Effect of Pretreatment on Durability of fct-Structured ... - ACS Publications

Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9809-9817

pubs.acs.org/journal/ascecg

Effect of Pretreatment on Durability of fct-Structured Pt-Based Alloy Catalyst for the Oxygen Reduction Reaction under Operating Conditions in Polymer Electrolyte Membrane Fuel Cells Won Suk Jung* and Branko N. Popov* Center for Electrochemical Engineering, Department of Chemical Engineering, University of South Carolina, 301 Main Street, Columbia, South Carolina 29208, United States S Supporting Information *

ABSTRACT: The effects of different pretreatments on performance and durability of the fct-structured Pt-based alloy catalyst were investigated under operating conditions in PEMFCs. The fct-structured PtCo catalyst (PtCo/CN) was prepared by impregnating transition metal salts into Pt/CN catalyst followed by a heat-treatment under a reducing atmosphere. To remove the excess amount of transition metal on the catalyst surface, a preleaching procedure was carried in 0.5 M H2SO4 solution to synthesize the L-PtCo/CN catalyst. Subsequently, the L-PtCo/CN catalyst was annealed under a reducing atmosphere at a mild temperature to synthesize the ALPtCo/CN catalyst. The intensive physicochemical analyses were performed before and after the durability test to evaluate the effects of the pretreatments on the catalyst durability. All catalysts were electrochemically tested for the ORR performance, while the durability test was carried out in a single cell by sweeping 30 000 potential cycles. The results indicated that the L-PtCo/CN catalyst contains a low percentage of metallic Pt(0), degrades faster, and exhibits unstable performance when compared to the AL-PtCo/CN catalyst. The L-PtCo/CN catalyst after the durability test shows poor catalyst particle distribution and catalyst particle detachment. On the other hand, the AL-PtCo/CN catalyst shows a remarkably stable performance of ECSA of 9% and only 16% in maximum power density loss after AST. KEYWORDS: polymer electrolyte membrane fuel cells, membrane electrolyte assembly, pretreatment, potential cycling, durability

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INTRODUCTION Polymer electrolyte membrane fuel cells (PEMFCs) are enticing clean power sources for automotive and stationary applications, due to intrinsic advantages such as low emission, high energy density, and high efficiency. However, there are still some issues to address before achieving commercialization, for example, the sluggish kinetic and poor durability of the cathode catalysts. Currently, Pt is the best catalyst of the pure metals for the oxygen reduction reaction (ORR). However, the durability of Pt catalyst suffers from the dissolution of the Pt1,2 catalyst, particle agglomeration,3,4 and migration of Pt.4,5 Through decades of development, the Pt-M (M = first-row transition metal) alloy catalysts have made remarkable progress with regard to the sluggish ORR kinetics.6−9 The enhancement of Pt-based alloy catalysts over the Pt catalyst is attributed to the modifications of electronic structure,10,11 suppression of Pt oxide formation,8,12 a decrease in the Pt−Pt distance,8 and the formation of Pt-skin on the topmost of the Pt-based alloy catalyst.13,14 Jia et al. reported that the high surface area carbonsupported dealloyed Pt1Co1 and Pt1Co3 catalysts exhibited the same levels of enhancement in oxygen reduction activity (∼4fold) and durability over pure Pt/C catalysts.9 Ex situ highangle annular dark field scanning transmission electron © 2017 American Chemical Society

microscopy (HAADF STEM) showed that the dealloyed Pt1Co1 catalyst was dominated by particles with solid Pt shells surrounding a single ordered PtCo core, while porous Pt shells surrounding multiple disordered PtCo cores with the local concentration of Co were observed in the dealloyed Pt1Co3 catalyst particles. Choi et al. utilized a chemical vapor deposition (CVD) technique to prepare the Pt3Co1 catalysts.7 The optimal Pt3Co1/C catalyst exhibited 5.4 times and 6.5 times higher mass activity and specific activity than that obtained for the Pt/C catalyst, respectively. Furthermore, the durability test showed no loss of electrochemical surface area (ECSA), and the HAADF-STEM imaging and energy dispersive X-ray spectroscopy (EDS) mapping showed that the particles maintained a homogeneous distribution of Pt and Co and did not significantly agglomerate. Yu et al. studied the durability of Pt/C and PtCo/C catalysts under a potential cycling test between 0.87 and 1.2 V (vs RHE).15 They found that cobalt dissolution neither detrimentally reduced the cell voltage nor dramatically affected the membrane conductance. Received: June 1, 2017 Revised: August 21, 2017 Published: September 11, 2017 9809

DOI: 10.1021/acssuschemeng.7b01728 ACS Sustainable Chem. Eng. 2017, 5, 9809−9817

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ACS Sustainable Chemistry & Engineering

then dried 80 °C overnight. The obtained powder was pyrolyzed under inert atmosphere at 1100 °C for 1 h followed by leaching in 0.5 M H2SO4 at 80 °C to remove the residual Fe. ICP confirmed that the CN contains ∼0.6 wt % Fe. Pt deposition was carried out by using a polyol reduction method for the preparation of 35% Pt/C catalyst. Initially, the support dispersed in 25 mL of ethylene glycol was sonicated for 30 min. A 100 mg amount of PtCl4 was added, and the pH was controlled to ∼11 by the addition of 0.5 M NaOH solution. The mixture thus prepared was refluxed at 160 °C for 3 h and cooled to room temperature. The catalyst was filtered and washed thoroughly with deionized (DI) water. Finally, the catalyst obtained was dried at 160 °C for 1 h. The Pt/CN catalyst was mixed with Co(NO3)2 at a ratio of 1:1 and stirred for 24 h at room temperature. After drying in the oven, the catalyst in an alumina crucible was transferred to a tubular furnace and annealed at 900 °C for 1 h under 5% H2 (balance N2) atmosphere (denoted as PtCo/CN). The PtCo/CN catalyst was preleached in 0.5 M H2SO4 at 80 °C for 4 h followed by washing with DI water (denoted as L-PtCo/ CN). The L-PtCo/CN catalyst was annealed at 500 °C for 1 h under 5% H2 (balance N2) atmosphere (denoted as AL-PtCo/CN). Physical Characterization. XRD analysis was performed using a Rigaku D/Max 2500 V/PC with Cu Kα radiation. A tube voltage of 30 kV and a current of 15 mA were used during the scanning. To estimate the particle size of samples, we employed the Scherrer equation:31

Cell performance enhancement by PtCo/C over Pt/C catalyst was sustained over 2400 cycles, and the performance of the PtCo/C was more stable than that of the Pt/C MEA. Gummalla et al. detected the signal from the Co in cationic form.16 They noted that the Co could not be reduced in the MEA due to nobility less than that of hydrogen. Dealloyed PtCo3 and PtCu3 catalysts were investigated under a number of potential cycles.17 They observed that the dealloyed PtCu3 possesses initial activity higher than that of the dealloyed PtCo3. However, due to Cu plating on the anode, the dealloyed PtCu3 catalyst showed poor durability in MEA tests. Recently, Pt-based alloy catalysts with a face-centered tetragonal (fct) structure have shown extraordinary durability in the ORR.18−22 In our previous works,18,19 PtCo catalyst prepared by controlled heat-treatment showed initial mass activity of 0.44 A mgPt−1 and 43% loss after 30 000 potential cycles between 0.6 and 1.0 V. The peak power density was exceptionally stable after the durability test (3% loss). As well, 64% of electrochemical surface area (ECSA) was retained after the durability test, which is highly active and stable as compared to the commercial Pt/C catalyst with a poor activity and durability. The catalyst with an fct structure is more durable than that with a face-centered cubic structure (fcc) due to their well-ordered intermetallic configuration.21 For the fct-structured FePt catalyst, the Fe and Pt are strongly interactive via spin−orbit coupling, and hybridization between Fe 3d and Pt 5d states makes the fct-structured FePt chemically much more stable.21,23,24 In addition, it was reported that the pretreatment could modify the Pt-based alloy such as Pt-skin structure, known for higher activity and durability than that of the Pt/C catalyst.25,26 The Pt-skin structure was formed by preleaching treatment to remove excessive Ni or Cu atoms.25−28 Further surface relaxation and restructuring of the topmost Pt atoms led to the transformation into a multilayered Pt-skin structure by thermal treatment.29 Thus, the objective of this work was to evaluate the impact of different pretreatments on performance and durability of the fct-structured Pt-based alloy catalyst as a cathode in PEMFCs. The fct-structured PtCo catalyst (PtCo/CN) was prepared by impregnating transition metal salts into Pt/CN catalyst followed by a heat-treatment under a reducing atmosphere. The leaching procedure is carried out in 0.5 M H2SO4 solution (LPtCo/CN) to remove the excess amount of transition metal salts that are used for the catalyst synthesis. Subsequently, the L-PtCo/CN catalyst was annealed under a reducing atmosphere at a mild temperature to synthesize the AL-PtCo/CN catalyst. All catalysts prepared by using different pretreatments were characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), and X-ray photoelectron spectroscopy (XPS) before and after the accelerated stress test (AST) to evaluate the effects of pretreatments on the catalyst durability. Next, the catalysts were electrochemically tested for the ORR activity and durability by using MEAs tests. The results were compared to those of the commercial catalyst.

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D=

kλ 10B cos θ

(1)

where D is the particle size in nanometers, k is a coefficient taken here as 0.9, λ is the wavelength of X-ray (0.15404 nm), B is the line broadening at half the maximum intensity in radians, and θ is the angle at the position of the maximum peak known as the Bragg angle. XPS was carried out with a Kratos AXIS 165 high-performance electron spectrometer on samples to analyze the elemental oxidation state and near-surface composition. Inductively coupled plasma atomic emission spectroscopy (ICP, PerkinElmer) analysis was used to determine the bulk composition of the catalysts. HR-TEM was used to study the morphology and particles size distribution of the catalysts using a Hitachi 9500 HRTEM operated at 300 kV accelerating voltage. X-ray fluorescence (XRF, Fischer XDAL) was used to determine the Pt loading in the catalyst-coated membrane. MEA Fabrication and Electrochemical Measurement. The inhouse catalysts were used as the cathode catalyst while the commercial Pt/C (TEC10E50E, Tanaka Kikinzoku Kogyo K.K) catalyst was used as the anode catalyst. The catalysts and Nafion ionomer (5% solution, Alfa Aesar) were ultrasonically mixed in IPA and DI water to prepare the catalyst ink. The catalyst inks were directly sprayed onto either side of the Nafion 212 membrane. The active area was 25 cm2. The Pt loading on the anode and cathode electrodes was fixed at 0.1 and 0.1 mg cm−2, respectively. The final ionomer content in the catalyst layer was fixed at 30% and 20% for the anode and cathode, respectively. The catalyst-coated membrane was hot pressed at 140 °C using a pressure of 20 kg cm−2 for 3 min in between the gas diffusion layers (Sigracet 10BC, SGL). The fuel cell polarization was performed using a fully automated fuel cell test station (Scribner Associates Inc., model 850e) at 80 °C. H2 and air prehumidified at 40% relative humidity (RH) were supplied to the anode and cathode at a stoichiometry of 1.5 and 1.8. During the measurement, a backpressure of 150 kPaabs for anode and cathode was applied. The electrochemical surface area (ECSA) was calculated using cyclic voltammetry (CV) conducted between 0.05 and 0.6 V (vs RHE) at 80 °C, while H2 and N2 with 100% RH were supplied to the anode and the cathode, respectively. In AST, the potential was swept between 0.6 and 1.0 V (vs RHE) at 50 mV s−1 in a triangle profile for up to 30 000 cycles while supplying 200 mL min−1 H2 and 75 mL min−1 N2 to the anode and cathode, respectively, because it has been estimated that the cathode catalysts go through 30 000 potential cycles under practical operating conditions of PEMFCs. For comparison purposes, the MEA with the commercial Pt/C catalyst as a cathode catalyst was also prepared and evaluated under the same experimental conditions.

EXPERIMENTAL SECTION

Preparation of Catalyst. The CN support used in this work was prepared by the pyrolysis of carbon black (Ketjen Black EC-300J) in the presence of 20 wt % Fe and 2 mL of ethylenediamine.30 Typically, the functionalized carbon black in 250 mL of isopropyl alcohol (IPA) is mixed with a desired amount of Fe(NO3)3 and ethylenediamine. The homogeneous mixture was refluxed at 85 °C under stirring and 9810


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RESULTS AND DISCUSSION Characterization of Alloy Catalysts. The elemental compositional analysis of catalysts is summarized in Table 1.

Scherrer equation based on the PtCo(112) plane indicated 3.2, 3.7, and 3.9 nm for PtCo, L-PtCo, and AL-PtCo catalysts, respectively. The HR-TEM images of Pt/CN, PtCo/CN, L-PtCo/CN, and AL-PtCo/CN catalysts are shown in Figure 2a−d,

Table 1. Pt:Co Atomic Compositions of PtCo/CN, L-PtCo/ CN, and AL-PtCo/CN Catalysts Given by ICP and XPS bulka near-surfaceb a

PtCo/CN

L-PtCo/CN

AL-PtCo/CN

1.1:1 0.9:1

2.6:1 4.6:1

2.6:1 2.9:1

Composition measured by ICP. bComposition measured by XPS.

The elemental compositions in the bulk of PtCo/CN, L-PtCo/ CN, and AL-PtCo/CN catalysts were determined using ICPAES. The initial Pt:Co atomic ratio for the PtCo/CN catalyst is 1.1:1, and the L-PtCo/CN and AL-PtCo/CN catalysts exhibit the same Pt:Co atomic ratios of 2.6:1. The results indicated that the Pt:Co atomic ratio is reduced after a preleaching process and has not been affected by the annealing. The nearsurface elemental composition by XPS shows that the initial Pt:Co atomic ratio is 0.9:1, indicating a similar ratio with the bulk analysis. However, the XPS of L-PtCo/CN and AL-PtCo/ CN catalysts indicated ratios of 4.6:1 and 2.9:1, respectively. The preleaching process makes a large difference between the bulk and near-surface composition for the L-PtCo/CN catalyst due to the loss of Co on the catalyst surface, while the mild annealing process leads to diffusion of Co into the catalyst surface, resulting in similar Pt:Co ratios in the bulk and nearsurface. The XRD patterns of Pt/CN, PtCo/CN, L-PtCo/CN, and AL-PtCo/CN catalysts are shown in Figure 1. The character-

Figure 1. Comparison of XRD patterns of Pt/CN, PtCo/CN, LPtCo/CN, and AL-PtCo/CN catalysts.

Figure 2. HR-TEM images and corresponding histograms of (a) Pt/ CN, (b) PtCo/CN, (c) L-PtCo/CN, and (d) AL-PtCo/CN catalysts. Scale bar represents 10 nm.

istic diffraction peaks of Pt at 39.8° and Co at 44.2° correspond to the (111) plane of the fcc structure, respectively. After the alloy formation, the peaks of Co and Pt are shifted to lower and higher angles, respectively, and merged into a single peak at ∼41° due to the strain caused by the Co doping into the Pt host lattice. The peak at ∼24° and 33° corresponding to the PtCo(001) and PtCo(100) superlattice planes (PDF#97-0102622), respectively, represents that the fresh PtCo shows the chemically ordered fct structure.32 PtCo peaks in the L-PtCo and AL-PtCo catalysts are slightly lower than those of PtCo catalyst. The observed difference probably is caused by the compositional difference induced by a leaching process. The fct characteristic peaks were observed in the XRD patterns of LPtCo and AL-PtCo catalysts. The crystallite size estimated by

respectively. Approximately 100 nanoparticles are used to determine the mean particle sizes and particle size distribution. The Pt nanoparticles for the Pt/CN catalyst are uniformly deposited and well-distributed on the supports, which is comparable to the commercial Pt/C catalyst in Figure S1. The mean particle sizes of Pt/CN, PtCo/CN, L-PtCo/CN, and ALPtCo/CN catalysts are 2.4, 4.9, 4.7, and 4.5 nm, respectively. For the Pt/CN catalyst, Pt nanoparticles are predominantly deposited on both supports in 2.5 nm around with the standard deviation (SD) of 0.4 nm. The SD is increased from 1.2 to 2.2 nm in the order of PtCo/CN, L-PtCo/CN, and AL-PtCo/CN catalysts. The majority of particles in PtCo/CN, L-PtCo/CN, 9811


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Figure 3. Deconvoluted XPS spectra of Pt 4f in (a) PtCo/CN, (b) L-PtCo/CN, (c) AL-PtCo/CN, and (d) commercial Pt/C catalysts.

and AL-PtCo/CN catalysts are in the range of 4−7 nm, while the particle distribution of the AL-PtCo/CN catalyst is relatively poor due to the annealing process. XPS has been used to analyze the oxidation state of Pt and its relative intensities. Figure 3a−d shows the Pt 4f spectra in PtCo/CN, L-PtCo/CN, AL-PtCo/CN, and commercial Pt/C catalysts, respectively, which is deconvoluted to three pairs of doublets corresponding to the Pt(0), Pt(II), and Pt(IV) states. Three deconvoluted doublets are roughly observed at binding energies of 71.1/74.4, 71.9/75.2, and 74.2/77.3 eV corresponding to Pt(0), Pt(II), and Pt(IV), respectively.33,34 The percentage of metallic Pt(0) in PtCo/CN catalyst, determined by the relative peak area of Pt(0) double peaks, shows 68%, which is even higher than that of commercial Pt/C (51%) due to the difference of electronegativity. The results are in good agreement with literature reports that alloying Pt with Co reduces the oxophilicity on Pt.35,36 The catalyst for the ORR does not need to strongly bind the O or OH formed on the catalyst surface for fast H2O desorption and high activity.37,38 The L-PtCo catalyst exhibits a metallic Pt(0) percentage lower than that of fresh PtCo catalyst, but after a mild annealing process, the metallic Pt(0) percentage increases to 69%. The results indicated that the Pt catalyst surface can be oxidized further by abundant oxygen species through the preleaching process which may be similar to the oxygen functionalization of carbon. When the catalyst was reduced by using lower temperatures under the reducing atmosphere, the percentage of metallic Pt(0) is fully recovered to the level of fresh PtCo catalyst before the pretreatments. Figure 4 shows the Co 2p spectra of Co in PtCo/CN, LPtCo/CN, and AL-PtCo/CN catalysts. The peaks at ∼778 and 793 eV correspond to the metallic Co(0), while the Co(II) is observed at ∼780 and 796 eV.39,40 The fresh PtCo/CN catalyst

Figure 4. Comparison of XPS spectra of Co 2p of Pt/CN, PtCo/CN, L-PtCo/CN, and AL-PtCo/CN catalysts.

shows that the Co(II) is dominant as compared to metallic Co(0). The L-PtCo/CN catalyst prepared by preleaching the fresh PtCo/CN catalyst exhibits a decreased peak intensity of Co(II) and the dominant peak of metallic Co(0). Because the Co is unstable in the air, the Co on the surface of the catalysts is in an oxidized state. The oxidized Co on the catalyst surface is removed by the leaching process, and the PtCo-core/Pt-skin structure is formed on the outmost layer of the catalyst,29 which results in the observation of dominant metallic Co(0) in the XPS measurement. Herein the Pt shell acts as a protective layer for the Co. The AL-PtCo/CN catalyst, obtained by annealing the L-PtCo/CN catalyst under the reducing atmosphere, shows a decrease in the peak intensity of Co(0) and an increase in the 9812


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Figure 5. (a) PEMFC polarization and power density curves of PtCo/CN, L-PtCo/CN, and AL-PtCo/CN catalysts at 80 °C and 40% RH. PEMFC polarization and power density curves of (b) L-PtCo/CN, (c) AL-PtCo/CN, and (d) commercial Pt/C catalysts before and after AST. The Pt loading at the cathode is fixed at 0.1 mgPt cm−2. H2 and air were supplied to the anode and cathode at a stoichiometry of 1.5 and 1.8, respectively. AST was performed under 30 000 potential cycles between 0.6 and 1.0 V, supplying fully humidified H2/N2 to the anode and cathode at 80 °C, respectively.

peak intensity of Co(II) as compared to the result of L-PtCo/ CN catalyst. The observed results are attributed to the diffusion of Co during the heat-treatment and the exposure to air.41 Results from Figures 3 and 4 are in agreement with those reported in the literature. Stamenkovic et al. studied the PtNi alloy catalysts treated with acidic solution followed by the mild annealing process.29 The composition line profiles obtained by energy-dispersive X-ray spectroscopy (EDX) showed that Pt and Ni are homogeneously dispersed throughout the particles in as-prepared catalysts. The acid-treated alloy catalysts exhibited a thick Pt shell (∼1 nm) due to removal of the transition metal by the acid solution. The decrease in the Pt shell (∼0.6 nm) was observed in compositional profiles after the subsequent mild annealing process. Advanced electron microscopy techniques were employed for compositional profiles of catalysts.41,42 The Pt shell thickness for the acidtreated PtCo catalysts corresponded to two to three monolayers of Pt, while the annealed catalysts exhibited a thinner Pt shell. The results obtained in this study could be explained by the surface energy difference between Pt (2.35 J m−2) and transition metals (Ni = 2.69 J m−2 and Co = 3.23 J m−2), which indicates that Pt atoms with a lower surface energy are prone to stay on the surface at the mild annealing temperature.43,44 Figure 5a shows the initial polarization and power density curves for the PtCo/CN, L-PtCo/CN, and AL-PtCo/CN catalysts as the cathode. The polarization curve of PtCo/CN catalyst exhibits performance lower than those of L-PtCo/CN and AL-PtCo/CN catalysts. As a result, the maximum power

density of the PtCo/CN catalyst is approximately 30% lower than the power densities of the L-PtCo/CN and AL-PtCo/CN catalysts. The results indicated that a high concentration of Co is initially present on the catalyst surface, thus indicating the necessity of the leaching process. As shown in Figure 5a and Figure S2, after removal of Co from the surface of the catalyst, the performance increases in the mass transport limitation region. For example, the initial current density of the PtCo/CN catalyst at 0.4 V is 870 mA cm−2, while those of the L-PtCo/ CN and AL-PtCo/CN catalysts are approximately 1200 mA cm−2. The corresponding power densities of the L-PtCo/CN and AL-PtCo/CN catalysts are comparable to that of commercial Pt/C catalyst as shown in Figure 5d. According to this result, catalysts showing a higher performance (L-PtCo/ CN and AL-PtCo/CN catalysts) proceeded to the AST for the evaluation of durability. The MEAs with low Pt loading (0.1 mgPt cm−2) were cycled between 0.6 and 1.0 V at 50 mV s−1 for up to 30 000 potential cycles, supplying fully humidified H2/N2 at 80 °C. Figure 5b,c shows the polarization curves and power density of the L-PtCo/CN and AL-PtCo/CN catalysts before and after AST, respectively. Initially, the L-PtCo/CN catalyst exhibits a polarization curve similar to that of the AL-PtCo/CN catalyst. However, after AST, the potential loss at 700 mA cm−2 is 48 mV for the AL-PtCo/CN catalyst, while that of the LPtCo/CN catalyst shows 125 mV (Figure S3). The maximum power density loss of the AL-PtCo/CN catalyst is 9%, while the L-PtCo/CN catalyst loses 28% of initial maximum power density, which indicates that the AL-PtCo/CN catalyst has superior stability when compared to the L-PtCo/CN catalyst. 9813


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ACS Sustainable Chemistry & Engineering For the comparison, the durability of commercial Pt/C catalyst is shown in Figure 5d. The potential loss at 800 mA cm−2 is not measurable, and the maximum power density decreases by 54% of the initial value. Thus, the Pt-based alloy catalysts are more stable than the commercial Pt/C catalyst after AST regardless of the pretreatments. The normalized ECSAs calculated for the L-PtCo/CN, ALPtCo/CN, and commercial Pt/C catalysts as a function of cycle number are shown in Figure 6. Initial ECSA values of 15, 17,

Figure 6. Normalized ECSAs of PtCo/CN, L-PtCo/CN, AL-PtCo/ CN, and commercial Pt/C catalysts as a function of cycle number.

and 63 m2 gPt−1 were measured for the L-PtCo/CN, AL-PtCo/ CN, and commercial Pt/C catalysts, respectively. The ECSA of the commercial Pt/C catalyst rapidly decreases when cycled up to 5000 potential cycles. After AST, the ECSA of the commercial Pt/C catalyst retains 12% of initial value only. On the other hand, the L-PtCo/CN catalyst shows slightly increased ECSA value at the early 100 potential cycles. Thereafter, the ECSA value is well maintained until 10 000 cycles but rapidly decreases. Finally, the retained ECSA of the L-PtCo/CN catalyst is 64% after AST, which indicates stability better than that of the commercial Pt/C catalyst. The ALPtCo/CN catalyst shows significantly increased ECSA at 5000 potential cycles as compared to the initial ECSA. The results can be explained by taking into account the electrochemically leached Co atoms diffused to the near-surface during the annealing process. The same phenomena were observed in fresh Pt-based alloy catalysts.7,45 After 5000 potential cycles, the ECSA of the AL-PtCo/CN catalyst decreases until 30 000 potential cycles. After 30 000 cycles, the ECSA of the AL-PtCo/ CN catalyst retains 84%. The L-PtCo/CN and AL-PtCo/CN catalysts exhibit ECSA more stable than that of the commercial Pt/C catalyst. The ECSA of the AL-PtCo/CN catalyst still retains a significant amount of ECSA even after 30 000 potential cycles. These results agree with those observed in Figure 5. HR-TEM images of the L-PtCo/CN, AL-PtCo/CN, and commercial Pt/C catalysts after AST are shown in Figure 7a−c, respectively. After AST, the mean particle size of the L-PtCo/ CN, AL-PtCo/CN, and commercial Pt/C catalysts increased to 6.8, 6.2, and 7.3 nm, respectively. The mean particle size of the L-PtCo/CN and AL-PtCo/CN catalysts increases by 47% and 38%, while that of the commercial Pt/C catalyst increases by approximately 200%. The SD of the AL-PtCo catalyst is wellmaintained at approximately 2 nm, but that of L-PtCo/CN catalyst increases from 1.4 to 2.5 nm. Also, detached catalyst particles are seen (Figure 7a). The detached particles are

Figure 7. HR-TEM images and corresponding histograms of (a) LPtCo/CN, (b) AL-PtCo/CN, and (c) commercial Pt/C catalysts after AST.

evidence that the carbon corrosion took place on the support.4 As shown in Figure 7 and Figure S4, the catalyst detachment is not observed in the images of the PtCo/CN, AL-PtCo/CN, and commercial Pt/C catalysts after AST. In the case of the LPtCo/CN catalyst, the poor particle distribution and weakened bonding between catalyst and support causes a larger ECSA loss as well as larger performance loss after the AST test when compared to the ECSA loss observed for the AL-PtCo/CN catalyst. To understand the durability of the L-PtCo/CN catalyst, the XPS analysis was applied to the electrodes after AST. Figure 8a,b shows the Pt 4f spectra of the L-PtCo/CN and AL-PtCo/ CN catalysts, respectively. The significant increase of Pt oxide in the L-PtCo catalyst is clearly observed as compared to the catalyst before AST (Figure 3b). The percentage of metallic Pt(0) falls from 59% to 36%, while those of Pt(II) and Pt(IV) 9814


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The carbon oxidation reaction in PEMFC thermodynamically occurs as low as at 0.207 V (vs RHE) in eq 2. C + 2H 2O → CO2 + 4H+ + 4e−

(2)

However, it is believed that the carbon materials are not significantly oxidized in the load-cycling operations as compared to the startup/shutdown operations in PEMFCs. The support used in this study is more corrosion-resistant than the commercial carbon support.30 However, a strong acid is used to provide the carbon surface with a variety of oxygen groups such as carboxylic, phenolic, etc. The carbon surface in the presence of those groups can be easily oxidized to CO or CO2. The carbon nanofiber (CNF), highly corrosion-resistant carbon material, used as support material for Pt catalysts was significantly oxidized when acid-treated. The preleached CNF for 1 and 4 h was oxidized approximately 2-fold and 3-fold faster than the pristine CNF, respectively. As a result, the carbon preleached with a strong acid resulted in the deleterious carbon oxidation of carbon-supported Pt catalysts.49 Therefore, the kinetic rate of the carbon oxidation reaction is slow at 0.207 V (vs RHE), while the reaction increases promptly at potentials above 0.9 V (vs RHE).50,51 In the presence of Pt, the potential can further decrease to about 0.6 V (vs RHE).50,52 During potential cycling, the metallic Pt(0) is oxidized to Pt oxides based on the following reactions:53,54 Pt + H 2O → PtO + 2H+ + 2e−

(3)

Pt + 2H 2O → PtO2 + 4H+ + 4e−

Figure 8. Deconvoluted XPS spectra of Pt 4f in (a) PtCo/CN, (b) LPtCo/CN, (c) AL-PtCo/CN, and (d) commercial Pt/C catalysts after AST.

+

Pt + H 2O → Pt−Oad + 2H + 2e

−

(4) (5)

Pt oxides are reduced to Pt by potential cycling from 1.0 to 0.6 V based on the following reactions:

rise from 25% and 16% to 31% and 33%, respectively. However, the AL-PtCo catalyst exhibits slight percentage changes on oxidation states of Pt. The percentage of metallic Pt(0) decreases by 3%, while those of Pt(II) and Pt(IV) increase by 1% and 2%, respectively. Moreover, the O/C atomic ratio for the L-PtCo catalyst increases by 17% after AST according to Table S1. Along with changes of Pt oxidation states, the percentage of C 1s decreases from 92% to 80% for the L-PtCo catalyst after AST. In the case of the AL-PtCo catalyst, the decrease in the percentage of C 1s by 4% is observed, while the O/C atomic ratio increases by 6% after AST, which indicates a low change of C and O contents as compared to the L-PtCo catalyst. According to the XPS results, the preleaching process may result in the acceleration of carbon corrosion and metallic Pt(0) deformation. The results are in good agreement with the catalyst detachment observed in Figure 7a, assuming that the catalyst detachment results from carbon corrosion.4 As well, catalyst particles are enlarged and aggregated by the catalyst dissolution/redeposition process induced by a number of iterations of Pt oxidation and reduction between 0.6 and 1.0 V. Hwang et al. proposed that the morphology of preleached PtCo could be damaged by strong dealloying, besides the fact that the Pt-skin shows a better catalytic performance.46 Chen et al. showed that the preleached PtCo catalyst after potential cycling transformed to alloy−core/Pt−shell particles.47 We observe a significant decrease of metallic Pt(0) and O/C atomic ratio. Wang et al. tested the durability of the Pt/C catalyst stored at 120 °C for 1000 h.48 They observed that the degraded activity for the ORR is attributed to the decreased percentage of metallic Pt(0), increased O/C atomic ratio, and decreased C contents.

PtO + 2H+ + 2e− → Pt + H 2O

(6)

PtO2 + 4H+ + 4e− → Pt + 2H 2O

(7)

Pt−Oad + 2H+ + 2e− → Pt + H 2O

(8)

The Pt dissolution possibly takes place when the Pt oxides are reduced by the following chemical and electrochemical reactions. PtO + 2H+ → Pt2 + + H 2O PtO2 + 4H+ + 2e− → Pt2 + + 2H 2O

(9) (10)

Pt redeposition causes the formation of a thick Pt shell and increases in particle size.2 In addition to the increase in particle size in Figure 7, as shown in Figure S5, no Co is observed on the catalyst layer by the XPS, which corresponds to the formation of a thick Pt shell.55 Therefore, the AL-PtCo/CN catalyst exhibits a lower increase in the particle size and particle distribution better than that of the L-PtCo/CN catalyst with a lower percentage of metallic Pt(0) and higher O contents, which explains its enhanced durability under the operating conditions.

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CONCLUSION The effects of pretreatment on the durability of Pt-based alloy catalysts are investigated under operating conditions in PEMFCs. The changes in the particle size, structure, oxidation states of metals, and electrochemical durability were analyzed by HR-TEM, XRD, XPS, and MEA tests. The results indicated 9815


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that the initial fct-structure of the fresh catalysts do not change to fcc-structure by preleaching or annealing treatments. The elemental oxidation states are varied by the pretreatments. Co(II) is observed only at the catalyst surface of the fresh PtCo/CN and AL-PtCo/CN catalysts. The oxidation states of Pt change depending on the preleaching and annealing process. The preleaching process decreases the metallic Pt(0) percentage. However, the metallic Pt(0) can be recovered to the initial level through an annealing process. The Pt dissolution/redeposition which occurs under practical operating conditions accelerates the catalyst degradation on the preleached Pt-based alloy catalyst. The L-PtCo/CN catalyst shows potential loss higher than that of the AL-PtCo/CN catalyst in MEA tests. The metallic Pt(0) percentage and C 1s contents significantly decrease, and O/C atomic ratio increases after AST. The L-PtCo/CN catalyst after AST shows poor catalyst particle distribution and catalyst particle detachment. On the other hand, the AL-PtCo/CN catalyst shows remarkably stable performance of ECSA of 9% and only 16% in maximum power density loss after AST. This study claims that the pretreatment of the Pt-based alloy catalysts has a great impact on the durability. We believe that this study can help to develop durable catalysts for the ORR.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01728. HR-TEM image, CV, LSV, polarization test of PtCo/ CN, Pt 4f XPS summary, and Co 2p XPS (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Won Suk Jung: 0000-0001-7443-0474 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of U.S. Department of Energy (contract no. DE-EE0000460) is gratefully acknowledged. REFERENCES

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