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Ordered Pt3Co Intermetallic Nanoparticles Derived from Metal-organic Frameworks for Oxygen Reduction Xiao Xia Wang, Sooyeon Hwang, Yung-Tin Pan, kate Chen, Yanghua He, Stavros G. Karakalos, Hanguang Zhang, Jacob S. Spendelow, Dong Su, and Gang Wu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b00978 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018
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Ordered Pt3Co Intermetallic Nanoparticles Derived from
Metal-organic
Frameworks
for
Oxygen
Reduction Xiao Xia Wang,†, ‡,1 Sooyeon Hwang,§,1 Yung-Tin Pan,⊥ Kate Chen, ‡ Yanghua He,‡ Stavros Karakalos,∥ Hanguang Zhang,‡ Jacob S. Spendelow,⊥ Dong Su, §,* and Gang Wu‡,*
†
School of Mechanical and Power Engineering, East China University of Science and
Technology, Shanghai 200237, China ‡
Department of Chemical and Biological Engineering, University at Buffalo, The State
University of New York, Buffalo, New York 14260, United States §
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York
11973, United States ⊥
Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos,
New Mexico 87545, United States ∥Department
of Chemical Engineering, University of South Carolina, Columbia, South Carolina
29208, United States Corresponding authors E-mail addresses:
[email protected] (G. Wu);
[email protected] (D. Su) 1
, These two authors contributed equally
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Abstract: Highly ordered Pt alloy structures are proved effective to improve their catalytic activity and stability for the oxygen reduction reaction (ORR) for proton exchange membrane fuel cells. Here, we report a new approach to preparing ordered Pt3Co intermetallic nanoparticles through a facile thermal treatment of Pt nanoparticles supported on Co-doped metal-organic framework (MOF)-derived carbon. In particular, the atomically dispersed Co sites, which are originally embedded into MOF-derived carbon, diffuse into Pt nanocrystals and form ordered Pt3Co structures. It is very crucial for the formation of the ordered Pt3Co to carefully control the doping content of Co into the MOFs and the heating temperatures for Co diffusion. The optimal Pt3Co nanoparticle catalyst has achieved significantly enhanced activity and stability, exhibiting a half-wave potential up to 0.92 V vs. RHE and only losing 12 mV after 30,000 potential cycling between 0.6 and 1.0 V. The highly ordered intermetallic structure was retained after the accelerated stress tests evidenced by atomic-scale elemental mapping. Fuel cell tests further verified the high intrinsic activity of the ordered Pt3Co catalysts. Unlike the direct use of MOF-derived carbon supports for depositing Pt, we utilized MOF-derived carbon containing 2 ACS Paragon Plus Environment
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atomically dispersed Co sites as Co sources to prepare ordered Pt3Co intermetallic catalysts. The new synthesis approach provides an effective strategy to develop active and stable Pt alloy catalysts by leveraging the unique properties of MOFs such as 3D structures, high surface areas, and controlled nitrogen doping.
Keywords: Pt3Co intermetallic; oxygen reduction reaction; electrocatalysis; metal-organic frameworks; atomically dispersed Co
Proton exchange membrane fuel cells (PEMFCs) have been considered as clean power sources for electric cars, consumer electronics, domestic power plants, and other applications. Nanosized Pt catalysts dispersed on carbon black supports (Pt/C) have been widely used to catalyze the sluggish oxygen reduction reaction (ORR) in the cathode.1,2 However, the high cost and scarcity of Pt are key obstacles for its widespread usage. Moreover, Pt catalysts often suffer from insufficient activity and degradation under the high potential and oxidative acidic environment in PEMFCs.3-5 Many efforts have been made to reduce the Pt loading and improve the stability of Pt/C catalysts, including Pt-M alloying (M: Fe,6 Co,7-9 Ni,10-12 Cu,13 or Cr14), and further controlling Pt-M alloys with core-shell structures,15-18 optimized morphology/facet,19-21 and ordered intermetallic structures.22,23 Because the ORR is highly depend on the surface electronic properties and atomic arrangement or coordination of the catalysts, crystal structure of catalysts is critical for their catalytic activity.24-26 Several recent studies demonstrated that structurally ordered intermetallic Pt-M nanoparticles showed promising improvement of ORR catalytic 3 ACS Paragon Plus Environment
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activity and stability.22,23,27,28 This ordering-induced catalytic performance enhancement has been widely reported in the systems of PtFe,22,29-31 PtCo,32-35 PtCu,36,37 and PdM.38 In these ordered structures, the specific sites are assigned for the atoms of each Pt and M constituent element, respectively, with defined stoichiometry and crystal structures. To date, the common methods to synthesize these intermetallic compounds are the co-reduction or impregnation reduction followed by a high temperature annealing. The PtM intermetallic nanoparticles were often prepared separately and then attached to the carbon supports. As a result, there is relatively weak control over the interfaces between metal nanoparticles and carbon supports. Obvious, it would be desirable if one can optimize the synthesis of these ordering nanostructures while enhancing bonding nature between PtM nanoparticles and carbon supports. Among studied PtM catalysts, PtCo catalysts have been widely considered the most promising ORR catalysts in PEMFCs due to their intrinsic activity and good stability.32,39,40 Recent studies on PtCo alloy or intermetallic catalysts are compared in Table S1 in terms of their synthesis, structures, and the corresponding ORR activity. Differing to previous reports, we developed a new approach to preparing ordered Pt3Co intermetallic catalysts on zeolitic imidazolate frameworks (ZIF-8)-derived nanocarbon (NC). During the synthesis, atomically dispersed Co sites were first embedded into the carbon derived from Co-doped ZIFs, which is further used for depositing Pt nanoparticles. A subsequent annealing treatment facilitates the diffusion of atomically dispersed Co into Pt nanocrystals and forms highly ordered Pt3Co intermetallic structures. Unlike the use of ZIF-derived carbon directly for Pt catalysts reported
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before,40 utilizing Co-doped ZIF-derived to prepare ordered Pt3Co intermetallic catalysts provides an effective strategy to develop highly active and stable Pt alloy catalysts for PEMFCs. Structures and Morphologies of Pt3Co Intermetallic Catalysts Co-doped ZIF-8 nanocrystals were prepared as the precursors for N- and Co- co-doped carbon supports.41,42 To optimize the atomic ratios of Pt and Co in the PtCo alloy catalysts, we synthesized Co-doped ZIF-8 precursors with different Co content followed by a carbonization process at 900oC. The dominant graphite phases shown in XRD patterns (Figure S1a) suggest that ZIF nanocrystals have been completely converted to carbon labeled as Co-NC. The N2 adsorption/desorption isothermal physisorption and pore size distribution for the carbon with different Co contents are shown in Figure S1b and 1c. The specific surface areas for 20Co-NC (i.e., 20% Co ions to replace Zn in ZIF-8) and 40Co-NC are around 600 m2/g. But for 60Co-NC, the specific surface area is decreased to 300 m2/g, because of the formation of Co aggregates in the carbon. All these Co-NC carbon samples have microporous, mesoporous, and macroporous structures. Based on our previous studies, Co species is likely dispersed as single atomic sites coordinated with N.41 As shown in Figure S2, uniformly dispersed atomic Co sites (white dots) were clearly observed throughout the carbon. However, when Co doping content is above 40 at.% vs. Zn, metallic Co clusters were observed by TEM images. The porous structure and polyhedral particle morphology of ZIF nanocrystal precursors are nearly retained after carbonization. The XRD patterns for Pt nanoparticles deposited onto various Co-NC carbon before the annealing treatment are shown in Figure S3. All samples exhibited three diffraction peaks at 5 ACS Paragon Plus Environment
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2θ=39.76°, 46.24°, and 67.45°, which were consistent with those of pure Pt metal with an fcc structure, corresponding to the (111), (200), and (220) planes. Noted that the peak positions of Pt (200) plane at 2θ=46.24° do not shift to higher angles for all samples, indicating no alloy phase formation. This result confirmed again that, before heat treatment, Co and Pt exist separately without interaction. After a heating treatment at 900°C, only Pt/40Co-NC-900 derived from optimal Co content exhibited a transformation from fcc Pt to an ordered primitive cubic (L12) Pt3Co intermetallic.34,43,44 XRD pattern exhibited a pronounced Pt3Co phase, especially at higher angles. Although the characteristic super lattice peaks of (100) and (110) planes for an ordered, intermetallic Pt3Co phase were not very obvious, because of the broad peak of graphite (002), other peaks at higher angles exhibited obvious positive shift and all diffraction peak positions were consistent with the standard JCPDS cards of the Pt3Co intermetallic phase (PDF card #29-0499) (Figure 1a and 1c) with the unit cell size being 0.3854 × 0.3854 × 0.3854 nm. For low Co-content (20 at.%) derived PtCo catalyst (Pt/20Co-NC-900), the peaks at higher angles shifted positively a little relative to individual Pt catalysts. This indicates that Co is incorporated into the Pt fcc structure to form an alloy phase with a concomitant lattice contraction, but not ordered Pt3Co structures. As for Pt/60Co-NC-900 with excess Co content (60 at%), except for the peaks related to Pt3Co, there were some additional impurity peaks, which cannot match with any standard PtCo intermetallic. After the annealing treatment, all the samples with a wide variety of Co content (20-60 at.%) lack any peaks associated with the single phase of Pt. This suggested that most Pt nanocrystal particles combine with Co and form alloy structures during annealing, but with different stoichiometry showing disordered structures. Thus, optimal Co 6 ACS Paragon Plus Environment
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content doped in ZIF-derived carbon is critical to forming ordered Pt3Co intermetallic structures during the annealing treatment. Figure 1b gives the XRD patterns for the samples of Pt/40Co-NC annealed at different temperatures. After heating at 700 and 800°C (denoted as Pt/40Co-NC-700 and Pt/40Co-NC-800, respectively), it is interesting to find that the peak intensity at about 40° increased obviously and the peaks at 67.45 and 81.28° shifted to higher angles slightly, suggesting particle size increase and a slight lattice contraction. However, the peaks over 65° were not completely in line with those from either Pt or Pt3Co intermetallic, which actually are between the peaks of them (Figure 1d). This indicates these samples are in the process of phase transformation from Pt to intermetallic Pt3Co. Notably, an increase in the annealing temperature resulted in a further shift of the peak position to higher angles. When the heating temperature reached 900°C, the phase structures are converted to Pt3Co intermetallic completely. This result is different from previous studies about PtCo or PtFe intermetallic nanoparticles that were reported before.22,30,32 By using this method, although Pt and Co existed separately in different areas, atomic Co sites diffuse into Pt nanoparticles during the annealing treatment at high temperature (>900oC) to form the highly ordered Pt3Co structures. However, for the sample heated at a higher temperature, i.e., 1000°C, the peaks intensity became stronger with narrow width, indicating a significant increase of particle size of Pt3Co intermetallic nanoparticles. Bright-field and HAADF images, as well as the STEM-EELS analysis of the Pt/40Co-NC sample before the annealing treatment are presented in Figure 2. The TEM images demonstrated that most of the Pt nanoparticles are well dispersed on the ZIF derived Co-NC support (Figure 7 ACS Paragon Plus Environment
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2a-2c). The particle size distribution of Pt, calculated from more than 200 nanoparticles, indicated that the mean size is about 3.0 nm in diameter (Figure S4). The STEM-EELS elemental mapping showed a uniform atomic dispersion of Co on the carbon (Figure 2d-2f). The line scanning (Figure S5) confirmed that Pt nanoparticles are isolated and separated with the atomically dispersed Co sites that are homogeneously embedded into the carbon matrix. Typically, Figure 2g shows the existence of both nanoparticles and atomically dispersed Co sites on the carbon support. In Figure 2h, the selected area EEL spectra for bright Pt particles (area 1) showed the dominant existence of Pt, while the spectra for the carbon matrix (area 2) exhibited only the atomic Co sites on support. These results proved again that, for the as-deposited sample before heat treatment, Pt and Co exist separately on the carbon supports. After the annealing treatment at optimal 900 oC, TEM images showed that the nanoparticles became a little larger (Figure 3a-c). Measurements of more than 200 nanoparticles indicated an average diameter of 5.0 nm as shown in Figure S6a-d, and some of particles are even larger than 10 nm. To obtain detailed structural information of Pt3Co intermetallic nanocrystals, the nanoparticles were characterized by aberration-corrected high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). In Figure 3d, a 5-nm Pt3Co nanoparticle is viewed along the [001] direction. Since HAADF-STEM images reflect the Z-contrast of materials, with Pt columns having a higher intensity than that of Co columns, the Pt/Co ordering is clearly indicated by the high (Pt) and low (Co) Z contrasts. As shown in Figure 3e and 3f, along the [001] axis, the unit cell consisted of a periodic square array of pure Co columns surrounded by Pt columns at the edges and corners of each unit cell. The fast Fourier 8 ACS Paragon Plus Environment
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transformation result (Figure 3g) identifies the ordered Pt3Co structure again, which is in accordance with the previous reports.32 Figure 3h-i and Figure S6e-f showed the line scanning profiles for several Pt particles taken along the dashed line, indicating that the Pt and Co uniformly distributed in the same single crystal. Figure 3j-m further showed the scanning STEM-EELS images of a Pt3Co nanoparticle, which indicated totally different element distribution to the particles before heat treatment shown in Figure 2. Therefore, the Pt and Co projected distributions throughout the particles are synchronous, confirming the coexistence of these two elements in one crystal in a highly ordered manner. More STEM images showed that the lattice fringes with an interplanar spacing of 0.385 nm correspond to the {100} of Pt3Co (Figure S6g-h).40 Thus, the combined characterization including XRD, STEM, and EELS unambiguously verified the formation of ordered intermetallic Pt3Co nanoparticles. The annealing treatment plays an essential role for the phase transformation from fcc Pt to ordered Pt3Co intermetallic through facilitating the diffusion of atomically dispersed Co sites into Pt nanocrystals. The X-ray fluorescence results showed that the Pt/Co atomic ratio for Pt/40Co-NC-900 is close to 3 (Table S2), confirming again the formation of Pt3Co intermetallic phase. X-ray photoelectron spectroscopy (XPS) was proved effective to determine the chemical components and bonding nature for different elements in the surface layers of catalysts. Figure S7 shows the wide scan spectra for the catalysts before and after the annealing treatment and Table S3 summarizes the elemental composition of different catalysts. Obviously, the N content for the Pt catalysts is decreased after the annealing treatment, especially for the Pt/40Co-NC-900 9 ACS Paragon Plus Environment
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with only 3.8 at% left (7.7 at% before post treatment), suggesting that original Co-N bonds are broken and the doped N atoms are removed. With an increase of heating temperature, the content of Co is increased from 0.3 to 0.8 at.%, while Pt content is decreased from 7.1 to 3.0 at.% for the sample annealed up to 1000oC. The possible reason is due to the diffusion of Co from the carbon matrix to Pt nanoparticles. Thus, more Co atoms could be detected. High-resolution XPS spectra were analyzed to further elucidate the chemical changes after the annealing treatment. As shown in Figure 4a and Figure S8a, the surface of Pt is primarily in the metallic state and the sample of Pt/40Co-NC-900 showed a higher metallic Pt content with slight shift to low binding energy direction relative to the sample before the annealing treatment (Table S4). For the Pt samples before the annealing treatment, in addition to pyridinic-N (~398.4 eV), graphitic-N (~401.0 eV), the N 1s spectrum exhibited Co-Nx at 399.6 eV (Figure 4b, S8b, and Table S5) confirms again the possible coordination between N with Co in the carbon supports. During the annealing treatment, Co-N bonds seem to be broken, thus leading to a significant decrease of N content especially the pyridinic N. The annealing treatment also leads to higher degree of graphitization for carbon supports (Figures 4c, and S8c) and the formation of Co species with higher valence (Figure 4d and Figure S8d). As for Pt3Co intermetallic compounds, the interactions between Pt and Co atoms include significant ionic bonds. Compared to the value of electronegativity of Co (1.8), Pt has higher value (2.2). In theory, compared to individual Pt nanoparticles, Pt in Pt3Co alloys should be more metallic with negative shift of binding energy, and Co in the alloy should have higher valence. Thus, the XPS analysis is in good agreement with the theoretical prediction, fully supporting the formation of Pt3Co intermetallic compounds. 10 ACS Paragon Plus Environment
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Catalyst Activity and Stability for the ORR Electrochemical measurements of the various Co-NC supports, Pt/Co-NC catalysts, and the commercial Pt/C were carried out to study ORR activity using rotating disc electrode (RDE) in a 0.1 M HClO4 electrolyte at room temperature. The ORR activity for different supports are included in Figure S9a. Obviously, 20Co-NC showed the highest activity with a half-wave potential (E1/2) of 0.8 V vs. RHE. The 40Co-NC showed slightly lower activity further followed by the 60Co-NC sample. The activity for the Pt catalysts on different Co-NC supports are similar before heat treatment. The E1/2 for these Pt catalysts is around 0.86 V vs. RHE, close to traditional Pt/C catalysts at a loading 60 µgPt/cm2 (Figure S9). These results indicate that the ORR active Co-NC supports barely contribute to the overall activity of the Pt/Co-NC catalysts. However, the annealing treatment leads to significant enhancement of ORR activity as shown in Figure 5a. The initial Co content doped in ZIF-derived carbon plays an important role in boosting activity. For example, the Pt/40Co-NC-900 catalyst derived from 40 at.% Co doping demonstrated the largest enhancement of ORR activity, when compared to the catalysts from 20 and 60 at% Co doping. In particular, the Pt/40Co-NC-900 catalyst exhibited the highest onset potential (Eonset) of 1.05 V, 30 mV higher than the state-of-the art Pt/C (1.02 V), indicating a higher intrinsic activity. It also showed a marked enhancement for ORR with an E1/2 of 0.92 V, about 60 mV higher than that of traditional Pt/C (e.g., 0.86 V). As discussed above, the optimal Co doping in the ZIF precursor is crucial for the formation of ordered Pt3Co intermetallic, corresponding to the highest ORR activity. At the initial stage to prepare Co-NC supports, the heating temperature to carbonize the Co-ZIF precursors also governs ORR activity of the 11 ACS Paragon Plus Environment
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eventually resulting PtCo catalysts. (Figure S10a). If the temperature is below 900°C, significant Zn would remain in the supports, thus inhibiting the formation of Pt3Co intermetallic. When the heating temperatures are too high (1000 or 1100°C), the atomically dispersed Co sites would be agglomerated and form metallic Co particles or clusters, which cannot effectively combine with Pt nanoparticles during the annealing treatment to form Pt3Co intermetallic (Figure S10b). The Pt catalyst supported on Co-free ZIF-8 derived carbon was also studied as a comparison (Figure S10c). Obviously, the Pt/ZIF-8-NC-900 exhibited a much lower E1/2 of 0.85 V relative Pt/40Co-NC-900, indicating the importance of Co doped into carbon. Figure 5b shows the effect of annealing temperatures on the ORR activity for the catalyst with optimal Co doping content, i.e. Pt/40Co-NC. Although the Pt/40Co-NC samples heated at 700 and 800°C also exhibited enhanced activity, the heating temperature of 900°C yielded the highest activity, which is in good agreement with XRD analysis indicating the exclusive formation of highly ordered Pt3Co intermetallic phase. When the temperature was continuously increased to 1000°C, the activity is decreased dramatically, which might be attributed to an increase in particle sizes of Pt3Co nanoparticles and additional phase formation. Several key activity parameters calculated from the kinetic current density (Ik) were compared in Figure 5c. Pt/40Co-NC-900 catalysts exhibited much higher mass and specific activities than pure Pt and other PtCo catalysts treated at other temperatures. In particular, the best performing Pt/40Co-NC-900 catalyst exhibited the highest specific activity of 5.1 mA/cmPt2 at 0.85 V and 1.15 mA/cmPt2 at 0.9 V, representing one of best performing PtM alloy catalysts. Furthermore, fuel cell tests were further conducted to evaluate the catalyst performance in real membrane 12 ACS Paragon Plus Environment
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assembly electrodes (MEAs). H2/air was used to measure the VI polarization plots. The open circuit voltage (OCV) is close to 0.95 V, comparable to commercial Pt/C catalysts. (Figure 5d), Pt/40Co-NC-900 exhibited encouraging fuel cell performance in the kinetic range and yielded a current density of 0.27 A/cm2 at 0.8 V, just under the DOE target of 0.3 A/cm2, which is a promising result given the low Pt loading (0.13 mgPt/cm2) and reasonable pressure (1.5 bar absolute). Under identical testing conditions, the commercial Pt/C catalyst (e.g., TEC10V20E) only generated 0.093 A/cm2. The ECSA measured in MEAs is 28 m2/g, which is still lower than that determined in liquid electrolytes by using RDE. This suggests that the catalyst surface is not being effectively utilized yet. Further optimization of ionomer interactions with catalysts will be a future focus for improvement of mass transport and Pt utilization. The accelerated stress tests (AST) were conducted to study catalyst stability by cycling the potentials in both low (0.6-1.0 V, 50 mV/s) and high (1.0-1.5 V, 500 mV/s) potential windows in liquid electrolytes by using RDE. In the low potential range, it was widely believed that the performance degradation comes from the dissolution, redepositing, migration, and agglomeration of Pt nanoparticles. While in the high potential window, degradation is often caused by the corrosion of carbon supports. As compared in Figure 5e, when the potential cycling was conducted from 1.0-1.5 V, the E1/2 loss of Pt/40Co-NC-900 was only 13 mV after 20000 cycles, which is much stable than the Pt/40Co-NC sample without annealing treatment (e.g., 57 mV) (Figure S11a, b). The degradation of Pt/40Co-NC-900 at high potential window was similar to commercial Pt/C (Figure S12a, b), indicating a similar corrosion resistance to commercial Vulcan XC-72 carbon supports. When the AST was conducted in the potential range between 0.6 13 ACS Paragon Plus Environment
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and 1.0 V, the Pt/40Co-NC-900 also demonstrated the best stability with only 12 mV loss in E1/2 after 30,000 cycles (Figure 5f). Oppositely, the commercial Pt/C catalyst suffers a larger degradation, i.e. 44 mV loss in E1/2 after 20,000 cycles (Figure S12c, d). It should be noted that the sample before annealing treatment (i.e., Pt/40Co-NC) is not stable and loses 43 mV in E1/2 after only 10000 cycles (Figure S11c, d). Along with the ASTs, cyclic voltammetry (CV) was applied to evaluate the ECSA. The initial ECSA for commercial Pt/C catalyst was about 80 m2/g, but only 40-60% was maintained after the AST. While for Pt/40Co-NC-900, the initial ECSA is relatively low at about 45 m2/g, but it almost remained at the same level during the ASTs (Figure S13). The enhanced ORR stability is probably due to the ordered intermetallic Pt3Co structure. The high degree of ordering and strong d-orbital interactions between Co and Pt tend to stabilize Pt, avoiding Co leaching in an acid solution. This hypothesis is further verified by analyzing the HRTEM and HAADF-STEM images of the best performing Pt/40Co-NC-900 catalyst after 30,000 potential cycles (0.6-1.0 V). The structure stability of Pt3Co nanoparticles was verified by HAADF-STEM and EELS elemental mapping (Figure 6). The Pt3Co ordering structure was generally maintained after long-term potential cycles. The Pt3Co nanoparticles dispersion remains similar in morphology to that before the AST (Figure 6a-6c). The ordered intermetallic structures were nearly preserved intact. In Figure 6d, a Pt3Co nanoparticle was viewed along close the [110] direction, which was directly identified from the simulated HAADF-STEM images (Figure 6e, f). And in Figure 6g, another nanoparticle was viewed along close to [100] axis direction. The STEM-EELS elemental mapping (Figure 6h-k) indicated that there was a little leaching of Co from the surface of particles, but still with an intermetallic Pt3Co core 14 ACS Paragon Plus Environment
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(Figure 6l, m). The carbon structures demonstrated short-range order in a single grapheme plane, which was almost the same as the structure before stability test (Figure S14). One of possible reasons for the improvement of stability may be the use of N-doping carbon as Pt3Co catalysts supports. As reported, the conjugation between the N lone-pair electron and the graphene π system may change the electronic structure of Pt3Co nanoparticles, leading to a high activity for the ORR. Also, this tailored electronic and mechanical properties might strengthen the interactions between the Pt-based nanoparticles and supports, increasing stability by avoiding agglomeration and coarsening.45-49 In summary, this work presents a new approach to preparing ordered intermetallic Pt3Co nanoparticle catalysts through a diffusion of Co sites atomically dispersed in the ZIF-derived carbon into Pt nanoparticles during an annealing treatment. Unlike other works, the sources of Co provided in this method is unique and directly from the atomically dispersed Co sites pre-doped in carbon. The careful controls of the Co doping content and the annealing temperatures are crucial for the formation of the ordered Pt3Co intermetallic structure. Compared with Pt/C and other PtCo alloy structures, the ordered Pt3Co particles derived from the Co-doped ZIF carbon achieved significantly enhanced activity and stability for the ORR in acidic media as evidenced by both traditional RDE and fuel cell tests. Compared to other reports regarding PtCo catalysts, Pt3Co of this work represents outstanding performance (Table S1). The excellent performance may relate to the unique geometric and electronic structures of the ordered Pt3Co intermetallic nanoparticles beneficial for catalytic activity enhancement and strong chemical stabilization on carbon support against Co etching and Pt dissolution. This study introduced a 15 ACS Paragon Plus Environment
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new strategy to prepare advanced PtM-based nanoparticle catalysts by using atomically dispersed transition metal doped carbon derived from metal-organic frameworks,42,50,51 which can further extend to studies of other ordered PtFe and PtNi catalysts potentially for PEMFC applications. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nano-lett.xxx Detailed experimental information on catalyst synthesis and characterization. Additional information on characterization to elucidate the correlation of catalyst structure and activity including electron microscopy, BET, XRD, XPS and electrochemical activity and stability measurements. AUTHOR INFORMATION Corresponding Authors *E-mail addresses:
[email protected] (G. Wu);
[email protected] (D. Su) Author Contributions G.W. and X.W. conceived the ideas and experiment design. G.W. coordinated the research work with the help from others. X.W., K.C., Y.H., and H.Z. conducted catalyst synthesis and all of electrochemical measurements. H. S and D.S. carried out electron microscopy analysis. S. K. performed XPS analysis. Y. P. and J.S.S. tested fuel cell performance. X.W., H.S., D.S., J.S.S., and G.W. wrote the paper. 16 ACS Paragon Plus Environment
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Notes The authors declare no competing financial interest Acknowledgements This work is financially supported from the start-up funding from the University at Buffalo, SUNY (G.W.) and U.S. DOE-EERE Fuel Cell Technologies Office (G.W. and J.S.S.) Electron microscopy research was conducted at the Center for Functional Nanomaterials at Brookhaven National Laboratory under Contract No. DE-SC0012704, which is DOE Office of Science User Facilities (D.S.). X. X. Wang thanks the Shanghai Natural Science Foundation of China under Contract No. 16ZR1408600 and the Fundamental Research Funds for the Central Universities under Grant No. 222201814024.
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Pt/40Co-NC-900
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Figure 1. XRD patterns for different Pt/Co-NC catalysts. (a), Pt deposited on different Co contents supports after heat treatment at 900 °C, (b) XRD patterns for Pt/40Co-NC annealed at different temperatures. Enlarged region of 2θ from 65 to 90° for (c) different Pt/Co-NC catalysts, and (d) Pt/40Co-NC annealed at different temperatures. The orange dash line and green dash line are corresponding to the peak positon of pure Pt and Pt3Co intermetallic, respectively.
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Figure 2. (a) TEM, (b) HRTEM, (c) STEM images of Pt/40Co-NC catalysts (before heat treatment). (d) ADF-STEM images and corresponding mapping of (e) Co and (f) Pt, (g-h) STEM images and EELS maps for different areas in the catalysts.
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Figure 3. (a) TEM, (b) HRTEM, (c) dark field STEM images of Pt/40Co-NC-900 catalyst. (d) Atomic resolution HAADF-STEM image of Pt3Co nanoparticle. (e) A crop of the super lattice feature from (d). (f) The simulated HAADF-STEM images of L12 ordered Pt3Co close along [100]. (g) FFT of highlighted area from (d), (h, i) line-scan of a Pt3Co nanoparticle. (j-m) HAADF-STEM image and elemental mappings for a Pt3Co nanoparticle.
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Figure 4. XPS analysis of (a) Pt 4f, (b) N 1s, (c) C1s, and (d) Co 2p spectra for optimal Pt/40Co-NC before and after a heat treatment at 900°C.
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Pt/C Pt/20Co-NC-900 Pt/40Co-NC-900 Pt/60Co-NC-900 Pt/ZIF67-NC-900
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13 mV loss Initial After 1k After 5k After 10k After 20k
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Figure 5. Steady-state polarization ORR plots in O2-saturated 0.1 M HClO4 at room temperature with rotation rate of 900 rpm. (a) Pt/C, Pt/20Co-NC-900, Pt/40Co-NC-900, Pt/60Co-NC-900, (b) Pt/40Co-NC and heated at different temperatures. (c) Comparison of mass activities and specific activities for Pt/C, Pt/40Co-NC-800 and Pt/40Co-NC-900 at 0.85 and 0.9 V. (d) Fuel cell performance with a comparison with a commercial Pt/C (TEC10V20E). Cathode loading is 0.13 mgPt/cm2; pressure 1.0 bar air; 80oC. RDE potential cycling stability tests for the best performing Pt/40Co-NC-900 (e) 1.0 and 1.5 V, scan rate 500 mV/s and (f) 0.6 and 1.0 V, scan rate 50 mV/s.
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Figure 6. TEM and STEM images for the Pt/40Co-NC-900 after potential cycling for 30000 cycles (0.6-1.0 V). (a) (b) TEM images, (c) dark field STEM images, (d) HAADF-STEM images of Pt3Co nanoparticles, (e) A crop of the super lattice feature from (d). (f) The simulated HAADF-STEM images of L12 ordered Pt3Co close along [110]. (g) Another particle close to [100] axis, (h-k) STEM image (h), 2D EELS maps of Pt (i), Co (j) and the composite Pt vs. Co map (k), and (l, m) line-scans of a Pt3Co nanoparticle after AST potential cycling. 27 ACS Paragon Plus Environment
