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An ingenious approach for ZIFs derived N-doped hierarchical porous carbon hybrids with FeCo alloy nanoparticles as efficient bifunctional oxygen electrocatalysts
Applied Surface Science 487 (2019) 496–502
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An ingenious approach for ZIFs derived N-doped hierarchical porous carbon hybrids with FeCo alloy nanoparticles as efficient bifunctional oxygen electrocatalysts Guoning Li, Kaitian Zheng, Chunjian Xu
T
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School of Chemical Engineering and Technology, State Key Laboratory of Chemical Engineering, Chemical Engineering Research Center, Tianjin University, Tianjin 300072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Porous carbon ZIF-8 Oxygen reduction reaction Oxygen evolution reaction FeCo alloy
An ingenious approach is proposed to achieve exceedingly efficient bifunctional oxygen electrocatalysts of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Hierarchical porous structure constituted by nanotubes and nanobulks with high specific surface area and encapsulated FeCo alloy nanoparticles is successfully prepared via the hybrid of Fe(II)-doped ZIF-8 (FeZIF) and cobalt (III) acetylacetonate (Co(acac)3), which is greatly redound to mass transfer and electron transport. Interestingly, Co(acac)3 plays dual-roles during preparation process, which not only generates new metal species but improves porous structure. Moreover, FeZIF acts as a self-sacrificing template and iron source. Owing to the coaction of the unique porous structure, bimetal-N active sites and FeCo alloy, the resultant catalyst carbonized at 850 °C exhibits a prominent ORR activity with a positive half-wave potential of 0.864 V (vs the reversible hydrogen electrode, RHE), and a low operating potential of 1.675 V at a current density of 10 mA cm−2 for OER in alkaline media. The potential gap (ΔE) of overall oxygen electrode is 0.81 V, which surpass that of numerous non-noble metal-based catalysts reported to date, indicating a promising bifunctional oxygen electrocatalyst.
1. Introduction With the fossil fuel consumption growing progressively, electrochemical energy conversation or storage devices, such as unitized regenerative fuel cells and rechargeable metal-air batteries, are gaining more and more attention [1–3]. The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which directly determine the overall energy efficiency, are pivotal to these devices [4,5]. Noble metal-based catalysts, especially Pt/C and RuO2, are considered as efficient electrocatalysts for ORR or OER. Unfortunately, the catalysts composed of a single noble metal cannot achieve acceptable activity for both ORR and OER simultaneously [6,7]. Moreover, their high cost and poor durability extremely hinder the cosmically practical commercialization. Hence, it is imperative to excogitate non-noble metal-based bifunctional oxygen catalysts. Recently, carbon-based materials, such as heteroatom-doped carbon, have been widely investigated as electrode materials [8–10]. Among these materials, transition metal based carbon materials (TM-N-C), especially Fe-N-C materials, are considered as potential electrocatalysts, exhibiting remarkable performance on ORR [11–14]. Meanwhile, It has been proved that the bimetallic TM-N-C
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materials could possess higher ORR activity than monometallic materials [15,16]. Wang et al. proposed a host-guest strategy to prepare ZIFderived FeCo-N-C materials for high ORR catalytic activity [17]. Moreover, bimetallic alloys (FeCo, FeNi, CoNi) exhibit outstanding catalytic activity and stability for OER [18,19]. An in situ coupling approach was developed by Zhu et al. to obtain FeCo alloys and Co4N hybrid as a highly efficient OER catalyst [20]. For the FeCo-based catalysts, the existence of Co can enhance the activity of Fe-based active sites and improve electrical conductivity [21–23]. Based on the above, bimetallic TM-N-C materials with encapsulated bimetallic alloy could suffice for excellent overall oxygen electrode activity for ORR and OER. Zeolitic imidazolate framework-8 (ZIF-8), as an important subclass of metal-organic frameworks (MOFs), is a 3D tetrahedral framework constituted by zinc ions and 2-methylimidazolate linkers. Owing to the unique features, especially large specific surface area, high nitrogen content, controllable size and shape, ZIF-8 has been widely used as pyrolysis precursors to prepare carbon-based materials [24]. However, the discrete nanoparticles and the porous structure mainly with micropores exist in the ZIF-derived carbon materials directly obtained by the pyrolysis of ZIF-8, which hinders mass transfer and electron
Corresponding author. E-mail address: [email protected] (C. Xu).
https://doi.org/10.1016/j.apsusc.2019.05.014 Received 12 February 2019; Received in revised form 10 April 2019; Accepted 2 May 2019 Available online 03 May 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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least 12 h.
transport as well as the accessibility of the active site, resulting in inferior electrochemical performance. Furthermore, the Fe-N-C materials have been successfully prepared by Fe-doped ZIF-8 or hybrids of ZIF-8 and iron source [25,26]. Fe2+ is superior to achieve homogeneously Fedoped ZIF-8 compared with Fe3+, resulting in Fe-N-C material with high Fe dispersion at atomic level via pyrolysis treatment [27]. Nevertheless, the introduction of Fe2+ or Co2+ can lead to ZIF-derived carbon materials constituted by larger size nanoparticles [28–30], which is detrimental to the exposure of active sites and mass transfer. Hence, reducing the crystal size and increasing meso/macropores for the ZIF-derived materials are gaining more attentions. Recently, Zhang et al. reported a facile method to prepare Fe-N-C catalyst with controllable size derived from Fe-doped ZIF precursors via tuning the concentration of metal precursors in methanol solution [31]. A poreexpansion strategy was proposed by Zhao et al. to enhance ORR activity, which could transform ZIF-8 with microporous structure into foam-like hierarchical porous carbons [32]. Therefore, it is sufficiently challenging and desirable to synthesize a bimetallic TM-N-C material with bimetals alloy and hierarchical pores only via the pyrolysis of ZIFs hybrids, which could be used as an efficient bifunctional oxygen electrocatalyst of ORR and OER. In this work, we present a facile and ingenious strategy comprising impregnation and pyrolysis treatments to achieve bimetallic TM-N-C catalysts by the hybrid of FeZIF and Co(acac)3. Via using local oxidation of FeZIF during impregnation, this work combines Fe(II)-doped ZIF-8 with Co(acac)3 to synthesize TM-N-C material for the first time. Interestingly, Co(acac)3 plays dual-roles during preparation process, which not only generates new metal species but improves porous structure. Moreover, FeZIF acts as a self-sacrificing template and iron source. The resultant catalyst prepared at 850 °C (FeCo-NC-850) shows a 3D interconnected hierarchical porous structure consist of nanotubes and nanobulks, which can efficiently facilitate the mass/electron transport during ORR and OER process. Benefiting from this unique nanostructure as well as FeCo alloy and bimetal-N active sites in carbon matrix, FeCo-NC-850 exhibits a superior overall oxygen electrode activity under alkaline conditions, manifesting a promising bifunctional catalyst for both ORR and OER.
2.4. Carbonization process FeCoZIF powder was pyrolyzed at different temperatures (750 °C, 850 °C, 950 °C) with a rate of 3 °C min−1 and held at the final temperature for 3 h under a nitrogen atmosphere in a tube furnace. The obtained material was soaked in 0.5 M H2SO4 solution for 24 h, washed with distilled water four times and ethanol two times, and finally dried under vacuum (80 °C) for at least 24 h. FeZIF powder was pyrolyzed at 850 °C and the remaining steps is the same with FeCoZIF. The FeCoZIFderived carbons are denoted as FeCo-NC-T, where T represents the pyrolysis temperature, and the FeZIF-derived carbon was denoted as FeNC-850. 2.5. Physicochemical characterization The morphology of material was observed by a scanning electron microscope (SEM, Hitachi S-4800) and transmission electron microscope (TEM, JEOL JEM-2100). The structure phase of samples was investigated by a D8 Advance Bruker X-ray diffractometer (XRD) at a scan rate of 10°min−1. Raman spectroscopy was performed using a Renishaw InVia Spectrometer with a 532 nm excitation laser. Nitrogen adsorption/desorption was performed on Micromeritic TriStarII 3020. XPS measurements were carried out by a Thermo Scientific Spectrometer with an Escalab 250 Xi X-ray. 2.6. Electrochemical performance evaluation All the electrochemical tests were measured via a CHI 660E workstation (CH Instruments, Shanghai) and RDE equipment (ALS/DY2323 Bi-potentiostat) with a standard three-electrode system. A glassy carbon rotating disk electrode (3 mm of disk diameter) covered up by catalysts, a platinum wire and a Ag/AgCl with saturated KCl were used as the working, counter and reference electrodes, respectively. The ORR and OER activity were both measured in 0.1 M KOH electrolyte. The mass loading of the prepared material on the working electrode was 0.28 mg∙cm−2, and that of reference substance (Pt for ORR and RuO2 for OER) was 0.12 mg cm−2. High purity oxygen or nitrogen was bubbled into the electrolyte for at least 0.5 h before the tests. The CV measurements were carried out in N2 or O2-saturated electrolyte with a scan rate of 50 mV s−1. The RDE tests were conducted with a scan rate of 10 mV s−1 at various rotation rates from 400 to 2025 rpm. The electron transfer number (n) for ORR was calculated by the KouteckyLevich equations:
2. Experimental 2.1. Material Zinc nitrate hexahydrate (Zn(NO3)2·6H2O) (99.95%) was purchased from Macklin. Cobalt (III) acetylacetonate (98%) was obtained from Tianjin Heowns Company. FeSO4·7H2O (99.5%), 2-Methylimidazole (99%) and methanol (99.9%) were produced by Tianjin Kermel Company. Nafion perfluorinated resin solution (5 wt%) was purchased from DuPont Company. RuO2 (99.9%) was purchased from Macklin and Pt/C (20 wt%) were used as received.
1 1 1 1 1 = + = + J JL JK Bω1/2 JK B = 0.62nFC0 D0 2/3υ−1/6 where J represents the measured current density. JL and JK are the kinetic current density and limiting diffusion current density, respectively. ω is the rotation rate of RDE test. F is the Faraday constant of 96,485C mol−1, C0 is the bulk concentration of O2 dissolved (1.2 × 10−3 mol L−1), D0 is the diffusion coefficient of O2 (1.9 × 10−5 cm2 s−1), and υ is the kinetic viscosity of 0.1 M KOH (0.01 cm2 s−1).
2.2. Synthesis of FeZIF Zn(NO3)2·6H2O (2.0 mmol) and FeSO4 (0.25 mmol) were dissolved in 30 ml N2-saturated methanol. Then, another methanol solution (30 ml) with 2-methylimidazole (11.4 mmol) was poured into the above solution. The N2-saturated mix solution was sealed and subsequently stirred for 24 h at room temperature. Afterward, the FeZIF was centrifuged, washed with methanol four times and finally dried under vacuum (60 °C) for at least 12 h.
3. Results and discussion Scheme 1 exhibits a facile and ingenious strategy for 3D interconnected hierarchical porous FeCo-Nitrogen-Carbon materials, which mainly includes the process of impregnation and carbonization. The morphology of the as-prepared catalysts was revealed by SEM and TEM images. As shown in Fig. S1, FeZIF displays a stacked nanocrystal structure with polyhedron shape, whereas FeCoZIF exhibits
2.3. Synthesis of FeCoZIF FeZIF and Co(acac)3 were mixed with a weight ratio of 11:1 in methanol under ultrasonication for 4 h and then the mixture was magnetic stirred for 24 h. Subsequently, the mixture was centrifuged, washed with methanol and finally dried under vacuum (60 °C) for at 497
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Scheme 1. Schematic illustration for the synthesis of FeCo-NC-850.
Fig. 1. SEM images of (a) Fe-NC-850, (b) FeCo-NC-850, (c) Co@Fe-NC-850. (d) TEM, (e) HRTEM and (f) Elemental mapping images of FeCo-NC-850.
improvement of the mass/electron transport. We can see that the introduction of Co(acac)3 results in a visible morphological change. To better understand the influence of cobalt source on the morphology, Co@Fe-NC-850 derived from the hybrid of Co(NO3)2 and FeZIF was prepared by the same preparation process as FeCo-NC-850. Different from FeCo-NC-850, larger bulk and many nanowires are observed in
disorderly smaller nanoparticles. Elemental mapping reveals that Fe and Co are homogenously distributed in FeCoZIF (Fig. S2). Fe-NC-850 derived from FeZIF still presents large irregular bulk with micron size (Fig. 1a). Remarkably, FeCo-NC-850 exhibits a 3D well-interconnected hierarchical porous structure built by nanotubes and nanobulks (Fig. 1b), which is conductive to electrochemical reaction owing to the 498
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located at 44.7, 65.1 and 82.2° which could be attributed to the Co3Fe7 alloy (JCPDS No. 48-1816), corresponding to the results of TEM. However, Co@Fe-NC-850 displays mightily weaker peak intensity of FeCo alloy, indicating negligible FeCo alloy. The Raman spectra of the as-prepared catalysts shows the D and G bands at about 1322 and 1587 cm−1 (Fig. 2b), corresponding to defective carbon and graphitic carbon, respectively [38–40]. The intensity ratio (ID/IG) decreases with the increased pyrolysis temperature, indicating that high temperature could improve the graphitic degree of carbon materials. Moreover, higher graphitization represents a better electrical conductivity of catalysts [41,42]. The values of ID/IG reveal FeCo-NC-850 has a superior conductivity compared with Fe-NC-850 and Co@Fe-NC-850. To further investigate the chemical composition and the interaction between metal and nitrogen for these catalysts, XPS measurement was conducted. The survey XPS spectrum of FeCo-NC-850 confirms the existence of N (6.18 at.%), Fe (0.67 at.%) and Co (0.12 at.%) elements (Fig. S4a). High-resolution N 1 s spectra of FeCo-NC-850 and the other catalysts are shown in Fig. 2c and S4. Based on the previous studies [26], the high- resolution N 1s spectra could be deconvoluted into five peaks assignable to pyridinic-N (398.3 ± 0.2 eV), metal-N (399.6 ± 0.2 eV), pyrrolic-N (400.8 ± 0.2 eV), graphitic-N (402 ± 0.2 eV) and N-oxidized (> 404 eV). Fig. 2d shows the percentage contents of different N species for the as-prepared catalysts. The content of pyridinic-N declines with pyrolysis temperature increasing from 750 to 950 °C, whereas that of graphitic-N increases significantly. Meanwhile, high pyrolysis temperature can form metal-N sites but overhigh temperature may lead to the decomposition of metal-N sites. Furthermore, the rational porous structure with high specific surface area is beneficial to the more metal-N active sites exposed on the surface of catalysts. As a result, FeCo-NC-850 exhibits the highest metal-N content (27.2%) among all the catalysts in this study, indicating a splendid ORR catalytic activity. To further certify the FeeN and CoeN coordination, the high-resolution Fe 2p and Co 2p XPS spectra are shown in Fig. 2e and S4 f. Presumably the broad and weak peaks in Co 2p spectra are ascribed to the very low Co content. Furthermore, two deconvoluted peaks at 707.4 and 720.1 eV are indexed to zero-valence
Co@Fe-NC-850 (Fig. 1c). Moreover, the X-ray photoelectron spectra (XPS) measurement was used to confirm the chemical states of surface Co and Fe for FeCoZIF (Fig. S3). The deconvoluted peaks located at 710.9, 714.0, 718.0, 723.9, 726 eV for Fe 2p are indicative of the Fe2+ 2p3/2, Fe3+ 2p3/2, satellite, Fe2+ 2p1/2, and Fe3+ 2p1/2, respectively [29]. The broad and weak peaks of Co 2p spectra are probably related to the low content of Co. The peak situated at 781.2 eV could be ascribed to Co2+-N or Co2+-O and the peak of 784.2 eV is related to the Co3+-N/O bonds [33]. Meanwhile, the CoeOH bonds are reflected by the peak at 787.2 eV. The XPS analysis indicates the existence of Fe2+, Fe3+, Co2+ and Co3+ in the FeCoZIF. Based on the above results and the previous studies [34,35], part of Fe2+ ions in FeZIF may be oxidated to Fe3+ by Co3+ from Co(acac)3 and oxygen dissolved in solution. Subsequently generated Co2+ would be immobilized by the coordination with exposed imidazole group. This local oxidation could result in the partial collapse and fragmentation for the framework of FeZIF. Finally, the 3D well-interconnected architecture constructed by nanobulks and nanotubes is formed by the catalysis of Fe and Co as well as the escape of metal Zn, Fe and Co during carbonization and acidetching treatments. The microscopic structure of FeCo-NC-850 was ulteriorly investigated by TEM and HRTEM. As revealed in Fig. 1d, the nano-size metallic particles are uniformly encapsulated in the carbon matrix and the meso/macropores are also readily observed. Fig. 1e shows the lattice spacing are 0.202 and 0.345 nm, corresponding to the (110) plane of the Fe3Co7 phase (JCPDS, No. 48-1816) and the (002) plane of the graphite phase, respectively. Elemental mapping images of FeCo-NC850 displays that element Fe and Co are evenly distributed in the carbon matrix (Fig. 1f), demonstrating the formation of FeCo alloy. The X-ray diffraction (XRD) patterns of three catalysts were investigated to analyze crystal structure (Fig. 2a). All catalysts exhibit a strong peak around 26° indexed to the (002) plane of graphitic carbon. The peaks at 37.7 and 43.7° are ascribed to Fe3C (JCPDS No. 35-0772), while two peaks around 50.8 and 74.6° correspond to FeN0.0324 (JCPDS No. 752127). It has been proved that the presence of Fe/Fe3C nanocrystals can enhance the catalytic activity of Fe-Nx for ORR and also intensify the OER activity [36,37]. FeCo-NC-850 features three obvious peaks
Fig. 2. (a) XRD patterns, (b) Raman spectra, (c) Percentage content of different N species of the as-prepared catalysts. (d) N 1 s and (e) Fe 2p high-resolution XPS spectra of FeCo-NC-850. (f) Nitrogen adsorption–desorption isotherms with DFT pore size distribution. 499
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state Fe and the peaks for Co with zerovalence state are centered at 778.5 and 793.9 eV, which suggest the presence of FeCo alloy in FeCoNC-850 [43]. The peaks at 711 eV could be attributed to Fe coordinated to N [44]. The deconvoluted Co 2p3/2 peaks at 780.9 and 783.9 eV reveal the presence of Co2+-N and Co3+-N, while the peak around 787.3 eV is observed owing to the CoeOH bonds [33,45]. Therefore, the XPS analysis demonstrates the existence of bimetal-N active sites and FeCo alloy in FeCo-NC-850, which is consistent with the results of XRD and TEM. The pore structure was further investigated by nitrogen adsorption/ desorption measurements. As revealed in Fig. 2 f and S5, three catalysts all exhibit type-IV isotherms with a distinct hysteretic loop, suggesting the coexistence of micro/meso/macropores [46,47]. The BET special surface area (SSA) of FeCo-NC-850 is calculated to be 553 m2 g−1, which is close to that of Fe-NC-850 (590 m2 g−1) but much higher than Co@Fe-NC-850 (399 m2 g−1). However, FeCo-NC-850 has an obviously larger pore volume (0.491 cm3 g−1) than Fe-NC-850 (0.366 cm3 g−1) and Co@Fe-NC-850 (0.377 cm3 g−1). The above results indicate that Co (acac)3 could increase the extra mesopores of the carbonized material compared with Fe-NC-850, resulting in the high SAA with large pore volume. The percentage of the micropore SSA and volume calculated by T-Method for FeCo-NC-850 are 63.0 and 31.5%, respectively, which is smaller than those of Fe-NC-850 (82.7 and 60.3%). Furthermore, FeCoNC-850 shows the presence of micro/mesopores centered at 2–3, 7–12 and 17–23 nm, whereas Fe-NC-850 and Co@Fe-NC-850 have a little narrow pore size distribution with inferior mesopore volume. Therefore, FeCo-NC-850 exhibits a hierarchical porous structure with high surface area and large pore volume, which is related to the escape of metal Fe, Zn and Co during carbonization and acid-etching treatment as well as the inheritance of micropores from Fe-doped ZIF-8. Cyclic voltammetry (CV) and Linear sweep voltammogram (LSV) measurements were performed in the N2- or O2-saturated 0.1 M KOH electrolyte to evaluate the ORR activity of the as-prepared catalysts. As shown in Fig. 3a, FeCo-NC-850 shows a quasi-rectangular voltammogram in N2-saturated electrolyte, and has an ORR peak at 0.873 V in O2
which is more positive than Pt/C (0.837 V), suggesting an outstanding ORR activity. From the LSV curves in Fig. 3b, FeCo-NC-850 exhibits a remarkable ORR performance with much more positive onset potential (Eonset) of 0.997 V, half-wave potential (E1/2) of 0.864 V and larger diffusion-limited current density than those of Pt/C (Eonset = 0.989 V, E1/2 = 0.838 V), Co@Fe-NC-850 (Eonset = 0.980 V, E1/2 = 0.783 V) and Fe-NC-850 (Eonset = 0.979 V, E1/2 = 0.803 V). It is worth noting that FeCo-NC-850 is the best ones of reported non-noble metal-based ORR catalysts in alkaline electrolyte (Table S1). The pyrolysis temperature as an important factor for catalytic activity was also investigated. Fig. S6 shows the LSV curves of the FeCo-NC materials prepared at different temperatures at 1600 rpm. FeCo-NC-850 exhibits a better catalytic activity than the catalysts carbonized at 750 and 950 °C. The inferior ORR activity of FeCo-NC-750 is possibly ascribed to the lower degree of graphitization which results in a poorer conductivity, whereas the low level of Metal-N active sites for FeCo-NC-950 could hinder ORR activity. Furthermore, the rotating disk electrode (RDE) measurements were performed at rotation rates speeds from 400 to 2025 rpm (Fig. 3c and S7). The corresponding Koutecky-Levich (K-L) plots of FeCo-NC850 show parallel and linear fitting lines (Fig. S8c), suggesting the firstorder reaction kinetics in respect of the dissolved oxygen concentration [48]. For comparison, LSV curves, K–L plots and the corresponding electron transfer number (n) of Pt/C and other samples are shown in Fig. S7 and S8. The calculated average n of FeCo-NC-850 is 3.93 same with that of Pt/C (3.93) and bigger than that of Fe-NC-850 (3.81) and Co@Fe-NC-850 (3.79), indicating that the ORR follows a four-electron pathway for FeCo-NC-850. These results demonstrate the appropriate pyrolysis temperature and cobalt source are favorable to the ORR activity. Furthermore, the electrochemical impedance spectroscopy (EIS) curves of these as-prepared catalysts are shown in Fig. S9. FeCo-NC-850 exhibits a smaller charge transfer resistance (Rct) of 64 Ω than FeCo-NC750 (89 Ω), FeCo-NC-950 (70 Ω), Fe-NC-850 (80 Ω), and Co@Fe-NC850 (81 Ω). The long-term stability of FeCo-NC-850 and Pt/C were evaluated by the cycle experiments (Fig. 3d and e). The E1/2 of FeCo-NC-850 displays
Fig. 3. (a) CV curves of FeCo-NC-850 and Pt/C. (b) LSV curves of FeCo-NC-850, Pt/C, Fe-NC-850 and Co@Fe-NC-850 at 1600 rpm. (c) RDE curves of FeCo-NC-850 at various rotation rates (inset: the electron transfer numbers at 0.2–0.5 V). LSV curves of (d) FeCo-NC-850 and (e) Pt/C before and after 5000 cycles. (f) The i–t chronoamperometric response of FeCo-NC-850 and Pt/C with addition of 2 M methanol. All the catalysts were measured in 0.1 M KOH solution. 500
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Fig. 4. (a) LSV curves (after IR compensation), (b) the corresponding Tafel slopes and (c) the overall LSV curves of FeCo-NC-850, Fe-NC-850, RuO2 and Pt/C at 1600 rpm in 0.1 M KOH.
4. Conclusions
a slight shift of 8 mV after 5000 cycles, whereas Pt/C has a distinct shift of 24 mV, manifesting that FeCo-NC-850 owns superb long-term stability. The durability of these two catalysts was also investigated by the chronoamperometry tests at 0.6 V vs RHE (Fig. S10). The relative current of FeCo-NC-850 remains 92.4% significantly more than that of Pt/ C (80.5%). Furthermore, we investigated methanol tolerance through the chronoamperometry tests (Fig. 3f). FeCo-NC-850 has an inappreciable change of current density with the addition of 2 M methanol into 0.1 M KOH solution, while Pt/C exhibits a tremendous positive current increase. Therefore, FeCo-NC-850 possesses the remarkable long-term stability and methanol tolerance, making it a towardly efficient ORR catalyst. Importantly, in addition to the prominent ORR performance, FeCoNC-850 also shows an appreciable OER activity, which was investigated by the LSV curve at 1600 rpm in 0.1 M KOH. The OER onset potential of FeCo-NC-850 is significantly lower than other catalysts but higher than RuO2 (Fig. 4a). Moreover, the operating potential to achieve the current density of 10 mA cm−2 (Ej=10) is widely used to evaluate the OER activity [49]. The Ej=10 of FeCo-CN-850 is 1.675 V, which is slightly inferior to 1.62 V of RuO2 but much better than that of other catalysts in this study. As revealed in Fig. 4b, the Tafel slope of FeCo-NC-850 (117 mV dec−1) is much closer to 99 mV dec−1 of RuO2 than 147 mV dec−1 of Fe-NC and 159 mV dec−1 of Pt/C, suggesting a respectable intrinsic OER kinetics [50,51]. FeCo-CN-850 possesses superior OER performance than Fe-NC-850 and Co@Fe-NC-850, which is mainly attributed to high surface area with rational pore structure and the existence of FeCo alloy. For the stability test, the Ej=10 of FeCo-NC850 after 1000 CV cycles has an apparently slighter shift than that of RuO2, indicating the superior stability of FeCo-NC-850 in alkaline electrolyte (Fig. S11). The overall oxygen electrode activity of bifunctional catalyst is assessed by the potential gap (ΔE) between the ORR half-wave potential (E1/2) and OER potential at 10 mA cm−2 (Ej=10) [7]. Fig. 4c shows the overall LSV curves of FeCo-NC-850, Fe-NC-850, commercial Pt/C and RuO2. Remarkably, FeCo-NC-850 has the smallest value of ΔE (0.81 V) among all the catalysts in this study which could be comparable with 0.78 V of the combination of commercial Pt/C and RuO2, suggesting superior bifunctional catalytic activity for ORR and OER. Furthermore, FeCo-NC-850 still exhibits a competitive performance compared with other non-noble metal-based catalysts in the previous reports (Table S2), indicating an ideal bifunctional catalyst toward both ORR and OER. Based on the above results, the outstanding overall oxygen activity of FeCo-NC-850 could be ascribed to the coaction of multi-ingredients, especially the unique hierarchical porous structure with high specific area, bimetal-N active sites, the existence of FeCo alloy and Fe/ Fe3C as well as high electrical conductivity.
In summary, we report a facile pyrolysis approach to prepare FeCoN‑carbon material with a unique hierarchical porous structure consisting of nanobulks and nanotubes as an efficient bifunctional catalyst for ORR and OER. Via using local oxidation of FeZIF during impregnation, this work combines Fe(II)-doped ZIF-8 with Co(acac)3 to synthesize TM-N-C material for the first time. Importantly, Co(acac)3 plays dual-roles during synthetic process, which not only generate FeCo alloy and bimetal-N active sites but improves porous structure. Owing to the unique structure with high surface area, FeCo alloy, bimetal-N active sites, the FeCo-NC-850 catalyst exhibits a superior overall oxygen electrode activity with 0.81 V of ΔE in 0.1 M KOH, indicating a towardly bifunctional oxygen catalyst. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.05.014. References [1] R. Raccichini, A. Varzi, S. Passerini, B. Scrosati, The role of graphene for electrochemical energy storage, Nat. Mater. 14 (2015) 271–279. [2] D. Ding, K. Shen, X. Chen, H. Chen, J. Chen, T. Fan, R. Wu, Y. Li, Multi-level architecture optimization of MOF-templated co-based nanoparticles embedded in hollow N-doped carbon polyhedra for efficient OER and ORR, ACS Catal. 8 (2018) 7879–7888. [3] Z.W. Seh, J. Kibsgaard, C.F. Dickens, I.B. Chorkendorff, J.K. Norskov, T.F. Jaramillo, Combining theory and experiment in electrocatalysis: insights into materials design, SCIENCE 355 (2017). [4] I.S. Amiinu, Z. Pu, X. Liu, K.A. Owusu, H.G.R. Monestel, F.O. Boakye, H. Zhang, S. Mu, Multifunctional Mo-N/C@MoS2 Electrocatalysts for HER, OER, ORR, and Zn-air batteries, Adv. Funct. Mater. 27 (2017). [5] J. Wang, H. Wu, D. Gao, S. Miao, G. Wang, X. Bao, High-density iron nanoparticles encapsulated within nitrogen-doped carbon nanoshell as efficient oxygen electrocatalyst for zinc-air battery, Nano Energy 13 (2015) 387–396. [6] J. Wang, C.F. Tan, T. Zhu, G.W. Ho, Topotactic consolidation of monocrystalline CoZn hydroxides for advanced oxygen evolution electrodes, Angew. Chem. Int. Edit. 55 (2016) 10326–10330. [7] Z. Huang, J. Wang, Y. Peng, C. Jung, A. Fisher, X. Wang, Design of efficient bifunctional oxygen reduction/evolution electrocatalyst: recent advances and perspectives, Adv. Energy Mater. 7 (2017). [8] A.L. Cazetta, L. Spessato, K.C. Bedin, I.P.F.A. Souza, R.A. Araujo, A.F. Martins, T.L. Silva, R. Silva, V.C. Almeida, Metal-free ovalbumin-derived N-S-co-doped nanoporous carbon materials as efficient electrocatalysts for oxygen reduction reaction, Appl. Surf. Sci. 467 (2019) 75–83. [9] W. Wang, D. Chai, J. Zhang, S. Xue, Y. Wang, Z. Lei, Ni5Sm-P/C ternary alloyed catalyst as highly efficient electrocatalyst for urea electrooxidation, J. Taiwan Inst Chem E. 80 (2017) 326–332. [10] W. Wang, Y. Yang, Y. Liu, Z. Zhang, W. Dong, Z. Lei, Hybrid NiCoOx adjacent to Pd nanoparticles as a synergistic electrocatalyst for ethanol oxidation, J. Power Sources 273 (2015) 631–637. [11] Z. Liu, J. Yu, X. Li, L. Zhang, D. Luo, X. Liu, X. Liu, S. Liu, H. Feng, G. Wu, P. Guo, H. Li, Z. Wang, X.S. Zhao, Facile synthesis of N-doped carbon layer encapsulated Fe2N as an efficient catalyst for oxygen reduction reaction, CARBON 127 (2018) 636–642. [12] J. Song, Y. Ren, J. Li, X. Huang, F. Cheng, Y. Tang, H. Wang, Core-shell Co/CoNx@C
501
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[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
nanoparticles enfolded by co-N doped carbon nanosheets as a highly efficient electrocatalyst for oxygen reduction reaction, CARBON 138 (2018) 300–308. S. Ratso, M. Kaarik, M. Kook, P. Paiste, V. Kisand, S. Vlassov, J. Leis, K. Tammeveski, Iron and nitrogen co-doped carbide-derived carbon and carbon nanotube composite catalysts for oxygen reduction reaction, CHEMELECTROCHEM 5 (2018) 1827–1836. Z. Lu, B. Liu, W. Dai, L. Ouyang, J. Ye, Carbon network framework derived ironnitrogen co-doped carbon nanotubes for enhanced oxygen reduction reaction through metal salt-assisted polymer blowing strategy, Appl. Surf. Sci. 463 (2019) 767–774. B.Y. Guan, Y. Lu, Y. Wang, M. Wu, X.W.D. Lou, Porous Iron-cobalt alloy/nitrogendoped carbon cages synthesized via pyrolysis of complex metal-organic framework hybrids for oxygen reduction, Adv. Funct. Mater. 28 (2018). X. Gao, J. Yang, K. Song, W. Luo, S. Dou, Y. Kang, Robust FeCo nanoparticles embedded in a Ndoped porous carbon framework for high oxygen conversion catalytic activity in alkaline and acidic media, J. Mater. Chem. A 6 (2018) 23445–23456. J. Wang, Z. Huang, W. Liu, C. Chang, H. Tang, Z. Li, W. Chen, C. Jia, T. Yao, S. Wei, Y. Wu, Y. Lie, Design of N-coordinated dual-metal sites: a stable and active Pt-free catalyst for acidic oxygen reduction reaction, J. Am. Chem. Soc. 139 (2017) 17281–17284. D. Friebel, M.W. Louie, M. Bajdich, K.E. Sanwald, Y. Cai, A.M. Wise, M. Cheng, D. Sokaras, T. Weng, R. Alonso-Mori, R.C. Davis, J.R. Bargar, J.K. Norskov, A. Nilsson, A.T. Bell, Identification of highly active Fe sites in (Ni,Fe)OOH for electrocatalytic water splitting, J. Am. Chem. Soc. 137 (2015) 1305–1313. G. Fu, Z. Cui, Y. Chen, L. Xu, Y. Tang, J.B. Goodenough, Hierarchically mesoporous nickel-iron nitride as a cost-efficient and highly durable electrocatalyst for Zn-air battery, Nano Energy 39 (2017) 77–85. X. Zhu, T. Jin, C. Tian, C. Lu, X. Liu, M. Zeng, X. Zhuang, S. Yang, L. He, H. Liu, S. Dai, 1 in situ coupling strategy for the preparation of FeCo alloys and Co4N hybrid for highly efficient oxygen evolution, Adv. Mater. 29 (2017). C. Su, H. Cheng, W. Li, Z. Liu, N. Li, Z. Hou, F. Bai, H. Zhang, T. Ma, Atomic modulation of FeCo-nitrogen-carbon bifunctional oxygen electrodes for rechargeable and flexible all-solid-state zinc-air battery, Adv. Energy Mater. (2017) 7. M. Zeng, Y. Liu, F. Zhao, K. Nie, N. Han, X. Wang, W. Huang, X. Song, J. Zhong, Y. Li, Metallic cobalt nanoparticles encapsulated in nitrogen-enriched graphene shells: its bifunctional electrocatalysis and application in zinc-air batteries, Adv. Funct. Mater. 26 (2016) 4397–4404. M.S. Burke, M.G. Kast, L. Trotochaud, A.M. Smith, S.W. Boettcher, Cobalt-iron (oxy) hydroxide oxygen evolution Electrocatalysts: the role of structure and composition on activity, stability, and mechanism, J. Am. Chem. Soc. 137 (2015) 3638–3648. P. Zhang, F. Sun, Z. Xiang, Z. Shen, J. Yun, D. Cao, ZIF-derived in situ nitrogendoped porous carbons as efficient metal-free electrocatalysts for oxygen reduction reaction, Energy Environ. Sci. 7 (2014) 442–450. Y. Chen, S. Ji, Y. Wang, J. Dong, W. Chen, Z. Li, R. Shen, L. Zheng, Z. Zhuang, D. Wang, Y. Li, Isolated single iron atoms anchored on N-doped porous carbon as an efficient electrocatalyst for the oxygen reduction reaction, angew. Chem. Int. Edit. 56 (2017) 6937–6941. Y. Ye, F. Cai, H. Li, H. Wu, G. Wang, Y. Li, S. Miao, S. Xie, R. Si, J. Wang, X. Bao, Surface functionalization of ZIF-8 with ammonium ferric citrate toward high exposure of Fe-N active sites for efficient oxygen and carbon dioxide electroreduction, Nano Energy 38 (2017) 281–289. X. Wang, H. Zhang, H. Lin, S. Gupta, C. Wang, Z. Tao, H. Fu, T. Wang, J. Zheng, G. Wu, X. Li, Directly converting Fe-doped metal organic frameworks into highly active and stable Fe-N-C catalysts for oxygen reduction in acid, Nano Energy 25 (2016) 110–119. Z. Hu, Z. Guo, Z. Zhang, M. Dou, F. Wang, Bimetal zeolitic imidazolite frameworkderived iron-, cobalt- and nitrogen-codoped carbon nanopolyhedra electrocatalyst for efficient oxygen reduction, ACS Appl Mater Inter 10 (2018) 12651–12658. Z. Wang, H. Jin, T. Meng, K. Liao, W. Meng, J. Yang, D. He, Y. Xiong, S. Mu, Fe, Cucoordinated ZIF-derived carbon framework for efficient oxygen reduction reaction and zinc-air batteries, Adv. Funct. Mater. 28 (2018). Y. Chen, C. Wang, Z. Wu, Y. Xiong, Q. Xu, S. Yu, H. Jiang, From bimetallic metalorganic framework to porous carbon: high surface area and multicomponent active dopants for excellent electrocatalysis, Adv. Mater. 27 (2015) 5010–5016. H. Zhang, S. Hwang, M. Wang, Z. Feng, S. Karakalos, L. Luo, Z. Qiao, X. Xie, C. Wang, D. Su, Y. Shao, G. Wu, Single atomic iron catalysts for oxygen reduction in
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49]
[50]
[51]
502
acidic media: particle size control and thermal activation, J. Am. Chem. Soc. 139 (2017) 14143–14149. R. Zhao, W. Xia, C. Lin, J. Sun, A. Mahmood, Q. Wang, B. Qiu, H. Tabassum, R. Zou, A pore-expansion strategy to synthesize hierarchically porous carbon derived from metal-organic framework for enhanced oxygen reduction, Carbon 114 (2017) 284–290. B. Cao, G.M. Veith, J.C. Neuefeind, R.R. Adzic, P.G. Khalifah, Mixed close-packed cobalt molybdenum nitrides as non-noble metal electrocatalysts for the hydrogen evolution reaction, J. Am. Chem. Soc. 135 (2013) 19186–19192. H. Su, H. Zhang, X. Tang, B. Liu, Y. An, High Q-factor NiCuZn ferrite with nanocrystalline ferrite particles and Co2O3 additives, Phys. Status Solidi A. 204 (2007) 576–580. K. Zhu, C. Jin, Z. Klencsar, A.S. Ganeshraja, J. Wang, Cobalt-iron oxide, alloy and nitride: synthesis, characterization and application in catalytic peroxymonosulfate activation for Orange II degradation, Catalysts 7 (2017). B.K. Barman, K.K. Nanda, Prussian blue as a single precursor for synthesis of Fe/ Fe3C encapsulated N-doped graphitic nanostructures as bi-functional catalysts, Green Chem. 18 (2016) 427–432. W. Yang, X. Liu, X. Yue, J. Jia, S. Guo, Bamboo-like carbon nanotube/Fe3C nanoparticle hybrids and their highly efficient catalysis for oxygen reduction, J. Am. Chem. Soc. 137 (2015) 1436–1439. T. Liu, L. Zhang, W. You, J. Yu, Core-Shell nitrogen-doped carbon hollow spheres/ Co3O4 Nanosheets as advanced electrode for high-performance supercapacitor, Small 14 (2018). W. Wang, Y. Dong, L. Xu, W. Dong, X. Niu, Z. Lei, Combining bimetallic-alloy with selenium functionalized carbon to enhance Electrocatalytic activity towards glucose oxidation, Electrochim. Acta 244 (2017) 16–25. W. Wang, W. Jing, F. Wang, S. Liu, X. Liu, Z. Lei, Amorphous ultra-dispersed Pt clusters supported on nitrogen functionalized carbon: a superior electrocatalyst for glycerol electrooxidation, J. Power Sources 399 (2018) 357–362. L. Zhang, Z. Su, F. Jiang, L. Yang, J. Qian, Y. Zhou, W. Li, M. Hong, Highly graphitized nitrogen-doped porous carbon nanopolyhedra derived from ZIF-8 nanocrystals as efficient electrocatalysts for oxygen reduction reactions, Nanoscale 6 (2014) 6590–6602. P. Hao, Z. Zhao, J. Tian, H. Li, Y. Sang, G. Yu, H. Cai, H. Liu, C.P. Wong, A. Umar, Hierarchical porous carbon aerogel derived from bagasse for high performance supercapacitor electrode, Nanoscale 6 (2014) 12120–12129. C. Su, H. Cheng, W. Li, Z. Liu, N. Li, Z. Hou, F. Bai, H. Zhang, T. Ma, Atomic modulation of FeCo-nitrogen-carbon bifunctional oxygen electrodes for rechargeable and flexible all-solid-state zinc-air BATTERY, Adv. Energy Mater. (2017) 7. Z. Wu, X. Xu, B. Hu, H. Liang, Y. Lin, L. Chen, S. Yu, Iron carbide nanoparticles encapsulated in mesoporous Fe-N-doped carbon nanofibers for efficient electrocatalysis, Angew. Chem. Int. Edit. 54 (2015) 8179–8183. Y. Wang, D. Liu, Z. Liu, C. Xie, J. Huo, S. Wang, Porous cobalt-iron nitride nanowires as excellent bifunctional electrocatalysts for overall water splitting, Chem. Commun. 52 (2016) 12614–12617. H. Zhong, J. Wang, Y. Zhang, W. Xu, W. Xing, D. Xu, Y. Zhang, X. Zhang, ZIF-8 derived graphene-based nitrogen-doped porous carbon sheets as highly efficient and durable oxygen reduction electrocatalysts, angew. Chem. Int. Edit. 53 (2014) 14235–14239. H. Zou, B. He, P. Kuang, J. Yu, K. Fan, Metal-organic framework-derived nickelcobalt sulfide on ultrathin Mxene Nanosheets for Electrocatalytic oxygen evolution, ACS Appl Mater Inter 10 (2018) 22311–22319. Y. Zhao, Q. Lai, J. Zhu, J. Zhong, Z. Tang, Y. Luo, Y. Liang, Controllable construction of core-shell polymer@Zeolitic Imidazolate frameworks fiber derived heteroatom-doped carbon nanofiber network for efficient oxygen Electrocatalysis, Small 14 (2018). J. Li, P. Hou, S. Zhao, C. Liu, D. Tang, M. Cheng, F. Zhang, H. Cheng, A 3D bifunctional porous N-doped carbon microtube sponge electrocatalyst for oxygen reduction and oxygen evolution reactions, Energy Environ. Sci. 9 (2016) 3079–3084. M. Ma, G. Zhu, F. Xie, F. Qu, Z. Liu, G. Du, A.M. Asiri, Y. Yao, X. Sun, Homologous catalysts based on Fe-doped CoP nanoarrays for high-performance full water splitting under benign conditions, ChemSusChem 10 (2017) 3188–3192. C. Tang, N. Cheng, Z. Pu, W. Xing, X. Sun, NiSe nanowire film supported on nickel foam: an efficient and stable 3D bifunctional electrode for full water splitting, angew. Chem. Int. Edit. 54 (2015) 9351–9355.
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Full length article
An ingenious approach for ZIFs derived N-doped hierarchical porous carbon hybrids with FeCo alloy nanoparticles as efficient bifunctional oxygen electrocatalysts Guoning Li, Kaitian Zheng, Chunjian Xu
T
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School of Chemical Engineering and Technology, State Key Laboratory of Chemical Engineering, Chemical Engineering Research Center, Tianjin University, Tianjin 300072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Porous carbon ZIF-8 Oxygen reduction reaction Oxygen evolution reaction FeCo alloy
An ingenious approach is proposed to achieve exceedingly efficient bifunctional oxygen electrocatalysts of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Hierarchical porous structure constituted by nanotubes and nanobulks with high specific surface area and encapsulated FeCo alloy nanoparticles is successfully prepared via the hybrid of Fe(II)-doped ZIF-8 (FeZIF) and cobalt (III) acetylacetonate (Co(acac)3), which is greatly redound to mass transfer and electron transport. Interestingly, Co(acac)3 plays dual-roles during preparation process, which not only generates new metal species but improves porous structure. Moreover, FeZIF acts as a self-sacrificing template and iron source. Owing to the coaction of the unique porous structure, bimetal-N active sites and FeCo alloy, the resultant catalyst carbonized at 850 °C exhibits a prominent ORR activity with a positive half-wave potential of 0.864 V (vs the reversible hydrogen electrode, RHE), and a low operating potential of 1.675 V at a current density of 10 mA cm−2 for OER in alkaline media. The potential gap (ΔE) of overall oxygen electrode is 0.81 V, which surpass that of numerous non-noble metal-based catalysts reported to date, indicating a promising bifunctional oxygen electrocatalyst.
1. Introduction With the fossil fuel consumption growing progressively, electrochemical energy conversation or storage devices, such as unitized regenerative fuel cells and rechargeable metal-air batteries, are gaining more and more attention [1–3]. The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which directly determine the overall energy efficiency, are pivotal to these devices [4,5]. Noble metal-based catalysts, especially Pt/C and RuO2, are considered as efficient electrocatalysts for ORR or OER. Unfortunately, the catalysts composed of a single noble metal cannot achieve acceptable activity for both ORR and OER simultaneously [6,7]. Moreover, their high cost and poor durability extremely hinder the cosmically practical commercialization. Hence, it is imperative to excogitate non-noble metal-based bifunctional oxygen catalysts. Recently, carbon-based materials, such as heteroatom-doped carbon, have been widely investigated as electrode materials [8–10]. Among these materials, transition metal based carbon materials (TM-N-C), especially Fe-N-C materials, are considered as potential electrocatalysts, exhibiting remarkable performance on ORR [11–14]. Meanwhile, It has been proved that the bimetallic TM-N-C
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materials could possess higher ORR activity than monometallic materials [15,16]. Wang et al. proposed a host-guest strategy to prepare ZIFderived FeCo-N-C materials for high ORR catalytic activity [17]. Moreover, bimetallic alloys (FeCo, FeNi, CoNi) exhibit outstanding catalytic activity and stability for OER [18,19]. An in situ coupling approach was developed by Zhu et al. to obtain FeCo alloys and Co4N hybrid as a highly efficient OER catalyst [20]. For the FeCo-based catalysts, the existence of Co can enhance the activity of Fe-based active sites and improve electrical conductivity [21–23]. Based on the above, bimetallic TM-N-C materials with encapsulated bimetallic alloy could suffice for excellent overall oxygen electrode activity for ORR and OER. Zeolitic imidazolate framework-8 (ZIF-8), as an important subclass of metal-organic frameworks (MOFs), is a 3D tetrahedral framework constituted by zinc ions and 2-methylimidazolate linkers. Owing to the unique features, especially large specific surface area, high nitrogen content, controllable size and shape, ZIF-8 has been widely used as pyrolysis precursors to prepare carbon-based materials [24]. However, the discrete nanoparticles and the porous structure mainly with micropores exist in the ZIF-derived carbon materials directly obtained by the pyrolysis of ZIF-8, which hinders mass transfer and electron
Corresponding author. E-mail address: [email protected] (C. Xu).
https://doi.org/10.1016/j.apsusc.2019.05.014 Received 12 February 2019; Received in revised form 10 April 2019; Accepted 2 May 2019 Available online 03 May 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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least 12 h.
transport as well as the accessibility of the active site, resulting in inferior electrochemical performance. Furthermore, the Fe-N-C materials have been successfully prepared by Fe-doped ZIF-8 or hybrids of ZIF-8 and iron source [25,26]. Fe2+ is superior to achieve homogeneously Fedoped ZIF-8 compared with Fe3+, resulting in Fe-N-C material with high Fe dispersion at atomic level via pyrolysis treatment [27]. Nevertheless, the introduction of Fe2+ or Co2+ can lead to ZIF-derived carbon materials constituted by larger size nanoparticles [28–30], which is detrimental to the exposure of active sites and mass transfer. Hence, reducing the crystal size and increasing meso/macropores for the ZIF-derived materials are gaining more attentions. Recently, Zhang et al. reported a facile method to prepare Fe-N-C catalyst with controllable size derived from Fe-doped ZIF precursors via tuning the concentration of metal precursors in methanol solution [31]. A poreexpansion strategy was proposed by Zhao et al. to enhance ORR activity, which could transform ZIF-8 with microporous structure into foam-like hierarchical porous carbons [32]. Therefore, it is sufficiently challenging and desirable to synthesize a bimetallic TM-N-C material with bimetals alloy and hierarchical pores only via the pyrolysis of ZIFs hybrids, which could be used as an efficient bifunctional oxygen electrocatalyst of ORR and OER. In this work, we present a facile and ingenious strategy comprising impregnation and pyrolysis treatments to achieve bimetallic TM-N-C catalysts by the hybrid of FeZIF and Co(acac)3. Via using local oxidation of FeZIF during impregnation, this work combines Fe(II)-doped ZIF-8 with Co(acac)3 to synthesize TM-N-C material for the first time. Interestingly, Co(acac)3 plays dual-roles during preparation process, which not only generates new metal species but improves porous structure. Moreover, FeZIF acts as a self-sacrificing template and iron source. The resultant catalyst prepared at 850 °C (FeCo-NC-850) shows a 3D interconnected hierarchical porous structure consist of nanotubes and nanobulks, which can efficiently facilitate the mass/electron transport during ORR and OER process. Benefiting from this unique nanostructure as well as FeCo alloy and bimetal-N active sites in carbon matrix, FeCo-NC-850 exhibits a superior overall oxygen electrode activity under alkaline conditions, manifesting a promising bifunctional catalyst for both ORR and OER.
2.4. Carbonization process FeCoZIF powder was pyrolyzed at different temperatures (750 °C, 850 °C, 950 °C) with a rate of 3 °C min−1 and held at the final temperature for 3 h under a nitrogen atmosphere in a tube furnace. The obtained material was soaked in 0.5 M H2SO4 solution for 24 h, washed with distilled water four times and ethanol two times, and finally dried under vacuum (80 °C) for at least 24 h. FeZIF powder was pyrolyzed at 850 °C and the remaining steps is the same with FeCoZIF. The FeCoZIFderived carbons are denoted as FeCo-NC-T, where T represents the pyrolysis temperature, and the FeZIF-derived carbon was denoted as FeNC-850. 2.5. Physicochemical characterization The morphology of material was observed by a scanning electron microscope (SEM, Hitachi S-4800) and transmission electron microscope (TEM, JEOL JEM-2100). The structure phase of samples was investigated by a D8 Advance Bruker X-ray diffractometer (XRD) at a scan rate of 10°min−1. Raman spectroscopy was performed using a Renishaw InVia Spectrometer with a 532 nm excitation laser. Nitrogen adsorption/desorption was performed on Micromeritic TriStarII 3020. XPS measurements were carried out by a Thermo Scientific Spectrometer with an Escalab 250 Xi X-ray. 2.6. Electrochemical performance evaluation All the electrochemical tests were measured via a CHI 660E workstation (CH Instruments, Shanghai) and RDE equipment (ALS/DY2323 Bi-potentiostat) with a standard three-electrode system. A glassy carbon rotating disk electrode (3 mm of disk diameter) covered up by catalysts, a platinum wire and a Ag/AgCl with saturated KCl were used as the working, counter and reference electrodes, respectively. The ORR and OER activity were both measured in 0.1 M KOH electrolyte. The mass loading of the prepared material on the working electrode was 0.28 mg∙cm−2, and that of reference substance (Pt for ORR and RuO2 for OER) was 0.12 mg cm−2. High purity oxygen or nitrogen was bubbled into the electrolyte for at least 0.5 h before the tests. The CV measurements were carried out in N2 or O2-saturated electrolyte with a scan rate of 50 mV s−1. The RDE tests were conducted with a scan rate of 10 mV s−1 at various rotation rates from 400 to 2025 rpm. The electron transfer number (n) for ORR was calculated by the KouteckyLevich equations:
2. Experimental 2.1. Material Zinc nitrate hexahydrate (Zn(NO3)2·6H2O) (99.95%) was purchased from Macklin. Cobalt (III) acetylacetonate (98%) was obtained from Tianjin Heowns Company. FeSO4·7H2O (99.5%), 2-Methylimidazole (99%) and methanol (99.9%) were produced by Tianjin Kermel Company. Nafion perfluorinated resin solution (5 wt%) was purchased from DuPont Company. RuO2 (99.9%) was purchased from Macklin and Pt/C (20 wt%) were used as received.
1 1 1 1 1 = + = + J JL JK Bω1/2 JK B = 0.62nFC0 D0 2/3υ−1/6 where J represents the measured current density. JL and JK are the kinetic current density and limiting diffusion current density, respectively. ω is the rotation rate of RDE test. F is the Faraday constant of 96,485C mol−1, C0 is the bulk concentration of O2 dissolved (1.2 × 10−3 mol L−1), D0 is the diffusion coefficient of O2 (1.9 × 10−5 cm2 s−1), and υ is the kinetic viscosity of 0.1 M KOH (0.01 cm2 s−1).
2.2. Synthesis of FeZIF Zn(NO3)2·6H2O (2.0 mmol) and FeSO4 (0.25 mmol) were dissolved in 30 ml N2-saturated methanol. Then, another methanol solution (30 ml) with 2-methylimidazole (11.4 mmol) was poured into the above solution. The N2-saturated mix solution was sealed and subsequently stirred for 24 h at room temperature. Afterward, the FeZIF was centrifuged, washed with methanol four times and finally dried under vacuum (60 °C) for at least 12 h.
3. Results and discussion Scheme 1 exhibits a facile and ingenious strategy for 3D interconnected hierarchical porous FeCo-Nitrogen-Carbon materials, which mainly includes the process of impregnation and carbonization. The morphology of the as-prepared catalysts was revealed by SEM and TEM images. As shown in Fig. S1, FeZIF displays a stacked nanocrystal structure with polyhedron shape, whereas FeCoZIF exhibits
2.3. Synthesis of FeCoZIF FeZIF and Co(acac)3 were mixed with a weight ratio of 11:1 in methanol under ultrasonication for 4 h and then the mixture was magnetic stirred for 24 h. Subsequently, the mixture was centrifuged, washed with methanol and finally dried under vacuum (60 °C) for at 497
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Scheme 1. Schematic illustration for the synthesis of FeCo-NC-850.
Fig. 1. SEM images of (a) Fe-NC-850, (b) FeCo-NC-850, (c) Co@Fe-NC-850. (d) TEM, (e) HRTEM and (f) Elemental mapping images of FeCo-NC-850.
improvement of the mass/electron transport. We can see that the introduction of Co(acac)3 results in a visible morphological change. To better understand the influence of cobalt source on the morphology, Co@Fe-NC-850 derived from the hybrid of Co(NO3)2 and FeZIF was prepared by the same preparation process as FeCo-NC-850. Different from FeCo-NC-850, larger bulk and many nanowires are observed in
disorderly smaller nanoparticles. Elemental mapping reveals that Fe and Co are homogenously distributed in FeCoZIF (Fig. S2). Fe-NC-850 derived from FeZIF still presents large irregular bulk with micron size (Fig. 1a). Remarkably, FeCo-NC-850 exhibits a 3D well-interconnected hierarchical porous structure built by nanotubes and nanobulks (Fig. 1b), which is conductive to electrochemical reaction owing to the 498
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located at 44.7, 65.1 and 82.2° which could be attributed to the Co3Fe7 alloy (JCPDS No. 48-1816), corresponding to the results of TEM. However, Co@Fe-NC-850 displays mightily weaker peak intensity of FeCo alloy, indicating negligible FeCo alloy. The Raman spectra of the as-prepared catalysts shows the D and G bands at about 1322 and 1587 cm−1 (Fig. 2b), corresponding to defective carbon and graphitic carbon, respectively [38–40]. The intensity ratio (ID/IG) decreases with the increased pyrolysis temperature, indicating that high temperature could improve the graphitic degree of carbon materials. Moreover, higher graphitization represents a better electrical conductivity of catalysts [41,42]. The values of ID/IG reveal FeCo-NC-850 has a superior conductivity compared with Fe-NC-850 and Co@Fe-NC-850. To further investigate the chemical composition and the interaction between metal and nitrogen for these catalysts, XPS measurement was conducted. The survey XPS spectrum of FeCo-NC-850 confirms the existence of N (6.18 at.%), Fe (0.67 at.%) and Co (0.12 at.%) elements (Fig. S4a). High-resolution N 1 s spectra of FeCo-NC-850 and the other catalysts are shown in Fig. 2c and S4. Based on the previous studies [26], the high- resolution N 1s spectra could be deconvoluted into five peaks assignable to pyridinic-N (398.3 ± 0.2 eV), metal-N (399.6 ± 0.2 eV), pyrrolic-N (400.8 ± 0.2 eV), graphitic-N (402 ± 0.2 eV) and N-oxidized (> 404 eV). Fig. 2d shows the percentage contents of different N species for the as-prepared catalysts. The content of pyridinic-N declines with pyrolysis temperature increasing from 750 to 950 °C, whereas that of graphitic-N increases significantly. Meanwhile, high pyrolysis temperature can form metal-N sites but overhigh temperature may lead to the decomposition of metal-N sites. Furthermore, the rational porous structure with high specific surface area is beneficial to the more metal-N active sites exposed on the surface of catalysts. As a result, FeCo-NC-850 exhibits the highest metal-N content (27.2%) among all the catalysts in this study, indicating a splendid ORR catalytic activity. To further certify the FeeN and CoeN coordination, the high-resolution Fe 2p and Co 2p XPS spectra are shown in Fig. 2e and S4 f. Presumably the broad and weak peaks in Co 2p spectra are ascribed to the very low Co content. Furthermore, two deconvoluted peaks at 707.4 and 720.1 eV are indexed to zero-valence
Co@Fe-NC-850 (Fig. 1c). Moreover, the X-ray photoelectron spectra (XPS) measurement was used to confirm the chemical states of surface Co and Fe for FeCoZIF (Fig. S3). The deconvoluted peaks located at 710.9, 714.0, 718.0, 723.9, 726 eV for Fe 2p are indicative of the Fe2+ 2p3/2, Fe3+ 2p3/2, satellite, Fe2+ 2p1/2, and Fe3+ 2p1/2, respectively [29]. The broad and weak peaks of Co 2p spectra are probably related to the low content of Co. The peak situated at 781.2 eV could be ascribed to Co2+-N or Co2+-O and the peak of 784.2 eV is related to the Co3+-N/O bonds [33]. Meanwhile, the CoeOH bonds are reflected by the peak at 787.2 eV. The XPS analysis indicates the existence of Fe2+, Fe3+, Co2+ and Co3+ in the FeCoZIF. Based on the above results and the previous studies [34,35], part of Fe2+ ions in FeZIF may be oxidated to Fe3+ by Co3+ from Co(acac)3 and oxygen dissolved in solution. Subsequently generated Co2+ would be immobilized by the coordination with exposed imidazole group. This local oxidation could result in the partial collapse and fragmentation for the framework of FeZIF. Finally, the 3D well-interconnected architecture constructed by nanobulks and nanotubes is formed by the catalysis of Fe and Co as well as the escape of metal Zn, Fe and Co during carbonization and acidetching treatments. The microscopic structure of FeCo-NC-850 was ulteriorly investigated by TEM and HRTEM. As revealed in Fig. 1d, the nano-size metallic particles are uniformly encapsulated in the carbon matrix and the meso/macropores are also readily observed. Fig. 1e shows the lattice spacing are 0.202 and 0.345 nm, corresponding to the (110) plane of the Fe3Co7 phase (JCPDS, No. 48-1816) and the (002) plane of the graphite phase, respectively. Elemental mapping images of FeCo-NC850 displays that element Fe and Co are evenly distributed in the carbon matrix (Fig. 1f), demonstrating the formation of FeCo alloy. The X-ray diffraction (XRD) patterns of three catalysts were investigated to analyze crystal structure (Fig. 2a). All catalysts exhibit a strong peak around 26° indexed to the (002) plane of graphitic carbon. The peaks at 37.7 and 43.7° are ascribed to Fe3C (JCPDS No. 35-0772), while two peaks around 50.8 and 74.6° correspond to FeN0.0324 (JCPDS No. 752127). It has been proved that the presence of Fe/Fe3C nanocrystals can enhance the catalytic activity of Fe-Nx for ORR and also intensify the OER activity [36,37]. FeCo-NC-850 features three obvious peaks
Fig. 2. (a) XRD patterns, (b) Raman spectra, (c) Percentage content of different N species of the as-prepared catalysts. (d) N 1 s and (e) Fe 2p high-resolution XPS spectra of FeCo-NC-850. (f) Nitrogen adsorption–desorption isotherms with DFT pore size distribution. 499
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state Fe and the peaks for Co with zerovalence state are centered at 778.5 and 793.9 eV, which suggest the presence of FeCo alloy in FeCoNC-850 [43]. The peaks at 711 eV could be attributed to Fe coordinated to N [44]. The deconvoluted Co 2p3/2 peaks at 780.9 and 783.9 eV reveal the presence of Co2+-N and Co3+-N, while the peak around 787.3 eV is observed owing to the CoeOH bonds [33,45]. Therefore, the XPS analysis demonstrates the existence of bimetal-N active sites and FeCo alloy in FeCo-NC-850, which is consistent with the results of XRD and TEM. The pore structure was further investigated by nitrogen adsorption/ desorption measurements. As revealed in Fig. 2 f and S5, three catalysts all exhibit type-IV isotherms with a distinct hysteretic loop, suggesting the coexistence of micro/meso/macropores [46,47]. The BET special surface area (SSA) of FeCo-NC-850 is calculated to be 553 m2 g−1, which is close to that of Fe-NC-850 (590 m2 g−1) but much higher than Co@Fe-NC-850 (399 m2 g−1). However, FeCo-NC-850 has an obviously larger pore volume (0.491 cm3 g−1) than Fe-NC-850 (0.366 cm3 g−1) and Co@Fe-NC-850 (0.377 cm3 g−1). The above results indicate that Co (acac)3 could increase the extra mesopores of the carbonized material compared with Fe-NC-850, resulting in the high SAA with large pore volume. The percentage of the micropore SSA and volume calculated by T-Method for FeCo-NC-850 are 63.0 and 31.5%, respectively, which is smaller than those of Fe-NC-850 (82.7 and 60.3%). Furthermore, FeCoNC-850 shows the presence of micro/mesopores centered at 2–3, 7–12 and 17–23 nm, whereas Fe-NC-850 and Co@Fe-NC-850 have a little narrow pore size distribution with inferior mesopore volume. Therefore, FeCo-NC-850 exhibits a hierarchical porous structure with high surface area and large pore volume, which is related to the escape of metal Fe, Zn and Co during carbonization and acid-etching treatment as well as the inheritance of micropores from Fe-doped ZIF-8. Cyclic voltammetry (CV) and Linear sweep voltammogram (LSV) measurements were performed in the N2- or O2-saturated 0.1 M KOH electrolyte to evaluate the ORR activity of the as-prepared catalysts. As shown in Fig. 3a, FeCo-NC-850 shows a quasi-rectangular voltammogram in N2-saturated electrolyte, and has an ORR peak at 0.873 V in O2
which is more positive than Pt/C (0.837 V), suggesting an outstanding ORR activity. From the LSV curves in Fig. 3b, FeCo-NC-850 exhibits a remarkable ORR performance with much more positive onset potential (Eonset) of 0.997 V, half-wave potential (E1/2) of 0.864 V and larger diffusion-limited current density than those of Pt/C (Eonset = 0.989 V, E1/2 = 0.838 V), Co@Fe-NC-850 (Eonset = 0.980 V, E1/2 = 0.783 V) and Fe-NC-850 (Eonset = 0.979 V, E1/2 = 0.803 V). It is worth noting that FeCo-NC-850 is the best ones of reported non-noble metal-based ORR catalysts in alkaline electrolyte (Table S1). The pyrolysis temperature as an important factor for catalytic activity was also investigated. Fig. S6 shows the LSV curves of the FeCo-NC materials prepared at different temperatures at 1600 rpm. FeCo-NC-850 exhibits a better catalytic activity than the catalysts carbonized at 750 and 950 °C. The inferior ORR activity of FeCo-NC-750 is possibly ascribed to the lower degree of graphitization which results in a poorer conductivity, whereas the low level of Metal-N active sites for FeCo-NC-950 could hinder ORR activity. Furthermore, the rotating disk electrode (RDE) measurements were performed at rotation rates speeds from 400 to 2025 rpm (Fig. 3c and S7). The corresponding Koutecky-Levich (K-L) plots of FeCo-NC850 show parallel and linear fitting lines (Fig. S8c), suggesting the firstorder reaction kinetics in respect of the dissolved oxygen concentration [48]. For comparison, LSV curves, K–L plots and the corresponding electron transfer number (n) of Pt/C and other samples are shown in Fig. S7 and S8. The calculated average n of FeCo-NC-850 is 3.93 same with that of Pt/C (3.93) and bigger than that of Fe-NC-850 (3.81) and Co@Fe-NC-850 (3.79), indicating that the ORR follows a four-electron pathway for FeCo-NC-850. These results demonstrate the appropriate pyrolysis temperature and cobalt source are favorable to the ORR activity. Furthermore, the electrochemical impedance spectroscopy (EIS) curves of these as-prepared catalysts are shown in Fig. S9. FeCo-NC-850 exhibits a smaller charge transfer resistance (Rct) of 64 Ω than FeCo-NC750 (89 Ω), FeCo-NC-950 (70 Ω), Fe-NC-850 (80 Ω), and Co@Fe-NC850 (81 Ω). The long-term stability of FeCo-NC-850 and Pt/C were evaluated by the cycle experiments (Fig. 3d and e). The E1/2 of FeCo-NC-850 displays
Fig. 3. (a) CV curves of FeCo-NC-850 and Pt/C. (b) LSV curves of FeCo-NC-850, Pt/C, Fe-NC-850 and Co@Fe-NC-850 at 1600 rpm. (c) RDE curves of FeCo-NC-850 at various rotation rates (inset: the electron transfer numbers at 0.2–0.5 V). LSV curves of (d) FeCo-NC-850 and (e) Pt/C before and after 5000 cycles. (f) The i–t chronoamperometric response of FeCo-NC-850 and Pt/C with addition of 2 M methanol. All the catalysts were measured in 0.1 M KOH solution. 500
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Fig. 4. (a) LSV curves (after IR compensation), (b) the corresponding Tafel slopes and (c) the overall LSV curves of FeCo-NC-850, Fe-NC-850, RuO2 and Pt/C at 1600 rpm in 0.1 M KOH.
4. Conclusions
a slight shift of 8 mV after 5000 cycles, whereas Pt/C has a distinct shift of 24 mV, manifesting that FeCo-NC-850 owns superb long-term stability. The durability of these two catalysts was also investigated by the chronoamperometry tests at 0.6 V vs RHE (Fig. S10). The relative current of FeCo-NC-850 remains 92.4% significantly more than that of Pt/ C (80.5%). Furthermore, we investigated methanol tolerance through the chronoamperometry tests (Fig. 3f). FeCo-NC-850 has an inappreciable change of current density with the addition of 2 M methanol into 0.1 M KOH solution, while Pt/C exhibits a tremendous positive current increase. Therefore, FeCo-NC-850 possesses the remarkable long-term stability and methanol tolerance, making it a towardly efficient ORR catalyst. Importantly, in addition to the prominent ORR performance, FeCoNC-850 also shows an appreciable OER activity, which was investigated by the LSV curve at 1600 rpm in 0.1 M KOH. The OER onset potential of FeCo-NC-850 is significantly lower than other catalysts but higher than RuO2 (Fig. 4a). Moreover, the operating potential to achieve the current density of 10 mA cm−2 (Ej=10) is widely used to evaluate the OER activity [49]. The Ej=10 of FeCo-CN-850 is 1.675 V, which is slightly inferior to 1.62 V of RuO2 but much better than that of other catalysts in this study. As revealed in Fig. 4b, the Tafel slope of FeCo-NC-850 (117 mV dec−1) is much closer to 99 mV dec−1 of RuO2 than 147 mV dec−1 of Fe-NC and 159 mV dec−1 of Pt/C, suggesting a respectable intrinsic OER kinetics [50,51]. FeCo-CN-850 possesses superior OER performance than Fe-NC-850 and Co@Fe-NC-850, which is mainly attributed to high surface area with rational pore structure and the existence of FeCo alloy. For the stability test, the Ej=10 of FeCo-NC850 after 1000 CV cycles has an apparently slighter shift than that of RuO2, indicating the superior stability of FeCo-NC-850 in alkaline electrolyte (Fig. S11). The overall oxygen electrode activity of bifunctional catalyst is assessed by the potential gap (ΔE) between the ORR half-wave potential (E1/2) and OER potential at 10 mA cm−2 (Ej=10) [7]. Fig. 4c shows the overall LSV curves of FeCo-NC-850, Fe-NC-850, commercial Pt/C and RuO2. Remarkably, FeCo-NC-850 has the smallest value of ΔE (0.81 V) among all the catalysts in this study which could be comparable with 0.78 V of the combination of commercial Pt/C and RuO2, suggesting superior bifunctional catalytic activity for ORR and OER. Furthermore, FeCo-NC-850 still exhibits a competitive performance compared with other non-noble metal-based catalysts in the previous reports (Table S2), indicating an ideal bifunctional catalyst toward both ORR and OER. Based on the above results, the outstanding overall oxygen activity of FeCo-NC-850 could be ascribed to the coaction of multi-ingredients, especially the unique hierarchical porous structure with high specific area, bimetal-N active sites, the existence of FeCo alloy and Fe/ Fe3C as well as high electrical conductivity.
In summary, we report a facile pyrolysis approach to prepare FeCoN‑carbon material with a unique hierarchical porous structure consisting of nanobulks and nanotubes as an efficient bifunctional catalyst for ORR and OER. Via using local oxidation of FeZIF during impregnation, this work combines Fe(II)-doped ZIF-8 with Co(acac)3 to synthesize TM-N-C material for the first time. Importantly, Co(acac)3 plays dual-roles during synthetic process, which not only generate FeCo alloy and bimetal-N active sites but improves porous structure. Owing to the unique structure with high surface area, FeCo alloy, bimetal-N active sites, the FeCo-NC-850 catalyst exhibits a superior overall oxygen electrode activity with 0.81 V of ΔE in 0.1 M KOH, indicating a towardly bifunctional oxygen catalyst. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.05.014. References [1] R. Raccichini, A. Varzi, S. Passerini, B. Scrosati, The role of graphene for electrochemical energy storage, Nat. Mater. 14 (2015) 271–279. [2] D. Ding, K. Shen, X. Chen, H. Chen, J. Chen, T. Fan, R. Wu, Y. Li, Multi-level architecture optimization of MOF-templated co-based nanoparticles embedded in hollow N-doped carbon polyhedra for efficient OER and ORR, ACS Catal. 8 (2018) 7879–7888. [3] Z.W. Seh, J. Kibsgaard, C.F. Dickens, I.B. Chorkendorff, J.K. Norskov, T.F. Jaramillo, Combining theory and experiment in electrocatalysis: insights into materials design, SCIENCE 355 (2017). [4] I.S. Amiinu, Z. Pu, X. Liu, K.A. Owusu, H.G.R. Monestel, F.O. Boakye, H. Zhang, S. Mu, Multifunctional Mo-N/C@MoS2 Electrocatalysts for HER, OER, ORR, and Zn-air batteries, Adv. Funct. Mater. 27 (2017). [5] J. Wang, H. Wu, D. Gao, S. Miao, G. Wang, X. Bao, High-density iron nanoparticles encapsulated within nitrogen-doped carbon nanoshell as efficient oxygen electrocatalyst for zinc-air battery, Nano Energy 13 (2015) 387–396. [6] J. Wang, C.F. Tan, T. Zhu, G.W. Ho, Topotactic consolidation of monocrystalline CoZn hydroxides for advanced oxygen evolution electrodes, Angew. Chem. Int. Edit. 55 (2016) 10326–10330. [7] Z. Huang, J. Wang, Y. Peng, C. Jung, A. Fisher, X. Wang, Design of efficient bifunctional oxygen reduction/evolution electrocatalyst: recent advances and perspectives, Adv. Energy Mater. 7 (2017). [8] A.L. Cazetta, L. Spessato, K.C. Bedin, I.P.F.A. Souza, R.A. Araujo, A.F. Martins, T.L. Silva, R. Silva, V.C. Almeida, Metal-free ovalbumin-derived N-S-co-doped nanoporous carbon materials as efficient electrocatalysts for oxygen reduction reaction, Appl. Surf. Sci. 467 (2019) 75–83. [9] W. Wang, D. Chai, J. Zhang, S. Xue, Y. Wang, Z. Lei, Ni5Sm-P/C ternary alloyed catalyst as highly efficient electrocatalyst for urea electrooxidation, J. Taiwan Inst Chem E. 80 (2017) 326–332. [10] W. Wang, Y. Yang, Y. Liu, Z. Zhang, W. Dong, Z. Lei, Hybrid NiCoOx adjacent to Pd nanoparticles as a synergistic electrocatalyst for ethanol oxidation, J. Power Sources 273 (2015) 631–637. [11] Z. Liu, J. Yu, X. Li, L. Zhang, D. Luo, X. Liu, X. Liu, S. Liu, H. Feng, G. Wu, P. Guo, H. Li, Z. Wang, X.S. Zhao, Facile synthesis of N-doped carbon layer encapsulated Fe2N as an efficient catalyst for oxygen reduction reaction, CARBON 127 (2018) 636–642. [12] J. Song, Y. Ren, J. Li, X. Huang, F. Cheng, Y. Tang, H. Wang, Core-shell Co/CoNx@C
501
Applied Surface Science 487 (2019) 496–502
G. Li, et al.
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
nanoparticles enfolded by co-N doped carbon nanosheets as a highly efficient electrocatalyst for oxygen reduction reaction, CARBON 138 (2018) 300–308. S. Ratso, M. Kaarik, M. Kook, P. Paiste, V. Kisand, S. Vlassov, J. Leis, K. Tammeveski, Iron and nitrogen co-doped carbide-derived carbon and carbon nanotube composite catalysts for oxygen reduction reaction, CHEMELECTROCHEM 5 (2018) 1827–1836. Z. Lu, B. Liu, W. Dai, L. Ouyang, J. Ye, Carbon network framework derived ironnitrogen co-doped carbon nanotubes for enhanced oxygen reduction reaction through metal salt-assisted polymer blowing strategy, Appl. Surf. Sci. 463 (2019) 767–774. B.Y. Guan, Y. Lu, Y. Wang, M. Wu, X.W.D. Lou, Porous Iron-cobalt alloy/nitrogendoped carbon cages synthesized via pyrolysis of complex metal-organic framework hybrids for oxygen reduction, Adv. Funct. Mater. 28 (2018). X. Gao, J. Yang, K. Song, W. Luo, S. Dou, Y. Kang, Robust FeCo nanoparticles embedded in a Ndoped porous carbon framework for high oxygen conversion catalytic activity in alkaline and acidic media, J. Mater. Chem. A 6 (2018) 23445–23456. J. Wang, Z. Huang, W. Liu, C. Chang, H. Tang, Z. Li, W. Chen, C. Jia, T. Yao, S. Wei, Y. Wu, Y. Lie, Design of N-coordinated dual-metal sites: a stable and active Pt-free catalyst for acidic oxygen reduction reaction, J. Am. Chem. Soc. 139 (2017) 17281–17284. D. Friebel, M.W. Louie, M. Bajdich, K.E. Sanwald, Y. Cai, A.M. Wise, M. Cheng, D. Sokaras, T. Weng, R. Alonso-Mori, R.C. Davis, J.R. Bargar, J.K. Norskov, A. Nilsson, A.T. Bell, Identification of highly active Fe sites in (Ni,Fe)OOH for electrocatalytic water splitting, J. Am. Chem. Soc. 137 (2015) 1305–1313. G. Fu, Z. Cui, Y. Chen, L. Xu, Y. Tang, J.B. Goodenough, Hierarchically mesoporous nickel-iron nitride as a cost-efficient and highly durable electrocatalyst for Zn-air battery, Nano Energy 39 (2017) 77–85. X. Zhu, T. Jin, C. Tian, C. Lu, X. Liu, M. Zeng, X. Zhuang, S. Yang, L. He, H. Liu, S. Dai, 1 in situ coupling strategy for the preparation of FeCo alloys and Co4N hybrid for highly efficient oxygen evolution, Adv. Mater. 29 (2017). C. Su, H. Cheng, W. Li, Z. Liu, N. Li, Z. Hou, F. Bai, H. Zhang, T. Ma, Atomic modulation of FeCo-nitrogen-carbon bifunctional oxygen electrodes for rechargeable and flexible all-solid-state zinc-air battery, Adv. Energy Mater. (2017) 7. M. Zeng, Y. Liu, F. Zhao, K. Nie, N. Han, X. Wang, W. Huang, X. Song, J. Zhong, Y. Li, Metallic cobalt nanoparticles encapsulated in nitrogen-enriched graphene shells: its bifunctional electrocatalysis and application in zinc-air batteries, Adv. Funct. Mater. 26 (2016) 4397–4404. M.S. Burke, M.G. Kast, L. Trotochaud, A.M. Smith, S.W. Boettcher, Cobalt-iron (oxy) hydroxide oxygen evolution Electrocatalysts: the role of structure and composition on activity, stability, and mechanism, J. Am. Chem. Soc. 137 (2015) 3638–3648. P. Zhang, F. Sun, Z. Xiang, Z. Shen, J. Yun, D. Cao, ZIF-derived in situ nitrogendoped porous carbons as efficient metal-free electrocatalysts for oxygen reduction reaction, Energy Environ. Sci. 7 (2014) 442–450. Y. Chen, S. Ji, Y. Wang, J. Dong, W. Chen, Z. Li, R. Shen, L. Zheng, Z. Zhuang, D. Wang, Y. Li, Isolated single iron atoms anchored on N-doped porous carbon as an efficient electrocatalyst for the oxygen reduction reaction, angew. Chem. Int. Edit. 56 (2017) 6937–6941. Y. Ye, F. Cai, H. Li, H. Wu, G. Wang, Y. Li, S. Miao, S. Xie, R. Si, J. Wang, X. Bao, Surface functionalization of ZIF-8 with ammonium ferric citrate toward high exposure of Fe-N active sites for efficient oxygen and carbon dioxide electroreduction, Nano Energy 38 (2017) 281–289. X. Wang, H. Zhang, H. Lin, S. Gupta, C. Wang, Z. Tao, H. Fu, T. Wang, J. Zheng, G. Wu, X. Li, Directly converting Fe-doped metal organic frameworks into highly active and stable Fe-N-C catalysts for oxygen reduction in acid, Nano Energy 25 (2016) 110–119. Z. Hu, Z. Guo, Z. Zhang, M. Dou, F. Wang, Bimetal zeolitic imidazolite frameworkderived iron-, cobalt- and nitrogen-codoped carbon nanopolyhedra electrocatalyst for efficient oxygen reduction, ACS Appl Mater Inter 10 (2018) 12651–12658. Z. Wang, H. Jin, T. Meng, K. Liao, W. Meng, J. Yang, D. He, Y. Xiong, S. Mu, Fe, Cucoordinated ZIF-derived carbon framework for efficient oxygen reduction reaction and zinc-air batteries, Adv. Funct. Mater. 28 (2018). Y. Chen, C. Wang, Z. Wu, Y. Xiong, Q. Xu, S. Yu, H. Jiang, From bimetallic metalorganic framework to porous carbon: high surface area and multicomponent active dopants for excellent electrocatalysis, Adv. Mater. 27 (2015) 5010–5016. H. Zhang, S. Hwang, M. Wang, Z. Feng, S. Karakalos, L. Luo, Z. Qiao, X. Xie, C. Wang, D. Su, Y. Shao, G. Wu, Single atomic iron catalysts for oxygen reduction in
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49]
[50]
[51]
502
acidic media: particle size control and thermal activation, J. Am. Chem. Soc. 139 (2017) 14143–14149. R. Zhao, W. Xia, C. Lin, J. Sun, A. Mahmood, Q. Wang, B. Qiu, H. Tabassum, R. Zou, A pore-expansion strategy to synthesize hierarchically porous carbon derived from metal-organic framework for enhanced oxygen reduction, Carbon 114 (2017) 284–290. B. Cao, G.M. Veith, J.C. Neuefeind, R.R. Adzic, P.G. Khalifah, Mixed close-packed cobalt molybdenum nitrides as non-noble metal electrocatalysts for the hydrogen evolution reaction, J. Am. Chem. Soc. 135 (2013) 19186–19192. H. Su, H. Zhang, X. Tang, B. Liu, Y. An, High Q-factor NiCuZn ferrite with nanocrystalline ferrite particles and Co2O3 additives, Phys. Status Solidi A. 204 (2007) 576–580. K. Zhu, C. Jin, Z. Klencsar, A.S. Ganeshraja, J. Wang, Cobalt-iron oxide, alloy and nitride: synthesis, characterization and application in catalytic peroxymonosulfate activation for Orange II degradation, Catalysts 7 (2017). B.K. Barman, K.K. Nanda, Prussian blue as a single precursor for synthesis of Fe/ Fe3C encapsulated N-doped graphitic nanostructures as bi-functional catalysts, Green Chem. 18 (2016) 427–432. W. Yang, X. Liu, X. Yue, J. Jia, S. Guo, Bamboo-like carbon nanotube/Fe3C nanoparticle hybrids and their highly efficient catalysis for oxygen reduction, J. Am. Chem. Soc. 137 (2015) 1436–1439. T. Liu, L. Zhang, W. You, J. Yu, Core-Shell nitrogen-doped carbon hollow spheres/ Co3O4 Nanosheets as advanced electrode for high-performance supercapacitor, Small 14 (2018). W. Wang, Y. Dong, L. Xu, W. Dong, X. Niu, Z. Lei, Combining bimetallic-alloy with selenium functionalized carbon to enhance Electrocatalytic activity towards glucose oxidation, Electrochim. Acta 244 (2017) 16–25. W. Wang, W. Jing, F. Wang, S. Liu, X. Liu, Z. Lei, Amorphous ultra-dispersed Pt clusters supported on nitrogen functionalized carbon: a superior electrocatalyst for glycerol electrooxidation, J. Power Sources 399 (2018) 357–362. L. Zhang, Z. Su, F. Jiang, L. Yang, J. Qian, Y. Zhou, W. Li, M. Hong, Highly graphitized nitrogen-doped porous carbon nanopolyhedra derived from ZIF-8 nanocrystals as efficient electrocatalysts for oxygen reduction reactions, Nanoscale 6 (2014) 6590–6602. P. Hao, Z. Zhao, J. Tian, H. Li, Y. Sang, G. Yu, H. Cai, H. Liu, C.P. Wong, A. Umar, Hierarchical porous carbon aerogel derived from bagasse for high performance supercapacitor electrode, Nanoscale 6 (2014) 12120–12129. C. Su, H. Cheng, W. Li, Z. Liu, N. Li, Z. Hou, F. Bai, H. Zhang, T. Ma, Atomic modulation of FeCo-nitrogen-carbon bifunctional oxygen electrodes for rechargeable and flexible all-solid-state zinc-air BATTERY, Adv. Energy Mater. (2017) 7. Z. Wu, X. Xu, B. Hu, H. Liang, Y. Lin, L. Chen, S. Yu, Iron carbide nanoparticles encapsulated in mesoporous Fe-N-doped carbon nanofibers for efficient electrocatalysis, Angew. Chem. Int. Edit. 54 (2015) 8179–8183. Y. Wang, D. Liu, Z. Liu, C. Xie, J. Huo, S. Wang, Porous cobalt-iron nitride nanowires as excellent bifunctional electrocatalysts for overall water splitting, Chem. Commun. 52 (2016) 12614–12617. H. Zhong, J. Wang, Y. Zhang, W. Xu, W. Xing, D. Xu, Y. Zhang, X. Zhang, ZIF-8 derived graphene-based nitrogen-doped porous carbon sheets as highly efficient and durable oxygen reduction electrocatalysts, angew. Chem. Int. Edit. 53 (2014) 14235–14239. H. Zou, B. He, P. Kuang, J. Yu, K. Fan, Metal-organic framework-derived nickelcobalt sulfide on ultrathin Mxene Nanosheets for Electrocatalytic oxygen evolution, ACS Appl Mater Inter 10 (2018) 22311–22319. Y. Zhao, Q. Lai, J. Zhu, J. Zhong, Z. Tang, Y. Luo, Y. Liang, Controllable construction of core-shell polymer@Zeolitic Imidazolate frameworks fiber derived heteroatom-doped carbon nanofiber network for efficient oxygen Electrocatalysis, Small 14 (2018). J. Li, P. Hou, S. Zhao, C. Liu, D. Tang, M. Cheng, F. Zhang, H. Cheng, A 3D bifunctional porous N-doped carbon microtube sponge electrocatalyst for oxygen reduction and oxygen evolution reactions, Energy Environ. Sci. 9 (2016) 3079–3084. M. Ma, G. Zhu, F. Xie, F. Qu, Z. Liu, G. Du, A.M. Asiri, Y. Yao, X. Sun, Homologous catalysts based on Fe-doped CoP nanoarrays for high-performance full water splitting under benign conditions, ChemSusChem 10 (2017) 3188–3192. C. Tang, N. Cheng, Z. Pu, W. Xing, X. Sun, NiSe nanowire film supported on nickel foam: an efficient and stable 3D bifunctional electrode for full water splitting, angew. Chem. Int. Edit. 54 (2015) 9351–9355.