Fabrication of ε-Fe2N Catalytic Sites in Porous Carbons Derived from

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Fabrication of ε‑Fe2N Catalytic Sites in Porous Carbons Derived from an Iron−Triazolate Crystal Yu-ichi Fujiwara,†,‡ Jet-Sing M. Lee,∥ Masahiko Tsujimoto,∥ Kanokwan Kongpatpanich,§ Taweesak Pila,§ Ken-ichi Iimura,⊥ Norio Tobori,† Susumu Kitagawa,*,∥ and Satoshi Horike*,‡,∥,# †

Functional Materials Science Research Laboratories, Research & Development Headquarters, Lion Corporation, 2-1, Hirai 7-chome, Edogawa-ku, Tokyo 132-0035, Japan ‡ Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ∥ Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Institute for Advanced Study, and #AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory (ChEM-OIL), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan § Department of Materials Science and Engineering, School of Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Rayong 21210, Thailand ⊥ Department of Material and Environmental Chemistry, Graduate School of Engineering, Utsunomiya University, 7-1-2 Yoto, Utsunomiya, Tochigi 321-8585, Japan S Supporting Information *

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pentaatomic tetrahedral secondary building unit with iron atoms at the center and at the vertices of the tetrahedron (Figure 1b). The framework was carbonized at 850 °C for 1 h at a heating rate of 10 °C min−1 under N2 flowing atmosphere, and the resultant matrix was etched with 0.5 M H2SO4 to remove excess metallic iron (denoted as C1-850Fe3C). We conducted two control experiments. Direct pyrolysis for the iron−triazolate under NH3 yielded metallic iron (Figure S2). Pyrolysis for Fe3(1,3,5-benzenetricarboxylate)2 (Fe-HKUST-1) under the same condition provided the black powder which ignites under the exposure to the air, probably due to the reactive Fe3C or Fe particles in the product. The results suggest the advantage of iron−triazolate crystal as a precursor to having air-stable carbon Fe3C composite. For the preparation of Fe3C particles, some reports use a pyrolysis temperature of about 500 °C.28,29 To provide sufficient ORR performance, the pyrolysis temperature under N2 was optimized between 750 and 950 °C for the iron− triazolate crystal, and the material at 850 °C showed the best ORR activity (Figure S3). The particle size of the pyrolyzed product at 850 °C was further reduced by ball milling the iron− triazolate crystal precursor and then performing the pyrolysis process, which was shown to produce better films on the electrode and resulted in a higher onset potential (Figures S4 and S5). This optimized material was further used for the ammonolysis experiments. The PXRD pattern of C1-850Fe3C shows several peaks between 37 and 49° derived from Fe3C diffractions and a sharp peak located at around 26.5°, which was assigned to the (002) diffraction of carbon (Figure 1c). The high-resolution transmission electron microscope (HR-TEM) image of incorporated nanoparticle shows crystalline lattices corresponding to the (001) and (100) planes of the Fe3C

olymer electrolyte fuel cells (PEFCs) have been regarded as a promising renewable energy technology to meet future societal demands, attributed to their environmentally friendly byproducts, high power densities, and energy conversion efficiency.1,2 Oxygen reduction reaction (ORR) in cathode is one of the key reactions in PEFCs; however, efficient ORR currently requires platinum group metal (PGM) based electrocatalysts.3−5 Transition-metal/nitrogen/carbon (M/N/ C), carbide, and metal oxide catalysts have thus been intensively explored over the past decade for the replacement of the PGM.6−10 Recently crystalline iron nitrides are regarded as a significant ORR catalyst due to their high onset potential and good durability.11−13 Iron nitride can be prepared by ammonolysis of iron oxide or iron complexes, and these show various crystal structures and phases, such as ζ-Fe2N, ε-Fe2N, εFe3N, and γ′-Fe4N.11−18 Among them, ε-Fe2N has been deduced as the most active nitride species from both experimental results and DFT calculations.14 However, εFe2N with anisotropic distortion prefers to transform to the ζFe2N with orthorhombic structure due to the thermodynamic stability;19,20 thus, it has been challenging to retain the catalyst in the ε-Fe2N phase. Carbon-encapsulated iron carbide (Fe3C) transforms to εFexN (2 ≤ x ≤ 3) depending on the ammonolysis temperature and concentration.16,21 Recently, iron-based metal−organic framework (MOF) crystals were used as precursors for the preparation of carbon-encapsulated Fe3C materials.22−26 We investigated the preparation of ε-Fe2N incorporated catalysts from an iron-based MOF. By optimization of the ammonolysis temperature, Fe3C derived from an iron−triazolate crystal predominantly transformed to the ε-Fe2N species. The ε-Fe2N incorporated catalysts exhibited excellent ORR activity with a superior durability compared to the benchmark Pt/C catalyst. To prepare Fe3C incorporated in porous carbon, we used an iron−triazolate framework [Fe(C2N3H2)2]n,27 (Figure 1a). The iron atoms are octahedrally coordinated to the nitrogen atoms of 1,2,3-triazole; thus, the framework is arranged forming a © 2018 American Chemical Society

Received: November 12, 2017 Revised: March 14, 2018 Published: March 14, 2018 1830

DOI: 10.1021/acs.chemmater.7b04762 Chem. Mater. 2018, 30, 1830−1834

Communication

Chemistry of Materials

PXRD measurements were performed to investigate the local structure of the C2-yFe2N samples. The C2-yFe2N samples show three notable peaks at 38, 41, and 43° (Figure 1d), which are assigned to ε-FexN diffractions, along with carbon diffractions. According to the optimization of the pyrolysis temperature, these diffractions from the (111) phase of ε-FexN shift toward the lower angle values, indicating the lattice expansion in ε-FexN.31,32 This suggests that the N-content increased in the lattice. The ε-FexN diffractions of C2-850Fe2N are located between the simulated ε-Fe2N and ε-Fe3N diffractions, whereas the ε-FexN diffraction of C2-800Fe2N matches with the simulated ε-Fe2N diffractions, suggesting the ε-Fe2N forms primarily. Fe3C diffraction patterns were also observed slightly in all C2-yFe2N samples. TEM was performed on all C2-yFe2N samples and C2800Fe2N as a representative sample (Figure 1e,f). The TEM image of C2-800Fe2N reveals nanoparticles of about 100 nm in diameter dispersed in carbon and that are covered with a graphitic carbon layer of 3−7 nm. From the HR-TEM image, the nanoparticles show crystalline lattices corresponding to the (001) and (101) planes of the ε-Fe2N. On the other hand, we partially observed ζ-Fe2N and Fe3C nanoparticles from the similar crystalline lattice analysis (Figure S6b−m). All of the other C2-yFe2N samples also show ε-Fe2N, ζ-Fe2N, and Fe3C additionally, therefore, suggesting the postammonolysis treatment generates these specific catalytic sites. X-ray photoelectron spectroscopy (XPS) measurements were performed to analyze the contents of N and Fe species on the surface of the C1-850Fe3C and the C2-yFe2N samples (Figure S7 and S8). The C1-850Fe3C and the C2-yFe2N samples show N content of 4.2−4.9%, whereas Fe content is negligible (≤0.7%) (Table S1). Such low iron surface content implies that the Fe3C or ε-FexN nanoparticles are exclusively encased by carbon layers and thus cannot be detected by the XPS measurements. The bulk content of Fe species which are encased by carbon layers was analyzed using thermogravimetric analysis (TGA) in air. The Fe content of C1-850Fe3C was found to be 7% (Table S2). However, the C2-yFe2N samples show an increased Fe content of 5−22% as it is known that ammonolysis can etch away carbon.33 The Brunauer− Emmett−Teller (BET) surface areas of all carbon samples were investigated by N2 adsorption at −196 °C and were found to be 527−637 m2 g−1 (Table S1). The isotherms show a steep increase of N2 uptake in the low relative pressure region, and a hysteresis loop between the adsorption and the desorption isotherms indicates the existences of micropores and mesopores (Figure S9). The isotherms can be categorized as a combination of the Type I and Type IV isotherms.34 X-ray adsorption fine structure (XAFS) measurements at the Fe K-edge were performed to investigate the valence state and local bonding environment of the Fe species in the C1850Fe3C and the C2-yFe2N samples. The X-ray adsorption near-edge structure (XANES) spectra for the C1-850Fe3C, iron foil, FeO, and Fe2O3 are shown in Figure S10. The Fe K-edge position of the C1-850Fe3C is located between the Fe foil and Fe2O3, indicating that the valence state of the C1-850Fe3C is in between that of Fe0 and Fe3+. We further measured XANES spectra for the C2-yFe2N samples to investigate the valence state of samples after ammonolysis to FexN (Figure 1g). The XANES spectra slightly shift to higher Fe K-edge position with increasing ammonolysis temperature; thus, the valence state slightly increased to that of Fe3+. Extended X-ray adsorption fine structure (EXAFS) spectra were performed for C1-

Figure 1. (a) Crystal structure of iron−triazolate framework, (b) tetrahedral secondary building unit of the framework, where the gray, blue, and orange colors denote C, N, and Fe, respectively, and H atoms are omitted. (c) PXRD patterns for C1-850Fe3C (black), C2850Fe2N (red), C2-750Fe2N (blue), C2-900Fe2N (green), and C2800Fe2N (purple) and (d) the enlarged PXRD patterns for C2850Fe2N (red), C2-750Fe2N (blue), C2-900Fe2N (green), and C2800Fe2N (purple). (e) TEM image of the C2-800Fe2N and (f) HRTEM and FFT (inset) images of a ε-FexN nanoparticle. (g) XANES and (h) FT-EXAFS oscillations for C1-850Fe3C (black), C2-750Fe2N (blue), C2-800Fe2N (purple), C2-850Fe2N (red), and C2-900Fe2N (green).

(Figure S6a). Thus, we concluded that Fe3C are formed in the porous carbon. Second, ε-FexN, 2 ≤ x ≤ 3, were prepared from the C1850Fe3C by pyrolysis under a pure NH3 flow. The pyrolysis treatments are conducted in the temperature of 750−900 °C, where NH3 is decomposed to N2 and H2 species.30 The obtained samples are denoted as C2-yFe2N (y = 750, 800, 850, and 900, where y refers to the ammonolysis temperature). 1831

DOI: 10.1021/acs.chemmater.7b04762 Chem. Mater. 2018, 30, 1830−1834

Communication

Chemistry of Materials 850Fe3C and the C2-yFe2N samples (Figure 1h). The k3 weighted Fourier transformation (FT) EXAFS oscillations for C1-850Fe3C show two peaks at 1.35 and 2.05 Å, which were assigned to Fe−C bonds and Fe−Fe bonds derived from Fe3C. In contrast, the C2-yFe2N samples show two peaks at the range of 1.8−2.6 Å, suggesting the existence of mainly two independent Fe−Fe bonds. From the theoretical bond length, the oscillation at 2.0 Å is assigned to Fe−Fe bonds from Fe3C and the oscillation at 2.4 Å is assigned to Fe−Fe bonds from εFexN. Therefore, the Fe3C incorporated porous carbon mostly converts to ε-FexN (2 ≤ x ≤ 3) by ammonolysis, and the PXRD pattern and HR-TEM image suggest that ε-Fe2N formation is dominant at 800 °C. In order to compare with the ORR behavior of ε-Fe2N incorporated carbon catalysts, electrochemical measurements in 0.1 M HClO4 electrolyte with a 0.14 mg cm−2 sample coated rotating disk electrode (RDE) were performed. Cyclic voltammetry (CV) in N2-saturated electrolyte curves shows no obvious redox peaks for all of the samples (Figure S11). In contrast, a well-defined cathodic peak appears at 0.70−0.72 V in O2-saturated electrolyte, indicating the electrocatalytic activity of oxygen molecules. C2-800Fe2N, which predominantly contains ε-Fe2N, shows the highest ORR peak potential (0.72 V). The ORR activities of the catalysts were further investigated by linear sweep voltammetry (LSV) at 1600 rpm (Figure 2a). C2-800Fe2N shows the highest onset potential (Eonset, 0.86 V) and a half-wave potential (E1/2, 0.66 V), consistent with the CV results. RDE measurements were performed at various rotating speeds, and the kinetic parameters were analyzed using the Koutecky−Levich (K−L) equation (Figure S11). The linearity of the K−L plots for all samples indicates first-order reaction kinetics toward the concentration of dissolved oxygen and similar electron transfer numbers (n) for ORR at various potentials. The values of n for C2-800Fe2N, C2-850Fe2N, and C2-900Fe2N were 3.9, 3.7, and 3.8, respectively, at 0.5 V, close to the theoretical value (4.0) of Pt/C. In contrast, the n values of C1-850Fe3C and C2-750Fe2N at the same potential were only 2.8 and 3.1, respectively, indicating poor electrocatalytic selectivity for these materials (Figure 2b). Moreover, the kinetic-limiting current density (jk) from the K−L plot of C2800Fe2N was 30.6 mA cm−2 at 0.5 V, much higher than those of C1-850Fe3C (7.4 mA cm−2), C2-750Fe2N (20.1 mA cm−2), C2-850Fe2N (18.2 mA cm−2), and C2-900Fe2N (23.4 mA cm−2). The relationship between jk and the 2 theta value of the ε-FexN (111) phase from PXRD is shown in Figure 2c. It was found that the catalytic activity increased with lower 2 theta angles toward the ε-Fe2N structure, indicating that the ε-Fe2N phase accelerates ORR activity. To compare with the activity of commercial 10% Pt/C, we evaluated the ORR activity of the C2-800Fe2N with a 0.42 mg cm−2 loading, which is the general loading amount.16,35 The curve of the C2-800Fe2N exhibits better performance with higher catalyst loading (E1/2 = 0.71 V vs 0.66 V) and is close to that of Pt/C (Figure 2d). The ORR activity of C2-800Fe2N is excellent and just second to the record iron nitride incorporated carbon catalyst, FePhen@ MOF-ArNH3 (Table S3).12 Durability tests in the range of 0.5−0.8 V were performed to investigate the current retention property of both the C2-800Fe2N and the Pt/C (Figure 2e). C2-800Fe2N shows excellent stability with a 76% current retention after 5000 cycles, whereas Pt/C shows poorer performance with only 51% current retention. Therefore, C2-

Figure 2. (a) LSV curves at 1600 rpm at a scan rate of 10 mV s−1 in O2 saturated 0.1 M HClO4, (b) kinetic-limiting current density (jk) and electron transfer number (n), and (c) relationship between jk and 2 theta value of ε-FexN (111) phase for C1-850Fe3C (black), C2750Fe2N (blue), C2-800Fe2N (purple), C2-850Fe2N (red), and C2900Fe2N (green). (d) LSV curves at 1600 rpm at a scan rate of 10 mV s−1 in O2 saturated 0.1 M HClO4 and (e) current durability at the range of 0.5−0.8 V for the C2-800Fe2N (black) and Pt/C (gray).

800Fe2N is a better candidate than Pt/C for long-term, real use in PEFCs due to its higher resistance to deactivation. In summary, an iron−triazolate framework was used as a carbonization precursor to synthesize a carbon-encapsulated Fe3C catalyst which will subsequently be converted to ε-FexN (2 ≤ x ≤ 3) incorporated catalysts by ammonolysis. The ammonolysis temperature was optimized to form ε-Fe2N active sites predominantly. Among the iron nitride incorporated porous carbons, C2-800Fe2N shows high catalytic activity, with an onset potential of 0.86 V, electron transfer number of 3.9, and a kinetic-limiting current density of 30.6 mA cm−2. Moreover, the optimized material shows only 24% degradation after 5000 reaction cycles, more stable than the benchmark Pt/ C. The results show a promising route to stabilize ε-Fe2N phase in the carbon matrix from an iron-based metal organic framework and is applicable to other electrocatalytic systems.

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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/acs.chemmater.7b04762. 1832

DOI: 10.1021/acs.chemmater.7b04762 Chem. Mater. 2018, 30, 1830−1834

Communication

Chemistry of Materials

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Experimental details and additional characterizations (PXRD, SEM, HR-TEM, XPS, N2 adsorption, XANES, CV, K−L plot) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(S.H.) E-mail: [email protected]. *(S.K.) E-mail: [email protected]. ORCID

Kanokwan Kongpatpanich: 0000-0002-4353-7057 Satoshi Horike: 0000-0001-8530-6364 Author Contributions

The manuscript was written through contributions of all authors. Funding

This work was supported by Grant-in-Aid for Specially Promoted Research (No. 25000007), Young Scientists (A) from the Japan Society of the Promotion of Science (JSPS), Advanced Program for Program Manager’s Candidate Hub (APPROACH); “Molecular Technology” of Strategic International Collaborative Research Program (SICORP) from the Japan Science and Technology Agency (JST); and Izumi Science and Technology Foundation. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS iCeMS is supported by World Premier International Research Initiative (WPI), MEXT, Japan. The authors are grateful to Ms. T. Sameshima for assistance with the synthesis of ORR catalysts. Y.-i.F. thanks Dr. S. Kabashima and Mr. H. Kurokawa for their helpful suggestion and discussion. J.-S.M.L is a JSPS International Research Fellow.

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