Research Article pubs.acs.org/acscatalysis
Nonprecious Bimetallic (Fe,Mo)−N/C Catalyst for Efficient Oxygen Reduction Reaction Ling Lin, Zheng Kun Yang, Yi-Fan Jiang, and An-Wu Xu* Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, China S Supporting Information *
ABSTRACT: Developing highly efficient catalysts for cathodes remains an important matter of the research of fuel cells. Here, we report a bimetallic iron- and molybdenum-based nitrogendoped carbon catalyst ((Fe,Mo)−N/C), acting as an efficient catalyst for oxygen reduction reaction (ORR) at the cathode. The catalyst was synthesized by pyrolysis of a complex precursor obtained through a facile ion-exchange process based on the hard−soft-acid−base (HSAB) principle. The dilution effect of Mo prevents Fe species from aggregation, leading to a high content of Fe. Besides, the synergistic catalysis effect of Fe and Mo enhances the graphitization degree of carbon, resulting in a high electrical conductivity of carbon at a relative low pyrolysis temperature (700 °C). Different initial Fe/Mo mole ratios were tested to determine the optimal catalyst. The (Fe,Mo)−N/C catalyst with Fe/Mo = 0.75 affords an excellent ORR activity, comparable to commercial Pt/C catalysts, and follows a four-electron mechanism under acidic conditions. Our present work demonstrates that both Fe−Nx and Mo−Nx can act as the active sites simultaneously. Notably, we have developed a versatile, new route toward the preparation of efficient catalysts with hierarchical porous structures for fuel cells. KEYWORDS: ion-exchange, bimetallic, nonprecious metal, low pyrolysis temperature, oxygen reduction reaction
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INTRODUCTION Global warming, environmental pollution, and energy scarcity have become the urgent problems of the 21st century. Developing clean and renewable energy technologies brooks no delay. Therefore, in recent years, plenty of efforts have focused on developing efficient catalysts for proton-exchange membrane fuel cells (PEMFCs), which offer a highly efficient technology to produce clean energy. Seeking suitable catalysts for the cathodic reaction, oxygen reduction reaction (ORR), is much more challenging, compared to the anodic reaction. Because ORR is a four-electron process and has sluggish kinetics that must overcome a high overpotential,1 even the most active platinum also has an overpotential of ∼300 mV.2 Driven by the scarcity and high cost of platinum, the development of nonprecious metal catalysts represents one of the major scientific problems. Transition-metal oxides,3,4 transition-metal nitrides/carbides,5−11 and transition-metal-based nitrogen-doped carbon materials (M−N/C)12−18 have been developed as promising alternatives for ORR. Previous studies on MoN,7 MoN2,8 and CoMoN9,10 demonstrated that molybdenum nitrides had comparatively good ORR activities and that the ORR proceeds through an almost-four-electron process, indicating that molybdenum is a promising nonprecious metal candidate for ORR catalyst. However, the research on Mo-containing catalysts has mainly focused on molybdenum nitrides and © XXXX American Chemical Society
carbides at present. M−N/C catalysts, which are the most studied nonprecious catalysts, are usually synthesized by pyrolysis of nitrogen-containing organic precursor, transitionmetal precursors, and carbon support in inert gas. Transition metals, such as Fe,12−14 Co,15,16 Mn,17 and Cu,18 are used to synthesize M−N/C catalysts and Fe- or Co-based catalysts have been proved to have the best performance among this type of catalyst,19 and are considered to be very promising ORR catalysts in PEMFCs, because of their facile preparation, lowcost, high activity, and corrosion resistance. However, the Mo− N/C catalyst has not been reported previously. Furthermore, previous reports suggest that some heat-treated M−N/C catalysts containing Fe and Co atoms show better catalytic activity and stability than that containing only a single metal.12,20,21 Keeping these points in mind, we aim to develop an Fe- and Mo-containing nitrogen-doped carbon catalyst ((Fe,Mo)−N/C) with desired ORR performance. In this work, a new type of (Fe,Mo)−N/C catalyst for ORR is reported. We develop a simple ion-exchange method to prepare (Fe,Mo)−N/C catalysts based on hard−soft acid−base (HSAB) theory, which is widely used for explaining and predicting stability of compounds and reaction mechanisms and Received: February 22, 2016 Revised: May 16, 2016
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DOI: 10.1021/acscatal.6b00535 ACS Catal. 2016, 6, 4449−4454
Research Article
ACS Catalysis
Figure 1. (a) Schematic diagram of of synthetic process of (Fe,Mo)−N/C catalysts. [Legend: 1, bidppz; 2, Mo-bidppz precursor; and 3, (Fe,Mo)bidppz precursor. Gray balls are C atoms, blue balls are N atoms, yellow balls represent the Mo atom, and green balls represent Fe atoms.] (b) XRD patterns of (Fe,Mo)−N/C catalysts before acid leaching. (c) XRD patterns of (Fe,Mo)−N/C catalysts after acid leaching.
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pathways.22 Hard acids and bases have a tendency to possess a high charge, a small ionic radius, and weak polarizability, while soft species usually have a low charge, a large ionic radius, and strong polarizability. Acids and bases that have characteristics between hard and soft are called “borderline”. Ordinarily, acids and bases interact to form compounds and the most stable compounds are formed by hard−hard and soft−soft interactions. (Fe,Mo)−N/C catalysts were synthesized by heat-treatment of a complex precursor containing Fe and Mo atoms. The precursor was prepared through coordinating Mo(V) with 11′-bis(dipyrido[3,2-a:2′,3′-c]phenazine (bidppz), an conjugated organic ligand with pyridine nitrogen atom as the ligating atom, to get bidppz coordination compound (Mobidppz), followed by an ion-exchange process using Fe(II) to replace Mo(IV). Mo5+ has a high oxidation state of 5, and the ionic radius of Mo5+ is 0.61 Å;23 thus, Mo5+ can be classified as a hard acid. According to the literature, Fe2+ is a borderline acid and pyridine is a borderline base.22 Pyridine is too soft for the rather hard Mo5+, so Fe2+ can replace Mo5+ in Mo-bidppz complex to produce a (Fe,Mo)-bidppz precursor. Since Fe and Mo atoms coexist in a catalyst, the ratio of iron to molybdenum is an important factor that would influence the ORR activity of a (Fe,Mo)−N/C catalyst. Therefore, different initial mole ratios of the reactants (FeCl2 and MoCl5) are applied to the catalyst preparation. The best ORR performance in acidic media achieved on (Fe,Mo)−N/C catalyst is comparable to that of commercial 20% Pt/C catalyst with an onset potential of 845 mV. The catalytic mechanism has been determined by the Koutecky−Levich equation,24 and the results indicate a fourelectron transfer pathway from O2 to H2O. Durability and methanol tolerance ability are also evaluated to prove that our newly developed (Fe,Mo)−N/C catalyst is stable enough to operate in the actual environments of a PEMFC or a direct methanol fuel cell (DMFC).
RESULTS AND DISCUSSION We have synthesized a new type of (Fe,Mo)−N/C catalyst containing two different metalsFe and Movia a simple ionexchange process; the flow diagram of fabricating the (Fe,Mo)−N/C catalyst is shown in Figure 1a. In this approach, Mo(V) was first coordinated with 11′-bis(dipyrido[3,2-a:2′,3′c]phenazine (bidppz) ligand through solvothermal method in dimethylformamide (DMF) to obtain the Mo-bidppz precursor. According to HSAB theory, Fe2+ could replace Mo5+ to form the (Fe,Mo)-bidppz precursor with random Fe2+ and Mo5+ coordinated with bidppz. Different initial Fe/Mo molar ratios (0.25, 0.5, 0.75, 1.0, and 1.25) were used during the synthesis of the precursor, and a series of (Fe,Mo)-bidppz precursors with an ionomer-like complex structure were obtained. Afterward, the (Fe,Mo)-bidppz precursors were calcined at 700 °C for 1.5 h in N2 and leached in 6 M hydrochloric acid (HCl) to obtain self-supporting (Fe,Mo)−N/C catalysts with different Fe/Mo ratios, labeled as (Fe,Mo)−N/C-1 to (Fe,Mo)−N/C-5, respectively. Our previous work showed that Fe2+ ions are easily to aggregate and produce iron particles, which is unfavorable for ORR activity.25 In this study, Fe2+ ions enter into (Fe,Mo)-bidppz precursor through ion exchange, Mo5+ ions may dilute Fe2+ ions and prevent iron from aggregation. For comparison, M−N/C catalysts containing only Mo or Fe were also synthesized. The X-ray photoelectron spectroscopy (XPS) was used to determine the elemental compositions of Mo−N/C, Fe−N/C, and (Fe,Mo)−N/C catalysts (Table S1 in the Supporting Information). The coexistence of Fe and Mo in all acid-leached (Fe,Mo)−N/C samples indicates that the ion exchange reaction is successful. It is worth noting that the content of Mo reduces gradually while the Fe content increases and then declines with increases in the amount of FeCl2 added. To understand the reason for the decrease of Fe content, the X-ray diffraction (XRD) analysis was performed. The XRD patterns 4450
DOI: 10.1021/acscatal.6b00535 ACS Catal. 2016, 6, 4449−4454
Research Article
ACS Catalysis
Figure 2. XPS spectra of (Fe,Mo)−N/C catalysts: (a) the survey scan, (b) the N 1s spectrum, (c) the Fe 2p spectrum, and (d) the Mo 3d spectrum of (the Fe,Mo)−N/C-3 catalyst.
C catalysts. The ratios of the D-band to G-band integrated intensities (ID/IG) are 3.17, 2.75, 2.20, 2.28, and 2.18 from (Fe,Mo)−N/C-1 to (Fe,Mo)−N/C-5, while the ID/IG ratios of Mo−N/C and Fe−N/C are 3.51 and 3.44, respectively (see Figure S5 in the Supporting Information). Raman spectra indicate that the coexistence of Fe and Mo leads to a higher degree of graphitization of the carbon support in (Fe,Mo)−N/ C catalysts. Based on chemical vapor deposition (CVD) growth for synthesizing carbon nanotubes, which is a procedure that converts carbon into graphitic layers, Fe−Mo was reported to be one of the best catalysts for the graphitization of carbon.27,28 Therefore, we propose that synergistic effect of Mo and Fe on catalyzing the graphitization of carbon in our (Fe,Mo)−N/C catalysts enhances the degree of graphitization of the carbon support and, subsequently, increases the electric conductivity of (Fe,Mo)−N/C catalysts, to some extent. Brunauer−Emmett− Teller (BET) specific surface area analysis reveals that the surface area of the Mo-bidppz precursor, (Fe,Mo)-bidppz precursor, and a representative (Fe,Mo)−N/C-3 catalyst are 45 m2 g−1, 50 m2 g−1, and 47 m2 g−1, respectively (Figure S6 in the Supporting Information), showing no significant difference between the precursors and final product. The Barrett− Joyner−Halenda (BJH) pore volume distribution curve in Figure S6d indicates that (Fe,Mo)−N/C-3 catalyst contains mesopores 21.1 nm in size and macropores with different diameters. Mesopores in carbon was verified to provide an accessible surface for charge transport and improve the electron transfer efficiency of the catalyst,29 while the macropores could shorten the diffusion distances of electrolyte to the interior surfaces, just like an in situ buffer reservoir.30 XPS survey scan analyses were used to provide an overview of the surface elemental imformation of (Fe,Mo)−N/C-3, and high-resolution XPS spectra of N 1s, Fe 2p, and Mo 3d provide more information on the valence state of these elements. The survey scan (Figure 2a) shows that (Fe,Mo)−N/C-3 catalyst contains five elements, including carbon (85.28%), nitrogen (12.15%), oxygen (1.55%), iron (0.43%), and molybdenum
of (Fe,Mo)−N/C catalysts before acid leaching showed in Figure 1b show the emergence of three peaks at 44°, 45°, and 51° when the initial FeCl2/MoCl5 molar ratio reaches 1.0, and with further increases in the molar ratio, the peaks become more obvious, indicating the formation of iron particles, similar to that observed in Fe−N/C catalyst (Figure S1a in the Supporting Information).26 In contrast, the XRD pattern of the Mo−N/C catalyst (Figure S1b in the Supporting Information) shows no Mo species, demonstrating that Mo ions do not aggregate into metallic particles under our pyrolysis conditions. The transmission electron microscopy (TEM) images (Figure S2 in the Supporting Information) of (Fe,Mo)−N/C samples before acid leaching show some metallic particles, even when the Fe/Mo ratio is
