Fundamental Aspects of Spontaneous Cathodic Deposition of Ru onto

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Fundamental Aspects of Spontaneous Cathodic Deposition of Ru onto Pt/C Electrocatalysts and Membranes under Direct Methanol Fuel Cell Operating Conditions: An in Situ X-ray Absorption Spectroscopy and Electron Spin Resonance Study Thomas M. Arruda,† Badri Shyam,‡ Jamie S. Lawton,† Nagappan Ramaswamy,† David E. Budil,† David E. Ramaker,‡ and Sanjeev Mukerjee*,† Department of Chemistry and Chemical Biology, Northeastern UniVersity Center for Renewable Energy Technology, Northeastern UniVersity, 360 Huntington AVenue, Boston, Massachusetts 02115, and Department of Chemistry, The George Washington UniVersity, 725 21st street N.W., Washington, DC 20052 ReceiVed: August 21, 2009; ReVised Manuscript ReceiVed: NoVember 30, 2009

In situ X-ray absorption spectroscopy (XAS), using both EXAFS and the ∆µ-XANES analysis procedures, is utilized to examine Ru deposition onto Pt/C cathodes at millimolar concentration of Ru in a 1 M HClO4 electrolyte. Also, electron spin resonance (ESR) spectroscopy is utilized to examine the effects of Ru3+ ion exchanged Nafion membranes. The ∆µ-XANES analysis of the data allows a determination of the coverage of Ru with time (minutes to hours) and also to identify the binding site of the deposited Ru species (atop/ bridged at low coverage and 3-fold at higher coverage) apparently onto the corners and edges initially and at higher coverage onto the faces of the cubooctahedral clusters when exposed to Run+ at OCP. The deposition of Ru appears to be inhibited at potentials where adsorbates (such as H and O(H)) usually adsorb, and Coulomb enhanced at OCP when substantial O exists on the surface. The ESR analysis of Ru3+ in the Nafion membrane indicates significant detrimental changes to the membrane in the presence of Ru ions; a decrease in the water uptake and an increase in the microviscosity of the fluid regions were noticed. Together these data indicate the critical nature of keeping the fuel cell under potential control and avoiding an uncontrolled shut down under direct methanol operating conditions. 1. Introduction Direct methanol fuel cells (DMFCs) offer the promise of high energy density power for both portable and stationary applications. Electronic devices are often noted as being the greatest beneficiaries of DMFCs, which could offer a 10-fold increase in power density in comparison to lithium-ion batteries.1 Despite the above-mentioned qualities, DMFCs have faced significant technological hurdles, hence impeding large-scale commercialization. Much of these issues rest on materials challenges such as activity and stability of anode electrocatalysts and the concomitant role of membranes. As indicated in prior reviews by Stuve et al.2 and Wieckowski et al.,3,4 electrocatalysis of the six-electron methanol oxidation reaction (MOR) can be considered as a two-step process. The first is the initial dehydrogenation step involving the abstraction of the first hydrogen by breaking the C-H bond in methanol (the next two dehydrogenation steps being more facile). The second is the oxidation of the CO and CHO moieties formed on the surface following the dehydrogenation steps. The current state of the art electrocatalysts rely on the “bifunctional approach”, in which a second element, such as Ru, initiates oxidation of the CO or CHO species by activating water (hence forming surface oxygenated species such as OH) at lower potentials. However, as reported previously,5 these dual electrocatalytic requirements cause a simple bifunctional catalyst with good nucleation of oxygenated species at lower overpotential to fail as a good electrocatalyst for methanol oxidation, * To whom correspondence should be addressed. E-mail: s.mukerjee@ neu.edu. † Northeastern University. ‡ The George Washington University.

despite excellent CO oxidation characteristics. This has been shown previously for PtSn electrocatalyst5,6 and more recently for PtMo.7 In the latter case, while PtMo/C exhibited more than 3-fold enhancement for CO oxidation as compared to the current state of the art PtRu/C, there was no concomitant increase in activity for methanol oxidation. The activity for MOR was closer to that for pure Pt despite its enhanced ability to oxidize CO. At the present moment, PtRu remains as the electrocatalyst of choice. However, in contrast to CO electro-oxidation, supported electrocatalysts (Pt/C, PtRu/C less than 30 wt % as commonly employed in H2/O2 fuel cells) have shown limited ability to sustain electrocatalytic activity beyond 0.3 A cm-2. This has necessitated the use of either unsupported electrocatalysts or those with high electrocatalyst loading, which can be an order of magnitude higher than the current state of the art low Pt loading electrodes (0.05 mg cm-2 for hydrogen anode and 0.4 mg cm-2 oxygen cathode).8 From an electrocatalytic perspective, the stability of PtRu in its various forms (supported, unsupported, and in some cases as decorated nanoparticles {i.e., Ru decorated on Pt and vice versa}) is of paramount interest not only from the perspective of its dissolution and other changes in anode electrocatalyst morphology but also from the perspective of any dissolved adducts migrating to the cathode electrode and associated effect on the membrane. In 2004, Piela and co-workers9,10 showed that PtRu anodes are susceptible to Ru dissolution in an actual working fuel cell stack. Although the concept of Ru dissolution had been previously known,11–17 the direct consequences of spontaneous Ru deposition onto the cathode catalyst from the anode had not been previously illustrated. To further complicate

10.1021/jp908082j  2010 American Chemical Society Published on Web 12/29/2009

Cathodic Deposition of Ru onto Pt/C Electrocatalysts matters, they9 also observed Ru ions in the polymer electrolyte membrane (PEM). As reported by us earlier, the deposition of Ru onto Pt under cathode operating conditions can result in an approximate 40-200 mV overpotential.18 To fully investigate the extent of Ru poisoning, Gancs et al.18 provided a fundamental account by means of rotating disk electrode (RDE) studies. It was noted that even micromolar quantities of dissolved Ru have dramatic negative effects on the oxygen reduction reaction (ORR) electrode kinetics. An additional overpotential of 160 mV was observed (ca. 170 µM) in comparison to pristine Pt, and the Ru remained stable on the surface in the entire ORR potential window (0-1.2 V vs RHE).18 In addition, the cyclic voltammograms (CV) of the Ru contaminated Pt revealed increasing double layer capacitance known to be caused by RuOx species,19,20 and decreases in Pt-O formation/reduction peaks and Hupd charge. Most of the earlier studies of Ru deposition on Pt either by electrochemical deposition21–26 or spontaneous deposition4,12,13,15,16,25–34 have attempted to exploit deposited Ru for the enhanced oxidation of methanol,12,17,26,29,32,35,36 ethanol,37 formic acid,38 and recently dimethyl ether.39 The latter method is particularly favored for producing surfaces that are quite stable upon voltammetric cycling. However, we are unaware of any comprehensive study investigating the effects of Ru adatoms on Pt surfaces during typical ORR operating conditions. Although the details may vary slightly, the overall theme centers on enhancement of methanol oxidation kinetics for Ru decorated Pt in comparison to Pt alone, or in some cases even PtRu alloys. For example, Waszczuk et al.32 showed that spontaneously deposited Ru on unsupported Pt nanoparticles produces an electrocatalyst that is twice as active as commercially available PtRu alloys for methanol oxidation. Their observations suggested that the electrocatalytic enhancement may be a direct result of Ru edge atoms being under-coordinated by Ru or surface Pt atoms, which could result in enhanced H2O activation and, hence, an improved bifunctional mechanism. In light of the importance of the spontaneous deposition of Ru, as it pertains to the above applications in electrocatalysis, many investigations have been conducted to elucidate the surface structure of the deposited Ru.30,31,40,41 Ex situ techniques such as Auger electron spectroscopy (AES),29 X-ray photoelectron spectroscopy (XPS),42 and low energy electron diffraction (LEED)43,44 have contributed greatly to the understanding of such surfaces and their properties. A series of scanning tunneling microscopy (STM) and electrochemical investigations by Crown et al.30,31 indicated that Ru deposits on low index Pt(hkl) surfaces without site preference. Surface coverages obtained for Pt(111), Pt(100), and Pt(110) after a single deposition process were 0.20 monolayers (ML), 0.22, and 0.10 ML, respectively, and found to be mostly in the form of monolayer-thick islands. The rate of island formation (although described as slow in comparison to Os/Pt(111) deposition) was shown to form 0.07 ML after 20 s with a maximum coverage obtained after 120 s. Iwasita et al.45 have shown that the coverage on such single crystals can be increased by a process of repeated spontaneous deposition. In another study, Ku et al.40 found that Ru formed a (3 × 3)R30° RuO+ adlayer on a Pt(111) surface, which is quite stable even after voltammetric cycling. A comprehensive investigation by Strbac et al.41 also employed in situ STM to study Ru and Os spontaneous deposition on Pt(111) and Au(111) surfaces. Interestingly, they found that the Ru island growth process is different on the two surfaces. On Au(111), Ru prefers to deposit on steps and terraces relatively quickly with a

J. Phys. Chem. C, Vol. 114, No. 2, 2010 1029 multilayer thickness and hexagonal surface structure. On Pt(111), the deposition time was also relatively fast; however, only monolayer thickness (0.18 ML saturation) could be observed after a single deposition period of 3 min. When a second 3 min deposition was applied, the coverage did not increase significantly (0.22 ML); however, the height of the Ru adlayer increased as evidenced in the STM cross section. Further, the Ru island size and shape (typically 2-5 nm in width) were shown to be dependent on Ru oxidation state and could be manipulated by varying the potential. Other in situ techniques such as electrochemical quartz crystal microbalance (EQCM) measurements have also been employed to study Ru deposition. Such a study was first carried out by Frelink et al.46 to measure the Ru surface content of electrodeposited Ru onto a Pt film electrode. They found a strong correlation between the potential of the surface oxide reduction peak of Pt and the Ru content and showed that it is possible to accurately monitor the surface Ru content using this technique. Another study by Vigier et al.,17 correlating measured growth rates with Fickian diffusion models, revealed a likely twodimensional deposition in nature. Furthermore, on the assumption that every Ru atom occupies one surface Pt atom (i.e., Ru occupies a one-fold or atop Pt surface site), they found that their estimates of surface coverage of Ru on Pt were in good agreement with other values in the literature. We will show later in the discussion that we also find atop adsorption albeit, chiefly at lower potentials, while at open circuit, we find that the Ru atoms at higher coverage prefer to be more highly coordinated and occupy 3-fold (i.e., one Ru ion is shared between three fcc Pt surface atoms) fcc sites on the oxygen covered surface. In the past, X-ray absorption spectroscopy (XAS) has been employed to study fundamental electrode processes in electrochemistry. XAS is an ideally suited method for examining nanoscale materials because it can be performed in situ with modern synchrotron facilities.47,48 Although XAS is traditionally a bulk-averaging method, nanoscale materials afford us the luxury of having ∼50% or more of their atoms on the surface (depending on size and geometry) where electrochemical processes occur. As such, small changes in coordination number (N) or bond distance (R) can be detected during an electrochemical reaction by analyzing the extended X-ray absorption fine structure (EXAFS). In addition, the newly developed ∆µ (sometimes referred to as ∆-XANES) method of X-ray absorption near edge structure (XANES) analysis has been successfully employed to provide fundamental accounts of adsorbate binding sites on Pt and Pt alloys.49–52 Other materials have also been probed by the ∆µ method with success including porphyrins53 and metal-chalcogenides.54,55 In other studies of Ru crossover, Ru cations have been observed in the solid electrolyte membrane by X-ray fluorescence spectroscopy (XRF).9 Investigation of Ru3+ exchanged into a Nafion membrane offers the possibility for fundamental studies of morphological changes caused by multivalent Ru ions leaching into the membrane from catalyst layers. PtRu composites in the membrane have been shown to decrease the proton conductivity of the membrane.56 To date, however, we are unaware of any comprehensive studies on the behavior of Ru ions inside the micropores of Nafion and the effects of such species. Previously, electron spin resonance (ESR) was used to measure the microviscosity of the fluid phase of the membrane.57 ESR has also been used to observe the effects of mono- and multivalent ions on the membrane.58 These investigations showed that different cations exchanged in the membrane alter the water uptake characteristics of the membrane as well as the

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microviscosity of the fluid phase, ultimately by changing the free volume available to the solvent. In this work, it is our intention to further the understanding of Ru poisoning by traditional electrochemical methods (CV and RDE analysis) as well as in situ XAS and ESR. For the first time, we have observed specifically adsorbed metal cations (Run+) on Pt in the ORR potential window using the ∆µ method. The presence of specifically adsorbed Run+ cations results in a lower diffusion limiting current (Levich) and a small increase in ORR overpotential in uncontaminated electrolyte. When ORR was performed in electrolyte contaminated with 2.0 mM Run+, the overpotential of the cathode was increased dramatically, and no diffusion limiting current was obtained. Further, the deposited Ru appears to be stable on the Pt surface and resists removal upon potential cycling. Interestingly, the spontaneous deposition of Ru occurs to a great extent when the electrodes are allowed to go to open circuit, while much less deposition occurs when the electrode potential is kept low (i.e., closer to anode electrode operating conditions). 2. Experimental Section 2.1. Electrochemical Characterization. Cyclic voltammetry and rotating disk electrode studies were carried out using a Pine Instruments MSR model dual contact RDE setup. All RDE measurements were conducted by a procedure, which has been discussed in great detail previously.59 Briefly, catalyst suspensions were comprised of 10.5 mg of 30 wt % Pt/C (BASF Fuel Cells, Inc., Somerset, NJ), 10 mL of 2-propanol (GFS Chemicals, 99.5% min), and 40 µL of 5 wt % Nafion in lower alcohols (Ion Power Inc.). Prior to the addition of 2-propanol, the catalyst powder was passivated with a few drops of deionized water to prevent spontaneous combustion of the support. The suspensions were magnetically stirred for 1 h and sonicated for 10 min prior to use. Thin films of catalyst were cast onto a polished glassy carbon (GC) RDE tip of 5.61 mm diameter (Pine Instruments). A total of 10 µL of suspension was used via two 5 µL applications, resulting in a final loading of 14 µg cm-2 Pt. All measurements were made at room temperature in a jacketed glass beaker type cell fit with a PTFE machined lid. Electrolyte used was 1 M HClO4 (GFS Chemicals, doubly distilled) for “clean electrolyte” experiments. Ru contamination experiments were conducted using the same 1 M HClO4 after it was subject to a sacrificial Ru electrode procedure, which will be outlined below. A typical three-electrode setup was used including the Pt/C modified GC disk working electrode (WE), Pt wire/mesh counter electrode (CE) with an area of 19.3 cm2 (by integration of the Hupd), and a sealed glass reference hydrogen electrode (RHE) containing clean 1 M HClO4. For the experiments where Ru contaminated electrolyte was used, a separate Pt wire (1.42 cm2) CE and RHE salt bridge (sealed with vycor frit, BAS Inc.) was used to avoid contamination of the clean cell. The electrolyte was purged with Ar gas for CV measurements and O2 for ORR (both UHP 5.0 grade, Middlesex Gasses). An Autolab PGSTAT 30 potentiostat (Metrohm USA, formerly Brinkman Instruments) equipped with a SCANGEN module was used for all electrochemical measurements. All electrochemistry experiments were carried out by activating the catalyst via potential cycling in clean 1 M HClO4 approximately 50 times at 50 mV s-1 or until a steady state CV was obtained. Also, CVs were recorded at 20 and 10 mV s-1 for Hupd integration to determine the electrochemically active surface area (ECSA). Once activated, the clean 1 M HClO4 was purged with O2, and ORR curves were measured via cyclic voltammetry prior to Ru contamination. Ru deposition was

Arruda et al. achieved by placing the RDE tip into a separate cell containing 1 M HClO4 + 2.0 mM Run+ and kept at open circuit potential, which was monitored by running zero-current chronopotentiometry for 1.5 h. Following Ru deposition, the electrode tip was rinsed off with copious amounts of deionized H2O and placed back into the clean 1 M HClO4 cell for postcontamination CVs and ORR measurements. All measurements were performed at room temperature (20 °C). It should be mentioned that we plot the CVs and ORR polarization curves using raw current obtained from the experiments to illustrate changes that occur only as a result of Ru deposition. Although a minimum of three experiments were performed for all cases, we only compare the electrochemical results obtained from a single catalyst deposition as current density normalization was not employed. A sacrificial Ru electrode was used to produce 1 M HClO4 contaminated with Run+ ions by the following procedure. A catalyst suspension was produced by mixing 90.1 mg of 60 wt % Ru/C electrocatalyst (BASF Fuel Cell Inc.) with 1 mL of deionized water and 1 mL of 2-propanol. The mixture was then sonicated for 10 min and applied to a piece of carbon weave (Panex 30, Zoltek Corp.) over an area of 10 cm2 for a final Ru loading of 5.4 mg cm-2 and capped off with 4 drops of 5 wt % Nafion solution. The electrode was placed into a beaker cell with clean 1 M HClO4 and cycled between 0.05 and 1.4 V vs RHE using a Pt wire CE and Ag/AgCl reference electrode (BAS Inc.). The Ag/AgCl reference electrode, which employed a vycor frit to retain the internal electrolyte, was also placed into an electrolyte bridge (fine glass frit) to ensure no chloride contamination would occur. After approximately 50 CVs, the potential was fixed at 1.2 V vs Ag/AgCl for 1 h. The resulting solution was filtered (Whatman 52 filter paper) three times to remove loose catalyst debris. Ru ion concentrations were determined by ion-coupled plasma mass spectrometry (ICP-MS) by contract at Geolabs Laboratories, Braintree, MA. Prior experiments in a working DMFC at different anode overpotentials as a function of time were used as a measure of expected Ru ion concentrations at the cathode electrode. These ranged between 0.1 and 3 mM. 2.2. Flow-Through Cell Design. All in situ XAS experiments were performed in a new flow-through type spectroelectrochemical cell designed to accommodate the introduction of contaminants without the need to disassemble the cell amid experiment. The cell was machined out of a Teflon block as depicted in Figure 1 with dimensions similar to those of the previously designed model.60 The cell is designed as a multifunction spectro-electrochemical cell, which can be used to collect data in either transmission or fluorescence mode. The thin wall side of the cell has a shallow internal electrolyte reservoir (3.81 cm × 4.064 cm × 0.254 cm) in which the electrode is affixed in place by a rectangular PTFE bracket. Directly across from the four corners of the holding bracket, on the thick wall side, are electrolyte flow holes, which terminate with standard PTFE compression fittings. All four corners are bored out to allow the cell to be ambidextrous in that fluorescence measurements may be performed at beamlines that have a detector on either the left or the right side of the cell. Two of the ports are used for the electrolyte inlet and return to the exterior reservoir, while a third port supplies a salt bridge to the reference electrode, which also resides in the exterior reservoir. The X-ray window is bored all of the way through the cell with a 45° chamfer on the rear of the thick side cell block. The windows are sealed off to prevent electrolyte leakage using PTFE tape (3M). A variable rate peristaltic pump (Cole

Cathodic Deposition of Ru onto Pt/C Electrocatalysts

Figure 1. Schematic illustration of the specially designed flow-through style, spectro-electrochemical XAS cell.

Parmer Masterflex L/P) is used to circulate electrolyte. The cell is sealed with a single precut silicone gasket (Auburn Chemical Co.), which is placed between the two cell halves. This cell configuration is particularly advantageous as it allows the user to oxygenate/deoxygenate the electrolyte or introduce poisons during the experiment, which was not possible with other cell models. 2.3. In Situ XAS Data Collection. The Pt WE were prepared by loading 4.97 mg cm-2 (metal loading) of 30% Pt/C (same lot as above) onto carbon weave and cutting into 3.5 cm × 0.5 cm size pieces. The electrode preparation method has been described in detail elsewhere.49 Briefly, the catalyst suspension was made by mixing preweighed catalyst powder with 1:1 deionized H2O/2-propanol and 5 wt % Nafion with a catalyst weight to Nafion weight ratio of 95:5. The Pt loading was chosen to yield an absorption cross section of ∼1. Prior to cell assembly, the electrodes were wetted via vacuum impregnation in 1 M HClO4. The cell also consisted of a prewashed (0.5 M H2SO4, 80 °C followed by DI H2O, 80 °C for 2 h) Grafoil (GrafTech International Inc.) CE, which was situated directly across from the WE. Grafoil was chosen as a CE because it is inert and does not significantly attenuate the X-ray beam. In all cases, 0.1 mm thick Au foil (99.999%, Alfa Aesar) was used as current collectors for both electrodes. The electrolyte inlet and outlet tubes (Viton, Cole Parmer) connected to the cell terminated in a 150 mL beaker containing the 1 M HClO4 or 1 M HClO4 + 2.0 mM Run+ electrolyte(s). An RHE was seated into a salt bridge sealed off with a vycor frit (BAS) to minimize Run+ ions migrating into the RHE. The entire RHE assembly was placed into the external electrolyte beaker where it was in ionic contact with the inside of the cell via the inlet/outlet tubes and salt bridge tube. Experiments were performed at beamlines X-18B, X11-A, and X-11B at the National Synchrotron Light Source, Brookhaven National Laboratory, Upton, NY. Prior to the start of each experiment, the working electrodes were activated by potential cycling (0.05 to 1.2 V vs RHE at 20 mV s-1) in clean 1 M HClO4 until a steady state CV was obtained. Full range Pt L3 EXAFS were collected (-200 eV to 18 k) with the working electrodes fixed at various static potentials as described by the following scheme: (i) xV, Ar sat. 1 M HClO4; (ii) xV, O2 sat. 1 M HClO4; (iii) xV, O2 sat. 1 M HClO4 + 2.0 × 10-3 M Run+ (1 h); (iv) xV, Ar sat. 1 M HClO4 (return scan). The scans

J. Phys. Chem. C, Vol. 114, No. 2, 2010 1031 recorded at open circuit observed an OCP between 0.9 and 0.95 V. Throughout the progression of the scans, potential control was maintained at all times. This was achieved as the electrolyte could be easily exchanged by draining the external reservoir of its contents down close to empty and replacing with the next electrolyte without having the RHE become disconnected. Prior to step (iv) above, the cell would be filled and drained with clean 1 M HClO4 three times (∼400 mL) to remove trace Ru contamination. Data were collected in transmission mode using the typical three gas ionization detector setup (I0, It, and Iref) with a nominal N2/Ar mixture to allow for 10% photon absorption in I0, 50-70% in It and Iref. The sample was placed between I0 and It, while Pt reference foil (4 µm, Alfa Aesar) EXAFS was collected between It and Iref. At each beamline, a Si(111) monochromator was employed and detuned by 40% to remove higher harmonics. 2.4. EXAFS Analysis. All EXAFS analyses were performed using the IFEFFIT suite61 version 1.2.9 (Copyright 2006, MatthewNewville,UniversityofChicago,http://cars9uchicago.edu/ ifeffit/). All scans were carefully aligned and calibrated using the reference foil to account for any changes in beam energy throughout the course of the experiment. Background subtraction and normalization was performed using the AUTOBK62 algorithm in Athena (Bruce Ravel, Copyright 2006), a subroutine of IFEFFIT. The normalized EXAFS data were then imported into the ARTEMIS program where EXAFS fits were carried out using a k-range window of 2.0-15 Å-1 (Kaiser-Bessel) and an R-window of 1.5-3.5 Å. 2.5. ∆µ Analysis. The ∆µ procedure has been described in great detail elsewhere.63–66 Briefly, all XAS scans are carefully aligned and normalized over a much more narrow energy range (∼25-130 eV) to only consider the XANES region. Difference spectra are calculated on the basis of the normalized XANES using the relationship:

∆µ ) µ(Pt-xelect., V)-µ(Pt-Ar, clean, V)

(1)

where µ(Pt-xelect., V) is the XANES at a particular electrode potential in either a clean or a Ru contaminated electrolyte (Ar purged), and µ(Pt-Ar, clean) is the reference scan in clean electrolyte at the same potential. This scan is chosen as the reference so as to remove any other electrode processes occurring simultaneously (i.e., H2O activation) and emphasize only the effect of Run+. This of course assumes that the O(H) adsorption levels are about the same with and without Ru, which is not necessarily true, but we will see below that this a reasonable assumption below 0.8 V. Experimental ∆µ spectra are then compared to theoretical ∆µ signatures calculated using the FEFF 8.0 code.67 This is achieved by calculating theoretical XANES curves using small Pt clusters, in this case the “Janin” Pt6 cluster,68 with and without the adsorbate placed in various geometries. The resulting XANES can then be subtracted using the relationship:

∆µt ) µ(Pt6X)-µ(Pt6)

(2)

where µ(Pt6X) is the theoretical XANES for the Pt6 cluster with adsorbate X in a particular binding geometry and µ(Pt6) is that of the blank Pt6 cluster. It should be noted that care needs to be taken to ensure that scans are properly aligned prior to subtraction. Also, for optimal comparison to the experimental data, theoretical signatures are sometimes shifted by 1-5 eV and/or scaled by a multiplication factor. 2.6. Electron Spin Resonance. Nafion 117 membranes were first purified by heating to 75 °C for 1 h in 3% hydrogen

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peroxide followed by 1 h in deionized water, 1 h in 0.5 M sulfuric acid, and again 1 h in deionized water. The membranes were ion exchanged to varying extents by soaking in Ru nitrosylnitrate (Alfa Aesar) solutions ranging in concentration from 0.5 to 100 mM for 2, 5, and 7 days, respectively. Upon removal from the Ru solution, the membranes were rinsed to remove any surface Ru and to terminate the exchange. The swollen membranes were weighed to gravimetrically determine the total water uptake. Before and after the exchange process, the membranes were thoroughly dried and weighed, and the extent of exchanges was determined gravimetrically using a Cahn C-33 microbalance. ESR measurements were accomplished by soaking the exchanged membranes in 0.1 mM 2,2,6,6-tetramethyl-4-piperidone N-oxide (TEMPONE) spin probe in water. The ESR spectra were collected on a Bruker EMX X-band spectrometer. For each spectrum, three scans of 2048 points were averaged using magnetic field modulation of 0.02 mT at 100 kHz. The fitting method utilized a MATLAB (MathWorks)-based version of EPRLL, the slow-motional line shape program of Freed and co-workers,69,70 and was used to determine the correlation time (τc) of the spin probe by monitoring the rotational rate of the probe (R). The relation between τc and R is τc ) 1/6R, where τc is related to the local viscosity of the solution around the spin probe through the Stokes-Einstein relationship:

τc )

4πr3e η 3kBT

(3)

where η is the effective local viscosity, re is the hydrodynamic radius of the rotating probe, and kB is the Boltzmann constant. 3. Results and Discussion 3.1. Electrochemical Characterization. The cyclic voltammograms (CVs) shown in Figure 2a reveal significant changes to the Pt surface following contact with Ru contaminated electrolyte. This is clearly visible in all three regions of the CV: (a) the Hupd region, (b) the Pt-O formation/reduction region, and (c) the double-layer charge region. In region (a), it is evident that the total charge of the Hupd has decreased due to a loss of electrochemically active surface area (ECSA) as it has become blocked by Ru. Likewise, in region (b), the Pt-O formation and subsequent reduction is muted by the presence of adsorbed Ru. The increase in double layer capacitance in region (c) suggests the adsorbed Ru likely exists in the form of some RuOx species, which are known to exhibit higher capacitance19,20 than Pt. To examine the reversibility of Ru deposition, a series of experiments were performed on the contaminated surface. This is of interest because some research has suggested that Ru can be at least partly removed.9 Figure 2b shows the CV of Pt/C in clean electrolyte in comparison to that of the Pt/C after a “cleaning step”. The cleaning step was performed by running 200 CVs between 0.05 and 1.2 V, followed by 50 CVs between 0.05 and 1.4 V on the deposited catalyst in clean electrolyte. It is evident in the Pt-O formation/reduction region (b) that some Ru has been removed as the Pt-O peaks have become better defined. However, the double layer capacitance was still widened and the Hupd did not fully recover either, indicating that some Ru remains on the surface. The ECSA for each of the above-mentioned situations has been calculated and displayed in Table 1. The ECSA was determined by a common practice of integrating the Hupd charge (after subtracting the double-layer capacitance) and dividing by the value of 210 µC cm-2. A total of 24.7% loss of ECSA was

Figure 2. Cyclic voltammograms of 30 wt % Pt/C taken in Ar purged 1 M HClO4. The Pt/C loading was 14 µg cm-2 onto a 5.56 mm diameter glassy carbon RDE tip with a rotation of 0 rpm, collected at a scan rate of 50 mV s-1 at 20 °C. (a) CV prior to contamination in 2.0 mM Run+ contaminated HClO4 (solid line) and after spontaneous Ru adsorption (OCP, 30 min), rinsing (DI H2O), and return to clean 1 M HClO4 (dashed). (b) Clean catalyst CV (solid) overlaid with the CV after Ru cleaning step (dashed). The cleaning step involved performing 200 potential cycles between 0.05 and 1.2 V, followed by 50 cycles between 0.05 and 1.4 V of clean 1 M HClO4 with a scan rate of 50 mV s-1.

TABLE 1: Electrochemically Active Surface Area Determination Resultsa clean Pt after contamination after cleaning step

average ECSA (m2 g-1)

% change (m2 g-1)

62.3 ( 6.3 47.0 ( 7.4 49.4 ( 4.3

na -24.6 +5.11

a Uncertainties reflect the standard deviation in ECSA determined as a result of performing at least three independent experiments.

observed as a result of Ru blocking Pt surface sites. Following the cleaning treatment, an increase in ECSA was observed (∼5%); however, clearly all of the Ru had not been entirely removed, even as the electrode had been cycled up to 1.4 V. This result is consistent with the observations of Piela et al.,9 who have observed partial Ru dissolution from contaminated cathodes when cycled to anodic potentials. The effects of deposited Ru on ORR are easily discernible by inspection of the ORR polarization curves in Figure 3. The clean Pt catalyst exhibits a commendable ORR activity with an onset potential ∼1.0 V and a well-defined diffusion limiting current as described by the Levich equation:

ilim ) 0.62neFAD2/3ω1/2ν-1/6Co

(4)

where ilim is the diffusion limiting current density, ne is the number of transferred electrons, F is Faraday’s constant, A is the electrochemically active surface area, D is the diffusion


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Figure 3. ORR polarization curves (anodic sweep) for 30 wt % Pt/C with a loading of 14 µg cm-2 on a 5.56 mm diameter glassy carbon disk in O2-saturated 1 M HClO4 with a 20 mV s-1 sweep rate at 900 rpm. The solid line represents the clean Pt/C prior to contamination, the dashed line has been exposed to 2.0 × 10-3 M Run+ and subsequently “cleaned” via the cycling procedure, and the dash-dot line was collected in 1 M HClO4 + 2.0 × 10-3 M Run+.

coefficient of O2 in the electrolyte, ω is the rotation rate of the RDE, ν is the kinematic viscosity, and Co is the concentration of O2. Although the ORR onset overpotential only increased by ∼15-20 mV going from the clean Pt to Ru deposited Pt, the decrease in ilim suggests a possible residual Ru presence blocking the Pt surface, and perhaps even an increased H2O2 production. Because of the difficulty in making reliable H2O2 measurements under these conditions, the increased H2O2 production cannot be confirmed. The ORR curve also shifts negative in the mixed kinetics mass-transport region as a result of some loss in activity. When ORR was conducted in Ru contaminated electrolyte (dash-dot line), the overpotential increased by more than 200 mV, and no discernible ilim was obtained. We believe this to be the result of a dynamic equilibrium established between continuing Ru adsorption occurring alongside ORR. The Tafel plots presented in Figure 4 were transformed from the ORR curves in Figure 3 after being treated by the mass transport correction equation:

ik ) ilim × i/(ilim - i)

(5)

where ik is the kinetic current, ilim is the diffusion limiting current described by eq 4, and i is the measured current during the ORR polarization (anodic sweep). Overall, the shapes of the Tafel curves remain relatively unchanged, indicating that there is no major change in the ORR mechanism, such as an increase in the H2O2 pathway.71 Although the Tafel slope fitting is not shown in the plot, values obtained were all close to the typical values of -60 mV decade-1 (high E region) and -120 mV decade-1 (low E region) for ORR on Pt. The decrease in the ORR activity observed in the Tafel curves reflects the increase in ORR overpotential as a result of adsorbed Ru, which is consistent with a site-blocking process.71 Thus, when considered collectively, the above electrochemical methods suggest Ru adsorption occurs on Pt in a fashion that could cause a significant decrease to the ORR kinetics. The following sections focus on determining the Ru adsorption site(s). 3.2. EXAFS Analysis. It is standard practice to fully analyze the extended X-ray absorption fine structure (EXAFS) prior to any ∆µ XANES analysis. The reason for this is to ensure that no major changes in Pt-Pt bond length occur under the given experimental conditions. The ∆µ-XANES analysis relies on

Figure 4. Mass transfer corrected Tafel plots shown at 900 rpm for the ORR polarization curves presented in Figure 3. Because of the changing active surface area, we utilize only geometric surface area for current density normalization. Pt loading was 14 µg cm-2.

crystallographic modeling using consistent bond lengths to generate realistic ∆µ simulations. The EXAFS data processing involves a normalization/background removal process using a background spline function (AUTOBK)62 in the ATHENA code.72 Once normalized, the EXAFS is imported into ARTEMIS, where the physical parameters are elucidated via leastsquares fitting. A representative fit is shown in Figure 5a,b for Pt/C at open circuit potential in 1 M HClO4 + 2.0 mM Run+. Although evidence suggests that deposited Ru exists on the surface (as will be illustrated by the ∆µ analysis below), it is not directly visible to the Fourier-transformed EXAFS (FTEXAFS) because it is too low in concentration and would likely be located in the region of the main Pt-Pt scattering (∼2.5 Å); nevertheless, the effects of the Ru deposition are evident in the NPt-Pt values. Table 2 offers a summary of EXAFS parameters obtained by the methods described above. To ensure a valid comparison of coordination numbers (NPt-Pt), the best value of σ2 (meansquare radial disorder) was fixed (5.05 × 10-3 Å2) and used for all fits. Although no significant changes to the Pt-Pt distance were observed, small changes in NPt-Pt were observed as the particles tend to distort when adsorbates are present as the Pt-Pt scattering near the surface is altered by adsorbates.49,52 These changes are illustrated more clearly in Figure 6 (left axis). The value of NPt-Pt of 7.6 appears to represent the clean cluster. The scan taken at 0.3 V shows a small increase in NPt-Pt, typical of that seen after H adsorption, and again a small increase at 0.80 V, which is typical of that seen when atop O(H) adsorption occurs. We have shown many times previously50,52 that atop anion (e.g., O(H)- and Cl-) adsorption generally increases NPt-Pt, and 3-fold adsorption decreases it. The increase occurs as a result of the overall morphology of the nanoparticle becoming more spherical in the presence of atop adsorbates and 3-fold adsorption generally directly decreases the Pt-Pt scattering. Note that the NPt-Pt values after Ru deposition are larger than for the clean at 0.3 and 0.4 V, smaller at 0.5 V, and then larger again at 0.7 V, that is, exactly opposite that expected for an anion. We have previously noted49,51 that cation or neutral species adsorption (H+ and S) even in n-fold sites (n ) 2 or 3) can increase NPt-Pt and apparently in 3-fold increase it. Thus, the changes in NPt-Pt are consistent with that expected for cation/neutral adsorbates in 3-fold sites at low coverage, except at 0.5 V when adsorption occurs more in atop sites as indicated by the ∆µ line-shapes discussed below. Interestingly, both values of NPt-Pt decrease at OCP because of some O adsorption in 3-fold sites as expected,

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Figure 6. Plot of NPt-Pt (solid lines, left axis) for Pt/C in 1 M HClO4 plotted as a function of potential. Also shown are the Ru ∆µ magnitudes (eq 1) for Ru deposition on Pt (dashed line, right axis). The dominant Ru adsorption site (n-fold or atop) as indicated by the ∆µ spectral lineshape is also given.

Figure 5. (a) Pt-L3 edge EXAFS spectrum (Kaiser-Bessel window 2.0 < k < 15 Å-1, k2 weighted) and corresponding least-squares fit for 30 wt % Pt/C in 1 M HClO4 + 2.0 × 10-3 M Run+ fixed at 0.80 V. (b) Fourier transformed EXAFS, fitting was performed in R space using a single shell Pt-Pt scattering path and a Kaiser-Bessel window (1.0 < R < 3.5 Å, k2).

TABLE 2: Summary of EXAFS Parameters Derived from First-Shell Fits NPt-Pt ∆N ) (0.28a

RPt-Pt, Å (∆R)

E0, eV (∆E0)

σ 2, Å 2 ×103

OCP clean Pt 1 h Ru exposure

7.16 6.24

2.73 (-0.045) 6.35 (0.77) 2.73 (-0.038) 7.20 (1.20)

5.05 5.05

0.80 V clean Pt 1 h Ru exposure

7.57 7.71

2.73 (-0.039) 7.29 (0.75) 2.73 (-0.040) 7.19 (0.66)

5.05 5.05


7.41 7.49

2.73 (-0.041) 7.21 (0.66) 2.73 (-0.041) 7.29 (0.71)

5.05 5.05


7.63 7.39

2.73 (-0.042) 7.62 (0.76) 2.73 (-0.039) 7.22 (0.73)

5.05 5.05


7.64 7.71

2.73 (-0.040) 7.59 (0.60) 2.73 (-0.039) 7.39 (1.05)

5.05 5.05


7.77 7.84

2.73 (-0.039) 7.86 (0.81) 2.73 (-0.040) 7.32 (0.97)

5.05 5.05

a Value represents the largest statistical error of all of the least-squares fits determined by ARTEMIS. NPt-Pt was calculated using the FEFF8 value of S20 (0.934) for Pt L3 edge.

but now the largest change in NPt-Pt between the clean and Ru deposited results also exists due to significant Ru deposition. It is a bit surprising, however, that now the Ru mostly adsorbed in 3-fold sites additionally decreases NPt-Pt relative to the clean. This may provide information about the charge on the Ru species in the presence of coadsorbed O atoms (i.e., less

Figure 7. Pt L3 edge ∆µ ) µ(2.0 × 10-3 M Run+, OCP) - µ(clean, 0.50 V) spectra for 30 wt % Pt/C using the µ obtained in 2.0 × 10-3 M Run+ in 1 M HClO4 at open circuit, and 0.50 V in clean HClO4.

positively charged and behaving more as an additional “anion”) at these potentials. The preferred Ru deposition site will be discussed in greater detail in the following section. 3.3. Experimental ∆µ Analysis. The ∆µ curves presented in Figure 7 illustrating spontaneous Ru deposition were constructed according to eq 1, where the electrode was maintained at open circuit and the electrolyte was 2.0 mM Run+ in 1 M HClO4 unless otherwise noted. After 20 min of exposure, a positive peak developed approximately 5 eV past the Pt L3 edge with a magnitude of ∼4% of the total XANES signal. The subsequent scans at 40 and 60 min reveal a negative dip that precedes the larger positive peak in the same region as the former. This can be explained by two separate, simultaneous processes: changes in the ∆µ magnitude typically indicate an increase of adsorbate surface coverage (or decrease depending on the direction of the change), and the modification of the line shape suggests there is an adsorbate binding site transformation. Interestingly, the “return” scan reveals little change in the ∆µ spectrum despite the cell being drained of all Ru contaminated electrolyte and rinsed with clean HClO4, although no cyclic potential scanning was performed. This suggests that adsorbed Ru is stable on the Pt, at least in the context of these experimental conditions. To determine the Ru binding site(s), the overall line shapes require modeling with FEFF8.0 and shall be discussed in full detail in section 3.4.



Figure 8. Pt L3 edge ∆µ ) µ(2.0 × 10-3 M Run+, xV) - µ(no Run+, 0.5 V) spectra for 30 wt % Pt/C taken after 60 min exposure to Run+ contaminated 1 M HClO4.

The ∆µ procedure was also used to investigate Ru deposition on Pt electrodes where potential control was maintained throughout the duration of the XAS measurements. The objective was to mimic the situation that fuel cell cathodes are subject to when operated under a constant load in the presence of Ru contamination. The results are presented in Figure 8. The ∆µ curves (calculated by eq 1) for the Pt/C electrodes were subject to Ru contamination for 60 min at the indicated static potentials and again flushed with clean electrolyte. For the sake of clarity, each curve in the figure was offset on the ∆µ axis by an increment of 0.02. All ∆µ magnitudes here are e0.02, indicating relatively low Ru adsorbate coverage. The spectral line-shapes reveal some inconsistencies that will be discussed in the theoretical section below. For further analysis, Figure 6 plots the amplitude of the experimental ∆µ (right axis) obtained from Figure 8, which signifies the change in relative Ru coverage with potential, along with the NPt-Pt as already discussed. While under potential control, the ∆µ amplitude reaches a maximum at 0.5 V, then decreases with potential until a sharp increase is observed at OCP. There are two factors that seem to affect Ru deposition as suggested in the Figure, the coverage of other adsorbates and Coulombic forces between Run+ in the electrolyte and already adsorbed species. The large deposition at OCP seems reasonable as one would expect that Run+ ions are not particularly attracted to a positively charged Pt surface, but as the oxide forms above 0.8 V, the Run+ ions are attracted to the negatively charged O atoms in the oxide layer and eventually co-deposit on the surface. To show that O(H) adsorption is still occurring on Pt with Ru present, and to determine the effect of this Ru on O(H) adsorption, we calculated the ∆µ in Figure 9a using the relationship:

∆µ ) µ(xV, Ru)-µ(0.5 V, Ru)

(6)

to isolate the Pt-O interactions by consequently subtracting out any Ru contributions. The value of 0.5 V was used as it resides in the double-layer region where typically no adsorbates are present. Recall that the 0.8 V line in Figure 8 did not reveal any Pt-O signature because it had been subtracted out. That particular ∆µ was calculated using the clean reference at the same potential (eq 1), which as mentioned above cancels out any process unassociated with Ru adsorption. The obtained line shapes in Figure 9a do indeed indicate the presence of adsorbed O, but, however, no Ru because that was removed by the difference. The ∆µ lines in Figure 9a indicate normal H2O

Figure 9. (a) Pt L3 edge O-adsorption ∆µ ) µ(2.0 × 10-3 M Run+, xV) - µ(2.0 × 10-3 M Run+, 0.5 V) spectra for 30 wt % Pt/C taken after 60 min exposure to Run+ contaminated 1 M HClO4. (b) Maximum magnitude of similar O ∆µ versus potential under three different indicated conditions: that is, when the 1 M HClO4 electrolyte was deoxygenated with Ar, when saturated with O2, and when saturated with O2 after 60 min of Run+ exposure. The shaded arrows indicate the dominant adsorbate as reflected in the ∆µ spectral line-shape and discussed in the text.

activation (atop Pt-O(H) at 0.7, 3-fold 0.8 V) and are in agreement with previous observations.50 Note that the ∆µ taken at 0.7 V reveals an additional shoulder (noted as OH(near)) about 2 eV to lower energy. This has been observed many times previously73–76 when Ru islands exist on the Pt surface and is attributed to OH bonded to Pt at sites next to the RuOx islands. It arises because of an electronic effect exerted by the RuOx on the nearby Pt atoms shifting the core-level binding energy and hence shifting the energy of the ∆µ feature. Similar O(H) ∆µ spectra (not shown) using eq 6 but taken before Ru deposition do not show this additional feature. The effects of Ru deposition on the oxide coverage are also observed (see Figure 9b). The absolute magnitudes of the oxide ∆µ line-shapes similar to those in Figure 9a (using eq 6) are plotted as a function of electrode potential. In the case of deoxygenated 1 M HClO4, H2O activation proceeds as previously observed in many of our studies.49,50,76 The O(H) coverage steadily increases with potential; the ∆µ line-shape below 0.7 V reflects atop OH, then 3-fold O, and finally above 0.8 V that of an oxide (with subsurface O). The data in 1 M HClO4, saturated with O2, show a similar trend; only the atop coverage increases much faster due to atop O adsorbed at cluster corner and edge sites. Such lower coordinated Pt sites are known50 to be more reactive with O2, and the adsorbed O in such sites will exhibit a lower coordination with Pt (i.e., atop-like). These Pt sites probably do not participate in the ORR (because they are

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Figure 10. FEFF 8.0 generated ∆µ ) µ(Pt6-Ru, site) - µ(Pt6) theoretical spectra for the indicated Ru adsorption sites. The Pt-Ru bond distances used were ∼2.6 Å.

Figure 11. Comparison of ∆µ spectra obtained after 60 min exposure in Run+ contaminated HClO4 with the theoretical 3-fold fcc adsorbed Pt6-Ru cluster.

blocked by strongly adsorbed O), but this adsorbed O on the corners/edges appears to decrease strongly the amount of 3-fold O on the Pt(111) planes above 0.7 V, which may enhance the ORR rate on those sites. Such differences between the in situ and operando O(H) coverage have been considered previously77 and shall not be further discussed here. The subsurface O appears similarly above 0.8 V with and without the presence of O2. The effect of Ru deposition is quite interesting: (a) It appears to slow O from going subsurface to form the oxide; (b) it definitely hinders atop adsorption below 0.7 V, apparently because the Ru and O compete for these sites; and (c) it appears to exert a ligand effect around 0.7 V, although it is difficult to separate the competing effects of atop O and Ru on the surface. However, it is clear from the ∆µ data just above 0.7 V that deposited Ru in the presence of O2 can increase the adsorption of O(H), which blocks sites for the ORR. Thus, the data here suggest that Ru adsorbs on Pt to block surface sites for H2O activation, however, not completely as O(H) is still visible using eq 6. To determine where Ru adsorbs on the Pt surface, FEFF8 modeling is provided below. 3.4. FEFF Modeling. As seen above, the ∆µ spectral lineshapes can provide valuable evidence of the binding sites of various adsorbates such as H, OH, and O on a surface using previously modeled ∆µ spectral line-shapes for O adsorption. No direct line-shape assignments can be made without first theoretically simulating the adsorption event, which has not been previously performed for the Ru ∆µ signature. Therefore, theoretical XANES modeling (and hence ∆µt) was performed for Ru/Pt using the spatial coordinates of the Janin Pt6/Pt6-Ruads clusters,68 along with the appropriate input parameters (HedinLundqvist potentials, NOHOLE card, etc.), and evaluated for full multiple scattering by FEFF8.0. The Ru-∆µ signatures for the commonly used 1-fold, 2-fold, and fcc 3-fold adsorption sites are presented in Figure 10. The 1- and 2-fold Ru line shapes are too similar and therefore will be treated as impossible to distinguish experimentally. The 3-fold signature, on the other hand, contains a negative feature just preceding the edge position, followed by a large positive peak that is in very close resemblance to the OCP scans in Figure 7 taken with a Ru exposure time >20 min. To further exemplify this, an overlay plot of the experimental OCP scan and the 3-fold theory curve are provided in Figure 11 for visualization purposes. We believe this to be compelling evidence that spontaneous Ru deposition occurs primarily in 3-fold geometries when Pt (along with adsorbed O) is allowed to remain at OCP in the presence of Run+ ions. Similar findings have also been established in the literature by means of voltammetry and AES. For example, a recent report

by Bonilla et al.36 revealed surface concentrations (θ) of Ru using the decreased Hupd charge and assuming that each Ru3+ adsorbed onto three Pt sites. Their θ values were comparable to those obtained using AES,45 which supports the 3Pt:1Ru ratio that we have described above. In further consideration, the ratio of Ru:Pt by this model is 1:3 or 0.33 ML. This value is consistent with many of the Ru coverage values observed in the literature as indicated in Table 3. Interestingly, the ∆µ at OCP in Figure 7 taken at a Ru exposure time of 20 min resembles the line-shape of the 1- or 2-fold Ru adsorption signatures. It is entirely reasonable to suggest that initially Ru adsorbs in lower coordinated sites (1 or 2) likely when O coverage is low, and subsequently fills in the 3-fold sites as more Ru adsorbs. However, in Figure 8, the ∆µ scan at 0.5 V, where the coverage is largest under potential control, reveals 1- or 2-fold spectral line-shape (atop or bridged), while those at lower coverage reflect a 3-fold fcc line-shape. Together, these data suggest that Ru prefers to deposit on atop/ bridged sites on clean Pt, but on 3-fold sites when coadsorbates (e.g., H, O(H)) are present. This is not surprising when one considers that the Hupd at low coverage (i.e., that at potentials above 0.3 V)78 and the OH adsorbed on the atop sites leave only 3-fold sites available for Ru deposition. When the H and OH coverage gets larger (at lower and higher potential, respectively, excluding OCP), Ru deposition apparently does not occur at all or only occurs to an extent that is undetectable by the ∆µ-XANES method. Likewise, at OCP, the Ru initially deposits on atop sites (probably along the corners/edges of the Pt clusters, see Figure 7) on O covered Pt (the O takes the 3-fold sites) and then moves over to the 3-fold sites at higher coverage. Thus, the Run+ deposition on an O covered Pt surface behaves remarkably similar to H adsorption on clean Pt. That is, adsorption occurs initially on corner/edges in atop sites and later on 3-fold sites because of lateral interactions.79 By comparing the ∆µt signatures to the experimental data, we believe that Ru initially deposits on clean Pt at 1,2-fold sites, followed by 3-fold fcc sites when the coverage is high. When coadsorbates are present, the ∆µ values suggest 3-fold adsorption is also preferred. 3.5. Deposition Time Dependence and Coverage. Finally, Figure 12 shows the time dependence of the Ru deposition at both OCP and 0.5 V. It is clear that the amount of Ru adsorbed at OCP is much larger than under potential control at 0.5 V. The marked difference in this time-dependence may also be reflecting a different deposition mechanism at the two potentials. At OCP, the Coulomb enhanced deposition (i.e., attractive



TABLE 3: Summary of Estimates of Ru Adsorption Coverage on Various Pt Catalysts type of electrode Pt film electrode Pt/C (E-TEK), 4.2 nm Pt(111) Pt(100) Pt(110) Pt(111) Pt(111) Pt(100) Pt(110) quartz supported Pt electrode Pt black Pt(111) Pc platinum wire Pt/C (E-TEK), 3.5 nm

deposition method spontaneous spontaneous electrochemical spontaneous spontaneous spontaneous electrochemical spontaneous spontaneous electrochemical spontaneous (multiple) spontaneous (multiple) spontaneous spontaneous

Ru coverage (ML) 0.32, 0.55 0.22 0.46 0.20 0.21 0.10 0.12, 0.18a 0.31b 0.10 0.24 0.05 0.10 max ca. 0.5c 0.20, 0.25, 0.35, 0.40d 0.11, 0.39, 0.63, 0.13e 0.22-0.25f 0.10, 0.33g

technique

reference

EQCM EQCM

46 26

STM STM

31 30

AES

29

electrochemical

13

EQCM

17

ICP, electrochemical electrochemical electrochemical XAS

32 45 36 this work

a 50 and 500 µM solutions of RuCl3 + 0.1 M HClO4 (2 min). b 2 mM Ru(NO)(NO3)3 + 0.5 M H2SO4 after a single voltammetric scan. c 0.5 M Ru(NO)(NO3)3 + 0.1 M HClO4 at 0.05 V vs RHE. d 1 mM RuCl3 + 0.1 M HClO4 (1 h) after 1, 2, 3, and 4 successive voltammetric scans. e 10 µM to 1.0 M RuCl3 + 0.1 M HClO4 (10 s to 10 min); θRu of 0.13 obtained with H2 redn. f 0.5 mM RuCl3 + 0.2 M H2SO4 at OCP (2 min). g 2 mM Run+ ions prepared in 1 M HClO4 (see Experimental Section).

Figure 12. Relative coverage of Ru on Pt at 0.5 V vs OCP (ca. 0.9 V) by comparison of experimental ∆µ-magnitudes at the two potentials.

interaction between Oδ- ions on the surface and Run+ ions in solution) apparently occurs quite rapidly, reaching an asymptote already after about 40 min and yielding a logarithmic type plot. On the other hand, the deposition at 0.5 V appears to increase in a linear fashion as illustrated by the regression line, and therefore controlled by a different, possibly slower diffusion process near the surface. In any event, this plot reveals the detrimental effect of bringing a cathode to OCP relative to maintaining potential control. Not only is the total coverage enhanced at OCP, it reaches this larger coverage all together, in a relatively shorter period of time. Spontaneous deposition saturation times have been reported26 to be on the order of seconds to minutes depending on Ru concentration in the bulk electrolyte, although much larger periods of 60 min or more have been observed in this and other studies.17 These discrepancies can likely be explained by factors such as the presence of anions (i.e., chlorides, nitrates, etc.), effective surface area, size of solvated Run+ ions, and catalytic activity of the Pt surface. For example, any pre- or coadsorbed anions would impede available sites for Ru deposition, hence requiring longer deposition times, lower coverage, or both. Many of the cited investigations have been performed on Pt(hkl) surfaces, which would have much lower electro-active surface

area than nanoparticles, possibly resulting in longer requisite exposure times to achieve saturation. Also, it has been well established that the various Pt(hkl) surfaces have different catalytic activity in terms of ORR and adsorption80 and therefore would be expected to yield different results (see Table 3). Finally, as the size of the solvation sheath of the Run+ ion increases, it would likely increase deposition time and/or decrease coverage due to steric hindrance and lower charge density. Figure 12 provides an estimate of the Ru coverage. The theoretical signature here has a comparable magnitude with the amplitude of the experimental line. Note that the Ru atom is in a 3-fold site on the surface for this calculation. Therefore, every Pt atom should “see” approximately 3 Ru atoms (the model only had 1 Ru), so, at full coverage, the theoretical line-shape should be approximately 3 times larger. Assuming the intensity of scattering off of neighboring atoms varies directly with the number of such neighbors, the estimated experimental coverage then becomes 1/3 or ca. 0.33 monolayers (ML). This can be compared to 0.1 ML when under potential control after 1 h, although the coverage appears to be linearly increasing still after 1 h. Here, it is also worth drawing a comparison to the experimental Hupd data shown in Table 1, which revealed that ∼25% of the ECSA was lost during spontaneous deposition of Ru. This value is consistent with the ∆µ magnitudes analysis, suggesting it is a relatively reliable method of determining adsorbate coverage. These values compare quite well with other estimates in the literature (see summary in Table 3) for saturation coverage in the case of spontaneous deposition of Ru on Pt. An estimate of the coverage on nanoparticles with ∆µ-XANES is only possible at very high Pt dispersion (>50%), thus giving rise to quite unambiguous ∆µ-XANES signatures with sufficiently high data quality. The Ru coverage on Pt has been shown to occur in both monolayer and multilayer fashion.15,28,32,45 Using the ∆µXANES method of determining coverage on nanoparticles, it is not possible to determine the nature of the adsorbed layer except for determining the binding site of the first layer of atoms on the catalyst surface. To the best of our knowledge, this is the first estimate of Ru coverage on a Pt catalyst using XAS. It is commonly accepted that X-ray methods are inherently bulk-

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Figure 13. Plot of gravimetrically measured water uptake versus extent of Ru exchange in Nafion membranes. Data were fit with a linear trend with a slope of -4.3 and y-intercept 11.5.

Figure 14. Plot of correlation time, τc, versus extent of Ru exchange in Nafion membranes calculated from the rotational diffusion of Tempone spin probe measured using X-band ESR spectroscopy. Data were fit with a linear trend with slope 1.0711 × 10-9 and y-intercept 1.4037 × 10-9.

averaging techniques and that deriving such information from either interfaces or surfaces is rather difficult.35 However, we show in this study that with sufficiently good data and appropriately designed experiments, the ∆µ-XANES analysis makes it possible to obtain surface-sensitive information from XAS and that such an analysis may be of value in cases where only in situ measurements are realistic. 3.6. ESR Results. While much of this work has focused on the result of Ru deposition on the cathode after dissolution at the anode and subsequent crossover through the membrane, in this section, the effect of Ru crossover in the membrane is considered. Figure 13 shows that as more Ru enters the membrane, a nearly linear decrease in water uptake occurs. This is consistent with the findings of Lawton et al.58 and Ahmed et al.,81 where the presence of Al3+ and Fe3+ in the Nafion

membrane caused much lower swelling than lower valence cations. Their conclusion,58 considering the modeling works of Niemark and Vishnyakov82,83 wherein trivalent ions were shown to rigidify the membrane’s backbone and side chain regions due to ionic cross-linking,84 was that the membrane interactions with the trivalent ions hinder swelling and minimize solvation. Lawton et al.58 also looked at the effects of hydration in the membrane using a free volume model. They observed that lower hydration levels lead to a lower rate of probe rotation. Figure 14 depicts the effect of Ru exchange on the τc of the probe. On the basis of the Stokes-Einstein relationship in eq 3, this suggests that the microviscosity of the fluid state increases linearly with the presence of Ru3+ in the membrane. This could result in slower vehicular diffusion across the membrane as a

Cathodic Deposition of Ru onto Pt/C Electrocatalysts result of lower hydration levels and an altering of the membrane’s free volume by ionic interactions with the Ru. Reports of Al3+ ionomer exchange82,83 and an in depth study of Ca2+ contamination in the Nafion membrane85 have indicated that the multivalent ions have a higher binding affinity than protons to the anion groups in the membrane. This suggests that over the lifetime of fuel cell operation, higher levels of Ru could build up inside the membrane. Ion contaminants with higher valence have been shown to reduce the proton conductivity as well as dehydrate the membrane,86 which is consistent with our findings. This is of further concern as transition metals existing in the membrane have been found to catalyze radical attack on the membrane that leads to degradation.87 Even when present at lower levels, the trivalent Ru ions decrease the equilibrium hydration level and increase the microviscosity of the fluid state, which could alter the water management attributes of the membrane in the fuel cell. More studies need to be accomplished to fully understand the longterm effects of Ru cross over considering membrane degradation, proton conductivity, and vehicular diffusion. 4. Summary and Conclusions Cyclic voltammograms of Pt taken after exposure (at open circuit) to 2.0 mM Run+ contaminated HClO4 reveal a significant increase in double-layer charge capacitance, decreased Pt-O(H) formation/reduction, and lower Hupd charge when compared to the CVs taken in clean HClO4. An attempt to remove the spontaneously adsorbed Ru revealed that some of the Ru could be removed by potential cycling to 1.4 V vs RHE, however, not all of the Ru could be removed. Integration of the Hupd charge area indicated that adsorbed Ru decreased the ECSA by approximately 25%, of which only 5% could be reclaimed upon the cleaning procedure. These results suggest that spontaneously deposited Ru leached out from a DMFC anode could impart irreversible damage to a fuel cell cathode via catalytic site blocking, particularly if the Ru is allowed to remain in the cathode electrolyte. ORR polarization sweeps reveal an increased overpotential by approximately 20 mV after being subject to 2.0 mM Run+ for 30 min at open circuit potential. Although a diffusion limited current was obtained, the magnitude of the current was slightly decreased, supporting a site blocking theory. When the ORR polarization was performed in the Ru contaminated electrolyte (2.0 mM), the overpotential increased by ∼150 mV, and no diffusion limiting current was obtained. That curve was provided to mimic the situation that might occur on a DMFC cathode if the electrode was not able to be “cleaned” of its deposited Ru. The Tafel curves revealed a normal Pt response with slopes close to -60 and -120 mV/decade for clean Pt, spontaneously deposited Ru/Pt, and ORR in Run+ contaminated HClO4. The Tafel line shapes were relatively unchanged, consistent with the theory that there is no overall change to the rate determining step of the reaction. However, the decrease in E (as well as exchange current density) illustrates the adverse effect that Ru contamination has on the ORR kinetics. The EXAFS analysis revealed small but significant changes to the coordination number NPt-Pt as a function of potential and the presence of Ru. The largest change to NPt-Pt occurred when the clean Pt electrode was subject to Ru contaminated electrolyte at OCP. Smaller changes in NPt-Pt occurred when the electrode potential was maintained, but even these small changes could be related to atop versus 3-fold Ru deposition, supporting the ∆µ results. The large decrease in NPt-Pt at OCP (7.16 to 6.24) is reflective of O adsorption in 3-fold sites, as well as Ru

J. Phys. Chem. C, Vol. 114, No. 2, 2010 1039 deposition primarily in the 3-fold fcc sites at higher coverage, resulting from the Pt particles becoming more flat. The ∆µXANES analysis supports the EXAFS results and suggests Ru adsorption may proceed by an initial atop/bridge Ru adsorption followed by adsorption onto the 3-fold fcc sites of the faces, when the electrode is maintained at OCP as suggested by the FEFF8.0 line shape assessment. The ∆µ curves, for which potential control was maintained, reveal smaller magnitudes (

Fundamental Aspects of Spontaneous Cathodic Deposition of Ru onto

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