Ruthenium-Alloy Electrocatalysts with Tunable ... - ACS Publications

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Ruthenium-Alloy Electrocatalysts with Tunable Hydrogen Oxidation Kinetics in Alkaline Electrolyte Samuel St. John,† Robert W. Atkinson, III,† Raymond R. Unocic,‡ Thomas A. Zawodzinski, Jr.,†,§ and Alexander B. Papandrew*,† †

Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States Center for Nanophase Materials Sciences and §Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States

‡

S Supporting Information *

ABSTRACT: High-surface-area ruthenium-based RuxMy (M = Pt or Pd) alloy catalysts supported on carbon black were synthesized to investigate the hydrogen oxidation reaction (HOR) in alkaline electrolytes. The exchange current density for hydrogen oxidation on a Ptrich Ru0.20Pt0.80 catalyst is 1.42 mA/cm2, nearly 3 times that of Pt (0.490 mA/cm2). Furthermore, RuxPty alloy surfaces in 0.1 M KOH yield a Tafel slope of ∼30 mV/dec, in contrast with the ∼125 mV/dec Tafel slope observed for supported Pt, signifying that hydrogen dissociative adsorption is rate-limiting rather than charge-transfer processes. Ru alloying with Pd does not result in modified kinetics. We attribute these disparate results to the interplay of bifunctional and ligand effects. The dependence of the rate-determining step on the choice of alloy element allows for tuning catalyst activity and suggests not only that a lowcost, alkaline anode catalyst is possible but also that it is tantalizingly close to reality.

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INTRODUCTION

alternative HOR catalysts in alkaline pH is thus a prerequisite for the technological promise of AEMFCs to be fulfilled. Different theoretical frameworks within which to interpret the change in Pt activity as the pH changes from acidic to alkaline and routes to more active anode catalysts have been proposed.1,9,10 Strmcnik et al.9 describe the bifunctional nature of an anode catalyst under alkaline conditions requiring active sites for both the adsorption of H2 and the adsorption of OH− suggestive of hydrogen dissociative adsorption (Tafel step) as rate determining. The authors presented high activity for a catalyst with a surface composition of Ru 0.15 Pt 0.85 as confirmatory of their hypothesis.11 Durst et al.,1 alternatively, present evidence that electron transfer (Heyrovsky or Volmer step) is the rate-determining step (rds) in both acidic and alkaline conditions, giving Tafel slopes ca. 120 mV/dec on Pt, Ir, and Pd catalysts. Within this framework, the change in activity can be wholly described by changes in metal−hydrogen (M−H) binding energy and its unexplained increase under alkaline conditions. Under this assumption reducing the overall M−H binding energy, i.e., shifting M−H oxidation to more cathodic potentials, would lead to improved catalyst activity. Changes with pH of electrochemical descriptors of M−H binding energy are well described in the literature,12,13 and they have recently been shown to correlate with changes in HOR activity on monometallic Pt throughout the pH range 0−13.14

Efficient electrochemical systems are required for robust, costefficient generation and storage of renewable energy. Protonexchange membrane fuel cells (PEMFCs) are the leading technology for automotive and stationary electrochemical energy conversion at low temperature. In these acidic electrolyte systems, the exchange current density (i0) for the hydrogen oxidation reaction on Pt is ∼102 mA/cm2Pt.1,2 This extremely high intrinsic activity has enabled the use of remarkably low amounts of Pt catalyst in state-of-the-art PEMFC anodes with loadings of 0.05 mgPt/cm2geo or lower.3 In contrast, in acidic environments the most active cathode catalysts reported in the literature only achieve exchange current densities ca. 10−3 mA/cm2metal,4,5 and the use of significant amounts of Pt catalyst in PEMFC cathodes is normal. Anion-exchange membrane fuel cells (AEMFCs) have attracted renewed attention with the demonstration of stable, conductive, solid-polymer anion exchange membranes.6 AEMFCs can potentially eliminate the use of Pt metal catalysts entirely from fuel cells because there exist more economical catalysts that possess comparable activities under alkaline conditions yet are otherwise not viable in acid, such as silver.7 However, gains at the cathode are offset by kinetic losses at the anode, where the HOR activity is reduced by ∼102 mA/ cm2Pt;1,8 consequently, an AEMFC would require a 10−100fold increase in Pt loading to achieve the same current density as is achieved under acidic conditions. Identifying suitable © XXXX American Chemical Society

Received: April 5, 2015 Revised: May 20, 2015

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DOI: 10.1021/acs.jpcc.5b03284 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Representative HR-TEM images of the as-synthesized electrocatalysts: (a, b) Ru0.80Pt0.20, (c, d) Ru0.80Pd0.20, (e, f) Ru0.20Pt0.80, and (g, h) Ru0.20Pd0.80.

vapor-phase deposition onto as-received Vulcan carbon XC72R supports (∼43%) using acetylacetonate precursors and similar methods to those previously published.19−21 Solid Ru(acac)3, Pt(acac)2, and Pd(acac)2 precursors (SigmaAldrich) were mechanically mixed in their appropriate stoichiometric ratios with as-received Vulcan XC-72R (Cabot) in a glass vial. The vial was placed in a vacuum oven along with a separate vial containing deionized water (Milli-Q, Millipore). The oven was sealed then purged with ultrahighpurity N2, evacuated to 0.30 bar, and heated to 240 °C. During the thermal treatment, the liquid water vaporized, the organometallic precursors sublimed and then decomposed, and metallic nanoparticles were deposited onto the support. After 15 h at 240 °C, the oven was cooled to room temperature with a N2 purge. The cooled samples were transferred to a quartz tube for reduction in a 4%-H2-in-Ar atmosphere at 240 °C for 1 h. As-synthesized C-PGM powders (5 mg) were dispersed in 25% isopropanol solutions (4 mL) with 5 mg of Nafion from a 5% solution (Ion Power). Spectroscopy/Microscopy. X-ray Diffraction. X-ray diffraction (XRD) patterns were recorded with a Bruker Phaser D2 diffractometer using Ni-filtered Cu Kα radiation (λ = 0.154 184 nm, 30 kV, 10 mA, 0.014° step, 1.0 s/step) in the Bragg−Brentano geometry fitted with a 0.6 mm antiscatter slit in the incident beam and a 2.5° Soller slit in the diffracted beam. The position and width of diffraction peaks were obtained by fitting to Voigt functions using IGOR Pro (Wavemetrics, Inc.). Transmission Electron Microscopy. High-resolution transmission electron microscopy was conducted on a Hitachi HF3300 TEM/STEM (300 kV). Atomic-scale energy dispersive spectroscopy (EDS) maps were collected using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). X-ray Absorption Spectroscopy. X-ray absorption spectroscopy (XAS) was conducted at beamline 20-BM at the

There is little experimental evidence exploring the HOR rds on alloys in alkaline electrolyte. Wang et al.15 present experimental data and propose that Pt surfaces generate sufficient amounts of adsorbed OH− under alkaline conditions as to disregard the bifunctional pathway in favor of the modification of the M−H binding energy. This argument, however, rests on a correlation of CO oxidation to hydroxyl adsorption, and the potential for Pt−OH formation in alkaline electrolyte is at higher potentials than the prepeak (onset) region for CO oxidation on pure Pt. The CO oxidation in this onset region has been demonstrated to rely on an Eley−Rideal mechanism where the adsorbed CO reacts with bulk water (or OH−) and not as evidence that OH− has adsorbed onto the catalyst surface.16 The E−R mechanism is important for H2 oxidation under alkaline conditions and is the preferred pathway for the Volmer/Heyrovsky rds.1 The data presented by Wang et al. may then ironically provide strong electrochemical evidence in support for increases in activity that occur at electrochemical potentials prior to the peak of M−OH formation whereby activity differences are owed to modifications in the M−H binding energy. In this contribution, we investigate the nature of the HOR/ HER on practical, high-surface area carbon-supported ruthenium alloys with variable compositions of Pt and Pd. Ruthenium is a well-known alloying element for Pt, and its usefulness for alkaline HOR has been demonstrated.17 Data are infrequently presented concerning Ru-rich alloys with more noble metals because such catalysts are typically not active for heterogeneous reactions of interest. Gasteiger et al. investigated Ru-rich alloys with Pt for CO and H2 oxidation under acidic conditions;18 however, to the best of our knowledge, this is the first time a study has been conducted under alkaline conditions.

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EXPERIMENTAL DETAILS Nanoparticle Synthesis. Pure Ru, Pt, and Pd, in addition to RuxMy samples, were synthesized via a single-step, chemicalB


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data it can be inferred that all of the materials have nanosized grains (consistent with the HR-TEM images) and structures appropriate to the host lattice. Mass balance results from the synthesis process suggest that virtually all of the metal from the respective precursors was deposited onto the carbon support. Diffraction peak positions are indicative of degrees of alloying less than unity based on a simple Vegard’s law approach, consistent with random alloy formation and suggestive of some intraparticle atomic segregation. The shift for the pure Pd catalyst relative to the reference positions is related to interstitial carbon absorbed during synthesis.20,21 Most of this carbon was removed using postsynthesis H2 annealing. The remaining carbon was removed prior to HOR activity testing by electrochemically cycling the catalyst in N2-saturated, 0.1 M KOH until consistent Pd−H formation peaks were observed. Hydrogen and carbon compete for the same interstitial sites;32,33 therefore forming Pd−H will displace the carbon from the Pd lattice. It is worth noting that electrochemical cycling may change the degree of alloying during electrochemical testing. The XRD patterns, therefore show only clear evidence that the as-synthesized nanoparticles possess the appropriate phase and expected nanosize. The forward Fourier transforms of the postedge oscillations, so-called EXAFS, are used to determine the interatomic distances in the nanoparticles and are illustrated in Figure 3a,b. The monometallic Pt and Pd nanoparticles have first-shell

Advanced Photon Source at Argonne National Laboratory (Argonne, IL). Pre-edge correction, normalization, and postedge subtraction via spline fitting were done in Athena.22 The k2-weighted χ(k) forward Fourier transform (FT) parameters were k-weight, 0.5; window, Hanning; k-range, 22−16 Å−1. The k2-weighted χ(R) backward FT parameters were R-range, 1.8−3 Å; window, Hanning. Atomic first-shell scattering paths were simulated for tetrahedral geometry (facecenter, close-packed systems) for the Pt, Pd, and Ru using FEFF23 and were fit to the EXAFS data in Artemis to determine scattering path lengths.24 Electrochemical Characterization. Electrocatalysts were tested at room temperature in a standard three-electrode electrochemical cell (Pine Instruments) with a double-junction Ag/AgCl reference electrode (Pine Instruments) and Pt-wire counter electrode in H2-saturated, 0.1 M potassium hydroxide (semiconductor grade, Sigma-Aldrich). Electrodes were made by depositing well-dispersed catalyst inks onto glassy-carbon electrodes (A = 0.196 cm2) with rotation while drying.25 Polarization curves were obtained over the range −0.1−1.025 V vs RHE. Potentiostatic electrochemical impedance spectroscopy (EIS) spectra were recorded from 200 kHz to 1 Hz at 0.6 V vs RHE with a 10 mV sine perturbation amplitude under masstransport-limited conditions and 1 atm of H2. The highfrequency intercept of the EIS spectrum was used to eliminate the ohmic resistance of the electrochemical cell from the polarization curves. Electrochemical active surface areas (ECSAs) were obtained using Cu-stripping techniques on electrodes with ∼25 μg Pt-group metal/cm2.26 ECSA has been measured on Pt, Ru, and Pd catalysts by a Cu-stripping method with a charge density of 420 μC/cm2metal.26,27

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RESULTS AND DISCUSSION Vapor-phase deposition yields highly dispersed, uniform catalysts with high activity from as-received materials in a single step that may provide benefits in contrast to solutionphase reduction where extensive pre- and post-treatments are necessary.28−30 HR-TEM micrographs in Figure 1a−h illustrate that the as-synthesized catalysts are nanosized and uniformly dispersed over the carbon support. These results are consistent with previous applications of this synthesis technique.19,20,31 We investigated the grain size and alloying degree for assynthesized materials using XRD (see Figure 2a,b). From these

Figure 3. Forward Fourier transforms of the EXAFS for the RuxPty (a) and RuxPdy (b) at the Pt L2 and Pd K edges, respectively. Data for the reference foil (black ◆) and their respective pure metal nanoparticles (orange ×) are included for comparison to the 20% Ru (green +) and 80% Ru (blue ●) nanoparticles. Gray guides illustrate foil EXAFS maxima. First-shell scattering path distances are reduced in the alloys where greater shifts are observed in the Ru-rich particles.

neighbors at the same radial position as that for their respective foils. Simultaneous edge fitting at the Pt L2,3 and Ru K edges, as well as at the Pd and Ru K edges, was conducted to determine self-consistent first-shell scattering path distances in the alloy nanoparticles. The results and quality of the fitting, given respectively in Supporting Information Table 1 and in k-space fits in Supporting Information Figure 1, show that the nearestneighbor distances are reduced upon alloying with Ru and that all of the first-shell scattering paths for each nanoparticle exhibit similar nearest-neighbor distances. Small differences are likely due to local enrichment of one element that is consistent with the random nature of alloy formation for this vapor synthesis technique. HAADF-STEM EDS confirms the atomic mixing of the as-synthesized nanoparticles (Supporting Information Figure 2).

Figure 2. Cu Kα XRD patterns for different RuxMy alloys and reference spectra on Vulcan carbon. Pure metal peak positions with relative intensities (gray sticks) included for reference (Ru: PDF 00006-0663; Pt: PDF 00-004-0802; Pd: PDF 00-005-0681). C


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cathodic shifts of the HUPD charging currents for the alloys imply that H is bound less strongly on the alloy surfaces than on the monometallic surfaces. Alloying shifted the primary M− HUPD oxidation peak, on average, lower by ∼43 mV for RuxPty and by ∼35 mV for RuxPdy. A shift of this magnitude and sign would result from a decrease in the M−H interaction strength by approximately 4.7 and 3.0 kJ/mol, respectively, for a oneelectron, reversible process. Such shifts have been suggested1 to yield a 3−7-fold increase in activity based on a Brønsted− Evans−Polanyi relationship between H-binding energy and changes in the activation energy for M−H oxidation where electron transfer is rate-determining. Hydrogen oxidation activity was measured under rotation at 2500 rpm at 10 mV/s (see Figure 4c,d). The resulting performance of the alloy catalysts was compared to the masstransport-limited current,34 calculated using eq 1:

When the HAADF-STEM, EXAFS, and XRD data are considered collectively, we can conclude that no surfaceenriched skins form on the alloy nanoparticles. The alloy results suggest some regions of partial enrichment of Ru, Pt, or Pd; however, random mixing persists throughout the structure. We, therefore conclude a surface morphology for the alloy nanoparticles reflective of the nominal composition. Similar morphological results were observed in the electrochemical data illustrated in Figure 4. Charging currents in N2-

ηdiffusion =

RT ⎛ i ⎞ ln⎜1 − ⎟ 2F ⎝ ilim ⎠

(1)

The HOR activity data for the pure Ru, Pt, and Pd catalysts are similar to those reported in the literature for monometallic nanoparticles,35 indicating that the as-synthesized catalysts made using our vapor-impregnation technique perform in a manner consistent with a clean, active nanoparticle surface. Additionally, the data suggest enhancement of the HOR activity by Ru incorporation with Pt or Pd. A Tafel analysis was used to determine the reaction rds for the HOR on the catalyst surfaces. Tafel plots were obtained by correcting the HOR branch of the activity data by subtracting the diffusion overpotential and determining the kinetically limited HOR currents.8 The reaction order was determined by analyzing normalized currents at several rotation speeds (400, 900, 1600, and 2500 rpm) using eq 2.

Figure 4. Electrochemical data collected in static, N2-saturated, 0.1 M KOH collected at 50 mV/s (a, b) or positive-going scans in H2saturated, 0.1 M KOH at 2500 rpm and 10 mV/s (c, d) for RuxPty (left) and RuxPdy (right). The M−H desorption peaks are demarcated with vertical lines (a, b) and indicate a shift in M−H oxidation peak relative to monometallic Pt or Pd, suggesting reduced H-binding energy. Shifts are determined relative to the monometallic nanoparticle of whichever element is richest in the alloy, e.g., Ru0.8Pt0.2 vs Ru or Ru0.2Pt0.8 vs Pt.

m ⎡ i ⎤ i = i k ⎢1 − ⎥ ilim ⎦ ⎣

saturated, 0.1 M KOH are consistent with alloy phases of Ru with Pt or Pd. The electrochemical data show the influence of Ru on the charging behavior of Pt (or Pd) with compositiondependent shifts of the double-layer charging region, especially in cathodic sweeps ∼0.5−0.75 V vs RHE. The double-layer capacitance for pure Ru is quite large with respect to either monometallic Pt or Pd, and the double-layer charge for the alloy nanoparticles decreases as Pt (or Pd) content increases. Further composition-dependent results were observed in the Cu-stripping currents (used to determine the electrochemically active surface area) and are given in Supporting Information Figure 3. The Cu-stripping currents for the alloy nanoparticles are intermediate to those obtained for their constituent monometallic nanoparticles and indicate consistency between surface and nominal compositions. Anodic shifts of the oxidation potential for adsorbed H on the alloy nanoparticle surfaces are observed in the N2-saturated, 0.1 M KOH charging data in Figure 4a,b. The initial behaviors of the Pt and Pd surfaces are consistent with surface disordered nanoparticles possessing a high concentration of stepped active sites.29 For example, H desorption from the stepped Pt (110) plane is resolved in the monometallic nanoparticle data and is shifted to more cathodic potentials upon alloying with Ru. For the Ru-rich species, we alternatively investigate the shift in the large and broad HUPD peak vs monometallic Ru. The consistent

(2)

The reaction order, m, is thus the slope from plots of log(i) vs the log(1 − i/ilim). We found reaction orders of ∼0.5 for the Ptcontaining catalysts and reaction orders of ∼0.2 for the Pdcontaining catalysts. Plots of kinetic current vs overpotential at all rotation speeds should be equivalent for the correct reaction order and were used to verify the results found using eq 3. The corrected kinetic currents collected at 2500 rpm, illustrated in Figure 4, were fit with a linear function to determine Tafel slope from the high current density region prior to the onset of the mass-transport limit. Additionally, both the HOR and HER branches of the corrected kinetic currents for the Pt, Pd, RuxPty, and RuxPdy nanoparticles were fit using the Butler−Volmer36 equation: i [mA/cm 2metal] = i0[e(αAF / RT )η − e(−αCF / RT )η]

(3)

The sum of the anodic (αA) and cathodic (αC) transfer coefficients is equal to one for a single-electron reaction at a given overpotential, η. For symmetric reactions, the anodic and cathodic transfer coefficients are both ∼0.5. Transfer coefficients can change throughout polarization;36 therefore conclusions drawn here with respect to reaction symmetry are limited to the regions where the kinetic currents are fit as illustrated in Figure 5a,b. The HOR branch of the corrected kinetic currents for the RuxPty nanoparticles were fit with the D


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transfer rds (Volmer/Heyrovsky).37 As illustrated in Figure 5, the RuxPty catalysts demonstrated a Tafel slope ∼30 mV/dec in accord with a Tafel rds, while the RuxPdy and Pd catalysts demonstrated a Tafel slope of ∼220 mV/dec similar to that on pure Pd. The performance characteristics for the tested nanoparticles are given in Table 1. They include the electrochemically active surface area (ECSA) determined by Cu stripping, the catalyst loading, exchange current density, specific activity at 50 mV overpotential, the mass activity per gram of Pt-group metal, and the anodic and cathodic transfer coefficients for the Butler− Volmer fits. The Tafel slopes were determined from linear fits to the kinetic current at high current density, and they are included for comparison to the given transfer coefficients and to literature results. Parameters based off of kinetic currents were not able to be determined for the Ru nanoparticle because it did not reach the mass-transport-limited current density. Transfer coefficients are not given for the Pt-alloy catalysts because they do not demonstrate an electron-transfer ratelimiting step. The approximate 2−3-fold increase in the exchange current density reported in Table 1 for the RuxPdy alloys vs Pd-only catalyst is consistent with the observed reduction in the M−H binding energy for the electron transfer rds revealed by the Tafel slope analysis. In Butler−Volmer kinetics, the transfer coefficient is a measure of the symmetry of the energy barrier for the forward and reverse reactions. Values range between 0 and 1 where transfer coefficients of 0.5 suggest a symmetric equilibrium.36 For Pt the energy barrier for the HOR/HER is nearly symmetric; however, the energy barrier is not symmetric for the Pd-containing catalysts. Low values of the anodic transfer coefficients, αA, and their corresponding high Tafel slopes suggest that hydrogen oxidation is more difficult than hydrogen evolution on the Pd-containing catalysts near the reversible potential. Difficulty for hydrogen oxidation, in addition to the apparent insensitivity to H2 (low reaction order) for the Pdcontaining catalysts, could be caused by the tendency of Pd to strongly absorb hydrogen according to eq 5.

Figure 5. Tafel slope analysis for RuxPty and RuxPdy in H2-saturated, 0.1 M KOH obtained at 10 mV/s and 2500 rpm. (a) Linear fits of the kinetic currents at high current density. RuxPty alloys exhibit Tafel slopes ∼30 mV/dec while RuxPdy alloys exhibit Tafel slopes ∼220 mV/dec. (b) Kinetic equation fits to the kinetic currents for either a Tafel limiting step (RuxPty alloys) or a Volmer/Heyrovsky limiting step (remaining samples).

rate equation for a Tafel rds for which the derivation has been provided by Krischer and Savinova,37 given in eq 4 ⎤ ⎡ e(2F / RT )η − 1 ⎥ i [mA/cm 2metal] = i0⎢ 0 ⎣ (θH + (1 − θH0)eηF / RT )2 ⎦

(4)

where θ H is the equilibrium H coverage. It is important to address the different types of adsorbed hydrogen discussed in the literature and consider if there is a different reaction intermediate for each type of ratedetermining step that we consider here. Historically, the HOR/HER equilibrium has been described with two reaction intermediates, namely hydrogen deposited at or below the equilibrium potential (so-called HOPD) and that deposited at or above the equilibrium potential (so-called HUPD).1 However, recent microkinetic modeling of the HOR/HER equilibrium has proven successful without invoking different species of adsorbed hydrogen.38 Additionally, HOR/HER exchange current densities have been determined directly from experimentally measured HUPD charge transfer resistance on monometallic Pt and Pd.1 If HOPD species need to be invoked to describe the HOR/HER equilibrium, then exchange current densities determined from HUPD charge transfer resistances would not match. Given this convincing evidence, we here interpret our results with respect to one type of adsorbed hydrogen, HUPD, for both Tafel and Volmer/Heyrovsky reaction intermediates. Recall our initial premise that a Tafel slope of ∼30 mV/dec is suggestive of a molecular hydrogen dissociative adsorption rds (Tafel) and a slope of ∼120 mV/dec would indicate an electron 0

Pd + e− + H 2O ⇋ Pd−H + OH−

(5)

Pd−H can form multiple hydride phases; alternatively, H can be subsurface absorbed or adsorbed.39 Hydride formation occurs readily under alkaline conditions40 and would provide an additional source of H attenuating the overall sensitivity of the HOR reaction to solution H2. Ru enhances the HOR activity of Pt electrocatalysts by lowering the activation energy of the Heyrovsky/Volmer step

Table 1. Catalyst Electrochemically Active Surface Area (ECSA), Loading, and Performance Characteristics for HOR Electrochemical Analyses tran coeff catalysts

ECSA (m2/g)

loading (μg/cm2disk)

exch cur, i0 (mA/cm2metal)

iη(50 mV) (mA/cm2metal)

iη(50 mV) (A/gPGM)

Tafel slope (mV/dec)

αA

αC

reaction order, m

Pt Ru Pd Ru0.80Pt0.20 Ru0.20Pt0.80 Ru0.80Pd0.20 Ru0.20Pd0.80

54.0 148 44.3 84.3 49.0 117 71.5

6.95 6.43 6.43 7.01 7.09 7.04 7.06

0.490 n/a 0.050 0.410 1.42 0.123 0.148

0.518 0.173 0.084 0.472 0.834 0.151 0.192

280 256 37 397 409 177 138

127 n/a 203 30 35 241 219

0.46 n/a 0.28 n/a n/a 0.24 0.27

0.54 n/a 0.72 n/a n/a 0.76 0.73

0.6 n/a 0.2 0.4 0.6 0.2 0.2

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Cu-stripping polarization curves. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03284.

native to Pt catalysts such that hydrogen dissociative adsorption (Tafel step) becomes rate-limiting; however, a rds change is not observed on Pd. Such behavior suggests that the reduction in hydrogen binding energy by alloying with Ru is insufficient to reduce the activation energy of the rds on Pd to where a rds change would be observed. Pd, therefore, maintains its native rds upon alloying with Ru albeit with enhanced HOR activity owed to a reduced hydrogen binding energy caused by ligand effects. Changes in activity for the RuxPty alloys are here interpreted for a dissociative H2 adsorption rds where high equilibrium H coverage (θ0H) would be observed.37 From fits to the kinetic data for the RuxPty alloys, the equilibrium H surface fraction is 0.86 and 0.61 for the Ru0.80Pt0.20 and Ru0.20Pt0.80 catalysts, respectively. The equilibrium H surface fraction corresponds closely to that calculated for a H2 dissociative adsorption rds and is the first experimental verification of the equilibrium coverage of adsorbed H under alkaline conditions.38 Higher coverage of adsorbed H corresponds to a lower exchange current density. This phenomenon is observed here for the RuxPty nanoparticles. Additionally, our experimental exchange current density for a Tafel rds closely matches those determined by microkinetic modeling for a polycrystalline (stepped) surface where a single HUPD type intermediate is considered, 3.7 mA/cm2metal (calculated) compared to 1.4 mA/ cm2metal (experimental) determined here.38 At high anodic overpotentials, the limiting current is hypothesized to depend on the surface fraction of H; however, this observation is here obscured by the mass-transport-limited current density.37 In the absence of mass-transport limitations, the analysis of the kinetic currents could reveal this consequence of a Tafel rds at high overpotential, in contrast to the traditional Volmer/Heyrovsky rds observed on the Pt, Pd, and RuxPdy alloys. Testing geometries free of the mass transport limitations of a RDE may be more appropriate for future studies of these highly active catalysts.41,42

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Corresponding Author

*Fax (865) 974-7076; Tel (865) 974-2421; e-mail [email protected] (A.B.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Office of Naval Research, N00014-12-1-0887; ZAF Energy Systems; and the NSF-funded, TN-SCORE program, EPS-1004083, under Thrust 2. Microscopy was conducted as part of a user proposal at ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02-06CH11357.

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REFERENCES

(1) Durst, J.; Siebel, A.; Simon, C.; Hasche, F.; Herranz, J.; Gasteiger, H. A. New Insights into the Electrochemical Hydrogen Oxidation and Evolution Reaction Mechanism. Energy Environ. Sci. 2014, 7, 2255− 2260. (2) Neyerlin, K. C.; Gu, W.; Jorne, J.; Gasteiger, H. A. Study of the Exchange Current Density for the Hydrogen Oxidation and Evolution Reactions. J. Electrochem. Soc. 2007, 154, B631−B635. (3) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Activity Benchmarks and Requirements for Pt, Pt-Alloy, and Non-Pt Oxygen Reduction Catalysts for PEMFCs. Appl. Catal. B: Environ. 2005, 56, 9−35. (4) Cui, C.; Gan, L.; Heggen, M.; Rudi, S.; Strasser, P. Compositional Segregation in Shaped Pt Alloy Nanoparticles and Their Structural Behaviour during Electrocatalysis. Nat. Mater. 2013, 12, 765−771. (5) Cui, C.; Gan, L.; Li, H.-H.; Yu, S.-H.; Heggen, M.; Strasser, P. Octahedral PtNi Nanoparticle Catalysts: Exceptional Oxygen Reduction Activity by Tuning the Alloy Particle Surface Composition. Nano Lett. 2012, 12, 5885−5889. (6) Varcoe, J. R.; Atanassov, P.; Dekel, D. R.; Herring, A. M.; Hickner, M. A.; Kohl, P. A.; Kucernak, A. R.; Mustain, W. E.; Nijmeijer, K.; Scott, K.; et al. Anion-Exchange Membranes in Electrochemical Energy Systems. Energy Environ. Sci. 2014, 7, 3135− 3191. (7) Blizanac, B. B.; Ross, P. N.; Marković, N. M. Oxygen Reduction on Silver Low-Index Single-Crystal Surfaces in Alkaline Solution: Rotating Ring DiskAg(hkl) Studies. J. Phys. Chem. B 2006, 110, 4735− 4741. (8) Rheinländer, P. J.; Herranz, J.; Durst, J.; Gasteiger, H. A. Kinetics of the Hydrogen Oxidation/Evolution Reaction on Polycrystalline Platinum in Alkaline Electrolyte Reaction Order with Respect to Hydrogen Pressure. J. Electrochem. Soc. 2014, 161, F1448−F1457. (9) Strmcnik, D.; Uchimura, M.; Wang, C.; Subbaraman, R.; Danilovic, N.; van der Vliet, D.; Paulikas, A. P.; Stamenkovic, V. R.; Markovic, N. M. Improving the Hydrogen Oxidation Reaction Rate by Promotion of Hydroxyl Adsorption. Nat. Chem. 2013, 5, 300−306. (10) Koper, M. T. M. Hydrogen Electrocatalysis: A Basic Solution. Nat. Chem. 2013, 5, 255−256. (11) Gasteiger, H. A.; Ross, P. N., Jr.; Cairns, E. J. LEIS and AES on Sputtered and Annealed Polycrystalline Pt-Ru Bulk Alloys. Surf. Sci. 1993, 293, 67−80. (12) Van der Niet, M. J. T. C.; Garcia-Araez, N.; Hernández, J.; Feliu, J. M.; Koper, M. T. M. Water Dissociation on Well-Defined Platinum

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CONCLUSION We have presented strategies to reduce Pt content and to improve activity of anode electrocatalysts in alkaline media by alloying Ru with Pt and Pd using a chemical vapor impregnation technique to generate highly active, dispersed catalysts on carbon supports. A Tafel analysis of the corrected kinetic currents reveals differences in the rate-determining step for hydrogen oxidation that depends on the solute atom (Pt or Pd) with Ru and can be explained by atomic-scale mixing of Pt (or Pd) and Ru surface atoms. A hydrogen dissociative rds (Tafel step) is found on RuxPty alloys and an electron transfer rds (Volmer/Heyrovsky step) on RuxPdy alloys. Though shifts in hydrogen binding energy are observed on all of the alloy catalysts, only Ru0.20Pt0.80 displays a significant increase in activity with respect to Pt. In this case ligand effects enhance the electron transfer step sufficiently to modify the HOR ratedetermining step. Therefore, both ligand and bifunctional effects interact in nanosized Ru−Pt systems. This behavior suggests that the degree of electron sharing, solute atom surface distributions, and adsorbate coverage contribute to the activity enhancements observed.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

S Supporting Information *

A table of fitted EXAFS parameters as well as illustrated k-space fits; STEM-EDS maps of the various supported catalysts and F


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The Journal of Physical Chemistry C Surfaces: The Electrochemical Perspective. Catal. Today 2013, 202, 105−113. (13) Gisbert, R.; García, G.; Koper, M. T. M. Adsorption of Phosphate Species on Poly-Oriented Pt and Pt(111) Electrodes over a Wide Range of pH. Electrochim. Acta 2010, 55, 7961−7968. (14) Sheng, W.; Zhuang, Z.; Gao, M.; Zheng, J.; Chen, J. G.; Yan, Y. Correlating Hydrogen Oxidation and Evolution Activity on Platinum at Different pH with Measured Hydrogen Binding Energy. Nat. Commun. 2015, 6. (15) Wang, Y.; Wang, G.; Li, G.; Huang, B.; Pan, J.; Liu, Q.; Han, J.; Xiao, L.; Lu, J.; Zhuang, L. Pt-Ru Catalyzed Hydrogen Oxidation in Alkaline Media: Oxophilic Effect or Electronic Effect? Energy Environ. Sci. 2015, 8, 177−181. (16) Spendelow, J. S.; Lu, G. Q.; Kenis, P. J. A.; Wieckowski, A. Electrooxidation of Adsorbed CO on Pt(111) and Pt(111)/Ru in Alkaline Media and Comparison with Results from Acidic Media. J. Electroanal. Chem. 2004, 568, 215−224. (17) Ohyama, J.; Sato, T.; Satsuma, A. High Performance of Ru Nanoparticles Supported on Carbon for Anode Electrocatalyst of Alkaline Anion Exchange Membrane Fuel Cell. J. Power Sources 2013, 225, 311−315. (18) Gasteiger, H. A.; Markovic, N. M.; Ross, P. N. H2 and CO Electrooxidation on Well-Characterized Pt, Ru, and Pt-Ru. 2. Rotating Disk Electrode Studies of CO/H2 Mixtures at 62 °C. J. Phys. Chem. 1995, 99, 16757−16767. (19) Papandrew, A. B.; Chisholm, C. R. I.; Elgammal, R. A.; Ö zer, M. M.; Zecevic, S. K. Advanced Electrodes for Solid Acid Fuel Cells by Platinum Deposition on CsH2PO4. Chem. Mater. 2011, 23, 1659− 1667. (20) Papandrew, A. B.; Chisholm, C. R. I.; Zecevic, S. K.; Veith, G. M.; Zawodzinski, T. A. Activity and Evolution of Vapor Deposited PtPd Oxygen Reduction Catalysts for Solid Acid Fuel Cells. J. Electrochem. Soc. 2013, 160, F175−F182. (21) Papandrew, A. B.; Wilson, D. L.; M. Cantillo, N.; Hawks, S.; Atkinson, R. W.; Goenaga, G. A.; Zawodzinski, T. A. Electrochemical Hydrogen Separation via Solid Acid Membranes. J. Electrochem. Soc. 2014, 161, F679−F685. (22) Ravel, B. EXAFS Analysis with FEFF and FEFFIT; University of Washington: Seattle, WA, 2001; Vol. 2. (23) Rehr, J. J.; Kas, J. J.; Vila, F. D.; Prange, M. P.; Jorissen, K. Parameter-Free Calculations of X-Ray Spectra with FEFF9. Phys. Chem. Chem. Phys. 2010, 12, 5503−5513. (24) Ravel, B. Crystallography for the X-Ray Absorption Spectroscopist; University of Chicago: Chicago, IL, 2002. (25) Garsany, Y.; Baturina, O. A.; Swider-Lyons, K. E.; Kocha, S. S. Experimental Methods for Quantifying the Activity of Platinum Electrocatalysts for the Oxygen Reduction Reaction. Anal. Chem. 2010, 82, 6321−6328. (26) Shao, M.; Odell, J. H.; Choi, S.-I.; Xia, Y. Electrochemical Surface Area Measurements of Platinum- and Palladium-Based Nanoparticles. Electrochem. Commun. 2013, 31, 46−48. (27) Shao, M.; Yu, T.; Odell, J. H.; Jin, M.; Xia, Y. Structural Dependence of Oxygen Reduction Reaction on Palladium Nanocrystals. Chem. Commun. 2011, 47, 6566−6568. (28) Xu, Y.; Xie, X.; Guo, J.; Wang, S.; Wang, Y.; Mathur, V. K. Effects of Annealing Treatment and pH on Preparation of CitrateStabilized PtRu/C Catalyst. J. Power Sources 2006, 162, 132−140. (29) St. John, S.; Angelopoulos, A. P. In Situ Analysis of Optimum Surface Atom Coordination for Pt Nanoparticle Oxygen Reduction Electrocatalysts. Electrochim. Acta 2013, 112, 258−268. (30) St. John, S.; Dutta, I.; Angelopoulos, A. P. Synthesis and Characterization of Electrocatalytically Active Platinum Atom Clusters and Monodisperse Single Crystals. J. Phys. Chem. C 2010, 114, 13515− 13525. (31) Papandrew, A. B.; Atkinson, R. W., III; Unocic, R. R.; Zawodzinski, T. Ruthenium as a CO-Tolerant Hydrogen Oxidation Catalyst for Solid Acid Fuel Cells. J. Mater. Chem. A 2015, 3, 3984− 3987.

(32) Vogel, W. Interaction of a Nanosized Pd Catalyst with Active C from the Carbon Support: An Advanced in Situ XRD Study. J. Phys. Chem. C 2010, 115, 1506−1512. (33) Tew, M. W.; Nachtegaal, M.; Janousch, M.; Huthwelker, T.; van Bokhoven, J. A. The Irreversible Formation of Palladium Carbide during Hydrogenation of 1-Pentyne over Silica-Supported Palladium Nanoparticles: In Situ Pd K and L3 Edge XAS. Phys. Chem. Chem. Phys. 2012, 14, 5761−5768. (34) Sheng, W.; Gasteiger, H. A.; Shao-Horn, Y. Hydrogen Oxidation and Evolution Reaction Kinetics on Platinum: Acid vs Alkaline Electrolytes. J. Electrochem. Soc. 2010, 157, B1529−B1536. (35) Sheng, W.; Myint, M.; Chen, J. G.; Yan, Y. Correlating the Hydrogen Evolution Reaction Activity in Alkaline Electrolytes with the Hydrogen Binding Energy on Monometallic Surfaces. Energy Environ. Sci. 2013, 6, 1509−1512. (36) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 2006. (37) Krischer, K.; Savinova, E. R. Fundamentals of Electrocatalysis. In Handbook of Heterogeneous Catalysis; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2008. (38) Skúlason, E.; Tripkovic, V.; Björketun, M. E.; Gudmundsdóttir, S.; Karlberg, G.; Rossmeisl, J.; Bligaard, T.; Jónsson, H.; Nørskov, J. K. Modeling the Electrochemical Hydrogen Oxidation and Evolution Reactions on the Basis of Density Functional Theory Calculations. J. Phys. Chem. C 2010, 114, 18182−18197. (39) Czerwiński, A.; Kiersztyn, I.; Grdeń, M. The Study of Hydrogen Sorption in Palladium Limited Volume Electrodes (Pd-LVE): Part II. Basic Solutions. J. Electroanal. Chem. 2000, 492, 128−136. (40) Enyo, M.; Biswas, P. C. Hydrogen Absorption in Palladium Electrodes in Alkaline Solutions. J. Electroanal. Chem. 1992, 335, 309− 319. (41) Kucernak, A. R.; Toyoda, E. Studying the Oxygen Reduction and Hydrogen Oxidation Reactions under Realistic Fuel Cell Conditions. Electrochem. Commun. 2008, 10, 1728−1731. (42) Chen, S.; Kucernak, A. Electrocatalysis under Conditions of High Mass Transport: Investigation of Hydrogen Oxidation on Single Submicron Pt Particles Supported on Carbon. J. Phys. Chem. B 2004, 108, 13984−13994.

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