Iron Nanoparticle Catalysts with Enhanced Oxygen Reduction Activity

Letter Cite This: ACS Appl. Energy Mater. 2018, 1, 324−330

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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*,†,‡,⊥ †

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 S Supporting Information *

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 Pt-rich 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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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 Pt-alloy 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

n 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 © 2018 American Chemical Society

Received: December 15, 2017 Accepted: January 30, 2018 Published: January 30, 2018 324

DOI: 10.1021/acsaem.7b00295 ACS Appl. Energy Mater. 2018, 1, 324−330

Letter

ACS Applied Energy Materials

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 in the figure has been reported in our previous study. Adapted from ref 23. Copyright 2015 Royal Society of Chemistry.

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 Figure 1C,D). 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 highperformance catalyst. 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 (asprepared 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 pattern of the as-prepared catalyst (Figure 2A-a) shows a peak at ca. 33°, which was 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 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%, 325


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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) ECpretreated 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.

Table 1. Structural Properties of the Pt/C and Connected Pt−Fe Catalysts after the EC Pretreatment, Estimated via in Situ XAS Analysis lattice constant catalyst Pt/C connected Pt−Fe

fcc fcc fct

Pt−Pt

unfilled d-states

Pt−Fe

composition (%)

a (Å)

c (Å)

CN

R (Å)

CN

R (Å)

100 73 27

3.98 3.83 3.84

3.98 3.83 3.68

9.5 3.1 1.1

2.77 2.72 2.72

5.4 2.0

2.66 2.66

(hTs) 0.33 0.36

three-to-four Pt atomic layers. In addition, the dissolution of a small fraction of Fe atoms from both the fct and fcc Pt−Fealloy 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. 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 k3weighted 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 Supporting Information Figure S1A. The EXAFS spectra were fitted using the structural parameters obtained in the Rietveld

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 highresolution 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 326


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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.

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 highenergy X-rays can pass through a nanosized sample. STEM− EDX line-scan measurements indicate the formation of a Ptrich 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 on the surface as catalytic reaction sites is 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 spherical-aberration-corrected STEM (Cs-corrected STEM) images. Figure 3 displays the overviews (Figure 3A-1,B-1,-2) and highly magnified views (Figure 3A-2,B-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 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 nanoparticle and the retention of high surface areas. Next, the lattice fringe spacings near the surfaces and inside the catalysts for the connected Pt−Fe (Figure 4A) and Pt/C 327


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

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 Pt−Pt bond distance (Å) catalyst

method

condition

surface

inside

EC-pretreated EC-pretreated as-prepared EC-pretreated EC-pretreated

2.76 ± 0.04

2.76 ± 0.05

connected Pt−Fe

lattice spacing EXAFS lattice spacing lattice spacing EXAFS

2.70 ± 0.09 2.76 ± 0.09

2.73 ± 0.10 2.72 ± 0.08

Pt/C

overall average 2.77

2.72

(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 ECpretreated 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. 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 Figure 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 three-to-four atomic Pt layers, which was within the range of the values reported in previous studies; one-to-five 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 328


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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.

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 The above-mentioned 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 carbon-support-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 three-to-four Pt atomic layers comprising high-index 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.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.7b00295. Experimental details, Pt L3-edge EXAFS and Pt 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 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hidenori Kuroki: 0000-0002-4602-3035 Takeo Yamaguchi: 0000-0001-9043-4408 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Kanagawa Institute of Industrial Science and Technology and Core Research for Evolutionary Science and Technology (Grant 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. 2016B5390 and 2017A5390. Cs-corrected 329


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