Refined Structural Analysis of Connected PlatinumâIron Nanoparticle

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Refined Structural Analysis of Connected Platinum–Iron Nanoparticle Catalysts with Enhanced Oxygen Reduction Activity Hidenori Kuroki, Takanori Tamaki, Masashi Matsumoto, Masazumi Arao, Yohei Takahashi, Hideto Imai, Yoshitaka Kitamoto, and Takeo Yamaguchi ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00295 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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ACS Applied Energy Materials

Refined Structural Analysis of Connected Platinum– Iron Nanoparticle Catalysts with Enhanced Oxygen Reduction Activity Hidenori Kuroki,†,‡ Takanori Tamaki,†,‡ Masashi Matsumoto,§ Masazumi Arao,§ Yohei Takahashi,§ Hideto Imai,§ Yoshitaka Kitamoto,†,# and Takeo Yamaguchi†,‡,┴ * †

Kanagawa Institute of Industrial Science and Technology (KISTEC), R1-17, 4259 Nagatsuta,

Midori-ku, Yokohama, Kanagawa 226-8503, Japan. ‡

Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of

Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8503, Japan. §

Device-functional Analysis Department, NISSAN ARC LTD., 1 Natsushima, Yokosuka,

Kanagawa 237-0061, Japan. #

Department of Materials Science and Engineering, Tokyo Institute of Technology, 4259

Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8502, Japan. ┴

Core Research for Evolutionary Science and Technology, Japan Science and Technology

Agency (JST-CREST), 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8503, Japan. *Prof. Takeo Yamaguchi, E-mail: [email protected] 1 ACS Paragon Plus Environment

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ABSTRACT

Carbon-free connected Pt–Fe nanoparticle catalysts exhibit excellent oxygen reduction reaction (ORR) activity. Unlike conventional ORR catalysts, connected Pt–Fe catalysts comprise a beaded network structure formed by the connection of Pt–Fe nanoparticles. The refined structural analyses reveal that the connected Pt–Fe network comprises a polycrystalline structure, whereas the commercial Pt nanoparticle on carbon is formed by a single crystal. In addition, a Ptrich thin layer made of three-to-four platinum atomic layers with high-index stepped facets was observed on the catalyst surface in the connected Pt–Fe catalyst after electrochemical treatment in an acid electrolyte. Lattice spacing analysis showed that the Pt‒Pt bond distance in the Pt-rich surface layer is longer than that inside the network of Pt–Fe alloys. Such surface structures play a beneficial role in enhancing ORR activities. The obtained results provide guidelines for catalyst structural designs that can improve ORR activity. KEYWORDS connected Pt–Fe nanoparticle catalyst, beaded network, polycrystalline, Pt-rich layer, lattice fringe spacing, oxygen reduction reaction, polymer electrolyte fuel cells

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In recent years, electrocatalysts have been developed extensively to improve the oxygen reduction reaction (ORR) activity and durability in fuel cell applications.1-3 Moreover, nanostructured Pt-based catalysts have attracted immense attention and many such structures have been reported, for example, Pt-based nanowires,4-7 nanotubes,8-10 nanostructured thin ﬁlms,11 aerogels,12 nanoframes,13 and porous structure catalysts.14-17 These catalysts possess unique structural features, unlike a conventional Pt nanoparticle catalyst, which is supported on carbon black (Pt/C, Figure 1A). Such Pt-bulk-like surfaces exhibit significantly enhanced ORR activities,11, 18-21 which lower the cost of fuel cells. Therefore, further improvement in the ORR catalyst performance will catapult the widespread use of fuel cells. In addition, carbon-free ORR catalysts have also attracted much attention. A conventional Pt catalyst involves carbon as a support material; however the electrochemical corrosion of a carbon support during the fuel cell start-stop cycles causes severe degradation of fuel cell performances.22 Carbon-free catalysts can eliminate the carbon-corrosion problem, resulting in an improved durability. We recently developed a carbon-free catalyst for a connected Pt–Fe nanoparticle catalyst with a porous, hollow capsule structure (see Figure 1B).23,

24

This connected Pt–Fe catalyst

comprises a beaded network formed by the connection of Pt–Fe alloy nanoparticles prepared via supercritical fluid treatment. The beaded network retains a nanosized crystallite size in the range of 6–7 nm. Therefore, the connected Pt–Fe catalyst has a high surface area. In addition, the Ptalloy metallic nanonetwork is electronically conductive; therefore, no carbon support is required. We succeeded in demonstrating a high fuel-cell performance and excellent durability for the carbon-free catalyst layer using the connected Pt–Fe catalyst against start/stop cycles in the membrane-electrode-assembly operation at 80 °C. Moreover, the ORR specific activity of the connected Pt–Fe catalyst was found to be nine times higher than that of commercial Pt/C (see 3 ACS Paragon Plus Environment


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Figures 1C and 1D). More importantly, the connected Pt–Fe catalyst exhibited enhanced specific activity in comparison to Pt–Fe alloy nanoparticles (without any nanoparticle connection) on carbon black (Pt–Fe/C). The result suggests that a beaded network and carbon-free structure in connected nanoparticle catalysts would be beneficial for ORR. However, previous studies have not clarified the detailed structures of the connected Pt– Fe network. Clarification on the network structure would allow us to determine the factors that could enhance its ORR activity. To understand the relation of the network structure with the ORR activity, it is important to know the catalyst surface structures at sites where ORRs occur in an activity test. The relation between catalyst structures and activities provides the guidelines to design and develop a high-performance catalyst.


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Figure 1. Schematic and TEM images for (A) a conventional Pt/C catalyst and (B) a carbon-free connected Pt–Fe catalyst with a hollow-capsule structure. (C) Linear sweep voltammetry curves in an O2-saturated 0.1 M HClO4 electrolyte solution for the catalysts of Pt/C (black dotted line), Pt–Fe/C (black solid line), and connected Pt–Fe (solid red line). (D) Comparison of their ORR specific activities. Part of the data in Figure 1 has been reported in our previous study (Reference 23 - Published by the Royal Society of Chemistry).

In this study, refined structural analyses at the atomic scale and nanoscale for the connected Pt–Fe nanoparticle catalyst were performed. By obtaining detailed structural information, we will be able to gain fresh insights into the network formation mechanism and identify the key factors for enhancing the ORR activity in the connected Pt–Fe catalyst. In particular, the catalyst surface structures after electrochemical (EC) pretreatment of cyclic voltammetry (CV) cycles (see the Supporting Information for the experimental details), whose condition is the same as that for the ORR activity tests in Figure 1, were carefully investigated. Subsequently the relation between the surface structures and ORR activity is discussed. The catalyst crystal structures were identified using Rietveld structure refinement analysis of X-ray diffraction (XRD) data for the connected Pt–Fe catalyst without any treatment (as-prepared sample) and with EC pretreatment. EC pretreatment was conducted using 50 cycles of CV in a 0.1 M HClO4 acid electrolyte, as described in the Supporting Information. As referred to in our previous literature,23 the XRD patterns of the as-prepared catalyst (Figure 2A-a) show peaks at ca. 33°, which were assigned as a L10-type face-centered tetragonal (fct) phase with chemical ordering of the Pt and Fe atomic planes (Figure 2A-i). Rietveld structure refinement 5 ACS Paragon Plus Environment


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revealed the presence of 48% fct phase in the as-prepared catalyst; the remaining structure was a chemically disordered face-centered cubic (fcc) phase (Figure 2A-ii). It was found from Figure 2A-b that the connected Pt–Fe catalyst after the EC pretreatment possessed an fct phase as well. After EC pretreatment, the percentage of the fct phase in the catalyst decreased to 27%, which indicates that the ordered fct phase partially transformed into fcc Pt and disordered fcc Pt–Fe alloy phases. Figure 2B shows the line-scan data obtained via high-resolution scanning transmission electron microscope–energy dispersive X-ray spectroscopy (STEM–EDX) for the connected Pt– Fe catalysts. In the case of the as-prepared sample (Figure 2B-a), Pt and Fe atoms were uniformly distributed throughout the network and Fe atoms were enriched near the surface. Whereas, after EC pretreatment, the surface changed to a Pt-rich layer (Figure 2B-b). The dissolution of Fe atoms near the surface upon EC pretreatment in the acid solution resulted in the formation of a Pt-rich layer. This EDX line scan result is in accordance with that reported in our previous literature.23 From the result in Figure 2B-b, the thickness of the Pt-rich layer was estimated to be about 0.8 nm. This corresponds to 3–4 Pt atomic layers. In addition, the dissolution of a small fraction of Fe atoms from both the fct and fcc Pt–Fe alloy phases near the catalyst surface induced changes in the fct and fcc ratios in the catalyst after the EC pretreatment; this was estimated using Rietveld refinement analysis.


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Figure 2. (A) Rietveld refinement analysis of the XRD patterns for the connected Pt–Fe catalysts using the (a) as-prepared sample and (b) EC-pretreated sample subjected to 50 cycles of CV. Schematic of the lattice structures for the chemically ordered fct (A-i) and disordered fcc (A-ii) structures. (B) STEM–EDX line scan of the connected Pt–Fe catalysts using the (a) as-prepared and (b) EC-pretreated catalysts. The scan lines for the catalysts are shown in the insets.

To estimate the structures at an atomic scale and the electronic structures of the catalysts, in situ X-ray absorption spectroscopy (XAS) measurements for the EC-pretreated catalysts were performed in the electrochemical environment. (Details are described in the Supporting Information.) The k3-weighted Fourier transforms of the Pt L3-edge extended X-ray absorption fine structure (EXAFS) spectra of the connected Pt–Fe and Pt/C after the EC pretreatment are shown in Figure S1A. The EXAFS spectra were fitted using the structural parameters obtained in 7 ACS Paragon Plus Environment


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the Rietveld analysis of the XRD data (See Table S1). The ratio of fcc and fct phases was determined by the Rietveld refinement, and constraint condition was imposed on coordination numbers to keep the phase ratio in the EXAFS curve fitting. The initial values of Pt–Pt and Pt–Fe bond-lengths in the EXAFS fitting were determined by the lattice constants obtained by the Rietveld analysis. E0 (absorption edge) values were taken from the previous result.25 The estimated bond distance (R) and the coordination number (CN) for Pt‒Pt and Pt‒Fe bonds are listed in Table 1. The Pt‒Pt bond distances for the connected Pt–Fe and Pt/C were 2.72 and 2.77 Å, respectively. The bond distance in the connected Pt–Fe was shorter than that in pure Pt. The X-ray absorption near edge structure (XANES) spectra for the EC-pretreated catalysts (Figure S1B) indicate that the Pt L3/L2 ratio in the white line area for the connected Pt–Fe catalyst was higher than that for Pt/C; this suggests that there were larger vacancies in the Pt 5d-orbitals for the connected Pt–Fe. The obtained results match the tendency shown by the previously reported Pt-alloy catalysts; alloying Pt with 3d transition metals (e.g., Fe, Co, and Ni) shortens the Pt‒Pt bond distances (because of the compressive lattice strain) and increases the vacancies in the Pt 5d orbital.26, 27 Such changes in the geometric (Pt‒Pt bond distance) and electronic (Pt 5d vacancy and d-band center) structures correlate with the ORR activity because they affect the adsorption behavior of oxygen species as ORR intermediates on Pt surfaces.26-30 However, the structural features estimated by the XAS analysis are average, including the catalyst surface and bulk, because high-energy X-rays can pass through a nanosized sample. STEM–EDX line-scan measurements indicate the formation of a Pt-rich layer on the surface of the connected Pt–Fe catalyst after the EC pretreatment; this implies that the surface structure can be different from the overall average structure. The structural information of the surface as catalytic reaction sites is 8 ACS Paragon Plus Environment

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important to discuss the factors responsible for enhanced ORR activity. Therefore, to clarify the surface structures, the lattice fringe of the catalysts are carefully analyzed using sphericalaberration-corrected STEM (Cs-corrected STEM) images.

Table 1. Structural properties of the Pt/C and connected Pt–Fe catalysts after the EC pretreatment, estimated via in situ XAS analysis.

Composition Catalyst

Pt/C Connected Pt–Fe

Lattice constant

Pt‒Pt

Unfilled d-states

Pt‒Fe

(%)

a (Å)

c (Å)

CN

R (Å)

CN

R (Å)

(hTs)

fcc

100

3.98

3.98

9.5

2.77

‒

‒

0.33

fcc

73

3.83

3.83

3.1

2.72

5.4

2.66 0.36

fct

27

3.84

3.68

1.1

2.72

2.0

2.66

Figure 3 displays the overviews (Figures 3A-1 and 3B-1, 2) and highly magnified views (Figure 3A-2 and 3B-3, 4) of the as-prepared catalysts observed via transmission electron microscope (TEM) and Cs-corrected STEM observations. First, a clear difference in the lattice fringe pattern can be seen between the Pt/C and the connected Pt-Fe catalysts. The Pt nanoparticle of Pt/C is formed by a single nanocrystal with a regular lattice pattern in the entire nanoparticle (Figure 3A-2), whereas the lattice fringes are randomly directed and multiple crystallites can be seen in the beaded network of the connected Pt–Fe (Figure 3B-4); thus, the network has a polycrystalline structure. This implies that a polycrystalline structure would be one 9 ACS Paragon Plus Environment


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of the key factors for the formation of a beaded network in the supercritical fluid process, which is the treatment in supercritical ethanol at high temperature and high pressure above the critical point. (Details are described in the Supporting Information.) Generally, nanoparticles with a single crystal tend to grow into a larger particle to minimize the total interfacial energy; this causes a decrease in the surface area. However, in the case of a polycrystalline structure, crystalline boundaries would impede complete coalescence between nanoparticles, thus enabling nanosized network formation via the partial fusion of each nanoparticles and the retention of high surface areas.


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Figure 3. TEM and Cs-corrected STEM images with corresponding illustrations of the crystal structures for (A-1), (A-2) Pt nanoparticle of a commercial Pt/C and (B-1)-(B-4) the connected Pt–Fe network.

Next, the lattice fringe spacings near the surfaces and inside the catalysts for the connected Pt–Fe (Figure 4A) and Pt/C (Figure S2) were estimated. The Pt‒Pt bond distances calculated based on the lattice spacings are listed in Table 2. For the connected Pt–Fe catalyst without treatment, the Pt‒Pt bond distance inside the network was 2.73 Å, which was slightly longer than that near the surface (2.70 Å). After the EC pretreatment, the Pt‒Pt bond distance inside the network (2.72 Å) was almost unchanged, whereas the bond distance near the surface increased to 2.76 Å. In the case of the EC-pretreated Pt/C, the Pt‒Pt bond distance near the surface of the Pt nanoparticles (2.76 Å) was the same as that on the inside (2.76 Å). In the two analytical methods, the average bond distance of the connected Pt–Fe catalyst (2.72 Å), estimated using EXAFS analysis, was the same as that inside the connected Pt–Fe network, estimated using lattice spacing; this distance was shorter than that of pure Pt in Pt/C because of a compressive lattice strain produced by the alloying with Fe. Whereas, the bond distance near the surface of the connected Pt–Fe after the EC pretreatment was nearly the same as that of pure Pt because of the formation of a Pt-rich layer.



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Figure 4. (A) Lattice fringe spacing analysis in the (a) as-prepared and (b) EC-pretreated Pt–Fe network. Figures (a-1) and (b-1) are the TEM images of the Pt–Fe networks, and Figs. (a-2), (a3), (b-2), and (b-3) are the Cs-corrected STEM images with interplanar spaceings. (B) Schematic of the surface atomic structures of the connected Pt–Fe networks for the (a) as-prepared and (b) EC-pretreated surfaces.


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Table 2. Comparison of the Pt‒Pt bond distances for the Pt/C and connected Pt–Fe catalysts, estimated using the lattice spacing and EXAFS analytical methods.

Catalyst

Method

Condition

Pt‒Pt bond distance (Å) Surface

Inside

2.76 ± 0.04

2.76 ± 0.05

Lattice spacing

EC-pretreated

EXAFS

EC-pretreated

Lattice spacing

As-prepared

2.70 ± 0.09

2.73 ± 0.10

Lattice spacing

EC-pretreated

2.76 ± 0.09

2.72 ± 0.08

EXAFS

EC-pretreated

Pt/C

Connected Pt–Fe

2.77

2.72

The lattice fringe spacing analysis revealed that the Pt-rich surface layer had a distinct structure from the inside (the overall average). As depicted in Fig. 4B, the as-prepared Pt–Fe network is composed entirely of alloy structures, whereas after the EC treatment, the inside of the network retained the alloy structures and only the surface changed to a Pt-rich layer. The estimated thickness of the Pt-rich layer was 3–4 atomic Pt layers, which was within the range of the values reported in previous studies; 1–5 Pt atomic layers on the Pt-alloy and Pt core/shell catalysts exhibited positive effects on ORR.13, 31-37 Thus, the underlying Pt–Fe alloys would alter the electronic structures of the topmost Pt surface in the connected Pt–Fe catalyst, leading to enhanced ORR activity. In addition, investigation of the outermost atomic layer based on the Cscorrected STEM image indicates that the high-index stepped facets of {211} and {311} were frequently present on the connected Pt–Fe network (see Figure S3). Such high-index atomic steps can contribute toward improving the ORR activity.6, 38, 39 13 ACS Paragon Plus Environment


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The abovementioned results show that the Pt–Fe alloys underneath the Pt-rich thin layer and the atomic-stepped surface provide beneficial effects for ORR activity. In addition, as indicated in Figure 1, Pt-bulk-like nanostructures, such as the beaded network and carbonsupport-free structure, enhance the ORR activity. Therefore, these three structural features of the connected Pt–Fe catalyst enabled us to achieve an ORR specific activity that was nine times higher than that of the Pt/C catalyst. Refined structural analyses at the atomic scale and nanoscale revealed detailed structures of the connected Pt–Fe catalysts. The connected Pt–Fe network is formed by a polycrystalline structure, which would be important for network formation via the partial connection of each nanoparticle in the supercritical fluid treatment. In addition, the connected Pt–Fe catalyst after the EC pretreatment possessed a Pt-rich surface layer with 3–4 Pt atomic layers comprising highindex atomic steps on the outermost surface. Such a surface structure and the Pt-bulk-like network structure led to enhanced ORR activity. More importantly, it was found that the Pt‒Pt bond distance (lattice spacing) of the Pt-rich thin layer is different from that inside the network. This suggests that controlling the thickness of the Pt-rich layer and the underlying Pt-alloy structures as well as more stabilization of alloyed metals can enhance the ORR activities of connected Pt-alloy catalysts. The knowledge obtained in this study will prove to be useful for analyzing and designing surface structures for advanced ORR electrocatalysts.

ASSOCIATED CONTENT Supporting Information The following files are available free of charge. Experimental details, Pt L3-edge EXAFS and Pt 14 ACS Paragon Plus Environment

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L3- and L2-edge XANES spectra, results of Rietveld analysis and supplementary Cs-corrected STEM images of the Pt/C and connected Pt–Fe catalysts. AUTHOR INFORMATION Corresponding Author *Prof. Takeo Yamaguchi, E-mail: [email protected] ORCID Hidenori Kuroki: 0000-0002-4602-3035 Takanori Tamaki: 0000-0002-7728-9397 Takeo Yamaguchi: 0000-0001-9043-4408 Funding Sources Kanagawa Institute of Industrial Science and Technology (KISTEC), Japan. Core Research for Evolutionary Science and Technology at the Japan Science and Technology Agency (JST-CREST), Japan Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by Kanagawa Institute of Industrial Science and Technology and Core Research for Evolutionary Science and Technology (JPMJCR1543) at the Japan Science and Technology Agency. Synchrotron XAS experiments were performed with the approval of Japan Synchrotron Radiation Research Institute (JASRI) with proposal nos. 15 ACS Paragon Plus Environment


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2016B5390, 2017A5390. Cs-corrected STEM observations and EDX analyses were conducted at Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, with proposal no. A-15-UT-0124.

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Effect of Particle Proximity on the Oxygen Reduction Rate of Size-Selected Platinum Clusters. Nat Mater 2013, 12, 919‒924. (21) Speder, J.; Altmann, L.; Bäumer, M.; Kirkensgaard, J. J. K.; Mortensen, K.; Arenz, M. The Particle Proximity Effect: from Model to High Surface Area Fuel Cell Catalysts. RSC Adv. 2014, 4, 14971‒14978. (22) Hashimasa, Y.; Shimizu, T.; Matsuda, Y.; Imamura, D.; Akai, M. Verification of Durability Test Methods of an MEA for Automotive Application. ECS Trans. 2013, 50, 723‒732. (23) Tamaki, T.; Kuroki, H.; Ogura, S.; Fuchigami, T.; Kitamoto, Y.; Yamaguchi, T. Connected Nanoparticle Catalysts Possessing a Porous, Hollow Capsule Structure as Carbon-Free Electrocatalysts for Oxygen Reduction in Polymer Electrolyte Fuel Cells. Energy Environ. Sci. 2015, 8, 3545‒3549. (24) Kuroki, H.; Tamaki, T.; Yamaguchi, T. Nanostructural Control and Performance Analysis of Carbon-Free Catalyst Layers Using Nanoparticle-Connected Hollow Capsules for PEFCs. J. Electrochem. Soc. 2016, 163, F927‒F932. (25) Kuroki, H.; Tamaki, T.; Matsumoto, M.; Arao, M.; Kubobuchi, K.; Imai, H.; Yamaguchi, T. Platinum–Iron–Nickel Trimetallic Catalyst with Superlattice Structure for Enhanced Oxygen Reduction Activity and Durability. Ind. Eng. Chem. Res. 2016, 55, 11458‒11466. (26) Mukerjee, S.; Srinivasan, S.; Soriaga, M. P.; McBreen, J. Role of Structural and Electronic Properties of Pt and Pt Alloys on Electrocatalysis of Oxygen Reduction: An In Situ XANES and EXAFS Investigation. J. Electrochem. Soc. 1995, 142, 1409‒1422. (27) Toda, T.; Igarashi, H.; Uchida, H.; Watanabe, M. Enhancement of the Electroreduction of Oxygen on Pt Alloys with Fe, Ni, and Co. J. Electrochem. Soc. 1999, 146, 3750‒3756.



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Figure 1. Schematic and TEM images for (A) a conventional Pt/C catalyst and (B) a carbon-free connected Pt–Fe catalyst with a hollow-capsule structure. (C) Linear sweep voltammetry curves in an O2-saturated 0.1 M HClO4 electrolyte solution for the catalysts of Pt/C (black dotted line), Pt–Fe/C (black solid line), and connected Pt–Fe (solid red line). (D) Comparison of their ORR specific activities. Part of the data in Figure 1 has been reported in our previous study (Reference 23 - Published by the Royal Society of Chemistry). 82x98mm (300 x 300 DPI)

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Figure 2. (A) Rietveld refinement analysis of the XRD patterns for the connected Pt–Fe catalysts using the (a) as-prepared sample and (b) EC-pretreated sample subjected to 50 cycles of CV. Schematic of the lattice structures for the chemically ordered fct (A-i) and disordered fcc (A-ii) structures. (B) STEM–EDX line scan of the connected Pt–Fe catalysts using the (a) as-prepared and (b) EC-pretreated catalysts. The scan lines for the catalysts are shown in the insets. 82x98mm (300 x 300 DPI)


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Figure 3. TEM and Cs-corrected STEM images with corresponding illustrations of the crystal structures for (A-1), (A-2) Pt nanoparticle of a commercial Pt/C and (B-1)-(B-4) the connected Pt–Fe network. 150x140mm (300 x 300 DPI)



Figure 4. (A) Lattice fringe spacing analysis in the (a) as-prepared and (b) EC-pretreated Pt–Fe network. Figures (a-1) and (b-1) are the TEM images of the Pt–Fe networks, and Figs. (a-2), (a-3), (b-2), and (b-3) are the Cs-corrected STEM images with interplanar spaceings. (B) Schematic of the surface atomic structures of the connected Pt–Fe networks for the (a) as-prepared and (b) EC-pretreated surfaces. 150x170mm (300 x 300 DPI)


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