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Nitrogen-coordinated single iron atom catalysts derived from metal organic frameworks for oxygen reduction reaction
Nano Energy 61 (2019) 60–68
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Nitrogen-coordinated single iron atom catalysts derived from metal organic frameworks for oxygen reduction reaction
T
Fei Xiaoa,1, Gui-Liang Xub,1, Cheng-Jun Sunc, Mingjie Xud,e, Wen Wenf, Qi Wangg, Meng Gug, Shangqian Zhua, Yueying Lih, Zidong Weii, Xiaoqing Pane, Jiangan Wanga,h, Khalil Amineb,j,∗, Minhua Shaoa,d,∗ a
Department of Chemical and Biological Engineering, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL, 60439, USA c X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439, United States d Fok Ying Tung Research Institute, Hong Kong University of Science and Technology, Guangzhou, 511458, PR China e Department of Chemical Engineering and Materials Science, University of California, Irvine, Irvine, CA, 92697, United States f Shanghai Synchrotron Radiation Facility, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Zhangjiang High Tech. Park, 201204, PR China g Department of Materials Science and Engineering, South University of Science and Technology of China, No.1088, Xueyuan Road, Shenzhen, Guangdong, 518055, China h State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials Science and Engineering, Northwestern Polytechnical University and Shaanxi Joint Laboratory of Graphene (NPU), Xi'an, 710072, China i College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 400044, China j Materials Science and Engineering, Stanford University, Stanford, CA, 94305, United States b
ARTICLE INFO
ABSTRACT
Keywords: Oxygen reduction reaction Metal organic framework Single-atom catalyst Non-precious metal catalysts
Iron and nitrogen co-doped carbon (Fe-N-C) catalysts hold great promise to replace platinum group metal used for the oxygen reduction reaction (ORR) in low-temperature fuel cells. However, general synthesis routes require tedious acid washing and extensive heat treatment, usually resulting in uncontrollable morphologies and undesirable compounds. In this work, a zeolitic imidazolate framework (ZIF-8) was employed as a self-template for one-pot synthesis of a Fe-N-C catalyst consisting of uniformly dispersed Fe single atoms. Atomically dispersed Fe atoms were well distributed along the edges of the porous carbon matrix. Each of the Fe atoms was coordinated with four N atoms in the plane and two O atoms in the axial direction. The optimized Fe-N-C catalyst showed excellent ORR activities with half-wave potentials of 0.81 and 0.90 V in acidic and alkaline solutions, respectively. The results may be important for the optimization of single-atom-based catalysts for various reactions.
1. Introduction Oxygen reduction reaction (ORR) electrocatalysts are commonly based on a platinum group metal (PGM) [1,2]. However, high cost due to the limited reserves impedes the wide adoption of PGMs. Over the past decades, great efforts have been made in the search for more costeffective materials. Since the discovery of the transition metal phthalocyanine complex in 1964 as an alternative ORR catalyst, non-PGM materials with M-N-C (M = transition metals) coordinated structures have been under development to reduce the material cost [3]. Among all M-N-C structures studied, Fe-N-C has attracted the greatest attention due to its high ORR activity [4,5]. However, a big gap in the catalytic performance still exists between Fe-N-C catalysts and PGM materials due to the limited active sites in the former. Thus, creating a higher
density of N-coordinated single Fe atoms is among the primary approaches to promote the performance of Fe-N-C catalysts. Fe-N-C catalysts can be synthesized by pyrolyzing various salts and polymer precursors [6–8]. However, the direct calcination of the mixture usually results in unexpected aggregated metallic compounds in the carbon shells. Acid washing followed by heat treatment is required to remove these inactive species and recover the damaged carbon structures. A template-assisted synthesis process can alleviate the aggregation of Fe atoms at high temperatures. A thin SiO2 layer [5], sodium salt [9], and nano-MgO [10] templates have been used during the pyrolysis to achieve a homogeneous distribution of Fe-N-C active sites. These significantly improved ORR activities as compared to the ones obtained through direct pyrolysis. However, removing templates by strong acid solutions (e.g., HF, HNO3) is undesirable in large-scale
Corresponding authors. Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL, 60439, USA.; E-mail addresses: [email protected] (K. Amine), [email protected] (M. Shao). 1 Equal contribution. ∗
https://doi.org/10.1016/j.nanoen.2019.04.033 Received 7 January 2019; Received in revised form 26 March 2019; Accepted 6 April 2019 Available online 11 April 2019 2211-2855/ © 2019 Elsevier Ltd. All rights reserved.
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production. Thus, developing novel templates that can be easily removed is still highly desired. A metal organic framework (MOF) can potentially serve as the template because of its diverse and tuneable functionalities by changing the metal nodes and organic ligands [11]. The zeolitic imidazolate framework (ZIF-8) is a commonly used template to dope Fe in the precursor and synthesize N-coordinated single Fe atoms. Zinc (Zn) nodes in ZIF-8 act as a “fence” to expand the spatial interval between two Fe atoms [12], and the sublimation of Zn above 907 °C contributes to a porous structure. The first step in the development of a new catalyst based on single atoms for ORR is to study the optimum parameters for its synthesis. Previous studies have found that the heat treatment conditions like temperature and atmosphere were pivotal to the successful generation of active sites [13]. Conventional Fe-N-C electrocatalysts derived from mixing iron salts, like iron porphyrin, with carbon black have been systematically studied by the Dodelet group. Their results demonstrated the challenge in achieving high activity and stability at the same time [14]. Besides, for thermally activated catalysts, the heat treatment sequence would also negatively affect the relationship between the formation of pores and active sites and their influence on ORR performance [15,16]. However, the impact of these parameters is not well understood for single-atom-doped Fe-N-C catalysts. For Fe-N-C synthesized from pyrolyzing a Fe-ZIF-8 precursor, the final carbon structure is determined by diffusivities of metals governed by the Kirkendall effect [17,18] and cavity diffusivity affected by gas diffusion. It is important to balance these two diffusivities by optimizing the synthesis conditions. In addition, because the Fe is surrounded with excess N ligands from the organic in the precursor [19], whether increasing the Ndoping content by secondary ammonia heat treatment will further promote the ORR is still an open question. With these considerations in mind, we have systematically investigated synthesis conditions for FeN-C. The catalyst obtained with optimized synthesis conditions showed superior ORR activities and stability in both acid and alkaline solutions. The half-wave potentials reached 0.81 and 0.90 V in acidic and alkaline solutions, respectively, with a catalyst loading of 0.56 mg cm−2.
2.2. Materials characterization The structures of the samples were examined by an X-ray diffractometer (XRD) equipped with a graphite monochromator and a CuKα radiation source. Morphologies were recorded with scanning electron microscopy (SEM, JEOL 6700) and transmission electron microscopy (TEM, JEM 2010F). X-ray photoelectron spectroscopy (XPS) was applied to determine surface composition. High-resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) with bright field (BF), high angle annular dark field (HAADF), and secondary electron imaging (SEI) were performed with an aberration-corrected JEOL JEM-ARM300CF S/STEM microscope. The SEI can reveal topographical information of a carbon-nitrogen matrix at a much higher resolution than conventional SEM, and HAADF Z-contrast is capable of distinguishing heavy Fe elements from carbon and nitrogen. The distance of the single Fe atoms can be easily resolved due to the high spatial resolution in the STEM mode (82 p.m. @ 300 kV). Fe-N-C mapping was conducted by using the Titan Themis (Double Cs-Correcter)-300 kV STEM mode. The surface area and pore size distribution of the catalysts were measured by the BrunauerEmmett-Teller (BET) method. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) characterizations were conducted at the Advanced Photon Source (APS) of Argonne National Laboratory (ANL). The fluorescence mode was applied to obtain the Fe signal in the sample [21]. Data reduction, analysis, and EXAFS fitting were processed with Athena and Artemis software [22,23]. 2.3. Electrochemical measurements The ORR activities were measured in a typical three-electrode system controlled by an electrochemical workstation (CHI 760). A piece of carbon paper was used as the counter electrode. Ag/AgCl and Hg/ HgO reference electrodes were applied in acidic and alkaline electrolytes, respectively. All potentials were calibrated with a reversible hydrogen electrode (RHE). The rotating disk electrode (RDE) technique was applied to measure the activity and stability. It is worth noting that the kinetic currents in this study were not IR compensation. To that end, 5.4 mg of final Fe-N-C catalysts was ultrasonically dispersed in 400 μL deionized water, 100 μL isopropanol (99.7%, VWR, UK), and 10 μL 5% Nafion. Then, 10 μL uniform ink was dropped on a glassy carbon RDE tip (diameter: 5 mm) and dried in air. For comparison, a 2.5 mg Pt/C (28.2%, TEC10E30E) catalyst was uniformly dispersed in 2 ml water, 0.5 ml isopropanol, and 10 μL 5% Nafion, then 10 μL ink was deposited on the same RDE tip and dried in air. Cyclic voltammograms (CVs) were obtained in Ar-saturated 0.1 M HClO4 and 0.1 M KOH solutions at a scan rate of 50 mV s−1 from 0 V to 1.2 V. To minimize the effect due to non-Faraday current, ORR activities were evaluated by steady-state polarization in O2-saturated 0.1 M HClO4 and 0.1 M KOH solutions with the potential width and duration of 25 mV and 30 s with a rotation speed of 1600 rpm. A stability measurement was performed in an O2-saturated acidic solution from 0.6 V to 1.0 V with a scan rate of 50 mV s−1. The electron transfer number from the RDE test was calculated from the Koutecky-Levich equation:
2. Experimental 2.1. Fe-N-C catalyst preparation The Fe-ZIF-8 precursors were synthesized by following the method in a previous report [20] to choose Fe2+ as the metal sources, except for removing surfactant and changing solvents to simplify the process. In a typical process for preparing Fe-ZIF-8 with a 0.95/0.05 molar ratio of Zn/Fe, 8 g 2-methylimidazole (98%, Aladdin, China) was dissolved in 100 ml methanol (99.9%, Merck, USA) in a flask. Then, 282.6 mg Zn (NO)3·6H2O (99.998% metals basis, Aladdin, China) and 14 mg FeSO4·6H2O (99.95% metals basis, Aladdin, China) were dissolved in 100 ml methanol in another flask. The two solutions were mixed uniformly after bubbling in Ar for 1 h. Then, the mixture was stirred under the protection of Ar for 10 h. The suspension was collected by centrifugation and then washed with absolute ethanol several times. The product was dried at 80 °C in a vacuum oven. For Fe-ZIF-8 precursors having different Zn/Fe molar ratios (0.97/ 0.03, 0.92/0.08 and 0.90/0.10), the experimental procedures were the same except for changing the amount of Zn (NO)3·6H2O and FeSO4·6H2O accordingly. A ZIF-8 precursor without Fe atoms was also synthesized by using 1.00 nmol Zn (NO)3·6H2O without adding FeSO4·6H2O. The Fe-N-C catalysts were prepared through a programmed pyrolysis in different atmospheres (Ar, N2,7%H2/Ar, and Ar + NH3). The heating rate was 5 °C per min and stayed at 1000 °C for 1 h (Ar, N2, and 7% H2/Ar). Secondary NH3 pyrolysis was applied with the same heating rate and kept at 900 °C for 15 min. Then, it was cooled naturally under the protection of the same atmosphere.
1 1 1 1 1 = + = + J Jk JL B 1/2 Jk where ω is the rotation rate applied in the test. Current density (j) and kinetic current density (jk) were determined from the steady-state polarization curves. The constant B was calculated from.where the diffusion coefficient of O2 (DO2) is 1.93 × 10−5 cm2 s−1; the kinetic viscosity ν equals 1.009 × 10−2 cm2 s−1; the concentration of O2 in electrolyte (CO2) is 1.26 × 10−3 mol L−1 [24,25]; and F and A are the Faraday constant and the electrode's geometric area (0.196 cm2), 61
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respectively. The rotating ring disk electrode (RRDE) test was applied to measure the hydrogen peroxide yield (H2O2%) and the electron transfer number (n). During the test, polarization curves were recorded by scanning the disk electrode with a rate of 1 mV s−1 in the voltage range of 1.0–0.125 V, where the ring electrode voltage was 1.20 V. Results were determined by the following equations:
H2 O2 (%) =
n=
200Ir Id N + Ir
4Id N Id N + Ir
Disk current (Id) and ring current (Ir) can be determined from polarization curves directly. The collection efficiency (N) equals 0.42. All the electrochemical measurements were done at room temperature. 3. Results and discussion 3.1. Synthesis of ZIF-8 and Fe-ZIF-8 precursors The ZIF-8 and Fe-ZIF-8 crystals with controllable morphologies were synthesized via a co-precipitation method. The Fe doping level was controlled by varying the Zn/Fe ratios. The sample with a Zn/Fe ratio of 0.95/0.05 was selected for further discussion. Both precursors with or without Fe doping had dodecahedral shapes, as shown in Fig. 1a and b, respectively. The particle size of the ZIF-8 precursor (length: ca. 50 nm) was smaller than that of the Fe-ZIF-8 precursor (length: ca. 80 nm), which could be attributed to more Zn being used in the former and resulting in more nucleation sites for crystallization [26]. The XRD results for ZIF-8 and Fe-ZIF-8 precursors presented in Fig. 1c imply similar crystal structures. After pyrolysis in Ar atmosphere at 1000 °C for 1 h, the observable peaks at 25° and 44° for both XRD patterns were assigned to the (002) and (101) planes of graphitic carbon (Fig. 1d). In addition, no peak related to metallic particles or metal carbides was detected. The ZIF-8 template not only prevented the formation of inactive species and simplified the synthesizing process, but maintained the morphologies of the precursors, as indicated by the TEM images in Fig. 2. Even though the dodecahedral shape was better maintained in NC (Fig. 2a) than in Fe-N-C (Fig. 2b), the homogeneously dispersed pores
Fig. 2. TEM images for (a) N-C and (b,c) Fe-N-C catalysts, and secondary electron images for (d) Fe-N-C catalyst. The Zn/Fe ratio in the precursor is 0.95/ 0.05.
in the well-defined framework resulted in a similar specific surface area for both samples (∼1090 m2 g−1). The main pores in the catalysts were mesoporous according to the BET measurements (Table S1 and Figure S1). The graphite layers can be clearly identified in the Fe-N-C catalyst shown in Fig. 2c. Topographical information for the Fe-N-C catalyst, which was explored by the SEI technique (Fig. 2d), shows a uniform particle size but rough surface. This also explains the high surface area of the catalyst. 3.2. Composition and atmosphere effects The poor ORR activity of Fe-N-C is commonly related to a limited number of active sites, which can potentially be resolved by increasing the Fe content. However, inactive metal clusters or even metallic nanoparticles are formed when excessive Fe is incorporated. Thus, optimizing Fe doping and synthesis conditions is important to the performance of the final catalysts. The effect of peak temperature on the final catalyst has been well studied previously. It was found that the inactive Zn species cannot be sublimated at too low temperature, while metal particles and carbides can be formed at too high temperature [21,27]. Thus, we focused on the effect of different protection atmospheres (Ar, N2, and 7% H2/Ar) in this study. Only carbon-related peaks were detected by XRD on samples with different Zn/Fe ratios (0.97/0.03, 0.92/ 0.08, and 0.90/0.10) or heat treatment atmospheres (Figure S2), indicating that the changes in these synthesis conditions did not produce detectable Fe particles or metal carbides. The SEM (Figure S3) and TEM (Figure S4) images confirmed the successful formation of well-defined porous matrixes in all samples regardless of the Zn/Fe ratio and the atmosphere. However, when the Fe doping level increased to 0.08 or higher, some clusters were observed in the framework (indicated by red circles in Figure S4c and e). Thus, a small amount of Fe doping and different heat treatment atmosphere would not change the morphologies of the precursors and the final products. However, the pore volume and surface area would change under different heat treatment conditions according to the BET measurements, as shown in Table S1. For the ZIF-8 framework, which already has cavities due to its unique structure [28], the distribution of pores should depend on the evaporation of Zn along with Fe movement and the resulting collapse of cavities during
Fig. 1. TEM images of (a) ZIF-8 and (b) Fe-ZIF-8 precursors; XRD patterns of (c) (Fe)-ZIF-8 and (d) (Fe)-N-C catalysts. The Zn/Fe ratio in the precursor is 0.95/ 0.05. 62
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Fig. 3. Steady-state polarization curves of (a) ORR catalysts derived from N-C, Pt/C, and Fe (Zn/Fe = 0.95/0.05)-N-C catalysts in Ar recorded in an O2-saturated 0.1 M HClO4 electrolyte, (b) different Zn/Fe compositions (0.97/0.03, 0.95/0.05, 0.92/ 0.08, and 0.90/0.10) in an O2-saturated 0.1 M HClO4 electrolyte, (c) different Zn/Fe compositions (0.97/0.03, 0.95/0.05, 0.92/ 0.08, 0.90/0.10) and Pt/C in an O2-saturated 0.1 M KOH electrolyte, and (d) Fe (Zn/Fe = 0.95/0.05)-N-C catalyst in Ar after different CV cycles in an O2-saturated 0.1 M HClO4 electrolyte. All tests were carried out on RDE with 1600 rpm rotating rate. The catalyst loading was 0.56 mg cm−2 for the Fe-N-C samples and the N-C sample and 0.051 mg cm−2 for the Pt/C sample (TKK, 28.5 wt%).
heat treatment. By comparison, the Fe (Zn/Fe=0.95/0.05)-N-C catalyst in Ar environment had the highest pore volume and pore surface area due to a better balance of relative diffusivities.
[30]. Thus, in the N2 atmosphere, the original pores formed in the precursor start to diffuse at an early stage. When the temperature increases, Zn starts to evaporate from the structure, which may result in the shrinkage of the original pores. This effect may also explain the poor stability of the N2-treated sample during the ORR. CVs of all the Fe-N-C catalysts showed a pair of well-developed redox peaks at 0.7–0.9 V, which were assigned to the redox couple of Fe3+/Fe2+ in an O2-saturated 0.1 M HClO4 electrolyte (Figure S6a, b). However, the CVs were virtually featureless in an O2-saturated 0.1 M KOH electrolyte (Figure S6c, d). When the doping level of Fe was increased, the redox peaks became more obvious, indicating more Fe species on the surfaces. In contrast to conventional Fe-N-C mixed with carbon black or Fe-N-C catalysts synthesized from ZIF-8 by ball milling with Fe sources [14,31], the secondary NH3 heat treatment did not promote the performance of the Fe-N-C catalyst (Figure S7a), which may be relative to the different environment of Fe in the precursor. Because of rich N ligands around Fe, solely increasing N doping in the carbon framework will not promote ORR performance [19]. Besides, NH3 will etch carbon HCN + H2 and C+ 2H2 CH4 according to the reactions C+ NH3 [14], damaging the pore structure formed in the first treatment. This effect is indicated by the near absence of Fe2+/Fe3+ redox peaks and a smaller double layer in the CV curve shown in Figure S7b. Next, we focus on the characterization and evaluation of the sample with the highest ORR activity: Fe (Zn/Fe = 0.95/0.05)-N-C in Ar, denoted as Fe-N-C. Durability testing was investigated in an O2-saturated 0.1 M HClO4 solution at a potential range of 0.6–1.0 V and rate of 50 mV s−1. Its half-wave potential was only lowered by 16 mV after 10,000 cycles (Fig. 3d). The activity decay may be due to Fe oxidation or dissolution as proposed previously [32]. It is worth noting that the stability of this catalyst is comparable to or better than the best ones reported in the literature [7,20,21]. The polarization curves recorded at different rotation rates in an anodic sweep of the catalyst in an O2saturated 0.1 M HClO4 solution are shown in Figure S8a. Figure S8b presents the inverse current density (j−1) as a function of the square root of the rotation rate (ω −1/2). Constant B calculated from the slope
3.3. Electrochemical evaluation The ORR activities of all samples were measured in O2-saturated 0.1 M HClO4 and 0.1 M KOH solutions. Among all the samples, the Fe (Zn/Fe = 0.95/0.05)-N-C catalyst in Ar environment exhibited the best performance in both acidic and alkaline electrolytes. An onset potential (Eonset) of 0.95 V and half-wave potential (E1/2) of 0.81 V were observed in the 0.1 M HClO4 solution. The activity in the 0.1 M KOH solution is much higher, with an Eonset above 1.0 V and E1/2 of 0.90 V, which are superior to those of commercial Pt/C (E1/2 = 0.875 V). Fig. 3 presents the steady-state polarization plots as a function of the Fe content in the precursors. As shown in Fig. 3a, the N-C catalyst without Fe doping exhibited a very poor ORR activity in acidic solution. The large discrepancy in activities between N-C and Fe-N-C catalysts suggested that Fe played an essential role in forming active sites. The optimal Zn/Fe ratio was 0.95/0.05 in the precursor for both acidic and alkaline solutions (as shown in Fig. 3b and c). Thus, the volcano plot of ORR activity with respect to Zn/Fe ratio indicated that a low Fe doping amount would result in a limited density of active sites, while agglomeration and formation of an unfavorable carbon matrix might occur with a high Fe doping content. With the optimized composition, the impacts of the pyrolysis atmosphere on the ORR activity are compared in Figure S5. Surprisingly, the activities of the catalysts treated in different atmospheres presented in the following order: N2 < 7% H2/Ar < Ar. According to Fick's law, N2 (28 g mol−1) and 7% H2 (2 g mol−1)/Ar gases have smaller mass than Ar (40 g mol−1), which resulted in quicker heat transfer and higher metal diffusion rates in the former two environments, which may increase the possibility of Fe aggregation [29]. Besides, the pore diffusivity has a strong inverse relationship with the kinetic diameter of the gas. The kinetic diameter of N2 (3.64 Å) is smaller than Ar (3.70 Å) 63
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Fig. 4. HAADF-STEM images of Fe-N-C catalyst in different areas (a,c,d). Corresponding intensity profiles (b) obtained on the zoomed areas in panel a. The Zn/Fe ratio in the precursor is 0.95/0.05.
Fig. 5. XPS spectrum for (a) N 1s and (b) Fe 2p in the Fe-N-C catalyst. The Zn/Fe ratio in the precursor is 0.95/0.05.
of the Koutecky-Levich plot is 0.093 mA s−1/2, which indicates a fourelectron reduction of oxygen [33]. The RRDE results presented in Figure S8c and d match well with the above RDE test results, the H2O2 yields always being below 2% from 0.6 to 0.9 V and electron transfer number being close to 4.
0.296 to 0.404 nm. The HAADF-STEM mapping images of Fe-N-C (Figure S9 and Figure S10) revealed that C, Fe, and N were distributed homogeneously in the carbon particle. Furthermore, the deconvoluted XPS spectrum of N 1s for the Fe-N-C catalyst (Fig. 5a) exhibits four major components, including three types of N: pyridinic-N (398.2 eV), graphitic-N (401.0 eV), and oxidized-N (404 eV). No pyroly-N was observed as it is unstable when the annealing temperature is above 700 °C [13]. Pyridinic-N and graphite-N are the main active sites in the N-doped carbon materials in acidic and alkaline conditions, respectively [35]. Pyridinic-N locates at the edge of graphite, creating Lewis basic sites that are active for ORR under the acidic conditions [36]. Interestingly, Fe-N bonding around 399.2 eV was also observed [37,38], indicating that single Fe atoms have a strong interaction with the neighboring doped N. Majority of the Fe atoms are in the 2 + form, with a small portion of 3+, as shown in Fig. 5b. To gain detailed structural and electronic information of Fe-N-C at
3.4. Structure properties The presence of Fe single atoms was confirmed by HAADF-STEM images (Fig. 4). The Fe atoms (bright dots) were uniformly distributed along the edge of the carbon matrix, which had been proved to be efficient active sites that promote the ORR [34]. Two selected areas from Fig. 4a in the summed through-focal HAADF images show different intensities at specific focal depths and were analyzed to determine the relative positions along the electron beam direction. As shown in Fig. 4b, the distance between two the closest Fe atoms ranges from 64
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Fig. 6. Comparison of XAS spectra of Fe-NC with Fe based references (red line for FeN-C catalysts, blue line for FeO, and black line for Fe2O3): (a) XANES spectra of Fe Kedge where the spectra are k2 weighted and (b) Fourier transforms for EXAFS spectra of Fe K-edge where the spectra are k3 weighted (a clear shift is indicated by a dashed blue line).
an atomic level, XANES and EXAFS data were recorded. Fig. 6a presents the ex-situ XANES spectra for Fe-N-C along with FeO and Fe2O3 for comparison. The position of the absorption edge is relative to the valence of the absorbing atom [39]. It moves to higher energy as the oxidation state of the atom increases. After high temperature pyrolysis, the Fe oxidation state is between 2 + and 3+, as evidenced from its edge position being between FeO and Fe2O3. This finding is consistent with the XPS data in Fig. 5b and qualitatively agrees with previous reports. A weak pre-edge peak at 7112 eV for Fe-N-C was assigned to the 1s → 3d transition [40]. Although this transition is electric dipole forbidden, the peak intensity can be gained from an electric quadrupole coupling mechanism [41]. The positions and intensities of the pre-edge peaks are directly related to sub-bands from 3d orbital splitting and the local coordination geometry of the metal atom, respectively. The Fe2O3 reference possesses C3V symmetry along with two sets of three Fe-O bonds of 1.96 and 2.08 Å to form the structure of octahedral Fe sites [42], similar to Fe phthalocyanine [43,44], which also has the same pre-peak at 7112 eV. Thus, the pre-peak suggests the formation of Fe-N and Fe-O bonds in the Fe-N-C catalyst. Because XANES is dependent on structural and electronic effects, the differences in the intensity of preedge peak are attributed to the differences in the particle size [45]. The Fourier transformation of the Fe K-edge EXAFS of Fe-N-C is shown in Fig. 6b and compared with that of Fe2O3. The peaks at 1.4 Å and 2.5 Å in the Fe2O3 reference stand for Fe-O and Fe-Fe bonds [46–49], respectively. A clear shift of the first shell bond length from 1.4 Å to 1.5 Å suggests that the Fe center is not solely coordinated with O atoms. Besides, there is no clear Fe−Fe bond feature found in the Fe-N-C catalysts in Fig. 6b. To gain further quantitative structural parameters of Fe in Fe-N-C catalysts, EXAFS fitting based on the first strong peak was carried out, and the results are shown in Figure S11. The coordination number of Fe and the mean bond length were found to be 6.0 ± 0.8 and 2.04 ± 0.04 Å, respectively. Based on the fitting data, we propose that isolated Fe atoms sited in the nitrogen-doped porous carbon framework by bonding with four N atoms in the plane and two O atoms in the axial direction. This configuration is consistent with other EXAFS measurements and density functional theory calculations [19,46,50–52].
alkaline solutions, respectively. The EXAFS data suggested that a single Fe atom might be coordinated with four N atoms in the plane and two O atoms in the axial direction. This work indicated that Fe-N-C with abundant Fe single atoms has considerable potential in shortening the gap between M-N-C (M = transition metal) and Pt-based catalysts. The results may be important for the optimization of single-atom-based catalysts for various reactions. Acknowledgements This work was supported by the National Key R&D Program of China (No 2017YFB0102900), the Research Grant Council (N_HKUST610/17) of the Hong Kong Special Administrative Region, the Shenzhen Science and Technology Innovation Commission (JCYJ20180507183818040), Guangdong Special Fund for Science and Technology Development (Hong Kong Technology Cooperation Funding Scheme (201704030019 and 201704030065)). G. Xu and K. Amine gratefully acknowledges support from the U. S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office. Argonne National Laboratory is operated by UChicago Argonne, LLC, for the U.S. Department of Energy under contract DE-AC02-06CH11357. This research used resources of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, and was supported by the U.S. DOE under Contract No. DE-AC0-06CH11357, and the Canadian Light Source and its funding partners. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.nanoen.2019.04.033. References [1] M. Shao, Q. Chang, J.-P. Dodelet, R. Chenitz, Recent advances in electrocatalysts for oxygen reduction reaction, Chem. Rev. 116 (2016) 3594–3657. [2] S.-I. Choi, S. Xie, M. Shao, J.H. Odell, N. Lu, H.-C. Peng, L. Protsailo, S. Guerrero, J. Park, X. Xia, Synthesis and characterization of 9 nm Pt–Ni octahedra with a record high activity of 3.3 A/mgPt for the oxygen reduction reaction, Nano Lett. 13 (2013) 3420–3425. [3] R. Jasinski, A new fuel cell cathode catalyst, Nature 201 (1964) 1212. [4] X. Yan, K. Liu, T. Wang, Y. You, J. Liu, P. Wang, X. Pan, G. Wang, J. Luo, J. Zhu, Atomic interpretation of high activity on transition metal and nitrogen-doped carbon nanofibers for catalyzing oxygen reduction, J. Mater. Chem. A. 5 (2017) 3336–3345. [5] Y.J. Sa, D.-J. Seo, J. Woo, J.T. Lim, J.Y. Cheon, S.Y. Yang, J.M. Lee, D. Kang, T.J. Shin, H.S. Shin, A general approach to preferential formation of active Fe–N x sites in Fe–N/C electrocatalysts for efficient oxygen reduction reaction, J. Am. Chem. Soc. 138 (2016) 15046–15056. [6] A. Akinpelu, B. Merzougui, S. Bukola, A.-M. Azad, R.A. Basheer, G.M. Swain, Q. Chang, M. Shao, A Pt-free electrocatalyst based on pyrolized vinazene-carbon
4. Conclusion In summary, Fe-N-C electrocatalysts based on atomically dispersed Fe single atoms were successfully prepared by using ZIF-8 as a selftemplate in a one-step pyrolysis method. The effects of the Zn/Fe ratio in the precursors on the ORR activities of Fe-N-C catalysts were systematically studied. The results showed that the Fe-N-C catalyst synthesized from the precursor with a Zn/Fe ratio of 0.95/0.05 and annealed in Ar at 1000˚C showed the highest ORR activity and good stability. The half-wave potentials were 0.81 and 0.90 V in acidic and 65
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Gui-Liang Xu received his Bachelor in July 2009 and a Ph.D. degree in June 2014 from Department of Chemistry of Xiamen University. He is now an assistant chemist at Argonne national laboratory. His research focuses on both fundamental understanding by operando synchrotron X-ray techniques and materials development relating to electrochemical energy storage systems.
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F. Xiao, et al. Cheng-Jun Sun is a physicist in the X-ray Science Division at Argonne National Laboratory, his research interests include X-ray absorption spectroscopy (XAFS), X-ray emission spectroscopy, high-energy-resolution fluorescence-detected XAFS, application of advanced spectroscopy on energy materials, and magnetic thin films and multiferroic materials.
Shangqian Zhu received his Bachelor in the Chemical Engineering and Technology from Beijing University of Chemical Technology (2013), and M.S. degree in Chemical and Biomolecular Engineering from the Hong Kong University of Science and Technology in 2016. He is now a Ph.D. candidate in chemical and biomolecular engineering at the Hong Kong University of Science and Technology under the supervision of Prof. Min-Hua Shao. His research is focused on electrochemical and in situ spectroscopic studies of the CO2 reduction reaction.
Mingjie Xu received his Bachelor in Material Science and Engineering from the University of Michigan (2009) and Mechanical Engineering from Shanghai Jiao Tong University (2009). He received his Ph.D. in Material Science and Engineering from the University of Michigan (2017). He is now a postdoc in professor Minhua Shao group in Hong Kong University of Science and Technology and an assistant specialist in University of California, Irvine Material Research Institute (UCIMRI). His main research interests are in situ transmission electron microscopy techniques and advanced catalytical materials for fuel cells.
Yueying Li received her Bachelor in Polymer Material from the Liaocheng University in 2013, and an M.S. degree in Material Engineering from Dalian Polytechnic University in 2015. She is now a Ph.D. candidate at Northwestern Polytechnical University under the supervision of Professor Bingqing Wei and Professor Jian-Gan Wang. She is interested in the development of nanostructured hybrid materials for solar energy conversion applications.
Wen Wen earned his B.S. from the University of Science and Technology of China (2000) and a Ph.D. from Northeastern University (2006). He was a post-doctoral fellow at Brookhaven National Laboratory from 2006 to 2008. Then he joined the Shanghai Synchrotron Radiation Facility in 2008. His research mainly focuses on the application and development of synchrotron diffraction related techniques for renewable energy, especially under operando experimental conditions, such as Lithium batteries and heterogeneous catalysis.
Zidong Wei received his Ph.D. from the Tianjin University in 1994. Currently, he is a full professor at Chongqing University. His research interests include electrocatalysis, computational study, electrochemical materials and their applications in catalysis and energy storage. He has been granted 26 patents associated with catalysis. He has served as an Editorial Board Member of 6 International and Chinabased journals.
Qi Wang received his Bachelor in 2011 and Ph.D. in 2017 from Central South University. He's currently doing his postdoctoral research in Dr. Gu's group at SUS Tech. His research interests focus on the TEM and structure-property relationship of materials in electrocatalysis.
Xiaoqing Pan earned his B.S. and M.S. degrees in Physics from Nanjing University, and a Ph.D. degree in Solid State Physics from the Saarland University. He is a Professor and Henry Samueli Endowed Chair in Engineering, at UC Irvine in The Henry Samueli School of Engineering's Department of Chemical Engineering & Materials Science and the School of Physical Sciences Department of Physics & Astronomy. He is also the inaugural director of the Irvine Materials Research Institute (IMRI).
Meng Gu received his Ph.D. degree in materials science from the University of California Davis. He is now an Associate Professor in SUS Tech focusing on the development of better catalysts, batteries, and advanced in-situ electron microscopy analysis of materials. Dr. Gu has been recognized with the Albert CREWE award from the Microscopy Society of America in 2015 for his outstanding research.
Jiangan Wang received his Ph.D. degree in Materials Science from Tsinghua University in 2013. Then he joined in the Center for Nano Energy Materials. He is currently an Associate Professor in the School of Materials Science and Engineering at Northwestern Polytechnical University. His research interests focus on novel nano-structured material science and nanotechnology for high-efficient energy storage systems and photovoltaics.
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F. Xiao, et al. Khalil Amine is an Argonne Distinguished Fellow and Manager of the Advanced Battery Technology programs at Argonne National Laboratory, where he is responsible for directing the research and development of advanced materials and battery systems for HEV, PHEV, EV, satellite, military, and medical applications. Dr. Amine currently serves a committee member of the U.S. National Research Consul, U.S. Academy of Sciences, on battery-related technologies. He is also the president of IMLB LLC, an international automotive battery conference.
Minhua Shao earned his B.S. (1999) and M.S. (2002) degrees in Chemistry from Xiamen University, and a Ph.D. degree in Materials Science and Engineering from the State University of New York at Stony Brook (2006). He received the Supramaniam Srinivasan Young Investigator Award from the ECS Energy Technology Division (2014). He is now an Associate Professor in the Department of Chemical and Biological Engineering and Associate Director of Energy Institute of the Hong Kong University of Science and Technology. His research is mainly focused on electrocatalysis, fuel cells, and advanced batteries.
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Full paper
Nitrogen-coordinated single iron atom catalysts derived from metal organic frameworks for oxygen reduction reaction
T
Fei Xiaoa,1, Gui-Liang Xub,1, Cheng-Jun Sunc, Mingjie Xud,e, Wen Wenf, Qi Wangg, Meng Gug, Shangqian Zhua, Yueying Lih, Zidong Weii, Xiaoqing Pane, Jiangan Wanga,h, Khalil Amineb,j,∗, Minhua Shaoa,d,∗ a
Department of Chemical and Biological Engineering, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL, 60439, USA c X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439, United States d Fok Ying Tung Research Institute, Hong Kong University of Science and Technology, Guangzhou, 511458, PR China e Department of Chemical Engineering and Materials Science, University of California, Irvine, Irvine, CA, 92697, United States f Shanghai Synchrotron Radiation Facility, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Zhangjiang High Tech. Park, 201204, PR China g Department of Materials Science and Engineering, South University of Science and Technology of China, No.1088, Xueyuan Road, Shenzhen, Guangdong, 518055, China h State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials Science and Engineering, Northwestern Polytechnical University and Shaanxi Joint Laboratory of Graphene (NPU), Xi'an, 710072, China i College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 400044, China j Materials Science and Engineering, Stanford University, Stanford, CA, 94305, United States b
ARTICLE INFO
ABSTRACT
Keywords: Oxygen reduction reaction Metal organic framework Single-atom catalyst Non-precious metal catalysts
Iron and nitrogen co-doped carbon (Fe-N-C) catalysts hold great promise to replace platinum group metal used for the oxygen reduction reaction (ORR) in low-temperature fuel cells. However, general synthesis routes require tedious acid washing and extensive heat treatment, usually resulting in uncontrollable morphologies and undesirable compounds. In this work, a zeolitic imidazolate framework (ZIF-8) was employed as a self-template for one-pot synthesis of a Fe-N-C catalyst consisting of uniformly dispersed Fe single atoms. Atomically dispersed Fe atoms were well distributed along the edges of the porous carbon matrix. Each of the Fe atoms was coordinated with four N atoms in the plane and two O atoms in the axial direction. The optimized Fe-N-C catalyst showed excellent ORR activities with half-wave potentials of 0.81 and 0.90 V in acidic and alkaline solutions, respectively. The results may be important for the optimization of single-atom-based catalysts for various reactions.
1. Introduction Oxygen reduction reaction (ORR) electrocatalysts are commonly based on a platinum group metal (PGM) [1,2]. However, high cost due to the limited reserves impedes the wide adoption of PGMs. Over the past decades, great efforts have been made in the search for more costeffective materials. Since the discovery of the transition metal phthalocyanine complex in 1964 as an alternative ORR catalyst, non-PGM materials with M-N-C (M = transition metals) coordinated structures have been under development to reduce the material cost [3]. Among all M-N-C structures studied, Fe-N-C has attracted the greatest attention due to its high ORR activity [4,5]. However, a big gap in the catalytic performance still exists between Fe-N-C catalysts and PGM materials due to the limited active sites in the former. Thus, creating a higher
density of N-coordinated single Fe atoms is among the primary approaches to promote the performance of Fe-N-C catalysts. Fe-N-C catalysts can be synthesized by pyrolyzing various salts and polymer precursors [6–8]. However, the direct calcination of the mixture usually results in unexpected aggregated metallic compounds in the carbon shells. Acid washing followed by heat treatment is required to remove these inactive species and recover the damaged carbon structures. A template-assisted synthesis process can alleviate the aggregation of Fe atoms at high temperatures. A thin SiO2 layer [5], sodium salt [9], and nano-MgO [10] templates have been used during the pyrolysis to achieve a homogeneous distribution of Fe-N-C active sites. These significantly improved ORR activities as compared to the ones obtained through direct pyrolysis. However, removing templates by strong acid solutions (e.g., HF, HNO3) is undesirable in large-scale
Corresponding authors. Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL, 60439, USA.; E-mail addresses: [email protected] (K. Amine), [email protected] (M. Shao). 1 Equal contribution. ∗
https://doi.org/10.1016/j.nanoen.2019.04.033 Received 7 January 2019; Received in revised form 26 March 2019; Accepted 6 April 2019 Available online 11 April 2019 2211-2855/ © 2019 Elsevier Ltd. All rights reserved.
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production. Thus, developing novel templates that can be easily removed is still highly desired. A metal organic framework (MOF) can potentially serve as the template because of its diverse and tuneable functionalities by changing the metal nodes and organic ligands [11]. The zeolitic imidazolate framework (ZIF-8) is a commonly used template to dope Fe in the precursor and synthesize N-coordinated single Fe atoms. Zinc (Zn) nodes in ZIF-8 act as a “fence” to expand the spatial interval between two Fe atoms [12], and the sublimation of Zn above 907 °C contributes to a porous structure. The first step in the development of a new catalyst based on single atoms for ORR is to study the optimum parameters for its synthesis. Previous studies have found that the heat treatment conditions like temperature and atmosphere were pivotal to the successful generation of active sites [13]. Conventional Fe-N-C electrocatalysts derived from mixing iron salts, like iron porphyrin, with carbon black have been systematically studied by the Dodelet group. Their results demonstrated the challenge in achieving high activity and stability at the same time [14]. Besides, for thermally activated catalysts, the heat treatment sequence would also negatively affect the relationship between the formation of pores and active sites and their influence on ORR performance [15,16]. However, the impact of these parameters is not well understood for single-atom-doped Fe-N-C catalysts. For Fe-N-C synthesized from pyrolyzing a Fe-ZIF-8 precursor, the final carbon structure is determined by diffusivities of metals governed by the Kirkendall effect [17,18] and cavity diffusivity affected by gas diffusion. It is important to balance these two diffusivities by optimizing the synthesis conditions. In addition, because the Fe is surrounded with excess N ligands from the organic in the precursor [19], whether increasing the Ndoping content by secondary ammonia heat treatment will further promote the ORR is still an open question. With these considerations in mind, we have systematically investigated synthesis conditions for FeN-C. The catalyst obtained with optimized synthesis conditions showed superior ORR activities and stability in both acid and alkaline solutions. The half-wave potentials reached 0.81 and 0.90 V in acidic and alkaline solutions, respectively, with a catalyst loading of 0.56 mg cm−2.
2.2. Materials characterization The structures of the samples were examined by an X-ray diffractometer (XRD) equipped with a graphite monochromator and a CuKα radiation source. Morphologies were recorded with scanning electron microscopy (SEM, JEOL 6700) and transmission electron microscopy (TEM, JEM 2010F). X-ray photoelectron spectroscopy (XPS) was applied to determine surface composition. High-resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) with bright field (BF), high angle annular dark field (HAADF), and secondary electron imaging (SEI) were performed with an aberration-corrected JEOL JEM-ARM300CF S/STEM microscope. The SEI can reveal topographical information of a carbon-nitrogen matrix at a much higher resolution than conventional SEM, and HAADF Z-contrast is capable of distinguishing heavy Fe elements from carbon and nitrogen. The distance of the single Fe atoms can be easily resolved due to the high spatial resolution in the STEM mode (82 p.m. @ 300 kV). Fe-N-C mapping was conducted by using the Titan Themis (Double Cs-Correcter)-300 kV STEM mode. The surface area and pore size distribution of the catalysts were measured by the BrunauerEmmett-Teller (BET) method. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) characterizations were conducted at the Advanced Photon Source (APS) of Argonne National Laboratory (ANL). The fluorescence mode was applied to obtain the Fe signal in the sample [21]. Data reduction, analysis, and EXAFS fitting were processed with Athena and Artemis software [22,23]. 2.3. Electrochemical measurements The ORR activities were measured in a typical three-electrode system controlled by an electrochemical workstation (CHI 760). A piece of carbon paper was used as the counter electrode. Ag/AgCl and Hg/ HgO reference electrodes were applied in acidic and alkaline electrolytes, respectively. All potentials were calibrated with a reversible hydrogen electrode (RHE). The rotating disk electrode (RDE) technique was applied to measure the activity and stability. It is worth noting that the kinetic currents in this study were not IR compensation. To that end, 5.4 mg of final Fe-N-C catalysts was ultrasonically dispersed in 400 μL deionized water, 100 μL isopropanol (99.7%, VWR, UK), and 10 μL 5% Nafion. Then, 10 μL uniform ink was dropped on a glassy carbon RDE tip (diameter: 5 mm) and dried in air. For comparison, a 2.5 mg Pt/C (28.2%, TEC10E30E) catalyst was uniformly dispersed in 2 ml water, 0.5 ml isopropanol, and 10 μL 5% Nafion, then 10 μL ink was deposited on the same RDE tip and dried in air. Cyclic voltammograms (CVs) were obtained in Ar-saturated 0.1 M HClO4 and 0.1 M KOH solutions at a scan rate of 50 mV s−1 from 0 V to 1.2 V. To minimize the effect due to non-Faraday current, ORR activities were evaluated by steady-state polarization in O2-saturated 0.1 M HClO4 and 0.1 M KOH solutions with the potential width and duration of 25 mV and 30 s with a rotation speed of 1600 rpm. A stability measurement was performed in an O2-saturated acidic solution from 0.6 V to 1.0 V with a scan rate of 50 mV s−1. The electron transfer number from the RDE test was calculated from the Koutecky-Levich equation:
2. Experimental 2.1. Fe-N-C catalyst preparation The Fe-ZIF-8 precursors were synthesized by following the method in a previous report [20] to choose Fe2+ as the metal sources, except for removing surfactant and changing solvents to simplify the process. In a typical process for preparing Fe-ZIF-8 with a 0.95/0.05 molar ratio of Zn/Fe, 8 g 2-methylimidazole (98%, Aladdin, China) was dissolved in 100 ml methanol (99.9%, Merck, USA) in a flask. Then, 282.6 mg Zn (NO)3·6H2O (99.998% metals basis, Aladdin, China) and 14 mg FeSO4·6H2O (99.95% metals basis, Aladdin, China) were dissolved in 100 ml methanol in another flask. The two solutions were mixed uniformly after bubbling in Ar for 1 h. Then, the mixture was stirred under the protection of Ar for 10 h. The suspension was collected by centrifugation and then washed with absolute ethanol several times. The product was dried at 80 °C in a vacuum oven. For Fe-ZIF-8 precursors having different Zn/Fe molar ratios (0.97/ 0.03, 0.92/0.08 and 0.90/0.10), the experimental procedures were the same except for changing the amount of Zn (NO)3·6H2O and FeSO4·6H2O accordingly. A ZIF-8 precursor without Fe atoms was also synthesized by using 1.00 nmol Zn (NO)3·6H2O without adding FeSO4·6H2O. The Fe-N-C catalysts were prepared through a programmed pyrolysis in different atmospheres (Ar, N2,7%H2/Ar, and Ar + NH3). The heating rate was 5 °C per min and stayed at 1000 °C for 1 h (Ar, N2, and 7% H2/Ar). Secondary NH3 pyrolysis was applied with the same heating rate and kept at 900 °C for 15 min. Then, it was cooled naturally under the protection of the same atmosphere.
1 1 1 1 1 = + = + J Jk JL B 1/2 Jk where ω is the rotation rate applied in the test. Current density (j) and kinetic current density (jk) were determined from the steady-state polarization curves. The constant B was calculated from.where the diffusion coefficient of O2 (DO2) is 1.93 × 10−5 cm2 s−1; the kinetic viscosity ν equals 1.009 × 10−2 cm2 s−1; the concentration of O2 in electrolyte (CO2) is 1.26 × 10−3 mol L−1 [24,25]; and F and A are the Faraday constant and the electrode's geometric area (0.196 cm2), 61
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respectively. The rotating ring disk electrode (RRDE) test was applied to measure the hydrogen peroxide yield (H2O2%) and the electron transfer number (n). During the test, polarization curves were recorded by scanning the disk electrode with a rate of 1 mV s−1 in the voltage range of 1.0–0.125 V, where the ring electrode voltage was 1.20 V. Results were determined by the following equations:
H2 O2 (%) =
n=
200Ir Id N + Ir
4Id N Id N + Ir
Disk current (Id) and ring current (Ir) can be determined from polarization curves directly. The collection efficiency (N) equals 0.42. All the electrochemical measurements were done at room temperature. 3. Results and discussion 3.1. Synthesis of ZIF-8 and Fe-ZIF-8 precursors The ZIF-8 and Fe-ZIF-8 crystals with controllable morphologies were synthesized via a co-precipitation method. The Fe doping level was controlled by varying the Zn/Fe ratios. The sample with a Zn/Fe ratio of 0.95/0.05 was selected for further discussion. Both precursors with or without Fe doping had dodecahedral shapes, as shown in Fig. 1a and b, respectively. The particle size of the ZIF-8 precursor (length: ca. 50 nm) was smaller than that of the Fe-ZIF-8 precursor (length: ca. 80 nm), which could be attributed to more Zn being used in the former and resulting in more nucleation sites for crystallization [26]. The XRD results for ZIF-8 and Fe-ZIF-8 precursors presented in Fig. 1c imply similar crystal structures. After pyrolysis in Ar atmosphere at 1000 °C for 1 h, the observable peaks at 25° and 44° for both XRD patterns were assigned to the (002) and (101) planes of graphitic carbon (Fig. 1d). In addition, no peak related to metallic particles or metal carbides was detected. The ZIF-8 template not only prevented the formation of inactive species and simplified the synthesizing process, but maintained the morphologies of the precursors, as indicated by the TEM images in Fig. 2. Even though the dodecahedral shape was better maintained in NC (Fig. 2a) than in Fe-N-C (Fig. 2b), the homogeneously dispersed pores
Fig. 2. TEM images for (a) N-C and (b,c) Fe-N-C catalysts, and secondary electron images for (d) Fe-N-C catalyst. The Zn/Fe ratio in the precursor is 0.95/ 0.05.
in the well-defined framework resulted in a similar specific surface area for both samples (∼1090 m2 g−1). The main pores in the catalysts were mesoporous according to the BET measurements (Table S1 and Figure S1). The graphite layers can be clearly identified in the Fe-N-C catalyst shown in Fig. 2c. Topographical information for the Fe-N-C catalyst, which was explored by the SEI technique (Fig. 2d), shows a uniform particle size but rough surface. This also explains the high surface area of the catalyst. 3.2. Composition and atmosphere effects The poor ORR activity of Fe-N-C is commonly related to a limited number of active sites, which can potentially be resolved by increasing the Fe content. However, inactive metal clusters or even metallic nanoparticles are formed when excessive Fe is incorporated. Thus, optimizing Fe doping and synthesis conditions is important to the performance of the final catalysts. The effect of peak temperature on the final catalyst has been well studied previously. It was found that the inactive Zn species cannot be sublimated at too low temperature, while metal particles and carbides can be formed at too high temperature [21,27]. Thus, we focused on the effect of different protection atmospheres (Ar, N2, and 7% H2/Ar) in this study. Only carbon-related peaks were detected by XRD on samples with different Zn/Fe ratios (0.97/0.03, 0.92/ 0.08, and 0.90/0.10) or heat treatment atmospheres (Figure S2), indicating that the changes in these synthesis conditions did not produce detectable Fe particles or metal carbides. The SEM (Figure S3) and TEM (Figure S4) images confirmed the successful formation of well-defined porous matrixes in all samples regardless of the Zn/Fe ratio and the atmosphere. However, when the Fe doping level increased to 0.08 or higher, some clusters were observed in the framework (indicated by red circles in Figure S4c and e). Thus, a small amount of Fe doping and different heat treatment atmosphere would not change the morphologies of the precursors and the final products. However, the pore volume and surface area would change under different heat treatment conditions according to the BET measurements, as shown in Table S1. For the ZIF-8 framework, which already has cavities due to its unique structure [28], the distribution of pores should depend on the evaporation of Zn along with Fe movement and the resulting collapse of cavities during
Fig. 1. TEM images of (a) ZIF-8 and (b) Fe-ZIF-8 precursors; XRD patterns of (c) (Fe)-ZIF-8 and (d) (Fe)-N-C catalysts. The Zn/Fe ratio in the precursor is 0.95/ 0.05. 62
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Fig. 3. Steady-state polarization curves of (a) ORR catalysts derived from N-C, Pt/C, and Fe (Zn/Fe = 0.95/0.05)-N-C catalysts in Ar recorded in an O2-saturated 0.1 M HClO4 electrolyte, (b) different Zn/Fe compositions (0.97/0.03, 0.95/0.05, 0.92/ 0.08, and 0.90/0.10) in an O2-saturated 0.1 M HClO4 electrolyte, (c) different Zn/Fe compositions (0.97/0.03, 0.95/0.05, 0.92/ 0.08, 0.90/0.10) and Pt/C in an O2-saturated 0.1 M KOH electrolyte, and (d) Fe (Zn/Fe = 0.95/0.05)-N-C catalyst in Ar after different CV cycles in an O2-saturated 0.1 M HClO4 electrolyte. All tests were carried out on RDE with 1600 rpm rotating rate. The catalyst loading was 0.56 mg cm−2 for the Fe-N-C samples and the N-C sample and 0.051 mg cm−2 for the Pt/C sample (TKK, 28.5 wt%).
heat treatment. By comparison, the Fe (Zn/Fe=0.95/0.05)-N-C catalyst in Ar environment had the highest pore volume and pore surface area due to a better balance of relative diffusivities.
[30]. Thus, in the N2 atmosphere, the original pores formed in the precursor start to diffuse at an early stage. When the temperature increases, Zn starts to evaporate from the structure, which may result in the shrinkage of the original pores. This effect may also explain the poor stability of the N2-treated sample during the ORR. CVs of all the Fe-N-C catalysts showed a pair of well-developed redox peaks at 0.7–0.9 V, which were assigned to the redox couple of Fe3+/Fe2+ in an O2-saturated 0.1 M HClO4 electrolyte (Figure S6a, b). However, the CVs were virtually featureless in an O2-saturated 0.1 M KOH electrolyte (Figure S6c, d). When the doping level of Fe was increased, the redox peaks became more obvious, indicating more Fe species on the surfaces. In contrast to conventional Fe-N-C mixed with carbon black or Fe-N-C catalysts synthesized from ZIF-8 by ball milling with Fe sources [14,31], the secondary NH3 heat treatment did not promote the performance of the Fe-N-C catalyst (Figure S7a), which may be relative to the different environment of Fe in the precursor. Because of rich N ligands around Fe, solely increasing N doping in the carbon framework will not promote ORR performance [19]. Besides, NH3 will etch carbon HCN + H2 and C+ 2H2 CH4 according to the reactions C+ NH3 [14], damaging the pore structure formed in the first treatment. This effect is indicated by the near absence of Fe2+/Fe3+ redox peaks and a smaller double layer in the CV curve shown in Figure S7b. Next, we focus on the characterization and evaluation of the sample with the highest ORR activity: Fe (Zn/Fe = 0.95/0.05)-N-C in Ar, denoted as Fe-N-C. Durability testing was investigated in an O2-saturated 0.1 M HClO4 solution at a potential range of 0.6–1.0 V and rate of 50 mV s−1. Its half-wave potential was only lowered by 16 mV after 10,000 cycles (Fig. 3d). The activity decay may be due to Fe oxidation or dissolution as proposed previously [32]. It is worth noting that the stability of this catalyst is comparable to or better than the best ones reported in the literature [7,20,21]. The polarization curves recorded at different rotation rates in an anodic sweep of the catalyst in an O2saturated 0.1 M HClO4 solution are shown in Figure S8a. Figure S8b presents the inverse current density (j−1) as a function of the square root of the rotation rate (ω −1/2). Constant B calculated from the slope
3.3. Electrochemical evaluation The ORR activities of all samples were measured in O2-saturated 0.1 M HClO4 and 0.1 M KOH solutions. Among all the samples, the Fe (Zn/Fe = 0.95/0.05)-N-C catalyst in Ar environment exhibited the best performance in both acidic and alkaline electrolytes. An onset potential (Eonset) of 0.95 V and half-wave potential (E1/2) of 0.81 V were observed in the 0.1 M HClO4 solution. The activity in the 0.1 M KOH solution is much higher, with an Eonset above 1.0 V and E1/2 of 0.90 V, which are superior to those of commercial Pt/C (E1/2 = 0.875 V). Fig. 3 presents the steady-state polarization plots as a function of the Fe content in the precursors. As shown in Fig. 3a, the N-C catalyst without Fe doping exhibited a very poor ORR activity in acidic solution. The large discrepancy in activities between N-C and Fe-N-C catalysts suggested that Fe played an essential role in forming active sites. The optimal Zn/Fe ratio was 0.95/0.05 in the precursor for both acidic and alkaline solutions (as shown in Fig. 3b and c). Thus, the volcano plot of ORR activity with respect to Zn/Fe ratio indicated that a low Fe doping amount would result in a limited density of active sites, while agglomeration and formation of an unfavorable carbon matrix might occur with a high Fe doping content. With the optimized composition, the impacts of the pyrolysis atmosphere on the ORR activity are compared in Figure S5. Surprisingly, the activities of the catalysts treated in different atmospheres presented in the following order: N2 < 7% H2/Ar < Ar. According to Fick's law, N2 (28 g mol−1) and 7% H2 (2 g mol−1)/Ar gases have smaller mass than Ar (40 g mol−1), which resulted in quicker heat transfer and higher metal diffusion rates in the former two environments, which may increase the possibility of Fe aggregation [29]. Besides, the pore diffusivity has a strong inverse relationship with the kinetic diameter of the gas. The kinetic diameter of N2 (3.64 Å) is smaller than Ar (3.70 Å) 63
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Fig. 4. HAADF-STEM images of Fe-N-C catalyst in different areas (a,c,d). Corresponding intensity profiles (b) obtained on the zoomed areas in panel a. The Zn/Fe ratio in the precursor is 0.95/0.05.
Fig. 5. XPS spectrum for (a) N 1s and (b) Fe 2p in the Fe-N-C catalyst. The Zn/Fe ratio in the precursor is 0.95/0.05.
of the Koutecky-Levich plot is 0.093 mA s−1/2, which indicates a fourelectron reduction of oxygen [33]. The RRDE results presented in Figure S8c and d match well with the above RDE test results, the H2O2 yields always being below 2% from 0.6 to 0.9 V and electron transfer number being close to 4.
0.296 to 0.404 nm. The HAADF-STEM mapping images of Fe-N-C (Figure S9 and Figure S10) revealed that C, Fe, and N were distributed homogeneously in the carbon particle. Furthermore, the deconvoluted XPS spectrum of N 1s for the Fe-N-C catalyst (Fig. 5a) exhibits four major components, including three types of N: pyridinic-N (398.2 eV), graphitic-N (401.0 eV), and oxidized-N (404 eV). No pyroly-N was observed as it is unstable when the annealing temperature is above 700 °C [13]. Pyridinic-N and graphite-N are the main active sites in the N-doped carbon materials in acidic and alkaline conditions, respectively [35]. Pyridinic-N locates at the edge of graphite, creating Lewis basic sites that are active for ORR under the acidic conditions [36]. Interestingly, Fe-N bonding around 399.2 eV was also observed [37,38], indicating that single Fe atoms have a strong interaction with the neighboring doped N. Majority of the Fe atoms are in the 2 + form, with a small portion of 3+, as shown in Fig. 5b. To gain detailed structural and electronic information of Fe-N-C at
3.4. Structure properties The presence of Fe single atoms was confirmed by HAADF-STEM images (Fig. 4). The Fe atoms (bright dots) were uniformly distributed along the edge of the carbon matrix, which had been proved to be efficient active sites that promote the ORR [34]. Two selected areas from Fig. 4a in the summed through-focal HAADF images show different intensities at specific focal depths and were analyzed to determine the relative positions along the electron beam direction. As shown in Fig. 4b, the distance between two the closest Fe atoms ranges from 64
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Fig. 6. Comparison of XAS spectra of Fe-NC with Fe based references (red line for FeN-C catalysts, blue line for FeO, and black line for Fe2O3): (a) XANES spectra of Fe Kedge where the spectra are k2 weighted and (b) Fourier transforms for EXAFS spectra of Fe K-edge where the spectra are k3 weighted (a clear shift is indicated by a dashed blue line).
an atomic level, XANES and EXAFS data were recorded. Fig. 6a presents the ex-situ XANES spectra for Fe-N-C along with FeO and Fe2O3 for comparison. The position of the absorption edge is relative to the valence of the absorbing atom [39]. It moves to higher energy as the oxidation state of the atom increases. After high temperature pyrolysis, the Fe oxidation state is between 2 + and 3+, as evidenced from its edge position being between FeO and Fe2O3. This finding is consistent with the XPS data in Fig. 5b and qualitatively agrees with previous reports. A weak pre-edge peak at 7112 eV for Fe-N-C was assigned to the 1s → 3d transition [40]. Although this transition is electric dipole forbidden, the peak intensity can be gained from an electric quadrupole coupling mechanism [41]. The positions and intensities of the pre-edge peaks are directly related to sub-bands from 3d orbital splitting and the local coordination geometry of the metal atom, respectively. The Fe2O3 reference possesses C3V symmetry along with two sets of three Fe-O bonds of 1.96 and 2.08 Å to form the structure of octahedral Fe sites [42], similar to Fe phthalocyanine [43,44], which also has the same pre-peak at 7112 eV. Thus, the pre-peak suggests the formation of Fe-N and Fe-O bonds in the Fe-N-C catalyst. Because XANES is dependent on structural and electronic effects, the differences in the intensity of preedge peak are attributed to the differences in the particle size [45]. The Fourier transformation of the Fe K-edge EXAFS of Fe-N-C is shown in Fig. 6b and compared with that of Fe2O3. The peaks at 1.4 Å and 2.5 Å in the Fe2O3 reference stand for Fe-O and Fe-Fe bonds [46–49], respectively. A clear shift of the first shell bond length from 1.4 Å to 1.5 Å suggests that the Fe center is not solely coordinated with O atoms. Besides, there is no clear Fe−Fe bond feature found in the Fe-N-C catalysts in Fig. 6b. To gain further quantitative structural parameters of Fe in Fe-N-C catalysts, EXAFS fitting based on the first strong peak was carried out, and the results are shown in Figure S11. The coordination number of Fe and the mean bond length were found to be 6.0 ± 0.8 and 2.04 ± 0.04 Å, respectively. Based on the fitting data, we propose that isolated Fe atoms sited in the nitrogen-doped porous carbon framework by bonding with four N atoms in the plane and two O atoms in the axial direction. This configuration is consistent with other EXAFS measurements and density functional theory calculations [19,46,50–52].
alkaline solutions, respectively. The EXAFS data suggested that a single Fe atom might be coordinated with four N atoms in the plane and two O atoms in the axial direction. This work indicated that Fe-N-C with abundant Fe single atoms has considerable potential in shortening the gap between M-N-C (M = transition metal) and Pt-based catalysts. The results may be important for the optimization of single-atom-based catalysts for various reactions. Acknowledgements This work was supported by the National Key R&D Program of China (No 2017YFB0102900), the Research Grant Council (N_HKUST610/17) of the Hong Kong Special Administrative Region, the Shenzhen Science and Technology Innovation Commission (JCYJ20180507183818040), Guangdong Special Fund for Science and Technology Development (Hong Kong Technology Cooperation Funding Scheme (201704030019 and 201704030065)). G. Xu and K. Amine gratefully acknowledges support from the U. S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office. Argonne National Laboratory is operated by UChicago Argonne, LLC, for the U.S. Department of Energy under contract DE-AC02-06CH11357. This research used resources of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, and was supported by the U.S. DOE under Contract No. DE-AC0-06CH11357, and the Canadian Light Source and its funding partners. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.nanoen.2019.04.033. References [1] M. Shao, Q. Chang, J.-P. Dodelet, R. Chenitz, Recent advances in electrocatalysts for oxygen reduction reaction, Chem. Rev. 116 (2016) 3594–3657. [2] S.-I. Choi, S. Xie, M. Shao, J.H. Odell, N. Lu, H.-C. Peng, L. Protsailo, S. Guerrero, J. Park, X. Xia, Synthesis and characterization of 9 nm Pt–Ni octahedra with a record high activity of 3.3 A/mgPt for the oxygen reduction reaction, Nano Lett. 13 (2013) 3420–3425. [3] R. Jasinski, A new fuel cell cathode catalyst, Nature 201 (1964) 1212. [4] X. Yan, K. Liu, T. Wang, Y. You, J. Liu, P. Wang, X. Pan, G. Wang, J. Luo, J. Zhu, Atomic interpretation of high activity on transition metal and nitrogen-doped carbon nanofibers for catalyzing oxygen reduction, J. Mater. Chem. A. 5 (2017) 3336–3345. [5] Y.J. Sa, D.-J. Seo, J. Woo, J.T. Lim, J.Y. Cheon, S.Y. Yang, J.M. Lee, D. Kang, T.J. Shin, H.S. Shin, A general approach to preferential formation of active Fe–N x sites in Fe–N/C electrocatalysts for efficient oxygen reduction reaction, J. Am. Chem. Soc. 138 (2016) 15046–15056. [6] A. Akinpelu, B. Merzougui, S. Bukola, A.-M. Azad, R.A. Basheer, G.M. Swain, Q. Chang, M. Shao, A Pt-free electrocatalyst based on pyrolized vinazene-carbon
4. Conclusion In summary, Fe-N-C electrocatalysts based on atomically dispersed Fe single atoms were successfully prepared by using ZIF-8 as a selftemplate in a one-step pyrolysis method. The effects of the Zn/Fe ratio in the precursors on the ORR activities of Fe-N-C catalysts were systematically studied. The results showed that the Fe-N-C catalyst synthesized from the precursor with a Zn/Fe ratio of 0.95/0.05 and annealed in Ar at 1000˚C showed the highest ORR activity and good stability. The half-wave potentials were 0.81 and 0.90 V in acidic and 65
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Gui-Liang Xu received his Bachelor in July 2009 and a Ph.D. degree in June 2014 from Department of Chemistry of Xiamen University. He is now an assistant chemist at Argonne national laboratory. His research focuses on both fundamental understanding by operando synchrotron X-ray techniques and materials development relating to electrochemical energy storage systems.
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F. Xiao, et al. Cheng-Jun Sun is a physicist in the X-ray Science Division at Argonne National Laboratory, his research interests include X-ray absorption spectroscopy (XAFS), X-ray emission spectroscopy, high-energy-resolution fluorescence-detected XAFS, application of advanced spectroscopy on energy materials, and magnetic thin films and multiferroic materials.
Shangqian Zhu received his Bachelor in the Chemical Engineering and Technology from Beijing University of Chemical Technology (2013), and M.S. degree in Chemical and Biomolecular Engineering from the Hong Kong University of Science and Technology in 2016. He is now a Ph.D. candidate in chemical and biomolecular engineering at the Hong Kong University of Science and Technology under the supervision of Prof. Min-Hua Shao. His research is focused on electrochemical and in situ spectroscopic studies of the CO2 reduction reaction.
Mingjie Xu received his Bachelor in Material Science and Engineering from the University of Michigan (2009) and Mechanical Engineering from Shanghai Jiao Tong University (2009). He received his Ph.D. in Material Science and Engineering from the University of Michigan (2017). He is now a postdoc in professor Minhua Shao group in Hong Kong University of Science and Technology and an assistant specialist in University of California, Irvine Material Research Institute (UCIMRI). His main research interests are in situ transmission electron microscopy techniques and advanced catalytical materials for fuel cells.
Yueying Li received her Bachelor in Polymer Material from the Liaocheng University in 2013, and an M.S. degree in Material Engineering from Dalian Polytechnic University in 2015. She is now a Ph.D. candidate at Northwestern Polytechnical University under the supervision of Professor Bingqing Wei and Professor Jian-Gan Wang. She is interested in the development of nanostructured hybrid materials for solar energy conversion applications.
Wen Wen earned his B.S. from the University of Science and Technology of China (2000) and a Ph.D. from Northeastern University (2006). He was a post-doctoral fellow at Brookhaven National Laboratory from 2006 to 2008. Then he joined the Shanghai Synchrotron Radiation Facility in 2008. His research mainly focuses on the application and development of synchrotron diffraction related techniques for renewable energy, especially under operando experimental conditions, such as Lithium batteries and heterogeneous catalysis.
Zidong Wei received his Ph.D. from the Tianjin University in 1994. Currently, he is a full professor at Chongqing University. His research interests include electrocatalysis, computational study, electrochemical materials and their applications in catalysis and energy storage. He has been granted 26 patents associated with catalysis. He has served as an Editorial Board Member of 6 International and Chinabased journals.
Qi Wang received his Bachelor in 2011 and Ph.D. in 2017 from Central South University. He's currently doing his postdoctoral research in Dr. Gu's group at SUS Tech. His research interests focus on the TEM and structure-property relationship of materials in electrocatalysis.
Xiaoqing Pan earned his B.S. and M.S. degrees in Physics from Nanjing University, and a Ph.D. degree in Solid State Physics from the Saarland University. He is a Professor and Henry Samueli Endowed Chair in Engineering, at UC Irvine in The Henry Samueli School of Engineering's Department of Chemical Engineering & Materials Science and the School of Physical Sciences Department of Physics & Astronomy. He is also the inaugural director of the Irvine Materials Research Institute (IMRI).
Meng Gu received his Ph.D. degree in materials science from the University of California Davis. He is now an Associate Professor in SUS Tech focusing on the development of better catalysts, batteries, and advanced in-situ electron microscopy analysis of materials. Dr. Gu has been recognized with the Albert CREWE award from the Microscopy Society of America in 2015 for his outstanding research.
Jiangan Wang received his Ph.D. degree in Materials Science from Tsinghua University in 2013. Then he joined in the Center for Nano Energy Materials. He is currently an Associate Professor in the School of Materials Science and Engineering at Northwestern Polytechnical University. His research interests focus on novel nano-structured material science and nanotechnology for high-efficient energy storage systems and photovoltaics.
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F. Xiao, et al. Khalil Amine is an Argonne Distinguished Fellow and Manager of the Advanced Battery Technology programs at Argonne National Laboratory, where he is responsible for directing the research and development of advanced materials and battery systems for HEV, PHEV, EV, satellite, military, and medical applications. Dr. Amine currently serves a committee member of the U.S. National Research Consul, U.S. Academy of Sciences, on battery-related technologies. He is also the president of IMLB LLC, an international automotive battery conference.
Minhua Shao earned his B.S. (1999) and M.S. (2002) degrees in Chemistry from Xiamen University, and a Ph.D. degree in Materials Science and Engineering from the State University of New York at Stony Brook (2006). He received the Supramaniam Srinivasan Young Investigator Award from the ECS Energy Technology Division (2014). He is now an Associate Professor in the Department of Chemical and Biological Engineering and Associate Director of Energy Institute of the Hong Kong University of Science and Technology. His research is mainly focused on electrocatalysis, fuel cells, and advanced batteries.
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