Fundamental Aspects of ad-Metal Dissolution and Contamination in

Article pubs.acs.org/JPCC

Fundamental Aspects of ad-Metal Dissolution and Contamination in Low and Medium Temperature Fuel Cell Electrocatalysis: A Cu Based Case Study Using In Situ Electrochemical X‑ray Absorption Spectroscopy Qingying Jia,† David E. Ramaker,‡ Joseph M. Ziegelbauer,§ Naggapan Ramaswamy,†,∥ Aditi Halder,† and Sanjeev Mukerjee*,† †

Department of Chemistry and Chemical Biology, Northeastern University, Boston, Massachusetts 02115, United States Department of Chemistry, George Washington University, Washington, DC 20052, United States § Electrochemical Energy Research Lab, General Motors Research and Development, Warren, Michigan 48090, United States ‡

ABSTRACT: Electrochemical methods (cyclic voltammetry and rotating ring disk electrode) and in situ X-ray absorption spectroscopy (XAS) were used in conjunction to study copper adsorption onto carbon-supported platinum nanoparticles over the operating potential range of proton exchange membrane fuel cell (PEMFC) cathodes and anodes (∼0.0−1.0 V vs the reference hydrogen electrode, RHE). Our purpose was to better understand the detrimental effects of Cu leaching from high-activity dealloyed PtCux electrocatalysts. These studies were conducted in CuSO4-doped 0.1 N solutions of HClO4 and H2SO4 under both inert and oxygenated conditions. Over the anode potential range (∼0.0−0.3 V vs RHE), concentrations of Cu2+ as low as 10 μM were found to coat the active Pt surfaces, thereby drastically inhibiting the hydrogen oxidation reaction. Over the Cu underpotential deposition region (0.35−0.70 V vs RHE), Cu2+ concentrations ≥0.05 mM resulted in Cu deposition onto Pt. This was found to lower the oxygen reduction reaction activity of Pt by skewing the reaction mechanism toward the twoelectron pathway (peroxide production) away from the desired four-electron pathway (water). Electrochemical methods were inconclusive as to the effects of Cu2+ at potentials greater than 0.84 V. While in situ Pt L3 edge extended X-ray absorption fine structure (EXAFS) revealed definitive Pt−Cu scattering paths below 0.84 V, Cu was not observed at higher potentials. The Δμ analysis of X-ray absorption near-edge structure (XANES) revealed that Cu(O) coadsorbs under high Cu2+ concentrations in HClO4, and that H2SO4 results in Cu(O) coadsorption at lower concentrations. Extending the Δμ analysis to lower potentials revealed the interplay of Cu2+, O(H), and H+ coadsorption with respect to potential, Cu2+ concentration, and sparging environment (inert or oxygenated). These studies verify that Cu leaching from PtCux-alloy electrocatalysts can have detrimental effects on both the anode and cathode sides of a PEMFC, and similar experiments can be extended to probe the adsorption effects of other transition metals from PtMx alloys.

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nanoparticles”, or “porous multiple-cores/shell particles” depending on the particle size of the dealloyed PtM precursor catalysts7 and, most likely, on dealloying conditions. These configurations induce compressive strain in the Pt-rich shell arising from lattice parameter mismatches with the PtMx-alloy core(s). Electronic-structure consequences of this strain decrease the Pt−O bond strength (negative shift of the Pt dband center) which in turn increases the turnover frequency for the ORR.5 While dealloyed PtCu catalysts have given good initial kinetic activities in fuel cells running on pure oxygen at low current densities, testing at General Motors has shown major problems with both durability and performance at high current density

INTRODUCTION

The first generation of mass-produced vehicles powered by proton exchange membrane fuel cells (PEMFCs) will almost certainly utilize platinum-based electrocatalysts. However, in order for PEMFC vehicles to reach cost-competitiveness with current internal combustion engine technologies, the Pt loadings must be reduced about 4-fold by increasing the oxygen reduction activity per unit mass of Pt. Pt-alloy catalysts have historically given activities for the oxygen reduction reaction (ORR) 2−3 times those of pure-Pt catalysts.1,2 Socalled “dealloyed” PtM (M = Cu, Ni, Co, etc.) catalysts have exhibited even higher activities.3−6 These catalysts are generated from carbon-supported, annealed PtM3 nanoclusters which then undergo dealloying through electrochemical cycling or acid leaching. Removal of most of the alloying 3d transition metal atoms has been shown, under favorable conditions, to result in “single core/shell nanoparticles”, “multiple-core/shell © 2013 American Chemical Society

Received: November 16, 2012 Revised: February 12, 2013 Published: February 13, 2013 4585

dx.doi.org/10.1021/jp311353u | J. Phys. Chem. C 2013, 117, 4585−4596

The Journal of Physical Chemistry C

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while running on air.8 Although most of the Cu was removed from the PtCu3 precursor by acid leaching of the catalyst powder prior to fabrication of the membrane-electrode assemblies (MEAs), copper still subsequently dissolved from the catalyst to form mobile Cu2+ ions in the ionomer of the electrodes and the membrane. Stripping voltammetry and AC impedance showed that some of the copper ions crossed through the membrane and deposited as Cu metal on the anode, poisoning the hydrogen oxidation reaction (HOR) and leading to a loss of cell voltage at high current densities. This effect would be limited to the more noble transition metals, such as Cu, Ag, and Au. Dissolved transition metal ions can also affect the cathode. At high current densities, they can electromigrate into the ionomer within the oxygen electrode and tie up the majority of the hydrogen-ion exchange sites. This effect, which is general for cations and is particularly evident under low-humidity conditions, can starve the oxygen reduction reaction of reactant hydrogen ions.9 As will be shown in this report, Cu2+ can also directly adsorb on the Pt surfaces of the cathode and alter the selectivity and kinetics of the oxygen reduction reaction over much of the operating potential window. Underpotential deposition (upd) of Cu on platinum has been extensively studied, in part to understand the fundamental mechanism of electrodeposition and to elucidate the specifics of Cu dissolution and adsorption. Studies of Cu upd have been performed with many techniques: electrochemical methods, reflectance spectroscopy, radioactive labeling, in situ scanning transmission microscopy, infrared spectroscopy, X-ray surface scattering, and several ex situ ultrahigh vacuum techniques (e.g., such as low energy electron diffraction (LEED), Auger electron spectroscopy, and X-ray photoelectron spectroscopy).10 Several studies have specifically focused on the influence of Cu upd on the ORR in acid media.11−13 A nearly complete inhibition of ORR was detected at full monolayer coverage at low potentials. Within the upd region, a strong but partial inhibition was observed; this was attributed to a transition from the favorable four-electron reduction pathway to a two-electron process. This phenomenon was widely observed with many other transition metal adsorbates on Pt, such as Ag, Ti, Bi, Pb, etc.11,14,15 However, most of these studies focused on the Cu upd occurring on Pt bulk polycrystalline electrodes or single-crystal surfaces. Very limited work, to our knowledge, on the upd of Cu on supported Pt nanoparticles (i.e., real commercial catalysts) has been reported,16 especially with regard to the poisoning effects of deposited Cu on the ORR. Recently, Price et al. conducted an in situ EXAFS study of Cu upd on a supported Au nanoparticle catalyst and showed it provides a comprehensive view of the structure of the Cu upd layer at different potentials.17 Further, there is a lack of in situ experimental evidence to fully explain the influence of Cu adatoms on the activity of real-life, carbon-supported platinum catalysts. Element-specific X-ray absorption spectroscopy (XAS) is a powerful tool to study the interactions between chemisorbed species and metal surfaces and to monitor the influence of adsorbed non-Pt metal atoms on the overall activity of the catalyst.18−20 Owing to the high photon flux of modern day synchrotron sources, XAS is capable of probing materials under in situ conditions even when the adsorber atom is a relatively minor species in the sample. The small changes in coordination number (N) or bond distance (R, generally ±0.005 Å), mean square-disorder term (Debye−Waller factor), and oxidation

states can be detected during an electrochemical reaction by analyzing the EXAFS and XANES. In addition, the newly developed Δμ (sometimes referred to as Δ-XANES) method of XANES analysis has been successfully employed to provide fundamental accounts of adsorbate chemistries and binding sites on Pt and Pt alloys.21−23 The Δμ analysis isolates surface/ adsorbate interactions and achieves surface sensitivity by taking difference spectra between samples held at two different electrochemical potentials when only the amount and/or adsorbate changes on the electrode surface. Once obtained, the Δμ spectra are compared to theoretical Δμ signatures generated on the basis of crystallographic models. Cu dissolution from dealloyed PtCux can significantly degrade the PEMFC performance as reported in previous work;8 however, a PEMFC is an extraordinarily complicated system and many other factors (such as carbon corrosion or deformation of the nanoscaled Pt-based nanoparticles) can contribute to the performance reduction concurrently. Therefore, to clearly establish the fundamental aspects of ad-metal dissolution and contamination in PEMFC electrocatalysis, more controlled experiments, yet under realistic environments such as those existing in PEMFCs, are essential. In this work, electrochemical methods (cyclic voltammetry and rotating-disk electrode analysis) in conjunction with in situ (Pt L3 edge) XAS in a spectro-electrochemical half cell were used to study Cu adsorption on carbon-supported Pt nanoparticles over a wide electrochemical window corresponding to both the anodic and cathodic operating potentials of a PEMFC. Similar studies have not been reported. The ORR performance of nanoscaled catalysts using the “thin-film” method in a RDE assembly has been widely adopted because the catalysts can be tested much more easily and more reproducibly this way than in a complex PEMFC. Gasteiger et al.24 demonstrated that the ORR activities measured in RDE at 60 °C are nearly equivalent with those measured in H2/O2 PEMFCs operating at 80 °C. Further, the RDE ORR activities measured at 25 °C on Pt-based alloys are lower by only a factor of 1.1 compared to those at 60 °C.25 Thus, the temperature effect on the ORR rate over this small range is small. In addition, the half cell experimental conditions are completely relevant to those existing in PEMFCs, and similar half cell experiments have been widely used to understand the electrocatalysis occurring at electrode materials in PEMFCs.26,27 The correlations between Cu2+ concentrations and the HOR/ORR activities were examined by doping 0.1 M HClO4 or 0.05 M H2SO4 electrolytes with varying concentrations of CuSO4. We have observed that the presence of Cu2+ results in a lower diffusion limited current, a higher ring current, and a significant increase in ORR overpotential compared to the uncontaminated electrolyte. In addition, XAS data were collected at different potentials to explore the nature of the Cu dissolution/deposition processes. EXAFS fitting was performed to explore the physical structure including the Pt coordination numbers and Pt−Cu bond distances as a function of potential. Meanwhile, the Δμ technique was also used to determine the adsorption sites of Cu on a Pt surface. For the first time, we have observed adsorbed Cu(O) coadsorbates at high potential (0.84 V) in HClO4 containing high concentrations of Cu2+ by using the Δμ technique. Similar studies in H2SO4 showed the presence of analogous Cu(SO4) coadsorbates. 4586



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were continuously circulated through the cells via a small peristaltic pump. The Pt/C working electrodes were activated by potential cycling (0.05−1.2 V vs RHE at 10 mV s−1) in clean 0.1 M HClO4 or 0.05 M H2SO4 electrolytes. Following 3−5 complete activation cycles, the “clean” (Cu-free) electrolytes were replaced with deoxygenated 0.1 N acid + x mM CuSO4 (x = 1, 10, and 100) electrolyte(s). Full range Pt L3 EXAFS data were taken at various static potentials along the anodic sweep of the CV. A full set of EXAFS spectra collected in Cu-free, Arsparged, 0.1 N acids was utilized as the baseline for comparisons to Cu-contaminated environments. The measurements in HClO4 were performed at beamline X19-A (National Synchrotron Light Source, Brookhaven National Laboratory, Upton, NY), and the H2SO4 data were collected at beamline XOR-9BM (Advanced Photon Source, Argonne National Laboratories, Argonne, IL). The data were collected simultaneously in both the transmission (ionization chamber, I1) and fluorescence (PIPS detector, Canberra) modes with a Pt reference foil positioned between ionization chambers I2 and I3 to aid in energy alignment. EXAFS and Δμ Analysis. The in situ XAS measurements were performed at the Pt L3 absorption edge. Measurements were made at different electrode potentials from 0.04 to 1.00 V. The data were processed and fitted using the Athena29 and Artemis30 programs. Scans were calibrated, aligned, and normalized with background removed using the IFEFFIT suite29 (version 1.2.9, IFEFFIT Copyright 2005, Matthew Newville, University of Chicago, http://cars9.uchicago.edu/ ifeffit/). The χ(R) transforms were modeled using single scattering paths calculated by the FEFF6 code.31 The Δμ analysis technique has been described in great detail elsewhere.27,32−35 Briefly, difference spectra were calculated using the equation

This work demonstrates several ways in which copper dissolved from a Pt-alloy catalyst can impede the operation of a fuel cell, at least some of which can be influenced by other ionic species in the electrolyte. The amount of alloying element dissolved likely depends upon the structure of the active nanoparticles (e.g., the continuity of a pure-Pt outer shell). Therefore, the durability of an alloy or dealloyed catalyst can critically depend upon the choice of the alloying element, the electrolyte, and the synthesis and dealloying processes employed. The in situ techniques described in this report can help to sort out these complex issues.

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EXPERIMENTAL SECTION Electrochemical Characterization. Cyclic voltammetry (CV) and rotating ring disk electrode (RRDE) studies were performed as described in detail elsewhere.28 In brief, catalysts were prepared by mixing 30 wt % Pt/C (2.7 nm diameter, ETEK, Vulcan XC72 carbon), 2-propanol (GFS Chemicals, 99.5% min), and 5 wt % solution of Nafion in lower alcohols (Aldrich). The catalyst suspensions (6.6 μL) were deposited onto a polished glassy carbon (GC) RDE tip of 5 mm diameter (Pine Instruments), resulting in a final loading of 15 μgPt cm−2. All measurements were taken at room temperature in either 0.1 M HClO4 (GFS Chemicals, doubly distilled) or 0.05 M H2SO4 as clean electrolytes to provide the baseline performances. Afterward, a 10 mM CuSO4 solution was injected into the electrolyte to reach the desired Cu2+ concentrations (0.01, 0.05, 0.25, 0.5, 1, and 2 mM, respectively). Copper sulfate was used even in the dilute perchloric acid electrolyte because of the hazards of handling perchlorate salts of metals. The same experiments were performed with 0.05 M H2SO4 in lieu of 0.1 M HClO4 to separate out the effects of Cu2+ from those of SO42− anions (with the concentration of SO42− being very different in the two cases). In all cases, the electrolyte was purged with Ar gas for CV measurements (0.02−1.0 V vs RHE) and with H2 or O2 for HOR or ORR polarization sweeps at rotation rates of 100, 400, 900, 1600, and 2500 rpm. The Au ring was held at 1.4 V vs RHE following several activation cycles (0.1−1.4 V). All of the measurements were conducted with an Autolab PGSTAT30 potentiostat (Metrohm USA, formerly Brinkman Instruments) equipped with a SCANGEN module. In Situ XAS Data Collection. All experiments were carried out at room temperature in an in situ spectro-electrochemical half cell of a previously reported design.27 For the experiments conducted in 0.1 M HClO4, the cell consisted of a 30 wt % Pt/ VXC72 (E-TEK) on a Zoltek carbon cloth working electrode (WE), an acid-washed Grafoil (GrafTech International Inc.) counter electrode (CE), and a platinum RHE reference electrode. The cell components for data collection in 0.05 M H2SO4 were slightly different; the WE was a 46.6 wt % Pt/ VXC72 (2.9 nm diameter, Tanaka) catalyst on a Zoltek carbon cloth, the CE was a high-purity gold wire (>99.9%, 0.5 mm diameter, Alfa Aesar), and the reference electrode was a sealed, saturated Ag/AgCl reference electrode (BAS, −0.283 V vs a dynamic hydrogen electrode in 0.05 M H2SO4). In either case, the total geometric Pt loadings of the working electrodes were 0.2−0.3 mgPt cm−2 in order to attain Pt L3 edge heights of at least 0.1. Preparation of the working electrodes has been described in detail elsewhere.26,27 In all cases, 0.1 mm thick Au foils (99.999%, Alfa Aesar) were used as the current collectors. The electrolytes (through which Ar, N2, or O2 was sparged)

Δμ = μ(V ) − μ(0.54 V)

(1)

where μ(V) is the absorption coefficient of the sample at a potential of interest and μ(0.54 V) is the reference signal at 0.54 V, which is considered the clean double layer region for platinum, i.e., free of any adsorbed H, O(H), or oxygen adsorbates. The Δμ spectra are then compared to theoretical curves (Δμt) calculated with the FEFF8 code: Δμt = μ(Pt6M) − μ(Pt6)

(2)

where M denotes Cu or O(OH) in different specified binding sites with respect to the absorbing Pt atom. These Pt6 clusters (from Janin36) with a Pt−Pt bond distance of 2.77 Å have been found to function as adequate representations of Pt and PtM surfaces37 for many purposes. The Pt−Cu bond distances were derived from the EXAFS fits (2.67 Å). In addition, to make optimal comparisons of the experimental and theoretical calculations, the theoretical Δμ curves were typically shifted by 1−5 eV and scaled by appropriate factors.38 The shifts are required because of uncertainties in the actual edge energy (accounted for by E0 in the normal EXAFS analysis) and the scaling factor to account for the different adsorbate coverages and surface area in the experimental data vs the full coverage assumed in the FEFF8 calculation.

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RESULTS AND DISCUSSION Electrochemical Characterization. Cyclic voltammograms (CVs) recorded after several cycles for Pt/C (E-TEK)

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Pt25Cu75 surfaces.42 Their study also indicated that the peak at ∼0.35 V arose from Cu bulk desorption, with the peak at ∼0.75 V attributed to Cu desorption from the bulk PtCu alloy. Strasser et al. has also observed the same features and phenomena when monitoring the dealloying of PtCu nanoparticles at different temperatures.6 They proposed that the consistency of the upd features for complex nanoparticle surfaces and single crystal surfaces can be explained by similarity of the binding sites; i.e., either the nanoparticles also have flat (111) regions, or the single crystal surfaces possess steps or kinks. Finally, the anodic peak at 0.45 V in Figure 1 was also observed in Pt25Cu75 alloys prepared at 600 °C by Strasser et al. According to their density functional theory (DFT) calculations based on simple Cu on the Pt(854) model, Cu atoms bound in greater proportion to Pt atoms than Cu atoms will dissolve at ∼0.45 V (the corresponding intermediate dissolution potential).6 The agreement between our measurements and literature reports indicates that Cu is on the Pt surface at 0.4−0.6 V, possibly extending into the operating range of a PEMFC cathode. Figure 2 shows a representative set of RRDE curves on Pt/C in room temperature, O2-saturated 0.1 M HClO4 containing 0−

in N2-sparged 0.1 M HClO4 (solid) and 0.1 M HClO4 + 2 mM CuSO4 (dotted) are shown in Figure 1. The Pt/C (solid line)

Figure 1. Room temperature cyclic voltammograms of 30 wt % Pt/C (E-TEK) taken in Ar-purged 0.1 M HClO4 (solid line) and 0.1 M HClO4 + 2 mM CuSO4 (dotted). The Pt/C loading was 15 μg cm−2 on a 5 mm diameter glassy carbon RDE, collected at a scan rate of 10 mV s−1 between 0.05 and 1.4 V.

shows the typical features of carbon-supported Pt nanoparticles in acid electrolyte: a Hupd region below 0.4 V, a double layer between 0.4 and 0.55 V, and Pt−O(H) formation and reduction above 0.55 V. When 2 mM Cu2+ (dotted line) was added to the electrolyte, however, the CV displayed significantly different features. In the anodic direction, the absence of the Hupd peak indicates the inhibition of H+ adsorption by adsorbed Cu.39 The prominent peak around 0.35 V is associated with desorption of Cu bulk atoms as follows: Cu(s) → Cu 2 + + 2e−

E 0 = 0.34 V vs NHE

(3)

This large, sharp peak indicates the initial presence of multilayers of Cu. The broad voltammetric Cu feature at 0.4− 0.8 V, with two distinct peaks around 0.45 and 0.7 V, corresponds to desorption of Cu adatoms that had been underpotentially deposited onto the nanoparticulate Pt surface during the cathodic sweep. The individual corrosion potentials can vary significantly under different chemical environments at the atomic scale. Above 0.8 V, the fairly flat shape of the CV suggests that all Cu adatoms are stripped from the surface (or drop to undetectable levels) when going anodically, and it indicates that the adsorption/desorption of Cu on Pt is a reversible process in acid media. During the cathodic sweep, the high overlap of the CVs above 0.8 V with and without Cu strongly implies that the reductive desorption of O(H) is relatively unaffected by the presence of Cu in the electrolyte at these potentials (≥0.8 V). At lower potentials (0.7 and 0.4 V), the deposition of Cu onto the Pt surface is evident. At around 0.2 V, bulk Cu deposition occurs. The overall features displayed above are highly analogous to what has been observed on Pt−Cu nanoparticles as well as Pt single crystals. A classic study of Cu upd on single-crystal Pt using combined voltammetric and LEED techniques unambiguously identified the anodic peak at 0.35 V as Cu bulk desorption off of Pt(111) and Pt(100) surfaces, and the peaks centered at ∼0.75 V as Cu monolayer stripping of upd Cu.40,41 In addition, the Cu bulk deposition peak around 0.2 V is also consistent with features observed on Pt single crystals. Pugh et al. performed Cu upd and dealloying of flat, arc-melted

Figure 2. Disk (top) and ring (bottom) current obtained on 30 wt % Pt/C with a loading of 15 μg cm−2 on a 5 mm diameter glassy carbon disk in O2-saturated 0.1 M HClO4 (dotted) plus 0.05 mM (dash-dotdot), 0.5 mM (long dash), and 1 mM (solid) Cu2+, respectively, with a 10 mV s−1 sweep rate at 900 rpm (anodic sweep).

1 mM CuSO4. The anodic sweeps were measured at a scan rate of 10 mV s−1 between 0.02 and 1.05 V (only the 900 rpm sweeps are shown for the sake of clarity). When the electrolyte was devoid of Cu2+, a well-defined limiting current was obtained, and the corresponding ring currents were also fairly flat until the potential approached the hydrogen adsorption/ desorption region below 0.4 V. On the other hand, after the addition of as little as 0.05 mM CuSO4, prominent peaks appeared between 0.2 and 0.7 V. The disk current peaks were 4588



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decreases, the performance declines compared to that in the same concentration of Cu-free H2SO4. This unambiguously indicates adsorption of Cu on the surface. Furthermore, the activities reduce as the CuSO4 concentration increases even in the high potential range (Figure 3). This result is consistent with the ORR polarization curves in Figure 2, showing that increasing Cu2+ concentration results in an increasing overpotential. Notably the shapes of the Tafel curves remain relatively consistent despite the different Cu2+ concentrations. This suggests that no major changes in the ORR mechanism, such as the shift to the unwanted H2O2 pathway,43 occur above 0.8 V. The performance reduction is mostly due to the absorbed Cu blocking the active Pt surface sites. Figure 4 presents representative Levich plots for 30% Pt/C in 0.1 M HClO4 doped with 0.05−1 mM CuSO4 (as well as

mirrored by increases in the ring current. The ring current is associated with the production of hydrogen peroxide at the electrocatalyst surface via a two-electron reduction pathway in lieu of the desired four-electron pathway. This correlation suggests that adsorbed Cu atoms on the Pt surface force the ORR four-electron pathway to the two-electron pathway, thereby inhibiting the reaction through modification of its mechanism. It is worth noting that these effects have also been observed on a Pt polycrystalline electrode surface.15 While Cu adsorption on Pt at potentials under 0.5 V is well established, it is notable that our experiments show a slight increase of the background ring current above 0.8 V. This raises the possibility that there might be a small amount of Cu on the surface above 0.8 V, which is well into the potential window for a PEMFC cathode electrocatalyst. The CVs in Figure 2 are unclear as to the extent of these effects, or from what process they arise from (Cu2+, bisulfate anion adsorption, or a combination of both). These issues will be explored by the Δμ results to be discussed below.

Figure 4. Representative Levich plot for the ORR on 30% Pt/C with 0.1 M HClO4 containing various concentrations of CuSO4. Theoretical plots are given for comparison. The current densities have been normalized to the electrochemical surface area of platinum on a 5 mm diameter glassy carbon disk. Figure 3. Mass-transfer-corrected Tafel plots for the ORR polarization curves presented in Figure 2. Scan rate: 10 mV s−1 at 900 rpm. Pt loading: 15 μg cm−2.

0.05 mM H2SO4) at rotation rates from 100 to 2500 rpm. As with the Tafel plots in Figure 3, concentrations of H2SO4 greater than 0.05 mM yielded nearly identical plots as those at 0.5 mM and are not shown here in the interest of clarity. The slope of each plot is directly related to the number of electrons (n) transferred per O2 molecule reacted.44 When only sulfate anions (without Cu2+) are present in the 0.1 M HClO4 electrolyte, the Levich plot nearly overlaps both the theoretical n = 4 and “clean” 0.1 M HClO4 plots. Any adverse effects on the kinetic performance resulting from low concentrations of SO42− can therefore be ignored, allowing the analysis to concentrate on the effects of Cu2+. As the concentration of Cu2+ increases, the slope decreases. This indicates that the presence of Cu in the electrolyte is shifting the ORR mechanism to the two-electron, H2O2-production pathway over the potential regime below 0.7 V. Analogous methods of electrochemical analyses were also applied to study the impact of Cu contamination on the HOR. Figure 5 presents CVs for 30 wt % Pt/C (E-TEK) in N2sparged 0.1 M HClO4 (solid line) and 0.1 M HClO4 + 10 μM CuSO4 (dashed line). We spiked the 0.1 M HClO4 electrolyte with CuSO4 (final concentration 10 μM) via a calibrated pipet, and then held the potential at 0.1 V (a typical operating potential for a PEMFC anode electrocatalyst) for a few hours without rotating the electrode to let the potentiostatic deposition of Cu reach equilibrium. Compared to the cathodic

Figure 3 presents the corresponding mass-transfer-corrected Tafel plots derived from Figure 2 according to the following relationship: ik = ilim × i/(ilim − i)

(4)

where ik is the kinetic current density, ilim is the diffusion limited current density determined by the ORR polarization, and i is the measured current density during the ORR anodic sweep. The ORR performance in clean 0.1 M HClO4 is clearly better than that in the CuSO4-doped electrolyte. Further, data were collected with a H2SO4-doped electrolyte in order to explore the isolated effects of only bisulfate anion adsorption at low SO42− concentrations (0.5 and 1 mM H2SO4 concentrations gave nearly identical results to those at 0.05 mM, and are not shown here for the sake of clarity). Performance loss with sulfate is not surprising because it is well-known that sulfate anions adsorb on the Pt surface, resulting in performance reduction by site-blocking. In 0.1 M HClO4 + 0.05 mM CuSO4 electrolyte, the performance was quite close to that in the 0.05 mM H2SO4 electrolyte in the high potential region, indicating only minimal Cu contamination at these potentials and at these low concentrations of Cu2+. However, as the potential 4589



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is no longer flat. At higher Cu concentrations, the negative shift of ilim increases, and Cu desorption manifests itself as a large positive peak at potentials above 0.25 V. Again, the effects of partial blocking of surface Pt sites by adsorbed Cu are clearly evident. The nature of the blocking of Pt sites by Cu will be elucidated by the EXAFS and Δμ analyses below. EXAFS Analysis. In situ, element-specific, Pt L3 edge EXAFS analysis was performed to explore the short-range atomic order of the Pt/C catalysts by determining the interatomic bond distances (R), coordination numbers (N), and atomic distributions of Pt and Cu. The overall fitting results at each potential with respect to Cu2+ concentrations are presented in Tables 1 and 2 (30 wt % Pt/C in HClO4 and 46.6 wt % Pt/C in H2SO4, respectively). Due to the high correlation between the Debye−Waller factors (σ2) and the coordination numbers, σ2 values of 0.0057 and 0.0058 Å2, obtained from the fits of the data without Cu at 0.54 V, were fixed for all of the fits to ensure the valid comparisons of the coordination numbers. The fit results indicate that the Pt−Pt interatomic distances (RPt−Pt) exhibited little change (within experimental uncertainties) with respect to both the electrochemical potential and electrolyte environment. This indicates that the bulk structures of both Pt/C catalysts were stable over our experimental conditions. The samples are therefore amenable to Δμ analysis. Although Cu on the surface at low potentials can be easily detected by electrochemical techniques, low Cu concentrations are not typically detectable by EXAFS, a bulk-sampling technique, because the structural distribution of Cu (e.g., only on the surface) results in an extremely low Pt−Cu scattering amplitude. To explore the sensitivity of both EXAFS and Δμ, three different concentrations of Cu2+ (1, 10, and 100 mM CuSO4) were used in the experiments. EXAFS could only discern Pt−Cu scattering at 100 mM Cu2+ concentrations in the low potential region. The presence of the Pt−Cu scattering confirms that the Cu coverage of the surface on the anode side is large, which is consistent with our electrochemical results. Figure 7 presents the fits of the Fourier-transformed (FT) EXAFS data at 0.04 V without Cu2+ (top) and with 100 mM Cu2+ (bottom) in 0.1 M HClO4 electrolyte. While a single Pt− Pt path provided a decent fit to the experimental data without Cu2+, a Pt−Cu path had to be included to achieve a reasonably good fit in the 100 mM Cu2+ case. Further, the contribution of the Pt−Cu path (long-dashed green line) to the overall fit is significant: NPt−Cu = 1.68. As a result of the contribution of the Pt−Cu scattering, the magnitude of the main peak (due to all metallic scattering) is clearly higher in 100 mM Cu2+. The overlap of the peaks occurs because of the roughly similar interatomic distances of Pt−Pt and Pt−Cu. This phenomenon, coupled with the results listed in Table 1, shows that the detected Pt−Cu bonds did not replace Pt−Pt bonds (NPt−Pt did not change), and suggests that most of the Cu atoms stay on the surface without significantly inhibiting the Pt−Pt scattering. The Pt−Cu interatomic distance determined by the fits was found to be 2.67 Å, and is consistent with previous reports.22 The trend in Pt−Pt coordination with potential in clean HClO4 was consistent with previous EXAFS results:27,45 the higher NPt−Pt at 0.04 V is due to the Cu and H absorption causing the nanoparticle to become more spherical or ordered,34 while the decrease of NPt−Pt at 0.84 V is due to the oxidation of Pt.46 Note that the decrease of NPt−Pt at 0.84 V in the presence of 100 mM Cu2+ is not as significant as in clean uncontaminated HClO4. In fact, it is almost the same as that at other potentials except 0.04 V. We will show below in the Δμ

Figure 5. Cyclic voltammograms of 30 wt % Pt/C (E-TEK) taken in Ar-purged 0.1 M HClO4 (solid line) and 0.1 M HClO4 + 10 μM CuSO4 deposited at 0.1 V for a few hours (dashed line). The Pt/C loading was 15 μg cm−2 on a 5 mm diameter glassy carbon RDE, and the data was collected at a scan rate of 10 mV s−1 between 0.02 and 1.0 V vs RHE at room temperature.

sweep obtained in clean 0.1 M HClO4, that obtained in 0.1 M HClO4 + 10 μM CuSO4 has flattened out in the Hupd region where the typical H/H+ desorption peaks fall. This region is followed at ∼0.35 V by a comparatively large Cu desorption peak. These results indicate that a Cu2+ concentration as low as 10 μm is sufficient to block the active sites for hydrogen adsorption and therefore probably also for the HOR. These results corroborate MEA measurements showing that crossover of Cu2+ from the cathode to the anode accounts in part for the drop-off in performance of dealloyed PtCu cathode catalystbased PEMFCs.8 Figure 6 directly explores the effects of Cu2+ concentration on the HOR performance of Pt/C catalysts by presenting a

Figure 6. Disk currents obtained on 30 wt % Pt/C with a loading of 15 μg cm−2 on a 5 mm diameter glassy carbon disk in H2-saturated 0.1 M HClO4 (solid) plus 0.05 mM (long dash), 0.5 mM (dash-dot), and 1 mM (dotted) Cu2+, respectively, at a 10 mV s−1 sweep rate at 1600 rpm (anodic sweep).

representative set of rotating ring-disk experiments performed at room temperature, in H2-saturated 0.1 M HClO4 containing different concentrations of CuSO4. The sweeps were measured at a scan rate of 10 mV s−1 at a rotation rate of 1600 rpm. When Cu2+ is not present in the electrolyte (solid line), ilim is typical (positive and flat). When the Cu2+ concentration is as low as 0.05 mM (dashed line), ilim has already shifted negative (due to reductive current arising from deposition of copper metal) and 4590



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Table 1. Summary of 30 wt % Pt/C (E-TEK) in 0.1 M HClO4 + x mM CuSO4 EXAFS Resultsa potential (V, RHE)

RPt−Pt (Å) (ΔR = ±0.01 Å)

RPt−Cu (Å) (ΔR = ±0.03 Å) No Cu

0.04 0.24 0.54 0.84

2.76 2.75 2.74 2.75

0.04 0.24 0.54 0.84

2.76 2.75 2.75 2.74

0.04 0.24 0.54 0.84

2.75 2.74 2.75 2.75

NPt−Pt (ΔN = ±0.8)a

NPt−Cu (ΔN = ±0.9)a

σPt−Pt2 (Å2)

2+

8.39 7.68 7.86 7.15 10 mM Cu2+ 2.67 2.67

0.0057 0.0057 0.0057 0.0057

8.30 8.73 8.84 7.43

100 mM Cu2+ 2.67 2.67

0.0057 0.0057 0.0057 0.0057

0.56

8.79 8.03 8.31 8.04

1.12 1.68

0.0057 0.0057 0.0057 0.0057

a Data collected in 0.1 M HClO4 + x mM CuSO4 under N2-sparging. S02 fixed at 0.934 as calculated by FEFF8.0. The Fourier-transformed (FT) EXAFS data were fitted under simultaneous k1,2,3 weighting, R range 1.02−3.2 Å, k range 2.703−10.858 Å−1. σPt−Cu2 was determined to be 0.011. The statistical errors of the least-squares fits were determined by ARTEMIS.

Table 2. Summary of 46.6 wt % Pt/C (Tanaka) in 0.05 M H2SO4 + x mM CuSO4 EXAFS Resultsa potential (V, RHE)

RPt−Pt (Å) (ΔR = ±0.01 Å)

RPt−Cu (Å) (ΔR = ±0.03 Å) No Cu

0.54 0.70 0.84 1.00

NPt−Pt (ΔN = ±0.6)a

NPt−Cu (ΔN = ±0.6)a

σPt−Pt2 (Δσ2 = 2 × 10−4 Å2)

2+

2.74 2.75 2.75 2.75

7.90 8.58 8.38 7.07

0.0058 0.0058 0.0058 0.0058

8.10 8.15 8.52 7.85

0.0058 0.0058 0.0058 0.0058

8.71 8.54 8.27 7.78

0.0058 0.0058 0.0058 0.0058

1 mM Cu2+ 0.54 0.70 0.84 1.00

2.75 2.75 2.75 2.75 10 mM Cu2+

0.54 0.70 0.84 1.00

2.75 2.75 2.75 2.75

0.54 0.70 0.84 1.00

2.75 2.75 2.75 2.75

100 mM Cu2+ 2.67 2.67 2.67 2.67

8.38 8.10 7.80 7.36

0.42 0.27 0.60 0.61

0.0058 0.0058 0.0058 0.0058

a Data collected in 0.05 M H2SO4 + x mM CuSO4 under N2-sparging. S02 fixed at 0.934 as calculated by FEFF8.0. The FT EXAFS data were fitted under simultaneous k1−3 weighting, R range 1.3−3.3 Å, k range 2.64−15.43 Å−1. σPt−Cu2 was determined to be 0.011. The statistical errors of the least-squares fits were determined by ARTEMIS.

responsible for the ORR activity enhancement of PtM bimetallic alloys.48 The inhibition of O(H) adsorption in 10 mM Cu2+ was not as significant as that in 100 mM due to the lower concentration of Cu2+. Δμ Analysis. Figure 8 shows the theoretical Δμ spectra for Cu adsorbed on a Pt6 cluster in atop, bridged, and 3-fold facecentered-cubic (fcc) sites, as calculated by eq 2. As shown, the overall features of the three sites are quite similar to each other. They all show a negative peak near 2 eV and a broader, positive feature between 8 and 15 eV. However, adequate differences still exist to distinguish the sites from each other, particularly in the 8−15 eV region. Compared to the bridged and fcc sites, the atop signature has a smaller negative peak at 2 eV, and the feature at 8−15 eV remains negative as well. In comparison, the 8−15 eV features for the bridged and fcc sites are both positive.

analysis that Cu(O) can stick to the Pt surface even at 0.84 V as long as the Cu2+ concentration is high enough. This effect can be even more drastic if there are SO42− and O2 in the media because their presence also stabilizes the Cu on the Pt surface (see below). This indicates that, when the concentration of CuSO4 is ≥100 mM, there will be a substantial amount of Cu(O) on the surface. The surface Cu(O) results in the substrate Pt particles becoming more uniform and spherical as discussed above. Moreover, the formation of Cu(O) strongly inhibits O(H) absorption on Pt, as shown by the Δμ data below. This inhibition has been attributed previously to lateral interactions between the Cu(O) and the OH on the surface.35,47 Both effects can account for the decreased reduction in NPt−Pt at 0.84 V. The suppression of O(H) adsorption from water activation is widely proposed to be 4591



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Figure 9. Pt L3 edge Δμ = μ(V, x mM Cu) − μ(0.54 V, no Cu) spectra for 30 wt % Pt/C in 0.1 M HClO4 at the indicated Cu2+ concentrations. The theoretical Δμ spectra in Figure 8 or Figure 11 and labeled as shown were also selectively embedded into the bottom of different plots for comparison. The horizontal dashed line indicates the maximum amplitude around 5 eV due to Cu, with the intensity above this line at 0.04 V being mostly due to coadsorbed H on Pt. Figure 7. Pt L3 edge FT EXAFS data and corresponding least-squares fits for 30 wt % Pt/C in 0.1 M HClO4 (top) and in 0.1 M HClO4 + 100 mM CuSO4 (bottom) held at 0.04 V. The fits were performed with k1−3 weighting.

The experimental spectra at 0.04 V include a combination of the Δμ components for both 3-fold upd H and Cu. While the Δμ signatures of the upd Cu (in fcc sites) are quite close to those observed for upd fcc (3-fold) H,34 we compare both theoretical signatures in Figure 9 (0.04 V), and the positive feature between 5 and 15 eV is much sharper for the upd H than for the upd Cu. The experimental spectrum without Cu at 0.04 V is much sharper than that at 100 mM Cu, consistent with this. We therefore attribute all of the experimental Δμ amplitude at 0.04 V above magnitude 0.025 to upd H, since, at 0.54 V, when the upd Cu is still fully present without H, it has this same amplitude (see dotted horizontal line in Figure 9). Therefore, a small amount of upd H (about 10% of that without Cu) can adsorb on Pt even in the presence of the upd Cu (i.e., they can coadsorb with the mobile H apparently finding geometrically constrained sites that the Cu cannot occupy). At 0.24 V, when most of the H has desorbed, the Δμ spectra become similar to those at 0.54 V. At 0.84 V, Pt−O[H] formation (water activation) begins, and the Δμ spectra change significantly. To show that the spectra are now dominated by the O[H] adsorption, the FEFF8-calculated theoretical signature for 3-fold fcc O on Pt is also given in Figure 9 (0.84 V). This theoretical line shape has been previously reported by Teliska et al.34,35 and is consistent with relevant DFT calculations34−36,49,50 for 3-fold fcc or bridged bonded O at these potentials. The strong adsorption of O[H] is also consistent with the results from the EXAFS fits, which show that the Pt−Pt coordination number is significantly reduced at 0.84 V, this occurring because O[H] adsorption decreases the Pt−Pt scattering (each 3-fold O decreases the Pt−Pt scattering more than each atop OH, but either OH or O adsorption decreases it some).27,35,51 The Δμ spectra for 100 mM Cu appear to have different features present, and this will be explained after we consider the results for upd Cu in sulfuric acid. Figure 10 shows similar experimental Δμ spectra but now in 0.05 M H2SO4 rather than in 0.1 M HClO4. The Δμ spectra in H2SO4 at 0.54 V are significantly different from those in HClO4 at the same potential. What can account for this change? At

Figure 8. Theoretical Δμ = μ(Cu/Pt6) − μ(Pt6) signatures for Cu in atop (solid), bridged (dashed), and 3-fold fcc (dotted) Pt sites.

Although the Δμ signatures for the bridged and fcc sites have the same negative dip at 2 eV, they do have significantly different positive features at 8−15 eV. Figure 9 shows the experimental Δμ spectra for the Pt/C electrodes held at the indicated potentials (0.04−0.84 V vs RHE) with different concentrations of Cu2+ in 0.1 M HClO4. At 0.54 V, the Pt electrode lies in the double-layer region where there is no adsorbed H, O[H] (O[H] means either OH or O), or ClO4− anion. Thus, the electrode is considered relatively “clean” when there is no Cu2+ in solution, and the difference between the experimental XANES spectra at this potential with and without Cu2+ in solution can be used to determine the binding site(s) of the upd Cu. The FEFF8 theoretical signature for the 3-fold fcc Cu is compared with the experimental spectra here, showing the best agreement, with the “doublet” feature between 8 and 15 eV in the theoretical signature reproduced even more sharply in the experimental spectra. This indicates that the upd Cu on Pt is in an fcc (3-fold) site. 4592



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HClO4 and H2SO4 are explained by SO4 coadsorption that occurs with the upd Cu. It should be noted here that the Cu2+ is introduced by adding CuSO4, so that even HClO4 has some SO4 anions present, but these are at the mM level (compared to 500 mM in 0.05 M H2SO4), and do not play a significant role in the case of the HClO4 experiments reported here. The experimental Δμ results at 0.54 V (Figure 10) also involve another coadsorbate, now SO42− anions along with H2O.55 At 0.40 V, the potential used as the reference for obtaining the experimental Δμ in H2SO4, very little H2O−SO4 adsorption has occurred, but by 0.54 V, an ordered overlayer has already developed. This overlayer began as a precursor to OH adsorption at higher potential.35 Again, the H2O adsorption occurs in the fcc sites along with SO4 in neighboring fcc sites, but the O atoms from the sulfate lie in the hcp sites.55 This was modeled in FEFF8 by placing an O atom in both the fcc and hcp sites of the Pt6 cluster, as illustrated in Figure 11. The resultant H2O−SO4 theoretical Δμ signature is given in Figure 11 and reproduced in Figure 10 (0.54 V). Again, reasonable agreement is found between this theoretical H2O− SO4 Δμ signature and the experimental Δμ results at 0.54 V in the absence of Cu. The results at 0.7 V are similar to those at 0.54 V and need not be further discussed here. The Δμ results in H2SO4 now explain the additional features found at 0.84 V and for 100 mM Cu seen in Figure 9 (0.84 V). The theoretical Δμ signature for Cu−SO4 is reproduced in Figure 9 (0.84 V), and clearly, the extra features at 0 and 13 eV can now be attributed to a component of this Δμ signature. Because sulfate anions were not present in significant concentrations, this feature is attributed to coadsorption of upd Cu and O atoms at this potential (i.e., Cu in fcc sites and nearby O in hcp sites). The Δμ spectra at 1.0 V in H2SO4 are clearly dominated by the OH/Pt, as found at 0.84 V in HClO4. However, the experimental Δμ results at 1.0 V in H2SO4 (Figure 10, 1.0 V) clearly show a shoulder at 0 eV (highlighted by the vertical line), suggesting that upd Cu either as Cu−SO4 or Cu−O is still on the surface at 1.0 V, even in H2SO4. The change in upd Cu adsorption with potential can best be followed by plotting the Δμ magnitude, |Δμ|, with potential, since Δμ does vary somewhat proportionally with Cu coverage. We show this in Figure 12. We use the Δμ magnitude in the 7− 10 eV range from Figure 9 for Cu deposition in HClO4, when the Cu is adsorbed in the fcc sites without further coadsorbates. We use the Δμ magnitude at around −1 eV from the Cu−SO4 signature to reflect the Cu coverage in H2SO4, but we divide this magnitude by 2 to normalize it to the same magnitude (coverage) obtained in HClO4 at 0.54 V. Figure 12 shows several important features: (1) At 0.04 V, the Cu coverage is the same for all Cu concentrations. This potential is below the opd Cu adsorption feature shown in the CV curve, Figure 1, so we determine this to represent 1 ML or greater coverage. Because of the relative sensitivity of the Δμ technique only to the first layer of Cu in contact with the Pt, the Δμ magnitude does not significantly change as the coverage increases beyond 1 ML. (2) Below 100 mM Cu concentration in the electrolyte, a full upd Cu monolayer does not form on the Pt surface. At 0.7 V, the Cu coverage drops for low Cu concentrations in HClO4, consistent with the large desorption feature seen in the CV curve, Figure 1. (3) Perhaps most significantly, the presence of H2SO4 strongly stabilizes the Cu on the surface due to the Cu−SO4 coadsorption. Further, at 100 mM Cu concentration in the

Figure 10. Pt L3 edge Δμ = μ(V, x mM Cu) − μ(0.54 V, no Cu) spectra for 46.6 wt % Pt/C in 0.05 M H2SO4 at the indicated Cu2+ concentrations. Theoretical Δμ spectra in Figure 8 or Figure 11 were also selectively embedded into the bottom of different plots for comparison and labeled as shown here. The vertical black lines are drawn as guides to the eye.

0.54 V, only upd Cu is adsorbed in HClO4, and good agreement was obtained between experiment and the theoretical signature for 3-fold fcc Cu. However, in H2SO4 environments, sulfate anions are known to adsorb with the upd Cu atoms. Results reported by Oda et al.52 and others53−57 reveal that Cu and sulfate anions form an ordered structure on Pt(111), as illustrated in the right-hand inset of Figure 11. Both

Figure 11. Plot of FEFF8 calculated Δμ = μ(ads/Pt6) − μ(Pt6), with the position of the Cu or O fcc site shown by the purple atom and the hcp O atom by the pink atom on the Pt6 cluster. The Cu−SO4 structure on a Pt 111 surface proposed by Oda et al.52 is also indicated, with the Cu atoms indicated by the red checkered spheres, the O by the red solid, and the S by the light blue solid sphere. Surface Pt atoms are located at each vertex of the background “diamond” pattern.

the upd Cu and sulfate anions lie in 3-fold fcc sites, but the O atoms of the sulfate actually are in the hcp sites with relatively longer Pt−O bond length than what is typical of Pt−O bonds, as illustrated in the inset. We therefore performed FEFF8 calculations assuming a Cu atom in the fcc site and an O atom in the hcp site of the Pt6 cluster, as shown in the left-hand inset of Figure 11. The resulting FEFF8-calculated signature is shown in Figure 11 and is reproduced in Figure 10 (0.54 V and 0.84 V). Reasonable agreement between the theoretical Cu− SO4 Δμ signature and the experimental Δμ data is now evident. Thus, the dramatic differences in Δμ signatures obtained in 4593



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higher potentials (>0.8 V), the adverse effects of low concentrations of Cu (1 mM) were not detected by XAS; however, a performance reduction was clearly observed for a higher concentration (100 mM). The stabilization of the upd Cu on the surface by coadsorption of Cu−SO4 or Cu−O(H) to potentials well above 0.7 V was confirmed with both the EXAFS and the Δμ techniques. This confirms that Cu can contaminate the cell even in the potential region corresponding to the normal operation of a fuel cell cathode. Overall, these data show that even small concentrations of Cu2+ in the electrolyte can cause severe detrimental effects on both the performance and durability of Pt nanocatalysts. This is critical because various bimetallic or dealloyed PtCu catalysts are being given increased consideration for the cathode, and some Cu leaching out into the electrolyte after formation of the MEA has been readily observed. Besides the site-blocking mechanism caused by the direct adsorption of the Cu ions on the Pt surfaces, as demonstrated in this work, it has been demonstrated that the excess transition metal cations concentrate in the ionomer in the cathode electrode with increasing current density, and the resultant decrease of the local proton concentration (proton starvation) can also hinder the performance of a fuel cell at high current density.59 Taken together, these results show that accumulation of Cu in the electrolyte can cause serious problems for both performance and durability in a fuel cell, and eliminate the prospect of using PtCu alloys as cathode catalysts in future commercial PEMFCs, unless the Cu leaching can somehow be drastically reduced. This discussion highlights the trade-offs between the activity and stability of alloyed and dealloyed PtM nanoparticles. Nonnoble metal ions leached from a catalyst can impede the operation of a fuel cell through several mechanisms. Therefore, the choice of the second transition metal, the electrolyte, and the final nature of the dealloyed PtM particles (e.g., core-full shell or core-porous shell) appears to be critical to fuel cell performance and durability.

Figure 12. Plot of Δμ magnitude at energies chosen to maximize sensitivity to adsorbed Cu (see text) vs electrochemical potential at the indicated Cu2+ concentrations (1 mM/blue, 10 mM/red, and 100 mM/black). Those found in HClO4 (Cu alone) are indicated by the solid lines and those obtained in H 2 SO4 involving a CuSO4 coadsorbate structure by the dashed lines. One symbol (open black square) is also indicated for the Cu−O coadsorbate found in HClO4 at 0.84 V. The |Δμ| of 0.025 at 0 V RHE corresponds to one monolayer or more of Cu (the Δμ is nearly the same for 1 or more ML), and atop H (attributed to upd H) begins to become evident on the surface below 0.2 V.

electrolyte, O coadsorption stabilizes the Cu. Thus, at 100 mM Cu concentrations or higher, sufficient Cu is able to remain on the surface in the O(H) region, and this Cu−O(H) coadsorption stabilizes the Cu on the surface. Figure 12 reveals why opd and upd Cu so strongly affect the HOR and ORR, as shown in Figures 5 and 1, respectively. At the anode side, the 1 ML or greater coverage of Cu could severely inhibit HOR by blocking all or most of the active sites. Under ORR conditions on the cathode side, O[H] adsorption comes not only from water activation but also from possible intermediates in the ORR process, namely, molecular O2 in an associative first step as proposed by Rossmeisl et al.58 or from a first dissociative reduction step 1

+

−

/2 O2 + H + e → OH/Pt

■

AUTHOR INFORMATION

Present Address

(5)

∥

N.R.: Nissan Technical Center North America, 39001 Sunrise Drive, Farmington Hills, MI 48331, United States.

in the double-trap ORR kinetic mechanism proposed by Wang et al.57 This first step (associative or dissociative makes little difference here) causes much larger OH or OO[H] coverage from 0.8 V all the way down below 0.5 V,27,35 and this could cause Cu−O(H) coadsorption that stabilizes the Cu on the surface.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Funding for this research was provided by the Fuel Cell Technologies Program in the Office of Energy Efficiency and Renewable Energy of the U.S. Department of Energy through contract DE-EE0000458. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0298CH10886. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357.

■

CONCLUSIONS Cu adsorption onto Pt/C nanoparticles in either 0.1 N HClO4 or H2SO4 over the operating potential range of a PEMFC was systematically investigated under half cell conditions to explore the poisoning effects of Cu on the ORR and HOR. Variations in the Cu coverage on the cathode catalyst with and without sulfate ions were monitored as a function of the potential. The results obtained from R(R)DE and the in situ Δμ XANES and EXAFS techniques were strongly correlated. In the low potential region below 0.1 V, even a low Cu2+ concentration (10 μM) led to full coverage of the active surface of Pt and might thereby disable the cell from the anode side. In the underpotential region, 0.35−0.7 V, very low concentrations of Cu (0.05 mM) result in upd Cu deposited on the Pt surface, and this can lower the ORR activity by converting the desired four-electron pathway to the unwanted peroxide pathway. At

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