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

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Fabrication of #-FeN 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 Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04762 • Publication Date (Web): 14 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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Chemistry of Materials

Fabrication of ɛ-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, 71-2 Yoto, Utsunomiya, Tochigi 321-8585, Japan ABSTRACT: The use of non-platinum group metals for oxygen reduction reactions (ORR) in acidic media is a challenging field due to their sluggish kinetics and fast deactivation. One of the key parameters in developing high performing ORR electrocatalysts is the careful consideration of highly active sites. Here, a facile approach to produce ɛ-FexN incorporated carbon catalysts from an iron−triazolate crystal is developed and the relationship between the iron nitride structure and their ORR activity investigated. The formation of ɛ-FexN sites could be systematically controlled by tuning of the ammonolysis temperature. These ɛ-FexN materials exhibited high ORR activity in acidic media and the best performing material exhibits an onset potential of 0.86 V and electron transfer number of 3.9. They also provided a high kinetic-limiting current density and a higher cycling stability compared with the benchmarked Pt/C. Carbon encapsulated ɛ-Fe2N is predominantly fabricated showing high catalytic reactivity from the iron−triazolate crystal.

Polymer electrolyte fuel cells (PEFCs) have been regarded as a promising renewable energy technology to meet future societal demands, attributed to their environmentally friendly by-products, 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 Transitionmetal/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, ɛ-Fe2N, ɛ-Fe3N, and γ'-Fe4N.11-18 Among them, ɛ-Fe2N has been deduced as the most active nitride species from both experimental results and DFT calculations.14 However, ɛ-Fe2N with anisotropic distortion prefers to transform to the ζ-Fe2N with orthorhombic structure due to the thermodynamic stabil-

ity,19-20 thus, it has been challenging to retain the catalyst in the ɛ-Fe2N phase. Carbon-encapsulated iron carbide (Fe3C) transforms to ɛ-FexN (2≤x≤3) depending on the ammonolysis temperature and concentration.16, 21 Recently, iron-based metalorganic framework (MOF) crystals were used as precursors for the preparation of carbon-encapsulated Fe3C materials.22-26 We investigated the preparation of ɛ-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 ɛ-Fe2N species. The ɛ-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 pentaatomic tetrahedral secondary building unit with iron atoms at the center and at the vertices of the tetrahedron (Figure 1b). The framework

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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] (FeHKUST-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 have air-stable carbon Fe3C composite.

TEM image of C2-800Fe2N reveals nanoparticles of about 100 nm in diameter dispersed in carbon and are covered with a graphitic carbon layer of 3−7 nm. From the HRTEM image, the nanoparticles show crystalline lattices corresponding to the (001) and (101) planes of the ɛ-Fe2N. On the other hand, we partially observed ζ-Fe2N and Fe3C nanoparticles from the similar crystalline lattices analysis (Figure S6b−m). All of the other C2-yFe2N samples also show ɛ-Fe2N, ζ-Fe2N and Fe3C additionally, therefore, suggesting the post-ammonolysis treatment generates these specific catalytic sites.

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 pyrolysed 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 (Figure 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 highresolution transmission electron microscope (HR-TEM) image of incorporated nanoparticle shows crystalline lattices corresponding to the (001) and (100) planes of the Fe3C (Figure S6a). Thus, we concluded that Fe3C are formed in the porous carbon. Secondly, ɛ-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). PXRD measurements were performed to investigate the local structure of the C2-yFe2N samples. The C2-yFe2N samples show notable three peaks at 38, 41, and 43° (Figure 1d), which are assigned to ɛ-FexN diffractions, along with carbon diffractions. According to the optimization of the pyrolysis temperature, these diffractions from (111) phase of ɛ-FexN shift towards the lower angle values, indicating the lattice expansion in ɛ-FexN.31-32 This suggests that the N-content increased in the lattice. The ɛ-FexN diffractions of C2-850Fe2N are located between the simulated ɛ-Fe2N and ɛ-Fe3N diffractions, whereas the ɛ-FexN diffraction of C2-800Fe2N matches with the simulated ɛFe2N diffractions, suggesting the ɛ-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−1f). The

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), C2-850Fe2N (red),C2-750Fe2N (blue), C2900Fe2N (green), and C2-800Fe2N (purple) and (d) the enlarged PXRD patterns for C2-850Fe2N (red),C2-750Fe2N (blue), C2-900Fe2N (green), and C2-800Fe2N (purple). (e) TEM image of the C2-800Fe2N and (f) HR−TEM and FFT (inset) images of a ɛ-FexN nanoparticle. (g) XANES and (h)


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Chemistry of Materials FT-EXAFS oscillations for C1-850Fe3C (black), C2-750Fe2N (blue), C2-800Fe2N (purple), C2-850Fe2N (red), and C2900Fe2N (green).

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 C2yFe2N samples show N-content of 4.2−4.9%, whereas Fe content are negligible (≤0.7%) (Table S1). Such low iron surface content implies that the Fe3C or ɛ-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 thermo−gravimetric 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 m2g−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 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 Fe K-edge were performed to investigate the valence state and local bonding environment of the Fe species in the C1-850Fe3C and the C2-yFe2N samples. The X-ray adsorption near-edge structure (XANES) spectra for the C1850Fe3C, iron foil, FeO, and Fe2O3 are shown in Figure S10. The Fe K-edge position of the C1-850Fe3C is located between 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 C1-850Fe3C and the C2-yFe2N samples (Figure 1h). The k3 weighted Fourier transformation (FT) EXAFS oscillations for C1850Fe3C 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 ɛ-FexN. Therefore, the Fe3C incorporated porous carbon mostly converts to ɛ-FexN (2≤x≤3) by ammonolysis and the PXRD pattern and HRTEM image suggest that ɛ-Fe2N formation is dominant at 800 °C. In order to compare with the ORR behavior of ɛ-Fe2N incorporated carbon catalysts, electrochemical measure-

ments in 0.1 M HClO4 electrolyte with a 0.14 mg cm−2 sample coated rotating disk electrode (RDE) was performed. Cyclic voltammetry (CV) in N2-saturated electrolyte curves show no obvious redox peaks for all of the samples (Figure S11). In contrast, well-defined cathodic peak appears at 0.70−0.72 V in O2-saturated electrolyte, indicating the electro-catalytic activity of oxygen molecules. C2-800Fe2N, which predominantly contains ɛ-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).

Figure 2. (a) LSV curves at 1600 rpm at a scan rate of 10 mV -1 s 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 ɛ-FexN (111) phase for C1-850Fe3C (black), C2-750Fe2N (blue), C2-800Fe2N (purple), C2-850Fe2N (red), and C2-900Fe2N (green). (d) LSV curves at -1 1600 rpm at a scan rate of 10 mV s 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).

C2-800Fe2N shows the highest an 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 value of C1-850Fe3C and C2-750Fe2N at the same potential was only 2.8 and 3.1, respectively, indicating poor electro-catalytic selectivity for these materials (Figure 2b). Moreover, the kineticlimiting 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 ɛ-FexN (111) phase from PXRD, is shown in Figure 2c. It was found that the catalytic activity increased with lower 2 theta angles towards the ɛ-Fe2N structure, indicating that the ɛ-Fe2N phase accelerate 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 C2800Fe2N is excellent and just second to the record iron nitride incorporated carbon catalyst, FePhen@MOFArNH3 (Table S3).12 Durability tests at the range of 0.5−0.8 V were performed to investigate the current retention property of both the C2-800Fe2N and 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, C2-800Fe2N 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 carbonencapsulated Fe3C catalyst which subsequently be converted to ɛ-FexN (2≤x≤3) incorporated catalysts by ammonolysis. The ammonolysis temperature was optimized to form ɛ-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 ɛ-Fe2N phase in carbon matrix from iron-based metal organic framework, and is applicable to other electrocatalytic systems.

ASSOCIATED CONTENT Supporting Information. Experimental details and additional characterizations (PXRD, SEM, HR-TEM, XPS, N2 adsorption, XANES, CV, K−L plot). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *[email protected]. *[email protected].

Author Contributions

The manuscript was written through contributions of all authors.

Funding Sources 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), and “Molecular Technology” of Strategic International Collaborative Research Program (SICORP) from the Japan Science and Technology Agency (JST), and Izumi Science and Technology Foundation.

ACKNOWLEDGMENT 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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Fabrication of Îµ-Fe2N Catalytic Sites in Porous Carbons Derived from

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