Highly Enhanced Electrocatalytic Oxygen Reduction Performance

Mar 27, 2012 - incorporation of Au into Pd and non-noble metal NPs as supports .... bimetallic Pd−Au pair sites at the surface in some cases and in ...
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Highly Enhanced Electrocatalytic Oxygen Reduction Performance Observed in Bimetallic Palladium-Based Nanowires Prepared under Ambient, Surfactantless Conditions Christopher Koenigsmann,† Eli Sutter,‡ Thomas A. Chiesa,† Radoslav R. Adzic,*,§ and Stanislaus S. Wong*,†,∥ †

Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794-3400, United States Center for Functional Nanomaterials, Brookhaven National Laboratory, Building 735, Upton, New York 11973, United States § Chemistry Department, Brookhaven National Laboratory, Building 555, Upton, New York 11973, United States ∥ Condensed Matter Physics and Materials Sciences Department, Brookhaven National Laboratory, Building 480, Upton, New York 11973, United States ‡

S Supporting Information *

ABSTRACT: We have employed an ambient, template-based technique that is simple, efficient, and surfactantless to generate a series of bimetallic Pd 1−xAu x and Pd 1−x Pt x nanowires with control over composition and size. Our asprepared nanowires maintain significantly enhanced activity toward oxygen reduction as compared with commercial Pt nanoparticles and other 1D nanostructures, as a result of their homogeneous alloyed structure. Specifically, Pd9Au and Pd4Pt nanowires possess oxygen reduction reaction (ORR) activities of 0.49 and 0.79 mA/cm2, respectively, which are larger than the analogous value for commercial Pt nanoparticles (0.21 mA/cm2). In addition, core−shell Pt∼Pd9Au nanowires have been prepared by electrodepositing a Pt monolayer shell and the corresponding specific, platinum mass, and platinum group metal mass activities were found to be 0.95 mA/cm2, 2.08 A/ mgPt, and 0.16 A/mgPGM, respectively. The increased activity and catalytic performance is accompanied by improved durability toward ORR. KEYWORDS: Nanowire, bimetallic, palladium, electrocatalysis, platinum monolayer, oxygen reduction reaction

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Nevertheless, despite the tangible gains in ORR activity and durability, a key challenge has been the generation of ORRactive 1D nanostructures using reliable synthetic techniques, which are also environmentally friendly and energy efficient.20 This effort is further complicated by the need for simultaneous control over crystallinity, surface structure, particle size, chemical composition, and the presence of residual surface impurities (e.g., surfactants) which all contribute to the electrocatalytic performance of as-prepared structures.21,22 Hard template-based techniques represent an exciting synthetic approach to meet this challenge, because particle dimensions and morphology can be manipulated directly by controlling the dimensions of the corresponding template material.1,2,19 In light of these considerations, we report for the first time a template-based synthetic approach (Figure S1, Supporting Information) coupled with electroless deposition for the

ne-dimensional metallic nanostructures, particularly those composed of noble metals, have become the focus of increasing attention in the literature, recently owing to their uniquely anisotropic structure.1,2 Accordingly, these materials have been widely demonstrated to be particularly effective as electrocatalysts for the oxygen reduction reaction (ORR) and alcohol electro-oxidation reactions in polymer electrolyte fuel cells (PEMFCs).3,4 Currently, conventional PEMFCs suffer from relatively low efficiencies and high costs primarily due to the nanoparticulate platinum based catalysts.5,6 In contrast with Pt nanoparticles (NPs), 1D noble metal nanowires (NWs), particularly those composed of Pt and Pd, have been shown to display structure-dependent enhancement in ORR activity.7−11 Additionally, recent reports have demonstrated that noteworthy enhancements in electrocatalytic performance can be achieved by manipulating the composition of 1D noble metal nanostructures.12−16 For example, the incorporation of Au into Pd and non-noble metal NPs as supports for Pt monolayers has been especially promising, because measurable enhancements in ORR activity, stability, and durability can be achieved.17−19 © 2012 American Chemical Society

Received: January 3, 2012 Revised: February 26, 2012 Published: March 27, 2012 2013

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Figure 1. Representative SEM images of the isolated Pd9Au NWs (A) and of a free-standing NW array (B). TEM image of a single Pd9Au NW (C) with a high-magnification image (D) highlighting the central region of the wire. The red box denotes where a high-resolution image (E) was obtained. Inset to panel E shows a selected area denoted by the black box, highlighting well resolved 111 lattice planes. Selected area electron diffraction pattern (F) corresponding to the images in (D) and (E) is shown.

the basis of the calculated lattice parameters (Figure S3A,D, Supporting Information). As expected, the composition of the Pd1−xAux nanostructures (Figure S3B, Supporting Information) correlates with that of the corresponding precursor solution employed during each synthesis, which is in excellent agreement with prior results.23,24 Composition measurements were also obtained on NW collections by scanning electron microscopy and energy dispersive X-ray analysis (SEM-EDAX). The observed trend (Figure S3C, Supporting Information) is in agreement with that of XRD results. In the case of Pd1−xPtx (Figure S3E−F, Supporting Information), the incorporation of Pd is favored slightly in as-prepared NWs, which may potentially arise from faster diffusion of the Pd precursor into the pore space.25,26 The corresponding SEM images obtained on the various Pd1−xAux and Pd1−xPtx NWs (Figure S4,

preparation of homogeneous bimetallic NWs. Our method avoids the need for surfactants, electrochemical equipment, toxic reaction media, and potentially complex physical vapor deposition techniques. Utilizing this method, we have prepared a series of bimetallic Pd1−xAux and Pd1−xPtx NWs with a variety of chemical compositions (x = 0.1, 0.25, 0.5, 0.75, and 0.9) and with control over surface texture and size. In addition, we have demonstrated that these template-based NWs display significant improvements in electrocatalytic activity and durability toward oxygen reduction. X-ray powder diffraction (XRD) obtained on the Pd1−xAux and Pd1−xPtx NWs (Figure S2A,B, Supporting Information) confirms that the NWs are homogeneous alloys with the desired face-centered cubic crystal structure. Vegard’s law was employed to estimate the chemical composition of the NWs on 2014

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Figure 2. Representative SEM images of isolated Pd3Pt7 NWs (A) and of a free-standing NW array (B). TEM image of a single Pd3Pt7 NW (C) with a high-magnification image (D) highlighting the central region of the wire. The red box denotes where a high-resolution image (E) was obtained. Inset to panel E shows a selected area outlined by the black box, highlighting the well resolved 111 and 200 lattice planes. The selected area electron diffraction pattern (F) corresponding to the images in (D) and (E) is shown.

before and after a brief heat treatment (cf. Figures S5 and S6, Supporting Information) revealed that after processing, the Pd9Au NWs are highly textured and largely single crystalline, with short polycrystalline segments restricted to the ends of the NW. The HRTEM image (Figure 1E) obtained along the central single crystalline segment indicates the presence of wellresolved equidistant lattice planes with a spacing of 0.230 nm. The diffraction data (Figure 1F) in combination with the HRTEM images suggest that the long axis of the NWs is oriented along the [111] crystallographic direction. Similar results are observed in the case of the Pd1−xPtx NWs and structural characterization of the Pd3Pt7 NWs is highlighted in Figure 2A−D, as a representative example. However, the diffraction data and associated HRTEM image (Figures 2E,F) reveal that these NWs are not actually single crystalline but rather are composed essentially of an aggregated ensemble of oriented crystallites.

Supporting Information) highlight the uniformity and homogeneity of the samples. It is also apparent that there is no significant difference in diameter, aspect ratio, or surface texture as a function of NW composition. Overall, as-prepared Pd1−xAux and Pd1−xPtx NWs maintain collective diameters of 50 ± 9 and 49 ± 8 nm, respectively, with lengths of up to 6 μm, consistent with the dimensions of the template pores. With the Pd9Au NWs serving as a representative example, asprepared NWs can be isolated as either individual NWs (Figure 1A) or as oriented free-standing NW arrays (Figure 1B), rendering these NWs as excellent candidates for sensing and electronics. Representative transmission electron microscopy (TEM) images (Figure 1C,D) of a single Pd9Au NW show that the NWs are dense and uniform with a distinctive texture and orientation. The surfaces are uniformly faceted and it is apparent that the facet sizes are inherently limited by the roughened uneven texture of the template’s pore wall.13 Selected area electron diffraction (SAED) patterns obtained 2015

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Figure 3. Representative HAADF image of the Pd9Au NWs (A). Immediately below the TEM image, the EDAX maps obtained from the area denoted by the red box of the Pd and Au L-edge are shown on the left and right, respectively, with the combined map shown in the center. A TEM image (B) of a representative cross-section of an as-prepared template membrane containing Pd9Au NWs. EDAX spectra (C) obtained on an individual isolated NW at various points (A)−(F) along the wire are shown, corresponding approximately to those areas spatially highlighted in the TEM image.

Information) reveals that the NWs may become slightly enriched with Pt as it elongates in the template pore. In previous reports by one of our groups, the electrocatalytic ORR performance of Pt monolayers deposited on Pd1−xAux NP/C was explored as a function of the NP composition and it was determined that an optimal performance could be achieved with a composition of Pd9Au.17,18 Hence, by analogy, in this work, we employed high quality, anisotropic Pd9Au NWs to explore the role of morphology in the performance of bimetallic Pd−Au nanostructures toward ORR. Data on the cathodic ORR kinetics of our Pd9Au and Pd NWs were obtained by the thin-layer rotating disk electrode method.27 Specifically, the cyclic voltammograms collected from Pd9Au and Pd NWs (Figure 4A) displayed the characteristic surface oxide formation (0.6−1.0 V) and hydrogen adsorption/desorption (Hads, 0.1 to 0.4 V) regions. The oxide reduction peak of the Pd9Au NWs (0.7963 V) is significantly shifted by ∼20 mV to higher potentials as compared with the Pd NWs (0.7729 V). This result suggests that the Pd9Au NWs should maintain improved ORR performance as a result of the weaker interaction with the adsorbed oxygen species.28,29 Additionally, it is apparent that the smooth shape of the Hads region of the Pd9Au NWs resembles that of the highly active Pt (111) surface.30

In addition to characterizing the crystallinity as a function of position along the NW, we also employed EDAX in scanning TEM mode to gain insight with respect to the uniformity of the NW’s composition. Figure 3A shows a high-angle annular dark field (HAADF) image of a portion of the Pd9Au NW. The contrast (sensitive to Z) is largely homogeneous, suggesting that the NWs maintain relatively uniform and consistent composition throughout their entirety. The few areas of lighter contrast result from the uneven texture of the NW surface as well as porosity within the NW itself. Representative EDAX maps of the Pd9Au and Pd3Pt7 NWs shown in Figure 3A and Figure S7A (Supporting Information) reveal that the spatial distributions of the elements are uniform throughout the NW and that no segregation of the metals into discrete phases is apparent. These results are consistent with the XRD and HRTEM data. EDAX spectra shown in Figure 3C were obtained at various points along the length of an individual isolated Pd9Au NW, corresponding to the locations shown in Figure 3B. The chemical compositions at each point are shown in Table S1 (Supporting Information) and it is apparent that the distribution of Pd and Au is uniform along the length of the entire NW. In contrast, an analogous examination of the Pd3Pt7 NWs summarized in Figures S7B,C and Table S2 (Supporting 2016

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Figure 4. Cyclic voltammograms (A) obtained from as-prepared Pd NWs, AuUPD∼Pd NWs, and as-prepared Pd9Au NWs, respectively. Polarization curves obtained from as-prepared Pd and Pd9Au NWs before (B) and after (D) Pt monolayer deposition. Potential vs specific activity (Jk) plots of Pd9Au NWs and Pt∼Pd9Au NWs are shown as insets to (B) and (D) with Pt NP/C and Pt∼Pd NWs serving as a comparison, respectively. Experimentally calculated kinetic currents normalized to catalyst surface area and platinum mass obtained at 0.9 V vs RHE are shown before (C) and after (E) Pt monolayer deposition.

On the basis of the polarization curves obtained in oxygen saturated 0.1 M HClO4 (Figure 4B), the Pd9Au NWs maintain significantly enhanced activity as compared with the Pd NWs alone. The measured kinetic currents at 0.9 V were normalized to the electrochemical surface area (ESA) (Figure 4C) to gain insight into the intrinsic activity of the Pd surface sites (cf. Supporting Information). The Pd9Au NWs display an outstanding specific activity (JK) of 0.49 ± 0.04 mA/cm2, which is more than double that of the Pd NWs alone (0.21 ± 0.02 mA/ cm2). The activity of the Pd9Au NWs also represents a 2-fold improvement over the corresponding value measured for Pt NPs (0.21 mA/cm2). In fact, it is also apparent from the potential versus kinetic current (E vs JK) plot shown as an inset to Figure 4B that the activity of the Pd active sites exceeds that of commercial Pt NP/C over the entire range of plausible operating potentials. This is a surprising and encouraging result, because we achieved activities greater than that of commercial Pt NPs alone with essentially no discernible Pt loading. The observed superior performance of our Pd9Au nanowires herein is in excellent agreement with a previous report demonstrating the enhanced activity of PdAu bimetallic nanotubes, as compared with conventional catalysts, toward ethanol electroxidation.16

In the existing literature regarding mixed Pd/Au electrocatalysts, the origin of enhanced ORR and alcohol electrooxidation performance has been attributed to the presence of bimetallic Pd−Au pair sites at the surface in some cases and in others, to the unique properties of the PdAu alloy phase.16−18,31,32 To explore the origin of enhancement in our alloyed NWs, we utilized Cu underpotential deposition (UPD) (AuUPD∼Pd NWs) and galvanic displacement (AuGD∼Pd NWs) reactions to deposit Au atoms at the surfaces of elemental Pd NWs.33 These methods were selected because they provide for two types of Pd−Au pair sites at the nanowire surface (e.g., gold clusters in the case of Cu UPD33 and a mixture of gold clusters and porous PdAu in the case of galvanic displacement). Because the Pd NWs and Pd9Au NWs maintain similar dimensions, crystallinity, and surface texture, the role of the Au additive is highlighted. Analysis of these samples by CV (Figure 4A) reveals that the gold modified Pd NWs with Pd−Au pair sites at the surface maintain Hads and oxide formation features that are similar to that of the Pd NWs. It is therefore not surprising that there are only negligible enhancements in the specific ORR activity of the Pd active sites in these samples (Figure 4C). Hence, these results suggest that the origin of enhancement in the Pd9Au NWs is due to their 2017

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Figure 5. Polarization curves (A) obtained from the Pt∼Pd9Au NWs over the course of the durability test. The results of the durability test (inset) are summarized as the percentage of the remaining specific activity over the course of the 30 000 cycles. Representative TEM images of the Pt∼Pd NWs (after 20 000 cycles) and Pt∼Pd9Au NWs (after 30 000 cycles) are also shown in (B) and (C), respectively.

of composition in the measured activity. The specific activity of the Pt∼Pd9Au NWs is also significantly enhanced as compared with the activity measured under membrane electrode assembly (MEA) conditions from analogous Pt∼Pd9Au NPs (i.e., 0.5 mA/cm2),17 thereby highlighting the improvement in activity as a result of use of the 1D NW motif. In addition, we find that the Pt∼Pd9Au NWs maintain a platinum group metal (PGM) activity of 0.16 A/mgPGM, which is slightly enhanced with respect to the value typically obtained from commercial Pt NPs/C with the same total loading. This result is especially encouraging because Pt, which is less abundant than Pd and Au, represents only 7.2% of the total mass of the Pt∼Pd9Au NW sample. The enhanced activity was accompanied by greatly improved stability and after 30 000 cycles of a durability test (cf. Supporting Information), the half-wave potential of the polarization curve shown in Figure 5A decreased by only 6 mV. The high stability is accompanied by a steady increase in the specific activity (inset to Figure 5A) of the Pt∼Pd9Au NWs to 1.53 mA/cm2, which is more than 1.5 fold higher than the original activity. To highlight the stability of the Pt∼Pd9Au NWs, TEM images obtained after 30 000 cycles (Figure 5C) reveal that there is essentially no perceptible change in the structural integrity and texture of the Pt∼Pd9Au NWs. In

homogeneous alloyed structure, as opposed to merely the presence of bimetallic sites localized on the NW surface. We believe that the combination of the advantageous 1D structural motif with the beneficial structural and electronic properties of PdAu alloys18,32,34 accounts for the enhanced performance in the case of ORR. In addition, we also employed our Pd9Au NWs as a substrate for the deposition of a Pt monolayer shell (Pt∼Pd9Au NWs) prepared by Cu UPD/galvanic displacement. It is apparent from the CVs (Figure S8, Supporting Information) that a Pt monolayer shell has been deposited and that the Pt∼Pd9Au NWs maintain a favorable shift in the oxide reduction peak (0.8093 V) when compared with the Pt∼Pd NWs (0.7975 V). The corresponding polarization curves (Figure 4D) as well as E vs JK curves (inset to Figure 4D) confirm the high ORR activity of the Pt∼Pd9Au NWs when compared with the Pt∼Pd NWs. As tangible evidence, specific and platinum mass activities at 0.9 V of the Pt∼Pd9Au NWs were determined to be 0.95 ± 0.03 mA/cm2 and 2.08 ± 0.05 A/mgPt, respectively. Figure 4E compares the measured specific and platinum mass activity of the Pt∼Pd9Au NWs with the corresponding activities obtained from Pt∼Pd NWs, Pt∼Pd NPs, and commercial Pt NPs, respectively. It is apparent that a 2-fold enhancement is achieved over the activity of Pt∼Pd NWs, highlighting the role 2018

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alloy results in significant and measurable enhancements in both ORR performance and durability. We also explored the cathodic ORR performance as a function of composition in the as-prepared Pd1−xPtx NWs, and the results are summarized in Figure 6. The CVs (Figure 6A) highlight a transition in the structure of the Hads and oxide region to that of the elemental Pt NWs, as the proportion of Pt is increased in the as-prepared alloy NW samples. In fact, the specific activity measured at 0.9 V (Figure 6C) of the NWs increases from 0.64 ± 0.01 to 0.79 ± 0.01 mA/cm2 as the Pt content correspondingly rises from 50 to 80%. This trend is further highlighted by the E vs JK curves shown as an inset to Figure 6B. Based on prior studies,12−15 it is not entirely surprising that the activity of these bimetallic catalysts greatly surpasses the corresponding activity of both commercial Pt NPs (0.21 mA/cm2) and elemental Pd NWs (0.20 mA/cm2). However, an unexpected finding is that the activity of our PtPd NWs (0.64 mA/cm2) also exceeds that of elemental Pt nanotubes with an outer diameter of 200 nm previously prepared and studied by our team under identical conditions,8 while only maintaining 50% of the Pt content. In addition, we find that the activity of the Pt9Pd NWs (0.79 mA/cm2) is essentially equivalent to the activity measured for analogous Pt NWs with approximately the same diameter (0.82 ± 0.04 mA/cm2). This fact is highlighted by the polarization curves (Figure 6B), which indicate that the PdPt4 NWs maintain performance almost identical to that of the Pt NWs when the same quantity of metal is present on the electrode. In contrast, previous reports regarding alloyed PtPd nanoparticles have demonstrated that the activity of Pt1−xPdx (x = 0.7 - 0.9) alloys exceeds that of pure Pt NPs as a result of a structural contraction induced by the incorporation of Pd atoms.35 In the case of NWs however, the enhanced activity of these systems has been generally attributed to a size-induced contraction of the nanowire surface, resulting in an advantageous change in the electronic properties of the nanowire.3,7,10,12,13,36 Hence, the observed activity trend for Pd1−xPtx NWs in this report is more complicated and suggests that the size-induced contraction phenomenon may be dependent upon and influenced by not only their diameter but also their inherent chemical composition. Although it is beyond the scope of this report, additional investigation of the electronic and structural properties of these 1D bimetallic catalysts will be required to more precisely evaluate the origin of the activity trend. In this report, we have employed the U-tube double diffusion device as an ambient and surfactantless method to prepare bimetallic nanowires with rational control over their composition, crystallinity, and spatial dimensions. As-prepared Pd1−xAux and Pd1−xPtx nanowires display superior electrocatalytic performance as oxygen reduction catalysts as compared with commercial Pt nanoparticles alone, and more importantly, the correlation between composition and activity has been examined in these highly 1D anisotropic systems. The measured structure−property correlation highlights the unique advantages of our one-step synthetic approach because high quality 1D catalysts can be generated with predictable structure and composition in an efficient manner. Hence, the U-tube synthetic approach has the potential for the sustainable generation of bimetallic nanostructures with broad applicability, particularly in the optimization of highly effective electrocatalysts.

Figure 6. Cyclic voltammograms (A) obtained from as-prepared Pd1−xPtx NWs. Polarization curves (B) obtained from as-prepared Pd, PdPt4, and Pt NWs at 1600 rpm (anodic sweep direction). The potential vs specific activity (E vs Jk) plot of various Pd1−xPtx NWs is shown as an inset with Pt NP/C serving as a comparison. (C) Electrochemical surface area activities (specific activities) of the series of Pd1−xPtx NWs are shown by comparison with both as-prepared, phase-pure Pt, and Pd NWs synthesized in an analogous manner as well as with previously reported Pt NTs.

contrast, the analogous Pt∼Pd NWs (Figure 5B) show an apparent evolution of a porous structure as a result of significant dissolution of the Pd core after only 20 000 cycles. Hence, the addition of 10% Au forming a stabilizing uniform 2019

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(17) Sasaki, K.; Naohara, H.; Cai, Y.; Choi, Y. M.; Liu, P.; Vukmirovic, M. B.; Wang, J. X.; Adzic, R. R. Angew. Chem., Int. Ed. 2010, 49 (46), 8602−8607. (18) Xing, Y.; Cai, Y.; Vukmirovic, M. B.; Zhou, W.-P.; Karan, H.; Wang, J. X.; Adzic, R. R. J. Phys. Chem. Lett. 2010, 1 (21), 3238−3242. (19) Liang, H.-W.; Liu, S.; Yu, S.-H. Adv. Mater. 2010, 22 (35), 3925−3937. (20) Patete, J. M.; Peng, X.; Koenigsmann, C.; Xu, Y.; Karn, B.; Wong, S. S. Green Chem. 2011, 13 (3), 482−519. (21) Gewirth, A. A.; Thorum, M. S. Inorg. Chem. 2010, 49 (8), 3557−3566. (22) Markovic, N. M.; Schmidt, T. J.; Stamenkovi, V.; Ross, P. N. Fuel Cells 2001, 1 (2), 105−116. (23) Zhang, F.; Sfeir, M. Y.; Misewich, J. A.; Wong, S. S. Chem. Mater. 2008, 20 (17), 5500−5512. (24) Zhang, F.; Wong, S. S. ACS Nano 2009, 4 (1), 99−112. (25) Cui, S. T. J. Chem. Phys. 2005, 123 (5), 054706−4. (26) Crank, J. The Mathematics of Diffusion; Oxford University Press: Oxford, U.K., 1975. (27) Garsany, Y.; Baturina, O. A.; Swider-Lyons, K. E.; Kocha, S. S. Anal. Chem. 2010, 82 (15), 6321−6328. (28) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H. J. Phys. Chem. B 2004, 108 (46), 17886−17892. (29) Nørskov, J. K.; Bligaard, T.; Rossmeisl, J.; Christensen, C. H. Nature Chem. 2009, 1 (1), 37−46. (30) Zhang, J.; Mo, Y.; Vukmirovic, M. B.; Klie, R.; Sasaki, K.; Adzic, R. R. J. Phys. Chem. B 2004, 108 (30), 10955−10964. (31) Cheng, F.; Dai, X.; Wang, H.; Jiang, S. P.; Zhang, M.; Xu, C. Electrochim. Acta 2010, 55 (7), 2295−2298. (32) Sarapuu, A.; Kasikov, A.; Wong, N.; Lucas, C. A.; Sedghi, G.; Nichols, R. J.; Tammeveski, K. Electrochim. Acta 2010, 55 (22), 6768− 6774. (33) Zhang, J.; Sasaki, K.; Sutter, E.; Adzic, R. R. Science 2007, 315 (5809), 220−222. (34) Damjanovic, A.; Brusic, V.; Bockris, J. O. M. J. Phys. Chem. 1967, 71 (8), 2741−2742. (35) Li, H.; Sun, G.; Li, N.; Sun, S.; Su, D.; Xin, Q. J. Phys. Chem. C 2007, 111 (15), 5605−5617. (36) Wang, S.; Jiang, S. P.; Wang, X.; Guo, J. Electrochim. Acta 2011, 56 (3), 1563−1569.

ASSOCIATED CONTENT

S Supporting Information *

Details of experimental procedures (including electrochemical protocols) and additional figures and data, including X-ray diffraction, electron microscopy, and electron diffraction results. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Research (including support for SSW and electrochemical experiments) was supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. We thank Dr. J. Quinn and Dr. A. Santulli for their assistance with obtaining SEM-EDAX measurements. We also acknowledge S. Van Horn and the Central Microscopy Imaging Center (CMIC) at Stony Brook University for assistance with preparing the microtome cross sections. Experiments in this manuscript were performed in part at the Center for Functional Nanomaterials located at Brookhaven National Laboratory, which is supported by the U.S. Department of Energy under Contract No. DE-AC02-98CH10886.

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