New Method to Synthesize Highly Active and ... - ACS Publications

Jul 3, 2017 - The fct-structured PtCo/CCCS at low-Pt loading (0.1 mgPt cm–2) shows 6% higher power density than that of the state-of-the-art commerc...
1 downloads 0 Views 4MB Size
Download PDF
Research Article www.acsami.org

New Method to Synthesize Highly Active and Durable Chemically Ordered fct-PtCo Cathode Catalyst for PEMFCs Won Suk Jung* and Branko N. Popov* Center for Electrochemical Engineering, Department of Chemical Engineering, University of South Carolina, Columbia, South Carolina 29208, United States S Supporting Information *

ABSTRACT: In the bottom-up synthesis strategy performed in this study, the Co-catalyzed pyrolysis of chelate-complex and activated carbon black at high temperatures triggers the graphitization reaction which introduces Co particles in the N-doped graphitic carbon matrix and immobilizes N-modified active sites for the oxygen reduction reaction (ORR) on the carbon surface. In this study, the Co particles encapsulated within the N-doped graphitic carbon shell diffuse up to the Pt surface under the polymer protective layer and forms a chemically ordered facecentered tetragonal (fct) Pt−Co catalyst PtCo/CCCS catalyst as evidenced by structural and compositional studies. The fct-structured PtCo/CCCS at low-Pt loading (0.1 mgPt cm−2) shows 6% higher power density than that of the state-of-the-art commercial Pt/C catalyst. After the MEA durability test of 30 000 potential cycles, the performance loss of the catalyst is negligible. The electrochemical surface area loss is less than 40%, while that of commercial Pt/C is nearly 80%. After the accelerated stress test, the uniform catalyst distribution is retained and the mean particle size increases approximate 1 nm. The results obtained in this study indicated that highly stable compositional and structural properties of chemically ordered PtCo/CCCS catalyst contribute to its exceptional catalyst durability. KEYWORDS: bottom-up synthesis, face-centered tetragonal catalyst, proton exchange membrane fuel cells, durability, oxygen reduction reaction

1. INTRODUCTION Cathode durability and performance are some of the key issues preventing the commercialization of proton exchange membrane fuel cells (PEMFCs). These deficiencies have been attributed to dissolution of platinum or alloying element, platinum particle aggregation, membrane degradation, electrochemical surface area (ECSA) loss, and large oxygen reversible potential resulting in the sluggish kinetics for oxygen reduction reaction1 and the decrease in the platinum catalysts activity.2,3In recent years, because of the high cost and limited supply of Pt, research has been reported on various Pt catalyst alloyed with 3d transition metals (M, e.g., Fe, Co, and Ni).4−15 The improvement in kinetic activity of the Pt catalyst by doping Pt with transition metals is attributed to diverse causes such as reduction of the Pt oxidation state, formation of a new electronic structure, shortening of the Pt−Pt bond distance and therefore energetically favorable sites for O2 adsorption, a Pt skin structure on the topmost of the Pt-alloy and the enhanced electronic configuration of the outmost Pt layers.4 Because of the electronegativity difference between Pt and Co (2.2 and 1.8, respectively), the content of metallic Pt0 increases in PtCo alloy. A relatively low coverage of oxygenated species on the catalyst surface drastically improves in the alloy which explains the observed increase in the activity of the catalyst.5,6 The Co dissolution neither detrimentally reduces the cell potential nor dramatically affects the membrane conductance.7,8 The particle © XXXX American Chemical Society

growth by the Pt dissolution/redeposition during potential cycling is a crucial issue for Pt/C catalyst since the decreased ECSA directly relates to a poor catalyst performance. However, the suppressed dissolution rate of Pt in PtCo catalyst causes Pt particle size and particle size distribution to be maintained well when comparing to the Pt/C catalyst.9,10 The chemically ordered face-centered tetragonal (fct) structure of catalyst shows higher activity and durability than disordered facecentered cubic (fcc) structure since the elemental compositions are retained in the alloy catalyst during potential cycling.11 Different nucleation and growth rates of the elements are obstacles for platinum alloy synthesis in solutions. More precisely because of the difference of reduction potential between Pt and 3d transition metals such as Co and Ni, the Pt precursor can be reduced with faster nucleation rate, which results in Pt nanoparticles separation or Pt-rich regions in the product.12,13 To avoid this issue and increase the catalyst activity, the conventional impregnation method is often used to prepare Pt-based alloys with a high ratio of M/Pt (≥1). However, since the Co precursor on Pt catalyst diffuses to form PtCo particles, it is inevitable to form the Co-rich topmost layer on the catalyst. Thus, to accomplish a high performance in H2/ Received: April 4, 2017 Accepted: June 19, 2017

A

DOI: 10.1021/acsami.7b04750 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic for the PtCo/CCCS prepared by the bottom-up synthesis strategy. HR-TEM images of (b) CCCS and (c) N-doped graphitic carbon shell thickness. temperature, the catalyst solution was filtered and washed with DI water. Ammonium peroxysulfate solution (Sigma-Aldrich) was added to the mixture of Pt/CCCS catalyst and aniline monomer at room temperature under vigorous stirring.22 After aging for 12 h, the resultant was heat-treated at 600, 700, 800, and 900 °C for 2 h supplying 5% H2 (balance N2) gas to the furnace. The catalysts thus synthesized are denoted as PtCo/CCCS-600, PtCo/CCCS-700, PtCo/CCCS-800, and PtCo/CCCS-900, respectively. For comparison, we prepared a catalyst by using the conventional impregnation method. Briefly, the commercial Pt/C (TEC10E50E, Tanaka Kikinzoku Kogyo K.K, Japan) catalyst was mixed with Co(NO3)2 at the atomic ratio of 1:1. The mixture was stirred for 12 h to obtain the homogeneity. The resultant was dried in the oven followed by heattreatment at 800 and 900 °C for 2 h under the same conditions as PtCo/CCCS catalysts. The catalysts thus obtained were denoted as PtCo/C-800 and PtCo/C-900, respectively. 2.2. Physical Characterization. X-ray diffraction (XRD Rigaku 405S5)) patterns were recorded to identify the crystalline structure of the supports and the catalysts. The compositional analysis was carried out by inductively coupled plasma atomic emission spectroscopy (ICP-AES, PerkinElmer) and X-ray fluorescence (XRF, Fischer XDAL) analysis, while the morphology and particles size distribution of the catalysts were analyzed by high resolution transmission electron microscope (HR-TEM). The oxidation states of the elements in the catalysts were obtained by X-ray photoelectron spectroscopy (XPS, Kratos AXIS 165). 2.3. Membrane Electrode Assembly (MEA) Fabrication and Electrochemical Measurement. The MEA fabrication and test is described in the previous study.23 Briefly, catalyst inks were ultrasonically prepared by dispersing appropriate amount of catalysts, DI water, IPA, and Nafion ionomer (5% solution, Alfa Aesar). The commercial Pt/C catalyst was used for the anode catalyst. The catalyst layers in the anode and cathode contained 30% and 20% Nafion, respectively. The MEAs were fabricated by the spray-coating method on the Nafion 212 membrane. The active area is 25 cm2. The catalyst loading was fixed at 0.1 mgPt cm−2 for both electrodes. The MEAa were hot-pressed at 140 °C using a pressure of 20 kg cm−2 for 4 min. The MEA test was conducted at 80 °C. The anode and cathode were supplied with H2 and air at the constant stoichiometry of 2 and 2 applying the backpressure of 170 kPaabs. The measurement was carried

air atmosphere, the acid pretreatment process is required since a high ratio of M/Pt typically reduces the number of active sites for the oxygen reduction reaction (ORR).14,15 The major disadvantage of acid treatment is that it causes the acceleration of corrosion at the high potential region due to adsorbed oxygenated functional groups on the carbon support.16 In this study, we synthesized chemically ordered PtCo catalyst with fct structure and mean particle size of ∼5 nm. The catalyst was synthesized using USC developed bottom-up synthesis instead of impregnation method. The catalysts were evaluated by using single cells with low-Pt loading (0.1 mgPt cm−2) in H2/air atmosphere. The accelerated stress test (AST) was carried out by potential cycling thus simulating real life load-cycle conditions. We expected that the new synthesis strategy will provide chemically ordered PtCo/CCCS catalyst with stable compositional and structural properties and better catalyst durability.

2. EXPERIMENTAL SECTION 2.1. Preparation of Support and Catalyst. The support which contains Co particles encapsulated with the N-doped graphitic carbon shells is synthesized using the method developed at University of South Carolina.17−21 Briefly, as-received carbon black (CB, Ketjen Black EC-300J) was activated with 10 M HNO3 solution at 80 °C overnight. After the mixture was filtered, washed with DI water, and dried at 80 °C, the activated carbon black (ACB) was obtained. The ACB was added to the Co(NO3)2 (480 mg) and ethylene diamine (EDA, 1 mL) in isopropyl alcohol (IPA, 200 mL). After vigorously stirring at 80 °C for 3 h, the mixture was dried with a rotary evaporator. The dried powder was heat-treated under N2 atmosphere at 800 °C for 1 h. The excess Co on the surface was removed by 0.5 M H2SO4 at 80 °C, which is denoted as CCCS. The support contains 13.7 wt % Co. A polyol reduction method was used to deposit a 30 wt % Pt on CCCS. Initially, the CCCS (100 mg) was dispersed in ethylene glycol (25 mL) in a sonication bath. PtCl4 (0.2 μmol) was dissolved and 0.1 M NaOH was used to control the pH value of 11. The solution was heated and refluxed at 160 °C for 3 h. After it was cooled to room B

DOI: 10.1021/acsami.7b04750 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces out at 50% relative humidity (RH). The ECSA and mass activity were evaluated on the basis of U.S. DOE’s requirements.24 AST was performed at a scan rate of 50 mV s−1 by sweeping potential between 0.6 and 1.0 V (vs RHE) in a triangle profile for up to 30 000 cycles.23,24 For comparison purposes, MEAs with PtCo catalyst prepared by a conventional impregnation method and commercial Pt/ C catalyst were also tested under the same conditions.

3. RESULTS AND DISCUSSION Figure 1a illustrates the schematic diagram that summarizes the steps used to synthesize chemically ordered PtCo/CCCS catalyst. In the conventional impregnation method, the transition metal precursor and Pt/C catalyst are homogeneously mixed in a solvent followed by drying in the oven. In the second step, Co species diffuse down to Pt catalyst during the heat-treatment to form alloy catalysts. In the bottom-up synthesis strategy performed in this study, the Co-catalyzed pyrolysis of chelate-complex and ACB at high temperatures triggers the graphitization reaction which introduces Co particles in the N-doped graphitic carbon matrix (Figure S1), and immobilizes N-modified active sites for the ORR on the carbon surface (Figures S2 and S3 and Table S1). In the next step, the unstable Co is removed by the acid leaching followed by the polyol process to deposit well-dispersed Pt nanoparticles. It appears that stable Co particles in CCCS are encapsulated within N-doped graphitic carbon shells of 3 nm thickness derived from EDA as shown in Figure 1b and 1c. Uniform Pt particles in the range of 2 to 3 nm are effectively deposited by polyol method. During the heat-treatment, the Co encapsulated with N-doped graphitic carbon shells in the mesoporous CCCS (Figures 1c, S4, and S5) diffuses up to the Pt surface and forms PtCo/CCCS catalyst. XRD patterns of CCCS, Pt/CCCS, and PtCo/CCCS prepared at 600, 700, 800, and 900 °C are shown in Figure 2a. The diffraction peaks at 39.8°, 46.3°, 67.7°, and 81.3° in the Pt/CCCS catalyst correspond to the (111), (200), (220), and (311) planes of the Pt metal, respectively, while the peaks observed at approximately 44°, 51°, and 76° are assigned to (111), (200), and (220) planes of the Co metal, respectively. The diffraction peaks of Pt/CCCS are observed at 39.8°, 46.3°, 67.7°, and 81.3° corresponding to the (111), (200), (220), and (311) planes of pure Pt, respectively. The peaks observed at 44.2°, 51.5°, and 75.8° are assigned to (111), (200), and (220) planes of pure Co, respectively. The PtCo/CCCS-600 and PtCo/CCCS-700 catalysts show the presence of split peaks. However, the PtCo/CCCS-800 catalyst does not show any split peaks. This catalyst clearly shows the presence of chemically ordered fct structure since the (001) and (100) superlattice planes observed at ∼24° and 33°, respectively, confirm the formation of the fct phase (PDF no. 97-010-2622). In the case of PtCo/C catalyst prepared by the conventional impregnation method (Figure S6 and Table S2), no split peak with the fct structure is observed at 900 °C, while multiple peaks are observed at 800 °C. Thus, the results presented in this study confirm that the bottom-up synthesis strategy is more efficient route to produce fct structure when compared to a conventional synthesis method. Deconvoluted XRD peaks of PtCo/ CCCS-600, PtCo/CCCS-700, PtCo/CCCS-800, and PtCo/ CCCS-900 catalysts are shown in Figure 2b. The peaks of PtCo/CCCS-600 catalyst are located at 39.9° and 42.4° corresponding to Pt and PtCo, respectively, indicating that the Co did not reach the Pt surface. For the PtCo/CCCS-700 catalyst, the peaks are located at 40.2°, 41.0°, and 41.6° and the

Figure 2. (a) XRD patterns of CCCS, Pt/CCCS, and PtCo/CCCS prepared from 600 to 900 °C. (b) Deconvoluted XRD peaks of PtCo/ CCCS-600, PtCo/CCCS-700, PtCo/CCCS-800, and PtCo/CCCS900. (c) Deconvoluted XPS spectra of the Pt 4f orbital of PtCo/ CCCS-800 and commercial Pt/C.

chemically ordered fct structure is starting to form as shown in Figure 2a. As the temperature increases, the two split peaks come close to approximately 41° and the symmetrical peak is observed at 41.2° at 800 °C. Figure 2a and 2b indicate that the Co slowly diffuses into the Pt particles as a function of temperature and that fct PtCo structure is formed at 800 °C. As a result of Co diffusion into Pt particles, the peaks of Pt in Pt/ CCCS have moved to higher angle thus reducing the lattice parameter (Table S3). The catalyst with fct structure is known for highly active and electrochemically stable due to the high chemical stability and low vacancy formation energy.11,25,26 The near-surface concentration measured by XPS is in good agreement with XRD results since the atomic ratio of Pt/Co = 14.7:1 in Pt/CCCS has reduced to atomic ratio Pt/Co = 1.2:1 in PtCo/CCCS-800 catalyst, while the bulk concentration remains constant. Since the Pt particles cover the surface of supports, the Co is hardly detected by the XPS. As the Co diffuses into the Pt lattice, the Co species reach the surface of C

DOI: 10.1021/acsami.7b04750 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 3. HR-TEM images of (a) Pt/CCCS and PtCo/CCCS-800 catalyst (b) without and (c) with protective coating at 800 °C. (d) The mean particle size and SD of PtCo/CCCS as a function of temperature. Scale bar represents 10 nm.

Figure 4. H2/air polarization curves of (a) PtCo/CCCS-800 and (b) commercial Pt/C polarization curves before and after AST. (c) ECSA losses of PtCo/CCCS-800 and commercial Pt/C catalysts as a function of cycle number. (d) Mass activities before and after AST for the PtCo/CCCS-800 and commercial Pt/C catalysts. AST was conducted by 30 000 potential cycles between 0.6 and 1.0 V.

D

DOI: 10.1021/acsami.7b04750 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 5. HR-TEM images of (a) PtCo/CCCS-800 and (b) commercial Pt/C catalysts after AST. (c) Comparison of XRD patterns of PtCo/CCCS800 and PtCo/C-900 catalysts after AST. (d) Deconvoluted peaks of PtCo/CCCS-800 and PtCo/C-900 catalysts after AST. Scale bar represents 10 nm.

catalyst without protective layer during the heat-treatment at 800 °C. However, the catalyst particles synthesized with protective layer are well-distributed in the range of 4−6 nm, which indicates that the particle size growth can be prevented by protective layer without collapsing the particle distribution (Figure 3c). As shown in Figure 3d, most of the catalyst particles in the range of 600−800 °C exhibit mean particle size of ∼5 nm with a low standard deviation (SD). However, the PtCo/CCCS-900 catalyst shows poor particle size distribution and large mean particle size (Figure S7). The MEA tests carried out for PtCo/CCCS-800 and commercial Pt/C catalysts using a low Pt cathode loading of 0.1 mgPt cm−2 are shown in Figure 4a and b, respectively. The catalyst durability was measured before and after 30 000 potential cycles between 0.6 and 1.0 V. Initially, the open circuit potential of PtCo/CCCS-800 is 26 mV higher than that of the commercial Pt/C catalyst. The PtCo/CCCS-800 and commercial Pt/C catalysts show 0.679 and 0.620 V at 600 mA cm−2, respectively. The maximum power density of PtCo/ CCCS-800 and commercial Pt/C catalysts are 603 and 571 mW cm−2, respectively. After AST, the potential and maximum power density of PtCo/CCCS-800 catalyst are 0.649 V at 600 mA cm−2 and 583.9 mW cm−2, respectively. Experimentally observed 30 mV loss and 3% power density loss after AST indicates that PtCo/CCCS-800 catalyst is exceptionally stable under laboratory fuel cell operation which mimics the conditions present in stationary and automotive applications. The potential loss of commercial Pt/C catalysts is 200 mV at 600 mA cm−2, while its maximum power density decreases by 55% after AST. PtCo/C-900 catalyst was tested under the same conditions (Figure S8). The observed low performance under H2/air, when compared to the commercial Pt/C, is due to the excess Co on the catalyst surface.

the catalyst and can be observed by the XPS. The strain and ligand effects by alloying the Pt with transition metals decrease the Pt−Pt interatomic distance and modify the electronic structure of Pt owing to heterometallic bonding interactions.27 To demonstrate the electronic structure, Pt 4f spectra of PtCo/ CCCS-800 and commercial Pt/C catalysts are deconvoluted to three pairs of doublets corresponding to Pt0, Pt2+, and Pt4+ as shown in Figure 2c. The metallic Pt0 for PtCo/CCCS-800 catalyst is observed at 71.1 (Pt 4f7/2) and 74.45 eV (Pt 4f5/2), while the doublets at 72.2/75.55 and 74.12/77.5 eV could be assigned to the Pt2+, such as PtO or Pt(OH)2, and Pt4+, such as PtO2, respectively.28,29 The deconvoluted Pt spectra indicate that the Pt binding energy (BE) of PtCo/CCCS-800 catalyst shifts to higher value by ∼0.07 eV. The positively shifted BE of Pt correlates to a downshift of the d-band center, which leads to a weak chemical interaction between oxygen species and catalyst surface.8−10,30 The percentage of Pt0 in PtCo/CCCS800 catalyst, determined by the relative peak area of Pt0 double peaks, shows 70.9%, which is even higher than that of commercial Pt/C (52%) due to the difference of electronegativity. The results are in good agreement with those reported in the literature that alloying Pt with Co reduces the oxophilicity on Pt.31,32 In fact, the catalyst for the ORR should not bind strongly the O or OH formed on the catalyst surface for fast H2O desorption and high activity.33,34 HR-TEM images of catalysts are shown in Figure 3. The mean particle sizes and particle size distribution were measured using the values obtained from over 100 nanoparticles. The pristine Pt particles in Figure 3a are uniformly distributed in the range of 2−3 nm. When subjected to the heat-treatment, the protective layer coating on catalysts is highly efficient in controlling the particle size and distribution. In Figure 3b, a drastic aggregation and large particles are observed for the E

DOI: 10.1021/acsami.7b04750 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

which impedes aggregation and coarsening of Pt particles.38,39 Since the ECSA and particle size and/or distribution are strongly correlated to the catalyst activity, the exceptionally high durability for the PtCo/CCCS-800 catalyst results from a combination of the ECSA and particle size. XRD was used to evaluate the structural changes of PtCo/ CCCS-800 and PtCo/C-900 catalysts after AST as shown in Figure 5c. After the AST, the PtCo peaks apparently show Bragg angle shift to lower angles near to the peak position of pure Pt. As shown in Figure 5d and Table S4, the peak at approximately 40° was deconvoluted into two peaks. Each deconvoluted peak contains Pt and PtCo species since the first one is the same as lattice parameter of pure Pt and the other exhibits lower lattice parameter for both catalysts. PtCo/Pt ratio determined by the relative peak areas for the PtCo/ CCCS-800 catalyst was estimated to be ∼4, while that for the PtCo/C-900 catalyst was estimated to be ∼3. The results indicate that the pure Pt is deposited during dissolution/ redeposition when subjected to potential cycling.40,41 However, the Co in PtCo/CCCS-800 catalyst is efficiently protected from electrochemical leaching during potential cycling. As shown in Table S5, the compositional analysis showed that the loss of Co in PtCo/CCCS-800 catalyst is less than that in PtCo/C-900 catalyst. Atomic Co/Pt ratio of PtCo/CCCS-800 catalyst remains 47% of initial ratio, while that of PtCo/C-900 loses 61% of initial ratio owing to the electrochemical dissolution of Co from the alloy catalyst in AST. A variety of studies have been reported that the dissolution of M leads to the alloy catalyst deactivation and fails to achieve a longer lifetime and higher durability under fuel cell operating conditions. As a result, the Pt alloy catalyst is transformed to the Pt skeleton catalyst within several hours of operation. After an extended period of time, a thick Pt shell is formed on Pt−M alloy catalyst due to the M dissolution and Ostwald ripening of Pt. This is evidenced by the decrease of M content in the bulk concentration and lower M content in near-surface alloy than in the bulk after AST.42−44 Therefore, the lattice parameter of pure Pt observed in the XRD measurement is attributed to the Pt dissolution/redeposition during the potential cycling. In addition, that the lattice parameter of PtCo/CCCS-800 catalyst is lower than that of PtCo/C-900 catalyst can be explained by taking into account the high chemical stability of the catalyst observed in the compositional analysis.

The ECSA losses of PtCo/CCCS-800 and commercial Pt/C catalysts as a function of cycle number are shown in Figure 4c and S9. The integration of hydrogen desorption area in the CV was carried out by subtracting the current density which results from double layer charging. The ECSA was calculated using the equation ECSA =

QH 0.21 × L Pt

where QH (mC cm−2) is the Coulombic charge for hydrogen desorption, LPt (mgPt cm−2) represents the Pt loading, and 0.21 mC cm−2 is the charge required to oxidize a monolayer of H2 on the Pt site.35 Initial ECSA values of 74 and 63 m2 gPt−1 were obtained for PtCo/CCCS-800 and commercial Pt/C catalysts, respectively. The ECSAs of PtCo/CCCS-800 decreases by approximately 23% after 10,000 potential cycles, while that of commercial Pt/C degrades by more than 50%. ECSA losses for PtCo/CCCS-800 and commercial Pt/C catalysts estimated after 30 000 cycles were approximately 36% and 78%, respectively. The mass activity of Pt is an important factor related to the cost reduction in PEMFCs. To demonstrate the mass activity and stability, the mass activities of PtCo/CCCS-800 and commercial Pt/C catalysts before and after AST are shown in Figure 4d. The mass activity was measured at 0.9 ViR‑free. The PtCo/CCCS-800 catalyst exhibits 0.39 A mgPt−1, which is approximately 3-fold higher mass activity than that of commercial Pt/C of 0.14 A mgPt−1. ECSA-normalized specific activities of the PtCo/CCCS-800 and commercial Pt/C catalysts are approximately 530 and 220 μA cm Pt −2 , respectively. After AST, the mass activity losses are 26% and 64% for PtCo/CCCS-800 and commercial Pt/C catalysts, respectively. Furthermore, the PtCo/CCCS-800 after AST still remained a higher mass activity (0.29 A mgPt−1) than the initial mass activity of commercial Pt/C catalyst. The results indicated the Co dissolution rate in the PtCo/CCCS-800 catalyst is significantly suppressed. Kim et al. investigated the transition metal content in fct PtFe in H2SO4 solutions.11 According to this investigation, the fct structure plays an important role in inhibiting the transition metal dissolution rate in the corrosive electrode interfaces such as PEMFC operating conditions. The results of this study indicated that the PtCo/CCCS-800 catalyst is remarkably more stable than the commercial Pt/C catalyst. The HR-TEM images of PtCo/CCCS-800 and commercial Pt/C catalysts after AST are shown in Figure 5a and 5b, respectively. As compared to the initial catalyst particles (Figures 2c and S10), the mean particle sizes of the PtCo/ CCCS-800 and commercial Pt/C catalysts after AST increase by 15 and 204%, respectively. As shown in Figure S11, the PtCo/CCCS-800 catalysts exhibit good catalyst dispersion and the narrow particle size distribution. The SD of PtCo/C-900 catalyst (5 nm) and commercial Pt/C (7 nm) significantly increased after AST in comparison to the PtCo/CCCS-800 catalyst (2 nm). Larger particles than 10 nm and extensive catalyst aggregation are observed in both PtCo/C-900 and commercial Pt/C catalysts. Pt dissolution/redeposition, Pt migration, and Ostwald ripening take place because of the formation of Pt oxides and their reduction during potential cycling between 0.6 and 1.0 V.36,37 The low ECSA loss and particle size growth observed in the PtCo/CCCS-800 catalyst results in a weak Pt dissolution/redeposition and coalescence. Also, nitrogen doped carbon supports provide strong binding

4. CONCLUSION A novel and effective bottom-up synthesis strategy was used to synthesize a highly active and durable chemically ordered PtCo/CCCS catalyst for the ORR. The chemically ordered fctstructured alloy catalyst exhibits a mean particle size of ∼5 nm with a low SD after pyrolysis. XRD, XPS, HR-TEM analysis and electrochemical studies indicated that the compositional and structural properties of chemically ordered PtCo/CCCS catalyst contribute to its exceptional durability. The chemically ordered fct-structured alloy catalyst exhibits a mean particle size of ∼5 nm with a low SD after pyrolysis. PtCo/CCCS-800 catalyst exhibits 0.39 A mgPt−1, which is 3-fold higher mass activity than that of commercial Pt/C of 0.14 A mgPt−1. After AST, the mass activity losses for PtCo/CCCS-800 and commercial Pt/C catalysts are ∼26% and 64%, respectively. The PtCo/CCCS-800 catalyst after AST still remained a higher mass activity (0.29 A mgPt−1) than the initial mass activity of commercial Pt/C catalyst. The ECSA loss for the PtCo/CCCS800 catalyst is 36%, while that for the commercial Pt/C catalyst F

DOI: 10.1021/acsami.7b04750 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

and PtCu3 Cathode Catalysts for Proton Exchange Membrane Fuel Cells. J. Phys. Chem. C 2012, 116, 19877−19885. (9) Hoshi, Y.; Tada, E.; Nishikata, A.; Tsuru, T. Effect of Potential Cycling on Dissolution of Equimolar Pt−M (M: Co, Ni, Fe) Alloys in Sulfuric Acid Solution. Electrochim. Acta 2012, 85, 268−272. (10) Hoshi, Y.; Yoshida, T.; Nishikata, A.; Tsuru, T. Dissolution of Pt−M (M: Cu, Co, Ni, Fe) Binary Alloys in Sulfuric Acid Solution. Electrochim. Acta 2011, 56, 5302−5309. (11) Kim, J.; Lee, Y.; Sun, S. Structurally Ordered FePt Nanoparticles and Their Enhanced Catalysis for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2010, 132, 4996−4997. (12) Chen, M.; Liu, J. P.; Sun, S. One-Step Synthesis of FePt Nanoparticles with Tunable Size. J. Am. Chem. Soc. 2004, 126, 8394− 8395. (13) Wang, C.; Markovic, N. M.; Stamenkovic, V. R. Advanced Platinum Alloy Electrocatalysts for the Oxygen Reduction Reaction. ACS Catal. 2012, 2, 891−898. (14) Han, B.; Carlton, C. E.; Kongkanand, A.; Kukreja, R. S.; Theobald, B. R.; Gan, L.; O’Malley, R.; Strasser, P.; Wagner, F. T.; Shao-Horn, Y. Record Activity and Stability of Dealloyed Bimetallic Catalysts for Proton Exchange Membrane Fuel Cells. Energy Environ. Sci. 2015, 8, 258−266. (15) Mani, P.; Srivastava, R.; Strasser, P. Dealloyed Binary PtM3 (M = Cu, Co, Ni) and Ternary PtNi3M (M = Cu, Co, Fe, Cr) Electrocatalysts for the Oxygen Reduction Reaction: Performance in Polymer Electrolyte Membrane Fuel Cells. J. Power Sources 2011, 196, 666−673. (16) Oh, H.-S.; Kim, K.; Ko, Y.-J.; Kim, H. Effect of Chemical Oxidation of CNFs on the Electrochemical Carbon Corrosion in Polymer Electrolyte Membrane Fuel Cells. Int. J. Hydrogen Energy 2010, 35, 701−708. (17) Popov, B. N.; Li, X.; Liu, G.; Lee, J.-W. Power Source Research at USC: Development of Advanced Electrocatalysts for Polymer Electrolyte Membrane Fuel Cells. Int. J. Hydrogen Energy 2011, 36, 1794−1802. (18) Li, X.; Liu, G.; Popov, B. N. Activity and stability of nonprecious metal catalysts for oxygen reduction in acid and alkaline electrolytes. J. Power Sources 2010, 195, 6373−6378. (19) Liu, G.; Li, X.; Ganesan, P.; Popov, B. N. Development of nonprecious metal oxygen-reduction catalysts for PEM fuel cells based on N-doped ordered porous carbon. Appl. Catal., B 2009, 93, 156−165. (20) Liu, G.; Li, X.; Lee, J.-W.; Popov, B. N. A review of the development of nitrogen-modified carbon-based catalysts for oxygen reduction at USC. Catal. Sci. Technol. 2011, 1, 207−217. (21) Nallathambi, V.; Lee, J.-W.; Kumaraguru, S. P.; Wu, G.; Popov, B. N. Development of high performance carbon composite catalyst for oxygen reduction reaction in PEM Proton Exchange Membrane fuel cells. J. Power Sources 2008, 183, 34−42. (22) Stejskal, J.; Gilbert, R. G. Polyaniline. Preparation of a Conducting Polymer (IUPAC Technical Report). Pure Appl. Chem. 2002, 74, 857. (23) Jung, W.; Xie, T.; Kim, T.; Ganesan, P.; Popov, B. N. Highly Active and Durable Co-Doped Pt/CCC Cathode Catalyst for Polymer Electrolyte Membrane Fuel Cells. Electrochim. Acta 2015, 167, 1−12. (24) Fuel Cell Tech Team Accelerated Stress Test and Plarization Curve Protocols for PEM Fuel Cells. https://energy.gov/eere/ fuelcells/downloads/fuel-cell-tech-team-accelerated-stress-test-andpolarization-curve (accessed Jan 30, 2017). (25) Chung, D. Y.; Jun, S. W.; Yoon, G.; Kwon, S. G.; Shin, D. Y.; Seo, P.; Yoo, J. M.; Shin, H.; Chung, Y.-H.; Kim, H.; Mun, B. S.; Lee, K.-S.; Lee, N.-S.; Yoo, S. J.; Lim, D.-H.; Kang, K.; Sung, Y.-E.; Hyeon, T. Highly Durable and Active PtFe Nanocatalyst for Electrochemical Oxygen Reduction Reaction. J. Am. Chem. Soc. 2015, 137, 15478− 15485. (26) Li, Q.; Wu, L.; Wu, G.; Su, D.; Lv, H.; Zhang, S.; Zhu, W.; Casimir, A.; Zhu, H.; Mendoza-Garcia, A.; Sun, S. New Approach to Fully Ordered fct-FePt Nanoparticles for Much Enhanced Electrocatalysis in Acid. Nano Lett. 2015, 15, 2468−2473.

is 78% after AST. The potential and maximum power density loss of the chemically ordered fct-structured PtCo/CCCS catalyst is negligible after 30 000 potential cycling tests. We believe that it can be considered as a promising candidate for practical fuel cell applications.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04750. Chelation of Co-EDA complex; XPS spectra of CCCS and CB; analysis of XPS N 1s peak; electrochemical characterization of CCCS; HR-TEM image of Pt/CCCS; XRD patterns of CCCS and CB; N2 adsorption/ desorption isotherms and pore-size distribution curves of supports; XRD patterns of PtCo/C-800 and PtCo/C900; structural characteristic data for PtCo/CCCS and PtCo/C; HR-TEM images of PtCo/CCCS with protective coating; H2/air polarization curves of PtCo/ C-900; HR-TEM images of PtCo/C-900 and commercial Pt/C before AST; histograms of catalysts after AST; and compositional analysis (PDF)

■

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.

■ ■

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

(1) 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, 9−35. (2) Ferreira, P. J.; la O’, G. J.; Shao-Horn, Y.; Morgan, D.; Makharia, R.; Kocha, S.; Gasteiger, H. A. Instability of Pt/C Electrocatalysts in Proton Exchange Membrane Fuel Cells: A Mechanistic Investigation. J. Electrochem. Soc. 2005, 152, A2256−A2271. (3) Wu, J.; Yuan, X. Z.; Martin, J. J.; Wang, H.; Zhang, J.; Shen, J.; Wu, S.; Merida, W. A Review of PEM Fuel Cell Durability: Degradation Mechanisms and Mitigation Strategies. J. Power Sources 2008, 184, 104−119. (4) Min, M.-k.; Cho, J.; Cho, K.; Kim, H. Particle Size and Alloying Effects of Pt-Based Alloy Catalysts for Fuel Cell Applications. Electrochim. Acta 2000, 45, 4211−4217. (5) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G.; Ross, P. N.; Markovic, N. M. Trends in Electrocatalysis on Extended and Nanoscale Pt-Bimetallic Alloy Surfaces. Nat. Mater. 2007, 6, 241−247. (6) Xia, B. Y.; Wu, H. B.; Li, N.; Yan, Y.; Lou, X. W.; Wang, X. OnePot Synthesis of Pt−Co Alloy Nanowire Assemblies with Tunable Composition and Enhanced Electrocatalytic Properties. Angew. Chem., Int. Ed. 2015, 54, 3797−3801. (7) Yu, P.; Pemberton, M.; Plasse, P. PtCo/C Cathode Catalyst for Improved Durability in PEMFCs. J. Power Sources 2005, 144, 11−20. (8) Yu, Z.; Zhang, J.; Liu, Z.; Ziegelbauer, J. M.; Xin, H.; Dutta, I.; Muller, D. A.; Wagner, F. T. Comparison between Dealloyed PtCo3 G

DOI: 10.1021/acsami.7b04750 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces (27) Kitchin, J. R.; Nørskov, J. K.; Barteau, M. A.; Chen, J. G. Role of Strain and Ligand Effects in the Modification of the Electronic and Chemical Properties of Bimetallic Surfaces. Phys. Rev. Lett. 2004, 93, 156801. (28) Liang, Y.; Zhang, H.; Zhong, H.; Zhu, X.; Tian, Z.; Xu, D.; Yi, B. Preparation and Characterization of Carbon-Supported PtRuIr Catalyst with Excellent CO-Tolerant Performance for ProtonExchange Membrane Fuel Cells. J. Catal. 2006, 238, 468−476. (29) Su, F.; Tian, Z.; Poh, C. K.; Wang, Z.; Lim, S. H.; Liu, Z.; Lin, J. Pt Nanoparticles Supported on Nitrogen-Doped Porous Carbon Nanospheres as an Electrocatalyst for Fuel Cells. Chem. Mater. 2010, 22, 832−839. (30) Choi, D. S.; Robertson, A. W.; Warner, J. H.; Kim, S. O.; Kim, H. Low-Temperature Chemical Vapor Deposition Synthesis of Pt−Co Alloyed Nanoparticles with Enhanced Oxygen Reduction Reaction Catalysis. Adv. Mater. (Weinheim, Ger.) 2016, 28, 7115−7122. (31) Xu, Z.; Zhang, H.; Zhong, H.; Lu, Q.; Wang, Y.; Su, D. Effect of Particle Size on the Activity and Durability of the Pt/C Electrocatalyst for Proton Exchange Membrane Fuel Cells. Appl. Catal., B 2012, 111− 112, 264−270. (32) Zeng, J.; Lee, J. Y. Effects of Preparation Conditions on Performance of Carbon-Supported Nanosize Pt-Co Catalysts for Methanol Electro-Oxidation under Acidic Conditions. J. Power Sources 2005, 140, 268−273. (33) Greeley, J.; Stephens, I. E. L.; Bondarenko, A. S.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T. F.; Rossmeisl, J.; Chorkendorff, I.; Nørskov, J. K. Alloys of Platinum and Early Transition Metals as Oxygen Reduction Electrocatalysts. Nat. Chem. 2009, 1, 552−556. (34) Kim, Y.; Kwon, Y.; Hong, J. W.; Choi, B.-S.; Park, Y.; Kim, M.; Han, S. W. Controlled Synthesis of Highly Multi-Branched Pt-Based Alloy Nanocrystals with High Catalytic Performance. CrystEngComm 2016, 18, 2356−2362. (35) Jung, W. S.; Popov, B. N. Hybrid cathode catalyst with synergistic effect between carbon composite catalyst and Pt for ultralow Pt loading in PEMFCs. Catal. Today 2017, DOI: 10.1016/ j.cattod.2017.06.019. (36) Chen, Z.; Waje, M.; Li, W.; Yan, Y. Supportless Pt and PtPd Nanotubes as Electrocatalysts for Oxygen-Reduction Reactions. Angew. Chem., Int. Ed. 2007, 46, 4060−4063. (37) Colón-Mercado, H. R.; Popov, B. N. Stability of Platinum Based Alloy Cathode Catalysts in PEM Fuel Cells. J. Power Sources 2006, 155, 253−263. (38) Maiyalagan, T.; Viswanathan, B.; Varadaraju, U. V. Nitrogen Containing Carbon Nanotubes as Supports for Pt−Alternate Anodes for Fuel Cell Applications. Electrochem. Commun. 2005, 7, 905−912. (39) Zhou, Y.; Pasquarelli, R.; Holme, T.; Berry, J.; Ginley, D.; O’Hayre, R. Improving PEM Fuel Cell Catalyst Activity and Durability Using Nitrogen-Doped Carbon Supports: Observations from Model Pt/HOPG Systems. J. Mater. Chem. 2009, 19, 7830−7838. (40) Hidai, S.; Kobayashi, M.; Niwa, H.; Harada, Y.; Oshima, M.; Nakamori, Y.; Aoki, T. Changes in Electronic States of Platinum− Cobalt Alloy Catalyst for Polymer Electrolyte Fuel Cells by Potential Cycling. J. Power Sources 2011, 196, 8340−8345. (41) Ohyagi, S.; Sasaki, T. Durability of a PEMFC Pt−Co Cathode Catalyst Layer during Voltage Cycling Tests under Supersaturated Humidity Conditions. Electrochim. Acta 2013, 102, 336−341. (42) Xin, H. L.; Mundy, J. A.; Liu, Z.; Cabezas, R.; Hovden, R.; Kourkoutis, L. F.; Zhang, J.; Subramanian, N. P.; Makharia, R.; Wagner, F. T.; Muller, D. A. Atomic-Resolution Spectroscopic Imaging of Ensembles of Nanocatalyst Particles Across the Life of a Fuel Cell. Nano Lett. 2012, 12, 490−497. (43) Dubau, L.; Maillard, F.; Chatenet, M.; Guetaz, L.; André, J.; Rossinot, E. Durability of Pt3Co/C Cathodes in a 16 Cell PEMFC Stack: Macro/Microstructural Changes and Degradation Mechanisms. J. Electrochem. Soc. 2010, 157, B1887−B1895. (44) Yu, Y.; Xin, H. L.; Hovden, R.; Wang, D.; Rus, E. D.; Mundy, J. A.; Muller, D. A.; Abruña, H. D. Three-Dimensional Tracking and Visualization of Hundreds of Pt−Co Fuel Cell Nanocatalysts During Electrochemical Aging. Nano Lett. 2012, 12, 4417−4423. H

DOI: 10.1021/acsami.7b04750 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX