Surface Properties of Pt and PtCo Electrocatalysts and Their Influence

J. Phys. Chem. C 2010, 114, 15823–15836

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Surface Properties of Pt and PtCo Electrocatalysts and Their Influence on the Performance and Degradation of High-Temperature Polymer Electrolyte Fuel Cells Antonino Salvatore Arico`,* Alessandro Stassi, Irene Gatto, Giuseppe Monforte, Enza Passalacqua, and Vincenzo Antonucci Istituto di Tecnologie AVanzate per l’Energia “Nicola Giordano”-CNR-ITAE, Via Salita S. Lucia sopra Contesse 5, 98126 Messina, Italy ReceiVed: May 18, 2010; ReVised Manuscript ReceiVed: August 4, 2010

An investigation of the behavior of carbon-supported Pt and PtCo cathode electrocatalysts was carried out with the aim to evaluate their performance and resistance to degradation under high temperature (110-130 °C) operation in a polymer electrolyte membrane fuel cell (PEMFC) based on a new ionomer membrane (Aquivion). Nanosized Pt and PtCo catalysts with similar crystallite size (2.7-2.9 nm) were prepared by using a colloidal route. A suitable degree of alloying and a face-centered-cubic (fcc) structure were obtained for the PtCo catalysts by using a carbothermal reduction. The surface properties were investigated by X-ray photoelectron spectroscopy (XPS) and low-energy ion scattering spectroscopy (LE-ISS, 3He+ at 1 kV). The formation of a Pt skin layer on the surface of the alloy electrocatalyst was obtained by using a preleaching procedure. Furthermore, the amount of Pt oxides on outermost atomic layers was much smaller in the PtCo than in the Pt catalyst. These characteristics appeared to influence catalysts’ performance and degradation. Accelerated tests (electrochemical cycling) at 130 °C in a pressurized PEMFC showed a better stability for the PtCo alloy as compared to Pt. Furthermore, better performance was obtained at high temperatures for the preleached PtCo/C as compared to the Pt/C cathode catalyst. At a moderate pressure of 1.5 bar (abs), maximum power densities of 800 and 700 mW cm-2 at 110 °C (H2-O2) were achieved for the PtCo cathode with 50% and 25% relative humidity (RH), respectively, by using 0.3 mg Pt cm-2 loading. At high pressure of 3 bar (abs), a maximum power density exceeding 1000 mW cm-2 was obtained at 130 °C and 100% RH. 1. Introduction The oxygen reduction process in acid electrolyte-based fuel cells is known to be enhanced on platinum alloys in relation to platinum metal.1–3 Leaching of non-noble metals from the alloy produces a surface roughening with a corresponding increase of the Pt surface area, but there is hardly any increase in the electrochemically active surface area (ECSA) for the platinum alloys as compared to the state of the art carbon-supported Pt metal. The observed improvements for the Pt alloys correspond to at least a doubling of the electrocatalytic activity in the Tafel region.4 This enhancement in electrocatalytic activity was differently interpreted, and several studies were made to analyze in depth the properties of several alloy combinations.2,5–7 Although a comprehensive understanding of the numerous reported evidence has not yet been reached, the observed electrocatalytic effects have been ascribed to several factors (interatomic spacing, preferred orientation, electronic interactions) which play, under fuel cell conditions, a favorable role in enhancing the oxygen reduction rate (ORR).2,7–12 As an example, the intrinsic electrocatalytic activity of Pt alloys (Pt-Cr, Pt-Ni, Pt-Co, Pt-Cu, Pt-Fe), with a lattice parameter smaller than that of Pt, was found to be higher than on the base metal.5–14 This effect was related to the nearest-neighbor distance of Pt-Pt atoms on the surface of the fcc crystals. The ratedetermining step was assumed to involve the rupture of the O-O bond through a dual site mechanism;3 accordingly, a decrease of the Pt-Pt distance favored the dual site O2 adsorption. In this regard, the formation of a tetragonal ordered structure upon * To whom correspondence should be addressed: Ph + 39090624237; Fax +39090624247; e-mail [email protected].

thermal treatment, in the case Pt-Co-Cr, was observed to lead to a more active electrocatalyst than that with a Pt fcc structure.8–12 Besides, an interplay between electronic and geometric factors (Pt d-band vacancy and Pt coordination numbers) and its relative effect on the OH chemisorption from the electrolyte was considered.15 Recently, a volcano-type relationship between the Pt d-band center for Pt3M alloys and the specific activity for the oxygen reduction reaction was observed.2 The d-band center appeared to determine the strength of the metal-adsorbate interaction. The highest electrocatalytic activity for the Pt3Co system in the Pt3M alloys series was the result of a favorable adsorption of reactive intermediates with respect to the surface coverage of blocking species.2 Moreover, the removal of transition metals from the surface of Pt alloys by acid treatment caused the occurrence of a Pt-skeleton structure in the near surface region.2 The enhancement of ORR activity in acid-leached Pt3Co relative to Pt was attributed to the formation of a percolated structure of Pt-rich and Pt-poor regions within the nanoparticles.16 It has been reported that most Pt alloys are not surface rich in the base-metal (M) component, and they may be essentially all Pt.2,17,18 Watanabe et al. reported that, after electrochemical testing of a smooth Pt-Fe alloy, the catalyst was covered by a thin Pt skin layer of less than 1 nm in thickness.18,19 The formation of the Pt skin probably occurs because iron on the surface leaches out of the alloy during operation in acidic electrolytes,18–20 while Pt atoms are redeposited and rearranged on the surface. Moreover, it was suggested18,19 that during the adsorption step a p orbital of O2 interacts with empty d orbitals of Pt, and consequent back-donation occurs from the partially

10.1021/jp104528q  2010 American Chemical Society Published on Web 08/30/2010

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filled orbital of Pt to p* (antibonding) molecular orbital of O2. Accordingly, the increase in d-band vacancies on Pt by alloying produces a strong metal-O2 interaction. This interaction weakens the O-O bonds, resulting in bond cleavage and bond formation between O and H+ of the electrolyte, thus improving the ORR. A tuning of surface characteristics in Pt alloys was developed by forming core-shell structures,21–25 whereas Chen et al.16 studied the effect of the acid treatment of Pt-Co catalyst in determining chemical and structure modifications in the nearsurface layers with corresponding enhancement of the ORR. A large-scale application of PEMFC technology requires a reduction of its high cost through a decrease of Pt loading and maximization of catalyst utilization as well as an improvement of performance and stability.1,26–28 Carbon-supported PtCo alloy catalysts have been recently demonstrated to be more stable than Pt/C to both carbon corrosion and metal area loss in PEMFCs.29–33 This system appears a promising alternative to Pt/C catalysts for practical applications. Beside the alloying effect, it is appropriate to mention the particle size effect. Various studies initially carried out on carbon-supported Pt electrocatalysts for oxygen reduction in phosphoric acid fuel cells showed that the electrocatalytic activity (mass activity, mA g-1 Pt, and specific activity, mA cm-2 real Pt surface area) depends on the mean particle size. The mass activity for oxygen reduction is maximized by a particle size of about 3 nm, corresponding closely to the maximum in the fraction of (111) and (100) surface atoms on Pt particles of cuboctahedral geometry.3,34 On the other hand, the specific activity increases gradually with an increase in Pt particle size and closely follows the trend observed between the surface fraction of (111) and (100) Pt atoms and the particle size.3,34 These results have indicated that the (111) and (100) surface atoms are more electrocatalytically active than Pt atoms located on high Miller index planes.34,35 Moreover, platinum atoms at edge and corner sites are considered to be less active than Pt atoms on the crystal faces.3,34 Accordingly, both mass and specific activity should decrease significantly as the relative fraction of atoms at edge and corner sites approaches unity.3,34,35 This occurs for a very small particle size. Several studies on the effect of the particle size are concerning carbon black supported Pt-based catalysts where a certain degree of agglomeration is present. Whereas, a recent study specifically addressing monodisperse Pt3Co catalysts indicates an optimum particle size of about 4 nm.36 Thus, it is appropriate to develop fuel cell catalysts with an optimal particle size, in the range of 3-4 nm, as a suitable compromise between dispersion (surface area) and intrinsic electrocatalytic activity. Several studies related to the surface properties of Pt alloys for oxygen electroreduction, especially those regarding the investigation of the surface chemistry of the topmost atomic layers, have been carried out on smooth ultrahigh-vacuumprepared electrodes;2,17–19 only recently, efforts have been addressed to investigate the topmost atomic layers for practical carbon-supported nanosized electrocatalysts as required for fuel cell applications.21–25 A surface analysis of practical fuel cell catalysts can provide useful insights into the surface properties of the carbon-supported Pt-Co alloy in relation to the Pt/C catalyst. Moreover, there is an increasing demand for PEMFC operation at high working temperatures to improve efficiency and tolerance of contaminants and for an easy thermal and water management.33 High-temperature operation influences significantly catalyst corrosion and sintering.3,33

Arico` et al. In this work, we have investigated the properties of the nearsurface region of Pt/C and PtCo/C catalysts with suitable particle size (∼3 nm) by using X-ray photoelectron spectroscopy and low-energy ion scattering spectroscopy with particular regard to their influence on performance and degradation of hightemperature polymer electrolyte fuel cells. High-temperature PEFCs represent the new frontier for automotive applications; thus, it appears appropriate to evaluate the most promising electrocatalysts under these conditions and possibly individuate relationships with the surface properties which play a paramount role in determining the activity and stability of the electrocatalysts. High-temperature electrochemical studies have been carried out on these electrocatalysts in the presence of new shortside-chain perfluorosulfonic membrane Aquivion (formerly called Hyflon37) E79-03S developed by Solvay-Solexis for hightemperature operation. This new polymer electrolyte is characterized by an equivalent weight of 790 g/equiv.38 The electrocatalytic activity for oxygen reduction and resistance to degradation under fuel cell conditions have been interpreted in the light of the physicochemical properties of the catalysts. Practical Pt/C and PtCo/C cathode catalysts with high metal surface area were prepared. The synthesis process was optimized to obtain suitable dispersions of the metal particles on the support for high metal concentrations (50 wt %) and a small mean particle size (about 3 nm). A preleaching procedure was studied to modulate the surface characteristics of PtCo catalysts.1,2,16,39–41 2. Experimental Section 2.1. Catalyst Preparation. 50 wt % Pt-Co/C catalysts with nominal alloy composition Pt3Co1 (at.) were prepared by incipient wetness from cobalt nitrate that was adsorbed on an amorphous PtOx/C catalyst.42 The PtOx/C was prepared by using a sulfite complex route.43 A Na6Pt(SO3)4 precursor was prepared from chloroplatinic acid. A Ketjenblack EC (KB) carbon black (BET area 850 m2 g-1) was suspended in distilled water and agitated in an ultrasonic water bath at about 80 °C to form a slurry. The appropriate amount of Na6Pt(SO3)4 was successively added to the slurry. The Pt sulfite complex solution was decomposed by adding H2O2 to form colloidal PtOx/C. After the cobalt impregnation step, a high temperature carbothermal reduction at 600 °C in inert (Ar) atmosphere was carried out to form the carbon-supported PtCo alloy.44 A 50% Pt/ Ketjenblack catalyst was similarly prepared for comparison. This catalyst was reduced under same conditions to obtain a particle size similar to the PtCo catalyst. A preleaching procedure at 80 °C in 0.5 M HClO4 was carried out for the PtCo catalyst. As reported in the literature,1,2,16,39–42 preleaching usually results in a better electrochemical activity. The initial amount of cobalt was varied for unleached and leached catalysts to obtain approximately a similar bulk metal composition (Pt/Co ∼ 3 at.). The following cathode catalysts were investigated: (i) 50% Pt-Co (3:1)/Ketjenblack EC; (ii) 50% Pt-Co (3:1)/Ketjenblack EC preleached in perchloric acid (0.5 M, 85 °C); (iii) 50% Pt/ Ketjenblack EC. 2.2. Physicochemical Analysis. The catalysts were characterized by X-ray diffraction (XRD) using a Philips X-pert 3710 X-ray diffractometer with Cu KR radiation operating at 40 kV and 30 mA. The peak profile of the (220) reflection in the facecentered-cubic structure of Pt and Pt alloy was analyzed by using the Marquardt algorithm, and it was used to calculate the crystallite size by the Debye-Scherrer equation.43 Instrumental broadening was determined by using a standard platinum

Surface Properties of Pt and PtCo Electrocatalysts

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TABLE 1: Catalyst Properties catalysts

treatment

50% PtCo/C 50% PtCo/C 50% Pt/C

unleached preleaching in HClO4 unleached

overall Pt/Co at. ratio (XRF)

alloy Pt/Co at. ratio (XRD)

2.9 3.4

4.4 3.2

sample. X-ray fluorescence analysis of the catalysts was carried out by a Bruker AXS S4 Explorer spectrometer operating at a power of 1 kW and equipped with a Rh X-ray source, a LiF 220 crystal analyzer, and a 0.12° divergence collimator. The Pt/Co atomic ratio for the alloy was determined by XRD using Vegard’s law. The overall Pt/Co ratio in the catalysts (both alloyed and unalloyed) was determined by XRF. The total metal content in the catalysts was determined by burning the carbon support in a thermal gravimetry experiment up to 950 °C in air and subsequent XRD analysis. Table 1 shows the main bulk physicochemical characteristics of the carbon-supported Pt and Pt-Co catalysts. Transmission electron microscopy (TEM) analysis was made by first dispersing the catalyst powder in isopropyl alcohol. A few drops of these solutions were deposited on carbon-film-coated Cu grids and analyzed with a FEI CM12 microscope. XRD and TEM investigations of the cathode after various degradation tests were made by scraping the powder from the catalytic layer and subsequent analysis as reported above. Scanning electron microscopy studies were carried out on cross sections of membrane-electrode assemblies (MEAs) with a SEM-FEG FEI XL30 instrument. X-ray photoelectron spectroscopy (XPS) measurements were performed by using a Physical Electronics (PHI) 5800-01 spectrometer. A monochromatic Al KR X-ray source was used at a power of 350 W. Spectra were obtained with pass energies of 58.7 eV for elemental analysis (composition) and 11.75 eV for the determination of the oxidation states. The pressure in the analysis chamber of the spectrometer was 1 × 10-9 Torr during the measurements. The Ag 3d5/2 peak of an Ag foil was taken, after argon sputtering, for checking the calibration of the binding energy (BE) scale. The quantitative evaluation of each peak was obtained by dividing the integrated peak area by atomic sensitivity factors, which were calculated from the ionization cross sections, the mean free electron escape depth, and the measured transmission functions of the spectrometer. XPS data have been interpreted by using the online library of oxidation states implemented in the PHI MULTIPAK 6.1 software and the PHI Handbook of X-ray Photoelectron Spectroscopy.45 For LE-ISS measurements, the polarity of the analyzer in the PHI 5800-01 spectrometer was switched from XPS to ISS mode, a 3He feed ion gun operating at low voltage (1 kV) was used. LE-ISS measurements were carried out by analyzing the energy of the scattered 3He+ ions at a scattering angle of 134.80°. A flat sample of catalyst powder was deposited on the sample holder which was tilted at 45° with respect to the analyzer. While the analysis chamber was under differential pumping, the sample surface was sputtered with 3He+ ions at 1 kV during the analysis and the counts N(E)/E at different energy ratios (eV/Ep) for scattered ions were recorded. The pressure in the analysis chamber during ISS measurements was 1 × 10-8 Torr. 2.3. Electrochemical Studies. The electrodes were prepared according to the procedure described in a previous paper;46 they consisted of carbon cloth backings, diffusion, and catalytic layers. In the present work, the catalytic layer was composed of 33 wt % Aquivion ionomer (790 g/equiv) and 67 wt % catalyst with Pt loading of 0.3 mg cm-2. MEAs were formed

A220, nm

crystallite size (XRD), nm

particle size (TEM), nm

ECSA (CV), m2/g

0.385 0.383 0.392

2.9 2.9 2.7

3.1 2.9 2.6

48 55 52

by a hot-pressing procedure and subsequently installed in a fuel cell test fixture. An experimental Aquivion E79-03S short-sidechain perfluorosulfonic membrane38 with a thickness of 30 µm (dry form) and an equivalent weight of 790 g/equiv recently developed by Solvay-Solexis for high-temperature operation was used. In the MEAs, the anode was maintained constant (50% Pt/KB) whereas the cathode was varied by using the catalysts described above. The cell test fixture was connected to a fuel cell test station including an Agilent HP6060B electronic load for polarization experiments, a digital memory oscilloscope, and an AUTOLAB Metrohm potentiostat/galvanostat equipped with a 20A current booster for electrochemical diagnostics. The humidifiers temperature was varied with respect to the cell temperature to change the relative humidity (RH). The cell temperature was measured by a thermocouple embedded in the cathodic graphite plate, close to the MEA. Steady-state galvanostatic polarization experiments in PEMFC were performed in the presence of H2-O2 in a 5 cm2 single cell at various temperature and pressure conditions. Cyclic voltammetry (CV) studies were carried out at 80 °C to determine the ECSA. In this experiment, hydrogen was fed to the anode that operated as both counter and reference electrode, whereas nitrogen was fed to the working electrode. The sweep rate was 20 mV s-1. The electrochemical active surface area was determined by integration of CV profile in the hydrogen adsorption region after correction for double-layer capacitance.33 Two different accelerated degradation tests in PEMFC, in terms of potential cycling, were carried out at 130 °C by using the configuration above-described for polarization and CV experiments, respectively; in these degradation tests both cell and humidifiers were pressurized (3 bar (abs)) and maintained at the same temperature. After electrochemical testing, the cathode layers were detached from the membrane and characterized ex situ by physicochemical analyses to evaluate Pt sintering and dissolution. 3. Results and Discussion 3.1. Structure and Morphology Analyses. The preparation of the catalysts was tailored to obtain a similar crystallite size and dispersion properties for the carbon-supported Pt and Pt-Co alloy as well as similar Co content in the bulk for unleached and acid-leached alloys. X-ray fluorescence indicated an overall Pt/Co atomic ratio of about 3 (Table 1), whereas thermal gravimetry confirmed a 50 ( 2 wt % metal concentration in the catalysts. The overall Pt/Co ratio in the bulk, as determined by XRF, was indeed slightly larger in the leached sample due to the fact that some Co was removed by the acid treatment. XRD patterns of the Pt3Co1/KB and Pt/KB catalysts are reported in Figure 1. The catalysts showed a cubic structure (fcc) for the alloy as well as for Pt and a hexagonal structure for the carbon support. Mean crystallite size, lattice parameter, atomic ratio in the true alloy, or degree of alloying was derived from XRD, particle size from TEM, and ECSA from CV. These properties are reported in Table 1. Line broadening analysis of the fcc 220 reflection (Figure 1b) indicated an appropriate

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Figure 1. (a) XRD patterns of Ketjenblack carbon-supported 50 wt % Pt, unleached 50 wt % Pt-Co, and preleached 50 wt % Pt-Co electrocatalysts. (b) Least-squares fitting of the diffraction peaks related to the (220) reflection of the fcc structure in the XRD patterns of the respective samples.

crystallite size of 2.7-2.9 nm for the catalysts. For both the PtCo catalysts, a large lattice (A220) contraction compared to Pt catalyst corresponding to a high degree of alloying was observed. The Co atomic concentration, as determined by XRD, was about 20 and 25% in the PtCo fcc structure of the unleached and preleached catalysts, respectively, as calculated from Vegard’s law. This was close to the nominal content (25%) for the alloy. A high alloying extent of Pt is considered a prerequisite to enhance the catalytic activity.47 By comparing the atomic Pt/Co ratios obtained from XRD and XRF, slight differences were observed due to the fact that a small amount of Co was not alloyed to Pt in the unleached sample. The unalloyed Co was probably located at the surface due to the Co nitrate impregnation procedure. The unalloyed Co was also probably removed by the preleaching in perchloric acid. TEM analysis (Figure 2) showed a good dispersion for all catalysts and mean particle size similar to the crystallite size determined by XRD (Table 1). 3.2. PEMFC Studies. In a previous study,33 the electrochemical and bulk properties of an unleached PtCo/C catalyst were investigated in comparison to Pt/C in the presence of a conventional Nafion membrane. Moreover, the catalysts previously investigated did not have similar crystallite size. The significant role of the particle size in determining mass and specific activity was mentioned in the Introduction.34–36 In the

present study, we have focused the attention on the surface properties of the catalysts. In this regard, a preleaching procedure of the PtCo catalyst in perchloric acid was investigated. Moreover, to avoid any effect of the particle size on the electrochemical behavior, we have investigated catalysts with similar mean crystallite size. The ECSA of these catalysts was studied by using conventional cyclic voltammetry (CV).33 The values determined by CV in the hydrogen adsorption region are reported in Table 1. The ECSA was around 50 m2/g for all the catalysts. To study the high-temperature behavior of these catalysts, the performances of 50% Pt/KB, 50% PtCo/KB, and preleached 50% PtCo/KB were compared in the presence of a new perfluorosulfonic acid membrane (Aquivion)38 at 130 °C under high pressure, i.e., 3 bar (abs) H2-O2 (Figure 3). The cell resistance was measured by the current interrupter method by using a digital memory oscilloscope. No large increase of ohmic resistance was recorded at 130 °C provided that proper pressure (3 bar (abs)) and humidification (100% RH) conditions were selected.33 The ohmic resistance for the various MEAs at 130 °C, 3 bar (abs), and 100% RH was similar (about 0.05 ohm cm2). Under these operating conditions, i.e., Aquivion electrolyte and high temperature (130 °C), the performance of the preleached 50% PtCo/KB was better than the 50% Pt/KB (Figure 3a). The gain in power density for the PtCo/KB vs Pt/KB



Figure 2. Transmission electron micrographs of Ketjenblack carbon-supported unleached 50 wt % Pt-Co (top), preleached 50 wt % Pt-Co (middle), and 50 wt % Pt (bottom) electrocatalysts.

cathode catalyst at the end of the activation controlled region in the polarization curve, e.g., at 0.75 V, was about 50%. The MEA containing the perchloric acid-leached PtCo cathode catalyst showed interesting performances at 130 °C, 3 bar (abs), and 100% RH achieving more than 1000 mW cm-2 in the presence of a Pt loading of 0.3 mg cm-2 in both electrodes (Figure 3a). However, the unleached PtCo/KB catalyst was much less performing than the preleached PtCo/KB (Figure 3b), indicating a paramount role of the surface properties. Accordingly, subsequent experiments were focused on the most performing preleached PtCo catalyst. Two different accelerated test procedures have been selected to evaluate the cathode catalyst stability:1,33 (A) Evaluation of Pt degradation in step cycles of 1 min between a cell voltage of 0.7 V (IR-free) and 0.9 V in the presence of H2 feed at the anode and O2 feed a the cathode. The operating conditions were: cell temperature 130 °C, pressure 3 bar (abs), 100% RH. (B) Evaluation of Pt degradation by continuous linear sweep cycling (20 mV s-1) between a cathode potential of 0.6 and 1.2 V vs RHE.

In procedure B, diluted H2 was supplied to the anode and N2 was fed to the cathode, the cell was operated at 130 °C, 3 bar (abs), and the gases were humidified at the same temperature of the cell (∼100% RH). Procedure A (a protocol adopted by the US Department of Energy1) resembles the practical (cycled) operation of a fuel cell. The potential cycling occurs between two limits represented by a cell potential close to the open circuit voltage (OCV, 0.9 V) and the designed operation point of a practical PEMFC (0.7 V). Since the corrosion rate appears to be mainly influenced by the electrochemical potential and to a less extent by the temperature and RH,33 the most severe operation condition of a PEMFC in terms of stability is represented by the OCV. Procedure B represents an assessment of the intrinsic stability of the catalyst. It is not directly related to the operating conditions, but the electrode is cycled in the region where Pt is less stable. It is known that the formation of a stable Pt oxide phase at potentials higher than 1.2 V RHE stabilizes Pt, whereas at potentials smaller than 0.6 V RHE, the corrosion effects are negligible.33 Furthermore, since nitrogen is fed to the cathode

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Figure 3. Comparison of the polarization behavior for the (a) Ketjenblack carbon-supported 50 wt % Pt and preleached 50 wt % Pt-Co and (b) unleached and preleached 50 wt % Pt-Co electrocatalysts in the presence of Aquivion membrane at 130 °C, 100% RH, and 3 bar (abs) H2-O2.

in the procedure B, there is no water produced electrochemically during this test. During the anodic dissolution of PtCo alloys, the initial step involves Co species on the surface that dissolve into the electrolyte at potentials lower than Pt. The second step involves hydrolysis of Pt2+ species.40 It is reported in the literature that the most significant degradation of Pt-based catalysts is associated with the Ostwald ripening, causing Pt dissolution and reprecipitation.30–33 This produces a growth of the metal particles. In both procedures A and B, a modification of the catalyst properties occurred during the accelerated tests as a consequence of the Pt dissolution and reprecipitation processes (particle growth).27–33 The growth of metal crystallites was studied by XRD (220) line broadening analysis of the cathode layer after 1500 cycles of degradation test A (Figure 4). It was observed that procedure A caused a degradation with consequent increase of crystallite size (Table 2). However, this increase was less significant for the preleached PtCo vs the Pt catalyst, i.e., 4 vs 6 nm (Figure 4, Table 2). Similar evidence was qualitatively confirmed by SEM observation of several electrode regions. An example of metal particle growth for the PtCo catalytic layer after the degradation test A is shown in Figure 5. The preleached PtCo appeared also more stable than Pt in terms of crystallite size

growth after 1500 cycles of degradation test B (Figure 6). The data concerning the crystallite size growth are summarized in Table 2. A comparison of the effects of the two different accelerated tests on the crystallite growth for the same preleached PtCo catalyst (1500 cycles in both procedures) is shown in Figure 7. These are compared with the steady-state operation for about 100 h at 130 °C, 0.7 V/cell (Figure 7). Moderate changes were observed after the steady-state operation at 0.7 V and 130 °C, 100% RH. The particle size after steady-state operation (3.3 nm) was essentially similar to that of the raw catalyst powder (2.9 nm). These small changes were due essentially to the MEA fabrication. The accelerated test protocol A caused moderate increase of the crystallite size as compared to the steady-state operation (4.0 vs 3.3 nm). The accelerated test B showed the largest sintering effect (6.0 vs 3.3 nm) and a partial dealloying as indicated by a shift of the diffraction peaks of the fcc structure toward low Bragg angles (Figure 7). The test B includes the effects of high electrochemical potential (1.2 V vs RHE) and high temperature (130 °C) as well as high relative humidity (RH 100%). These results were confirmed by TEM analysis of several cathode regions in the various samples (Figure 8). The results related to the increase of mean particle size as determined by TEM analysis are reported in Table 3.



Figure 4. (a) XRD patterns of 50 wt % Pt and preleached 50 wt % Pt-Co catalyst-based electrodes after 1500 cycles of the degradation test A (cell voltage 0.9-0.7 V IR-free). (b) Least-squares fitting of the diffraction peaks related to the (220) reflection of the fcc structure in the XRD patterns of the respective samples.

TABLE 2: Crystallite Size Growth in Degradation Tests As Determined from XRD Analysis

catalysts 50% PtCo/C preleached 50% Pt/C

crystallite size after crystallite size after crystallite size 1500 cycles degradation test 1500 cycles degradation test after 100 h steadycrystallite size raw catalyst, nm state operation at 130 °C, nm at 130 °C, 0.9-0.7 V/cell, nm at 130 °C, 0.6-1.2 V vs RHE, nm 2.9 2.7

3.3 3.4

It is well-known that water accelerates the corrosion rate favoring the dissolution of metal ions from small particles and reprecipitation on large particles. The occurrence of the Ostwald ripening mechanism during the degradation tests appears evident in the TEM picture of the Pt/KB catalyst (Figure 8) showing several regions with the carbon support not covered by metal particles and the growth of large particles concentrated in other regions. As reported in the literature, under harsh conditions the Pt particles may also dissolve in the electrolyte or precipitate in the membrane, especially near the cathode layer, without redeposition on larger metal particles in the catalyst.27,48,49 The present TEM micrographs show the presence of uncovered carbon support regions after electrochemical degradation at 130 °C mainly in the 50% Pt/C catalyst. From these considerations one would expect a paramount effect of water on the degradation; however, by comparing the results of particle growth for the same catalyst subjected to the two accelerated procedures, the effect of the electrochemical potential appears predominant. In procedure B, nitrogen is fed

4.0 6.0

6.0 8.0

to the cathode, and there is no water produced electrochemically during the experiment. The polarization curves carried out at 130 °C before and after the procedure A (1500 cycles) showed larger degradation for the Pt catalyst vs PtCo (Figure 9). The small OCV observed for the MEA based on the Pt catalyst after degradation may be also associated with a degradation of the electrolyte after significant dissolution of Pt and migration into the thin (30 µm) membrane33,49 which causes short circuiting effects and an increase of crossover.50 Although, the results are in part affected by a polymer membrane swelling under these conditions (130 °C, RH 100%), it clearly results that the PtCo catalyst causes lower degradation than Pt. Of course, high pressure and high RH are conditions not aimed for fuel cell cars. However, these accelerated testing conditions represent, beside the temperature, a useful and fast method for screening fuel cell catalyst in terms of stability. The effect of the relative humidity on the MEA degradation is shown in Figure 10. The relative decrease of peak power

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Figure 5. Scanning electron micrographs of the cathode layer based on the preleached 50 wt % Pt-Co/C catalyst-Aquivion ionomer before (top) and after (bottom) the degradation test A (cell voltage 0.9-0.7 V IR-free). Magnifications are about 260K× (a) and 125K× (b).

density was lower in the presence of a smaller RH for MEAs subjected to the same accelerated degradation procedure. The peak power density decreased by about 49% in the presence of 50% RH and 18% in the case of 25% RH (Figure 10). This confirmed that there was also a relevant effect of water content on the MEA degradation. An investigation of the catalyst performance for automotive applications requires high-temperature testing at pressures below 1.5 bar (abs) and low relative humidity.1,50 The most appropriate temperature for investigating catalyst performance under practical automotive conditions, in the presence of the Aquivion E7903S, was 110 °C. Figure 11 shows both the steady-state polarization and power density curves for the most performing preleached PtCo catalyst at high temperature and different relative humidity. The polarization curves were carried out from OCV to the maximum current densities, and the potential at each current was collected when a steady-state condition was reached. At low current densities, the potential losses were small in the presence of a high relative humidity (RH 100%); however, at high current densities, the flooding effect due to the large amount of water produced at the cathode caused an increase of potential losses in the presence of high RH (100%). The largest peak power density of 800 mW cm-2 was indeed obtained in the presence of 50% RH (Figure 11). For what concerns the steady-state polarization curve recorded with 25% RH (Figure 11), large potential losses were observed at low current densities;

Arico` et al. however, as the MEA was internally humidified by the electrochemically produced water, the potential losses diminished and the occurrence in the polarization curves of a sudden increase of potential at certain values of current densities was envisaged. (The polarization curve was recorded from OCV to the maximum current, and the internal humidification spontaneously increased with the current.) These results showed the advantage of the occurrence of internal humidification for the thin (30 µm) short-side-chain Aquivion membrane. The small membrane thickness promotes back-diffusion of the water produced at the cathode favoring anode humidification. The water retention and proton conductivity were also favored by the low equivalent weight resulting from the short side chain, whereas mechanical resistance was assured by the enhanced crystalline properties of the polymer.37,38 At high current density, the obtained peak power density of 700 mW cm-2 at 110 °C, 25% RH, and 1.5 bar (abs) H2-O2 (Figure 11) appears promising for automotive applications. Yet, further losses were recorded in the presence of air feed at the cathode (not shown) due to the increased mass transport constraints and the decrease of oxygen solubility at the catalyst-ionomer electrolyte interface as the temperature was increased. These aspects need to be addressed and a proper improvement of the electrode structure for high-temperature operation is required.1,38 3.3. Surface Studies. To get more insights into the factors influencing the different behavior of carbon-supported PtCo and Pt catalysts, the surface properties of 50% Pt/KB, unleached 50% PtCo/KB, and preleached 50% PtCo/KB were studied by XPS and LE-ISS. Survey XP spectra of the Pt-Co catalysts are shown in Figure 12. The typical Auger and photoelectron lines of Pt, Co, C, and O are observed (Figure 12a). C and O signals derive from the carbon support including the functional groups51,52 as well as from the organic impurities present on the catalyst surface. By comparing the Pt 4f and Co 2p photoelectron lines in the acid-treated and untreated samples, it clearly appeared that there was a significant enrichment of the Pt atoms on the surface for the leached catalyst (Figure 12b). This result is in agreement with the study carried out by Wieckowski et al.53 The surface atomic Pt/Co ratios were 5 and 3.1 for the leached and unleached samples, respectively, whereas, as above-reported, the atomic Pt/Co ratio in the bulk was about 3 in both catalysts as determined by XRF analysis (Table 1). To investigate more in depth these aspects, we have sputtertreated (Ar+, 5 kV) the leached PtCo catalyst in successive steps of 5-10 min. A comparison of the XP survey spectra is shown in Figure 13. The sputtering procedure caused a slight shift of the BE scale in the XP spectra due to the sample charging. However, this did not affect the quantitative evaluation of the elements on the surface. The Pt/Co ratio decreased significantly in the XP spectra after the first sputtering and progressively with the successive sputtering steps to reach a Pt/Co ratio similar to the nominal one (it was indeed slightly lower). On the contrary, there was no significant change of composition in the XP spectra for the unleached PtCo after several sputtering steps (not shown). Although this result clearly shows an enrichment of Pt on the surface of the preleached PtCo sample, it is considered that the XPS technique may provide only a partial information about the composition of the topmost atomic layers which govern the electrochemical behavior. In fact, the analysis depth of XPS is 3-5 nm.45 LE-ISS is actually more surface sensitive than XPS. We have thus studied the unleached and acid leached 50% Pt-Co catalyst by LE-ISS (Figure 14). To investigate selectively the outermost atomic layers, we have used



Figure 6. (a) XRD patterns of 50 wt % Pt and preleached 50 wt % Pt-Co catalysts based electrodes after 1500 cycles of the degradation test B (electrode potential 0.6-1.2 V RHE). (b) Least-squares fitting of the diffraction peaks related to the (220) reflection of the fcc structure in the XRD patterns of the respective samples.

a low-energy source, i.e., 3He+ at 1 kV. During LE-ISS analysis, the sample was gently sputtered and the scattered ions were analyzed. Thus, an increase of the sputtering/analysis time corresponds to an increasing analysis depth of the outermost atomic layers generally reported as depth profile analysis. Because of the high catalyst dispersion and the absence of appropriate standards, we could not determine how many atomic layers have been analyzed in each measurement; however, it was possible to determine the change of the atomic Pt/Co atomic ratio for the topmost surface layers. The unleached catalyst (Figure 14a) showed during the first cycle (topmost atomic layer) mainly the occurrence of C (I/I0 ) 0.38 eV/Ep) and O (I/I0 ) 0.48 eV/Ep) elements due to the oxygenated surface functional groups on carbon.51,52 Possibly, Pt (I/I0 ) 0.94 eV/Ep) and Co (I/I0 ) 0.82 eV/Ep) atoms were covered by adsorbed organic species. In the second cycle, both Pt and Co were clearly detected (Figure 14a) with the Pt peak only moderately prevailing in intensity. This suggested an excess of unalloyed Co on the surface with respect to the nominal amount (Pt:Co ) 3:1) that may be responsible for the lower electrochemical activity of the unleached catalyst. On the contrary, the preleached PtCo catalyst (Figure 14b), the first cycle, corresponding to the topmost atomic layer, clearly showed a Pt peak with an intensity not significantly different than the O peak, whereas the Co peak was almost absent. The

latter increased slightly in intensity as compared to Pt, C, and O signals during successive cycles (i.e., with increasing analysis depth). However, the intensity of the Co peak remained significantly smaller than that of Pt for the top layers. The ISS results clearly indicated that there was a skin layer of Pt on the surface of practical PtCo catalysts subjected to an acid preleaching procedure, whereas this evidence was not observed in the unleached PtCo. Figure 15 summarizes the XPS and ISS depth profiles for the preleached (a) and unleached (b) Pt-Co electrocatalysts. It is pointed that the analysis depth is about 1 order of magnitude larger in the XPS than ISS analysis;45 in other words, ISS is significantly more surface sensitive than XPS. In the preleached sample, the Pt/Co ratio did not vary significantly in the ISS depth analysis, whereas it is reduced to about one-half in the XPS depth analysis study (Figure 15a). On the contrary, in the unleached sample, the Pt/Co ratio appeared almost constant in the XPS depth analysis (Figure 15b). ISS results indicated a slightly larger content of Co on the surface only in the first cycle whereas the signal ratio was almost constant in the successive cycles (Figure 15b). It is worthy noting that during a prolonged operation as cathode in PEMFC the unleached PtCo catalyst may also give rise to a leaching effect; however, the released Co ions may exchange the protons in the membrane, causing degradation as well as an increase of ohmic resistance. The preleaching

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Figure 7. (a) Comparison of the XRD patterns of the preleached 50 wt % Pt-Co/C electrocatalyst after 100 h operation at 130 °C-0.7 V cell voltage and after 1500 cycles of the degradation tests A and B. (b) Least-squares fitting of the diffraction peaks related to the (220) reflection of the fcc structure in the XRD patterns of the respective samples.

procedure appears to promote the formation of a Pt skin layer2 that appears, as observed from the electrochemical results,42 beneficial in terms of performance and resilience to degradation. The LE-ISS results were substantially in agreement with the XPS analysis if we consider the different analysis depths. This confirmed that the XPS data were also representative of the catalyst surface even if not totally of the outermost layer. The additional information from LE-ISS was that there was almost no Co on the topmost surface atomic layers in the preleached catalyst, whereas XPS just indicated for this catalyst an enrichment of Pt on the surface. On the contrary, a slight enrichment of Co with respect to the nominal content was detected by ISS for the outermost layer in the unleached catalyst. This may explain why the unleached alloy catalysts are less performing than the leached ones.1,2 Specific information about the chemical oxidation states was derived from XPS analysis (Table S1, Figures S1 and S2). Highresolution Pt 4f spectra for the Pt/C and unleached PtCo/C catalysts as well as for the PtCo/C unleached and preleached catalysts are compared in Figure 16, a and b, respectively. It is clearly observed that the Pt 4f doublet in the PtCo/KB sample is negatively shifted as compared to the same doublet in the Pt/KB catalyst (Figure 16a). There is a larger extent of metallic Pt on the surface for the alloy catalyst, whereas oxygen chemisorption appears to occur easily at step and kink sites54

present on the surface of the Pt clusters in the Pt/KB sample (Table S1, Figure S1). This determines a larger amount of oxidized Pt sites in the Pt/KB with respect to PtCo/KB. No significant difference is envisaged in terms of Pt oxidation states for the leached and unleached PtCo. The chemical states of Co were also analyzed by XPS. Co 2p X-ray photoelectron spectra of the carbon-supported Pt-Co catalysts are shown in Figure 17. From a comparison with the literature,45,53,55 in both catalysts, the Co 2p profile appeared characterized by the presence of electron vacancies (Table S1, Figure S2). Such aspect was more relevant for the unleached catalyst (shift to higher BE). This was possibly associated with the fact that there was a larger occurrence of Co atoms on the outermost layers of the unleached catalyst. The amount of the platinum oxides in the alloyed electrocatalysts decreased with respect to the Pt/C possibly due to an intra-alloy electron transfer from Co to Pt.55 It is worth noting that Co is an electropositive element in relation to platinum, and hence, Pt atoms could produce an electron-withdrawing effect from the neighboring Co atoms in the alloy bringing about an oxide-cleansing action. 4. Conclusions Bulk and surface properties of carbon-supported Pt and PtCo oxygen reduction electrocatalysts have been investigated with



Figure 8. Transmission electron micrographs of various cathode layers after degradation tests: preleached 50 wt % Pt-Co electrocatalysts after 1500 cycles of the degradation test B (top); preleached 50 wt % Pt-Co electrocatalysts after 1500 cycles of the degradation test A (middle); 50 wt % Pt/C after 1500 cycles of the degradation test A (bottom).

TABLE 3: Metal Particle Size Growth in Degradation Tests As Determined from TEM Analysis

catalysts 50% PtCo/C preleached 50% Pt/C

particle size raw catalyst, nm

particle size after 1500 cycles degradation test at 130 °C, 0.9-0.7 V/cell, nm

particle size after 1500 cycles degradation test at 130 °C, 0.6-1.2 V vs RHE, nm

2.9

3.7

5.6

2.6

5.5

8.0

regard to their application in high-temperature polymer electrolyte fuel cells. An appropriate synthesis method was developed to obtain PtCo electrocatalysts with a small particle size and suitable degree of alloying. A face-centered-cubic crystallographic structure was observed for the electrocatalysts, and the preparation procedure was tailored to obtain a similar

crystallite size and similar dispersion. This allowed to compare the catalysts in terms of chemical properties mainly. A preleaching treatment was adopted to improve the catalytic activity. Accelerated high-temperature (130 °C) electrochemical degradation procedures showed that the particle size of all catalysts increased after these tests. However, the HClO4 preleached PtCo catalyst showed the best characteristics in terms of electrochemical activity and resistance to particle growth. High-temperature electrochemical tests carried out in a polymer electrolyte fuel cell in the presence of a new short side chain perfluorosulfonic membrane confirmed what it was already shown in the literature for other polymer electrolytes and/or at low temperatures; i.e., the carbon supported PtCo alloy is more stable than Pt/C.29–33 A small Co dissolution possibly increases the surface roughness,1,56 thus limiting the effect of electro-

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Figure 9. Comparison of the polarization behavior before and after 1500 cycles degradation test A for the 50 wt % Pt/C and preleached 50 wt % Pt-Co/C electrocatalysts in the presence of Aquivion membrane at 130 °C, 100% RH, and 3 bar (abs) H2-O2.

Figure 10. Power density curves before and after 1500 cycles degradation test A recorded at two different RH values (25, 50%) for the preleached 50 wt % Pt-Co/C electrocatalyst in the presence of Aquivion membrane at 130 °C and 3 bar (abs) H2-O2.

Figure 12. (a) Assignment of Auger and photoelectron lines in the survey XPS spectrum of a PtCo/C catalyst. The arrows indicate the Pt 4f and Co 2p lines used for analytical purposes. (b) Comparison of survey spectra for Ketjenblack carbon-supported unleached 50 wt % Pt-Co and preleached 50 wt % Pt-Co electrocatalysts. The calculated surface Pt/Co atomic ratio is indicated.

Figure 11. Steady-state polarization behavior at different RH values (25, 50, 100%) for the preleached 50 wt % Pt-Co/C electrocatalyst in the presence of Aquivion membrane at 110 °C and 1.5 bar (abs) H2-O2. The curves were collected from OCV to the maximum current.

chemical sintering. However, the present results indicate that the different electrochemical behavior is governed by quite different surface properties for the Pt and PtCo catalysts. ColonMercado and Popov31 individuated particle migration for pure Pt and Ostwald ripening for PtCo as the main degradation mechanisms. However, from the present analysis, dissolution of Pt particles into the electrolyte during high-temperature operation is not discarded. In the present study, a significant surface Pt enrichment was observed for the leached Pt-Co electrocatalyst. This would indicate that the active phase is mainly composed of Pt sites,

Figure 13. Survey X-ray photoelectron spectra of a sputter-treated Ketjenblack carbon-supported preleached 50 wt % Pt-Co electrocatalyst.

and Co sites from the underlying layers exert their electronic effect onto Pt as proposed by Watanabe et al.17,18 for other Pt alloys and recently discussed by Markovic et al.2



Figure 14. Low-energy ion scattering spectroscopy (3He+, 1 kV) of (a) unleached 50 wt % Pt-Co and (b) preleached 50 wt % Pt-Co electrocatalysts.

Figure 16. Pt 4f X-ray photoelectron spectra of (a) carbon-supported 50 wt % Pt and unleached 50 wt % Pt-Co and (b) unleached and preleached 50 wt % Pt-Co electrocatalysts.

Figure 17. Co 2p X-ray photoelectron spectra of Ketjenblack carbonsupported unleached and preleached 50 wt % Pt-Co electrocatalysts.

Figure 15. XPS and ISS depth profiles for the preleached (a) and unleached (b) 50 wt % Pt-Co electrocatalysts.

The presence of a metallic Pt skin layer on the outermost surface of practical catalysts would indicate a better thermodynamic stability also enhanced by an intra-alloy charge transfer from the atoms in the bulk.55 This causes a modification of the surface properties especially with regard to the extent of oxide formation and oxidation.54 The higher Pt-metal fraction observed on the surface of the alloy electrocatalysts also causes an alteration of oxygen adsorption and electroreduction properties during fuel cell tests.54 The lower oxidation state of Pt in the

alloy, as compared to pure Pt, and Pt-skin layer formation are both the primary factors which determine a high electrocatalytic activity toward the oxygen-reduction reaction and proper stability in the fuel cell environment for the present preleached alloy electrocatalyst. The lower amount of Pt oxides on the PtCo electrode possibly reflects a lower adsorption strength for the oxygen species and their easier electroreduction to water on the surface of this catalyst. Similar effects were also observed for PtNi catalysts.57 On the contrary, the presence of strongly adsorbed oxygen species such as Pt oxides on the surface of Pt/C catalysts are believed to reduce the oxygen reduction activity.58–62 The lower amount of these oxide species is thus associated with an increase of specific activity for oxygen reduction in practical carbon supported electrocatalysts.

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Acknowledgment. The authors acknowledge the financial support of the EU through the Sixth Framework Autobrane (SES6-CT-2005-020074) project. The authors are also grateful to M. Gebert, M. Corasaniti, and A. Ghielmi of Solvay-Solexis for the supply of the Aquivion E79-03S short-side-chain perfluorosulfonic membrane. Supporting Information Available: Quantitative analysis of the surface oxidation states for Pt/KB and PtCo/KB catalysts. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Appl. Catal. B: EnViron. 2005, 56, 9. (2) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G.; Ross, P. N.; Markovic, N. M. Nature Mater. 2007, 6, 241. (3) Arico`, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J.-M.; Van Schalkwijk, W. Nature Mater. 2005, 4, 366. (4) Mukerjee, S.; Srinivasan, S. J. Electroanal. Chem. 1993, 357, 201. (5) Paffet, M. T.; Beery, G. J.; Gottesfeld, S. J. Electrochem. Soc. 1988, 135, 1431. (6) Watanabe, M.; Tsurumi, K.; Mizukami, T.; Nakamura, T.; Stonehart, P. J. Electrochem. Soc. 1994, 141, 2659. (7) Freund, A.; Lang, J.; Lehman, T.; Starz, K. A. Catal. Today 1996, 27, 279. (8) Jalan, V.; Taylor, E. J. J. Electrochem. Soc. 1983, 130, 2299. (9) Gottesfeld, S.; Paffett, M. T.; Redondo, A. J. Electroanal. Chem. 1986, 205, 163. (10) Beard, B. C.; Ross, P. N. J. Electrochem. Soc. 1990, 137, 3368. (11) Kim, K. T.; Hwang, J. T.; Kim, Y. G.; Chung, J. S. J. Electrochem. Soc. 1993, 140, 31. (12) Mukerjee, S.; Srinivasan, S.; Soriaga, M. P.; McBreen, J. J. Electrochem. Soc. 1995, 142, 1409. (13) Ren, X.; Sringer, T. E.; Gottesfeld, S. In Proton Conducting Membrane Fuel Cells - Second International Symposium; Gottesfeld, S., Fuller, T. F., Eds.; The Electrochemical Society: Pennington, NJ, 1999; Vol. 98-27, p 341. (14) Dohle, H.; Divisek, J.; Mergel, J.; Oetjen, H. F.; Zingler, C.; Stolten, D. In Fuel Cell Seminar Abstracts, 2000; Oct 30-Nov 2, Portland, OR, p 126. (15) Zinola, C. F.; Castro Luna, A. M.; Triaca, W. E.; Arvia, A. J. J. Appl. Electrochem. 1994, 24, 119. (16) Chen, S.; Sheng, W.; Yabuuchi, N.; Ferreira, P. J.; Allard, L. F.; Shao-Horn, Y. J. Phys. Chem. C 2009, 113, 1109. (17) Toda, T.; Igarashi, H.; Uchida, H.; Watanabe, M. J. Electrochem. Soc. 1999, 146, 3750. (18) Toda, T.; Igarashi, H.; Watanabe, M. J. Electroanal. Chem. 1999, 460, 258. (19) Uchida, H.; Ozuka, H.; Watanabe, M. Electrochim. Acta 2002, 47, 3629. (20) Li, W.; Zhou, W.; Li, H.; Zhou, Z.; Zhou, B.; Sun, G.; Xin, Q. Electrochim. Acta 2004, 49, 1045. (21) Zhang, J.; Vukmirovic, M. B.; Xu, Y.; Mavrikakis, M.; Adzic, R. R. Angew. Chem., Int. Ed. 2005, 44, 2. (22) Zhang, J.; Vukmirovic, M. B.; Sasaki, K.; Nilekar, A. V.; Mavrikakis, M.; Adzic, R. R. J. Am. Chem. Soc. 2005, 127, 12480. (23) Koh, S.; Strasser, P. J. Am. Chem. Soc. 2007, 129, 12624. (24) Lai, F.-J.; Sarma, L. S.; Chou, H.-L.; Liu, D.-G.; Hsieh, C.-A.; Lee, J.-F.; Hwang, B.-J. J. Phys. Chem. C 2009, 113, 12674. (25) Sarkar, A.; Manthiram, A. J. Phys. Chem. C 2010, 114, 4725. (26) Jaffray, C.; Hards, G. A. In Handbook of Fuel CellssFundamentals, Technology and Applications; Vielstich, W., Lamm, A., Gasteiger, H., Eds.; Wiley: Chichester, UK, 2003; Vol. 3, Chapter 41, p 509. (27) Borup, R. L.; Davey, J. R.; Garzon, F. H.; Wood, D. L.; Inbody, M. A. J. Power Sources 2006, 163, 76. (28) Cai, M.; Ruthkosky, M. S.; Merzougui, B.; Swathirajan, S.; Balogh, M. P. J. Power Sources 2006, 160, 977.

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