- Email: [email protected]
C nanoparticles as an electrocatalyst for oxygen reduction reaction
Applied Surface Science 257 (2011) 6353–6357
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Exploration of bimetallic Pt-Pd/C nanoparticles as an electrocatalyst for oxygen reduction reaction A. Maghsodi a,b,∗ , M.R. Milani Hoseini a , M. Dehghani Mobarakeh b , M. Kheirmand c,∗∗ , L. Samiee b , F. Shoghi b , M. Kameli d a
Electrochemistry Research Center, Department of Analytical Chemistry, Faculty of Chemistry, Iran University of Science and Technology (IUST), Narmak, Farjam, Tehran, Iran Renewable Energy Department, Energy Research Center, Research Institute of Petroleum Industry (RIPI), Tehran, Iran Department of Chemistry, School of Basic Science, Yasouj University, Yasouj, Iran d Inhibitors Department, Materials Protection Division, Research Institute of Petroleum Industry (RIPI), Tehran, Iran b c
a r t i c l e
i n f o
Article history: Received 18 December 2010 Received in revised form 19 January 2011 Accepted 22 January 2011 Available online 4 February 2011 Keywords: Electrocatalyst Polymer electrolyte membrane fuel cell Oxygen reduction reaction (ORR) Nanoparticles
a b s t r a c t In this study, carbon supported Pt and Pt-Pd were synthesized as oxygen reduction reaction electrocatalysts for polymer electrolyte membrane fuel cells (PEMFCs). Pt and Pt-Pd nanoparticles have been synthesized by reduction of metal precursors in presence of NaBH4 . Various techniques such as X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX) and scanning electron microscopy (SEM) were utilized to study the prepared samples. Furthermore, electrochemical properties of the prepared samples were evaluated from cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry and electrochemical impedance spectroscopy (EIS). The results showed, the crystallite size of electrocatalysts (Pt and Pt-Pd) is below 10 nm. The higher catalytic activity was detected for Pt-Pd/C electrocatalyst for oxygen reduction reaction (ORR). In addition, it is believed that the better performance of electrocatalyst is related to the synergic effect between Pt and Pd nanoparticles, weakening of the O O bond on Pd-modified Pt nanoparticles in ORR, uniform dispersion of Pd and Pt on the carbon support and higher electrochemical active surface area (EAS) of Pt-Pd/C electrocatalyst. © 2011 Elsevier B.V. All rights reserved.
1. Introduction PEMFCs attract attentions in terms of alternative power sources for electric vehicles and portable devices due to their low temperature working condition, high efficiency and environmentally friendly characteristics [1]. The sluggish kinetics of molecular oxygen on various electrocatalytic surfaces has been caused to more attention to ORR [2]. One of the major challenges in this field is the development of high-performance cathode catalysts in order to reduce the produced over potential during the ORR. Platinum and its alloys supported on carbon black and bimetallic nanoparticles are extensively used as cathode electrodes because
∗ Corresponding author at: Renewable Energy Department, Energy Research Center, Research Institute of Petroleum Industry (RIPI), Tehran, Iran. Tel.: +98 21 48253224; fax: +98 21 44739725. ∗∗ Corresponding author at: Department of Chemistry, School of Basic Science, Yasouj University, Yasouj, Iran. E-mail addresses: akram [email protected] (A. Maghsodi), [email protected] (M.R. Milani Hoseini), [email protected] (M. Dehghani Mobarakeh), [email protected] (M. Kheirmand), [email protected] (L. Samiee), N [email protected] (F. Shoghi), [email protected] (M. Kameli). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.01.094
of their catalysis ability in four-electron reduction reaction of O2 to form H2 O and improvement of catalytic properties in relative to the isolated metals in low and medium temperature PEMFCs [3–5]. Based on simple thermodynamic principles, Fernandez et al. provided guidelines for the design of binary and multicomponent electrocatalytic materials for the reduction of oxygen, which combine one metal with another, which based on, they will easily break the O O bond of O2 and reduce adsorbed atomic oxygen [6]. The alloyed of transition metals, such as V, Cr, Co, Ti and Ni, with platinum exhibit significantly higher electrocatalytic activities towards the ORR than platinum alone in low temperature fuel cells [7–10]. Metallic palladium presents considerable catalytic activity for the ORR in acid electrolyte, which preferentially proceeds through a four-electron pathway. The use of Pd is interest as it is at least 50 times more abundant than Pt on the earth and they belong to similar group in the periodic table. The enhancement of the activity for Pt in the presence of Pd has been found by CV and single direct ethanol fuel cell (DMFCs) measurements and these results originate from the weakening of the O O bond on Pd-modified Pt nanoparticles. Recently, several researches were aimed towards the study of interaction of oxygen with supported and unsupported Pd and Pt catalysts. These materials showed significant selectivity towards
6354
A. Maghsodi et al. / Applied Surface Science 257 (2011) 6353–6357
OH and O adsorption, with decreasing the OH coverage on Pt and enhancing the ORR kinetics [11,12]. Lopesa et al. prepared carbon supported Pt-Pd alloy electrocatalysts with formic acid method and explorated ORR in DMFCs [11]. Joo et al. investigated Methanol-tolerant PdPt/C alloy catalyst for ORR [13]. Cho et al. synthesized Pd-based PdPt (19:1)/C electrocatalyst by sodium borohydride reduction combined with freeze–drying, also and studied it in single cell [14]. Sodium borohydride is widely used as a mild reducing agent in organic synthesis of stereo specific action [15]. Because of toxicity problems of other reducing agents such as hydrazine, formaldehyde, sodium borohydride is a safe and high efficient one. It is normally suitable for use without excluding atmospheric oxygen or moisture, too. Therefore, in the present study, carbon supported Pt and Pt-Pd electrocatalysts were prepared by reduction of metal precursors with NaBH4 method. Then, they were applied onto the gas diffusion layer (GDL). Finally, gas diffusion electrode (GDE) was formed as working electrode (0.6644 cm2 area). By considering the effectiveness of NaBH4 as a reducting agent, we were therefore stimulated to study of catalytic activity of carbon supported Pt-Pd electrocatalysts and then compare to pure Pt. 2. Experimental 2.1. Chemicals used Chloroplatinic acid hydrate (H2 PtCl6 ·xH2 O), sulfuric acid (H2 SO4 ), sodium borohydride (NaBH4 ) and 2-propanol ((CH3 )2 CHOH) were received from Merck. PdCl2 and nafion solution (5 wt%) were obtained from Alfa Aesar. Polytetrafluoroethylene (PTFE) and Vulcan carbon (XC-72R) have been produced from Aldrich and Cabot Company respectively. All chemicals used were of analytical grade. 2.2. Synthesis of catalysts Vulcan carbon (XC-72R) was ultrasonically dispersed in de-ionized water and 2-propanol. Then metal precursors (H2 PtCl6 ·xH2 O + PdCl2 ) were added to the result ink. Molar ratio of Pt to Pd was achieved 1:1 and the total metal content in the electrocatalyst was 40 wt% vs. carbon support. The mixture was heated up to 80 ◦ C in presence of NaBH4 solution (reducing agent) under vigorous agitation. Then mixture was filtered, washed and dried under vacuum at 120 ◦ C for 4 h. For comparison, 20 wt% Pt/C was prepared with the same method, too. 2.3. Preparation of gas diffusion layer To preparation of GDLs, PTFE solution and Vulcan carbon (XC72R) was sonicated into mixture of 2-propanol, water and glycerol for 20 min and then homogeneous suspension was painted onto carbon paper (Electrochem). The resulting layer was dried at 80 ◦ C and sintered at 350 ◦ C for 30 min. The loading of GDLs was obtained equal to 2 mg cm−2 . 2.4. Preparation of gas diffusion electrode The catalyst ink was produced by ultrasonically mixing the desired amounts of synthesized electrocatalysts with 2-propanol, de-ionized water and 5 wt% nafion solution (Dupont). After painting the electrocatalyst ink on the GDLs, the electrodes were dried at 130 ◦ C for 1 h. The as prepared electrodes were known as GDEs. The total metal loading on GDLs was 0.5 mg cm−2 .
Fig. 1. XRD patterns of (a) Pt/C and (b) Pt-Pd/C electrocatalysts.
2.5. Physical and electrochemical characterizations of the electrocatalysts X-ray diffraction (XRD) pattern of all samples were collected in 2 range between 10◦ and 90◦ by using Philips PW1840 Diffrac˚ The crystallite sizes of tometer (Cu K␣1 radiation, K␣1 = 1.5406 A). the samples were estimated by applying Scherrer equation to (1 1 1) diffraction peak of Pt [16]. Scanning electron microscopy (SEM) images were obtained with Philips XL30, 17 kV. Electrochemical behavior of electrocatalysts was examined by CV, LSV, chronoamperometry and EIS techniques in H2 SO4 solution. Electrochemical measurements were carried out using an Autolab potentiostat/galvanostat PGSTAT30, Ag/AgCl saturated KCl as reference electrode and a Pt foil as a counter electrode. The GDE was mounted in a teflon holder containing a graphite ring as the current collector and having the provision for the oxygen or nitrogen supply at the back of the electrode. 3. Results and discussion 3.1. Physical characterization of the catalysts Fig. 1 shows the XRD patterns of Pt/C and Pt-Pd/C electrocatalysts. The peak appeared at 2 = 24◦ is related to use of Vulcan carbon (XC-72R) as a support. The characteristic crystalline peaks of electrocatalysts are at 2 = 39.7◦ (1 1 1), 46.2◦ (2 0 0), 67.4◦ (2 2 0), 81.2◦ (3 1 1) [17]. The XRD patterns of both electrocatalysts show the characteristic peaks of the face-centered cubic (fcc) crystalline Pt and Pd. The formation of Pt/Pd alloy was confirmed by slightly shift of XRD patterns to higher angles. This shift reveals that the alloy formation between Pt and Pd is due to incorporation of Pd in the fcc structure of Pt. The values of the lattice parameter of pure Pt and Pd metals are 0.3923 nm and 0.3890 nm, respectively. The lattice parameters of carbon supported Pt-Pd catalysts, commonly used in fuel cells, are higher than the value for pure Pd but smaller than that of for pure Pt which these indicate the contraction of the lattice is due to the particle substitution of Pt by Pd in the fcc structure in the formation of the Pt-Pd alloy [11,18]. The calculated lattice parameters of the Pt/C and Pt-Pd/C alloy, from the (1 1 1) peak, are listed in Table 1. Table 1 Crystallite size and lattice parameter of Pt/C and Pt-Pd/C electrocatalysts. Catalyst
Crystallite size (nm)
Lattice parameter (nm)
Pt/C Pt-Pd/C
6.3 7.8
0.3918 0.3904
A. Maghsodi et al. / Applied Surface Science 257 (2011) 6353–6357
6355
Fig. 2. SEM images of (a) Pt/C and (b) Pt-Pd/C electrocatalysts.
The average particle size of the nanoparticles (d) was estimated by using Scherrer equation (1) after background subtraction from (1 1 1) peak at 2 of ∼40◦ also [16]: d=
k ˇ cos
(1)
where k is the coefficient, generally taken as 0.9, is wavelength of ˚ ˇ is full width at half-maximum X-ray radiation equal to 1.54051 A, (FWHM) measured in radians, and is the angle measured at the position of platinum peaks. Table 1 shows the crystalline size of the electrocatalysts. Fig. 2 represents SEM images of the synthesized electrocatalysts. SEM graphs revealed the high dispersion of Pt and Pt/Pd nanoparticles on Vulcan carbon support with an average diameter about 20 nm (approximate value). As can be seen, in the SEM graphs Pt and Pt-Pd alloyed nanoparticles were agglomerated and as a result, the achieved size is larger than crystalline size that has obtained from XRD data. The bulk atomic compositions of electrocatalysts were investigated by using energy dispersive X-ray (EDX) Spectroscopy (Fig. 3). EDX results showed presence of 20 wt% and 19.45 wt% Pt, in Pt/C and Pt-Pd/C electrocatalysts respectively. The obtained atomic ratio for Pd in Pt-Pd/C electrocatalyst was around 16.5 wt% also which is in good agreement with nominal value.
Fig. 3. EDX analysis of (a) Pt/C and (b) Pt-Pd/C electrocatalysts.
1 V relative to an Ag/AgCl reference electrode in 0.5 M sulfuric acid. Between 0.05 and 0.25 V, hydrogen atoms are adsorbed and desorbed on the cathodic and anodic sweeps. Above 0.8 V, the formation and removing of oxide film is done on the surface of platinum electrocatalysts, on anodic sweep and cathodic sweep, respectively. It can be seen that the Pt-Pd/C electrocatalyst produces higher currents in the hydrogen region, compared with that of Pt/C elec-
3.2. Electrochemical evaluation of catalysts 3.2.1. Cyclic voltammetry Fig. 4 shows the cyclic voltammograms of the Pt and Pt-Pd nanoparticles supported on Vulcan carbon (XC-72R) in the nitrogen atmosphere with scan rate of 50 mV s−1 between −0.2 and
Fig. 4. Cyclic voltammograms of (a) Pt/C and (b) Pt-Pd/C electrocatalysts in N2 saturated 0.5 M H2 SO4 aqueous solution with potential scan rate of 50 mV s−1 and 20 ◦ C.
6356
A. Maghsodi et al. / Applied Surface Science 257 (2011) 6353–6357
Table 2 Electro active surface area, Tafel slope, exchange current density and Cottrell parameters of Pt/C and Pt-Pd/C electrocatalysts. EAS (m2 g−1 )
Catalyst Pt/C Pt-Pd/C
16.26 31.6
Tafel slope (mV/dec) 162 107
Exchange current density (A cm−2 ) −4
7.55 × 10 13 × 10−4
trocatalyst. The electrochemical active surface area (EAS) of the electrocatalysts could be estimated according to the following equation also [19]: EAS =
QH 0.21[Pt]
(2)
where [Pt] represents the platinum loading (mg cm−2 ) in the electrode, QH is the charge for hydrogen desorption and 0.21 shows the charge required for oxidation of a monolayer of H2 on bright Pt (mC cm−2 ). The calculations showed that the Pt-Pd/C electrocatalyst has highest EAS in relative to Pt/C electrocatalyst. So, with increasing of available active surface area, catalytic activity of electrocatalyst towards ORR has been raised. In spite of, larger particle size of Pt-Pd/C electrocatalyst, this sample has a higher EAS whereas generally, it occurs conversely. This matter can be related to synergetic effect, which is playing a critical role in hydrogen adsorption/desorption reaction and is not only dependent to particle size parameter. The synergetic effect of the simultaneous presence of Pd and Pt in the electocatalyst can be explained by the spill-over effect of hydrogen and oxygen from Pt surface to Pd and resultant freedom for Pt active sites [20]. Another electrochemical results confirm the better electrocatalytic performance of Pt-Pd/C electrocatalyst, too. The electrochemical active surface area of electrocatalysts is listed in Table 2. 3.2.2. Linear sweep voltammetry Fig. 5 depicts the polarization curves for electrocatalysts, in a conventional three electrode cell. Polarization curves were produced at a scan rate of 1 mV s−1 between 1 and −0.3 V relative to the Ag/AgCl in oxygen saturated in 2 M H2 SO4 solution. The kinetics parameters of the ORR were extracted by using the Tafel equation [21]:
Cottrell slope (A s1/2 )
D1/2 C (mol cm−2 s−0.5 )
0.007 0.009
1.93 × 10−7 2.5 × 10−7
is Faraday’s constant. The equation can be simplified as: = a + b log i
(4)
where b = 2.303RT/˛nF and called Tafel slope and a = b log i0 . This equation implies that in a certain current density range, over potential is linearly dependant on the logarithm of current density. The exchange current density can be obtained from the intercept at the current density axis. In the higher Tafel slope, kinetics of reaction is slower. The results showed that the ORR current density in Pt-Pd/C GDE is higher than in Pt/C GDE, which indicated that the GDE containing Pt-Pd/C has a much better performance which has been caused by weakening of the O O bond on Pd-modified Pt nanoparticles [11], synergic effect between Pt and Pd nanoparticles, and uniform dispersion of Pd and Pt on the carbon support. The kinetic parameters have been obtained in Table 2. The lower Tafel slope and higher exchange current density in Pt-Pd/C GDE can be attributed to an enhanced electrocatalytic activity of Pt-Pd/C electrocatalyst in relative to Pt/C electrocatalyst. 3.2.3. Chronoamperometry The oxygen diffusion coefficients from electrodes were determined by chronoamperometry. Chronoamperograms were obtained by holding the potential of the electrode at 1.2 V for 60 s and then holding it at 0.4 V relative to the Ag/AgCl electrode for 10 s with oxygen flowing along the back of the electrode. With plotting i vs. t−1/2 , the linear dependence relationship was obtained for different electrodes [22]: i(t) =
nFACD0.5 (t)0.5
(5)
where = (E − E0 ) is the over potential, R is gas constant, T is the absolute temperature, ˛ is the transfer coefficient, i0 is exchange current density, i is current density, n is number of electrons, and F
where n is the number of electrons involved in overall reaction of ORR, F is the Faraday constant, A is the surface area of the electrode, and D and C are the diffusion coefficient and the concentration of the reacting species, respectively. Fig. 6 shows the chronoamperograms of GDEs at 0.4 V in relative to the Ag/AgCl in oxygen saturated in 2 M H2 SO4 solution. The results confirmed the higher Cottrell slope and D for Pt-Pd/C GDE. Cottrell parameters are listed in Table 2, also.
Fig. 5. Linear potential sweep curves of ORR on (a) Pt/C and (b) Pt-Pd/C electrocatalysts in O2 saturated 2 M H2 SO4 aqueous solution, at potential scan rate of 1 mV s−1 and 20 ◦ C.
Fig. 6. Chronoamperograms of the (a) Pt/C and (b) Pt-Pd/C electrocatalysts in O2 saturated 2 M H2 SO4 aqueous solution at 0.4 V vs. Ag/AgCl electrode and 20 ◦ C.
=
2.303RT ˛nF
log
i i0
(3)
A. Maghsodi et al. / Applied Surface Science 257 (2011) 6353–6357
6357
ment in the Energy Research Center of Research Institute of Petroleum Industry (RIPI). References
Fig. 7. Impedance responses of the (a) Pt/C and (b) Pt-Pd/C electrocatalysts from 100 kHz to 100 mHz, in O2 saturated 2 M H2 SO4 aqueous solution at 0.3 V vs. Ag/AgCl electrode and 20 ◦ C.
3.2.4. Impedance spectroscopy The Nyquist plots of the different electrodes were obtained as be shown in Fig. 7. Impedance spectra were collected for frequencies between 100 kHz and 100 mHz, in oxygen saturated 2 M H2 SO4 solution at 20 ◦ C and 0.3 V relative to the Ag/AgCl. An AC potential of 5 mV was superimposed on the DC potential. The low frequency impedance is corresponded to the kinetic impedance of the ORR and it becomes smaller as the cathode over potential increases, whereas the high-frequency is independent of the over potential, which indicates the impedance dependency to an Ohmic process. The high-frequency impedance reflected the combination of the double-layer capacitance in the catalyst layer and the effective charge transfer resistance [23]. As can be seen, Pt-Pd/C GDE has minimum electronic resistance and slightly better performance towards ORR in relative to the Pt/C GDE. 4. Conclusion Nanoparticles of Pt/C and Pt-Pd/C have been prepared via a synthetic procedure by reduction in presence of NaBH4 . These nanoparticles were formed in range of 6–8 nm average size. XRD Patterns revealed the nanoparticles formation in fcc structure. The electrochemical results showed the higher catalytic activity of Pt-Pd/C as compared to Pt/C electrocatalyst towards ORR in PEMFCs. This enhanced electroactivity was attributed to weakening of the O O bond on Pd-modified Pt nanoparticles, synergetic effect between Pt and Pd nanoparticles, uniform dispersion of Pd and Pt on the carbon support and higher EAS of the alloyed electrocatalyst. Acknowledgements The authors are grateful to the support of Iran University of Science and Technology (IUST), and the Renewable Energy Depart-
[1] J. Larminie, D. Dicks, Fuel Cell Systems Explained, third ed., Wiley & Sons, Chichester, 2000. [2] W.E. Mustain, J. Prakash, Kinetics and mechanism for the oxygen reduction reaction on polycrystalline cobalt–palladium electrocatalysts in acid media, J. Power Sources 170 (2007) 28–37. [3] M. Gustavsson, H. Ekstroem, P. Hanarp, G. Lindbergh, E. Olsson, B. Kasemo, Thin film Pt/TiO2 catalysts for the polymer electrolyte fuel cell, J. Power Sources 163 (2007) 671–678. [4] A. Wieckowski, E.R. Savinova, C.G. Vayenas, Catalysis and Electrocatalysis at Nanoparticle Surfaces, Marcel Dekker Inc., New York, 2003. [5] K.S. Nagabhushana, C. Wiedenthaler, S. Hocevar, D. Strmcnik, M. Gaberscek, A.L. Antozzi, G.N. Martelli, Kinetics of oxygen reduction reaction on nanosized Pd electrocatalyst in acid media, J. New Mater. Electrochem. Syst. 9 (2006) 73–81. [6] J.L. Fernandez, D.A. Walsh, A.J. Bard, Thermodynamic guidelines for the design of bimetallic catalysts for oxygen electroreduction and rapid screening by scanning electrochemical microscopy. M–Co (M: Pd, Ag, Au), J. Am. Chem. Soc. 127 (2005) 357–365. [7] M.T. Paffett, G.J. Berry, S. Gottesfeld, A new approach to the problem of carbon monoxide poisoning in fuel cells operating at low temperatures, J. Electrochem. Soc. 135 (1988) 2651–2652. [8] B.C. Beard, P.N. Ross, The structure and activity of Pt-Co alloys as oxygen reduction electrocatalysts, J. Electrochem. Soc. 137 (1990) 3368–3374. [9] T. Toda, H. Igarashi, H. Uchida, M. Watanabe, Enhancement of the electroreduction of oxygen on Pt alloys with Fe, Ni, and Co, J. Electrochem. Soc. 146 (1999) 3750–3756. [10] S.C. Zignani, E. Antolini, E.R. Gonzalez, Evaluation of the stability and durability of Pt and Pt–Co/C catalysts for polymer electrolyte membrane fuel cells, J. Power Sources 182 (2008) 83–90. [11] T. Lopesa, E. Antolini, E.R. Gonzaleza, Carbon supported Pt–Pd alloy as an ethanol tolerant oxygen reduction electrocatalyst for direct ethanol fuel cells, Int. J. Hydrogen Energy 33 (2008) 5563–5570. [12] L. Zhang, K. Lee, J. Zhang, The effect of heat treatment on nanoparticle size and ORR activity for carbon-supported Pd–Co alloy electrocatalysts, J. Electrochim. Acta 52 (2007) 3088–3094. [13] J.B. Joo, Y.J. Kim, W. Kim, N.D. Kim, P. Kim, Y. Kim, J. Yi, Methanol-tolerant PdPt/C alloy catalyst for oxygen electro-reduction reaction, Korean J. Chem. Eng. 25 (2008) 770–774. [14] Y.H. Cho, B. Choi, Y.H. Cho, H.S. Park, Y.E. Sung, Pd-based PdPt(19:1)/C electrocatalyst as an electrode in PEM fuel cell, Electrochem. Commun. 9 (2007) 378–381. [15] W. Buchner, H. Niederprum, Sodium borohydride and amine-boranes, commercially important reducting agents, Pure Appl. Chem. 49 (1977) 733–743. [16] V. Radmilovic, H.A. Gasteiger, J.P.N. Ross, Structure and chemical composition of a supported Pt-Ru electrocatalyst for methanol oxidation, J. Catal. 154 (1995) 98–106. [17] JCPDS International Center for Diffraction Data, U.S.A. Powder diffraction files (Inorganic Volumes), 4-802. [18] E. Antolini, Palladium in fuel cell catalysis, Energy Environ. Sci. 2 (2009) 915–931. [19] A. Pozio, M.D. Francesco, A. Cemmi, F. Cardellini, L. Giorgi, Comparison of high surface Pt/C catalysts by cyclic voltammetry, J. Power Sources 105 (2002) 13–19. [20] C. Wang, B. Peng, H.N. Xie, H.X. Zhang, F.F. Shi, W.B. Cai, Facile fabrication of Pt, Pd and Pt-Pd alloy films on Si with tunable infrared internal reflection absorption and synergetic electrocatalysis, J. Phys. Chem. C 113 (2009) 13841–13846. [21] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications, 2nd ed., John Wiley & Sons, New York, 1980. [22] J. Mirzazadeh, E. Saievar-Iranizad, L. Nahavandi, An analytical approach on effect of diffusion layer on ORR for PEMFCs, J. Power Sources 131 (2004) 194–199. [23] X.-Z. yuan, H. wang, PEM fuel cell electrocatalysts and catalyst layers: fundamentals and applications, in: J. Zhang (Ed.), PEM Fuel Cell Fundamentals, Springer, Vancouver, Canada, 2008, pp. 76–77.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Exploration of bimetallic Pt-Pd/C nanoparticles as an electrocatalyst for oxygen reduction reaction A. Maghsodi a,b,∗ , M.R. Milani Hoseini a , M. Dehghani Mobarakeh b , M. Kheirmand c,∗∗ , L. Samiee b , F. Shoghi b , M. Kameli d a
Electrochemistry Research Center, Department of Analytical Chemistry, Faculty of Chemistry, Iran University of Science and Technology (IUST), Narmak, Farjam, Tehran, Iran Renewable Energy Department, Energy Research Center, Research Institute of Petroleum Industry (RIPI), Tehran, Iran Department of Chemistry, School of Basic Science, Yasouj University, Yasouj, Iran d Inhibitors Department, Materials Protection Division, Research Institute of Petroleum Industry (RIPI), Tehran, Iran b c
a r t i c l e
i n f o
Article history: Received 18 December 2010 Received in revised form 19 January 2011 Accepted 22 January 2011 Available online 4 February 2011 Keywords: Electrocatalyst Polymer electrolyte membrane fuel cell Oxygen reduction reaction (ORR) Nanoparticles
a b s t r a c t In this study, carbon supported Pt and Pt-Pd were synthesized as oxygen reduction reaction electrocatalysts for polymer electrolyte membrane fuel cells (PEMFCs). Pt and Pt-Pd nanoparticles have been synthesized by reduction of metal precursors in presence of NaBH4 . Various techniques such as X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX) and scanning electron microscopy (SEM) were utilized to study the prepared samples. Furthermore, electrochemical properties of the prepared samples were evaluated from cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry and electrochemical impedance spectroscopy (EIS). The results showed, the crystallite size of electrocatalysts (Pt and Pt-Pd) is below 10 nm. The higher catalytic activity was detected for Pt-Pd/C electrocatalyst for oxygen reduction reaction (ORR). In addition, it is believed that the better performance of electrocatalyst is related to the synergic effect between Pt and Pd nanoparticles, weakening of the O O bond on Pd-modified Pt nanoparticles in ORR, uniform dispersion of Pd and Pt on the carbon support and higher electrochemical active surface area (EAS) of Pt-Pd/C electrocatalyst. © 2011 Elsevier B.V. All rights reserved.
1. Introduction PEMFCs attract attentions in terms of alternative power sources for electric vehicles and portable devices due to their low temperature working condition, high efficiency and environmentally friendly characteristics [1]. The sluggish kinetics of molecular oxygen on various electrocatalytic surfaces has been caused to more attention to ORR [2]. One of the major challenges in this field is the development of high-performance cathode catalysts in order to reduce the produced over potential during the ORR. Platinum and its alloys supported on carbon black and bimetallic nanoparticles are extensively used as cathode electrodes because
∗ Corresponding author at: Renewable Energy Department, Energy Research Center, Research Institute of Petroleum Industry (RIPI), Tehran, Iran. Tel.: +98 21 48253224; fax: +98 21 44739725. ∗∗ Corresponding author at: Department of Chemistry, School of Basic Science, Yasouj University, Yasouj, Iran. E-mail addresses: akram [email protected] (A. Maghsodi), [email protected] (M.R. Milani Hoseini), [email protected] (M. Dehghani Mobarakeh), [email protected] (M. Kheirmand), [email protected] (L. Samiee), N [email protected] (F. Shoghi), [email protected] (M. Kameli). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.01.094
of their catalysis ability in four-electron reduction reaction of O2 to form H2 O and improvement of catalytic properties in relative to the isolated metals in low and medium temperature PEMFCs [3–5]. Based on simple thermodynamic principles, Fernandez et al. provided guidelines for the design of binary and multicomponent electrocatalytic materials for the reduction of oxygen, which combine one metal with another, which based on, they will easily break the O O bond of O2 and reduce adsorbed atomic oxygen [6]. The alloyed of transition metals, such as V, Cr, Co, Ti and Ni, with platinum exhibit significantly higher electrocatalytic activities towards the ORR than platinum alone in low temperature fuel cells [7–10]. Metallic palladium presents considerable catalytic activity for the ORR in acid electrolyte, which preferentially proceeds through a four-electron pathway. The use of Pd is interest as it is at least 50 times more abundant than Pt on the earth and they belong to similar group in the periodic table. The enhancement of the activity for Pt in the presence of Pd has been found by CV and single direct ethanol fuel cell (DMFCs) measurements and these results originate from the weakening of the O O bond on Pd-modified Pt nanoparticles. Recently, several researches were aimed towards the study of interaction of oxygen with supported and unsupported Pd and Pt catalysts. These materials showed significant selectivity towards
6354
A. Maghsodi et al. / Applied Surface Science 257 (2011) 6353–6357
OH and O adsorption, with decreasing the OH coverage on Pt and enhancing the ORR kinetics [11,12]. Lopesa et al. prepared carbon supported Pt-Pd alloy electrocatalysts with formic acid method and explorated ORR in DMFCs [11]. Joo et al. investigated Methanol-tolerant PdPt/C alloy catalyst for ORR [13]. Cho et al. synthesized Pd-based PdPt (19:1)/C electrocatalyst by sodium borohydride reduction combined with freeze–drying, also and studied it in single cell [14]. Sodium borohydride is widely used as a mild reducing agent in organic synthesis of stereo specific action [15]. Because of toxicity problems of other reducing agents such as hydrazine, formaldehyde, sodium borohydride is a safe and high efficient one. It is normally suitable for use without excluding atmospheric oxygen or moisture, too. Therefore, in the present study, carbon supported Pt and Pt-Pd electrocatalysts were prepared by reduction of metal precursors with NaBH4 method. Then, they were applied onto the gas diffusion layer (GDL). Finally, gas diffusion electrode (GDE) was formed as working electrode (0.6644 cm2 area). By considering the effectiveness of NaBH4 as a reducting agent, we were therefore stimulated to study of catalytic activity of carbon supported Pt-Pd electrocatalysts and then compare to pure Pt. 2. Experimental 2.1. Chemicals used Chloroplatinic acid hydrate (H2 PtCl6 ·xH2 O), sulfuric acid (H2 SO4 ), sodium borohydride (NaBH4 ) and 2-propanol ((CH3 )2 CHOH) were received from Merck. PdCl2 and nafion solution (5 wt%) were obtained from Alfa Aesar. Polytetrafluoroethylene (PTFE) and Vulcan carbon (XC-72R) have been produced from Aldrich and Cabot Company respectively. All chemicals used were of analytical grade. 2.2. Synthesis of catalysts Vulcan carbon (XC-72R) was ultrasonically dispersed in de-ionized water and 2-propanol. Then metal precursors (H2 PtCl6 ·xH2 O + PdCl2 ) were added to the result ink. Molar ratio of Pt to Pd was achieved 1:1 and the total metal content in the electrocatalyst was 40 wt% vs. carbon support. The mixture was heated up to 80 ◦ C in presence of NaBH4 solution (reducing agent) under vigorous agitation. Then mixture was filtered, washed and dried under vacuum at 120 ◦ C for 4 h. For comparison, 20 wt% Pt/C was prepared with the same method, too. 2.3. Preparation of gas diffusion layer To preparation of GDLs, PTFE solution and Vulcan carbon (XC72R) was sonicated into mixture of 2-propanol, water and glycerol for 20 min and then homogeneous suspension was painted onto carbon paper (Electrochem). The resulting layer was dried at 80 ◦ C and sintered at 350 ◦ C for 30 min. The loading of GDLs was obtained equal to 2 mg cm−2 . 2.4. Preparation of gas diffusion electrode The catalyst ink was produced by ultrasonically mixing the desired amounts of synthesized electrocatalysts with 2-propanol, de-ionized water and 5 wt% nafion solution (Dupont). After painting the electrocatalyst ink on the GDLs, the electrodes were dried at 130 ◦ C for 1 h. The as prepared electrodes were known as GDEs. The total metal loading on GDLs was 0.5 mg cm−2 .
Fig. 1. XRD patterns of (a) Pt/C and (b) Pt-Pd/C electrocatalysts.
2.5. Physical and electrochemical characterizations of the electrocatalysts X-ray diffraction (XRD) pattern of all samples were collected in 2 range between 10◦ and 90◦ by using Philips PW1840 Diffrac˚ The crystallite sizes of tometer (Cu K␣1 radiation, K␣1 = 1.5406 A). the samples were estimated by applying Scherrer equation to (1 1 1) diffraction peak of Pt [16]. Scanning electron microscopy (SEM) images were obtained with Philips XL30, 17 kV. Electrochemical behavior of electrocatalysts was examined by CV, LSV, chronoamperometry and EIS techniques in H2 SO4 solution. Electrochemical measurements were carried out using an Autolab potentiostat/galvanostat PGSTAT30, Ag/AgCl saturated KCl as reference electrode and a Pt foil as a counter electrode. The GDE was mounted in a teflon holder containing a graphite ring as the current collector and having the provision for the oxygen or nitrogen supply at the back of the electrode. 3. Results and discussion 3.1. Physical characterization of the catalysts Fig. 1 shows the XRD patterns of Pt/C and Pt-Pd/C electrocatalysts. The peak appeared at 2 = 24◦ is related to use of Vulcan carbon (XC-72R) as a support. The characteristic crystalline peaks of electrocatalysts are at 2 = 39.7◦ (1 1 1), 46.2◦ (2 0 0), 67.4◦ (2 2 0), 81.2◦ (3 1 1) [17]. The XRD patterns of both electrocatalysts show the characteristic peaks of the face-centered cubic (fcc) crystalline Pt and Pd. The formation of Pt/Pd alloy was confirmed by slightly shift of XRD patterns to higher angles. This shift reveals that the alloy formation between Pt and Pd is due to incorporation of Pd in the fcc structure of Pt. The values of the lattice parameter of pure Pt and Pd metals are 0.3923 nm and 0.3890 nm, respectively. The lattice parameters of carbon supported Pt-Pd catalysts, commonly used in fuel cells, are higher than the value for pure Pd but smaller than that of for pure Pt which these indicate the contraction of the lattice is due to the particle substitution of Pt by Pd in the fcc structure in the formation of the Pt-Pd alloy [11,18]. The calculated lattice parameters of the Pt/C and Pt-Pd/C alloy, from the (1 1 1) peak, are listed in Table 1. Table 1 Crystallite size and lattice parameter of Pt/C and Pt-Pd/C electrocatalysts. Catalyst
Crystallite size (nm)
Lattice parameter (nm)
Pt/C Pt-Pd/C
6.3 7.8
0.3918 0.3904
A. Maghsodi et al. / Applied Surface Science 257 (2011) 6353–6357
6355
Fig. 2. SEM images of (a) Pt/C and (b) Pt-Pd/C electrocatalysts.
The average particle size of the nanoparticles (d) was estimated by using Scherrer equation (1) after background subtraction from (1 1 1) peak at 2 of ∼40◦ also [16]: d=
k ˇ cos
(1)
where k is the coefficient, generally taken as 0.9, is wavelength of ˚ ˇ is full width at half-maximum X-ray radiation equal to 1.54051 A, (FWHM) measured in radians, and is the angle measured at the position of platinum peaks. Table 1 shows the crystalline size of the electrocatalysts. Fig. 2 represents SEM images of the synthesized electrocatalysts. SEM graphs revealed the high dispersion of Pt and Pt/Pd nanoparticles on Vulcan carbon support with an average diameter about 20 nm (approximate value). As can be seen, in the SEM graphs Pt and Pt-Pd alloyed nanoparticles were agglomerated and as a result, the achieved size is larger than crystalline size that has obtained from XRD data. The bulk atomic compositions of electrocatalysts were investigated by using energy dispersive X-ray (EDX) Spectroscopy (Fig. 3). EDX results showed presence of 20 wt% and 19.45 wt% Pt, in Pt/C and Pt-Pd/C electrocatalysts respectively. The obtained atomic ratio for Pd in Pt-Pd/C electrocatalyst was around 16.5 wt% also which is in good agreement with nominal value.
Fig. 3. EDX analysis of (a) Pt/C and (b) Pt-Pd/C electrocatalysts.
1 V relative to an Ag/AgCl reference electrode in 0.5 M sulfuric acid. Between 0.05 and 0.25 V, hydrogen atoms are adsorbed and desorbed on the cathodic and anodic sweeps. Above 0.8 V, the formation and removing of oxide film is done on the surface of platinum electrocatalysts, on anodic sweep and cathodic sweep, respectively. It can be seen that the Pt-Pd/C electrocatalyst produces higher currents in the hydrogen region, compared with that of Pt/C elec-
3.2. Electrochemical evaluation of catalysts 3.2.1. Cyclic voltammetry Fig. 4 shows the cyclic voltammograms of the Pt and Pt-Pd nanoparticles supported on Vulcan carbon (XC-72R) in the nitrogen atmosphere with scan rate of 50 mV s−1 between −0.2 and
Fig. 4. Cyclic voltammograms of (a) Pt/C and (b) Pt-Pd/C electrocatalysts in N2 saturated 0.5 M H2 SO4 aqueous solution with potential scan rate of 50 mV s−1 and 20 ◦ C.
6356
A. Maghsodi et al. / Applied Surface Science 257 (2011) 6353–6357
Table 2 Electro active surface area, Tafel slope, exchange current density and Cottrell parameters of Pt/C and Pt-Pd/C electrocatalysts. EAS (m2 g−1 )
Catalyst Pt/C Pt-Pd/C
16.26 31.6
Tafel slope (mV/dec) 162 107
Exchange current density (A cm−2 ) −4
7.55 × 10 13 × 10−4
trocatalyst. The electrochemical active surface area (EAS) of the electrocatalysts could be estimated according to the following equation also [19]: EAS =
QH 0.21[Pt]
(2)
where [Pt] represents the platinum loading (mg cm−2 ) in the electrode, QH is the charge for hydrogen desorption and 0.21 shows the charge required for oxidation of a monolayer of H2 on bright Pt (mC cm−2 ). The calculations showed that the Pt-Pd/C electrocatalyst has highest EAS in relative to Pt/C electrocatalyst. So, with increasing of available active surface area, catalytic activity of electrocatalyst towards ORR has been raised. In spite of, larger particle size of Pt-Pd/C electrocatalyst, this sample has a higher EAS whereas generally, it occurs conversely. This matter can be related to synergetic effect, which is playing a critical role in hydrogen adsorption/desorption reaction and is not only dependent to particle size parameter. The synergetic effect of the simultaneous presence of Pd and Pt in the electocatalyst can be explained by the spill-over effect of hydrogen and oxygen from Pt surface to Pd and resultant freedom for Pt active sites [20]. Another electrochemical results confirm the better electrocatalytic performance of Pt-Pd/C electrocatalyst, too. The electrochemical active surface area of electrocatalysts is listed in Table 2. 3.2.2. Linear sweep voltammetry Fig. 5 depicts the polarization curves for electrocatalysts, in a conventional three electrode cell. Polarization curves were produced at a scan rate of 1 mV s−1 between 1 and −0.3 V relative to the Ag/AgCl in oxygen saturated in 2 M H2 SO4 solution. The kinetics parameters of the ORR were extracted by using the Tafel equation [21]:
Cottrell slope (A s1/2 )
D1/2 C (mol cm−2 s−0.5 )
0.007 0.009
1.93 × 10−7 2.5 × 10−7
is Faraday’s constant. The equation can be simplified as: = a + b log i
(4)
where b = 2.303RT/˛nF and called Tafel slope and a = b log i0 . This equation implies that in a certain current density range, over potential is linearly dependant on the logarithm of current density. The exchange current density can be obtained from the intercept at the current density axis. In the higher Tafel slope, kinetics of reaction is slower. The results showed that the ORR current density in Pt-Pd/C GDE is higher than in Pt/C GDE, which indicated that the GDE containing Pt-Pd/C has a much better performance which has been caused by weakening of the O O bond on Pd-modified Pt nanoparticles [11], synergic effect between Pt and Pd nanoparticles, and uniform dispersion of Pd and Pt on the carbon support. The kinetic parameters have been obtained in Table 2. The lower Tafel slope and higher exchange current density in Pt-Pd/C GDE can be attributed to an enhanced electrocatalytic activity of Pt-Pd/C electrocatalyst in relative to Pt/C electrocatalyst. 3.2.3. Chronoamperometry The oxygen diffusion coefficients from electrodes were determined by chronoamperometry. Chronoamperograms were obtained by holding the potential of the electrode at 1.2 V for 60 s and then holding it at 0.4 V relative to the Ag/AgCl electrode for 10 s with oxygen flowing along the back of the electrode. With plotting i vs. t−1/2 , the linear dependence relationship was obtained for different electrodes [22]: i(t) =
nFACD0.5 (t)0.5
(5)
where = (E − E0 ) is the over potential, R is gas constant, T is the absolute temperature, ˛ is the transfer coefficient, i0 is exchange current density, i is current density, n is number of electrons, and F
where n is the number of electrons involved in overall reaction of ORR, F is the Faraday constant, A is the surface area of the electrode, and D and C are the diffusion coefficient and the concentration of the reacting species, respectively. Fig. 6 shows the chronoamperograms of GDEs at 0.4 V in relative to the Ag/AgCl in oxygen saturated in 2 M H2 SO4 solution. The results confirmed the higher Cottrell slope and D for Pt-Pd/C GDE. Cottrell parameters are listed in Table 2, also.
Fig. 5. Linear potential sweep curves of ORR on (a) Pt/C and (b) Pt-Pd/C electrocatalysts in O2 saturated 2 M H2 SO4 aqueous solution, at potential scan rate of 1 mV s−1 and 20 ◦ C.
Fig. 6. Chronoamperograms of the (a) Pt/C and (b) Pt-Pd/C electrocatalysts in O2 saturated 2 M H2 SO4 aqueous solution at 0.4 V vs. Ag/AgCl electrode and 20 ◦ C.
=
2.303RT ˛nF
log
i i0
(3)
A. Maghsodi et al. / Applied Surface Science 257 (2011) 6353–6357
6357
ment in the Energy Research Center of Research Institute of Petroleum Industry (RIPI). References
Fig. 7. Impedance responses of the (a) Pt/C and (b) Pt-Pd/C electrocatalysts from 100 kHz to 100 mHz, in O2 saturated 2 M H2 SO4 aqueous solution at 0.3 V vs. Ag/AgCl electrode and 20 ◦ C.
3.2.4. Impedance spectroscopy The Nyquist plots of the different electrodes were obtained as be shown in Fig. 7. Impedance spectra were collected for frequencies between 100 kHz and 100 mHz, in oxygen saturated 2 M H2 SO4 solution at 20 ◦ C and 0.3 V relative to the Ag/AgCl. An AC potential of 5 mV was superimposed on the DC potential. The low frequency impedance is corresponded to the kinetic impedance of the ORR and it becomes smaller as the cathode over potential increases, whereas the high-frequency is independent of the over potential, which indicates the impedance dependency to an Ohmic process. The high-frequency impedance reflected the combination of the double-layer capacitance in the catalyst layer and the effective charge transfer resistance [23]. As can be seen, Pt-Pd/C GDE has minimum electronic resistance and slightly better performance towards ORR in relative to the Pt/C GDE. 4. Conclusion Nanoparticles of Pt/C and Pt-Pd/C have been prepared via a synthetic procedure by reduction in presence of NaBH4 . These nanoparticles were formed in range of 6–8 nm average size. XRD Patterns revealed the nanoparticles formation in fcc structure. The electrochemical results showed the higher catalytic activity of Pt-Pd/C as compared to Pt/C electrocatalyst towards ORR in PEMFCs. This enhanced electroactivity was attributed to weakening of the O O bond on Pd-modified Pt nanoparticles, synergetic effect between Pt and Pd nanoparticles, uniform dispersion of Pd and Pt on the carbon support and higher EAS of the alloyed electrocatalyst. Acknowledgements The authors are grateful to the support of Iran University of Science and Technology (IUST), and the Renewable Energy Depart-
[1] J. Larminie, D. Dicks, Fuel Cell Systems Explained, third ed., Wiley & Sons, Chichester, 2000. [2] W.E. Mustain, J. Prakash, Kinetics and mechanism for the oxygen reduction reaction on polycrystalline cobalt–palladium electrocatalysts in acid media, J. Power Sources 170 (2007) 28–37. [3] M. Gustavsson, H. Ekstroem, P. Hanarp, G. Lindbergh, E. Olsson, B. Kasemo, Thin film Pt/TiO2 catalysts for the polymer electrolyte fuel cell, J. Power Sources 163 (2007) 671–678. [4] A. Wieckowski, E.R. Savinova, C.G. Vayenas, Catalysis and Electrocatalysis at Nanoparticle Surfaces, Marcel Dekker Inc., New York, 2003. [5] K.S. Nagabhushana, C. Wiedenthaler, S. Hocevar, D. Strmcnik, M. Gaberscek, A.L. Antozzi, G.N. Martelli, Kinetics of oxygen reduction reaction on nanosized Pd electrocatalyst in acid media, J. New Mater. Electrochem. Syst. 9 (2006) 73–81. [6] J.L. Fernandez, D.A. Walsh, A.J. Bard, Thermodynamic guidelines for the design of bimetallic catalysts for oxygen electroreduction and rapid screening by scanning electrochemical microscopy. M–Co (M: Pd, Ag, Au), J. Am. Chem. Soc. 127 (2005) 357–365. [7] M.T. Paffett, G.J. Berry, S. Gottesfeld, A new approach to the problem of carbon monoxide poisoning in fuel cells operating at low temperatures, J. Electrochem. Soc. 135 (1988) 2651–2652. [8] B.C. Beard, P.N. Ross, The structure and activity of Pt-Co alloys as oxygen reduction electrocatalysts, J. Electrochem. Soc. 137 (1990) 3368–3374. [9] T. Toda, H. Igarashi, H. Uchida, M. Watanabe, Enhancement of the electroreduction of oxygen on Pt alloys with Fe, Ni, and Co, J. Electrochem. Soc. 146 (1999) 3750–3756. [10] S.C. Zignani, E. Antolini, E.R. Gonzalez, Evaluation of the stability and durability of Pt and Pt–Co/C catalysts for polymer electrolyte membrane fuel cells, J. Power Sources 182 (2008) 83–90. [11] T. Lopesa, E. Antolini, E.R. Gonzaleza, Carbon supported Pt–Pd alloy as an ethanol tolerant oxygen reduction electrocatalyst for direct ethanol fuel cells, Int. J. Hydrogen Energy 33 (2008) 5563–5570. [12] L. Zhang, K. Lee, J. Zhang, The effect of heat treatment on nanoparticle size and ORR activity for carbon-supported Pd–Co alloy electrocatalysts, J. Electrochim. Acta 52 (2007) 3088–3094. [13] J.B. Joo, Y.J. Kim, W. Kim, N.D. Kim, P. Kim, Y. Kim, J. Yi, Methanol-tolerant PdPt/C alloy catalyst for oxygen electro-reduction reaction, Korean J. Chem. Eng. 25 (2008) 770–774. [14] Y.H. Cho, B. Choi, Y.H. Cho, H.S. Park, Y.E. Sung, Pd-based PdPt(19:1)/C electrocatalyst as an electrode in PEM fuel cell, Electrochem. Commun. 9 (2007) 378–381. [15] W. Buchner, H. Niederprum, Sodium borohydride and amine-boranes, commercially important reducting agents, Pure Appl. Chem. 49 (1977) 733–743. [16] V. Radmilovic, H.A. Gasteiger, J.P.N. Ross, Structure and chemical composition of a supported Pt-Ru electrocatalyst for methanol oxidation, J. Catal. 154 (1995) 98–106. [17] JCPDS International Center for Diffraction Data, U.S.A. Powder diffraction files (Inorganic Volumes), 4-802. [18] E. Antolini, Palladium in fuel cell catalysis, Energy Environ. Sci. 2 (2009) 915–931. [19] A. Pozio, M.D. Francesco, A. Cemmi, F. Cardellini, L. Giorgi, Comparison of high surface Pt/C catalysts by cyclic voltammetry, J. Power Sources 105 (2002) 13–19. [20] C. Wang, B. Peng, H.N. Xie, H.X. Zhang, F.F. Shi, W.B. Cai, Facile fabrication of Pt, Pd and Pt-Pd alloy films on Si with tunable infrared internal reflection absorption and synergetic electrocatalysis, J. Phys. Chem. C 113 (2009) 13841–13846. [21] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications, 2nd ed., John Wiley & Sons, New York, 1980. [22] J. Mirzazadeh, E. Saievar-Iranizad, L. Nahavandi, An analytical approach on effect of diffusion layer on ORR for PEMFCs, J. Power Sources 131 (2004) 194–199. [23] X.-Z. yuan, H. wang, PEM fuel cell electrocatalysts and catalyst layers: fundamentals and applications, in: J. Zhang (Ed.), PEM Fuel Cell Fundamentals, Springer, Vancouver, Canada, 2008, pp. 76–77.