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Mar 22, 2018 - nanopolyhedra electrocatalysts (Fe,Co,N-CNP) by one-step pyrolysis of a new type of Fe/Co bimetal zeolitic imidazolate framework (Fe,Co...
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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Bimetal Zeolitic Imidazolite Framework-Derived Iron‑, Cobalt- and Nitrogen-Codoped Carbon Nanopolyhedra Electrocatalyst for Efficient Oxygen Reduction Zhaowen Hu,† Zhiyuan Guo,† Zhengping Zhang,*,†,‡ Meiling Dou,*,†,‡ and Feng Wang*,†,‡ †

State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, and ‡Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China S Supporting Information *

ABSTRACT: Replacing precious metal electrocatalysts with high-performance and low-cost nonprecious metal electrocatalysts (NPMCs) is crucial for the commercialization of fuel cell technologies. Herein, we present a novel and facile route for synthesis of iron-, cobalt-, and nitrogen-codoped carbon nanopolyhedra electrocatalysts (Fe,Co,N-CNP) by one-step pyrolysis of a new type of Fe/Co bimetal zeolitic imidazolate framework (Fe,Co-ZIF) crystals that were self-assembled by oxygen-free solvothermal reaction of Fe2+ and Co2+ with 2methylimidazole. During the pyrolysis process, the Fe2+ ions in Fe,Co-ZIF not only effectively inhibit the aggregation of Co nanoparticles but also increase the specific surface area (SSA) and N content of the resultant electrocatalysts. The optimized Fe,Co,N-CNP(0.3) (Fe/Co molar ratio of 0.3 in Fe,Co-ZIF) electrocatalyst exhibited a highly promising activity for oxygen reduction reaction (ORR) with a positive half-wave potential (E1/2) of 0.875 V (29 mV higher than that of the commercial Pt/ C), excellent methanol tolerance, and electrochemical stability in the alkaline electrolyte. Also, Fe,Co,N-CNP(0.3) presents comparable ORR catalytic activity as Pt/C in the acidic electrolyte with E1/2 of 0.764 V and superior methanol tolerance and electrochemical stability. The outstanding ORR performance of Fe,Co,N-CNP(0.3) is ascribed to the synergistic contribution of homogeneous Fe, Co, and N codoping structure, high SSA, and hierarchically porous structure for rapid mass transport. This novel and rational methodology for controlled synthesis of ZIFs-derived nitrogen-doped porous carbon nanopolyhedras offers new prospects in developing highly efficient NPMCs for ORR. KEYWORDS: oxygen reduction reaction, electrocatalysts, iron, cobalt and nitrogen codoped carbon nanopolyhedra, bimetallic zeolitic imidazolate framework crystals, one-step carbonization

1. INTRODUCTION

Recently, metal−organic framework (MOF) materials have attracted significant interest for their appealing properties including permanent porosity, three-dimensional ordered framework structure, and diverse metals and organic linkers.11−13 Among various MOFs, zeolitic imidazolate frameworks (ZIFs) with a large specific surface area (SSA) and periodically ordered TM−N4 coordination substructures have been regarded as the promising precursors to synthesize highly efficient TM−N−C electrocatalysts for ORR.14−16 However, ZIFs usually suffer from uneven decomposition during the pyrolysis process, which causes severe agglomeration of metalbased nanoparticles (NPs), low N-doping, and sharp decrease of the SSA for prepared TM−N−C.17,18 These disadvantages obstruct the formation of uniform distribution and sufficient exposure of active sites, which might cause a relatively

The bottleneck of the commercialization for fuel cell technologies lies in the high expense of platinum group metal (PGM) electrocatalysts for catalyzing the oxygen reduction reaction (ORR) at the cathode.1−4 To address this issue, the exploration of high-performance and cost-effective non-PGM electrocatalysts is of vital importance.5 Transition-metal and nitrogen codoped nanocarbons (TM−N−C, TM: Fe, Co, Ni, Cu, etc.) are regarded as one kind of promising non-PGM ORR electrocatalysts because of their outstanding electrocatalytic activity, superior electrochemical stability, and methanol tolerance.6,7 So far, however, most reported TM−N−C electrocatalysts still exhibit an inferior ORR activity compared with the commercial Pt/C electrocatalyst especially in the acidic electrolyte,8,9 which is probably ascribed to their inhomogeneous distribution of limited TM−Nx ORR active sites (e.g., Fe−Nx and Co−Nx) and unfavorable porous structure to promote efficient mass transfer.10 © XXXX American Chemical Society

Received: January 10, 2018 Accepted: March 22, 2018

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DOI: 10.1021/acsami.8b00512 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

the electrocatalyst was ultrasonically blended with ethanol (2.0 mL) and Nafion (20 μL, 5 wt %, Dupont) for 0.5 h to generate a uniform electrocatalyst ink. Then, the electrocatalyst ink (20 μL) was transferred onto the working electrode, leading to a geometric loading of 0.788 mg cm−2. The Pt/C working electrode was used as a reference with the Pt loading of 20 μg cm−2.

insufficient ORR activity. Therefore, a rational design of the ZIF precursors by engineering their compositions and microstructures is a rational and effective way to synthesize highperformance TM−N−C electrocatalysts. In this work, we develop a novel and facile route to synthesize iron- cobalt- and nitrogen-codoped carbon nanopolyhedra (Fe,Co,N-CNP) as electrocatalysts for ORR via directly pyrolyzing Fe/Co bimetal ZIF (Fe,Co-ZIF) crystals without any posttreatment. To the best of our knowledge, very few works reported the successful synthesis of Fe,Co-ZIF crystals and their utilization as precursors for TM−N−C electrocatalysts. The presence of Fe2+ can effectively restrain the aggregation of Co NPs to form the well-dispersed metal NPs during pyrolysis, resulting in a uniform codoping of Fe, Co, and N, and a large SSA for Fe,Co,N-CNP. By controlling the molar ratio of Fe2+ to Co2+ (Fe/Co), the dispersion of metal NPs, SSA, and porous structure can be well-tuned and hence the ORR performance. The optimal Fe,Co,N-CNP (Fe/ Co = 0.3) exhibits higher ORR performance than that of the commercial Pt/C (20 wt % of Pt, Johnson Matthey) in the alkaline electrolyte and comparable ORR performance with Pt/ C in the acidic electrolyte.

3. RESULTS AND DISCUSSION 3.1. Physical Characterizations. The synthetic process in Scheme 1 involved the self-assembly of Fe,Co-ZIF(X) (where Scheme 1. Schematic Illustration of the Synthetic Process of the Fe,Co-ZIF(X) Precursors and the Resultant Fe,Co,NCNP(X) Electrocatalysts

2. EXPERIMENTAL SECTION 2.1. Synthesis of the Fe,Co,N-CNP Electrocatalyst. Typically, 0.728 g of Co(NO3)2 and 0.139 g of FeSO4 were dissolved in N2saturated methanol (160 mL) to form a clear solution (the total amount of TM2+ was 3 mmol with different molar ratios of Fe/Co). Then, the abovementioned solution was poured into another N2saturated methanol solution (160 mL) containing 2-methylimidazole (MeIM) (3.941 g) under stirring. Afterward, the solution was continuously bubbled with N2 and kept at 30 °C for 12 h, and the obtained precipitate was centrifugated, washed with methanol, and dried, generating the Fe,Co-ZIF(X) crystals where X (X = 0.1, 0.2, and 0.3, respectively) represents the molar ratio of Fe/Co. The as-prepared Fe,Co-ZIF(X) was pyrolyzed and carbonized at high temperatures (700−1000 °C) for 2 h in an argon atmosphere to generate Fe,Co,NCNP(X) electrocatalysts. Besides, ZIF-67 crystals were also synthesized by identical solvothermal reaction of Co(NO3)2 (0.873 g) and MeIM (3.941 g) in 320 mL of methanol, and the ZIF-67-derived Co,N-CNP electrocatalyst was also synthesized by pyrolysis at 800 °C for 2 h in an argon atmosphere for comparison. 2.2. Material Characterizations. Powder X-ray diffraction (XRD) patterns were profiled on a D/max-2500 X-ray diffractometer (Rigaku, Japan) using Cu Kα radiation as the source. Scanning electron microscopy (SEM) images were recorded on an FE-JSM6701F (JEOL, Japan) microscope, and high-resolution transmission electron microscopy (HR-TEM) images were taken on a JSM-2100 (JEOL, Japan) microscope. Aberration-corrected high-resolution scanning transmission electron microscopy (HR-STEM) images were obtained from a JEM-ARM200F (JEOL, Japan) microscope. The Brunauer−Emmett−Teller SSA and the pore structure were characterized by nitrogen adsorption−desorption measurements with a Quantachrome Autosorb-SI instrument. Raman spectra were profiled on a HORIBA Jobin Yvon LabRam HR800 confocal microscope (632.8 nm laser). Elemental analyses were conducted on a Thermo Fisher Scientific ESCALAB 250 X-ray photoelectron spectroscopy (XPS) spectrometer and an inductively coupled plasma−optical emission spectrometry (ICP−OES) system. 2.3. Electrochemical Measurements. All the electrochemical measurements were carried out on an RRDE-3A workstation (ALS/ DY2323 Bi-potentiostat) with a standard three-electrode system. A glassy carbon rotating disk electrode coated with an electrocatalyst, a saturated calomel electrode, and a Pt wire was employed as the working, reference, and counter electrodes, respectively. The reported potentials in this work were in reference to the reversible hydrogen electrode. In a typical preparation of the working electrode, 10 mg of

X represents the molar ratio of Fe/Co) followed by one-step pyrolysis for synthesis of Fe,Co,N-CNP(X) electrocatalysts. First, the Fe,Co-ZIF(X) crystals with various Fe/Co molar ratios were synthesized via the solvothermal reaction of Co(NO3)2 and FeSO4 with MeIM in methanol solution at 30 °C in a N2 atmosphere. We found that the O2-free environment is critical for the homogeneous coordination of Fe2+ and Co2+ with MeIM by preventing the oxidation of Fe2+ (under ambient conditions, we only obtained disordered aggregates, as indicated in Figure S1). The Fe/Co ratio was restricted to less than 0.5 because a higher Fe/Co ratio resulted in severe structure damage (Figure S2), probably because of the unfavorable coordination of excess Fe2+ with MeIM. Then, Fe,Co,N-CNP(X) electrocatalysts were synthesized by one-step pyrolysis of Fe,Co-ZIF(X) at 800 °C in an Ar atmosphere without any posttreatment (such as acid pickling and/or second heat treatment). The ordered MeIM organic linker in Fe,CoZIF(X) acted as the carbon and nitrogen source to form highly porous nitrogen-doped carbon networks, while metal ions can be converted to Fe−Nx and Co−Nx moieties, with a part of them thermally reduced to metallic Fe and Co NPs. The ICP− OES analysis indicated that the Fe/Co molar ratio was approximately 0.03, 0.11, and 0.15 for Fe,Co,N-CNP(0.1), Fe,Co,N-CNP(0.2), Fe,Co,N-CNP(0.3), respectively. Thermogravimetric analysis (Figure S3) showed that the decomposition and carbonization of Fe,Co-ZIF mainly took place at the temperature range of 500−600 °C, as indicated by the sharp decrease of weight, which was attributed to the pyrolysis of MeIM organic linkers at high temperatures.19 Besides, the residual weight of Fe,Co-ZIF (∼50%) was significantly higher than that of ZIF-67 (∼28%), indicating a better thermal stability of Fe,Co-ZIF. The pyrolysis temperature of 800 °C for B

DOI: 10.1021/acsami.8b00512 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) XRD patterns and (b) nitrogen adsorption−desorption isothermal curves (inset shows the PSDs) of ZIF-67 and Fe,Co-ZIF.

synthesis of Fe,Co,N-CNP(X) was selected according to the optimal experiments (Figure S4), which indicated a better ORR activity of Fe, Co,N-CNP obtained at 800 °C. The successful synthesis of Fe,Co-ZIF(X) (X = 0.1, 0.2 and 0.3) crystals was confirmed by their XRD patterns with the analogous topological information to that of ZIF-67 (Figure 1a). The negative shift of Fe,Co-ZIF(X) diffractions compared to ZIF-67 was probably caused by the lattice expansion because of partial replacement of Co2+ with larger Fe2+ ions into the ZIF-67 framework. The SEM images (Figure S5) showed that the Fe,Co-ZIF(0.1) and Fe,Co-ZIF(0.2) crystals exhibited welldefined rhombic dodecahedral morphologies similar to those of ZIF-67, while the Fe,Co-ZIF(0.3) displayed a slightly different morphology with quadrate facet grown at the vertex. The diameter of Fe,Co-ZIF(0.1), Fe,Co-ZIF(0.2), and Fe,CoZIF(0.3) is approximately 0.6, 0.7, and 1 μm, respectively, all larger than that of ZIF-67 (approximately 0.4 μm). The increased diameter of Fe,Co-ZIF(X) nanopolyhedra is probably attributed to the elevated Gibbs free energy caused by lattice distortion of partial substitution of Co2+ by Fe2+. Element mapping (Figure 2a,b) of representative Fe,Co-ZIF(0.3) indicated the uniform distribution of Fe, Co, N, and C, indicating a homogenous substitution of Co2+ by Fe2+. Nitrogen adsorption−desorption measurements (Figure 1b) indicated that all the Fe,Co-ZIF crystals have microporous structures similar to that of ZIF-67 with large SSAs of 1456.3, 1447.4, and 1450.8 m2 g−1 for Fe,Co-ZIF(0.1), Fe,Co-ZIF(0.2), and Fe,Co-ZIF(0.3), respectively (Table S1) and narrow pore size distribution (PSD) centered at approximately 1.1 nm. The morphologies of Fe,Co,N-CNP(X) samples were revealed by TEM and HR-STEM images. It is clear that all the Fe,Co,N-CNP samples well-inherited the polyhedron morphology of Fe,Co-ZIF crystals but with Fe and Co NPs well-dispersed on the porous carbon frameworks. As the Fe/Co ratio of Fe,Co,N-CNP(X) increased, the average diameters of Fe and Co NPs decreased, and their dispersion became more uniform (Figure S5). The HR-STEM image (Figure 2c) showed that Fe,Co,N-CNP(0.3) possessed the smallest Co and Fe NPs with an average diameter of approximately 4.8 nm without any obvious aggregation. In contrast, the Co NPs in Co,N-CNP from ZIF-67 exhibited severe aggregation with a diameter ranging from 5 to 70 nm (average diameter of 16.7 nm, Figures S5b and S6a−c). These results were most probably due to that the presence of Fe2+ species which could enter into the Co2+-based frameworks, resulting in the effective inhibition of the Co NP aggregation during the pyrolysis of Fe,Co-ZIF crystals.18 HR-STEM elemental mapping analysis (Figure 2d) suggested a more uniform doping of Fe, Co, and N in the carbon frameworks for Fe,Co,N-CNP(0.3) than that of Co,NCNP with Co and N doping (Figure S6), implying the probable

Figure 2. Representative HR-STEM and corresponding element mapping images of (a,b) Fe,Co-ZIF(0.3) and (c,d) Fe,Co,NCNP(0.3). (e) HR-TEM and (f) SAED pattern of Fe,Co,N-CNP(0.3).

formation of homogeneously distributed Fe−Nx and Co−Nx moieties during the pyrolysis of Fe,Co-ZIF(0.3), which is generally considered to be favorable to boost the ORR catalytic activity.20−22 The energy-dispersive spectrometer spectrum analysis indicated that the content of Fe and Co was determined as 1.7 and 11.7 at. %, respectively. The HR-TEM images in Figure 1e displayed that some well-dispersed metallic NPs were wrapped by the graphitic carbon layers with a (002) crystal lattice spacing of 0.334 nm, which is ascribed to the catalytic graphitization of MeIM at the surface by Fe and Co NPs. The lattice spacings of 0.202 and 0.204 nm were assignable to the (110) and (111) planes of face-centered cubic C

DOI: 10.1021/acsami.8b00512 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. XPS survey spectra of (a) Co,N-CNP and Fe,Co,N-CNP(X) and (b) corresponding N content. High-resolution (c) N 1s and (d) Fe 2p and Co 2p spectra of Fe,Co,N-CNP(0.3). (e) Nitrogen adsorption−desorption isothermal curves and (f) PSDs of Co,N-CNP and Fe,Co,NCNP(X).

presence of Fe is crucial to ensure the high-content N-doping in Fe,Co,N-CNP(X). The Fe,Co,N-CNP(0.3) possessed a N content of 9 at. %, which is the highest among all the samples. The deconvoluted N 1s spectra (Figure 3c) of Fe,Co,NCNP(0.3) showed that the doped N was primary pyridinic N (48.9%) and pyrrolic N (18.9%), which are considered as the decent contributors for ORR activity because of their electron effect within the carbon frameworks.24,25 The deconvoluted XPS spectra of Fe 2p and Co 2p (Figure 3d) also verified the existence of Fe−Nx and Co−Nx moieties and metallic Fe and Co NPs. The high content and homogeneous N-doping with Fe−Nx and Co−Nx moieties are expected to contribute significantly to the improvement of ORR activity. The SSA and the porous structure of Fe,Co,N-CNP(X) were investigated by nitrogen adsorption−desorption measurements (Figure 3e,f). All the Fe,Co,N-CNP(X) samples exhibited a unique hierarchically porous structure with the co-existence of micropores and mesopores. The SSAs were determined as 413.5, 425.3, and 422.0 m2 g−1 for Fe,Co,N-CNP(0.1), Fe,Co,N-CNP(0.2), and Fe,Co,N-CNP(0.3), respectively, all of which were rather higher compared with that of Co,N-CNP (347.4 m2 g−1). The enlarged SSA of Fe,Co,N-CNP(X) was attributed to the better retention of the microporous networks of Fe,Co-ZIF because of their more uniform pyrolysis as compared with ZIF-67 (Table S1). It is clear that adopting bimetal Fe,Co-ZIF(X) as the precursor can not only promote the uniform dispersion of metal NPs but also increase the SSA

metallic (fcc) Co (JCPDS: no. 15-0806) and body-centered face (bcc) metallic Fe (JCPDS: no. 06-0696), respectively. XRD (Figure S7) and selected area electron diffraction (SAED) patterns (Figure 2f) also reflected the high crystallinity of metal NPs with clear (111), (200), and (220) diffractions for fcc Co and (110), (200), and (211) diffraction, respectively, for bcc Fe. Raman analyses indicated that all the Fe,Co,N-CNP(X) samples possessed lower ID/IG ratios [1.29, 1.25, and 1.11 for Fe,Co,N-CNP(0.1), Fe,Co,N-CNP(0.2), and Fe,Co,NCNP(0.3), respectively] compared with that of Co,NCNP(1.42) (Figure S8), indicating a higher graphitization degree of Fe,Co,N-CNP. Besides, the graphitization degree of Fe,Co,N-CNP(X) increased with the increase of the Fe/Co ratio, suggesting that the presence of Fe can significantly improve the graphitization degree by catalyzing the carbonization process from MeIM to pyrolyzed carbon. To explore the chemical properties of the surface state, we conducted XPS measurement for all the Fe,Co,N-CNP(X) samples and Co,N-CNP. It is clear that all the electrocatalysts showed the existence of Fe, Co, N, C, and O elements (Figure 3a), indicating the doping of Fe, Co, and N in the carbon frameworks by one-step pyrolysis of the Fe,Co-ZIF crystals. Notably, the N content of all the Fe,Co,N-CNP(X) samples was higher compared with that of Co,N-CNP and positively correlated with the Fe/Co ratio (Figure 3b and Table S2), probably because of the easy bonding of Fe−N than Co−N during the pyrolysis process.23 This result indicates that the D

DOI: 10.1021/acsami.8b00512 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. (a) LSV curves of Co,N-CNP, Fe,Co,N-CNP(X), and Pt/C electrocatalysts in O2-saturated 0.1 M KOH at a rotation rate of 1600 rpm. (b) Corresponding half-wave potentials and kinetic current densities for these electrocatalysts. (c) LSV curves for Fe,Co,N-CNP(0.3) at various rotation rates (inset: the corresponding K−L plots at different potentials). (d) Tafel plots and the corresponding slopes in the high potential region. (e) Current−time chronoamperometric responses of Fe,Co,N-CNP(0.3) and Pt/C in O2-saturated 0.1 M KOH with a rotation rate of 1600 rpm at 0.8 V. (f) Current−time chronoamperometric responses of Fe,Co,N-CNP(0.3) and Pt/C at 0.8 V in O2-saturated 0.1 M KOH with and without 3 M methanol.

of the resultant Fe,Co,N-CNP(X). The PSD indicated that the pores of Fe,Co,N-CNP(X) were all primarily in the range of 0− 2 and 2−10 nm. Because the TM−Nx active sites are probably embedded into the sub-2 nm micropores of porous carbonbased networks, the higher the content of the micropores, the more active the sites are in the Fe,Co,N-CNP(X) electrocatalyst. In particular, Fe,Co,N-CNP(0.3) possessed the highest microporous SSA (205 m2 g−1) among all the samples (Table S1), which is beneficial to the anchoring of Fe−Nx and Co−Nx moieties and the increase of active site density.26 Besides, the remarkably higher content of mesopores in Fe,Co,N-CNP(0.3) compared with Fe,Co,N-CNP(0.1), Fe,Co,N-CNP(0.2), and Co,N-CNP is advantageous to improve the mass transport of ORR reactants. 3.2. Electrochemical Performance. To investigate the ORR activity of Fe,Co,N-CNP(X), cyclic voltammetry (CV) measurements were conducted in the N2- and O2-saturated 0.1 M KOH electrolyte (Figure S9). In the O2-saturated electrolyte, all Fe,Co,N-CNP(X) electrocatalysts showed obvious ORR peaks at approximately 0.8−0.9 V and exhibited higher peak potential than that of Co,N-CNP, suggesting the improvement of ORR activity. Besides, the peak at potential higher than 1.0 V in the CV profile was attributed to the redox reaction of surface Co species for Co,N-CNP, Fe,Co,NCNP(0.1), and Fe,Co,N-CNP(0.2), and no obvious Co species

redox peak was observed for Fe,Co,N-CNP(0.3), which was probably ascribed to the low content of surface Co. Linear sweep voltammetry (LSV) measurements were further performed to examine the ORR activity (Figure 4a). All the Fe,Co,N-CNP(X) electrocatalysts exhibited remarkably improved activity with higher onset potentials (Eonset) and halfwave (E1/2) potentials, as well as larger diffusion-limited current densities (Jd) and kinetic current density (Jk) than those of Co,N-CNP electrocatalysts derived from ZIF-67 (Eonset: 0.893 V, E1/2: 0.825 V, Jd: 4.742 mA cm−2, Jk: 1.620 mA cm−2 at 0.85 V) (Figure 4b). This result indicated that the presence of Fe played a vital role in the enhancement for ORR activity. The activity tended to increase with the increase of the Fe/Co ratio, and the best activity was achieved at the Fe/Co ratio of 0.3. The Eonset and E1/2 of Fe,Co,N-CNP(0.3) were determined as 0.979 and 0.875 V, respectively (Table S3), which are higher than those of Pt/C (Eonset of 0.941 V, E1/2 of 0.846 V) and recently reported MOF-derived ORR electrocatalysts (Table S4). The Jk of Fe,Co,N-CNP(0.3) (12.55 mA cm−2 at 0.85 V) calculated according to the Koutecky−Levich (K−L) equation was 2.4-fold of Pt/C (5.15 mA cm−2 at 0.85 V) and significantly better compared with Co,N-CNP. Furthermore, the Fe,Co,NCNP(0.3) also possessed a high ORR electron transfer number (3.96) according to the K−L equation (Figure 4c), which is very close to Pt/C (3.99, Figure S10) and higher compared E

DOI: 10.1021/acsami.8b00512 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. (a) LSV curves for Co,N-CNP, Fe,Co,N-CNP(X), and Pt/C electrocatalysts in O2-saturated 0.1 M HClO4 at a rotation rate of 1600 rpm. (b) Corresponding half-wave potentials and kinetic current densities for these electrocatalysts. (c) Tafel plots and the corresponding slopes in the high potential region. (d) The electron transfer number (n, top) and the H2O2 yield (bottom) of Co,N-CNP, Fe,Co,N-CNP(0.3) and Pt/C. (e) Chronoamperometric responses of Fe,Co,N-CNP(0.3) and Pt/C in O2-saturated 0.1 M HClO4 with a rotation rate of 1600 rpm at 0.8 V. (f) Chronoamperometric response of Fe,Co,N-CNP(0.3) and Pt/C in O2-saturated 0.1 M HClO4 with and without 3 M methanol.

highest ORR catalytic activity (Eonset: 0.871 V, E1/2: 0.764 V, and Jd: 5.79 mA cm−2) (Table S5), which was significantly higher than Co,N-CNP (Eonset: 0.780 V, E1/2: 0.671 V, and Jd: 4.97 mA cm−2) and recently reported MOFs-derived ORR electrocatalysts (Table S6), and even comparable with Pt/C (Eonset: 0.901 V, E1/2: 0.814 V, and Jd: 5.93 mA cm−2). The HO2− yield of Fe,Co,N-CNP(0.3) was also measured to be as low as 2.5%. The electron transfer number was 3.95−3.98 at various rotation rates, which is comparable to Pt/C and rather higher than Co,N-CNP (3.10−3.60), suggesting a four-electron pathway for ORR. Furthermore, the Fe,Co,N-CNP(0.3) also demonstrated a higher electrochemical stability and superior methanol tolerance in comparison with Pt/C in the acidic electrolyte. Notably, the activity of Fe,Co,N-CNP(0.3) in the acidic electrolyte was indeed not as good as it was in the alkaline electrolyte. The E1/2 of the LSV curve for Fe,Co,NCNP(0.3) in the acidic electrolyte was approximately 120 mV lower than that in the alkaline electrolyte. The chronoamperometric test of Fe,Co,N-CNP(0.3) in the acidic electrolyte showed more significant degradation of ORR current than that in the alkaline electrolyte, indicating that Fe,Co,N-CNP(0.3) was less stable in the acidic electrolyte. It was reported that the Fe- or Co-based electrocatalyst tended to suffer from the degradation in the acidic electrolyte, which might be attributed to the loss of active TM−Nx sites and N-doped carbon that occurs at high potential,28 leading to a decayed ORR kinetic.

with Co,N-CNP (3.85, Figure S11), suggesting a four-electron transfer ORR pathway. The plotted Tafel curves in Figure 4d indicated that Fe,Co,N-CNP(0.3) afforded a similar Tafel slope (75 mV dec−1) to Pt/C (69 mV dec−1) in the high potential region, indicating that the ORR rate-determining step might be the first electron transfer.27 High electrochemical stability and good methanol tolerance of ORR electrocatalysts are also of great significance for the commercialization of fuel cell technologies. The electrochemical stability of Fe,Co,N-CNP(0.3) was evaluated by chronoamperometry at 0.8 V in the O2-saturated 0.1 M KOH electrolyte. Fe,Co,N-CNP(0.3) showed a retention current of approximately 90.2% after the test for 10 000 s (Figure 4e), indicating a superior electrochemical stability. For the commercial Pt/C, an obvious current loss of approximately 34.3% was detected under the identical condition. Moreover, Fe,Co,N-CNP(0.3) also showed a superior methanol tolerance with negligible deviation of current density upon the rapid injection of methanol (Figure 4f), while Pt/C showed an obvious decrease in the current density. The ORR performance of Fe,Co,N-CNP(X) electrocatalysts was also investigated in the acidic electrolyte by measuring the LSV curves in O2-saturated 0.1 M HClO4 (Figure 5). A similar trend with obvious improvement of activity with the increasing Fe/Co ratio was also observed, indicating that the presence of Fe effectively enhanced the ORR activity in the acidic electrolyte. Notably, Fe,Co,N-CNP(0.3) still exhibited the F

DOI: 10.1021/acsami.8b00512 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces On the basis of the abovementioned results, pyrolysis of the bimetal Fe,Co-ZIF can afford a significantly superior ORR activity with higher electron transfer number for Fe,Co,N-CNP compared with Co,N-CNP derived from the single-metal ZIF67 in both alkaline and acidic electrolytes. The synergistic contribution of Fe−Nx and Co−Nx active sites played a positive role in the activity and selectivity of ORR; first, the co-doping of Fe−Nx and Co−Nx has promoted the homogeneous dispersion of ORR active sites because the introduction of Fe hindered the aggregation of Co NPs during pyrolysis. Second, the co-existence of Fe−Nx and Co−Nx active sites can increase the total number of ORR active sites because of the modified pyrolysis behavior of Fe,Co-ZIF with increased N-doping content. Third, the dual Fe−Nx and Co−Nx sites have been reported to be favored for activation of the O-O band by reducing the cleavage barrier,29 which is beneficial for the improvement of ORR activity and especially crucial for high selectivity with a four-electron pathway.29 Besides, the homogenous dispersion of active sites, large SSA, and hierarchically micro- and mesoporous structure for favorable mass transportation also contributed to the improvement of ORR activity.

4. CONCLUSIONS To summarize, we have designed and synthesized a bimetal Fe,Co-ZIF as the precursor to fabricate Fe,Co,N-CNP electrocatalysts by one-step pyrolysis. The presence of Fe2+ in Fe,Co-ZIF inhibited the aggregation of Co NPs and endowed the resultant Fe,Co,N-CNP with a uniform Fe, Co, and Ndoping. By optimizing the Fe/Co ratio, the best ORR activity was achieved with Fe,Co,N-CNP(0.3), which possessed the smallest diameter of Fe and Co NPs, highest N content, and largest microporous SSA. The Fe,Co,N-CNP(0.3) exhibited a remarkable ORR activity with the Eonset of 0.979 V (38 mV higher than Pt/C), E1/2 of 0.875 V (29 mV higher than Pt/C), and Jk of 12.55 mA cm−2 at 0.85 V (2.4 times of Pt/C) in the alkaline electrolyte, and a higher ORR activity with E1/2 of 0.764 V comparable to Pt/C in the acidic electrolyte. More importantly, Fe,Co,N-CNP(0.3) also showed better electrochemical stability and superior methanol tolerance in both alkaline and acidic electrolytes. It is expected that the bimetal Fe,Co-ZIF-derived Fe,Co,N-CNP electrocatalysts hold great promise and can be extended to the application of batteries, sensors, and supercapacitors. In addition, this novel and facile synthetic route offers an effective strategy for the design and synthesis of high-performance nonprecious metal electrocatalysts toward ORR.

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ZIF(0.2), and (g) Fe,Co-ZIF(0.3). HR-TEM images of (b) Co,N-CNP, (d) Fe,Co,N-CNP(0.1), (f) Fe,Co,NCNP(0.2), and (h) Fe,Co,N-CNP(0.3); HR-STEM (a) and the corresponding elemental mapping (b) of Co,NCNP, and particle distribution of metal NPs in Co,NCNP(c) and Fe,Co,N-CNP(0.3) (d); XRD patterns of Co,N-CNP and Fe,Co,N-CNP(X); Raman spectra of Co,N-CNP and Fe,Co,N-CNP(X); comparative CV curves of Co,N-CNP and Fe,Co,N-CNP(X) in N2(dotted line) and O2- (solid line) saturated 0.1 M KOH (scan rate: 50 mV s−1); LSV curves of the commercial Pt/C in O2-saturated 0.1 M KOH (scan rate: 5 mV s−1) under various rotation rates (inset: the corresponding K−L plots); LSV curves of Co,N-CNP in O2-saturated 0.1 M KOH (scan rate: 5 mV s−1) under different rotation rates (inset: the corresponding K−L plots); pore characteristics of ZIF-67 and Fe,Co-ZIF(X), and the corresponding electrocatalysts; N content and configuration determined by XPS in Co,N-CNP and Fe,Co,NCNP(X); the ORR activity of Co,N-CNP and Fe,Co,NCNP(X) in 0.1 M KOH; summary of ORR activity of the nonprecious electrocatalysts in the 0.1 M KOH electrolyte reported in the literature; the ORR activity of Co,NCNP and Fe,Co,N-CNP(X) in 0.1 M HClO4; and summary of ORR activity of the nonprecious electrocatalysts in 0.1 M HClO4 reported in the literature (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone/Fax: 8610-64451996 (Z.Z.). *E-mail: [email protected] (M.D.). *E-mail: [email protected] (F.W.). ORCID

Feng Wang: 0000-0002-7901-3693 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Funds of China (nos. 51502013 and 51432003). REFERENCES

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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/acsami.8b00512. Low (a) and high (b) magnification SEM images of Fe,Co-ZIF(0.2) synthesized under ambient conditions; SEM image of Fe,Co-ZIF with the Fe/Co ratio of 0.5 (a) and 1.0 (b); TG analysis of Fe,Co-ZIF(0.3) in comparison with ZIF-67 under argon atmosphere (temperature ramp rate: 10 °C min−1); LSV curves of Fe,Co,N-CNP prepared at various pyrolysis temperatures in O2-saturated 0.1 M KOH (scan rate: 5 mV s−1, electrode rotation rate: 1600 rpm); representative SEM images of (a) ZIF-67, (c) Fe,Co-ZIF(0.1), (e) Fe,CoG

DOI: 10.1021/acsami.8b00512 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.8b00512 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX