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Au-doped Stable L1 Structured Platinum Cobalt Ordered Intermetallic Nanoparticle Catalysts for Enhanced Electrocatalysis Kurian A. Kuttiyiel, Shyam Kattel, Shaobo Cheng, Ji Hoon Lee, Lijun Wu, Yimei Zhu, Gu-Gon Park, Ping Liu, Kotaro Sasaki, Jingguang G Chen, and Radoslav R. Adzic ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00555 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 3, 2018

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ACS Applied Energy Materials

Au-doped Stable L10 Structured Platinum Cobalt Ordered Intermetallic Nanoparticle Catalysts for Enhanced Electrocatalysis Kurian A. Kuttiyiel †§, Shyam Kattel §, Shaobo Cheng ‡, Ji Hoon Lee †, Lijun Wu ‡, Yimei Zhu*‡, Gu-Gon Park †⊥⊥, Ping Liu †, Kotaro Sasaki*†, Jingguang G. Chen*†§, Radoslav R. Adzic†

†

Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USA

§

Department of Chemical Engineering, Columbia University, New York, NY 10027, USA

‡

Department of Condensed Matter Physics and Materials Science, Brookhaven National

Laboratory, Upton, NY 11973, USA ⊥

Fuel Cell Laboratory, Korea Institute of Energy Research, Daejeon 305-343, South Korea

KEYWORDS:

electrocatalysis, fuel cells, intermetallics, core-shell nanoparticles, oxygen

reduction reaction

ABSTRACT

Bimetallic Pt3Co alloys are the commercial electrocatalysts for the oxygen reduction reaction in a fuel cell, but their high Pt loading and durability are a concern. Working towards the goal of reducing the amount of Pt and simultaneously increasing the activity and stability of the catalyst

ACS Paragon Plus Environment

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we describe two new structures of ordered intermetallics consisting of Pt4Co5 nanocatalyst protected by Au atoms. Varying the temperature for the formation of the intermetallics, two distinct PtCo structural characteristics were observed in the nanoparticles: one with a simple intermetallic structure and the other with an intermetallic structure core protected by a shell of Pt atoms. The improved electrocatalytic activity and durability are attributed to the atomically ordered structure of PtCo nanoparticles along with protective surface Au atoms as confirmed by DFT and experimental results.

INTRODUCTION

Improving electricatalysts to alleviate the slow kinetics of oxygen reduction reaction (ORR) has been one of the important issues in development of polymer electrolyte membrane fuel cells (PEMFCs)1-3 and metal-air batteries.4-5 Platinum (Pt) and its alloys are the best catalysts for ORR cathodes.6-8 However, these catalysts are expensive and undergo low utilization efficiency; such the barriers of ORR catalysts still hamper the widespread adoption in PEMFCs. A number of methods have been proposed for the fabrication of electrocatalysts to mitigate these problems.9-13

Recently, intermetallics consisting of Pt alloys, especially the AuCu3-type

structure (L12), have attracted attention as electrocatalyst for the ORR.14-15

Particularly,

bimetallic Pt3Co catalyst consisting of L12 ordered intermetallic structures and random alloyed nanoparticles has been widely studied as readily viable electrocatalysts.16-20 However, these catalysts have high Pt content and poor durability as the harsh oxidative conditions of the fuel cells decrease their performance. Although recent studies of doping transition metal oxides have


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greatly improved the catalyst performance,21 these catalysts still have large amount of Pt content and their stability still remains a concern. Modern computational screening techniques confirm that surface atomic arrangement plays an important role on the kinetics of the ORR.22-24 Changes in the surface structure of the nanoparticle (NP) by engineering core-shell and intermetallic structures can have large impacts on the ORR activity.16, 25-27 We introduce here two distinct PtCo NP structures that emphasize the important role on the kinetics of the ORR. Pt3Co is known to form stable ordered intermetallic structures, but reducing the Pt content can drastically change its structural stability.17,

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Here we design and synthesize stable AuCu-type structure (L10) ordered Pt4Co5

NPs with highly hierarchical structures by doping a small amount of Au atoms to the alloy. Au atoms (10 at-%) play an important role in reducing the overall Pt content in the nanostructures, and they also behave like a matrix or stabilizer shells while restructuring PtCo atoms.26, 29 The restructuring of NPs were controlled by heat treatment of the chemically synthesized core-shell structured NPs under flowing H2/Ar gas atmosphere. Varying the temperature, two distinct characteristics of structurally ordered NPs were obtained. Based on X-ray powder diffraction (XRD) and scanning transmission electron microscopy (STEM) coupled with high angle annular dark field detector (HAADF) results, the NPs at 700°C tend to form a core-shell structure with the L10 PtCo ordered-intermetallic structured core surrounded by a thin layer of Pt shell, whereas NPs treated at 800°C form L10 PtCo ordered intermetallic structure. Using this strategy the NPs exhibited the highest Pt mass activities for the ORR among the Pt-Co systems reported in the literature16 under similar testing conditions along with greatly enhanced durability, making them promising candidates for advancing the proton exchange membrane fuel cells.


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EXPERIMENTAL PROCEDURES

Vulcan carbon XC72R was ultrasonically mixed with Co(HCO2) 2H2O salt in an aqueous solution. Trisodium citrate dehydrate (Na3C6H5O7•2H2O, 99%) at a mass ratio of 2:1 with respect to the Co salt was added to the solution. After the mixture was purged with Ar under ultrasonication for one hour, NaBH4 was added to reduce the Co salt. After five minutes of reaction time, the K2PtCl4 and HAuCl4 H2O mixture was added to the solution and sonicated for one hour to obtain AuPtCo core-shell NPs. The mixture was washed, rinsed with Millipore water, and then dried. The core-shell NPs obtained had a molar fraction of Au10Pt40Co50. To achieve atomic structure ordering these NPs were annealed at 700°C and 800°C in H2/Ar stream for 30 minutes in a tube furnace. All other technical details of experimental procedures such as electrochemical testing, XRD measurements, high-resolution TEM and Computational Methods are described in the Supporting Information.


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Figure 1. XRD patterns of AuPtCo/C NPs with its zoomed-up profiles in the inset. Also shown is a schematic of the L10 PtCo ordered-intermetallic structure. The formation of ordered PtCo NPs with two distinct structures by varying the temperature was first identified by XRD. As shown in the XRD pattern of Figure 1, the as-synthesized (AuPtCo/C-AS, black line) NPs have a typical face-centered cubic (fcc) features with distinct Au and PtCo peaks. Upon the heat treatments at 700°C (denoted as AuPtCo/C-700, red line) and 800°C (denoted as AuPtCo/C-800, blue line), the peaks around 41o shifted toward higher 2-theta values with strengthened peak intensities due to the significant crystalization during the heat treatment. Notably, AuPtCo/C-700 shows prominent peaks at 23o and 33o, originating from (001) and (100) planes in a tetragonal symmetry (P4/mmm, a=b=2.68 Å, c=3.80 Å), respectively. The peak intensity ratio (I100/I101) of (100) and (101) reflections, as a measure of the extent of the ordering of Pt and Co, was calculated 13.2%, which is similar to those in the previous reported.30-31 However, those peaks become weakened in AuPtCo/C-800, indicating that such an ordering of Pt and Co would be less favorable at a high temperature treatment (i.e., 800 oC), which is in the same line with the lowered (I100/I101) value (5.9%). Based on (200) peaks for both samples, those particle sizes are calculated to ~7 nm using the Sherrer equation.


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Figure 2. a) STEM-HAADF image of an AuPtCo/C-700 NP along with its EDS mapping. b) Line profiles extracted from the white boxed area in (a) showing the thickness (~0.7 nm) of surface Pt. c) High resolution STEM-HAADF image viewed along [100] direction, showing the Pt surface layer and the inner intermetallic structure of PtCo. The top-left part is the fast Fourier transform (FFT) pattern, which can be indexed as the (100) zone of the tetragonal structure of the given sample. The bottom-left part is the magnified image from the red boxed area with the [100] projection of the atomic structure embedded. The red and green spheres represent Pt and Co, respectively. d) Intensity profile for the line marked as Line 1 and Line 2 in (c) showing the regularly repeated arrangement of Pt and Co atoms (Line 1) and Pt surface atoms (Line 2), respectively.

Low magnification TEM images from the NPs reveal that both AuPtCo/C-700 and AuPtCo/C800 NPs have overall spherical shape (Supporting Information Figure S1). Although the particle


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size distribution ranges from 5 nm to 30 nm, the average size is around 7 nm, consistent with the value estimated from the XRD measurements. The microstructure of both the NPs was studied by STEM-HAADF images along with two-dimensional energy dispersive spectroscopy (EDS) spectrum imaging. Figure 2 represents the microstructure of AuPtCo/C-700 NPs indicating 2-3 monolayer thick Pt shell covering the PtCo ordered intermetallic core. STEM-HAADF image of a single representative AuPtCo/C-700 particle along with its EDS mapping is shown in Figure 2a. The Pt (green) versus the Co (red) distributions within the particle demonstrates a welldispersed alloy with Pt on the surface. A line profile (Figure 2b) extracted from the edges of the NP to the interior (white boxed area in Figure 2a) reveals that the Pt shell thickness is 0.7 nm, which are 2-3 atomic layers. This unique core-shell structure of the NP can be further confirmed by atomic resolution STEM-HAADF image with its intensity proximately proportional to Z1.7 (Z is the atomic number) along the atom column, as shown in Figure 2c. The regularly-positioned bright and weak dots correspond to Pt and Co atoms, respectively, which is a direct visualization of the ordered intermetallic structure. Along the edge of the NP core, a 1~2 unit-cell-thick Pt surface layer is observed to cover the inner intermetallic PtCo structure. The fast Fourier transform (FFT) patterns from the images in Figure 2c and Figure S2 (another NP), viewed along the [100] direction, confirm the tetragonal structure. The line intensity profile gives a better understanding of the surface layers as shown in Figure 2d. The Line 1 intensity profile taken from a selected area in the STEM-HAADF image (Line 1 arrow in Figure 2c) indicates the alternating arrangement of Pt and Co. The high intensity peaks are for Pt atoms while the low intensity ones correspond to Co atoms suggesting the ordered arrangement of PtCo atoms. On the other hand, the Line 2 intensity profile taken from the surface of the NP (Line 2 arrow in


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Figure 2c) reveals the arrangement of only Pt atoms on the surface, which is in good agreement with the EDS line profile (Figure 2b).

Figure 3. a) STEM-HAADF image of AuPtCo/C-800 NP along with its EDS mapping. b) Line profiles extracted from the white boxed areas in (a) showing no Pt shell layers. c) High resolution STEM-HAADF image viewed along [100] direction (the left) for the red box area in (a), showing intermetallic structure of PtCo. The magnified image of the red box area is shown in the bottom-right with the atomic model. The top-right is the FFT pattern of the image. d) Intensity profile for white rectangle area in (c) showing the modulated structure for PtCo.

Similar STEM studies were carried out for the AuPtCo/C-800 NPs as represented in Figure 3. The STEM-HAADF image of a single AuPtCo/C-800 NP along with its EDS mapping (Figure 3a) shows the well-dispersed distribution of Pt and Co atoms in the NP. Its corresponding EDS


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line mapping taken from the edge of NP to the interior (white boxes) clearly shows that the Pt and Co line scans overlap each other (Figure 3b). It indicates that the situation where Pt segregates along the NP core as in the case of AuPtCo/C-700, is not thermodynamically favorable due to the high temperature treatment. This is further verified by the intensity line scan profile (Figure 3d) taken from the zoomed-up STEM HAADF image (Figure 3c). The intensity line profile representing the Z-contrast of materials clearly shows the alternating arrangement of Pt and Co atoms starting right from the edge of the NP. This along with EDS mapping ensures that the surface of the AuPtCo/C-800 NPs is not fully terminated with Pt layers. The FFT pattern from the NP clearly confirm again the tetragonal structure of the intermetallic PtCo phase. A careful inspection on image intensity, however, indicates that the intensity difference between Pt and Co sites in AuPtCo/C-800 NP (Figure 3c) is smaller than that in AuPtCo/C-700 NP (Fig. 2c), especially in the interior of the nanopaticle. This implies that Pt and Co becomes less ordered in AuPtCo/C-800 NP, consistent with the XRD result (Figure 1). Based on the combined XRD and STEM analyses, it is notable that the simple control in heating temperatures can manipulate the resultant Pt shell along the NPs even with the same composition, enabling us to set a structural platform to see how it would affect the electrocatalytic performance.


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Figure 4. a) CV curves for the intermetallic NPs in 0.1M HClO4 acid (sweep rate: 20 mV s-1). b) ORR polarization curves for both the AuPtCo/C NPs along with Pt/C catalyst (scan rate: 10 mV s-1). c) ORR polarization and CV curves (inset) of AuPtCo/C-800 before and after cycling. d) Comparison between specific and mass activities at 0.9V for Pt/C, AuPtCo/C-700 and AuPtCo/C-800 catalyst. The cyclic voltammetry (CV) curves for the thin-film electrodes of AuPtCo/C-700 and AuPtCo/C-800 recorded in Ar-saturated 0.1 M HClO4 acid solution show remarkable differences in their features (Figure 4a). Though the hydrogen adsorption/desorption (Hupd) and oxygenspecies adsorption/desorption regions are similar to those of Pt/C, their magnitudes are much


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smaller (suppressed). The suppression of the Hupd region could be caused by the presence of Au atoms on the surface of the NP. Even though the amount of Au is the same for both catalysts, the suppression of the Hupd region for the AuPtCo/C-800 catalyst is higher compared to AuPtCo/C700 justifying the presence of Pt shell on the latter.32 Moreover, due to the absence of the Pt shell on AuPtCo/C-800 NPs the surface Co/Co oxide atoms quickly dissolve in the acid conditions without affecting the electrocatalytic activity of these NPs by creating a Pt-skin surface. The CO stripping curves recorded for electrochemical oxidation of adsorbed CO (COads) monolayer obtained from RDE (Supporting Information Figure S3) show a positively shifted value of 0.94 V for both intermetallic NPs. This positive shift compared to Pt was well explained using theoretical calculations affirming that due to the larger lattice constant of Au, the d-band center of surface Pt is shifted closer to the Fermi level resulting in poor oxidation of COads.33-35 The delaying of COads oxidation suggests that the nobility of the intermetallic surface is preserved at such high potential making the Pt surface facile to catalyze and release the oxygen from the surface in the form of H2O. The ORR polarization curves for the catalysts in O2-saturated 0.1 M HClO4 at 1600rpm are presented in Figure 4b. The half-wave potential (E1/2) measured for the core-shell structured AuPtCo/C-700 ordered intermetallic NPs from the ORR curves reveal a 14 mV increase than AuPtCo/C-800 NPs. The mass and specific activity (Figure 4d) at 0.9V calculated from the kinetic currents determined from the intercepts of the Koutecky–Levich plot (Supporting Information Figure S4) shows that AuPtCo/C-700 NPs has the highest activity. Owing to the altered electronic/adsorption properties of the intermetallic surface and in order to diminish the underestimation from Hupd, the charge calculated by integration of the area under the CO stripping peak was used to estimate the electrochemical surface area (ECSA) (Supporting Information Table S1). The Pt mass activities of the AuPtCo/C-700 and AuPtCo/C-800


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electrocatalyst were 0.68 A mg-1 and 0.49 A mg-1 respectively at 0.9 V/RHE, whereas their specific activities were 0.53 mA cm-2 and 0.45 mA cm-2, respectively. The precious metals (Pt and Au) mass activities of the AuPtCo/C-700 and AuPtCo/C-800 electrocatalysts were 0.54 A mg-1 and 0.39 A mg-1, respectively. The stability tests were performed by applying potential cycles between 0.6 and 1.0 V in air-saturated 0.1 M HClO4 at a sweep rate of 50 mV s–1. For the AuPtCo/C-800 electrocatalyst (Figure 4c), the Hupd region between +0.05 and +0.40 V increased slightly after 5,000 potential cycles, most likely due to the dissolution of some Co/Co oxides on the catalyst surface. Further cycling up to 10,000 potential cycles shows no change in the Hupd region suggesting a stable Pt shell on the surface. However, this was not the case for the AuPtCo/C-700 electrocatalyst, which showed no change in the Hupd region after 5,000 cycles indicating that the AuPtCo/C-700 nanoparticle surfaces were Pt-rich right after preparation (Supporting Information Figure S5). The commercial Pt/C catalyst, after 10,000 cycles (Supporting Information Figure S6) exhibited a shift of 12 mV for the E1/2 and 18.0 % loss of the initial mass activity due to the Pt dissolution and Pt NP agglomeration.36 On the other hand, both the intermetallic catalysts showed an increase in its ECSA (Figure 4c (inset) and Supporting Information Figure S5a) and also exhibited negligible loss in mass activity (Figure 4d).


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Figure 5. a) DFT optimized geometries of a) PtCo intermetallic NP, b) cross section of PtCo@Pt core-shell NP, c) O on the PtCo@Pt core-shell NP and d) OH on the PtCo@Pt coreshell NP. e) DFT calculated binding energies (in eV) of O and OH at the most stable sites of the Pt, PtCo-intermetallic and PtCo@Pt core-shell NPs. f) Au surface segregation energies and the position of an Au atom on the subsurface and surface of the PtCo@Pt core-shell NP. (Pt: blue, Co: purple, Au: gold, O: red and H: white).


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To better understand the enhanced ORR activity of the AuPtCo/C-700 (PtCo@Pt) core-shell NP, DFT calculations were performed using ~2.2 nm sphere-like truncated octahedral Pt [Pt405] NP, PtCo-intermetallic [Pt197Co208] NP, and PtCo@Pt [Pt365Co40] core-shell NP as shown in Figure 5a and 5b. The PtCo@Pt core-shell NP consists of two monolayers of Pt shell, which are consistent with the Pt shell thickness of AuPtCo/C-700 NP from the TEM measurements. The binding energy of the ORR intermediates, oxygen (O) and hydroxyl (OH), previously predicted as descriptors of ORR activity,37-38 were calculated on the (111) terraces of Pt, PtCo-intermetallic and PtCo@Pt core-shell NPs, which are found to be more active than the step edges toward the ORR.23, 39 Au was not explicitly included in PtCo NPs models since the trend in O/OH binding energies has been shown to be unaltered with or without Au present on a Pt alloy nanoparticle.40 The DFT calculations show that PtCo@Pt core-shell NP binds atomic O 0.2 eV more weakly than Pt NP, as shown in Figure 5e. Such amount of weakening in oxygen binding energy lies close to that to achieve the maximum ORR activity according to the volcano plot identified previously.37-38 The weakened oxygen binding is able to accelerate the reduction of O to OH on the PtCo@Pt core-shell NP, and the destabilized OH on the PtCo@Pt core-shell NP suggests that the OH reduction to H2O, the final product of ORR in acid medium, is also facilitated on the PtCo@Pt core-shell NP. In comparison, the DFT calculations show PtCo-intermetallic NP, representing AuPtCo/C-800, binds O (Figure 5c) and OH (Figure 5d) more strongly by 1.37 and 1.11 eV, respectively, than the Pt NP (Figure 5e) due to the presence of Co on the shell. Consequently, the reduction of O/OH and therefore the ORR is likely more difficult on PtCointermetallic NP than on Pt NP. This is in contradiction to the experimental results. This inconsistency can be due to the absence of Co/Co oxide surface atoms for the PtCo-intermetallic NPs (AuPtCo/C-800), which easily dissolve in the acid conditions during the initial stage of


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ORR, creating a Pt-skin on the surface.16 Overall, the DFT results predict that the presence of the Pt shell on the intermetallic core can affect the ORR activity and it should be superior to the Pt NP due to slightly weakened O and OH binding, in line with the corresponding experimental measurements. DFT calculations and experimental results confirm that atomic-level surface morphology plays an important role on the kinetics of ORR. Stable L10 structured PtCo nanoparticles can be synthesized by controlling the restructuring of its atoms by varying the temperature and doping of Au atoms. Experimental surface electrochemistry has shown that surface Co dissolution from the AuPtCo/C-800 catalyst creates a Pt-skin, which has an impact on the initial ORR activity, suffering 28% (mass activity) compared to the core-shell structured AuPtCo/C-700 NPs. The overall structural integrity of the NPs was maintained due to the presence of Au atoms on the surface. Even though Au is not an active catalyst for ORR, recent studies have shown that Au clusters can stabilize Pt and its alloys.36, 41-42 DFT calculations were carried out to identify the preferential position of Au in the AuPtCo intermetallic NPs. Since Au concentration in our synthesized AuPtCo NPs is low, to this end, segregation of single Au atom from the subsurface to the surface of the PtCo@Pt core-shell NP was considered in the DFT calculations. The surface segregation energy of Au in the PtCo@Pt core-shell NP was calculated as an energy difference between the total energy of the PtCo@Pt core-shell NP with an Au atom in the subsurface and the PtCo@Pt core-shell NP with an Au atom on the surface as shown in Figure 5f. It was found, as expected,42 that Au prefers to segregate to the surface of the PtCo@Pt core-shell NP. The DFT calculated segregation energy in Figure 5f indicates that Au preferentially occupies corners and edges of the NP. Thus, when the concentration of Au is increased in AuPtCo NPs, corners and edges would most likely be first populated by Au followed by the segregation of Au to the


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terraces. Surface low-coordination sites such as corners and edges are considered to be vulnerable to oxidation/dissolution.21 It may well be that the preferential segregation of Au atoms at corners and edges blocks the initiation of Pt oxidation, and thereby improves the stability of surface Pt.36, 22] Co dissolution from the cores may be effectively suppressed, since segregated Au atoms also block the dissolution paths for Co.

CONCLUSIONS

We successfully developed a method to synthesize structurally ordered active and stable L10 PtCo NP catalysts with addition of some Au atoms. Our analyses reveal that its enhanced activity and stability are attributed to the ensemble between the three constituents of the electrocatalyst. The doping of Au preserves the nobility of the intermetallic surface, and modifies surface Pt to a slightly weakened O and OH binding energy, facilitating the release of oxygen from the surface in the form of H2O. This study offers a new approach for synthesizing fuel cell catalysts with less Pt while exhibiting both exceptional electrocatalytic activity and enhanced stability.

ASSOCIATED CONTENT

Supporting Information. Experimental procedures, electrochemical data and TEM imaging

AUTHOR INFORMATION

Corresponding Authors *Kotaro Sasaki ([email protected]); Jingguang G. Chen ([email protected]); Yimei Zhu ([email protected]).


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ACKNOWLEDGMENT This manuscript has been authored by employees/guests of Brookhaven Science Associates,

LLC under Contract No. DE-SC0012704 with the U.S. Department of Energy. The publisher by accepting the manuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. This work was also supported by the International Collaborative Energy Technology R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20158520030830). The DFT calculations were performed using computational resources at the Center for Functional Nanomaterials, a user facility at Brookhaven National Laboratory, and at the National Energy Research Scientific Computing Center (NERSC), which is supported by the Office of Science of the U.S. DOE under Contract No. DE-AC02-05CH11231. Authors from Columbia University acknowledge support by the U.S. Department of Energy (DE-FG0213ER16381).

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Au-doped Stable - ACS Publications

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