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Sep 13, 2010 - VancouVer, B.C. V6T 1Z3, Canada, and Ballard Power Systems, 9000 Glenlyon .... g-1 graphitized carbon (provided by Ballard Power System...
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16488

J. Phys. Chem. C 2010, 114, 16488–16504

Pt-SnO2-Pd/C Electrocatalyst with Enhanced Activity and Durability for the Oxygen Reduction Reaction at Low Pt Loading: The Effect of Carbon Support Type and Activation Anna Ignaszak,† Carolyn Teo,† Siyu Ye,‡ and Elo˝d Gyenge*,† Department of Chemical and Biological Engineering, The UniVersity of British Columbia, 2360 East Mall, VancouVer, B.C. V6T 1Z3, Canada, and Ballard Power Systems, 9000 Glenlyon Parkway, Burnaby B.C. V5J 5J8, Canada ReceiVed: May 16, 2010; ReVised Manuscript ReceiVed: July 28, 2010

A simple-to-use method of carbon support surface activation using a PdCl2-SnCl2 solution (referred to as the Shipley solution) was developed, for improving the activity and durability of low Pt loading (0.1 mg cm-2) catalysts toward the oxygen reduction reaction (ORR). Three types of carbon supports were investigated: Denka, Graphitized Carbon (GC) and Vulcan XC-72R. Pt nanoparticles were synthesized by a modified polyol process and adsorbed on Shipley solution treated carbon supports. The resulting catalyst composition was Pt-SnO2-Pd/C with beneficial atomic ratios of Pt/Sn g 12 and Sn/Pd g 2. Employing extensive potential cycling as accelerated degradation testing, we found that the Pt-SnO2-Pd/C catalysts maintained excellent electrocatalytic activity for ORR with respect to control samples without carbon support treatment (both commercial and in-house prepared). The Pt area-specific oxygen reduction current density at 0.9 VRHE and 25 °C was most improved in the case of the GC support. The activity of commercial Pt/GC after stability testing decreased by 21.5% to 0.055 A m-2Pt while for Pt-SnO2-Pd/GC, 0.078 A m-2Pt was measured, identical to that of the fresh catalyst. The electrochemical results are supported by detailed surface analysis. Pt-Pd bimetallic nanoclusters were identified, and the kinetically beneficial role of the partially reduced SnO2 surface for oxygen and water adsorption is discussed. The oxygen mass transfer limiting current densities in the porous rotating disk electrodes (PRDE) were qualitatively correlated with changes of the catalyst layer morphology during stability testing in terms of the Bonnecaze and Ahlberg models. Introduction One of the most challenging problems facing polymer electrolyte membrane (PEM) fuel cells involves the oxygen reduction reaction (ORR) at temperatures below 100 °C and the need to find cost-effective, highly active, and durable electrocatalysts. Recently, promising results were reported using as cathode catalyst Pd-Co-Au (7:2:1 atomic ratio, heat treated at 750 °C in a reducing atmosphere), Fe-N-C, and Co-polypyrrole.1-3 In a H2-air PEM cell with 0.06 mg cm-2 Co loading (in the form of Co-polypyrrole), the peak power density was 70 mW cm-2 at 80 °C and 200 mA cm-2. Furthermore, the Co-polypyrrole was stable during 100 h of continuous operation at 0.4 V cell voltage.3 This result warrants further studies offering a realistic prospect for replacing Pt as the PEM fuel cell catalyst of choice. In spite of its cost, platinum remains for now the preferred catalyst for practical PEM fuel cell development, due to superior activity and stability compared to those of most nonplatinum catalysts. The goal is to reduce the cathode Pt loading to (or below) 0.1 mg cm-2 by improving the intrinsic kinetic activity while at the same time minimizing the loss of active surface area during extended fuel cell operation.4-10 Many investigations have demonstrated both by computational approach (e.g., density functional theory) and experimental methods that Pt based bimetallic catalysts (e.g., Pt3M where M ) Co, Ni, Fe, Pd, V, Ti) exhibit enhanced activity for ORR in comparison with pure * Corresponding author. E-mail: [email protected]. † The University of British Columbia. ‡ Ballard Power Systems.

Pt by a factor of 1.5 to 3 times, expressed in terms of active area-specific current density at 0.9 VRHE.11-26 The principle explanations for the enhanced ORR activity could be summarized as being due to (i) modification of the electronic structure of Pt (5d orbital vacancies);13,21 (ii) changes in the surface arrangement of the Pt atoms and coordination number (i.e., contraction of the Pt-Pt bond induced by the alloying element produces favorable atomic configuration for dissociative oxygen adsorption);11,12,14 and (iii) increased availability of Pt surface sites for oxygen adsorption due to weakening of the bond between Pt and the nonreactive oxygenated species present in the electrolyte (e.g., such mechanism was proposed in the case of Pt3Ni(111)).5,23 For binary or ternary Pt-alloy catalyst formulations, techniques promoting surface segregation of Pt and formation of a special Pt skin with modified atomic arrangement have received attention, such as high temperature annealing, acid leaching, and voltammetric anodic stripping of the less noble alloying elements (e.g., Cu, Co, and Co-Cu).27-29 Some of these techniques were beneficial for creating very active surfaces generating up to ten times higher ORR activity with respect to unmodified Pt as confirmed also by fuel cell experiments. Corresponding to a cell voltage of 0.9 V at 80 °C, the voltammetrically dealloyed Pt25Co75 catalyst generated a current density of 550 µA cm-2Pt compared to 180 µA cm-2Pt for 30 wt % Pt/C.28 There are, however, concerns whether the surface restructured Pt sites can maintain their stability over longer periods of time, and the applicability of some of these methods for large scale Pt catalyst synthesis is questionable. In order to control the morphological characteristics of the catalyst, the Pt nanoparticle or nanostructure preparation condi-

10.1021/jp104456j  2010 American Chemical Society Published on Web 09/13/2010

Pt-SnO2-Pd/C Electrocatalyst tions are crucial.30 A large variety of methods have been investigated including chemical reduction, ultraviolet photolysis, thermal decomposition, chemical vapor deposition, electrochemical synthesis (i.e., electrodeposition, electroless deposition, and electrophoresis), and sonochemical synthesis.31 Among these techniques, the polyol method, where the polyol has a dual role of reductant and colloidal metal encapsulating agent, has many advantages. It is fairly simple to execute and scale-up, and very importantly, it enables the accurate control of the size and shape of the metal nanoparticles.32-44 Controlling the pH, temperature, reaction time, and the composition of the organic-aqueous phases in the polyol process (e.g., the presence of certain additives), metal nanoparticles with typical sizes between 2 and 6 nm can be deposited on different substrates, including carbon nanotubes,40,41 carbon blacks,42 and metal oxides.45,46 In the present work, we address the question as to whether it would be possible to sensitize and activate by chemical treatment various carbon black supports (Vulcan XC-72R, Denka, and graphitized carbon (GC)) such that in conjunction with the polyol method one can obtain Pt/C catalysts with superior activity and stability toward ORR at low Pt loading (0.1 mg cm-2). We selected a SnCl2-PdCl2 solution, referred to as a Shipley-type solution,47-49 for carbon support activation. It was previously demonstrated that the Shipley solution treatment of graphite electrodes improved the dispersion and attachment of electrodeposited Pt and Pd particles.48,50 There is also some indication in the literature that SnOx supports could be beneficial for ORR in conjunction with Pt and Au catalysts, respectively.51,52 This is the first study where a detailed investigation is performed by electrochemical and surface analytical techniques with respect to the Sn-Pd activation of carbon black supports and the associated effect on ORR. In order to evaluate the wider applicability of the technique, three supports were selected with different specific surface areas and surface functional properties: Denka, GC, and Vulcan XC-72R. Thus, the aim is to gain insights into novel support-catalyst interaction effects and to provide an easy-to-use method for enhancing the electrocatalytic activity and durability of Pt/C toward ORR. Experimental Procedures Materials. Hexachloroplatinic acid hydrate H2PtCl6 · 6H2O (Sigma-Aldrich), palladium(II) chloride 99.9% (Sigma-Aldrich), and tin(II) chloride 95% (Sigma-Aldrich) were employed as metal precursors. The following chemical stock solutions were used: sulphuric acid, 95-98% reagent A.C.S grade (Fisher Scientific), glacial acetic acid (Fisher Scientific), hydrochloric acid, 36.5% reagent A.C.S grade (Fisher Scientific), acetone (CHROMASOLV, Sigma-Aldrich), ethylene glycol 99% (SigmaAldrich), and Nafion (5% solution in a mixture of lower aliphatic alcohols and water, Sigma-Aldrich). Ultra pure water (18 MΩ, Cole-Parmer Instrument Company, US) was used throughout the catalyst preparation procedures and electrochemical experimental work. High purity N2 and O2 (Praxair, Canada) gases were employed in the electrochemical measurements. Carbon blacks with different Brunnauer-Emmett-Teller (BET) surface areas were used as catalyst support: 225 m2 g-1 Vulcan XC-72R (from Cabot Corporation, US), 69 m2 g-1 Denka (from Denka Kagaku Kogyo K.K., Japan), and 164 m2 g-1 graphitized carbon (provided by Ballard Power Systems Inc.). Commercial catalysts: Pt/Vulcan XC-72R (HiSPEC4000 from Johnson Matthey, UK), Pt/Denka (provided by Ballard Power Systems Inc.), and Pt/GC (provided by Ballard Power Systems Inc.) were also tested as reference samples. The Pt contents of the commercial catalysts were 40 wt % for Pt/Vulcan

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16489 XC-72R and Pt/Denka, and 50 wt % for Pt/GC, respectively. The physicochemical properties of the three carbon supports and the commercial Pt catalysts were investigated in detail by the present authors in a previous related publication.53 Shipley-Type (SnCl2-PdCl2-HCl) Treatment of the Carbon Supports. Two hundred fifty milligrams of carbon black (Vulcan XC 72R, Denka, or GC) was immersed in 50 mL of a Shipley-type solution under three sets of experimental conditions. In the first treatment procedure (assigned as S), the carbon powders were immersed in Shipley solution for 5 min at 22 °C. The Shipley solution was prepared by dissolving in 12.5 wt % HCl appropriate amounts of PdCl2 and SnCl2 giving a Sn2+/Pd2+ molar concentration ratio of 40:1. For the second treatment condition (assigned as S1), the same Sn2+ and Pd2+ molar concentration ratio of 40:1 was used, but the temperature was increased to 50 °C, and the reaction time was extended to 10 min. The last treatment method (assigned as S2) was performed with higher Pd2+ content, namely, a Sn2+/Pd2+ molar concentration ratio of 40:5, while the temperature and time were 50 °C and 10 min, respectively (the same as in method S1). The HCl concentration was 12.5 wt % for all cases. The experiments were performed with magnetic stirring of the carbon support suspension. Following the Shipley solution treatment, the carbon supports were thoroughly washed in deionized water, filtered, and dried. Pt Catalyst Synthesis: Modified Ethylene Glycol (EG) Method. A modified ethylene glycol (EG) method was developed to prepare Pt/C with 40 wt % metal content. The polyol synthesized Pt was deposited on both Shipley treated and untreated carbon black supports (Denka, GC, and Vulcan XC72R). The catalyst samples on the untreated supports together with the commercial catalysts served as controls. In the EG method, first hexachloroplatinic acid was dissolved in a mixture of 5%vol EG in water so as to achieve a ratio of 1.9 mg of Pt per 1 mL of EG. Separately, the carbon support was ultrasonically dispersed in EG in a 1:1 ratio of carbon support weight to EG volume. The Pt precursor solution was added dropwise (over 10 min) under mechanical stirring to the ultrasonically dispersed carbon support-EG suspension. The pH of the mixture was adjusted to 11 by dropwise addition of 0.1 M NaOH under stirring. Afterward, the temperature was increased to 130 °C, and the reaction was carried out for 2 h. The entire catalyst preparation procedure was conducted under N2 atmosphere in order to minimize the unwarranted oxidation of either Pt or EG. After filtration and washing several times with deionized water, the filter cake was dried at 60 °C for 12 h followed by heat treatment at 200 °C for 1 h in N2 flow.37,39 Catalyst Characterization. X-ray Diffraction (XRD). A D8 Advance Bruker diffractometer with Cu KR1 radiation (1.54058 Å) was used to identify the crystallographic structure and phase composition of the catalysts. Measurements were performed at room temperature in the angle range of 10-85° with an angular resolution of 0.04° and step time of 1.5 s. XRD analysis was carried out in order to obtain all diffraction peaks of Pt catalysts and for the phase identification of carbons after pretreatment in the Shipley solution. The Pt (2 2 0) signal was scanned finely again and fitted using the Gaussian function to calculate the unit cell parameters for selected catalysts. X-ray Photoelectron Spectroscopy (XPS). XPS measurements of catalysts and supports were performed on Leybold Max 200 spectrometer using a nonmonochromatic Mg KR radiation (1253.6 eV) operated at 200 W (10 kV, 20 mA). All data were calibrated with respect to the C 1s excitation (284.6

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eV) to account for charging effects by the electron beam. Prior to individual elemental scans, survey scans were acquired with a 1 eV step to estimate the surface (depth 200 °C) burning of the catalyst-tocarbon contact catalyzed by Pt could take place, weakening the attachment of the catalyst to the support. The polyol synthesized catalysts in this work were not heated above 200 °C.

The Shipley-type support treatment methods (S, S1, or S2), in the case of the as-prepared Pt polyol catalysts, had virtually no effect on the ESA for the intrinsically higher surface area supports such as Vulcan and GC (Table 2). However, for the low surface area Denka support, the Shipley-type surface treatment method S had a profound beneficial impact on the Pt area (compare DP and DSP, Table 2). The DSP catalyst had smaller particle diameters,

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Figure 10. Cyclic voltammograms of Pt/C (a) and Pt-SnO2-Pd/C (b) catalysts supported on Vulcan XC-72R. 0.5 M H2SO4; N2 purge; 25 °C. Scan rate: 50 mV s-1.

between 1.9 and 4.5 nm with a mean of 3.48 nm (Fig. 6e), while the DP sample was characterized by larger particle sizes, between 2.9 and 6.4 nm with a mean of 4.11 nm (Fig. 6b). With respect to the stability of ESA following extensive cycling, the carbon support treatment methods were generally beneficial again for the high-surface area supports Vulcan XC72R and GC (Table 2). The best ESA stability was observed in the case of the GC support subjected to treatment method S2 when the ESA decreased by only 31.4% (GCS2P). The cyclic voltammograms used for the Pt area calculation (exemplified in Figure 10 for Pt/Vulcan XC-72R) reveal, in the case of the pretreatment methods that generated higher amounts of Pd on the surface such as S2 (i.e., VS2P) and S1 (i.e., VS1P), the formation of a characteristic sharp peak at 50 mVRHE ascribed to hydrogen adsorbed as the β phase on the Pd metal67 (Figure 10b). This peak was absent for the sample prepared by method S (Figure 10b) in accordance with the much lower Pd content (Table 1), and the peak at 50 mVRHE was also not present obviously in case of the commercial Pt/Vulcan XC-72R (Figure 10a). Hence, for samples S2 and S1 (to a lesser extent), there is free Pd on the carbon support (not covered by Pt) generating a characteristic hydrogen adsorption/desorption peak in cyclic voltammetry. Porous Rotating Disk Electrode (PRDE) Study of the Oxygen Electroreduction: Implications of the Mass Transfer Limiting Current Density. Figure 11 shows the linear voltammograms for ORR recorded at 1600 rpm for the as-prepared Pt/C, Pt-SnO2-Pd/C, and Pt/C commercial catalysts. The mass transfer limiting superficial current densities (i.e., expressed per geometric electrode area) were about -5 mA cm-2 indicating good reproducibility (less than 10% difference) of the oxygen mass transfer conditions for all of the as-prepared (i.e., fresh) electrodes. Furthermore, the mass transfer limited superficial

Ignaszak et al.

Figure 11. RDE voltammograms for O2 reduction on the as-prepared Pt/C and Pt-SnO2-Pd/C catalysts. Pt loading 0.1 mg cm-2, Nafion loading 0.09 mg cm-2 mixed in the catalyst layer. 0.5 M H2SO4; 25 °C, 1 atm O2 pressure, O2 saturated electrolyte; 1600 rpm scan rate, 5 mV s-1.

current densities for the porous electrodes are in agreement with the value reported for the polycrystalline flat Pt disk: -5.8 mA cm-2 at 1600 rpm, 333 K.19c PRDE studies were also carried out for ORR after the catalysts were subjected to accelerated degradation tests (Figure 12). It is important to note that after extensive potential cycling the commercial catalysts gave lower superficial current densities at E e 0.9 VRHE compared to the in-house prepared Pt polyol catalysts regardless of the support and support treatment methods (Figure 12). Furthermore, for the commercial catalysts the mass transfer limiting superficial current densities were lower by as much as almost 50% compared to that of the polyol synthesized Pt/C, (e.g., in the case of commercial Pt/GC, -2.8 mA cm-2 was measured vs -5 mA cm-2 for Pt/GCS2P; Figure 12b). The mass transfer limiting superficial current densities for the three commercial catalysts after the stability tests were in the order of Pt/Denka > Pt/Vulcan XC-72R > Pt/GC (Figure 12). Comparing also with the data for the as-prepared commercial catalysts (Figure 11), the smaller limiting superficial current densities for the commercial catalysts after the stability test can be attributed to the morphological degradation of the catalyst layer hampering the oxygen mass transfer in the porous layer. In contrast, the polyol synthesized Pt catalysts with Shipleytype support pretreatment (specifically using methods S2 and S1) generated the same oxygen mass transfer limiting superficial current densities of about -5 mA cm-2 before and after the stability tests (compare, for example, Pt/VS2P or Pt/GCS2P before and after stability tests; Figures 11 and 12). In order to better understand the oxygen mass transfer limiting superficial current density and to correlate it with changes in

Pt-SnO2-Pd/C Electrocatalyst

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16499

(

B ) 2kh

9ω9 R2ν5D4

)

1/6

(2)

where B is a perturbation parameter with typical values between 0.01 and 1,69 Cb is the bulk concentration of the reactive species, D is the bulk diffusion coefficient of the species in the fluid phase, h is the thickness of the porous catalyst layer, k is the permeability of the porous electrode, R is a constant equal to 0.51, ν is the fluid kinematic viscosity, and ω is the angular velocity. For an impermeable electrode k ) 0 and B ) 0, the Nam-Bonnecaze eq 1 simplifies to the Levich equation. When porous electrode effects are considered, the Nam-Bonnecaze model suggests higher ilim compared to the Levich case coupled with a nonlinear dependence of ilim versus ω1/2 as a result of eq 2. Relating the porous rotating electrode model to the data in Figure 12, it is observed that after the stability test for certain electrodes (most importantly for the commercial Pt/GC and Pt/ Vulcan XC-72R), ilim was lower compared to the value corresponding to the flat RDE based on the Levich equation and obtained also for the as-prepared electrodes (about -5 mA cm-2). Ahlberg et al. demonstrated both experimentally and theoretically that lower ilim is obtained when the RDE is covered by a porous electrochemically inactive film (referred to as a recessed electrode).70 The ratio of ilim in the absence (2-D Levich) and presence of a porous electrochemically inactive film (3-D recessed) is given by:70 Figure 12. RDE voltammograms for O2 reduction on the Pt/C and Pt-SnO2-Pd/C catalysts after stability testing. 0.5 M H2SO4; 25 °C, 1 atm O2 pressure, O2 saturated electrolyte; 1600 rpm; scan rate, 5 mV s-1.

the catalyst layer morphology as a result of degradation, we have to consider the theory of porous rotating disk electrode developed by Bonnecaze, Heller, and co-workers.68,69 Typically, in the case of Nafion coated thin-film catalyst layers deposited on RDE with Pt loadings of the order of 10-20 µg cm-2, the PRDE polarization response was approximated with that of a smooth and flat RDE where the mass transfer limiting current density is described by the Levich equation.19 This is justified by the thin catalyst layer (below 2 µm thickness) offering virtually 100% catalyst utilization efficiency19 and basically a two-dimensional reaction zone (i.e., absence of any porous media effects). However, in the present case with 0.1 mg cm-2 Pt loading (40 wt % Pt/C) mixed with 0.09 mg cm-2 Nafion, the thickness of the porous catalyst layer is approximately between 10 to 20 µm. According to Nam and Bonnecaze69 for porous electrode thickness to disk radius ratios (h/R) smaller than 0.1 and at low rotation rates (e.g., below 1000 rpm for a set of conditions used in ref 69), the PRDE behavior can be described by a model where the liquid boundary layer extends from the vicinity of the outer electrode surface into the porous electrode. This model leads to the following equation for the mass transfer limiting superficial current density:69

ilim ) 0.62nF(1 + B)D2/3ν-1/6Cbω1/2 with

(1)

ilim ,2D deff ω1/2 )1+ 1/3 1/6 ilim ,3D,recessed 1.61D ν (1 - φ)

(3)

where deff is the effective pore length (including both pore entrance and tortuosity effects),70 and φ is the area blocking factor expressing the fraction of disk geometric area that is blocked by the electrochemically inactive porous layer. In the present case of the catalyst layer, one could envisage a recessed porous electrode structure similar to the Ahlberg et al. model70 after the electrode has been subjected to extensive potential cycling causing the dislocation, receding, and agglomeration of the Pt nanoparticles at the bottom of the pores in contact with the disk surface. In this scenario, the Nafion and carbon support composite would constitute the electrochemically inactive porous layer from the point of view of the four-electron ORR. On the basis of Figure 12, this could be the situation for the commercial catalysts after extensive potential cycling when ilim decreased significantly compared to that of the fresh, as-prepared, electrode. Figures 13 and 14 present the superficial mass transfer limiting current density dependence on the square root of the angular velocity for both fresh (as-prepared) and after stability test catalysts. These figures compare the mass transfer performance of the commercial catalysts and selected in-house synthesized catalysts that generated the highest ilim values measured at 0.4 VRHE in Figures 11 and 12. The goal was to obtain some insights with respect to the differences in the PRDE behavior in the mass transfer limited domain based on the theoretical framework presented above. A quantitative data interpretation was beyond the objective, and it would require further experiments with varying porous catalyst layer thicknesses and compositions. For the as-prepared commercial Pt/GC and Pt/Vulcan XC72R, Figure 13a shows the mass transfer limiting current

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Ignaszak et al. interesting differences between the commercial and polyol prepared catalysts were found (Figures 13b and 14b, respectively). For the polyol synthesized catalysts, the ilim vs ω1/2 function was similar to the fresh electrode case (Figure 14b) showing the durability of the catalyst morphology during extensive potential cycling (i.e., 3800 cycles, see Experimental Procedures). In contrast, all commercial catalysts generated lower ilim values after being subjected to potential cycling, and the slope of ilim vs ω1/2 was dependent on ω (Figure 13b). It is proposed that the commercial electrode morphology after the stability test was characterized by a high effective intraporous layer diffusion length deff coupled possibly with an area blocking factor φ approaching one, thus causing large deviation from the 2-D Levich case (eq 3). Hence, on the basis of the mass transfer limiting current behavior, it can be inferred that during stability testing the porous commercial catalysts suffered more significant morphological degradation than polyol-synthesized Pt using Shipley activated supports. These modifications comprised receding and agglomeration of the Pt nanoparticles at the bottom of the pores generating an electrochemically inactive porous composite layer (Nafion-carbon). Related to this, the oxygen permeability of the porous electrode changed during the stability test in a way that is dependent on the carbon support porous structure, its chemical functionality, and Nafion content.53 Furthermore, it is likely that the oxidation of the support during

Figure 13. Mass transfer limiting superficial current densities (measured at 0.4 VRHE) for O2 reduction on commercial Pt/C as a function of angular velocity: 0.5 M H2SO4; 25 °C; 1 atm O2 pressure, O2 saturated electrolyte. (a) As-prepared electrodes; (b) after stability tests. Note: the lines serve only as a guide to the application of the Levich model.

densities reached a plateau at ω1/2 ) 16 (rad s-1)1/2. For the polyol synthesized catalysts (Figure 14a), a plateau was most notably observed in case of Pt/Denka (DS1P) at about ω1/2 ) 11 (rad s-1)1/2. The critical rotation rate where the ilim plateau appears is controlled by the porosity, thickness, and permeability of the catalyst layer.68,69 In the present case, these variables are a function of the catalyst support’s physicochemical properties and Nafion content.53 The critical rotation rate determining the ilim plateau can be understood from eq 4, which expresses the fact that at a certain critical angular velocity the limiting current of the recessed electrode is virtually independent of the rotation rate and it is dominated by intraporous layer diffusion:70

1 ilim ,3D,recessed

)-

deff 1.61ν1/6 1 · nF(1 - φ)DCb nFD2/3Cb ω1/2

(4) These observations qualitatively indicate differences in the porous catalyst layer morphologies as a function of support type and Pt catalyst preparation procedure. Future studies involving a thorough quantitative approach are necessary. Moving on to the mass transfer limiting current behavior of the catalysts after the stability test, very pronounced and

Figure 14. Mass transfer limiting superficial current densities (measured at 0.4 VRHE) for O2 reduction on polyol synthesized Pt/C and Pt-SnO2-Pd/C as a function of angular velocity. 0.5 M H2SO4; 25 °C; 1 atm O2 pressure, O2 saturated electrolyte: (a) as-prepared catalysts and (b) catalysts after stability tests. Note: the lines serve only as a guide to the application of the Levich model.

Pt-SnO2-Pd/C Electrocatalyst

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16501 TABLE 3: ORR Exchange Current Density and Apparent Tafel Slope for Selected Pt-SnO2-Pd/C Catalysts and the Commercial Pt/GCa Tafel slope (mV dec-1)

exchange current density i0 (A mgPt-1) catalyst

as-prepared

after stability test

asprepared

after stability test

DS1P GCS1P VS1P PtGC com

2.8 × 10-6 4.0 × 10-6 4.8 × 10-6 7.2 × 10-6

8.0 × 10-6 1.3 × 10-5 9.6 × 10-6 9.6 × 10-6

94 95 98 103

96 99 102 113

a 25 °C. 0.5 M H2SO4 saturated with O2 at 1 atm. Pt loading 0.1 mg cm-2 with 0.09 mg cm-2 Nafion mixed in the catalyst layer.

Figure 15. Electrochemical area-specific ORR activity at 0.9 VRHE. (a) Denka supported catalysts, (b) GC supported catalysts, and (c) Vulcan XC-72R supported catalysts. 0.5 M H2SO4; 25 °C; 1 atm O2 pressure; O2 saturated electrolyte.

the potential cycling protocol was different for Denka, GC, and Vulcan XC-72R and that the support oxidation affected not only the Pt nanoparticle stability but also the hydrophile-lypophile balance of the porous structure. The topic of PRDE behavior particularly with regard to degradation deserves further attention by coupling experimental studies with mathematical modeling. If a thorough mathematical description of the phenomena can be achieved, then the PRDE could become an important off-situ tool for studying PEM FC catalyst layer degradation. Electrocatalytic Activity for ORR. From an intrinsic electrode kinetic perspective, it was important to evaluate the performance of the various supported catalysts and the effect of the Shipley-type support treatment procedures. Figure 15 shows the area-specific (i.e., per ESA) current densities at 0.9 VRHE before and after the accelerated degradation tests. The percentage difference between fully replicated runs (i.e., with newly prepared electrodes) was typically below 10% for all of the electrodes. Among the commercial catalysts, Pt/Denka had the highest activity both before and after the stability tests, and its activity remained virtually constant at 0.11 A m-2Pt (Figure 15a). The

most significant loss of activity due to extensive potential cycling was found for Pt/GC where the area-specific activity dropped by 21.5% after the stability test (Figure 15b). The area-specific activity of commercial Pt/Vulcan XC-72R was also fairly stable, around 0.055 A m-2Pt (Figure 15c). For fresh commercial Pt/ Vulcan XC-72R catalysts with Pt loading of 14 µg cm-2 and without Nafion in the catalyst layer, Paulus et al. reported mass and area specific activities of 15 A g-1Pt and 0.23 A m-2Pt, respectively, at 0.9 V and 20 °C.19b The latter values can be regarded as the intrinsic kinetic maximum because due to ultralow Pt loading and absence of Nafion, the catalyst utilization efficiency was 100%.19a Clearly, these conditions are not met in operating PEM fuel cells. Hence, Figure 15 offers a more realistic scenario for the catalyst activity that could be extrapolated to fuel cell application. The activation of the carbon supports, particularly method S1, had a significant beneficial effect with regard to specific activity after the stability test. The effect was very pronounced in the case of Pt/GC. The treatment method S1 stabilized the Pt/GC specific activity at about 0.078 A m-2Pt; thus, there was virtually no difference before and after the stability test (GCS1P, Figure 15b). Similarly, in the case of both Denka and Vulcan XC-72R, the pretreatment method S1 was the most effective for maintaining high specific activity for ORR after extensive potential cycling, and the corresponding area specific current densities were 0.108 A m-2Pt (DS1P) and 0.067 A m-2Pt (VS1P), respectively (Figure 15a and c). Employing the kinetic region of the PRDE polarization data (0.85 V e E e 0.95 V), Tafel plots were constructed in terms of mass activity, and the corresponding exchange current densities and apparent Tafel slopes were calculated before and after the stability tests. The exchange current densities were obtained by extrapolating the Tafel lines in the high-potential region (E g 0.85 V) to E0 ) 1.23 V. Table 3 summarizes the kinetic parameters for Pt-SnO2-Pd/C catalysts with support treatment method S1 and for the commercial Pt/GC. The apparent Tafel slopes for ORR on the as-prepared catalysts were between 94 and 103 mV dec-1 (Table 3). In the literature on Pt/C at temperatures between 20 and 25 °C, ORR Tafel slopes of 60 mV dec-1 to 65 mV dec-1 were typically reported at potentials above 0.85 V (i.e., low overpotential region) in the absence of Nafion in the catalyst layer and with ultralow Pt loading, assuring 100% catalyst utilization efficiency.19b,71 The apparent Tafel slopes obtained here for the asprepared electrodes (Table 3), although higher than the typical values, are not uncommon in the literature.72,73 Considering the Nafion presence mixed with Pt/C, it is possible that in spite of utilizing the kinetically controlled region, nonelectrode kinetics related effects (e.g., ohmic potential drop) could have increased the apparent Tafel slope.74 From an intrinsic kinetic point of

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view, the adsorption of Nafion on Pt could affect the oxygen adsorption isotherm, the OHad surface coverage, and influence, and therefore, the Tafel slope (see below). After the stability test, the general trend was that the apparent Tafel slopes for ORR increased (Table 3). The exchange current density followed a compensatory trend with the Tafel slope, namely, the higher the Tafel slope, the higher was the exchange current density. It is hypothesized that the same factors that increased the Tafel slope contributed to an increase of the exchange current density as well. One possible explanation could be the effect of OHad surface coverage. Adzic and co-workers determined that high OHad surface coverage leads to an increase of the Tafel slope.75 Surface conditions that favor high OHad surface coverage could increase the exchange current density by increasing the rate of the backward reaction at equilibrium (1/2O2 + H+ + e- r OHad). Considering that at equilibrium the rates of the forward and backward reactions must be equal and determine the exchange current density, a new surface equilibrium state is established, which is characterized by higher exchange current density. Hence, a compensatory effect is formed between the exchange current density and the Tafel slope. The enhanced ORR activity for certain Pt-SnO2-Pd/C catalysts compared to commercial samples (Figure 15) can be explained by the presence of optimal amounts of Pd and SnO2 in the catalyst formulation. Yang et al.76 ascribed the improvement of the ORR activity to the Pt-Pd electronic interaction in Pt-Pd/C catalysts with a Pt-rich shell. Li et al.36 carried out a Mulliken population analysis of s, p, and d orbitals of pure Pt, Pd and PtmPdn clusters revealing that the s-orbital population of Pt atoms is increased, while the d-orbital population for the Pd atoms is decreased in comparison with their population in the pure Pt and Pd clusters. This indicates that the charges are transferred from the d-orbital of Pd to the s and p orbital of Pt. Furthermore, with respect to the O2 adsorption and dissociation on the Pt-Pd nanoclusters, it was found that oxygen is more readily adsorbed and dissociated on the Pd-modified Pt surface due to weakening of the O-O bond. In calculations performed by Li et al., the Pt-Pt bridge site modified by the underlying Pd atoms is considered to be the most active site for the dissociation of O2.36 In the present work, for the Pt-SnO2-Pd/C catalysts the k3-weighted Fourier transforms of the Pd K-edge in the EXAFS spectra showed splitting of the first shell peak of the Pd-Pd bond indicating the formation of bimetallic Pd-Pt nanoclusters. In addition to electronic Pd-Pt interaction, Arvia and coworkers reported the importance of Pd mesoscale structural effects such as ramified dendritic structures for improving the rate of ORR.77 Pursuing the same idea of structural effects, Xia and co-workers found that Pd-Pt nanodendrites generated high short-term activity, but unfortunately no comparison was provided with proper Pd-Pt control samples with other structural features.78 Hence, in the context of this study, the emphasis is on the electronic interaction between Pd-Pt, while the impact of the catalyst nanostructural feature, presumably significant for enhancing the ORR activity, requires further investigation. Furthermore, the role of SnO2 (mixed with SnO or Sn(OH)2) must also be considered in the discussion of the intrinsic kinetic activity. Watanabe et al.79 reported higher specific activity for oxygen reduction on Pt/SnO2 in comparison with metallic platinum and referred this improvement to the catalyst-support interaction along the lines of typical metal oxide support interaction effects.80 However, in more recent studies the

Ignaszak et al. enhanced ORR activity on hydrous SnOx (mainly SnO) supported catalysts was ascribed to the so-called bifunctional mechanism.52 This hypothesis assumes that oxygen is adsorbed on tin oxide where it is converted to OHad, which is further reduced to water on the metal particles. The bifunctional effect was originally proposed for the Au-SnOx/C catalyst.52 For the Pt-SnO2-Pd/C catalyst developed in this work, this mechanism is open to discussion. Taking into account the use of SnO2 in solid-state gas sensing devices where the adsorption of both O2 and H2O on SnO2 play very important roles81,82 it would be ill-advised to disregard similar effects when explaining the catalytic effect toward ORR. While the rate of O2 chemisorption on Pt is faster than on the SnO2,51 a possible secondary role for SnO2 sites in promoting O2 adsorption in conjunction with dissociative H2O adsorption82 cannot be excluded. Moreover, especially after extensive potential cycling during the stability test when the Pt surface sites are rendered less effective as shown for both the commercial and in-house synthesized Pt without support treatment, the coadsorbant role of SnO2 for oxygen and water can become very important for maintaining high ORR activity. On SnO2 (1 1 0), the most stable and majority component for polycrystalline SnO2, oxygen adsorption takes place on bridging-oxygen vacancies induced by the presence of Sn2+.81 In other words, oxygen adsorption occurs on the reduced SnO2 surface sites, while the fully stoichiometric SnO2 is inactive. On the basis of our XPS results for the as-prepared PtSnO2-Pd/C (Figure 8 a), the strongest intensities correspond to Sn4+, but the presence of Sn2+ cannot be distinguished due to the small difference in binding energies (0.18 eV) between Sn4+ and Sn2+. Therefore, it is difficult to determine the relative amounts of Sn2+ and Sn4+. However, it is plausible to assume that some vacancies were formed in the SnO2 structure by two routes: (a) during rinsing of the treated carbon particle supports, the residual SnCl2 hydrolyzed to Sn(OH)2 or SnO · H2O as shown by the higher than stoichiometric Sn/Pd atomic ratio (Table 1), and (b) during electrode conditioning, the potential reached 0 V (see Experimental Procedures), which is close the standard potential for SnO2/Sn2+ (-0.09 VSHE at 298 K). Therefore, it is plausible to consider the SnO2 surface in the present system as partially reduced, containing bridging-oxygen vacancies that act as active sites for oxygen adsorption. Regarding H2O adsorption on SnO2, the situation is the opposite, in the sense that molecular water adsorbs more strongly on the stoichiometric surface and only weakly on the reduced surface.82 On the stoichiometric SnO2 surface, the dissociative water adsorption generating OHad is energetically favored by 0.53 eV molecule-1 compared to molecular adsorption.82,83 It is important to note that Pt (or Pd) deposition on the SnO2 surface is not likely to affect either the reduced or the stoichiometric sites since adsorption of the metal on SnO2 is most favored on the in-plane oxygen vacancies that have no role in oxygen or water adsorption.84 Thus, from the point of view of adsorption processes the partially reduced SnO2 surface can play a dual role, favoring oxygen adsorption on the bridge-oxygen vacancies and dissociative water adsorption on the stoichiometrically occupied sites. Both processes are important for ORR and can assist the main Pt catalytic sites especially after the degradation occurring during stability testing. Obviously, there must be an optimum ratio of Pd and SnO2 on the surface in order for the effect in relation to Pt to be effective. On the basis of Figure 15, the surface treatment methods S or S1 generating Sn/Pd atomic ratios greater than or equal to 2

Pt-SnO2-Pd/C Electrocatalyst and Pt/Sn atomic ratios greater than or equal to 12 (Table 1) generated the highest area-specific ORR activities before and after the stability test, virtually independent of the carbon support type. Method S2, however, generating Sn/Pd atomic ratio of 1:1 and Pt/Sn atomic ratio less than or equal to 9 (Table 1), and leading to cyclic voltammetric characteristics representative of free Pd (Figure 10), was generally less effective for ORR activity enhancement. Conclusions A simple-to-use method of carbon support surface activation with a PdCl2-SnCl2 solution (referred to as the Shipley-type solution47) was developed and investigated for improving the activity and durability of Pt/C catalysts toward ORR. Three different activation conditions were applied (denoted by S, S1, and S2; see Abbreviations for detailed descriptions) to three types of carbon supports (Denka, graphitized carbon, and Vulcan XC-72R). Pt nanoparticles were synthesized by a modified polyol process and deposited at a loading of 0.1 mg cm-2 on both Shipley treated and as-received carbon supports. The Pt supported on as-received (untreated) carbons together with commercial Pt/Denka, Pt/GC, and Pt/Vulcan XC-72R were employed as controls. The catalyst layers contained 0.09 mg cm-2 Nafion as well. The goal was to perform the experiments with catalyst layer compositions that are closely applicable to fuel cells. XPS, XRD, and TEM characterization showed that the Shipley-type treatment of the support in conjunction with the modified polyol method generated a catalyst that could be described as Pt-SnO2-Pd/C with Pt particles of diameters between 2.5-4.1 nm (depending on the support type) and larger nanoparticles of Pd and SnO2 with diameters between 5 and 8 nm. The detailed composition of the various catalysts formulations is presented in Table 1. The SnO2 surface can be considered partially reduced, also containing Sn2+ sites. On the basis of high-resolution TEM images, we concluded that during the Shipley-type pretreatment of the carbon support, SnO2 and Pd deposited mostly on separate carbon sites but some on-the-top deposition occurred as well, generating core-shell structures with a SnO2 shell and a Pd metal core. The formation of separate SnO2 and Pd clusters is also supported by theoretical calculations indicating that the binding energy between Pd atoms exceeds the interaction energy of Pd adatoms with the SnO2 surface.84 Both SnO2 and Pd clusters can serve as Pt nucleation and deposition centers during the polyol method. The activation of the supports (Denka, GC, and Vulcan XC72R) by the Shipley-type methods (S, S1, and S2) triggered significant beneficial effects with respect to the Pt electrochemical active area and ORR electrocatalytic activity, particularly with regard to the durability as revealed by accelerated degradation testing. The loss of the Pt electrochemically active area was reduced for selected pretreated supports, and the most stable was the Pt/GC subjected to treatment methods S1 and S2 (GCS1P and GCS2P, respectively). The Sn-Pd activation of GC introduced heterogenic anchoring centers for Pt on the outer graphene layers, providing enhanced stability for the Pt nanoparticles. For the as-prepared catalysts, the highest area-specific ORR activity at 0.9 VRHE and 25 °C was typically obtained for the Denka supported Pt-polyol catalysts, about 0.11 A mPt-2. The support treatment method yielding the Pt-SnO2-Pd/C system was especially beneficial for maintaining high ORR activity after stability testing independent of the support type, thus demonstrating the wide applicability of the described technique. The

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16503 area-specific activity after stability testing was either higher (by up to 23.8% for Pt/Denka pretreated by method S1) or virtually the same as that for the fresh catalyst, indicating intrinsic kinetic contributions from the secondary components (SnO2 and Pd) of the catalytic system. It is proposed that the effect of SnO2 is mainly by providing secondary adsorption sites for O2 on the partially reduced SnO2 surface (at the bridging-oxygen vacancies associated with Sn2+) and dissociative H2O adsorption on the stoichiometric SnO2 sites. The role of Pd is mostly due to electronic effect. The optimum atomic ratios are Pt/Sn g 12 and Sn/Pd g 2. These ratios correspond to activation methods S or S1 (Table 1). The oxygen mass transfer limiting current density behavior for the porous rotating disk electrodes was analyzed in terms of the Bonnecaze et al.68,69 and Ahlberg et al.70 models. It was shown that morphological changes suffered by the commercial catalyst layers during stability testing could impact the transport properties of the porous media in a manner that was consistent with the recessed rotating porous electrode model.70 Thus, the Pt nanoparticles receded and agglomerated at the bottom of the catalyst layer, exposing a layer of electrochemically inactive porous carbon-Nafion composite. The catalyst layers on Shipley-activated supports were morphologically much more resilient and generated mass transfer limiting current densities virtually identical to those of the as-prepared (fresh) electrodes. Acknowledgment. We gratefully acknowledge the generous financial support by Ballard Power Systems Inc. (Burnaby, Vancouver) and funding from the Natural Sciences and Engineering Research Council of Canada to C.T. Abbreviations com ESA D GC S

S1

S2

P V

commercially available catalyst electrochemically active specific surface area Denka carbon support graphitized carbon support Shipley treatment of the carbon support using the Sn(II)/Pd(II) molar ratio in a solution of 40:1, at 22 °C for 5 min. Shipley treatment of the carbon support using the Sn(II)/Pd(II) molar ratio in a solution of 40:1, at 50 °C for 5 min. Shipley treatment of the carbon support using the Sn(II)/Pd(II) molar ratio in a solution of 40:5, at 50 °C for 10 min. Pt nanoparticles prepared by the modified polyol method Vulcan XC-72R support

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