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Cite This: Chem. Mater. 2019, 31, 4205−4212
Homogenized Bimetallic Catalysts from Metal−Organic Framework Alloys Jet-Sing M. Lee,† Yu-ichi Fujiwara,‡ Susumu Kitagawa,*,† and Satoshi Horike*,†,§,∥,⊥
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†
Institute for Integrated Cell-Material Sciences, 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 ‡ Functional Materials Science Research Laboratories, Research & Development Headquarters, LION Corporation, 2-1, Hirai 7-chrome, 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 ⊥ Department of Materials Science and Engineering, School of Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Rayong 21210, Thailand S Supporting Information *
ABSTRACT: Metal−organic frameworks (MOFs) are an important class of materials for the preparation of oxygen reduction reaction (ORR) catalysts but are limited in that only a single metal ion is used. By alloying MOFs of various metal ions, catalytic properties of a material could be enhanced compared to its individual components. However, traditional solution-based synthesis using two different metal ions results in heterogeneous domains of both species. We found that mechanically alloying [Fe(1,2,3-triazolate)2] with isostructural [Cu(1,2,3-triazolate)2] results in tunable, homogeneously dispersed, bimetallic MOF alloys, which when pyrolyzed, retain bimetallic homogeneity and can be used as ORR catalysts. The bimetallic catalysts synergize and show exceptional ORR performance, outperforming the benchmark Pt/C, attributed to the homogenous distribution and control of the Fe2+ and Cu2+ composition within the MOF crystals, which cannot be obtained by solution-based synthesis. The findings show that MOFs described in literature could be alloyed together and show synergistic properties that outperform the individual components.
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Mo,17 and Fe/Co18 catalysts showing excellent properties for ORR. This remains a relatively unexplored route; thus, a strategy to precisely control the Fe alloy composition with use of advanced materials may yield unique, high-performing ORR catalysts. Metal−organic frameworks (MOFs) have emerged as an exciting class of materials synthesized from metal ions and organic linkers connected by coordination bonds to produce extended networks.19 Their tunable crystal structures and properties have allowed access to many applications. Among the applications, the strategy of pyrolyzing MOF crystals is a significant approach to prepare predesigned metal-containing carbon catalysts with properties (composition, functionality, morphology) derived from the parent MOF. For example, the ORR catalyst derived from [Co(imidazolate)2] showed notable ORR catalytic behavior: an onset potential (Eonset) of up to 0.86 V, kinetic-limiting current density (jK) of ∼5 mA cm−2, and electron transfer number (n) of 3.8.20 Pyrolysis of [Fe3O(H2N-BDC)] (H2N-BDC = 2-aminoterephthalic acid), known as MIL-88B-NH2, produces a high-performing ORR
rives in society to move away from fossil fuels and toward renewable and clean energy sources has attracted immense attention. This has led to huge efforts in developing next-generation electrochemical technology because of its low environmental impact and use in many applications.1 In particular, proton exchange membrane fuel cells,2,3 direct alcohol fuel cells,4 and metal−air batteries5−7 have exciting opportunities as energy storage and conversion devices but are still not widely commercialized due to various challenges. The oxygen reduction reaction (ORR) step which occurs on the cathode is known to have sluggish kinetics, therefore, platinum group metal (PGM) catalysts are currently used to mitigate this.8 PGM catalysts are the current leading technology which show high performance toward 4e− reductions. Despite PGM catalysts being benchmark materials, key issues are their high cost, limited supply, low stability, and susceptibility to deactivation. Hence, research into high performing nonPGM-based materials that use cheaper and abundant elements is of high interest.9 Recently, ORR catalysts which use Fe−N have garnered much attention due to their high onset potential, near 4e− transfer reaction, and good stability.10−12 Notably, bimetallic catalysts with a second metal have shown improved properties for ORR compared to the pristine single-metalbased catalysts.13,14 Only in the last few years has this strategy been applied to Fe, with reports on Fe/Ni,15 Fe/Mn,16 Fe/ © 2019 American Chemical Society
Received: March 18, 2019 Revised: May 17, 2019 Published: May 22, 2019 4205
DOI: 10.1021/acs.chemmater.9b01093 Chem. Mater. 2019, 31, 4205−4212
Article
Chemistry of Materials
and direct Fe−N bonds.32 Fe2+ of Fe-Tz are octahedrally coordinated to the nitrogen atoms of 1,2,3-triazolate, which forms a framework with pentaatomic tetrahedral secondary building units with Fe2+ at the center and vertices of the tetrahedron. As a counterpart, copper-triazolate ([Cu(1,2,3triazolate)2]: Cu-Tz) was chosen as the co-alloy. They have the same space group and similar unit cell parameters,32 which is advantageous to prepare the MOF alloy. Guest-free Fe-Tz and Cu-Tz were mechanically alloyed together via ball-milling (400 rpm and 1 h) at ratios of x and y, respectively, and denoted as FeCu-x:y (Scheme 1). Powder X-ray diffraction (PXRD) patterns of pristine Fe-Tz and Cu-Tz and FeCu-x:y showed crystalline peaks, indicating long-range structural periodicity (Figure S1). As the main (101) peak at ∼9° was relatively broad and hard to differentiate by the laboratory PXRD, synchrotron PXRD measurements were performed (Figure 1). Cu-Tz has a larger cell size than Fe-Tz due to the longer metal-N coordination bond length accompanied with the Jahn−Teller effect (Figures S2 and S3). These frameworks were found to have low elastic moduli and retain their crystallinity upon milling (Figure S4). The overall peaks of FeCu-x:y closely match those of Fe-Tz. A sharp single (101) peak at ∼6° was observed with each alloy, with the peak incrementally shifting with increasing co-alloy, suggesting the production of homogenized structures (Figure 1a). Incremental shifting to higher angles rather than in between Fe-Tz and Cu-Tz suggests that all alloys retain a cell parameter similar to Fe-Tz, with the cell size decreasing with increasing Cu-Tz co-alloy because of the smaller ionic radii and labile character of the Cu ions.33−35 The (202) peak at 12° also shifts incrementally (Figure 1b). The gradual shifts of the peaks indicate that Cu2+ in FeCu-x:y are homogeneously mixed in the atomic scale. X-ray fluorescence spectroscopy (XRF) was performed to investigate the ratios of metal ions of FeCu-x:y (Table S1). Fe and Cu compositions are close (
