Understanding Oxygen Reduction on Tantalum Oxyphosphate and

Excet, Inc., 8001 Braddock Road, Suite 105, Springfield, Virginia 22151, United States. § Chemistry Division, Naval Research Laboratory, Washington, ...
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Understanding Oxygen Reduction on Tantalum Oxyphosphate and Tantalum Oxide Supported Platinum by X‑ray Absorption Spectroscopy Anna Korovina,† Yannick Garsany,‡ Albert Epshteyn,§ Andrew P. Purdy,§ Karren More,∥ Karen E. Swider-Lyons,§ and David E. Ramaker*,† †

Department of Chemistry, The George Washington University, Washington, D.C. 20052, United States Excet, Inc., 8001 Braddock Road, Suite 105, Springfield, Virginia 22151, United States § Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375, United States ∥ Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ‡

S Supporting Information *

ABSTRACT: The Pt activity for the oxygen reduction reaction (ORR) is improved when Pt nanoparticles are supported on nanoscale layers of tantalum oxyphosphate (nominally, TaOPO4) on Vulcan carbon (VC) and heated at high temperature (660 °C) in reducing conditions. We attempt to explain the increased activity of the PtTaOPO4/VC “HT” by comparison to other less-active electrocatalysts comprising Pt on tantalum oxide (Pt-Ta2O5/VC) and Pt-TaOPO4/VC heated to 200 °C in air. Our toolbox for this analysis contains the rotating disk electrode methodology for characterization of the ORR and high-angle annular dark-field scanning transmission electron microscopy with high-resolution energy-dispersive X-ray spectroscopy (EDS) capabilities. The adsorption of molecular species on the Pt and Ta is determined from the Δμ XANES (X-ray absorption near-edge structure) adsorbate isolation technique of X-ray absorption spectroscopy (XAS) data for electrocatalsyts in situ, whereby H and OH adsorption products from water activation can be used to infer how an electrocatalyst would react for oxygen reduction. The Δμ XANES analysis at the Pt L2 edge suggests that interfacial hydrogen exists between the Pt and the support for Pt-Ta2O5/VC and Pt-TaOPO4/VC “HT” at potentials > 0.3 V vs RHE even after surface H is removed. More importantly, in the most active sample, Pt-TaOPO4/VC “HT”, the onset potential for O(H) adsorption is highest in the Δμ XANES. The XAS and microscopy/EDS results show that the Pt-TaOPO4/VC “HT” is PtTa2O5/VC with polyphosphate groups directly associated with the Pt. From the body of results, we surmise that the nanoscale layer of polyphosphate on the Pt-TaOPO4/VC “HT” facilitates proton conduction to the Pt particles and hence moves the equilibrium for OH adsorption on the Pt to higher potentials.

1. INTRODUCTION A significant area of research in proton-exchange membrane fuel cells (PEMFCs) is the electrocatalysis of the kinetically limited oxygen reduction reaction (ORR) (eq 1). O2 + 4e− + 4H+ = 2H 2O

catalysts, oxide and phosphate supports have exhibited a remarkable range of mechanisms for altering the rates of chemical reactions occurring over the catalyst. Various noble metals supported on metal oxides have enhanced reactivity for many reactions in gas-phase catalysis.7−10 The primary mechanism for this enhancement, although somewhat controversial over the years, is now accepted as arising from a metal−support interaction (MSI), that is, an electronic polarization or charge rearrangement within the Pt particle brought about by the negative charge on the nearest support oxygen atoms,8,11−14 but the spillover mechanism (i.e., when adsorbates, formed on the Pt particle, migrate onto the support and/or the reverse) is also still being cited (see, for example, Awaludin et al.15 and references therein). Recently, we reported on the high mass and surface specific activity for the ORR, using a tantalum oxyphosphate/Vulcan

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The most effective catalyst in the highly oxidizing and corrosive environment of the PEMFC cathode is Pt, which has a high cost. A higher platinum loading is needed at the fuel cell cathode to catalyze the ORR. The cathode Pt loading has been decreased significantly by using high-surface-area Pt and Pt alloy nanoparticles that are supported on electronically conductive carbon and are mixed with a perfluorosulfonic acid ionomer for proton conductivity. However, the durability of such nanomaterials is poor at the PEMFC cathode. Inspiration for new ideas for high activity, durable catalysts can come from the field of heterogeneous catalysis, where it is common to support platinum and other noble metals on metal oxides1−4 or metal phosphates5,6 to improve their activity and durability. Unlike the carbon supports normally used in fuel cell © 2012 American Chemical Society

Received: March 1, 2012 Revised: July 31, 2012 Published: July 31, 2012 18175

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Table 1. Description of the Catalysts and Their Heat Treatment and Electrochemical Attributesa sample Pt/VC Pt-Ta2O5/VC Pt-TaOPO4/VC Pt-TaOPO4/VC “HT”

heating conditions

wt % Pt

Pt particle size (nm)

RDE Pt loading (μgPt/cm2 ± 0.5)

electrochemical surface area (ECSA) (m2/gPt ± 3)

specific activity (μA/cm2Pt ± 50)

mass activity (A/mgPt ± 0.05)

200 °C air 200 °C air 200 °C air 660 °C 10% H2/N2

18.5 16.7 16 18

2.1 1.8 1.8 2.4

20 20 18 28

72 103 108 76

333 202 309 625

0.24 0.24 0.32 0.46

a

The Pt mass and specific activity for the ORR are determined by RDE at 0.9 V vs RHE in O2-saturated 0.1 M HClO4 electrolyte at a scan rate of 20 mV s−1 and a electrode rotation rate of 1600 rpm.

with high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and high-resolution energy-dispersive X-ray spectroscopy (EDS).

carbon supported Pt catalyst heat treated in a H2/N2 (1:9) atmosphere at 660 °C in a reducing atmosphere (referred to here as Pt-TaOPO4/VC “HT”, where the “HT” indicates high temperature). This catalyst showed very high performance for the ORR activity with rotating disk electrode (RDE) voltammetry (0.46 A/mgPt at 0.9 V in 0.1 M HClO4 electrolyte, 1600 rpm, and scan rate of 20 mV/s).16 In contrast, a similarly prepared Pt-TaOPO4/VC heated at 200 °C in air, and a PtTa2O5/VC catalyst heated at 200 °C in air, showed no significant improvement over the standard Pt/VC. This paper will attempt to explain the affect of the heat treatment and presence of tantalum and phosphate groups on the Pt electrocatalytic activity. The catalyst ORR activity is established using the RDE methodology. Adsorbates and adsorbate coverage are elucidated as a function of potential by in situ X-ray absorption spectroscopy (XAS), through Δμ XANES (X-ray absorption near-edge structure) analysis17,18 at the Pt L2, Pt L3, and Ta L3 edges. The Δμ XANES analysis of an in situ electrocatalyst is compared to theoretical Δμ signatures of model Janin19 (Pt6) clusters with adsorbates from FEFF8 calculations. In our case, we monitor hydrogen and oxygen adsorbates arising from water activation as a function of potential, which then can be correlated to ORR activity. In the oxidizing region, the water activation reaction in acid results in OH adsorption on the Pt from about 0.7 to 1.0 V, and then O adsorption at higher potentials: H 2O + Pt = Pt‐OH + H+ + e−

2. EXPERIMENTAL PROCEDURES 2.1. Catalysts Synthesis. Four catalysts are evaluated in this work: Pt/VC made from a standard platinum colloid supported on Vulcan carbon (Pt/VC, NRL standard), platinum tantalum phosphate on VC heated to both 200 °C in air (PtTaOPO4/VC) and 660 °C in H2/N2 (Pt-TaOPO4/VC “HT”), and platinum tantalum oxide on VC heated to 200 °C in air (Pt-Ta2O5/VC). The same Pt colloid solution is used to make the Pt nanoparticles for all four samples, and its preparation is described elsewhere.4 The Pt-TaOPO4/VC samples were prepared from the Pt colloids, tantalum ethoxide, and polyphosphoric acid, as previously described,16 and the preparation of the Pt-Ta2O5/VC material is a variation on that same method. Additionally, standards of the Ta-oxide/VC and Ta-oxyphosphate/VC compounds were made without the addition of Pt. More complete synthetic details are given in the Supporting Information. 2.2. Electrocatalyst Evaluation. The catalysts were characterized in a three-electrode cell using 0.1 M HClO4 electrolyte as described in our previous work.16 All results are referenced to the reversible hydrogen electrode (RHE) of Pt| H2. The chemical composition of the catalysts was determined by inductively coupled plasma (ICP) atomic emission spectroscopy performed by Columbia Analytical Services, Inc., and checked by CALI Laboratories, Inc. of Parsipanny, NJ, as well as by our own analyses. The platinum content of the samples varied from 16% to 18%, and are listed in Table 1. The Ta content of the Ta-containing sample is 3.0 (±0.5)%. The phosphorus content is about ∼1%, and the remainder is carbon. Platinum particle sizes were estimated by X-ray diffraction (XRD, Bruker D8 Advance diffractometer [Cu Kα radiation: 40 KV, 40 mA]) using the Scherrer equation. The Pt particle sizes from XRD were 1.8 and 2.1 nm for the three samples heated in air, and 2.4 nm for the Pt-TaOPO4/VC “HT” sample (Table 1). These particle sizes agree well with those found from EXAFS analysis, as shown in Figure S-1 (Supporting Information (SI)), and discussed in the SI. The physical morphology of the Pt-TaOPO4/VC “HT” catalyst was surveyed with high-area HAADF-STEM using a JEOL 2100F STEM at Oak Ridge National Laboratory. Thin-film electrodes were prepared for electrochemical analysis by cyclic and RDE voltammetry in a standard threeelectrode electrochemical cell using a method adapted from the literature.16,20,21 Additional information is given in the Supporting Information.

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In the hydrogen region (∼0−0.3 V), water is activated to form hydrogen adsorbates on the Pt: H+ + Pt + e− = Pt‐H

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It has previously been shown that the Pt L3 edge is preferred for Δμ XANES analysis because it allows for adsorbate identity, coverage, and binding site information with changing potential due to the allowed transition (2p3/2 → 5d5/2, 5d3/2).3,17,18 However, the Ta L1 (11 682 eV) edge interferes with the Pt L3 (11 564 eV) edge. Although 118 eV is normally sufficient to perform Δμ XANES analysis, it is not sufficient for EXAFS analysis, which can provide helpful structural information. Therefore, the Pt L2 edge (i.e., the 2p1/2 → 5d3/2 transition) was measured, sacrificing some Δμ XANES binding site information in order to include the structural EXAFS (extended X-ray absorption fine structure) information. Consequently, the normal difference between the Pt L3 Δμ XANES signature for OH/Pt and O/Pt cannot be distinguished (i.e., difference between atop vs fcc binding) because this binding site information is lost at the L2 edge; therefore, the OH and O coverage will be written as O(H) collectively. The findings of the XAS and electrochemical results are supported by analysis of the Pt-TaOPO4/VC “HT” material 18176

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3. X-RAY ABSORPTION SPECTROSCOPY XAS measurements of the electrodes were measured in situ in a proven electrochemical cell22 in 1 M HClO4 at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory on beamline X11A. The NSLS storage ring operated at 2.8 GeV with the ring current between 100 and 300 mA. A Si(111) double-crystal monochromator was used to measure the Pt L2 and Ta L3 edges. The double-crystal monochromator was detuned by 10−15% to minimize the presence of higher harmonics in the beam. The electrodes for XAS measurements were made from either Pt/VC, Pt-Ta2O5/VC, Pt-TaOPO4/VC, Pt-TaOPO4/VC “HT”, Ta2O5, or TaOPO4, which were individually mixed with 1 M HClO4 for about 1 h to a paste-like consistency. This paste was made into thick film electrodes on a carbon gas diffusion layer (E-TEK) and assembled into an in situ cell with a platinum counter electrode and a Ag/AgCl reference electrode. The potential of the cell was set using a computer-controlled bipotentiostat (Pine) AFCBP1. All potentials were corrected versus a reversible hydrogen electrode (RHE). Spectra were obtained at specific applied potentials in succession at about 0.1 V intervals; see Figure 1a. Measurements at the Pt L2 edge

were collected in the anodic direction only. The cell was not sparged with O2, and so the only electrocatalytic reaction was water activation. Pt loadings of ∼7 mgPt/cm2 were used to allow measurement of transmission spectra at the Pt L2 edge; however, the inherent lower Ta loadings (∼1 mgTa/cm2) in the sample required Ta L3 data collection in fluorescence mode for the Pt-Ta2O5/VC and Pt-TaOPO4/VC samples. The Ta L3 edge was collected in transmission mode for the Pt-TaOPO4/VC “HT”, Ta2O5, and TaOPO4 samples. Simultaneous transmission spectra were collected by placing the cell and reference foil between a series of detectors, I0 (intensity of the incident beam), It (the beam transmitted by the sample), and Iref (the beam subsequently transmitted by the reference foil). The cell was placed between the I0 and It detectors while a Pt or Ta reference foil was placed between It and Iref for energy calibration. Fluorescence was measured by placing the sample surface between the I0 and It detectors at a 45° angle to the fluorescence detector (If), while simultaneous Ta reference foil data were obtained in transmission mode for energy calibration. 3.1. EXAFS Analysis. EXAFS analysis of the results at the Pt L2 and Ta L3 edges was carried out using the ATHENA and ARTEMIS codes23 from the IFEFFIT software package.24 More details are given in the Supporting Information. 3.2. XANES, Extraction of Δμ. The absorption coefficient, μ, was obtained from the raw XAS data using the ATHENA code of Ravel and Newville.23 The pre-edge background was removed using the AUTOBK function, which is described more fully elsewhere.25 A normalization procedure was conducted over the ∼30−100 eV range, relative to the Pt L2 edge (13 273 eV), for XANES analysis. This was done for the samples, standards, and foils no matter the mode, that is, transmission or fluorescence. Because energy calibration is critical for the success of the Δμ technique to guarantee full cancellation of the atomic contribution in the XANES region, any slight shifts in the data were removed by careful alignment of the reference foil data and then transferring any shifts to the sample data. The total change in the experimental XANES spectra due to adsorption by some adsorbate “Ads”, Δμ = μ(Ads/Pt) − μ(Pt), was isolated by subtracting the XANES spectra from a reference Pt spectrum measured for the “1st cathodic” cycle (∼0.54 V), where the Pt surface is relatively adsorbate-free, μ(Pt). The Pt experimental Δμ signatures qualitatively match the theoretical Δμ signatures for model Pt6 clusters obtained using the FEFF826,27 code. The Ta L3 edge (9881 eV) XANES data were analyzed similarly to those at the Pt L2 edge. Correspondingly, a reference potential had to be chosen. For the Ta compounds, the reference condition does not represent a “clean” surface but rather a reference level for oxidation/surface hydration. The tantalum-supported catalysts and tantalum supports by themselves (i.e., without Pt) appear to be the least oxidized at higher potentials above 0.9 V (we will discuss this below). Therefore, for the Pt-Ta2O5/VC catalysts, the spectra at 0.9 V was used as a reference during the anodic cycle, for the PtTaOPO4/VC catalyst and the Ta2O5 and TaOPO4 supports, 1.2 V in the anodic direction was used, and for the Pt-TaOPO4/ VC “HT” catalyst, the spectrum at 1.1 V in the anodic direction, was used. The Δμ expressions utilized are specifically defined with the references indicated in eqs 4−8.

Figure 1. (a) The 27 potentials at which water activation activity was measured in situ with XAS at the Pt L2 edge for all Ta-supported samples and the Pt/VC standard. These 27 points were only collected at the Ta L3 edge for the Pt-TaOPO4/VC “HT” sample. The Ta L3 edge was measured for the Pt-TaOPO4/VC only in the 1st cathodic and anodic directions (16 potentials), and in the anodic direction only (10 points) for the Pt-Ta2O4/VC sample. (b) Selected Δμ results for Pt-TaOPO4/VC and comparison to the theoretical Δμ signatures for H on Pt and O(H) on Pt. H is prevalent on the surface of the Pt in the Pt-TaOPO4/VC at 0.1 V. At 0.9−1.2 V, the Δμ signature for O(H) adsorption on Pt increases with potential. The Δμ amplitudes, |Δμ|, are shown for Δμ at 0.1 and 1.2 V to represent H and O(H) adsorption, respectively.

began at the OCP (open-circuit potential) and continued in steps downward to 0.05 V; these will be labeled “1st cathodic”. The measurements then continued upward from 0.1 to 1.2 V (labeled “anodic”) and then back downward from 1.1 to 0.05 V (labeled “2nd cathodic”). The Ta L3 edge was collected in this same anodic and cathodic manner for the Pt-TaOPO4/VC “HT” sample only. The Pt-TaOPO4/VC data were collected only in the first cathodic and anodic directions, while data for Pt-Ta2O5/VC and the support materials, Ta2O5 and TaOPO4,

Δμ Pt‐Ta 2O5/VC = μ(V) − μ(0.9 V anodic) 18177

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Figure 2. Electrochemical evaluation of Pt/VC, Pt-Ta2O5/VC, Pt-TaOPO4/VC, and Pt-TaOPO4/VC “HT” samples. (a) Cyclic voltammetry in N2purged 0.1 M HClO4 at a scan rate of 20 mV/s. The current is normalized to the Pt loadings (see Table 1). (b) Oxygen reduction reaction at 1600 rpm, with a sweep rate of 20 mV/s in O2-saturated 0.1 M HClO4 at 30 °C. (c) Pt mass activity. (d) Pt specific activity.

Δμ Pt‐TaOPO4 /VC = μ(V) − μ(1.2 V anodic)

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Δμ Ta 2O5/VC = μ(V) − μ(1.2 V anodic)

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Δμ TaOPO4 /VC = μ(V) − μ(1.2 V anodic)

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ones, making it difficult to distinguish between O and OH adsorbates. The Δμ at the Ta L3 edge for the Ta supports was modeled with an octahedral TaO6 cluster with Ta as the absorber.33 The theoretical Δμ that best matched the experimental Δμ used to represent the oxidation state of the support is written in eq 9. The equation for the theoretical signature for H on Ta is in eq 10.

Δμ Pt‐TaOPO4 /VC “HT” = μ(V) − μ(1.1 V anodic) (8)

Δμ Theory Ta‐O = μ(TaO6 ) − μ(TaO5)

3.3. FEFF Calculations for Pt L2 and Ta L3. A thorough discussion of the theoretical parameters used for the Pt L3 FEFF calculations has been given previously by Teliska et al.,18 so they are only briefly mentioned here. A six-Pt-atom Janin cluster contains both fcc and hcp sites.28 Because of the lower cluster symmetry compared with a highly symmetric octahedral cluster,26,29,30 problematic surface states near the Fermi level do not arise. Consequently, a Janin Pt6 cluster was utilized with a Pt−Pt distance of 2.77 Å, a Pt−H distance of 1.80 Å, and a Pt− O distance of 2.0 Å to carry out the FEFF8 calculations. The theoretical result, ΔμTheory = μ(Ads/Pt6) − μ(Pt6), simulates the experimental Δμ, where the reference μ is now the clean Pt6 Janin cluster, μ(Pt6), and μ(Ads/Pt6) is that for an adsorbate in a specific binding site utilizing the distances specified above. The photon absorber is always the Pt, to which the adsorbate is bound, assuming the normal single-adsorber model, as discussed previously.31,32 The Pt L2 edge results were compared against the theoretical signatures calculated at the Pt L3 edge, as the Pt L2 edge and Pt L3 edges produce essentially the same signatures. The only difference is that the Pt L2 experimental signatures have lower intensity than the Pt L3

Δμ Theory Ta‐H = μ(TaO5H) − μ(TaO5)

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4. RESULTS AND ANALYSIS 4.1. Electrochemical Characterization of the Catalysts. Figure 2a displays the cyclic voltammograms (CVs) recorded for the Pt/VC standard, Pt-Ta2O5/VC, Pt-TaOPO4/VC, and Pt-TaOPO4/VC “HT”. The current is normalized to the electrode Pt mass in Figure 2a, because the Pt loading on the RDE varied between 18 and 28 μgPt cm−2 (see Table 1). The Pt-Ta2O5/VC, Pt-TaOPO4/VC, and Pt-TaOPO4/VC “HT” catalysts exhibit the characteristic hydrogen adsorption and desorption peaks of nanoscale Pt at a potential range between 0.05 and 0.40 V. The hydrogen adsorption/desorption region is followed by the double-layer potential region at 0.40 V ≤ E ≤ 0.70 V and the Pt−OH adsorption/reduction potential region at 0.70 V ≤ E ≤ 1.25 V. There is no change in the onset potential for OH adsorption for the oxide- and phosphatesupported Pt samples versus that of the Pt/VC standard. 18178

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amount of each adsorbate on the catalyst surface as a function of potential. We have found that, in cases where we have as many as 27 spectra, an overlay or waterfall plot is confusing and insufficient. With the |Δμ| approximation, we can succinctly compare the 10s of spectra measured in multiple cathodic and anodic sweeps by plotting the amplitude, |Δμ|, of the major peak. The actual Δμ signature is still used to identify the type of adsorbate, and these differences are indicated in the plots by the different background colors. Figure 3a shows a plot of such |Δμ|

The Pt electrochemical surface area (ECSA) values measured for the different catalysts are summarized in Table 1. The ECSA of the Pt/VC is 72 m2 gPt−1, whereas those of the tantalumsupported catalysts heated to 200 °C are 103−108 m2 gPt−1, a nearly 30% increase. The different ECSAs resulting from the different particle sizes can be compared based on simple geometric considerations of the surface area of a sphere, its volume, and the density of Pt. We can write the equation, surface area (m2 g−1) = [6/ρd] = [279/d], where ρ is the density of Pt and d is the particle diameter in nanometers. A 2.1 nm particle (Pt/VC standard) would correspond to a specific surface area of ∼133 m2 g−1, and a 1.8 nm particle (Pt-Ta2O5/ VC and Pt-TaOPO4/VC) would have a specific surface area of ∼156 m2 g−1. Therefore, one should expect about a 17% increase in the Pt ECSA when the Pt particle size decreases from 2.1 nm to 1.8 nm versus the observed 30% increase. A higher metal surface area is common for metal oxides and phosphorus-containing catalysts34−36 and has been attributed to geometric or chemical changes35 or higher hydrophilicity imparted by the support near the Pt, which increases its wetting properties and thus water coverage.37,38 Figure 2b compares the ORR for the Pt/VC, Pt-Ta2O5/VC, Pt-TaOPO4/VC, and Pt-TaOPO4/VC “HT” materials in an O2-saturated 0.10 M HClO4 electrolyte at 30 °C, at a rotation rate of 1600 rpm and a scan rate of 20 mV s−1. A single, steep reduction wave with a well-developed limiting current density plateau (Jlim) near −6 mA cm−2 is measured for all the electrocatalysts. The ORR catalytic activity of the Pt-TaOPO4/ VC “HT” electrocatalyst is highest, as indicated by its high onset potential of O2 reduction (0.97 V vs RHE) and its high half-wave potential (E1/2), which is 0.92 V vs RHE. The E1/2 for the Pt-Ta2O5/VC and Pt-TaOPO4/VC is 0.90 V, within experimental uncertainties identical to that of the Pt/VC. The Pt mass-specific (MA) and area-specific (SA) activities are reported in Table 1 at E = 0.90 vs RHE and also indicated in Figure 2c,d. The MA and SA values of the Pt/VC agree well with values reported in the literature by Gasteiger et al.21 for Pt/carbon electrocatalysts. The MA activity measured at E = 0.90 V vs RHE for the Pt-TaOPO4/VC “HT” is equal to 0.46 A mgPt−1, compared with 0.24 A mgPt−1 for the Pt/VC, 0.24 A mgPt−1 for the Pt-Ta2O5/VC, and 0.32 A mgPt−1 for the PtTaOPO4/VC. The higher MA of the Pt-TaOPO4/VC “HT” electrocatalyst is probably due to its high SA of 625 μA cmPt−2, which is about 1.9 times higher than that of the 18% Pt/VC standard (333 μA cmPt−2). Within experimental errors, the MA and SA values measured for the Pt-Ta2O5/VC and PtTaOPO4/VC electrocatalysts are close to those estimated for the Pt/VC standard. 4.2. XAS Analysis. 4.2.1. Δμ and |Δμ| at the Pt L2 Edge. Figure 1b shows the theoretical Δμ signatures for n-fold H (thin blue line) and O(H) (thick green line) adsorption at the Pt L2 edge, along with experimental Δμ results for Pt-TaOPO4/ VC at 0.1, 0.9, 1.0, 1.1, and 1.0 V. This result is representative for all the catalysts, which had Δμ signatures of the same shape and quality. The Δμ signatures are qualitatively equivalent to those seen on Pt/VC in this and previously reported works at the Pt L3 edge.17,18 Comparison to models indicates that H is present on the surface of Pt at the lower potential (0.05 V), whereas O(H) dominates at higher potentials. Also shown in Figure 1b are the Δμ amplitudes, |Δμ|, of the curves measured at 0.1 and 1.2 V. We have shown in previous work that the |Δμ| values are roughly proportional to the adsorbate coverage17,18 and can be used to track the relative

Figure 3. Relative adsorbate coverage, |Δμ|, vs potential for the (a) Pt/ VC standard and Pt-TaOPO4/VC and (b) Pt/VC standard and PtTa2O5/VC, from in situ XAS at the Pt L2 edge. The shaded regions highlight the identity of the different adsorbates that dominate in that specific potential region as determined by the individual Δμ signatures measured at each potential (see Figure 1b) where H adsorption on Pt is prevalent below 0.4 V, and O(H) adsorption on Pt dominates the Δμ signatures above 0.7 V. Data were not collected below 0.2 V for Pt/VC due to experimental difficulties.

values of adsorbate coverage with potential for the Pt/VC standard. By comparison of the individual Δμ spectra to the FEFF8 adsorbate models in Figure 1b, we know that H adsorbates dominate at potentials below 0.4 V and O(H) adsorbates above 0.7 V. As expected, we observe the least adsorbates on the Pt in the potential region between 0.4 and 0.7 V. During the “1st cathodic” cycle from 0.8 to 0.2 V, most of the (O)H on the surface has left by 0.6 V and the |Δμ| is at a minimum from 0.6 to 0.4 V. Some residual oxygen species remain on the Pt surface and are not fully removed until about 0.1 V, as is known from ORR evaluation of the Pt with RDE (see Figure 8.33 in ref 39). Although H adsorption is generally reversible, this is not seen as clearly in this data set, as the electrode was only taken down to 0.2 V rather than to 0.05 V due to experimental difficulties with H2 evolution; anions and other oxygen-containing species, and impurities on the surface at this potential region, can have a significant effect on the adsorption properties.40−44 A large irreversibility in the O adsorption is seen, as is generally evident in the CVs. The |Δμ| amplitude is shown as a function of potential in Figure 3a for the Pt-TaOPO4/VC, and in Figure 3b for PtTa2O5/VC, both versus Pt/VC catalysts for comparison of their relative adsorbate coverage. The initial cathodic cycle (1st cathodic) is excluded, because this only shows that the samples are highly oxidized at the open-circuit conditions and reduce to a lower state at 0.05 V. In the anodic cycle, the onset for O(H) adsorption occurs around ∼0.6−0.7 V for all three samples. 18179

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The O(H) adsorbate from water activation poisons Pt sites for the ORR, and a higher onset potential of the Δμ for O(H) adsorption and lower O(H) coverage has been correlated to higher ORR catalyst activity in alloys such as Pt3Co.45,46 Assuming that OH adsorption and coverage play a role in the catalyst activity, it is not surprising that, with the same Δμ for O(H) adsorption, these three catalysts have similar ORR activities (see Table 1). The |Δμ| results measured for Pt-Ta2O5/VC (Figure 3b) in the H region in the anodic sweep are quite different from those of Pt/VC and Pt-TaOPO4/VC (Figure 3a). Surface H usually desorbs off the Pt surface by 0.3 V, yet the |Δμ| signature of the Pt-Ta2O5/VC suggests that hydrogen does not desorb from this material until ∼0.6 V. This phenomenon can be attributed to interfacial H, which lies between the Pt and the Ta2O5 support and requires higher potentials to be driven off. Such interfacial H has been reported in the gas phase for H/Pt on oxide supports,47 and needs higher temperatures for desorption, and thus is consistent with the higher potentials needed to desorb H from the Pt on Ta2O5. An additional mechanism for H adsorption for Pt-Ta2O5/VC is consistent with its high ECSA values. Figure 4 shows a similar |Δμ| plot with potential, but which now compares the ORR-active Pt-TaOPO4/VC “HT” sample

Figure 5. Representative Δμ signatures from in situ XAS at the Ta L3 edge for Pt-TaOPO4/VC “HT”, as defined by eq 6, with comparison to theoretically calculated (FEFF8) Δμ signatures, as defined by eqs 7 and 8. The inset shows the structure used to calculate TaO6.

Ta L3 edge representing adsorption of either O or H to Ta in apparent O surface vacancies, that is, on TaO5. The adsorption of H or O gives indistinguishable Δμ signatures in this case, in contrast with that on Pt where H and O adsorption give very different Δμ signatures. The only notable difference in the Δμ for H and O adsorption lies in the amplitude of their signatures, which can be compared by plotting the |Δμ| amplitudes versus potential. Such a plot is given in the Supporting Information. The |Δμ| plot of the Ta-edge data shows only subtle changes with potential, but suggests that the tantalum oxide and phosphate materials heated to 200 °C saturate with water at high potentials, but the Pt-TaOPO4/VC “HT” does not saturate, suggesting that it has a different interaction with the electrolyte than the other compounds. There is also some additional evidence for interfacial H between the Pt and TaO compounds coming from the Ta |Δμ| plots (see discussion in the Supporting Information). The EXAFS results for the tantalum-supported Pt compounds at the Ta L3 edge are more revealing than the Δμ analysis. As shown in Figure 6, the structure in the FT(χ) of the

Figure 4. Relative adsorbate coverage, |Δμ|, vs potential for TaOPO4/ VC “HT” vs Pt-Ta2O5/VC from in situ XAS at the Pt L2 edge.

to the Pt-Ta2O5/VC results from Figure 3b. The initial cathodic cycle (1st cathodic) is again excluded for clarity. The PtTaOPO4/VC “HT” appears to have the same interfacial H as the Pt-Ta2O5/VC, but extended up to 0.7 V, while it was not observed in the Pt-TaOPO4/VC in Figure 3a. Further, the onset potential for O(H) adsorption in the Pt-TaOPO4/VC “HT” occurs at 0.1 V higher potential than for the sample heated at 200 °C in air. These results show that the heating to high temperature in H2 has introduced significantly different H and OH adsorption properties to the Pt-TaOPO4/VC “HT”. These results correlate well with the electrochemical results: both the Pt-Ta2O5/VC and the Pt-TaOPO4/VC “HT” samples have high ECSA values and high amounts of adsorbed H by Δμ analysis. The only sample that shows high ORR activity is the Pt-TaOPO4/VC “HT” sample, which also has a higher onset potential for OH adsorption by Δμ analysis. 4.2.2. Δμ at the Ta L3 Edge. Figure 5 shows the experimental Δμ for the Ta L3 edge, defined by eq 8, at several potentials for Pt-TaOPO4/VC “HT” and theoretically calculated FEFF8 signatures, as defined by eqs 9 and 10. The theory qualitatively matches the experimental signatures at the

Figure 6. Ta L3-edge EXAFS Fourier transform of χ(k) at 1.2 V for the Pt-Ta2O5/VC and Pt-TaOPO4/VC “HT” catalysts.

Pt-TaOPO4/VC “HT” most resembles that of the Pt-Ta2O5/ VC sample, further suggesting decomposition of the TaOPO4 during high-temperature heating. Figure S-3 (Supporting Information) shows the Fourier transform of the Ta L3 EXAFS function, FT(χ), for all three Ta-containing catalysts. To help identify similar peaks, vertical lines identify similar peaks in the high-temperature-treated and Pt-Ta2O5/VC 18180

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samples. Clearly, the 660 °C heat treatment has significantly changed the basic crystal structure of the TaOPO4, and changed it to something similar to that found in Ta2O5. The striking similarity between the high-temperature-heated TaOPO4 and Ta2O5 also suggests some nanoscale phase separation between the Ta-containing phase and the Pcontaining phase, because, otherwise, some scattering from the P atoms should be present. This is also consistent with the Ta L3 Δμ data that suggest an interlayer between the Pt and the Ta2O5 that blocked the adsorption of water and blocked the EXAFS scattering from the interfacial H on the Pt (discussed in the Supporting Information). 4.3. HAADF-STEM, EDS Elemental Mapping. The nanoscale phase separation between the Ta- and P-containing phases is also suggested by the results from high-resolution energy-dispersive X-ray spectroscopy (EDS) mapping of the catalyst surface. Figure 7a shows a HAADF-STEM image of the

both the Pt-Ta2O5/VC and the Pt-TaOPO4/VC “HT” samples are composed of a TaOx-like material interspersed between and among the Pt particles. The presence of a Ta2O5 structure is also present in both samples, according to the EXAFS analysis. Third, the onset potential for OH adsorption is about 100 mV higher in the Δμ analysis of the active Pt-TaOPO4/VC “HT” samples compared with that of the Pt/VC, Pt-Ta2O5/VC, and Pt-TaOPO4/VC materials. This upward shift of OH adsorption is not observed by cyclic voltammetry (Figure 2a). Note that the difference in the onset for OH adsorption cannot be reconciled by the different particle sizes of the Pt-TaOPO4/ VC “HT” (2.4 nm), Pt/VC (2.1 nm), Pt-Ta2O5/VC (1.8 nm), and Pt-TaOPO4/VC (1.8 nm) samples. The first two trends suggest that the heat treatment has caused the TaOPO 4 to decompose into some Ta 2 O 5 orthophosphate-like material, namely 2TaOPO4 /VC → Ta 2O5:Px Oy

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In such metal oxide−orthophosphate moieties, the presence of water normally produces hydration, causing the formation of orthophosphoric acid (P2O5 + 2H2O → H4P2O7) or further polymerization to polyphosphoric acids.48 At higher temperatures, dehydration occurs and can produce long polyphosphate chains, such as that shown in Scheme 1,48 where each of the O− groups can add a H+ to produce an −OH moiety (Scheme 1). Scheme 1

Because the polyphosphate surfaces are highly populated with protons, they are highly acidic; plus, proton conduction can occur along the surface via chaining reactions.49 This concentration of protons at the intersection of the polyphosphate and Pt likely leads to the higher ORR activity observed in the Pt-TaOPO/VC “HT” material. Studies on polycrystalline platinum show that the current density of the ORR increases with increasing proton concentrations50 and that protons play a critical role in the multiple oxygen reduction steps. Oxygen reduction at low current densities in acidic electrolyte follows eq 12, where A is a constant, pO2 is the oxygen partial pressure, F is Faraday’s constant, V is potential, R is the ideal gas constant, and T denotes temperature.50

Figure 7. (a) A HAADF-STEM image of the Pt-TaOPO4/VC “HT” catalyst; elemental mapping for (b) Pt, (c) P, and (d) Ta. The circles represent the same viewing area.

Pt-TaOPO4/VC “HT” catalyst, while Figure 7b−d show the corresponding EDS elemental mapping acquired through this technique. Figure 7 shows the Pt particles (panel b) along with the Ta atoms (panel c) dispersed as thin, trail-like, discontinuous films across the carbon surface between Pt nanoparticles, while there is essentially no Ta atom density around the Pt particles. Figure 7d shows the elemental P mapping, which shows the P atoms to be in close association with the Pt particles. On the basis of this data, on average, P and Ta are not closely associated in this material and are separated as TaOx and POx, further indicating that the POx phases are present more at the Pt particle surface. This is also consistent with the Ta L3 Δμ data, which suggested an interlayer between the Pt and the Ta2O5 that blocked the adsorption of water and blocked the view of the interfacial H on the Pt.

⎛ −FV ⎞ ⎟ i = ApO [H+]3/2 exp⎜ 2 ⎝ RT ⎠

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A similar equation is written for the ORR at high current densities, although with a new constant and the power law for the H+ concentration changing from 3/2 to 1. The increase in specific activity from 333 μA/cm2Pt on Pt/VC to 623 μA/cm2Pt on the Pt-TaOPO4/VC “HT” in 0.1 M HClO4 at 0.9 V (low current density) could thus be rationalized by the effective Pt surface H+ concentration increasing, for example, from 0.1 to 0.15 M, assuming that all of the other constants remain the same. If the H+ concentration only has a power law of 1, an effective pH at the Pt surface of 0.19 would be needed for these higher current densities. Note that our preliminary attempt to

5. DISCUSSION Three significant experimental trends are seen in the results. First, Pt-Ta2O5/VC and Pt-TaOPO4/VC “HT” samples both have significant evidence for increased H adsorption from their ECSA and Δμ analysis, and the H appears to be present as interfacial hydrogen. Second, the HAADF analysis shows that 18181

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potential for OH adsorption from water activation, whereas the Δμ XANES shows that the equilibrium amount of O(H) on the Pt is lower than that on pure Pt on carbon and Pt-Ta2O5 and Pt-TaOPO4 heated only to 200 °C. Both Δμ XANES and HAADF-STEM/EDS results provide evidence of polyphosphate surface segregation in the Pt-TaOPO4/VC “HT”. The highly acidic polyphosphate layer can provide a source of proton conduction to the Pt, and thereby reduce the OH poisoning of the Pt by conversion to H2O, or via H+ involvement directly in the ORR rate-determining step.

verify higher ORR activity on Pt/VC and Pt-TaOPO/VC with small changes of pH was unsuccessful by RDE voltammetry, suggesting that the support (e.g., carbon, Nafion, polyphosphate, etc.) dominates the local pH at the Pt. An effective increase in H+ concentration from the polyphosphoric acid groups could also directly enhance the ORR rate since, in the Wang et al.51 double-trap ORR mechanism, the rate-determining step (RDS) involves the dissociative adsorption step in eq 13: 1 O2 + Pt + H+ + e− → OH/Pt 2

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(13)

Using similar Δμ studies on working fuel cells with H2 and methanol as fuels,52 it has been confirmed that the rate of this RDS is directly enhanced by the availability of H+ ions. Thus, the increased ORR activity exhibited by the high-temperatureheated Pt-TaOPO4/VC “HT” catalysts can arise not only from a reduced OH poisoning (which is directly shown by the data) but perhaps also from an increased rate of the RDS, which we did not directly confirm in this work. The electrochemical CV of the Pt-TaOPO4/VC “HT” material does not change compared to the Pt/VC standard (see Figure 2a), because this conduction mechanism differs significantly from the ligand and electronic effects coming from the M atoms on water activation with Pt observed in PtM alloys. The higher proton concentration provided by the ortho(poly)phosphate may remove OH from the Pt surface by the reactions in eqs 14−16, whereby the H+ from PxOy combines with the OH− on the Pt to make water with no net electron transfer, explaining why there is no change in the CV of the more active materials. H 2O + Pt → Pt‐OH− + H+

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PtxOy + H+ → Px Oy H+

(15)

Pt‐OH− + Px Oy H+e− → Pt + Px Oy + H 2O

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ASSOCIATED CONTENT

* Supporting Information S

Experimental details of catalyst synthesis, methods for electrochemical characterization, and EXAFS analysis (in-depth Pt and Ta edges). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 202-994-6934. Fax: 202-994-5873. E-mail: ramaker@ gwu.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Office of Naval Research for support of this research. The synchrotron measurements were successful due to the help of Dr. Kumi Pandya. The National Synchrotron Light Source is supported by the U.S. Department of Energy, Division of Material Sciences and Division of Chemical Sciences, under contract number DE-AC02-98CH10886. The X11 beamline is supported by the Naval Research Laboratory and contributions from Participating Research Team (PRT) members. Microscopy is supported by the Fuel Cell Technologies Program, Office of Energy Efficiency and Renewable Energy, the U.S. Department of Energy, and Oak Ridge National Laboratory’s Shared Research Equipment (ShaRE) User Facility, which is sponsored by the Office of Basic Energy Sciences, U.S. Department of Energy.

+

The H on the phosphate could be continuously replenished from the acid electrolyte. Once associated with the hydrophilic polyphosphate, the protons would be fully wetted and more accessible to the Pt than H+ directly from the electrolyte. Evaluation of these materials in full fuel cells is now critical, as the proton concentration in a fuel cell is set by the flux of protons from the anode to the cathode and there is not the same excess of protons as exist in a wet electrochemical cell, resulting in no net increase in proton concentration at the Pt. The change in the [H+] reaction order from 3/2 to 1 with increasing current density might also be probed in full fuel cells and electrochemical cells. Further spectroscopic analysis of these materials with infrared spectroscopy or Raman would be useful to further elucidate the mechanism and determine how the adsorbates and reaction intermediates change.

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REFERENCES

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6. CONCLUSIONS We attribute the high ORR activity of nanoscale platinum supported on tantalum oxyphosphate/carbon and heated to 660 °C to nanoscale segregation of polyphosphates at the Pt surface, and a subsequent enrichment of the concentration of available protons at the Pt surface. This mechanism comes from the comparison of in situ XAS (Δμ XANES and EXAFS) results to electrochemical and HAADF-STEM/EDS observations. The electrochemical CV shows no change in the onset 18182

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