Research Article pubs.acs.org/journal/ascecg
Confined-Space-Assisted Preparation of Fe3O4‑NanoparticleModified Fe−N−C Catalysts Derived from a Covalent Organic Polymer for Oxygen Reduction Jianing Guo, Yuanhui Cheng,* and Zhonghua Xiang* State Key Laboratory of Organic−Inorganic Composites, College of Chemical Engineering, College of Energy, Beijing University of Chemical Technology, North Third Ring Road 15, Chaoyang District, Beijing 100029, China S Supporting Information *
ABSTRACT: Iron−nitrogen−carbon (Fe−N−C) has been considered as one of the most promising nonprecious metal catalysts for the oxygen reduction reaction (ORR) in fuel cells and metal−air batteries. Herein, we prepare a highly active Fe3O4/Fe−N−C catalyst (named COP@K10-Fe-900), for the ORR from a layered tetraphenylporphyrin-based (TPP-based) covalent organic polymer (COP) grown in nanoconfined space as precursors, followed by iron ion incorporation and a pyrolysis process. The nanoconfined space, i.e., montmorillonite (K10) template, contributes to the unique layered structure of designed precursors and enables Fe3O4 nanoparticles to disperse uniformly in the resulting layered Fe−N−C catalyst. The nanoconfined space reduces the iron-based particle size from ∼50−150 to ∼10 nm. An enhancement of 50 mV was obtained after using layer space confinement for half-wave potential. Moreover, the half-wave potential of the newly developed COP@K10-900 exceeds 20 mV as compared to the benchmark Pt/C in alkaline media. Therefore, we believe that this work can provide an important guideline for designing highly active M−N−C catalysts that can be widely used in energy conversion and storage devices. KEYWORDS: Covalent organic polymers, Fuel cell, Chemical energy materials, Electrocatalysis, Space confinement
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the ordered mesoporous structure.27 Although the catalytically active sites in the Fe−N−C catalyst remain controversial, they are widely believed to be iron-related species.28,29 A high content of active iron species in the Fe−N−C catalysts is important to ensure a superior ORR activity to some extent. Iron species are usually encapsulated into or supported on nitrogen-doped carbon materials. The carbon support also has a significant effect on the structure, distribution, and stability of the active iron species. Wu et al. find that the three-dimensional (3D) N-doped graphene-aerogel-supported Fe3O4 nanoparticles exhibit more positive onset potential, higher cathodic current density, better stability, and higher electron transfer numbers than Fe3O4 nanoparticles on N-doped carbon black or N-doped graphene sheets.19 Therefore, an ideal carbon support should feature a large surface area and appropriate porous structure, which are critical for the mass transport of ORRrelevant species and exposure of active sites.29−33 Currently, Fe−N−C catalysts are usually prepared by pyrolysis of the composites with carbon, nitrogen, and iron precursors in an inert atmosphere.34−36 However, it is difficult to efficiently control the porous structure of the carbon
INTRODUCTION Fuel cells are considered as promising power sources for mobile cars and stationary power stations because of their high power density, high efficiency, and zero carbon emissions.1−4 However, the oxygen reduction reaction (ORR) at the cathodic electrode has sluggish reaction kinetics needing Pt-based catalysts.5−7 So far, the key steps toward the commercialization of fuel cell technology have been constrained by the high cost, poor durability, and scarcity of Pt-based catalysts.8−10 Therefore, it is necessary to explore novel catalysts with low cost, high activity, and good stability for the ORR.11−14 Recently, extensive efforts have been devoted to developing low-cost nonprecious metal catalysts (NPMCs) with a high performance close to that of the Pt-based catalysts.15−18 Among various NPMCs, iron−nitrogen−carbon (Fe−N−C) catalysts have attracted considerable attention as promising substitutes to Pt-based catalysts for the ORR because of their superior catalytic activity and low cost.19−22 Despite numerous studies, the exact nature of active sites in these catalysts still remains debatable.23−25 Wei et al. reveal that the high ORR catalytic activity of the Fe@C-FeNC catalyst should be attributed to the fact that Fe/Fe3C nanocrystals boost the activity of Fe−Nx.26 Li et al. believe that the outstanding catalytic performance is ascribed to the formation of active Fe (Fe3O4) and N (N2) active sites with their interaction as well as © 2017 American Chemical Society
Received: May 3, 2017 Revised: July 4, 2017 Published: July 30, 2017 7871
DOI: 10.1021/acssuschemeng.7b01367 ACS Sustainable Chem. Eng. 2017, 5, 7871−7877
Research Article
ACS Sustainable Chemistry & Engineering materials after pyrolysis.37,38 Simultaneously, most iron species are nonuniformly encapsulated or supported on the surface of N-doped carbon materials, and it is easy for them to aggregate together forming large crystals, which can reduce the ORR activity and stability.39 Recently, covalent organic polymers (COPs)40,41 have been emerging as rich carbon precursors due to their highly tunable structure, high surface area, and colorful controlled synthesis and molecular engineering approaches. After introducing nonprecious transition metals (e.g., Fe and Co) into the porphyrin-based COP precursors, we obtained efficient ORR electrocatalytic activities via a 4e− path in both alkaline and acid media with excellent long-term stability and free from methanol crossover and CO-poisoning effects, outperforming the benchmarked Pt/C catalysts.22 However, the layer-stack effect in the layered porphyrin-based COP precursors makes the particles of metal species aggregated (∼60 nm) during carbonization, which reduces reactive surface area and electroactivity. Here, we demonstrate a facile and scalable method to prepare a highly active Fe3O4-encapsulated Fe−N−C ORR catalyst, i.e., COP@K10-Fe-900, via COP growth in layered nanoconfined space. COP@K10-Fe-900 was constructed by Fe−Nx configurations and uniformly encapsulated Fe3O4 nanoparticles in N-doped carbon materials with a porous layered structure. The layered tetraphenylporphyrin-based (TPP-based) COP material acted as the N sources, the C sources, and a matrix for incorporating iron, and is prepared through a simple and low-cost Scholl reaction using a unique layered structure and ordered porous montmorillonite (K10)42 as templates. The porous layered COPs not only permit iron ions to incorporate into the pyrrole ring homogeneously, but also serve as a skeleton structure and template for subsequent carbon production. After the pyrolysis process, both Fe−Nx sites and Fe3O4 nanoparticles are created and dispersed uniformly in the porous layered carbon materials. Notably, we found that Fe3O4 nanoparticles disperse uniformly onto both internal and external surfaces of COP@K10-Fe-900 catalysts and can boost the catalytic activity of the neighboring Fe−Nx sites. Importantly, the synthesized COP@K10-Fe-900 catalyst exhibits higher ORR activity in alkaline solutions than the commercial Pt/C (20%) catalyst. Accordingly, this work opens a new path for designing highly active nonprecious metal catalysts.
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black precipitates were collected, washed with methanol 3 times, etched in hydrofluoric acid (10 wt %) for 12 h at room temperature to remove the templates, finally Soxhlet extracted with ethanol for 24 h, and then dried in a vacuum oven at 75 °C for 24 h. Synthesis of COP@K10-Fe. The COP-K10 (0.05 g) and FeCl3 (0.40 g) [or FeCl2 (0.31 g)] were dissolved in 5 mL of DMF with magnetic stirring at 95 °C for 24 h, and the resulting precipitate was washed with ethanol and deionized water, and then dried in a vacuum oven at 100 °C for 24 h. In addition, COP-Fe was prepared via the above method. Synthesis of COP@K10-Fe-x. The obtained polymer was then carbonized at various temperatures to obtain COP@K10-Fe-x, where x represents carbonization temperatures of 800, 900, and 1000 °C. COP-TPP(Fe)@K10 powder was first heated to 350 °C for 2 h, with a heating rate of 6 °C min−1 from room temperature under Ar flow, and then at a heating rate of 6 °C min−1 up to 800, 900, or 1000 °C for 2 h. The COP@K10-900 is also prepared by carbonizing the COP polymer at 900 °C without an Fe source. In addition, COP-Fe-900 was prepared by carbonizing the COP-Fe at 900 °C. Physicochemical Characterization. Powder X-ray diffraction (PXRD) measurements were recorded on a D/MAX 2000 X-ray diffractometer with a Cu Kα line (λ = 1.541 78 Å) as the incident beam. X-ray photoelectron spectroscopy (XPS) analysis was obtained on an ESCALAB 250 device operated at 150 W and 200 eV with monochromated Al Kα radiation. Scanning electron microscopy (SEM) images were performed on an S4700 SEM instrument. Transmission electron microscopy (TEM) images were obtained on an H800 instrument. The high-resolution transmission electron microscopy (HRTEM) images were taken on a 2100F device. Raman spectra were tested by the LabRAM Aramis Raman spectrometer (Horiba Jobin Yvon) using a 514 nm laser as the excitation source. Electrochemical Measurements. The ORR performance for catalysts was measured using a rotating Pt-ring-disk electrode (RRDE) technique (Pine Inc., GC, d = 5.6 mm) with a CH Instrument-760e electrochemical workstation in O2-saturated 0.1 M KOH at room temperature. The electrochemical measurements were carried out in a standard three-electrode cell, with a saturated calomel electrode (SCE) as the reference electrode, a Pt wire as the counter electrode, and a rotating ring-disk electrode as the working electrode. Before the tests, the RRDE was mechanically polished with a 0.5, 0.15, and 0.05 μm alumina slurry (CH Instrument, Inc.) in turn to obtain a mirrorlike surface, subsequently rinsed with deionized water and ethanol, and dried in air. For preparation of the catalyst ink, 5 mg of the catalyst was ultrasonically dispersed in a mixture of ethanol (1 mL) and 0.5% Nafion solution (0.05 mL) for about 30 min to form a homogeneous ink. Then, 10 μL of catalyst ink was pipetted onto the glassy carbon (GC) disk electrode giving a loading of 0.2 mg cm−2. The commercial carbon-supported Pt/C (20 wt %) was used for comparison with the same loading of ∼0.2 mg cm−2. In RDE (rotating disk electrode) tests, steady cyclic voltammetry (CV) characterization of the catalysts was conducted in O2-saturated 0.1 M KOH electrolyte with the potential ranging from 0 to 1.2 V versus the reversible hydrogen electrode (RHE) at a scan rate of 100 mV s−1. Meanwhile, a flow of O2 was maintained over the electrolyte solution for continued O2 saturation. In RRDE experiments, the potential was cycled from 1.1 to 0 V (versus RHE) at a scan rate of 5 mV s−1 with an electrode rotation speed of 1600 rpm to record the polarization curves. To detect the H2O2 yield and the number of electrons transferred (n) during the ORR process, a potential of 1.4 V versus RHE was applied to the Pt-ring electrode to measure the H2O2 production using the rotating ring-disk electrode (RRDE). The H2O2 yield and the number of electrons transferred (n) in the ORR are calculated using the following equations, respectively.
EXPERIMENTAL SECTION
Material Synthesis. Synthesis of the COP Material. A COP (known as SMPs-4) was prepared according to the previous method with a slight modification.43 In a glovebox, meso-tetraphenylporphine (0.12 g, 0.20 mmol) was added into 3 mL of CHCl3 with magnetic stirring at room temperature for 3 h; then, AlCl3 (0.50 g, 3.20 mmol) was mixed in the solution, and the reaction was allowed to proceed for 48 h at 58 °C. The resulting precipitate was washed three times with ethanol, and stirred in HCl (6 M) for 2 h at room temperature to remove the AlCl3. The final product was obtained after Soxhlet extraction for 24 h with ethanol and dried in a vacuum oven at 75 °C for 24 h. Synthesis of COP@K10. Montmorillonite template was ionexchanged in 0.1 M NaCl aqueous solution (20.00 g of template/L) for 24 h before washing with distilled water and drying at 160 °C overnight. The synthesis process was carried out in a glovebox under an inert atmosphere. First, meso-tetraphenylporphine (0.12 g, 0.20 mmol) and montmorillonite (0.12 g) were dispersed in 3 mL of CHCl3 with magnetic stirring at room temperature for 3 h; then, AlCl3 (0.50 g, 3.20 mmol) was added into the solution, and the reaction proceeded for 48 h at 58 °C under a Ar atmosphere. The resulting
%HO−2 = 200 × n=4× 7872
Ir / N Id + Ir /N
Id Id + Ir /N
(1) (2) DOI: 10.1021/acssuschemeng.7b01367 ACS Sustainable Chem. Eng. 2017, 5, 7871−7877
Research Article
ACS Sustainable Chemistry & Engineering Here, the terms are as follows: Id is disk current, Ir is ring current and N = 0.37 is the current collection efficiency of the Pt ring. The RDE tests are conducted at different rotating speeds from 400 to 2025 rpm, and the Koutecky−Levich (K−L) plots are obtained from polarization curves at different potentials. The fitting linearity of the K−L plots indicates first-order reaction kinetics toward the electron transfer numbers for the ORR at different potentials.
1 1 1 1 1 = + = + J JL JK JK Bω1/2
(3)
B = 0.62nFCO(DO)2/3 v−1/6
(4)
Here, the terms are as follows: J is the measured current density, JK and JL are the kinetic- and diffusion-limiting current densities, ω is the angular velocity of the rotating electrode, n is transferred electron number per oxygen molecule, B is the reciprocal of the slope, F is the Faraday constant (F = 96 485 C mol−1), CO is the bulk concentration of O2 (1.2 × 10−3 mol L−1), DO is the O2 diffusion coefficient (1.9 × 10−5 cm2 s−1), and v is the kinematic viscosity of the electrolyte (1 × 10−2 cm2 s−1). The constant 0.62 is adopted when the rotation speed is expressed in rad s−1.
Figure 2. SEM images of (a) COP@K10 and (b) COP@K10-Fe-900. (c) TEM and HRTEM (insert) images of COP@K10-Fe-900. SEM images of (d) COP and (e) COP-Fe-900 without template. (f) TEM image of COP-Fe-900.
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happens in the COP without using the K10 template (Figure 2d). The iron species only distribute on the surface of COP-Fe900 and reunite together easily (Figure 2e,f), which reduces the effective surface area for the ORR. This indicates that the ordered porous template (K10) greatly contributes to reducing the stack effect, and the resulting porous layered COPs not only permit iron ions to incorporate into the pyrrole ring homogeneously, but also serve as a skeleton structure and template for the final carbon production. In addition, Figure S4 clearly shows that the sizes of Fe3O4 in COP@K10-Fe-900 are the smallest compared to those in COP@K10-Fe-800 and COP@K10-Fe-1000. Notably, some Fe3O4 nanoparticles agglomerate into large particles in COP@K10-Fe-800 and COP@K10-Fe-1000, leading to poor performance of the ORR. Aside from the above factors, COP@K10 and COP@K10-Fe900 also have high BET surface areas of 875.6 and 352.2 m2 g−1, respectively (Figure S5 and Table S1). The composition of COP@K10-Fe-900 was characterized by powder X-ray diffraction (PXRD), Raman spectra, and X-ray photoelectron spectroscopy (XPS). The PXRD result reveals that the COP@K10-Fe-900 has well-formed graphitic structures with two obvious broad diffraction peaks at 25.7°and 42.9° indexed to the (002) and (101) diffraction peaks of graphitic carbon, respectively (Figure 3a).17 Iron species are confirmed to be Fe3O4 crystallites (JCPDS 85-1436) with five characteristic diffraction peaks at 30.1° (220), 35.5° (311), 53.4° (422), 57° (511), and 65.6° (440) (Figure 3a),27,44 indicating the coexistence of Fe2+ and Fe3+ in COP@K10-Fe900. Raman spectra analysis was carried out to understand the crystal structure of the obtained carbon. As shown in Figure 3b, all prepared catalysts have a clear characteristic D band and G band at around 1353 and 1588 cm−1, respectively.45 The D band is designated to disordered structure, and the G band is related to the vibration of graphitic phases of the carbon layers.46 A lower ID/IG ratio is usually attributable to a higher degree of graphitization.47,48 The ID/IG ratio of COP@K10-Fe900 (0.97) is the lowest compared to those of COP@K10-Fe800 (1.05) and COP@K10-Fe-1000 (0.985), indicating that COP@K10-Fe-900 has the highest degree of graphitization and higher electron conductivity. The XPS results suggest that there are C, O, N, and Fe components in COP@K10-Fe-900 (Figure S6a). The high-resolution N 1s spectrum can be divided into four signal peaks at 398.5, 399.4, 401.2, and 405.0 eV (Figure 3c), which are attributed to pyridinic N (18%), N−Fe bonding
RESULTS AND DISCUSSION As shown in Figure 1, to overcome the layer-stack effect, COP material was first synthesized in layered confined space;
Figure 1. Schematic diagram of the synthesis of COP@K10-Fe-900. TPP = meso-tetraphenylporphine.
subsequently, the iron species were incorporated and carbonation was performed to produce the highly active Fe3O4-encapsulated Fe−N−C ORR catalyst, i.e., COP@K10Fe-900. The structure and morphology of prepared catalysts were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM) (Figure 2). The SEM images (Figure 2b) reveal that the COP@K10-Fe-900 still displays similar sheetlike structures with COP@K10 (Figure 2a and Figure S1) even after pyrolysis. Iron species with a size of tens of nanometers with a darker contrast have been evenly confined and wrapped in the layered carbon materials (Figure 2c). In addition, the lattice spacing of 0.25 nm in the dark center part can be ascribed to Fe3O4 (211), while the lattice spacing of 0.34 nm around Fe3O4 can be ascribed to carbon from the HRTEM images (Figure S2a). We can clearly see that Fe3O4 was encapsulated in layered carbon material. Moreover, the HAADF-STEM (high-angle annular dark-field STEM) image and EDX-mapping of Fe and C elements in COP@K10-Fe900, and of Fe, N, and C elements in Fe3O4-removed COP@ K10-Fe-900, show that Fe3O4 nanoparticles and Fe−Nx sites are uniformly distributed in carbon materials (Figures S2b,c and S3). In contrast, the remarkable aggregation of iron species 7873
DOI: 10.1021/acssuschemeng.7b01367 ACS Sustainable Chem. Eng. 2017, 5, 7871−7877
Research Article
ACS Sustainable Chemistry & Engineering
Figure 3. (a) PXRD patterns of carbonized COP@K10-Fe-900. (b) Raman spectra of carbonized COP@K10-Fe-x (x = 800, 900, and 1000) at different pyrolysis temperatures. High-resolution XPS spectra of (c) N 1s and (d) Fe 2p of COP@K10-Fe-900.
Figure 4. (a) Cyclic voltammograms of COP@K10-Fe-900, COP-Fe900, and COP@K10-900 in N2- or O2-saturated 0.1 mol L−1 KOH at 100 mV s−1. (b) LSV curves and (c) Tafel plots of COP@K10-900, COP-Fe-900, COP@K10-Fe-900, and Pt/C in O2-saturated 0.1 mol L−1 KOH at 5 mV s−1. (d) LSV curves of COP@K10-Fe-800, COP@ K10-Fe-900, and COP@K10-Fe-1000 in O2-saturated 0.1 mol L−1 KOH at 5 mV s−1.
(13%), graphitic N (21%), and oxidized N (48%), respectively.23,44,49 Notably, this result indicates that Fe−Nx active sites are present in COP@K10-Fe-900, which is beneficial for a high catalytic activity for the ORR. In addition, the pyridinic N was reported to play a crucial role in oxygen reduction,50 which is able to change the electron distribution of adjacent carbon atoms and enhance its catalytic activity. According to previous reports, the Fe 2p peak is deconvoluted into five peaks at 711.4, 713.9, 718.6, 724.3, and 725.6 eV (Figure 3d).34,51,52 The peaks at 711.4 and 713.9 eV correspond to the 2p3/2 binding energy of Fe2+ and Fe3+, respectively. The peaks at 724.3 and 725.6 eV can be attributed to the 2p1/2 binding energy of Fe2+ and Fe3+, respectively. The peak at 718.6 eV is assigned to a satellite peak. This demonstrates that both Fe2+ and Fe3+ coexist in the catalyst, which is consistent with the result of PXRD. The electrocatalytic properties of the catalysts toward the ORR were evaluated by cyclic voltammetry and linear scan voltammetry. All catalysts were first pretreated by repeated cycling in the potential between 0 and 1.2 V versus a reversible hydrogen electrode (RHE) at a sweep rate of 100 mV s−1 to remove surface contamination. As depicted in Figure 4a, there is only double-layer charging current in N2-saturated 0.1 mol L−1 KOH for all catalysts, whereas the ORR current peak of the COP@K10-Fe-900 shifts positively to around 0.816 V compared to those of COP-Fe-900 (0.780 V) and COP@ K10-900 (0.730 V) in O2-saturated 0.1 mol L−1 KOH solution, suggesting its superior activity. For further investigation of the ORR electrocatalytic activity of the prepared catalysts, linear scan voltammetry measurements of different catalysts were carried out on a rotating disk electrode (RDE). For comparison, a commercial Pt/C (20 wt % Pt) catalyst was also tested under the same conditions. As shown in Figure 4b, the COP@K10-Fe-900 exhibits the highest ORR activity with high onset potential (0.97 V) and half-wave potential (0.85 V), which are much more positive than those of COP-Fe-900 (0.91 and 0.8 V). Importantly, the newly developed COP@K10-Fe900 obtains an even positive onset potential (0.97 V) and halfwave potential (0.85 V) to those of the benchmark commercial Pt/C catalyst (0.97 and 0.83 V for onset and half-wave potential, respectively). This clearly indicates that iron species and the porous layered structure are both beneficial to improve
ORR activity of N-doped carbon materials. The higher ORR activity of COP@K10-Fe-900 is further confirmed by a smaller Tafel slope (66 mV decade−1 versus 65.7 mV decade−1 for commercial Pt/C, 72 mV decade−1 for COP-Fe-900, and 79 mV decade−1 for COP@K10-900; Figure 4c). It is noteworthy that the annealing temperature is also a key factor in the ORR activity for electrocatalysts. In view of these considerations, our catalysts were heat-treated under different temperatures ranging from 800 to 1000 °C. The COP@K10-Fe-900 catalyst exhibits the highest ORR catalytic activity in terms of highest onset potential and half-wave potential among three heat-treated temperatures (Figure 4d). This is because low-temperature carbonization lead to a low graphitization degree with low electron conductivity, while high-temperature carbonization could destroy the layered structure of the COP frameworks with a concomitant reduction in ORR catalytic activity. The yield of H2O2 reflects the reaction process and mechanism of the ORR. H2O2 yield of the COP@K10-Fe900 catalyst is monitored and calculated via RRDE, which is the smallest among those of COP-Fe-900, COP@K10-900, COP@ K10-Fe-800, and COP@K10-Fe-1000 (Figure S7). The corresponding electron transfer number is 3.69−4 over the potential range 0.3−0.9 V (versus RHE) for COP@K10-Fe-900 (Figure 5a), which is consistent with the results derived from the Koutecky−Levich plots based on the RRDE (Figure S8). This means that the COP@K10-Fe-900 favors a 4e− ORR process and exhibits extremely high efficiency to catalyze the ORR. Remarkably, low yield of H2O2 also prevents the catalyst from damaging its structure. As a result, the ORR activity of COP@K10-Fe-900 shows almost no degradation in the onset potential and half-wave potential even after 5000 continuous cycles by cycling the potential between 0.6 and 1.0 V (versus RHE) in oxygen-saturated 0.1 mol L−1 KOH at 100 mV s−1 (Figure 5b), indicating a high durability of COP@K10-Fe-900 in an alkaline medium. To further investigate the active sites of the catalyst, we first treated COP@K10-Fe-900 with acid to remove Fe 3 O 4 7874
DOI: 10.1021/acssuschemeng.7b01367 ACS Sustainable Chem. Eng. 2017, 5, 7871−7877
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ACS Sustainable Chemistry & Engineering
Figure 5. (a) The electron transfer number during the ORR process on different catalysts measured by the RRDE in the alkaline solution. (b) LSV curves of COP@K10-Fe-900 before and after a 5000 cycle test. (c) LSV curves of COP@K10-Fe-900, treated with 0.5 M HCl or 10 mM added KSCN in electrolyte, and simultaneously treated with 0.5 M HCl and 10 mM added KSCN in electrolyte.
Figure 6. (a, b) HRTEM images of COP@K10-Fe-900. (c) PXRD patterns of COP@K10-Fe-900 before and after being treated with 0.5 mol L−1 HCl. (d, e) HRTEM images and (f) XPS spectra of N 1s of COP@K10-Fe-900 treated with 0.5 mol L−1 HCl.
nanoparticles. After being treated with 0.5 mol L−1 HCl at 80 °C for about 8 h, Fe3O4 nanoparticles on COP@K10-Fe-900 are removed from the HRTEM and PXRD results (Figure 6a− e). XPS spectra disclose that Fe−Nx sites are still presented in the Fe3O4-removed COP@K10-Fe-900 catalyst (Figure 6f). Simultaneously, the Fe3O4-removed COP@K10-Fe-900 catalyst shows a negative shift of the half-wave potential by 35 mV and a decrease of the diffusion-limited current by about 14% (Figure 5c) compared to the COP@K10-Fe-900 catalyst. Fe3O4 itself also has poor catalytic activity for the ORR (Figure S9). This suggests that Fe3O4 nanoparticles or Fe−Nx itself can not deliver high activity for the ORR, and catalysts with both Fe3O4 nanoparticles and Fe−Nx exhibit high activity for the ORR. Additionally, it is well-known that SCN− ions can strongly coordinate with iron and hence poison the Fe−Nx sites. After 10 mM of SCN− ions are added in the alkaline electrolyte, the ORR catalytic activities of Fe3O4-removed COP@K10-Fe-900 and COP@K10-Fe-900 both dramatically decrease. This proves that there are Fe−Nx species in COP@K10-Fe-900 catalysts other than Fe3O4 nanoparticles, which are also presented in the XPS result (Figure 6f). On the basis of the above results, we believe that Fe−Nx itself is not the only high ORR catalytic activity in the COP@K10-Fe-900 catalyst, and the formed
Fe3O4 nanoparticles can also boost the ORR catalytic performance of the Fe−N−C catalyst.
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CONCLUSIONS In summary, we have reported a facile and scalable approach to prepare a porous layered Fe3O4/Fe−N−C (i.e., COP@K10-Fe900) catalyst from designed TPP-based covalent organic polymers within nanoconfined space. The unique layered structure of the designed precursors enables Fe3O4 nanoparticles and Fe−Nx species to disperse in the porous layered carbon framework uniformly. The nanoconfined space reduces the iron-based particle size from ∼50−150 to ∼10 nm. The optimal catalyst COP@K10-Fe-900 exhibits high ORR catalytic activity with an onset potential of 0.97 V (versus RHE) and a half-wave potential of 0.85 V (versus RHE), which are even higher than those of benchmark commercial 20 wt % Pt/C. Moreover, COP@K10-Fe-900 displays good stability and a four-electron transfer pathway, suggesting the direct reduction of oxygen to water. We also unambiguously find that Fe3O4 nanoparticles dramatically promote the ORR catalytic activity of neighboring Fe−Nx sites. Together with previous studies, this finding suggests that it is essential to dope abundant iron sources forming active iron compounds (e.g., metallic iron,53 iron carbide,26 or iron oxide) around Fe−Nx sites. We believe 7875
DOI: 10.1021/acssuschemeng.7b01367 ACS Sustainable Chem. Eng. 2017, 5, 7871−7877
Research Article
ACS Sustainable Chemistry & Engineering
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that this work can provide an important guideline for designing highly active M−N−C catalysts that can be widely used in energy conversion and storage devices.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01367. HRTEM image, EDX-mapping, N2 adsorption isotherm, XPS results, as well as LSV curves at different rotating rates (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Zhonghua Xiang: 0000-0002-0709-4527 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the financial support from the NSF of China (51502012, 21676020, and 21606015), Beijing Natural Science Foundation (2162032, The Start-Up Fund for Talent Introduction of Beijing University of Chemical Technology (buctrc201420), Talent Cultivation of State Key Laboratory of Organic−Inorganic Composites, The Fundamental Research Funds for the Central Universities (ZD1502, buctrc201524, and buctrc201714), BUCT Fund for Disciplines Construction and Development (XK1502), The 111 Project (B14004), and the Distinguished Scientist Program at BUCT.
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