loading (0.8 mg cm−2 ), O2 500 cm3 min−1 .
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Fig. 10. Nyquist plots of the full-cell impedance with different temperatures at the cell voltage of 0.3 V. Anode: PtSnO2 /C with a Pt loading (3.4 mg cm−2 ), 2 M CH3 OH 2 ml min−1 . Cathode: Pt/C with a Pt loading (0.8 mg cm−2 ), O2 500 cm3 min−1 .
reactions. The PtSnO2 and PtRu nanoparticles, which were uniformly dispersed on carbon, were 2–7 nm in diameters. The PtSnO2 /C and PtRu/C catalysts prepared as such displayed the characteristic diffraction peaks of a Pt face-centered cubic (fcc), excepting that the 2θ values were shifted to slightly lower value for PtSnO2 /C and higher value for PtRu/C. XPS analysis revealed that the PtSnO2 /C contained mostly Pt(0) and Sn(IV), with traces of Pt(II), Pt(IV) and Sn(0), and the PtRu/C contained mostly Pt(0) and Ru(0), with traces of Pt(II), Pt(IV) and Ru(IV). Preliminary data from a DMFC single stack test cell using the PtSnO2 /C catalyst as anode showed higher power density with respect to Pt/C and PtRu/C catalysts. The cell performance increased with the increasing Pt loading in the PtSnO2 /C anode. When the Pt loading was further increased, the performance started to decline at high current densities. The electrode impedance plots showed the distorted semicircles occurring at higher frequencies correspond to the impedance behavior of methanol electro-oxidation kinetics and the inductance loops at very low frequencies are due to the adsorbed intermediate species. References
case, mass transport limitations have been eliminated by using higher fuel flow rates. Fig. 9 shows the effect of different temperatures on the polarization curves (a) and output power (b) of single cell for the PtSnO2 /C anode. The open circuit voltage, performance and power density increased with temperature. These behaviors of DMFC can be explained by the increase in the temperature reduced the activation overvoltage according to the Arrhenius relation. The power-current curve forms a peak at the region of medium discharge current. The highest power density of 48 mW cm−2 was obtained from the cell with a Pt loading of 3.4 mg cm−2 in the PtSnO2 /C anode at 80 ◦ C. The impedance plots for the DMFCs with different temperature are shown in Fig. 10. The size of the plot decreased with increasing temperature, and the shape of the plot remained similar in each one. As shown in Fig. 10, the anode kinetics was improved with temperature. The oxidation of methanol and intermediates are faster at elevated temperatures. Therefore, inductance loops corresponding to the adsorbed intermediate species at very low frequencies is also smaller. 4. Conclusion A microwave assisted rapid heating method was used to prepare carbon supported Pt, PtSnO2 and PtRu nanoparticles with high electrocatalytic activities in direct methanol fuel cell
[1] B.D. McNicol, J. Electroanal. Chem. 118 (1981) 71. [2] R. Parsons, T. VanderNoot, J. Electroanal. Chem. 257 (1988) 9. [3] A.K. Shukla, M.K. Ravikumar, K.S. Gandhi, J. Solid State Electrochem. 2 (1998) 117. [4] A.K. Shukla, A.S. Aric`o, K.M. El-Khatib, H. Kim, P.L. Antonucci, V. Antonucci, Appl. Surf. Sci. 137 (1999) 20. [5] L. Jiang, Z. Zhou, W. Li, W. Zhou, S. Song, H. Li, G. Sun, Q. Xin, Energy Fuels 18 (2004) 866. [6] F. Vigier, C. Coutanceau, F. Hahn, E.M. Belgsir, C. Lamy, J. Electroanal. Chem. 563 (2004) 81. [7] L. Jiang, G. Sun, Z. Zhou, W. Zhou, Q. Xin, Catal. Today 93–95 (2004) 665. [8] W. Zhou, S. Song, W. Li, Z. Zhou, G. Sun, Q. Xin, S. Douvartzides, P. Tsiakaras, J. Power Sources 140 (2005) 50. [9] Z. Liu, B. Guo, L. Hong, T.H. Lim, Electrochem. Commun. 8 (2006) 83. [10] Z. Liu, J.Y. Lee, W. Chen, M. Han, L.M. Gan, Langmuir 20 (2004) 181. [11] J.T. M¨uller, P.M. Urban, W.F. H¨olderich, J. Power Sources 84 (1999) 157. [12] R.C. Weast, Handbook of Chemistry and Physics, 47th ed., The Chemical Rubber Co., Cleveland, Ohio, 1966. [13] F. Fievet, J.P. Lagier, B. Blin, B. Beaudoin, M. Figlarz, Solid State Ionics 32/33 (1989) 198. [14] W.J. Zhou, S.Q. Song, W.Z. Li, G.Q. Sun, Q. Xin, S. Kontou, K. Poulianitis, P. Tsiakaras, Solid State Ionics 175 (2004) 797. [15] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, MN, 1992. [16] R. Woods, J. Electroanal. Chem. 9 (1976) 1. [17] C. Roth, M. Goetz, H. Fuess, J. Appl. Electrochem. 31 (2001) 793. [18] C. Lamy, E.M. Belgsir, J.-M. Leger, J. Appl. Electrochem. 31 (2001) 799. [19] N. Nakagawa, Y. Xiu, J. Power Sources 118 (2003) 248.