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Nanoporous PtFe Nanoparticles Supported on N‑Doped Porous Carbon Sheets Derived from Metal−Organic Frameworks as Highly Efficient and Durable Oxygen Reduction Reaction Catalysts Kang Yang,† Peng Jiang,† Jitang Chen,† and Qianwang Chen*,†,‡ †

Hefei National Laboratory for Physical Science at Microscale, Department of Materials Science & Engineering, and Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei 230026, China ‡ The Anhui Key Laboratory of Condensed Mater Physics at Extreme Conditions, High Magnetic Field Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China S Supporting Information *

ABSTRACT: Designing and exploring catalysts with high activity and stability for oxygen reduction reaction (ORR) at the cathode in acidic environments is imperative for the industrialization of proton exchange membrane fuel cells (PEMFCs). Theoretical calculations and experiments have demonstrated that alloying Pt with a transition metal can not only cut down the usage of scarce Pt metal but also improve performance of mass activity compared with pure Pt. Herein, we exhibit the preparation of nanoporous PtFe nanoparticles (np-PtFe NPs) supported on N-doped porous carbon sheets (NPCS) via facile in situ thermolysis of a Pt-modified Fe-based metal−organic framework (MOF). The np-PtFe/NPCS exhibit a more positive half-wave potential (0.92 V) compared with commercial Pt/ C catalyst (0.883 V). The nanoporous structure allows our catalyst to possess high mass activity, which is found to be 0.533 A·mgPt−1 and 3.04 times better than that of Pt/C (0.175 A·mgPt−1). Moreover, the conversion of PtFe NPs from porous to hollow structure can maintain the activity of electrocatalyst. Our strategy provides a facile design and synthesis process of noble−transition metal alloy electrocatalysts via noble metal modified MOFs as precursors. KEYWORDS: metal−organic framework, nanoporous, bimetallic, alloy, ORR

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the first choice for developing appropriate materials in the near term.7 For the sake of widespread adoption of fuel cells, high Pt mass activity (catalytic activity per given mass of Pt) must be obtained. Two factors determine Pt mass activity: one is the specific activity (SA, normalized by surface area) and the other is the electrochemically active surface area (ECSA, normalized by mass).5 Previous computational and experimental research has revealed that the strong bonding energy between oxygenated species and Pt atoms impedes the activity of pure Pt.8 The high ECSA requires optimized geometric factors, thereby increasing utilization efficiency (UE) of platinum.5 Therefore, Pt−transition metal alloys with unique structures, like caged, hollow, and porous nanoparticles (NPs), have drawn more and more attention. On one hand, the d-band center of Pt goes down when it is alloyed with first-row transition metals such as Fe, Co, and Ni, which leads to a reduction of bonding strength between the oxygenated species and Pt.9 On the other hand, compared with those solid nanoparticles, the hollow interior exposes more precious metal atoms, and their unique

INTRODUCTION Proton exchange membrane fuel cells (PEMFCs) have been of great concern as a potential energy supply source for equipment such as low-emission electric automobiles and various household electric generators.1 However, there are still substantial impediments that need to be resolved before these devices can be designed as viable technologies. One of the most important problems is to overcome the sluggish kinetics of oxygen reduction reactions (ORR) at the cathode.2 Platinum has been researched comprehensively as an effective cathode catalyst to decrease undesired overpotentials in ORR due to its extraordinary electrocatalytic activities. However, the worldwide technological scalability of PEMFC is significantly restricted by the high price and natural rarity of platinum.3−5 Therefore, designing and exploring low-Pt even non-Pt ORR electrocatalysts with elevated catalytic activity and high stability is imperative for the industrialization of fuel cells. It is worth noting that either non-Pt or low-Pt ORR catalysts ought to satisfy the rigorous requirements for absolute activity and durability challenges.6 It is found that few electronically conductive non-noble catalysts are steady in the acidic (and fluoride-ion-containing) PEMFC environment; therefore, decreasing the amount of Pt in the cathode catalysts is still © 2017 American Chemical Society

Received: July 1, 2017 Accepted: August 25, 2017 Published: August 25, 2017 32106

DOI: 10.1021/acsami.7b09428 ACS Appl. Mater. Interfaces 2017, 9, 32106−32113

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Synthetic Process and Model of Nanoporous PtFe Nanoparticles on N-Doped Porous Carbon Sheetsa

a (a, b) Pt-modified Fe-based metal−organic framework. (c) Annealing product of Pt/Fe-ZIF. (d) Enlarged model of PtFe NPs loaded on N-doped porous carbon sheets. (e) Enlarged model of one nanoporous PtFe NP.

increases the possibility of fabricating catalysts with excellent performance.18 Our group had developed a process of preparing alloys that consist of transition metals and precious metals using MOF compounds as precursors, and we demonstrated that those catalysts possess outstanding activity in the hydrogen evolution reaction (HER).20−22 For example, Su et al.21 prepared RuCo@NC as an effective HER catalyst via adding RuCl3 solution into Co3[Co(CN)6]2 precursor. Chen et al.22 also fabricated PdCo@NC through adding PdCl2 solution into Co3[Co(CN)6]2 precursor. After annealing in inert gases, they directly got the electrocatalysts without further operation. Inspired by this method, we have designed a facile way to fabricate nanoporous PtFe alloys supported on N-doped porous carbon sheets via in situ themolysis of Pt-modified Fe-based metal−organic framework. Benefiting from the H2assisted high-temperature pyrolysis of MOF, which provides high Gibbs free energy and large diffusion speed, np-PtFe NPs with uniform alloy composition are successfully synthesized, and their electrocatalytic activity for ORR in acidic electrolytes is much better than that of Pt/C.

geometry provides an approach for adjusting physical and chemical properties.10 Typically, these particular structures are made intentionally through the following methods: treatment based on the Kirkendall effect, template-directed protocol relying on the removal of microbeads and nanobeads, galvanic displacement reaction, and dealloying of Pt−transition metal alloys.10,11 Yang and co-workers10 transformed crystalline PtNi3 polyhedra to Pt3Ni nanoframes through interior erosion, which led to surfaces that could offer three-dimensional molecular accessibility. Wei and co-workers12 synthesized hollow PtFe nanoparticles with Pt-skin surface through structural evolution of solid Pt nanoparticles after treatment of space-confined pyrolysis and the nanoscale Kirkendall effect. Although those methods have shown advantages in the activity of synthesized Pt−transition metal alloy electrocatalysts with particular structures, the synthetic procedures and complex contaminants removal greatly increased production costs, thereby resulting in economically unfeasible techniques. Recently, metal−organic frameworks (MOFs) have exhibited tremendous advantages for use as templates to fabricate various carbon-based composite materials via thermolysis.13−17 Different kinds of organic ligands and tunable metal ion centers (such as Fe, Co, and Ni) produce a variety of precursors for synthesizing alloys consisting of different metal compositions.18 During the calcination process, high annealing temperature provides enough Gibbs free energy for diffusion of different metal ions to form alloys; meanwhile, the organic ligands of MOF precursor will be catalyzed to form in situ carbon materials.12,19 It is well-known that only when they are supported on carbon materials or other bases with good conductivity can metal nanoparticles be used as efficient electrocatalysts. With this advantage, MOF calcination that fabricates electrocatalysis materials is undoubtedly a feasible method in the electrical catalysis field. Furthermore, different elements (N, P, and S atoms) in changing organic ligands would make graphene layers with different dopants, which

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EXPERIMENTAL SECTION

Synthesis of Nanoporous PtFe/N-Doped Porous Carbon Sheets. Scheme 1 shows the synthetic route for preparing nanoporous PtFe nanoparticles supported on N-doped porous carbon sheets (npPtFe/NPCS) as electrocatalysts. First, 328 mg of 2-methylimidazole and 20 mg of poly(vinylpyrrolidone) (PVP) are dissolved in 25 mL methanol to form solution A, while 404 mg of ferric nitrate [Fe(NO3)3·9H2O] is dissolved in 25 mL of methanol to form solution B. After solution A is added to solution B via a syringe (10 mL) drop by drop, 1 mL of the synthesized solution containing Pt NPs was added, and the reaction mixture was stirred for 5 min. After the reaction was held for 24 h without any interruption at room temperature, we centrifuged the solution and washed the precipitation with methanol. The resulting precipitate was finally dried in a vacuum drying oven overnight. Figure S1 shows the X-ray diffraction (XRD) pattern of the resulting Fe-zeolitic imidazolate framework (ZIF) 32107

DOI: 10.1021/acsami.7b09428 ACS Appl. Mater. Interfaces 2017, 9, 32106−32113

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) XRD pattern, (b) Raman spectrum, and (c) nitrogen adsorption isotherm at 77 K for np-PtFe/NPCS. (c, inset) Pore-size distribution calculated from the BJH equation from N2 adsorption. (d) XPS survey spectrum, (e) N 1s XPS spectra, and (f) Fe 2p XPS spectra of np-PtFe/NPCS catalyst. precursor with two broad peaks, which are similar to the reported ZIF pattern, suggesting successful synthesis of the targeted Fe-ZIF.23 In order to obtain np-PtFe/NPCS, the resulting Pt/Fe-ZIF composites were heated to 120 °C at 5 °C·min−1 under flowing 5% H2/95% Ar, held at 120 °C for 60 min, and then heated at a rate of 5 °C·min−1 to 700 °C and held at that temperature for 20 min. After slow cooling of the sample to room temperature, the pyrolyzed samples were etched with 3 M HCl to remove the redundant Fe species. After being washed with deionized water and alcohol several times, the resulting products were then dried in a vacuum drying oven for 24 h.

are between 2 and 20 nm in diameter and are large enough for free diffusion of O2, which possesses a kinetic diameter of 0.346 nm.12 The atomic percentage of N element in np-PtFe/NPCS was estimated to be 2.4% (Figure 1d). Meanwhile, bands for graphitic N (ca. 401 eV), pyrrolic N (ca. 399.8 eV), and pyridinic N (ca. 398.5 eV) could be distinctly resolved from the high-resolution N 1s X-ray photoelectron spectroscopic (XPS) spectrum (Figure 1e),28 which indicates the formation of Ndoped carbon after thermolysis of Fe-ZIF. Figure 1f shows the Fe 2p XPS spectra of np-PtFe/NPCS catalyst. Fe(0) of the PtFe alloy can be identified by the low-energy peak at 707.7 eV, while the high-energy peaks at 710.3 and 712.8 eV can be attributed to Fe2+ and Fe3+ in the FeNx/C composites.12 The weight contents of Pt and Fe for np-PtFe/NPCS were determined to be 19.31% and 2.32%, respectively, according to inductively coupled plasma-atomic emission spectrometric (ICP-AES) analysis. Figure 2 shows the transmission electron microscopic (TEM) and scanning electron microscopic (SEM) images of as-prepared np-PtFe/NPCS as well as the corresponding energy-dispersive spectroscopic (EDS) element mapping. The SEM image (Figure 2a) of np-PtFe/NPCS at low magnification displays the sheet morphology of carbon bases, and the highermagnification image (Figure 2b) clearly exhibits the rough surface of the carbon sheets because of themolysis and washing with 3 M HCl solution.29 The TEM image (Figure 2c) further confirms the sheetlike feature of the carbon bases. Remarkably, as shown in Figure S2 and Figure 2c, the carbon sheets consist of several graphene layers, which corresponds to the Raman results. Moreover, the np-PtFe particles are uniformly dispersed on the NPCS. At the same time, we can also notice that some bright dots also disperse uniformly on the carbon bases, which demonstrates that the carbon bases are porous again. Figure 2d shows the high-resolution transmission electron microscopic (HRTEM) image of an individual PtFe nanoparticle of about 15 nm diameter, which has a spongy multihollow structure. The

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RESULTS AND DISCUSSION The XRD pattern of synthesized np-PtFe/NPCS is illustrated in Figure 1a. It exhibited three characteristic peaks, which were assigned to (111), (200), and (220) of Pt with face-centered cubic (fcc) structure.24 Compared with pure Pt, the higher angles of peaks’ positions indicate that Fe successfully permeated into the Pt fcc structure and led to an alloy phase, resulting in the contracted lattice distance. Remarkably, we can identify no apparent diffraction peak between 20° and 30°, indicating np-PtFe particles were in fact embedded on the graphene surface and suppressed the stacking of graphene layers.25 Raman spectroscopy is a nondestructive method to characterize disordered and ordered crystal structures of carbon materials.26 A characteristic feature of graphitic layers is the G band, which corresponds to tangential vibration of the carbon atoms, while the disordered carbon or defective graphitic structures could be identified by the D band.27 The ratio of D band to G band integrated intensity (ID/IG) for np-PtFe/NPCS was 1.4 (Figure 1b), suggesting that generation of abundant defective edges arises from the pores of carbon sheets. Moreover, the broad and weak 2D band indicates that the carbon sheets consist of several layers of graphene.21 The specific surface area of np-PtFe/NPCS is 34.7 m2·g−1, demonstrated in the Brunauer−Emmett−Teller (BET) analysis of Figure 1c. The corresponding pore-size distribution analysis of np-PtFe/NPCSs (inset, Figure 1c) clearly revealed that a great number of nanopores existed in the catalyst. Most pores 32108

DOI: 10.1021/acsami.7b09428 ACS Appl. Mater. Interfaces 2017, 9, 32106−32113

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a, b) SEM images and (c) TEM image of np-PtFe/NPCS. (d) HRTEM image of an individual np-PtFe. (e−h) Elemental mapping of N, C, Pt, and Fe, respectively. (i) EDX line-scanning profile of an individual np-PtFe.

high-angle annular dark-field scanning transmission electron microscopic (HAADF-STEM) image of np-PtFe/NPCS (Figure S3) clearly shows a bright contrast at the edge and some dark contrast dots in the interior of PtFe particles, which further confirms the formation of multihollow structure. The HAADF cross-sectional compositional line profiles (Figure 2i and Figure S4) also exhibit nanoporous characteristics of the PtFe nanoparticles. With regard to the line-scanning, the signal of Pt rises significantly on the side edge of the individual particle and reaches its peak at 9, 15, and 20 nm, and two minimum Pt signals can be observed at 11 and 17.5 nm. Meanwhile, the Fe/Pt atomic ratio (Figure S5) shows that, whether in the interior or at the edge of the multihollow PtFe NP, the Pt and Fe atoms are dispersed uniformly. The distance between adjacent lattice fringes is 0.22 nm, which matched well with that of (111) planes in fcc PtFe alloy, suggesting that the PtFe NPs have been successfully synthesized through the annealing of Pt-modified Fe-ZIF. Figure 2g,h displays HAADFSTEM micrographs and EDX maps of Pt and Fe elements, which also show that Pt and Fe elements are evenly dispersed in the whole nanoparticle, revealing a uniform alloy composition. Furthermore, the dark areas observed in the

element maps also manifest that the nanoparticles are nanoporous. Figure 2e,f shows HAADF-STEM micrographs and EDX maps of C and N elements. For the same reason, the dark area indicates the porous structure of carbon sheets. We carried out both cyclic voltammetry (CV) and linear sweep voltammetry (LSV) tests to evaluate the electrocatalytic performance of np-PtFe/NPCS and chose commercial 20% Pt/ C catalysts as reference. Figure 3a shows CV curves of npPtFe/NPCSs catalyst and commercial 20% Pt/C in N2-purged 0.1 M HClO4 solution at a scan rate of 50 mV·s−1. After measuring the Coulombic charge for hydrogen adsorption/ desorption, we can obtain the value of electrochemical surface area (ECSA). The specific value of ECSA normalized to Pt mass for np-PtFe/NPCS is estimated to be 111.58 m2·g−1, while the value for commercial 20% Pt/C catalyst is only 90 m2· g−1. The high specific value of ECSA of np-PtFe/NPCS may be ascribed to two merits of np-PtFe/NPCS. First, the np-PtFe NPs are loading on the porous carbon sheets and no carbon layers are around the nanoparticles. Moreover, the porous structure of PtFe NPs exposes more Pt atoms on the surface of NPs, which can adsorb H+ and be used for oxygen reduction reaction. Figure 3b shows the electrochemical polarization 32109

DOI: 10.1021/acsami.7b09428 ACS Appl. Mater. Interfaces 2017, 9, 32106−32113

Research Article

ACS Applied Materials & Interfaces

Figure 3. (a) Electrochemical CV curves of np-PtFe/NPCSs and Pt/C catalysts recorded at a scan rate of 50 mV·s−1 in N2-saturated 0.1 M HClO4 solution. (b) Electrochemical polarization curves of np-PtFe/NPCS and Pt/C catalysts recorded with a sweep rate of 10 mV·s−1 and a rotation rate of 1600 rotations·min−1 in O2-purged 0.1 M HClO4 solution at room temperature. (c) Specific activity and (d) mass activity at 0.9 V versus reversible hydrogen electrode (RHE) for np-PtFe/NPCS and Pt/C catalysts.

literature in Table S1.11,12,31−33 From the table, it was found that we obtained a catalyst with promising catalytic activity via a facile synthesis process, which makes this technique economically feasible. The greatly enhanced performance of np-PtFe/ NPCS catalyst is primarily ascribed to two reasons. First, previous calculations have shown that charge transfer from Fe to Pt in the PtFe alloy plays an important role in modulating electronic structure over the strain effect, which results in a significant decrease in surface average charge and thereby the decline of oxygenated species’ adsorption energy.12,34 Second, nanoporous architectures not only expose more active sites for ORR and increase the utilization efficiency (UE) of platinum but also provide integral molecular transport paths.33 In addition to the catalytic activity, stability is another important evaluation criterion that determines whether the electrocatalyst can be employed in practical applications. So, we further performed an accelerated durability test (ADT) between 0.6 and 1.1 V in O2-saturated 0.1 M HClO4 to evaluate the electrochemical durability of the catalyst. The 20% commercial Pt/C catalyst was also used as a baseline catalyst for comparison. In Figure 4a, np-PtFe/NPCS catalyst shows negligible attenuation of diffusion-limited current density (Jd) and half-wave potential (E1/2) after 2000 CV cycles, while those of Pt/C showed apparent decay (Figure 4b). The mass activity of aged np-PtFe/NPCS catalyst was still as high as 0.444 A· mgPt−1 (Figure 4d), showed only 16.7% decrease from the initial mass activity, while the aged Pt/C (0.047 A·mgPt−1) retained only 26.9% of the initial mass activity (0.175 A· mgPt−1). In order to understand the reason for the superior stability, we utilize TEM (Figure S8 and Figure 4c) to

curves of np-PtFe/NPCS and Pt/C catalysts operated with a rotation rate of 1600 rotations·min−1 in O2-purged 0.1 M HClO4 at room temperature. It can be seen that np-PtFe/ NPCS exhibit a more positive half-wave potential (0.92 V) compared with commercial Pt/C catalyst (0.883 V). In the meantime, for the sake of the fact that the ORR catalytic activity originates from np-PtFe rather than the Fe-Nx/C structure, we test the sample annealed from Fe-ZIF without adding Pt NPs (Figure S6). From the LSV result, we can see that the sample annealed from Fe-ZIF without added Pt NPs possesses inferior catalytic activity to ORR in acid environment. The Tafel plots in Figure S7 show slopes of 40 and 48 mV· decade−1 for np-PtFe/NPCS and Pt/C, respectively. The npPtFe/NPCS possesses a smaller slope, which demonstrates improved kinetics for ORR.5 The specific activity is calculated by normalizing kinetic currents to both ECSA and the loading amount of metal Pt. The values of specific activity for np-PtFe/ NPCS and Pt/C help us further investigate different catalysts for better catalytic activity. As shown in Figure 3c, the specific activity of the np-PtFe/NPCSs catalyst is 0.477 mA·cm−2 at 0.9 V, which is 2.44 times greater than that of Pt/C (0.195 mA· cm−2). After normalization of kinetic currents to the loading amount of Pt, the mass activity of np-PtFe/NPCS (Figure 3d) catalyst is found to be 0.533 A·mgPt−1, which is 3.04 times greater than that of Pt/C (0.175 A·mgPt−1) and surpasses the U.S. Department of Energy’s 2017 target (0.440 A·mgPt−1).30 We also applied np-PtFe/NPCS in the PEMFC (Figure S10), and the result also demonstrated that np-PtFe/NPCS has better eletrocatalytic activity than 20% Pt/C. We compared our material with some disordered PtFe catalysts from the previous 32110

DOI: 10.1021/acsami.7b09428 ACS Appl. Mater. Interfaces 2017, 9, 32106−32113

Research Article

ACS Applied Materials & Interfaces

Figure 4. ORR polarization curves of (a) np-PtFe/NPCS and (b) 20% Pt/C catalysts before and after 2000 potential cycles between 0.6 and 1.1 V versus RHE. (Insets) CV curves of np-PtFe/NPCS and 20% Pt/C before and after 2000 CV cycles. (c) TEM image and (inset) particle-size distribution of aged np-PtFe/NPCS catalyst after 2000 CV cycles. (d) Changes in mass activities of np-PtFe/NPCS and 20% Pt/C before and after 2000 potential cycles.

characterize the morphology of np-PtFe/NPCS sample before and after 2000 CV cycles. After 2000 CV cycles, the diameter of PtFe NPs increases from 5−14 nm to 12−24 nm and exhibits a broad size distribution. However, we can also clearly identify that np-PtFe NPs have converted into hollow PtFe NPs with a Pt skin. This unique alloy structure with a nearly pure Pt skin results from the reconstruction of neighboring surface Pt atoms because of gradual erosion of Fe atoms on the surface during CV cycling.33 A previous study demonstrated that, during PEMFC operation, Pt3Co nanoparticles are inclined to convert into hollow Pt nanoparticles due to depletion of transition metal in the acid environment.35 Remarkably, this hollow structure with Pt-skin surface can effectively decrease the effect of low coordination sites, where the binding strength between oxygenated species and Pt atoms is very strong, and thereby weaken the adsorption of nonreactive oxygenated species on the surface of Pt, retaining high electrocatalytic activity.12,35

of Pt; therefore, we are able to use equivalent Pt content of commercial 20% Pt/C to obtain higher activity. Furthermore, the conversion of PtFe NPs from porous to hollow structure can maintain the activity of electrocatalyst. We envision that high performance and economically feasible catalysts for proton exchange membrane fuel cells could be achieved via the abovementioned facile approach.

CONCLUSIONS In conclusion, we designed a facile synthetic route to fabricate a double nanoporous structure comprising nanoporous PtFe nanoparticles supported on N-doped porous carbon sheets through the pyrolysis of Pt-modified Fe-based metal−organic framework. The synergistic effect between the open framework structure of nanoporous PtFe nanoparticles and porous carbon sheets not only creates more active sites but also is beneficial for mass and charge transport, which facilitates the electrochemical catalytic process. In particular, the porous structure of PtFe NPs immensely enhanced the utilization efficiency (UE)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b09428. Additional text describing syntheses, material characterization, and electrocatalytic measurements; 10 figures showing XRD, HRTEM, HAADF-STEM, EDX, atomic ratio, LSV, Tafel, TEM, particle-size distribution, XPS, polarization, and durability data; one table listing ORR activity data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. ORCID

Kang Yang: 0000-0002-8800-5601 Jitang Chen: 0000-0002-1607-7075 Notes

The authors declare no competing financial interest. 32111

DOI: 10.1021/acsami.7b09428 ACS Appl. Mater. Interfaces 2017, 9, 32106−32113

Research Article

ACS Applied Materials & Interfaces

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation (NSFC; 21271163 and U1232211), CAS/SAFEA International Partnership Program for Creative Research Teams, and CAS Hefei Science Center (2016 HSC-IU011). We thank Yanyan Gao of Dalian Institute of Chemical Physics, Chinese Academy of Sciences, for help on the PEMFC test.

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REFERENCES

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DOI: 10.1021/acsami.7b09428 ACS Appl. Mater. Interfaces 2017, 9, 32106−32113

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DOI: 10.1021/acsami.7b09428 ACS Appl. Mater. Interfaces 2017, 9, 32106−32113