PlatinumâIronâNickel Trimetallic Catalyst with ... - ACS Publications

Subscriber access provided by BOSTON UNIV

Article

Platinum–Iron–Nickel Trimetallic Catalyst with Superlattice Structure for Enhanced Oxygen Reduction Activity and Durability Hidenori Kuroki, Takanori Tamaki, Masashi Matsumoto, Masazumi Arao, Kei Kubobuchi, Hideto Imai, and Takeo Yamaguchi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02298 • Publication Date (Web): 14 Oct 2016 Downloaded from http://pubs.acs.org on October 15, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Platinum–Iron–Nickel Trimetallic Catalyst with Superlattice Structure for Enhanced Oxygen Reduction Activity and Durability Hidenori Kuroki,†,‡ Takanori Tamaki,†,‡ Masashi Matsumoto,§ Masazumi Arao,§ Kei Kubobuchi,§ Hideto Imai,§ and Takeo Yamaguchi†,‡,* †

Kanagawa Academy of Science and Technology, R1-17, 4259 Nagatsuta-cho, Midori-ku,

Yokohama, Kanagawa 226-8503, Japan. ‡

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

Technology, R1-17, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan §

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

Kanagawa 237-0061, Japan *Prof. T. Yamaguchi, E-mail: [email protected]

KEYWORDS. Superlattice Structure, Trimetallic Catalyst, High-temperature Annealing, Metal-Composition Variation, Oxygen Reduction Reaction, Polymer Electrolyte Fuel Cell

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 33

ABSTRACT.

We develop platinum–iron–nickel (PtFeNi) trimetallic alloy catalysts having a chemically ordered L10-type superlattice structure. The PtFeNi catalyst annealed at 800 °C possesses a superlattice structure, and exhibits much higher oxygen-reduction-reaction (ORR) activity compared with the disordered PtFeNi catalysts and a commercial Pt/C. Moreover, the ORR specific activities in the PtFeNi trimetallic systems are more enhanced than those in the PtFe and PtNi bimetallic systems. Analyses of in situ X-ray absorption spectroscopy reveals shorter Pt–Pt bond distances in the PtFeNi annealed at 800 °C (2.70~2.72 Å) than in a commercial Pt (2.77 Å), leading to a suitable balance of oxygen-species chemisorption energies on the Pt atoms. In addition, high durability of the PtFeNi catalyst with a superlattice structure is demonstrated. Therefore, tuning the atomic structures of Pt-alloy catalysts by the formation of a superlattice structure and trimetallic system effectively enhances both ORR activity and durability.


2

Page 3 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION Polymer electrolyte fuel cells (PEFCs) are excellent candidates as next-generation electric generators for vehicles, residential applications, etc. because they can potentially convert chemical energy to electric power with high efficiency, even at low temperatures. However, there are issues preventing the widespread commercial use of PEFCs. One such issue involves the catalysts for the oxygen reduction reaction (ORR) at the cathode. Although Pt catalysts are commonly used as cathode catalysts, it is known that they contribute to the high cost of PEFCs and exhibit low ORR activity and low durability, leading to poor PEFC performance. Numerous studies regarding the enhancement of ORR activity and improvement of catalyst durability have been reported.1–5 The alloying of Pt with other metals is an effective approach for enhancing its ORR activity due to alteration of the atomic structure.6–11 It has been reported that the ORR activity of bimetallic Pt-alloy catalysts is strongly related to the Pt–Pt bond distance due to a compressive strain effect and the electronic effect resulting from d-band vacancies in the Pt 5d-orbital.12 Alloying Pt with 3d transition metals such as Ni, Co, Fe, Cr, and Mn, shortens the Pt–Pt bond distance and increases the d-orbital vacancies in the platinum 5d orbital.12,13 Such changes in the surface atomic configuration and d-band center position affect the O2, O, and OH chemisorption energies on the catalyst surfaces, whose interplay and balance determined ORR activity.14–18 Another approach to the enhancement of ORR activity is heat treatment of Pt19 and Pt-alloy catalysts20–22 for the modification of the atomic surface structure. Furthermore, note that when the Pt:M ratio is 3:1, 1:1, or 1:3, high-temperature treatment can induce a change in the crystal structure from a disordered face-centered cubic (fcc) phase to an ordered L10 facecentered tetragonal (fct) or ordered L12-type phase with ordering of the Pt and M (M = Fe, Co, Cu, etc.) atomic planes. These ordered crystal structures are referred to as superlattice structures.


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 33

Recently, such chemically ordered intermetallic Pt-alloy catalysts have attracted much interest as ORR catalysts because they exhibit enhanced activity and/or superior durability for the ORR.23– 33

However, guidelines for the design of superlattice catalysts with both high ORR activity and

superior durability are still unclear. For example, PtFe bimetallic catalysts with superlattice structures achieve high ORR activity; however, they have the problem of low durability because iron can be easily leached out, resulting in the drastic decrease of the ORR activity.33 In this study, in order to achieve enhanced ORR activity and high durability, we focus on the PtFeNi trimetallic system having a chemically ordered superlattice structure. The selected alloyed metal is nickel because PtNi bimetallic catalysts exhibit high ORR activity22,34 and nickel, which has a higher standard redox potential than iron, would inhibit the dissolution of alloyed metals. We prepare Pt50M50 trimetallic or bimetallic catalysts by alloying Fe and Ni to ensure a L10 type superlattice structure. Their catalyst structures are finely controlled by varying high-temperature annealing condition and the metal composition of Pt50FexNi50−x. By examining the effects of the catalyst structures on ORR activities and durabilities, we develop a PtFeNi trimetallic catalyst, which has a superlattice structure and exhibits both high ORR activity and superior durability. Systematic investigations of Pt-alloy superlattice catalysts by changing both annealing temperature and metal composition have never been conducted. Therefore, the obtained results in this study can provide useful guidelines for the design of ORR catalysts with enhanced activity and durability.


4

Page 5 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation and Characterization The PtFeNi catalysts supported on carbon black (Ketjen black) were prepared using a solid-state impregnation method.31,33 Here, a solid mixture of the metal salts and carbon black was ground to make it homogeneous, and the resulting mixture was annealed in a tube furnace under 30 mL min−1 H2 and 100 mL min−1 N2 dry gas flows at temperatures ranging from 400 to 800 °C for 2 h. The time to reach the target annealing temperature from room temperature was fixed at 2 h. The Pt:M (M = Fe, Ni) ratio was adjusted to 1:1 to ensure the formation of fct (superlattice) ordered structures. The metal loading on carbon black was constant at 40 wt%. Hydrogen hexachloroplatinate (IV) hexahydrate (H2Cl6Pt·6H2O, Wako Pure Chemical Industries), vinylferrocene (C12H12Fe, Sigma-Aldrich), and nickel (II) acetate tetrahydrate ((CH3COO)2Ni·4H2O, Wako Pure Chemical Industries) were used as the Pt, Fe, and Ni metal salts, respectively. The PtFeNi atomic compositions of the prepared samples were determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Shimadzu, ICPS-8100), and the metal loading was estimated by thermogravimetric analysis (TGA, PerkinElmer, Pyris 1 TGA). The size and distribution of the catalyst nanoparticles on the carbon black were observed by transmission electron microscopy with an acceleration voltage of 200 kV (TEM, Hitachi High Technologies Corporation, H-8100). The metal profiles in three individual catalyst nanoparticles were determined by scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDX) line-scan measurements using an HD-2700 CS-corrected STEM (Hitachi High Technologies Corporation). The crystal structures of the prepared catalysts were


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 33

identified by XRD analysis (Rigaku, Ultima IV). The XRD patterns were obtained over the 2θ range from 10° to 90° with a scan rate of 2° min−1 using Cu Kα radiation (λ = 0.15406 nm, 40 kV, 40 mA). The structure at the atomic scale for the PtFeNi trimetallic catalysts were also analyzed by in situ X-ray absorption spectroscopy (XAS) combined with Rietveld structure refinement of powder X-ray diffraction data. In situ XAS analyses were performed using synchrotron radiation X-rays at the Pt L3-edge in a transmission configuration (Beamline: BL16B2, SPring-8).35 The catalyst electrodes were prepared using a method similar to that described in Ref. 35. Prior to XAS measurements, the catalyst electrodes were electrochemically cleaned through CV scans. In situ XAS measurements were then conducted in an electrochemical cell designed for XAS analysis using a 0.5 M H2SO4 electrolyte solution. The in situ XAS data were collected at 0.4 V vs. reversible hydrogen electrode (RHE). Extended X-ray absorption fine structure (EXAFS) analyses were performed using the Athena and Artemis programs in the IFEFFIT package.36 Initial structural parameters and the ratios of fcc and fct structures required for the EXAFS analyses were determined by the Rietveld structure refinement of laboratory XRD data. The XRD data were collected using an X-ray diffractometer (Rigaku SmartLab) with Cu Kα1 (λ = 1.5406 Å) radiation produced at 45 kV and 120 mA.

2.2. Electrochemical Measurements The catalyst electrodes were prepared on a glassy carbon disk electrode (geometric area = 0.196 cm2) using a previously reported procedure.37 Electrochemical measurements (cyclic voltammetry: CV; linear sweep voltammetry: LSV) were performed using a potentiostat (HZ3000, Hokuto Denko, or Sorlartron 1287, Toyo Corporation) and an electrode rotating system


6

Page 7 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(HZ-200, Hokuto Denko). An RHE and a Pt wire were used as the reference and counter electrodes, respectively. As the electrolyte, 0.1 M HClO4 solution was used. The electrochemical surface areas (ECSAs) of platinum on the catalysts were estimated from the peaks of hydrogen desorption on platinum in the CV curves obtained in an N2-saturated electrolyte solution. First, the CV sweep from 0.05 to 1.2 V vs. RHE was performed at a rate of 50 mV s−1 for approximately 40 cycles to clean the catalyst surface. Three CV cycles at a sweep rate of 20 mV s−1 were then conducted to estimate the ECSA. LSV measurements were performed in an O2saturated electrolyte solution at room temperature with a sweep rate of 20 mV s−1 and a rotation rate of 1600 rpm. The ORR activities were determined using the kinetic current at 0.9 V in the background-corrected LSV curves. In addition, the electron transfer numbers for ORR were estimated by the Koutecky-Levich (K-L) plots (refer the session of S.1. in the supporting information).38,39 The electrochemical analysis of a commercial Pt catalyst supported on carbon black (Pt/C, Tanaka Kikinzoku Kogyo, TEC10E50E, Pt loading = 45.8 wt%) was also conducted to obtain reference data. The obtained ECSA values and the ORR mass and specific activities for commercial Pt/C were within the range of previously reported values,37,40 indicating that our electrode preparation and electrochemical analyses were appropriate. The load-cycle durability tests with accelerated dissolution of Pt and alloyed metals were performed according to FCCJ (Fuel Cell Commercialization Conference of Japan) protocol; square-wave potential cycle (0.6 V for 3 s and 1.0 V for 3 s) in an N2-saturated 0.1 M HClO4 electrolyte at 60 °C.41 After specific load cycles, the ECSA and mass ORR activity were estimated for the validation of the catalyst stability. The catalyst structures after the durability tests (10,000 load cycles) were observed by TEM and STEM-EDX line-scan measurements.


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 33

3. RESULTS AND DISCUSSION 3.1. Effect of Annealing Temperature PtFeNi trimetallic catalysts supported on carbon black were prepared using the abovementioned procedure at different annealing temperatures: 400 °C (PtFeNi–400), 600 °C (PtFeNi–600), and 800 °C (PtFeNi–800). The feedstock was Pt:Fe:Ni = 50:35:15. As listed in Table 1, the actual metal compositions and loadings of all of the catalysts (PtFeNi–400, –600, and –800) were nearly the same as the feed ones. The TEM images of the catalysts are shown in Figure 1. From the TEM observations of the catalysts at several different areas, it was found that for all of the PtFeNi/C catalysts, most of the catalyst particles were approximately 2~3 nm in diameter and small numbers of particles with diameters greater than 10 nm were detected. The one-step procedure for simultaneous formation of the PtFeNi nanoparticles and heat treatment would produce catalyst nanoparticles with smaller sizes. STEM-EDX line-scan measurements for three individual particles of the as-prepared PtFeNi–800 catalyst revealed that this catalyst had an alloying structure, not a core-shell structure, because the Fe and Ni atoms were distributed throughout the nanoparticles; these distributions were nearly equal to that of the Pt atoms as shown in Figure 2. The XRD patterns of the PtFeNi catalysts (PtFeNi–400, –600, –800) and the commercial Pt/C catalyst are shown in Figure 3. In all patterns, there are very small and broad peaks around 25° derived from C(002) of carbon black. The (111) peak position for PtFeNi–400 was 40.3°, which was higher than that of the commercial Pt/C catalyst (39.2°). The shift of this peak to a higher angle by alloying the Pt with Fe and Ni indicates a shortening of the lattice distance. Notably, an increase in the annealing temperature also resulted in a further shift of the (111) peak position to higher angles. These results suggest that the lattice distance (Pt–Pt bond distance) decreased as the annealing temperature increased. In addition, the (001) and (110)


8

Page 9 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


peaks appeared at approximately 24° and 33°, respectively. These two peaks were not obvious in the XRD patterns for PtFeNi–400 and PtFeNi–600, but were clearly observed in the XRD pattern for PtFeNi–800. As previously reported,31,33 these peaks are due to an fct phase (chemically ordered L10-type superlattice structure) wherein the Pt and Fe/Ni atoms are arranged in alternating atomic planes. Other peaks derived from the fct phase were also seen in PtFeNi–800, including the (002), (201), (112), and (202) planes. Therefore, high-temperature treatment (> 600 °C) can induce the transformation from a disordered fcc structure to an ordered fct structure. It was also found that the (111) peak in the XRD pattern for PtFeNi–800 was asymmetric, i.e., two different crystalline structures (disordered fcc and ordered fct) were present in the catalyst. The Rietveld analysis of the XRD pattern further revealed that the PtFeNi–800 catalyst consisted of 27% fct and 73% fcc phases.

Table 1. Structural properties of the Pt50Fe35Ni15 catalysts annealed at different temperatures.

Catalyst

Annealing temperature (°C)

Metal composition Pt:Fe:Ni (mole%)

Metal loading (wt%)

Metal particle size (nm)

Pt loading on electrode (µ µg cmgeo−2)

Pt/C

-

100:0:0

45.8

2.5 ± 0.6

17.3

(a) PtFeNi–400

400

48:35:17

38.8

2.2 ± 0.8

11.1

(b) PtFeNi–600

600

50:34:16

40.8

2.3 ± 0.9

11.9

(c) PtFeNi–800

800

48:37:15

47.7

2.8 ± 1.1

12.8


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

(b)

20 nm

(c)

Page 10 of 33

20 nm

(d)

20 nm

20 nm

Figure 1. TEM images of the Pt50Fe35Ni15/C catalysts prepared at annealing temperatures of (a) 400 °C (PtFeNi‒400), (b) 600 °C (PtFeNi‒600), (c) 800 °C (PtFeNi‒800), and (d) a commercial Pt/C catalyst as a reference.


10

Page 11 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. STEM-EDX line scan of the Pt50Fe35Ni15 catalyst annealed at 800 °C (PtFeNi–800). (left) Counts and (right) normalized counts are indicated along with the scan line for the nanoparticle shown in the inset. X-ray counts for each element (Pt, Fe, and Ni) are normalized by the corresponding maximum counts near the centers of the catalyst nanoparticle.

Figure 3. XRD patterns of the Pt50Fe35Ni15/C catalysts prepared at annealing temperatures of (a) 400 °C (PtFeNi–400), (b) 600 °C (PtFeNi–600), (c) 800 °C (PtFeNi–800). XRD pattern of a commercial Pt/C catalyst is also shown for reference.

Electrochemical analyses of the PtFeNi catalysts and the commercial Pt/C were performed using the rotating disk electrode (RDE) method at room temperature in a 0.1 M HClO4 electrolyte solution. Figures 4A and 4B show the CV curves and the electrochemical surface areas (ECSAs) for the catalysts, respectively. The ECSA values for the PtFeNi catalysts were approximately 60


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 33

m2 gPt−1, which was slightly lower than but close to the ECSA value for the commercial Pt/C catalyst. The sufficiently high ECSA values of the PtFeNi catalysts can be attributed to the small size (~ 3 nm) of most of the catalyst particles, though high-temperature annealing was used for catalyst preparation. Figure 4C shows the LSV curves of the catalysts. The mass activities (ORR activity per mass of Pt) and specific activities (ORR activity per electrochemical surface area of Pt) of the catalysts are shown in Figure 4D. The positive influence of high-temperature annealing on the ORR activity can be observed. As the annealing temperature increased, both the mass and specific activities of the PtFeNi catalysts were dramatically enhanced. The mass and specific activity of the PtFeNi–800 catalyst with a superlattice structure were 0.63 A mgPt−1 and 1.1 mA cmPt−2, respectively, and were more than double the values for the disordered PtFeNi–400 catalyst. Moreover, the mass activity of the PtFeNi–800 catalyst was approximately three times greater than that of the commercial Pt/C catalyst and higher than that of the 2017 U.S. Department of Energy (DOE) target of 0.44 A mgPt−1. The sufficiently high ECSA value and superior specific activity of the PtFeNi–800 resulted in a higher ORR mass activity. In addition, the analyses using the K-L plots revealed that the electron transfer numbers for ORR were approximately four in three PtFeNi catalysts; thus four-electron reduction pathway from O2 to H2O was predominant on the PtFeNi catalysts.


12

Page 13 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Electrochemical measurements. (A) CV curves, (B) ECSA values, (C) LSV curves, and (D) ORR mass and specific activities for PtFeNi–400, PtFeNi–600, PtFeNi–800, and a commercial Pt/C catalyst.

3.2. Effect of Metal Composition Subsequently, five PtFeNi catalysts with different Fe and Ni contents (Pt50Ni50, Pt50Fe15Ni35, Pt50Fe25Ni25, Pt50Fe35Ni15, and Pt50Fe50) on carbon black were prepared. The annealing temperature was fixed at 800 °C. It was confirmed by the ICP-AES and TG analyses that the desired metal compositions and loadings were obtained for all of the catalyst. The actual metal


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 33

compositions and loadings of the prepared catalysts are listed in Table 2. TEM observations (Figure 5) revealed that the particle sizes in the prepared catalysts were small at approximately 3 nm, and small numbers of particles larger than 10 nm were observed, except for Pt50Ni50. Although most of the particles of the Pt50Ni50 catalyst were approximately 3 nm, larger particles (> 10 nm) were also readily detected in the TEM image of this catalyst. XRD patterns of the five catalysts with different metal compositions and the commercial Pt/C are shown in Figure 6. The (111) peak position for all five catalysts was shifted above that of the commercial Pt/C catalyst. These results indicate that the lattice distances of the prepared catalysts were shortened. The PtFeNi and PtFe catalysts exhibited (001) peaks at ca. 24° and (110) peaks at ca. 33° in their XRD patterns, whereas the PtNi catalyst did not. Thus, the PtFeNi and PtFe catalysts have an fct crystal (L10 type superlattice) structure with an ordered arrangement of the Pt and Fe/Ni planes (or Fe planes in the PtFe catalyst). Rietveld analyses of the XRD patterns for the PtFeNi trimetallic catalysts indicated the presence of approximately 30% of an fct phase in each of the PtFeNi catalysts.

Table 2. Structural properties of the Pt-alloy catalysts annealed at 800 °C.

Annealing temperature (°C)

Metal composition Pt:Fe:Ni (mole%)

Metal loading (wt%)

Metal particle size (nm)

Pt loading on electrode (µ µg cmgeo−2)

(a) Pt50Ni50

800

48:0:52

48.3

4.0 ± 3.0

13.8

(b) Pt50Fe15Ni35

800

50:16:34

46.7

2.8 ± 0.9

13.7

(c) Pt50Fe25Ni25

800

50:25:25

47.9

2.6 ± 1.1

13.3

(d) Pt50Fe35Ni15

800

48:37:15

47.7

2.8 ± 1.1

12.8

(e) Pt50Fe50

800

50:50:0

44.2

2.7 ± 1.0

13.0

Catalyst


14

Page 15 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b)

10 nm

(c)

20 nm

(d)

(e)

20 nm

20 nm

20 nm

Figure 5. TEM images of the Pt-alloy catalysts on carbon-supports annealed at 800 °C. (a) Pt50Ni50/C, (b) Pt50Fe15Ni35/C, (c) Pt50Fe25Ni25/C, (d) Pt50Fe35Ni15/C, and (e) Pt50Fe50/C.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 33

Figure 6. XRD patterns of the Pt-alloy catalysts on carbon-supports annealed at 800 °C. (a) Pt50Ni50/C, (b) Pt50Fe15Ni35/C, (c) Pt50Fe25Ni25/C, (d) Pt50Fe35Ni15/C, and (e) Pt50Fe50/C. XRD pattern of a commercial Pt/C catalyst is also shown for reference.

The ECSA values and mass and specific ORR activities of the five prepared catalysts in a 0.1 M HClO4 electrolyte solution are shown in Figure 7. The ECSA values for the catalysts, except the Pt50Ni50 catalyst, were sufficiently high, suggesting that most of the catalysts have small particle sizes. The lower ECSA value for the Pt50Ni50 catalyst was attributed to partial aggregation of the catalyst particles, leading to a larger average particle size. The ECSA value for the Pt50Fe35Ni15 catalyst was the highest of the prepared Pt-alloy catalysts, probably due to the smallest number of aggregated catalyst nanoparticles. In addition, the ORR specific activities of all the Pt-alloy catalysts were 3~4 times greater than that of the commercial Pt/C catalyst. It was also confirmed by the K-L plot analyses that the ORR on the Pt-alloy catalysts predominantly proceeded via four-electron pathway. Both a superior specific activity and a high ECSA value were necessary


16

Page 17 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


for improving the mass activity; thus, the Pt50Fe35Ni15 catalyst exhibited the highest mass activity. Noting that the specific activities of the PtFeNi trimetallic catalysts were higher than those of the PtFe and PtNi bimetallic catalysts; thus, fine-tuning the catalyst structures by the trimetallic alloy systems of PtFeNi enhanced the ORR activities more, compared with the bimetallic alloy systems of PtFe and PtNi.

Figure 7. Electrochemical measurements. (A) CV curves, (B) ECSA values, (C) LSV curves, and (D) ORR mass and specific activities for the Pt-alloy catalysts on carbon-supports annealed


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 33

at 800 °C; (a) Pt50Ni50/C, (b) Pt50Fe15Ni35/C, (c) Pt50Fe25Ni25/C, (d) Pt50Fe35Ni15/C, and (e) Pt50Fe50/C. The data of a commercial Pt/C is also shown for reference.

3.3. Atomic Structures of PtFeNi Catalyst The atomic structures of the PtFeNi trimetallic catalysts with annealing at 800 °C, which exhibited more enhanced specific activities herein, were estimated by the in situ XAS measurement. The original and fitting data for the k3-weighted Fourier transforms of Pt L3-edge EXAFS spectra of the PtFeNi catalysts at 0.4 V are shown in Figure S2. Pt-oxide peaks were not observed in these spectra. The obtained bond distances (R) and coordination numbers (CN) for the Pt–Pt, Pt–Fe, and Pt–Ni bonds are listed in Table 3. The estimated distances of Pt–Pt bond for the fcc- and fct-PtFeNi were 2.71~2.72 Å and 2.70 Å, respectively. These values were shorter than that of the commercial Pt (2.77 Å). It was also found that the Pt–Pt bond distances of the fct structures were shorter than those in the fcc structures for all of the prepared PtFeNi catalysts. This result can be explained by the greater compressive strain effect in the fct structures, which likely affected the Pt–Pt bond distances more strongly than that in the fcc structures. As described in the introduction, it has been reported that the Pt–Pt bond distances in Pt-alloy catalysts are strongly correlated with their ORR activities, and the volcano-plot relationship between the Pt–Pt bond distances and the ORR activities was observed due to the balance of chemisorption energies for oxygen species on Pt atoms.12,42 The Pt–Pt bond distances of the fcc and fct structures in the PtFeNi catalysts were 2.70~2.72 Å, which were close or slightly shorter than the optimum bond distance according to the previously reported volcano-plot trend for the ORR.12 As indicated in Figure 6, the PtNi bimetallic catalyst did not have a superlattice structure.


18

Page 19 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


On the other hand, the (111) peak position of the PtFe bimetallic catalyst with a superlattice structure was lower angle than those of the PtFeNi trimetallic catalysts, i.e., the PtFe catalyst had longer Pt‒Pt bond (lattice) distances than the PtFeNi catalysts. Longer Pt‒Pt bond distances led to strong chemisorption of the oxygen species on platinum, which depressed the ORR specific activity. Therefore, it was indicated that the atomic structures of the Pt50M50 alloy catalysts can be tuned by the formation of a superlattice structure through high-temperature annealing and metal composition variation to trimetallic systems, thereby enabling the development of an ORR catalyst with enhanced activity.

Table 3. Structural properties of the Pt and PtFeNi catalysts. Compositions and lattice constants of the fcc and fct structures, bond distances (R), and coordination numbers (CN) for the Pt–Pt, Pt–Fe, and Pt–Ni bonds.

Compo Lattice constant sition

Catalyst Pt Pt50Fe15Ni35

Pt50Fe25Ni25

Pt50Fe35Ni15

Pt–Pt

(%)

a (Å)

c (Å)

CN

R (Å)

fcc

100

3.98

3.98

9.5

2.77

fcc

68

3.73

3.73

3.2

fct

32

3.80

3.63

fcc

70

3.85

fct

30

fcc fct

Pt–Fe

Pt–Ni

CN

R (Å)

CN

R (Å)

2.71

0.8

2.71

2.4

2.71

1.5

2.70

0.4

2.61

1.1

2.61

3.85

3.2

2.72

1.6

2.71

1.6

2.71

3.82

3.65

1.4

2.70

0.7

2.61

0.7

2.61

73

3.85

3.85

3.4

2.72

2.5

2.71

0.9

2.71

27

3.83

3.68

1.3

2.70

0.9

2.61

0.3

2.61


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 33

3.4. Durability of PtFeNi Catalyst For ORR catalysts in PEFCs, both high ORR activity and high durability are necessary. Here, the load-cycle durability of the Pt50Fe35Ni15 trimetallic catalyst with annealing at 800 °C was studied in an N2-saturated 0.1 M HClO4 electrolyte at 60 °C. Figure 8 displays the ECSA, mass activity, and their retentions as a function of load-cycle numbers up to 10,000 cycles. (Figure S3 shows the CV and LSV curves for the PtFeNi and commercial Pt catalysts after the specific loadcycles.) The ECSA loss of the Pt50Fe35Ni15 catalyst after the load-cycles was smaller than that of the commercial Pt catalyst; approximately 65% of ECSA in the PtFeNi catalyst retained even after 10,000 load-cycles, whereas the ECSA of the commercial Pt was decreased to less than half of the initial value. As shown in Figure 9 of the TEM images before and after the 10,000 loadcycle durability tests, the particle size distribution for both the PtFeNi and commercial Pt catalysts became wider after the durability tests. Additionally, the mean particle size of the PtFeNi catalyst was increased from 2.8 ± 1.1 nm to 4.3 ± 1.3 nm after the 10,000 load-cycles, while the commercial Pt nanoparticles was slightly more enlarged from 2.5 ± 0.6 nm to 5.2 ± 1.9 nm. The retentions of the mass activity in both the Pt50Fe35Ni15 and commercial Pt catalysts were almost the same as each other; thus, three times higher mass activity of the PtFeNi catalyst remained even after the 10,000 load cycles. The EDX line scans of the catalyst nanoparticles before and after the 10,000 load-cycles (Figure 10) provided the evidence of the sufficient high stability for ORR activity in the Pt50Fe35Ni15 catalyst. The Pt enriched layer was observed in the metal distribution profiles of Figure 10B after the load cycle test. (Figure 4S shows the normalized count data of the EDX-line scans.) It


20

Page 21 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


indicated iron and nickel atoms around the catalyst surface were dissolved out after the load potential cycles; however, most of the iron and nickel remained in the catalyst even after the durability test, keeping the alloyed effect for the enhanced ORR activity (specific activity of the PtFeNi catalyst after the 10,000 cycles = ca. 0.9 mA cmPt−2). The stability of the Pt50Fe35Ni15 catalyst would be due to the chemically ordered superlattice structure, leading to the suppression of leaching-out of the alloyed metals, as reported in our literature regarding the chemically ordered Pt-alloy catalysts.31,33 These results allow us to conclude that a PtFeNi trimetallic catalyst having a superlattice structure achieves both enhanced ORR activity and excellent durability.


21


Figure 8. Load-cycle durability of the Pt50Fe35Ni15/C annealed at 800 °C and a commercial Pt/C in 0.1 M HClO4 electrolyte at 60 °C. (A) ECSA, (B) the retention of ECSA, (C) ORR mass activity, and (D) the retention of ORR mass activity as a function of the load-cycle numbers.

(A) PtFeNi/C, before

(B) PtFeNi/C, after

20 nm

20 nm

30

30

20

10

0

(B) PtFeNi/C, after Frequency (%)

Frequency (%)

(A) PtFeNi/C, before

2

4

20

10

0

6 8 10 12 14 Particle size (nm)

(C) Pt/C, before

2

4


(D) Pt/C, after

20 nm

20 nm

40

40

(C) Pt/C, before

30

Frequency (%)

Frequency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 33

20 10 0

2

4


(D) Pt/C, after

30 20 10 0

2

4



22

Page 23 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9. TEM images of (A, B) the Pt50Fe35Ni15/C catalyst prepared at annealing temperature of 800 °C and (C, D) a commercial Pt/C catalyst (A, C) before and (B, D) after the durability tests. The graphs under the images show the frequency of the particle sizes.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 33

Figure 10. STEM-EDX line scans of the Pt50Fe35Ni15 catalyst annealed at 800 °C (A) before and (B) after the 10,000 load-cycles at 60 °C. Counts are indicated along with the scan lines for the three individual nanoparticles shown in the insets.

4. CONCLUSIONS In summary, we succeeded in developing the PtFeNi trimetallic alloy catalysts having chemically ordered L10-type superlattice structures with enhanced ORR activity and durability. Annealing of the PtFeNi catalysts at 800 °C induced the formation of a L10-type superlattice structure, leading to much higher ORR activity than the disordered PtFeNi catalysts annealed at lower temperatures and a commercial Pt/C. Furthermore, the ORR specific activities in the PtFeNi trimetallic systems were more enhanced than those in the PtFe and PtNi bimetallic systems. The PtFeNi catalysts annealed at 800 °C was found using in situ XAS analysis to possess shorter Pt–Pt bond distances than a commercial Pt/C catalyst. The Pt–Pt bond distances in the PtFeNi catalysts were 2.70~2.72 Å, which are close to the topmost bond lengths for ORR according to a previously reported volcano plot trend; such bond distances result in a suitable balance of energies for the chemisorption of oxygen species on the Pt atoms. In addition, the PtFeNi catalyst with a superlattice structure showed high durability against load potential cycles. The chemically ordered superlattice structure would suppress the leaching-out of the alloyed metals; thus, the three-fold improvement of the mass activity remained even after the 10,000 load-cycles. The above results indicate that trimetallic systems of Pt50M50 and formation of a superlattice structure can enhance both ORR activity and durability. They also demonstrate that use of a high


24

Page 25 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


annealing temperature in combination with variation of the metal composition of Pt‒alloy catalysts is an effective approach for tuning the catalyst structure at the atomic scale. The obtained knowledge in this study can be applied to the design of platinum alloys using a wide range of metals for the development of advanced electrocatalysts with more enhanced ORR activity and durability.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. LSV curves and K-L plots for the Pt50Fe35Ni15 catalyst, Pt L3-edge EXAFS spectra of the PtFeNi catalysts, CV and LSV curves of the PtFeNi/C and Pt/C in the load-cycle durability tests, and metal profiles in the PtFeNi nanoparticles before and after the load-cycle durability test.

AUTHOR INFORMATION Corresponding Author *Prof. Takeo Yamaguchi, E-mail: [email protected] Funding Sources Kanagawa Academy of Science and Technology (KAST). Notes The authors declare no competing financial interest.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 33

ACKNOWLEDGMENT We gratefully acknowledge the financial support of this study by Kanagawa Academy of Science and Technology (KAST). We thank Material Analysis Suzukake-dai Center, Technical Department, Tokyo Institute of Technology, for the ICP-AES, and the Foundation for Promotion of Materials Science and Technology (MST) of Japan for the STEM-EDX line scan measurements. Synchrotron XAS experiments were performed under approval of Japan Synchrotron Radiation Research Institute (JASRI) with proposal nos. 2013A5392, 2013B5391, 2014A5391, 2014B5390, and 2014B5391.

REFERENCES (1) Bing, Y.; Liu, H.; Zhang, L.; Ghosh, D.; Zhang, J. Nanostructured Pt-Alloy Electrocatalysts for PEM Fuel Cell Oxygen Reduction Reaction. Chem. Soc. Rev. 2010, 39, 2184. (2) Debe, M. K. Electrocatalyst Approaches and Challenges for Automotive Fuel Cells. Nature 2012, 486, 43. (3) Dhavale, V. M.; Kurungot, S. Tuning the Performance of Low-Pt Polymer Electrolyte Membrane Fuel Cell Electrodes Derived from Fe2O3@Pt/C Core–Shell Catalyst Prepared by an in Situ Anchoring Strategy. J. Phys. Chem. C 2012, 116, 7318. (4) Rabis, A.; Rodriguez, P.; Schmidt, T. J. Electrocatalysis for Polymer Electrolyte Fuel Cells: Recent Achievements and Future Challenges. ACS Catal. 2012, 2, 864. (5) Oezaslan, M.; Hasché, F.; Strasser, P. Pt-Based Core–Shell Catalyst Architectures for Oxygen Fuel Cell Electrodes. J. Phys. Chem. Lett. 2013, 4, 3273.


26

Page 27 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(6) Yang, H.; Alonso-Vante, N.; Léger, J. M.; Lamy, C. Tailoring, Structure, and Activity of Carbon-Supported Nanosized Pt−Cr Alloy Electrocatalysts for Oxygen Reduction in Pure and Methanol-Containing Electrolytes. J. Phys. Chem. B 2004, 108, 1938. (7) Loukrakpam, R.; Luo, J.; He, T.; Chen, Y.; Xu, Z.; Njoki, P. N.; Wanjala, B. N.; Fang, B.; Mott, D.; Yin, J.; Klar, J.; Powell, B.; Zhong, C. J. Nanoengineered PtCo and PtNi Catalysts for Oxygen Reduction Reaction: An Assessment of the Structural and Electrocatalytic Properties. J. Phys. Chem. C 2011, 115, 1682. (8) Loukrakpam, R.; Wanjala, B. N.; Yin, J.; Fang, B.; Luo, J.; Shao, M.; Protsailo, L.; Kawamura, T.; Chen, Y.; Petkov, V.; Zhong, C. J. Structural and Electrocatalytic Properties of PtIrCo/C Catalysts for Oxygen Reduction Reaction. ACS Catal. 2011, 1, 562. (9) Wang, C.; Chi, M.; Li, D.; Strmcnik, D.; van der Vliet, D.; Wang, G.; Komanicky, V.; Chang, K. C.; Paulikas, A. P.; Tripkovic, D.; Pearson, J.; More, K. L.; Markovic, N. M.; Stamenkovic, V. R. Design and Synthesis of Bimetallic Electrocatalyst with Multilayered PtSkin Surfaces. J. Am. Chem. Soc. 2011, 133, 14396. (10) Wang, C.; Chi, M.; Wang, G.; van der Vliet, D.; Li, D.; More, K.; Wang, H. H.; Schlueter, J. A.; Markovic, N. M.; Stamenkovic, V. R. Correlation Between Surface Chemistry and Electrocatalytic Properties of Monodisperse PtxNi1-x Nanoparticles. Adv. Funct. Mater. 2011, 21, 147. (11) Kakade, B. A.; Wang, H.; Tamaki, T.; Ohashi, H.; Yamaguchi, T. Enhanced Oxygen Reduction Reaction by Bimetallic CoPt and PdPt Nanocrystals. RSC Adv. 2013, 3, 10487.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 33

(12) 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. (13) 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. (14) Stamenkovic, V.; Mun, B. S.; Mayrhofer, K. J.; Ross, P. N.; Markovic, N. M.; Rossmeisl, J.; Greeley, J.; Norskov, J. K. Changing the Activity of Electrocatalysts for Oxygen Reduction by Tuning the Surface Electronic Structure. Angew. Chem. Int. Ed. Engl. 2006, 45, 2897. (15) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J.; Lucas, C. A.; Wang, G.; Ross, P. N.; Markovic, N. M. Trends in Electrocatalysis on Extended and Nanoscale PtBimetallic Alloy Surfaces. Nat. Mater. 2007, 6, 241. (16) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jónsson, H. Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode. J. Phys. Chem. B 2004, 108, 17886. (17) Greeley, J.; Stephens, I. E.; Bondarenko, A. S.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T. F.; Rossmeisl, J.; Chorkendorff, I.; Norskov, J. K. Alloys of Platinum and Early Transition Metals as Oxygen Reduction Electrocatalysts. Nat. Chem. 2009, 1, 552. (18) Okamoto, Y.; Sugino, O. Hyper-Volcano Surface for Oxygen Reduction Reactions over Noble Metals. J. Phys. Chem. C 2010, 114, 4473.


28

Page 29 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(19) Chung, D. Y.; Chung, Y. H.; Jung, N.; Choi, K. H.; Sung, Y. E. Correlation between Platinum Nanoparticle Surface Rearrangement Induced by Heat Treatment and Activity for an Oxygen Reduction Reaction. Phys. Chem. Chem. Phys. 2013, 15, 13658. (20) Wang, C.; Wang, G.; van der Vliet, D.; Chang, K. C.; Markovic, N. M.; Stamenkovic, V. R. Monodisperse Pt3Co Nanoparticles as Electrocatalyst: The Effects of Particle Size and Pretreatment on Electrocatalytic Reduction of Oxygen. Phys. Chem. Chem. Phys. 2010, 12, 6933. (21) Wanjala, B. N.; Loukrakpam, R.; Luo, J.; Njoki, P. N.; Mott, D.; Zhong, C. J.; Shao, M.; Protsailo, L.; Kawamura, T. Thermal Treatment of PtNiCo Electrocatalysts: Effects of Nanoscale Strain and Structure on the Activity and Stability for the Oxygen Reduction Reaction. J. Phys. Chem. C 2010, 114, 17580. (22) Chung, Y. H.; Chung, D. Y.; Jung, N.; Park, H. Y.; Yoo, S. J.; Jang, J. H.; Sung, Y. E. Origin of the Enhanced Electrocatalysis for Thermally Controlled Nanostructure of Bimetallic Nanoparticles. J. Phys. Chem. C 2014, 118, 9939. (23) Kim, J.; Lee, Y.; Sun, S. Structurally Ordered FePt Nanoparticles and Their Enhanced Catalysis for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2010, 132, 4996. (24) Chen, L.; Bock, C.; Mercier, P. H. J.; MacDougall, B. R. Ordered Alloy Formation for Pt3Fe/C, PtFe/C and Pt5.75Fe5.75Cuy/CO2-Reduction Electro-Catalysts. Electrochim. Acta 2012, 77, 212. (25) Li, X.; An, L.; Wang, X.; Li, F.; Zou, R.; Xia, D. Supported Sub-5nm Pt–Fe Intermetallic Compounds for Electrocatalytic Application. J. Mater. Chem. 2012, 22, 6047.


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 33

(26) Oezaslan, M.; Hasché, F.; Strasser, P. Oxygen Electroreduction on PtCo3, PtCo and Pt3Co Alloy Nanoparticles for Alkaline and Acidic PEM Fuel Cells. J. Electrochem. Soc. 2012, 159, B394. (27) Wang, D.; Xin, H. L.; Hovden, R.; Wang, H.; Yu, Y.; Muller, D. A.; DiSalvo, F. J.; Abruna, H. D. Structurally Ordered Intermetallic Platinum-Cobalt Core-Shell Nanoparticles with Enhanced Activity and Stability as Oxygen Reduction Electrocatalysts. Nat. Mater. 2013, 12, 81. (28) Jia, Q.; Caldwell, K.; Strickland, K.; Ziegelbauer, J. M.; Liu, Z.; Yu, Z.; Ramaker, D. E.; Mukerjee, S. Improved Oxygen Reduction Activity and Durability of Dealloyed PtCoxCatalysts for Proton Exchange Membrane Fuel Cells: Strain, Ligand, and Particle Size Effects. ACS Catal. 2015, 5, 176. (29) Zhang, S.; Zhang, X.; Jiang, G.; Zhu, H.; Guo, S.; Su, D.; Lu, G.; Sun, S. Tuning Nanoparticle Structure and Surface Strain for Catalysis Optimization. J. Am. Chem. Soc. 2014, 136, 7734. (30) Wanjala, B. N.; Fang, B.; Luo, J.; Chen, Y.; Yin, J.; Engelhard, M. H.; Loukrakpam, R.; Zhong, C. J. Correlation between Atomic Coordination Structure and Enhanced Electrocatalytic Activity for Trimetallic Alloy Catalysts. J. Am. Chem. Soc. 2011, 133, 12714. (31) Arumugam, B.; Kakade, B. A.; Tamaki, T.; Arao, M.; Imai, H.; Yamaguchi, T. Enhanced Activity and Durability for the Electroreduction of Oxygen at a Chemically Ordered Intermetallic PtFeCo Catalyst. RSC Adv. 2014, 4, 27510.


30

Page 31 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(32) Tamaki, T.; Minagawa, A.; Arumugam, B.; Kakade, B. A.; Yamaguchi, T. Highly Active and Durable Chemically Ordered Pt–Fe–Co Intermetallics as Cathode Catalysts of Membrane– Electrode Assemblies in Polymer Electrolyte Fuel Cells. J. Power Sources 2014, 271, 346. (33) Arumugam, B.; Tamaki, T.; Yamaguchi, T. Beneficial Role of Copper in the Enhancement of Durability of Ordered Intermetallic PtFeCu Catalyst for Electrocatalytic Oxygen Reduction. ACS Appl. Mater. Interfaces 2015, 7, 16311. (34) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Improved Oxygen Reduction Activity on Pt3Ni(111) via Increased Surface Site Availability. Science 2007, 315, 493. (35) Imai, H.; Izumi, K.; Matsumoto, M.; Kubo, Y.; Kato, K.; Imai, Y. In Situ and Real-Time Monitoring of Oxide Growth in a Few Monolayers at Surfaces of Platinum Nanoparticles in Aqueous Media. J. Am. Chem. Soc. 2009, 131, 6293. (36) Newville, M. IFEFFIT: Interactive XAFS Analysis and FEFF Fitting. J. Synchrotron Rad. 2001, 8, 322. (37) Garsany, Y.; Baturina, O. A.; Swider-Lyons, K. E.; Kocha, S. S. Experimental Methods for Quantifying the Activity of Platinum Electrocatalysts for the Oxygen Reduction Reaction. Anal. Chem. 2010, 82, 6321. (38) Levich, V. G. Physicochemical Hydrodynamics; Prentice-Hall: Englewood Cliffs, NJ, 1962. (39) Sasaki, K.; Zhang, L.; Adzic, R. R. Niobium Oxide-Supported Platinum Ultra-Low Amount Electrocatalysts for Oxygen Reduction. Phys. Chem. Chem. Phys. 2008, 10, 159.


31


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 33

(40) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Activity Benchmarks and Requirements for Pt, Pt-alloy, and Non-Pt Oxygen Reduction Catalysts for PEMFCs. Appl. Catal., B, 2005, 56, 9. (41) Ohma, A.; Shinohara, K.; Iiyama, A.; Yoshida, T.; Daimaru, A. Membrane and Catalyst Performance Targets for Automotive Fuel Cells by FCCJ Membrane, Catalyst, MEA WG. ECS Trans. 2011, 41, 775. (42) Min, M.; Cho, J.; Cho, K.; Kim, H. Particle Size and Alloying Effects of Pt-Based Alloy Catalysts for Fuel Cell Applications. Electrochim. Acta 2000, 45, 4211.


32

Page 33 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents Only


33

PlatinumâIronâNickel Trimetallic Catalyst with ... - ACS Publications

Recommend Documents

PlatinumâIronâNickel Trimetallic Catalyst with ... - ACS Publications