C Intermetallic Electrocatalysts

Research Article www.acsami.org

Synthesis of Chemically Ordered Pt3Fe/C Intermetallic Electrocatalysts for Oxygen Reduction Reaction with Enhanced Activity and Durability via a Removable Carbon Coating Chanwon Jung,† Changsoo Lee,† Kihoon Bang,† JeongHoon Lim,† Hoin Lee,† Ho Jin Ryu,‡ EunAe Cho,*,† and Hyuck Mo Lee*,† †

Department of Materials Science and Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea Department of Nuclear and Quantum Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea

‡

S Supporting Information *

ABSTRACT: Recently, Pt3M (M = Fe, Ni, Co, Cu, etc.) intermetallic compounds have been highlighted as promising candidates for oxygen reduction reaction (ORR) catalysts. In general, to form those intermetallic compounds, alloy phase nanoparticles are synthesized and then heat-treated at a high temperature. However, nanoparticles easily agglomerate during the heat treatment, resulting in a decrease in electrochemical surface area (ECSA). In this study, we synthesized Pt−Fe alloy nanoparticles and employed carbon coating to protect the nanoparticles from agglomeration during heat treatment. As a result, Pt3Fe L12 structure was obtained without agglomeration of the nanoparticles; the ECSA of Pt−Fe alloy and intermetallic Pt3Fe/C was 37.6 and 33.3 m2 gPt−1, respectively. Pt3Fe/C exhibited excellent mass activity (0.454 A mgPt−1) and stability with superior resistances to nanoparticle agglomeration and iron leaching. Density functional theory (DFT) calculation revealed that, owing to the higher dissolution potential of Fe atoms on the Pt3Fe surface than those on the Pt−Fe alloy, Pt3Fe/C had better stability than Pt−Fe/C. A single cell fabricated with Pt3Fe/C showed higher initial performance and superior durability, compared to that with commercial Pt/C. We suggest that Pt3M chemically ordered electrocatalysts are excellent candidates that may become the most active and durable ORR catalysts available. KEYWORDS: Pt3Fe intermetallic compounds, Pt−Fe nanoparticles, ordered phase, oxygen reduction reaction, electrocatalysts

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superior durability in ORR.17−24 Previous studies have been conducted on disordered alloys, which have randomly occupied atomic positions. They have various compositions and randomly distributed active sites on their surface plane.25,26 Unlike disordered Pt−M alloys, ordered intermetallic compounds have well-defined compositions and structures that facilitate predictable control of electronic27,28 and geometric effects.29,30 Despite the obvious advantages of ordered intermetallic compounds, synthesizing these intermetallic catalysts is still challenging. To obtain ordered intermetallic compounds, heat treatment at high temperature is indispensable.19,31,32 Metal nanoparticles loaded on a support easily agglomerate with each other during the heat treatment. Particle agglomeration results in decreased electrochemical surface area (ECSA) and poorer catalytic performance. To overcome this limitation, various strategies have been suggested, such as the use of oxide coating,33−35 KCl matrices,36,37 and carbon coating.21,38 The Sun group used an MgO coating as an agglomeration barrier before heat treatment and successfully

INTRODUCTION Owing to worldwide interest in sustainable energy, many researchers and scientists are conducting studies on sustainable energy techniques, such as photovoltaics,1,2 biomass,3,4 wind energy,5,6 and fuel cells.7,8 Among them, proton exchange membrane fuel cells (PEMFCs) are one of the most attractive sustainable energy conversion technologies, because they directly convert fuel into electricity efficiently without generating any pollutants. However, commercialization of PEMFCs is limited by the slow rate of the oxygen reduction reaction (ORR) at cathodes. Large amounts of platinum catalyst are required, and the high cost of platinum is problematic. To attain a competitive price, many studies have been performed to increase ORR activity and decrease the amount of platinum used by alloying it with 3d transition metals (e.g., Fe, Ni, Co, Cu, etc.).9−13 However, Pt−M (M is the 3d transition metal) alloys easily dissolve in electrolyte solution because of the harsh conditions of the cathode.14,15 As a result, less noble element M is leached from the Pt−M alloys, thus causing catalytic degradation and poisoning of the electrodes and membranes.4,15,16 Recently, many studies have reported that Pt−M intermetallic compounds have excellent catalytic activity and © 2017 American Chemical Society

Received: May 30, 2017 Accepted: August 29, 2017 Published: August 29, 2017 31806

DOI: 10.1021/acsami.7b07648 ACS Appl. Mater. Interfaces 2017, 9, 31806−31815

Research Article

ACS Applied Materials & Interfaces synthesized a PtFe intermetallic compound.35 Similarly, the Lee group has reported a Pt3Ti intermetallic compound that serves as a highly active and durable catalyst by coating it with TiO2 layers.20 However, these oxide coating strategies require an additional oxide removal process, and brittle oxide layers hinder atomic movement in nanoparticles, thus making phase transformation from disordered alloys to ordered intermetallic compounds more difficult. In contrast, the Hyeon group has synthesized PtFe ordered L10 intermetallic compounds by using a dopamine coating process.21 In carbon coating methods, the coated layers hinder the movement of the metal atoms less and facilitate formation of ordered intermetallic compounds. In addition, they do not require an additional step to remove the coating layer and they can be synthesized relatively easily. In this study, we synthesized chemically ordered Pt3Fe/C intermetallic catalysts by coating a carbon layer to act as an agglomeration barrier during heat treatment. Then, ORR performance of the as-synthesized Pt3Fe/C was measured and compared with that of disordered Pt−Fe alloy catalysts and commercial Pt/C by using a rotating disk electrode (RDE) test. An accelerated durability test (ADT) was carried out to confirm the durability of catalysts. Furthermore, ORR activity and durability of the synthesized Pt3Fe/C were evaluated through single cell tests, in comparison with commercial Pt/C. Finally, we demonstrated that chemically ordered Pt3Fe/C is a highly active and durable catalyst and explained the relationship between chemically ordered structures and catalytic performances.

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In order to prevent nanoparticle agglomeration, Pt−Fe/C catalysts were coated with a carbon layer before heat treatment. 50 mg of Pt− Fe/C catalysts was mixed with 5 mL of hexane to achieve better dispersion. The suspension was ultrasonicated for 5 min. The resultant suspension was injected into a mixture of 50 mL of DPE, 250 mg of 1,2-hexadecanediol, and 1 mg of Fe(acac)3. The mixture was heated to 259 °C for 30 min. After the reaction, 20 mL of ethanol was added to induce precipitation. The resultant mixture was centrifuged at 10 000 rpm for 30 min and washed one more time with a mixture of hexane and ethanol. The carbon coated Pt−Fe/C catalysts were dried in a vacuum oven at 50 °C for 5 h. Then, heat treatment was performed at 973 K under a reductive atmosphere (H2 4% + Ar 96%). As a result, we obtained uniformly dispersed Pt3Fe/C electrocatalysts without an additional removal step. Characterization. The morphologies of the Pt−Fe nanoparticles and as-synthesized catalysts were observed by transmission electron microscopy (TEM; Tecnai G2 F30 S-Twin, FEI). High-angle annular dark-field scanning transmission electron microscopy (HAADFSTEM) and energy-dispersive X-ray spectroscopy (EDS) measurements were carried out on a 300 kV Cs-corrected TEM (Titan cubed G2 60-300, FEI). The metal content and composition were measured by inductively coupled plasma-optical emission spectrometry (ICPOES; ICAP 6500, Thermo Elemental). A microwave digestion system (Ethos-plus, Milestone) was used to facilitate dissolution of metals. To investigate crystal structures, high resolution X-ray diffractometry (HRXRD; SmartLab, RIGAKU) was used. The diffraction patterns were recorded on a 2D detector (Hypix-3000) and high speed 1D detector (D/tex Ultra 250) with Cu Kα radiation operated at 45 kV and 200 mA. X-ray photoelectron spectroscopy (XPS; K-Alpha+, Thermo Fisher Scientific) measurements were performed to investigate the chemical state of the elements. The calibration of the binding energy was carried out using C 1s as the reference (binding energy of C 1s = 284.8 eV).40 Electrochemical Measurements. Electrochemical measurements were carried out at room temperature (298 K) using a three electrode electrochemical cell and potentiostat (Interface 1000, Gamry). Pt wire and Ag/AgCl electrode were used as the counter and reference electrode, respectively. Measured potentials were converted to a reversible hydrogen electrode (RHE) scale by calibration in a H2purged 0.1 M HClO4 solution prior to measurement. Ten mg of catalysts was dispersed in a mixture of 0.5 mL of IPA, 2.0 mL of DI water, and 50 μL of Nafion solution under ultrasonic wave agitation for 20 min. The resultant suspension (10 μL) was dropped onto a glassy carbon electrode with a diameter of 5 mm (Pine Instruments) and dried for 30 min under an IR lamp. All catalysts were activated by repeated cyclic voltammetry in the range of 0.05−1.2 V with a scan rate of 100 mV/s for 50 cycles before each cyclic voltammetry measurement. CV curves were obtained by cycling the potential between 0.05 and 1.2 V in a N2-saturated 0.1 M HClO4 solution with a scan rate of 50 mV/s. ECSA was calculated by integration of the hydrogen adsorption region between 0.05 and 0.4 V using 210 μC/ cm2 of monolayer hydrogen adsorption charge on platinum.41 The ORR polarization curves were measured by sweeping the potential between 0.3 and 1.05 V in an O2-saturated 0.1 M HClO4 solution with scan rate of 10 mV/s and rotating speed of 1600 rpm. The kinetic current (jk) was calculated by using the Koutecky-Levich equation,42 which is expressed by

EXPERIMENTAL SECTION

Materials. Platinum acetylacetonate (Pt(acac)2, 97%), iron(0) pentacarbonyl (Fe(CO)5, >99.99%), iron acetylacetonate (Fe(acac)3, 97%), 1,2-hexadecanediol (HDD, 90%), 1-octadecene (ODE, 90%), oleylamine (OAm, 70%), oleic acid (OAc, 90%), diphenyl ether (DPE, 99%), toluene (C6H5CH3, >99.5%), perchloric acid (HClO4, 70%), and Nafion solution (5 wt % Nafion dissolved in alcohol) were purchased from Aldrich. Hexane (C6H14, 95%) was purchased from Junsei. A commercial Pt/C catalyst (nominally 20 wt % on carbon black, Johnson Matthey Company) was purchased from Alfa Aesar. All other reagents were purchased from Samchun Chemical. All chemicals were used as received without further purification. Synthesis of Pt−Fe Nanoparticles. To synthesize Pt−Fe nanoparticles,39 200 mg of Pt(acac)2 and 20 mL of ODE were mixed in a 200 mL three-neck flask and the mixture was degassed under vacuum at 120 °C for 15 min. Then, 0.2 mL of Fe(CO)5 was dissolved in a certain amount of OAc (0.2, 0.5, or 1.0 mL) and was injected into the reaction mixture under a flow of Ar. After 5 min, a certain amount of OAm (0.5, 1.0, or 2.0 mL) was injected into the reaction mixture. Then, the reaction mixture was heated at 210 °C for an hour. Then, the Pt−Fe nanoparticles were precipitated from the resultant mixture by addition of acetone and washed three times with a mixture of acetone and hexane. After the washing procedure, the Pt− Fe nanoparticles were dried in a vacuum oven at 50 °C for 5 h. Preparation of Pt−Fe/C Catalysts. To load Pt−Fe nanoparticles on carbon support, 140 mg of carbon black (Vulcan XC 72R) was mixed with 70 mL of toluene under ultrasonic wave agitation for an hour. Then, 35 mg of Pt−Fe nanoparticles (OAc: 0.5 mL; OAm: 1.0 mL) was added to the mixture and ultrasonicated for an hour. After that, mixing was continued with magnetic stirring agitation for 10 h. The resulting Pt−Fe/C catalysts were precipitated by addition of 20 mL of ethanol and were centrifuged at 10 000 rpm for 20 min. After centrifugation, the Pt−Fe/C catalysts were dried in a vacuum oven at 50 °C for 5 h. Preparation of Pt3Fe/C Catalysts. To convert random alloy structure to chemically ordered structure, heat treatment is required.

1 1 1 1 1 = + = + j jk jd jk 0.62nF(DO2)2/3 v−1/6CO2ω1/2 where j is the measured current density, jd is the diffusion-limited current density, n is the number of electrons transferred, F is Faraday’s constant (96 485 C/mol), DO2 is the diffusion coefficient of O2 in 0.1 M HClO4 solution (1.93 × 10−5 cm2/s), v is the kinematic viscosity of the electrolyte (1.01 × 10−2 cm2/s), CO2 is the concentration of oxygen in 0.1 M HClO4 solution (1.26 × 10−6 mol/cm3), and ω is the angular rate of the rotating disk electrode. The accelerated durability test (ADT) was performed by cycling the potential between 0.6 and 1.0 V 31807


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ACS Applied Materials & Interfaces for 4000 and 8000 times in an O2-saturated 0.1 M HClO4 solution with a scan rate of 100 mV/s. Fabrication of Single Cells. Catalyst ink was prepared by mixing catalyst powders (Pt3Fe/C or Pt/C; Johnson Matthey Company, 20 wt %), IPA, and 5 wt % Nafion dispersion. Then, the mixture was sonicated for 30 min and sprayed onto both sides of Nafion 211 membrane to fabricate membrane-electrode assembly (MEA). The cathode was prepared using Pt3Fe/C or Pt/C. For the anode, Pt/C was used. Pt loading was 0.15 mgPt/cm2 for both anode and cathode with an active area of 5 cm2. A single cell was assembled with the prepared MEA, gas diffusion layers (JNT20-A3, JNTG), gaskets, current collectors, and graphite flow fields with quintuple-serpentine channels. Single Cell Tests. The single cells were operated on fully humidified hydrogen and air at 65 °C. Hydrogen and air were fed to the single cells at a flow rate of 209 and 660 sccm, respectively, without back pressure after passing bubble-type humidifiers. A PEMFC test station (CNL Energy Co.) and an electric loader (ESL-300Z, E.L.P tek) were employed to measure i−V (current−voltage) curves of the single cells. To evaluate durability of the single cells, a triangular voltage sweep cycle was performed at 50 mV/s from 0.6 to 1.0 V.43 During the voltage cycling, fully humidified hydrogen and nitrogen gas were supplied to the anode and cathode, respectively. Flow rates of hydrogen and nitrogen were 20 and 100 sccm, respectively. After 3000 and 10 000 voltage cycles, i−V curves were measured for the single cells.

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RESULTS AND DISCUSSION Characterization of Pt−Fe Nanoparticles. To synthesize Pt−Fe nanoparticles with proper size and composition, we controlled the amount of surfactant (OAc and OAm) in ODE. These surfactants combine with the metal atoms and form metal−ligand complexes in the nucleation step, thus hindering the nucleation of the metal atoms.44,45 As a result, the amount of surfactants affects the nucleation rate. We synthesized Pt−Fe nanoparticles by changing the amount of oleic acid and oleylamine. The detailed experimental results are shown in Figures S1 and S2. We selected Pt−Fe nanoparticles that were synthesized with 0.5 mL of OAc and 1.0 mL of OAm. Figure S3a shows the TEM image of the Pt−Fe nanoparticles. These particles were of suitable size (5.57 nm) and were obtained in high yield. The synthesized Pt−Fe nanoparticles were composed of 23 at. % iron and 77 at. % platinum, as measured by ICP-OES. HRTEM analysis was conducted to determine the lattice constant, as shown in Figure S3b. The lattice spacings of (200) and (111) were 0.192 and 0.221 nm, respectively. From these lattice spacings, the lattice constant of the as-synthesized Pt−Fe nanoparticles was calculated to be 0.384 nm, which was smaller than the lattice constant of bulk Pt (0.392 nm) owing to smaller Fe atoms.46 To confirm the crystal structure of the Pt− Fe nanoparticles, XRD analysis was performed. Figure S3c illustrates an XRD pattern of the Pt−Fe nanoparticles. The main peaks were shifted to a higher angle, and there was no peak separation. Those XRD results support that Pt and Fe are well alloyed, in accord with TEM analysis. Characterization of Pt3Fe/C Catalysts. The as-synthesized Pt−Fe nanoparticles were loaded onto a carbon support. As a result of this procedure, highly dispersed Pt−Fe/C catalysts were prepared as shown in Figure 1a,b. To investigate the composition of Pt−Fe/C catalyst, ICP-OES analysis was used. As-prepared Pt−Fe/C catalysts were composed of 18.84 wt % of platinum and 1.51 wt % of iron in weight percent. This weight percent of platinum was similar to commercial 20 wt %

Figure 1. TEM images of (a, b) Pt−Fe/C, (c, d) carbon coated Pt− Fe/C, and (e, f) Pt3Fe/C at low magnification and high magnification.

Pt/C. Thus, it is reasonable to compare performances of the prepared Pt−Fe/C with those of commercial 20 wt % Pt/C. In general, high-temperature heat treatment is inevitable to synthesize intermetallic structures.35 However, high-temperature heat treatment causes metal nanoparticles to agglomerate. In this study, we employed a carbon layer coating to prevent particle agglomeration. We used HDD and Fe(acac)3 as carbon sources.47,48 After the carbon coating process, a thin carbon layer was coated on the nanoparticles uniformly, as shown in Figure S4. Figure 1c,d shows there was no serious nanoparticles agglomeration during the additional coating process. After the coating process, we did reductive heat treatment at 973 K. There were two purposes of the heat treatment process, which were to convert the disordered Pt−Fe alloys to ordered Pt3Fe intermetallic L1 2 structures and to remove the unnecessary carbon layer.31 As a result, the carbon layer was removed and the shapes of the nanoparticles were changed to cuboctahedron which have lower surface energy (111) facets than the (100) facets in cubic, as shown in Figure 1e,f. Figure S5 compares TEM images of carbon-coated Pt−Fe/C and bare Pt−Fe/C after heat treatment. Clearly, carbon-coated Pt−Fe/C was protected from agglomeration. From Figure S5, particle size was measured and presented in Figure S6. Bare Pt−Fe/C that underwent heat treatment had a larger particle size (10.74 nm), and the deviation of the particle size was increased by particle agglomeration. Therefore, we concluded that the 31808


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Figure 2. Structural analysis of as-synthesized Pt3Fe/C ordered intermetallic catalysts. (a) XRD patterns of Pt−Fe/C and Pt3Fe/C catalysts (Pt3Fe: PDF card # 29-0716). (b) HAADF-STEM images of Pt3Fe/C close to [011]. (c) EDS mapping images of Pt3Fe/C close to [011].

precisely shows the aligned Pt and Fe layer structure. Hence, chemically ordered intermetallic compounds serve as uniform active sites on the surface, and these are different from randomly distributed alloys (Figure S8). These differences confer special properties, in terms of both activity and stability on intermetallic compound catalysts. ORR Catalytic Performance. To evaluate catalytic performance, rotating disk electrode (RDE) measurements were performed. Each catalyst was coated on a glassy carbon electrode used as the working electrode. Figure 3 illustrates

carbon layer acted as a good agglomeration barrier. Composition of as-synthesized Pt3Fe/C catalysts measured by ICP-OES analysis was 19.12 wt % of platinum and 1.85 wt % of iron in weight percent, which was similar to composition of Pt− Fe/C, as shown in Table S1. XPS analysis was conducted to investigate the chemical states of carbon. The core-level C 1s spectra were deconvoluted into four peaks at approximately 284.8, 285.9, 287.2, and 289.1 eV, which were assigned to C−C, C−OH, C−O−C, and CO, respectively,49,50 as shown in Figure S7. Area ratio of the peaks associated with the carbon bondings is summarized in Table S2. Carbon-coated Pt−Fe/C displayed more C−OH and CO bonding than Pt−Fe/C because of the −OH groups from the diol (HDD) and dehydration of alcohol, respectively. It supported the fact that carbon layer was formed by a coating process. After heat treatment, the chemical bonding characteristics of carbon were similar to those of Pt−Fe/C before carbon coating, thus indicating that the coated carbon layer was removed by pyrolysis during treatment. To confirm the crystal structures, XRD analysis was conducted. Figure 2a shows that disordered Pt−Fe nanoparticles were transformed to a Pt3Fe ordered intermetallic compound by heat treatment. In the low angle area, there was a broad peak originating from the amorphous carbon support.51 After heat treatment, new peaks such as (100), (110), (210), (211), (300), and (310) appeared, thus indicating that disordered FCC was converted into the chemically ordered L12 structure. Moreover, HAADF-STEM analysis was carried out to determine the atomic structure of the Pt3Fe intermetallic compounds. In the STEM images, the platinum atoms and iron atoms appeared with different contrasts; platinum atoms appeared bright, and iron atoms appeared dark. Figure 2b illustrates the chemically ordered Pt3Fe nanoparticles at [110]. In the [110] direction, chemically ordered L12 structures appeared as a layer-by-layer structure.31 To distinguish the different atoms directly, we used EDS analysis. Figure 2c

Figure 3. Electrochemical data for the initial ORR polarization curves of Pt/C, Pt−Fe/C, and Pt3Fe/C. The inset shows the mass activity plots of the catalysts.

typical polarization curves for the ORR, showing the diffusionlimiting region below 0.8 V and the kinetic-diffusion control region between 0.8 and 1.0 V. The measured half-wave potential was changed in the following order: Pt/C < Pt−Fe/C < Pt3Fe/C. Pt3Fe/C exhibited the highest half-wave potential of 0.927 V, which is 39 mV higher than that of Pt/C. Pt−Fe/C exhibited an E1/2 value of 0.899 V. The kinetic current density 31809


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Figure 4. Electrochemical data during the accelerated durability test (ADT). CV curves for (a) Pt/C, (b) Pt−Fe/C, and (c) Pt3Fe/C. (d) Changes in ECSA for Pt/C, Pt−Fe/C, and Pt3Fe/C during the ADT.

the decrease in ECSA of Pt/C and Pt−Fe/C was strongly related to particle agglomeration during the ADT. During the ADT, nanoparticles on the carbon support underwent rapid potential changes, which induced the dissolution and redeposition of metals.55 This harsh environment forced the nanoparticles to agglomerate and decreased ECSA. In addition to agglomeration of catalyst nanoparticles, corrosion of carbon support might deactivate the catalyst.16,55 Thermodynamically, carbon oxidation can occur at PEMFC 0 = 0.207 VNHE at T = 298 K). cathode potentials (ECO 2/C However, due to high overpotential and sluggish kinetics, carbon oxidation occurs notably only if the electrode potential is higher than 1.0−1.1 VNHE.57,58 On the other hand, it is known that, if carbon oxidation occurs, double layer capacitance increases in CV.59 However, there were negligible changes in double layer capacitance in Figure 4a−c. Therefore, it can be concluded that carbon corrosion played minor roles in deactivating the catalysts in this work. The ORR polarization curves of Pt/C, Pt−Fe/C, and Pt3Fe/ C are presented in Figure 6a−c from the initial measurements and after the ADT of 4000 and 8000 cycles. After the ADT of 8000 cycles, all of the catalysts exhibited loss of mass activity and specific activity, as shown in Figure 6d,e. After the ADT of 8000 cycles, the amount of mass activity loss was in the following order: Pt3Fe/C < Pt−Fe/C < Pt/C. Pt3Fe/C exhibited the smallest degradation of mass activity (0.343 A mgPt−1), and its final value of mass activity was much higher than that of Pt/C (0.030 A mgPt−1) and Pt−Fe/C (0.109 A mgPt−1). EDS analysis was carried out to investigate compositional changes of the catalysts during the ADT. Figure 6f shows Fe contents in Pt3Fe/C and Pt−Fe/C measured before and after the ADT of 8000 cycles. Pt3Fe/C underwent less of a

was calculated using the Koutecky-Levich equation. The inset in Figure 3 shows that Pt3Fe/C had the highest mass activity (0.454 A mgPt−1), which was a 4-fold higher mass activity than commercial Pt/C. Many studies have reported that chemically ordered intermetallic structures show outstanding ORR activities.18,52 Unlike disordered alloys, intermetallic structures provide better control over composition, structure, and electronic effects.27,53,54 This improved control leads to the presence of uniform active sites on the surface plane. As a result, the intermetallic compounds have superior catalytic activities.18 Electrochemical Stability of Catalysts. In addition to catalytic activity, long-term stability is a crucial issue in developing new catalysts.16,55 To evaluate long-term stability of the catalysts, cyclic voltammetry (CV) and ORR polarization curves were measured for Pt/C, Pt−Fe/C, and Pt3Fe/C before and after the ADT of 4000 and 8000 cycles. From the CV curves (Figure 4a−c), we determined the degradation of ECSA during the ADT, as shown in Figure 4d. After the ADT 4000 cycles, normalized ECSA of Pt3Fe/C was slightly increased to 1.03, as a result of surface roughening due to dissolution of iron.56 After the ADT of 8000 cycles, Pt3Fe/C maintained a normalized ECSA of 0.988. However, the ECSAs of Pt−Fe/C and Pt/C decreased to 0.8 and 0.7 of the initial values. To investigate why ECSA was decreased during the ADT cycles, we analyzed particle morphology by TEM analysis. For the Pt/C and Pt−Fe/C catalysts, pronounced particle agglomeration was observed after 8000 cycles, as shown in Figure 5a−d. In contrast, significant particle agglomeration was not observed in Pt3Fe/C catalysts after the ADT of 8000 cycles (Figure 5e,f). In addition, the morphology of Pt3Fe/C particles was almost unchanged during ADT. These results reveal that 31810


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change in iron content (24.3 → 19.6 at. %). However, Pt−Fe/ C showed a substantial change in iron content (23.7 → 11.9 at. %). These compositional changes could decrease the specific activity. Therefore, Pt3Fe/C showed less specific activity degradation, owing to the small change in iron content. However, Pt−Fe/C showed a large specific activity change, owing to the considerable change in iron content. In short, we concluded that the degradation of mass activity during the ADT of 8000 cycles resulted from the decrease in ECSA and specific activity, which originated from compositional changes.60 In the case of Pt3Fe/C, reduction of specific activity affected mass activity degradation during ADT. However, degradation of both ECSA and specific activity caused mass activity degradation in Pt−Fe/C and Pt/C. As a result, Pt−Fe/C and Pt/C had lost much larger mass activity than Pt3Fe/C and this result is closely related to superior stability of intermetallic phase. To demonstrate ORR activity and durability of Pt3Fe/C in a single cell, membrane-electrode assemblies (MEAs) were fabricated using Pt3Fe/C and commercial Pt/C as a cathode catalyst. Figure 7a,b compares i−V curves for Pt3Fe/C and Pt/ C measured before and after 3000 and 10 000 voltage cycles from 0.6 to 1.0 V. The single cell prepared with Pt3Fe/C exhibited higher performance than that with Pt/C; at a cell voltage of 0.6 V, current density was 581 and 475 mA/cm2 for Pt3Fe/C and Pt/C, respectively. With repetition of the voltage cycle, current density at 0.6 V was kept almost constant up to 3000 cycles and then slightly lowered to 539 mA/cm2 at 10 000 cycles for the single cell using Pt3Fe/C (7.2% reduction). In contrast, performance of the single cell using Pt/C decreased rapidly to 435 and 373 mA/cm2 (21.5% reduction) at 0.6 V in 3000 and 10 000 cycles, respectively, as shown in Figure 7c. These results demonstrate that the synthesized intermetallic Pt3Fe/C had higher activity and durability than commercial Pt/ C and previously reported Pt3Fe/C in the single cell as well as the RDE test (Table S3).

Figure 5. TEM images before the ADT of (a) Pt/C, (c) Pt−Fe/C, and (e) Pt3Fe/C and after the ADT of 8000 cycles of (b) Pt/C, (d) Pt− Fe/C, and (f) Pt3Fe/C.

Figure 6. Catalytic performances during the ADT. The ORR polarization curves of (a) Pt/C, (b) Pt−Fe/C, and (c) Pt3Fe/C and their catalytic performances in terms of (d) mass activity and (e) specific activity and (f) change in Fe composition. 31811


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Figure 7. Single cell test for (a) Pt3Fe/C and (b) Pt/C and (c) current density (0.6 V) during the voltage cycling.

Figure 8. Dissolution potential of (a) Pt and (b) Fe on the faceted planes of Pt−Fe and Pt3Fe.

To support the excellent stability of the Pt3Fe intermetallic catalyst, we performed density functional theory (DFT) calculation by using the Vienna ab initio simulation package (VASP) code. Dissolution potential of metal element can be used as a measure of stability of catalysts.61,62 Detailed information on the computation is available in the Supporting Information. As presented in Figures S1−S5, Pt−Fe alloys were mainly faceted in the (100) plane and intermetallic Pt3Fe nanoparticles were faceted with (100) and (111) owing to the surface reconstruction during the heat treatment. Thus, dissolution potential of Pt and Fe was calculated for Pt−Fe alloy (100) and intermetallic Pt3Fe (100) and (111). It should be noted that, in the disordered Pt−Fe alloy structure, dissolution potential of Pt and Fe has broad distribution due to random arrangement of atoms with different coordination numbers and chemical environment. Figure 8a reveals that the dissolution potential of Pt atoms on the intermetallic Pt3Fe (100) and (111) plane was higher than that on the Pt−Fe alloy (100) plane. Also, dissolution potential of Fe atoms on the Pt− Fe (100) is lower than that of Pt3Fe (111) in all cases and lower or higher than that of Pt3Fe (100) depending on the configuration of Pt−Fe alloys, as shown in Figure 8b. Since the Fe atoms on Pt−Fe (100) with lower dissolution potential would dissolve preferentially, it can be concluded that both Fe and Pt atoms would more readily dissolve from Pt−Fe alloy than from intermetallic Pt3Fe. To confirm the DFT calculation results, the amount of Pt and Fe dissolved in the electrolyte during the ADT was measured by ICP-MS (inductively coupled plasma-mass spectroscopy). Figure S9 demonstrates that the amount of dissolved Pt was very small and similar in both samples because dissolution potential of Pt was higher than the applied potential

during ADT (0.6−1.0 V). However, the amount of Fe dissolved from Pt−Fe/C was almost four times higher than that from Pt3Fe/C, in a good agreement with the DFT calculation results shown in Figure S9. Considering the decreases in Fe content shown in Figure 6f, these results imply that Pt skin layers could be formed on the surface of Pt3Fe nanoparticles with dissolution of surface Fe atoms. On the other hand, in the case of Pt−Fe alloy, about 50% of Fe atoms was dissolved during the ADT (Figure 6f) with dissolution of Fe atoms inside of the nanoparticles. Formation of Pt skin layers on Pt3Fe nanoparticles can be observed by using EDS line scanning (Figure S10) and CO-stripping voltammetry (Figure S11). It was reported that, if Pt skin layers are formed, ECSA obtained by CO-stripping (ECSACO) is higher than that obtained by Hupd (ECSAHupd).63 From the CO-stripping voltammograms shown in Figure S11, ECSACO was calculated to be 36.7 m2/gPt, which is higher than ECSAHupd (32.9 m2/gPt). Finally, we concluded that activity degradation was caused by two factors: ECSA degradation due to particle agglomeration and specific activity degradation due to changes in the composition of the nanoparticles. These factors are not independent of each other. Furthermore, these factors affect not only activity but also stability. Therefore, making stable nanoparticle is very important to ensure both activity and stability. All data related to the ADT are presented in Table S4.

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CONCLUSIONS We synthesized Pt3Fe/C oxygen reduction reaction catalyst. The intermetallic structure was formed by heat treatment without causing substantial particle agglomeration. To prevent particle agglomeration, we coated a carbon layer as agglomeration barrier. Before heat treatment, the particle size 31812


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of Pt−Fe alloy was 5.57 nm. After heat treatment, the average particle size of carbon coated Pt3Fe/C catalysts was 6.77 nm. However, bare Pt−Fe/C grew to a particle size of 10.74 nm after heat treatment, which successfully supported the carbon layer acting as an agglomeration barrier. As a result, ECSA of the Pt3Fe/C was almost maintained and the Pt3Fe/C catalysts exhibited four times higher mass activity (0.454 A mgPt−1) than Pt/C (0.110 A mgPt−1) in the initial measurements. This higher mass activity originated from unique intermetallic properties such as the chemically ordered structure and uniform active sites on the surface plane. Moreover, this Pt3Fe/C catalyst showed superior durability, owing to the excellent stability of the intermetallic phase. After the ADT of 8000 cycles, ECSA was almost completely maintained in Pt3Fe/C because there was no particle agglomeration during the ADT of 8000 cycles. Specifically, Pt3Fe/C showed 11 times higher mass activity (0.343 A mgPt−1) than Pt/C (0.030 A mgPt−1) after the ADT of 8000 cycles. In contrast, disordered Pt−Fe/C showed 50% degradation of mass activity after the ADT of 8000 cycles, owing to substantial particle agglomeration and iron leaching which is supported by DFT calculation. Furthermore, we proved Pt3Fe had excellent durability by the single cell test under the actual operating environment. The single cell prepared using Pt3Fe/C exhibited only 7.2% reduction in current density at 0.6 V while commercial Pt/C demonstrated a 21.5% decrease after 10 000 voltage cycles. Therefore, intermetallic compound catalysts may be a promising solution for ORR catalysts with higher activity and superior durability because of their uniform chemical composition and excellent phase stability.

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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/acsami.7b07648. More information and details about the first-principle calculations, CO-stripping method, analysis of the synthesized Pt−Fe nanoparticles, TEM images of Pt/C, Pt−Fe/C, and Pt3Fe/C, C 1s XPS spectra of catalysts, HADDF-STEM images of Pt−Fe/C and Pt3Fe/C, ICP results of Pt−Fe/C and Pt3Fe/C after ADT of 8000 cycles, and EDS line scanning results (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hyuck Mo Lee: 0000-0003-4556-6692 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by a National Research Foundation (NRF) of Korea grant funded by the Korean Government (MSIP) (No. NRF-2015R1A5A1037627)

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REFERENCES

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