Highly Efficient Oxygen Reduction Reaction Catalyst Derived from Fe

Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 10224−10237

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Highly Efficient Oxygen Reduction Reaction Catalyst Derived from Fe/Ni Mixed-Metal−Organic Frameworks for Application of Fuel Cell Cathode Xiulan Qin,†,‡,§ Ying Huang,*,†,‡,§ Ke Wang,†,‡,§ Tingting Xu,§ Yanli Wang,§ Mingyue Wang,†,‡,§ Ming Zhao,†,‡,§ and Qiao Gao†,‡,§

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†

MOE Key Laboratory of Material Physics and Chemistry under Extrodinary Conditions, School of Natural and Applied Science, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China ‡ Shaanxi Engineering Laboratory for Graphene New Carbon Materials and Applications, School of Natural and Applied Science, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China § Department of Applied Chemistry, School of Natural and Applied Science, Northwestern Polytechnical University, Xi’an, Shaanxi 710129, People’s Republic of China S Supporting Information *

ABSTRACT: An effective route was originally reported to synthesize a series of iron−nickel−nitrogen codoped carbon nanocomposites, derived from bimetallic iron/nickel metal−organic frameworks (Fe/Ni-MOFs) and nitrogen-doped graphene (NG). Notably, bimetallic Fe/Ni-MOFs/NG nanocomposites as oxygen reduction reaction (ORR) catalysts have been for the first time investigated for cathodic materials in fuel cells. Interestingly, Fe/Ni-MOFs are not only as the ORR catalyst itself, but also as the cocatalyst carriers with NG in our work. The Fe/Ni-MOFs/NG-20 exhibits excellent ORR performances with an onset potential of 1.09 V and a limiting current density of 8.56 mA cm−2 at a rotation rate of 1600 rpm. The remaining relative current density after 12 000 s is kept at 93.69%. The Tafel slope value is 58.17 mV dec−1. Thus, Fe/Ni-MOFs/NG-20 can be as a novel promising ORR catalyst for cathodes in fuel cells and metal−air batteries to solve the problems of sluggish reaction kinetics, low durability, and high costs.

1. INTRODUCTION Fuel cells as the most promising alternative, renewable, and sustainable devices for energy conversion have been paid everincreasing attention in all kinds of fields, ranging from portable electronics, to transportation, to industrial stationary power.1−3 Both basically theoretical research and considerably applied research have been carried out.4−6 However, the sluggish reaction kinetics of the oxygen reduction reaction (ORR) at the cathode is still the most important problem that needs to be solved thoroughly. This has severely restricted wide and extensive applications and hindered large-scale development of fuel cells. Therefore, research tendencies in recent years have focused on how to accelerate the sluggish kinetics of the ORR at the cathode in a fuel cell.7 Obviously, exploiting highperformance electrocatalysts for the ORR are considered to be very necessary for the cathode. A highly efficient and inexpensive catalytic carrier and catalyst system will greatly © 2019 American Chemical Society

promote the large-scale industrial and commercial applications of fuel cells. In previous research, attempts were made to develop all kinds of various electrocatalysts including precious metal materials,8 3d transition metals and their oxides,9,10 and conducting polymers11,12 and their composites.13,14 More recently, comparatively greater attraction has been transferred to a catalyst carrier or a catalyst system,15,16 which has a great influence on the activity, durability, and efficiency of electrocatalysts. Metal−organic frameworks (MOFs) are regarded as the most promising catalyst carriers and precursors, especially in the preparation of porous carrier materials.17−20 MOFs as Received: Revised: Accepted: Published: 10224

March 13, 2019 May 27, 2019 May 28, 2019 May 28, 2019 DOI: 10.1021/acs.iecr.9b01412 Ind. Eng. Chem. Res. 2019, 58, 10224−10237

Article

Industrial & Engineering Chemistry Research

dimethylformamide (DMF) and stirred for 30 min in air at room temperature to solution form B. Afterward, solutions A and B were mixed to form the suspended mixture. Second, the above mixture was transferred into a 100 mL Teflon-lined autoclave, and kept at 120 °C for 8 h. After that, it was washed by deionized water and ethanol several times, centrifuged, and dried under vacuum at 65 °C for 24.0 h for further use. In order to study the growth mechanism of Fe/Ni-MOFs, we prepared samples that were heated at 150 °C for 2, 6, and 20 h in solvothermal reaction processes, which were marked as Fe/ Ni-MOFs-2, Fe/Ni-MOFs-6, and Fe/Ni-MOFs-20, respectively. 2.2. Preparation of Fe/Ni-MOFs/NG Nanocomposites. Graphene oxide (GO) was prepared by the modified Hummers’ method.36 Typically, 25 mL of suspension solution of GO (0.30 mg/mL) was obtained by vigorous ultrasonication to form A. Simultaneously, as-prepared Fe/Ni-MOFs-20 was dispersed into ethanol solution stirred for 0.5 h to form B. The above two suspensions were mixed together to form C; meanwhile 1.2 g of urea and the 15 mL of glycol were also added into C. Then, mixed C was transferred into a 100 mL Teflon-lined autoclave, and solvent thermal reactions were kept at 100 °C for 10.0 h and afterward cooled to room temperature. The blackish brown Fe/Ni-MOFs/NG-20 was collected, and washed several times with ethanol. Finally, the resulting products were dried at 65 °C under vacuum. In order to compare the ORR performances for a series of the Fe/NiMOFs/NG nanocomposites, the same processes above have been done for Fe/Ni-MOFs-2 and Fe/Ni-MOFs-6; the obtained products were labeled Fe/Ni-MOFs/NG-2 and Fe/ Ni-MOFs/NG-6, respectively. 2.3. Chemical Characterization. Crystalline phases of the as-synthesized samples were identified by X-ray powder diffraction (XRD; model D/max-2500 system, Rigaku). The valence state and surface chemical composition of elements were measured by X-ray photoelectron spectroscopy (XPS). Images of the morphologies and structures were produced by transmission electron microscopy (TEM; Talos F200X) and by field emission scanning electron microscopy (FESEM; Helios G4 CX). Energy dispersive X-ray spectra (EDS) and their mappings were analyzed on a JEOL JEMOARF200F SEM (Talos F200X, FEI Co., USA). Pore surface areas, sizes, porosities, and volumes of the samples were recorded by a Belsorp-Mini II, MicrotracBel. The Raman spectra were produced by a Renishaw Invia Raman microscope. Thermogravimetric analysis (TGA) curves of the samples were obtained by a METTLER TOLEDO TGA2 in N2 atmosphere and at a heating rate of 8 °C min−1. 2.4. Electrochemical Measurements. Electrochemical measurements were carried on a Gamry Interface 5000E electrochemical station with the normal three-electrode system (Gamry Instruments Co. Ltd., USA). The standard three electrodes were used, where a platinum wire was the counter electrode, Ag/AgCl electrode (saturated KCl) was the reference electrode, and glassy carbon (3 mm) was the working electrode, respectively. All values of the potentials in our work were calculated into reversible hydrogen electrode (RHE), and electrochemical measurements were conducted in 0.1 M KOH electrolyte. The active area of the electrode was 0.07056 cm−2, and the load of the working electrode was 0.141 mg·cm−2. The Nernst equation was used for the conversion of the obtained potential.

catalysts possess remarkable performances, such as modularly tunable structures, tunable pore sizes, high crystallinity, and versatile functionalities.21−23 In recent years, MOFs have been used as catalytic carriers and catalysts in asymmetric catalysis for organic reactions,24 in photocatalysis,25,26 in electrochemical catalysis,27 in drug delivery,28,29 and in sensing and luminescence.30,31 For most MOFs catalyst carriers and electrochemical catalysts, postpyrolysis processes of high temperature are often introduced in the synthesis process to improve their electrocatalytic performances.32−34 However, these high temperature treatments of annealing technologies can bring breakdowns wholly or in a part of porous structures in MOFs. Moreover, porous frameworks and carbon− heteroatom bonds can partly collapse. Thus, the load-carrying capacity of the catalyst support is greatly debased. Therefore, the catalyst nanoparticles are agglomerated and the active sites are dramatically reduced. The migration and penetration of ions and electrons are greatly blocked. These greatly impact the performances of ORR catalysts. Herein, to address the above two problems, a series of nanocomposites derived from bimetallic iron/nickel MOFs and nitrogen-doped graphene (NG), and denoted as Fe/NiMOFs/NG, have been successfully synthesized. Here, the bimetallic Fe/Ni-MOFs belong to M-MOFs (M-MOFs are defined as mixed-metal−organic frameworks35). The metalloligands in the M-MOFs are the potential binding active sites to further coordinate with the other functional active groups in graphene oxide structures (such as −OH, −COC−, and −COOH) or to further bridge linkers of other metal ions or clusters. These make the Fe/Ni-MOFs by in situ growth and trapped tightly in the layers of graphene. Further, compared with the more extensively studied monometallic MOFs, MMOFs not only have more potentially active sites for catalysis, but also the second metal constitutes more cornerstones to form stable three-dimensional (3D) networks with porous dimensions. Besides, the synergistic effects of the metallic ions in the Fe/Ni-MOFs/NG and π−π conjugated structures of graphene can together improve the ORR performance. More importantly, the use of urea for nitridation further induced the formation of the nitrogen-doped, graphitic, and hierarchically porous carbon structures. As a result, three-dimensional Fe/ Ni-MOFs/NG with a hierarchically porous size not only allows reactants to enter the networks and to access the ORR active sites, but also promotes the migration and penetration of electrons and ions at the electrode for a fuel cell. Therefore, Fe/Ni-MOFs/NG acts as a novel promising ORR catalyst for cathodes in fuel cells and metal−air batteries to solve the problems of sluggish reaction kinetics, low durability, and high costs.

2. EXPERIMENTAL SECTION Commercial 20% Pt/C was from Johnson Matthey, and Nafion solution (5 wt %) was from DuPont. All other reagents in our work were purchased from Sinopharm Chemical Reagent Co., Ltd., Xi’an, China, as analytical reagent grade. All chemical reagents were directly applied as purchased without any postprocessing. 2.1. Preparation of Fe/Ni-MOFs. In a typical preparation process, first, nickel(II) acetate tetrahydrate (2.5 mmol) and ferric nitrate (2.5 mmol) were dispersed into ethanol (EtOH) (10.0 mL) to form solution A, and then benzene-1,3,5tricarboxylic acid (H3BTC) (1.2 mmol) was dissolved into a mixed solution with 12 mL of EtOH and 18 mL of N,N10225

DOI: 10.1021/acs.iecr.9b01412 Ind. Eng. Chem. Res. 2019, 58, 10224−10237

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Figure 1. Formation mechanism of the fabrication process for hierarchically porous Fe/Ni-MOFs/NG nanocomposites.

Figure 2. (a−c) SEM images of Fe/Ni-MOFs with different growth times for 2, 6, and 20 h: (a) Fe/Ni-MOFs-2, (b) Fe/Ni-MOFs-6, and (c) Fe/ Ni-MOFs-20. (d, e) SEM images of Fe/Ni-MOFs/NG-20 nanocomposite with (d) 1 μm and (e) 500 nm. (f−h) TEM images of Fe/Ni-MOFs/ NG-20 nanocomposite with (f) 500, (g) 100, and (h) 50 nm. (i) TEM image of the porous−lamellar structure of nitrogen doped graphene.

Fe/Ni-MOFs/NG, resulting in the formation of NG and a hierarchically porous structure. On the other hand, the addition of urea blocks to some degree the aggregation of graphene nanolayers. The morphologies and microstructures of as-prepared samples were investigated by SEM and TEM. As shown in Figure 2a−c, the Fe/Ni-MOFs grow into different morphologies with different times of the solvothermal reaction. Clearly, Fe/Ni-MOFs-2 in Figure 2a exhibits a bulk morphology with 2 h growth time of the solvothermal reaction, not forming a complete Fe/Ni-MOFs structure. The original morphology of Fe/Ni-MOFs has been preliminarily formed after 6 h of the solvothermal reaction, but its diameter size is relatively small and not obvious enough, as seen in Figure 2b. As shown in Figure 2c, the formation of granular and obvious morphology is Fe/Ni-MOFs-20 with diameter sizes of 30−50 nm, after 20

E RHE = EAg/AgCl + 0.0591pH + 0.197 = EAg/AgCl + 0.964 (for 0.1 M KOH)

(1)

3. RESULTS AND DISCUSSION The formation mechanism of the Fe/Ni-MOFs/NG nanocomposites is seen in Figure 1. First, the bimetallic Fe/NiMOFs were synthesized by a one-step solvothermal process. Then, as-prepared Fe/Ni-MOFs were dispersed in a suspension of GO ethanol solution. Simultaneously, urea was also added in the above solution as a specific additive by the final hydrothermal reduction process. Finally, the hierarchically porous Fe/Ni-MOFs/NG nanocomposites were obtained. During the hydrothermal process, on the one hand, use of urea for nitridation can further lead to structural evolution of 10226


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Figure 3. (a) SEM image of Fe/Ni-MOFs/NG-20; its elemental mappings of (b) Fe, (c) Ni, (d) C, (e) O, and (f) N, respectively; and (g) its corresponding EDS spectrogram from area #1 in (a). (i) XRD patterns of as-prepared samples.

and Fe/Ni-MOFs/NG-20. The diffraction peaks of the Fe/NiMOFs have been well consistent with the XRD peaks of NiMOFs and Fe-MOFs reported.37−39 XRD results further confirm well-formed bimetallic Fe/Ni-metal−organic frameworks. In order to further reveal the formation, structure, and composition of bimetallic Fe/Ni-MOFs-20, TEM with different magnifications, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and its corresponding compositional line scanning profiles for compositional elements were employed. The results are seen in Figure 4a−e. As are shown in Figure 4a,b,d, the morphology of bimetallic Fe/Ni-MOFs-20 exhibits irregular hexagonal property, agreeing well with the TEM results of Fe/Ni-MOFs20/NG (Figure 2f−h). The fast Fourier transform (FFT) patterns with diffraction radius rings reveal that Fe/Ni MOFs20 exhibits polycrystalline nature, as seen in Figure 4c. Line scanning profiles in Figure 4e further reveal the distribution of each element in bimetallic Fe/Ni-MOFs-20, which illustrates that Fe, Ni, O, and C elements are the main components, while the content of N element is very low. The results of line scanning profiles in Figure 4e are consistent with the TEM− EDS results in Figure 4g. Moreover, the black HAADF line spectrum in Figure 4e further indicates that the average radius width of hexagonal Fe/Ni-MOFs-20 is about 37 nm. Most importantly, elemental mapping images of HAADF-STEM are further observed where each element of Fe, Ni, O, C, and N is uniformly distributed within Fe/Ni-MOFs-20. The results are

h of hydrothermal reaction. SEM images of Fe/Ni-MOFs/NG20 are shown in Figure 2d,e. It can be revealed that the Fe/NiMOFs are embedded and wrapped by flexible NG nanosheets to further form the 3D network with hierarchically porous nanostructures. The results are consistent with the TEM images of the Fe/Ni-MOFs/NG-20 nanocomposite, as seen in Figure 2f−h. TEM images further verify hierarchically porous structures of the Fe/Ni-MOFs/NG-20 nanocomposite, and there are no large agglomeration phenomena. Moreover, the mesoporous structures are an obvious formation in external surfaces and edges of the nitrogen-doped ultrathin and flexible graphene nanolayers, as shown in Figure 2i. SEM and TEM images are also suggested that the experimental protocols are feasible to synthesize the Fe/Ni-MOFs/NG nanocomposites. The element distribution of the Fe/Ni-MOFs/NG-20 nanocomposite was investigated by energy dispersive spectroscopy (EDS), as shown in Figure 3a−g. Its atomic content ratio of Fe, Ni, O, N, and C is 16.33:11.19:57.27:5.90:9.30. The mappings of EDS indicate that the Fe/Ni-MOFs/NG-20 nanocomposite is made up of the Fe, Ni, O, N, and C elements, and the distributions of all elements are consistent and uniform. Clearly, these observations are in accord with the results of the XPS analysis (seen in Figure 6). According to the XRD patterns of samples in Figure 3i, with the increase time of solvothermal reaction, the crystal structures of Fe/Ni-MOFs gradually form. It is shown that there exists incomplete crystalline structure in Fe/Ni-MOFs-2, while obvious crystalline structures are found in Fe/Ni-MOFs-6, Fe/Ni-MOFs-20, 10227


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Figure 4. (a, b) TEM images of Fe/Ni-MOFs-20 with different magnifications and (c) its corresponding FFT patterns. (d) HAADF-STEM image of Fe/Ni-MOFs-20 and (e) and its corresponding compositional line scanning profiles extracted from the boxed area. (f) TEM−EDS spectrogram of Fe/Ni-MOFs-20 from Area #1 in (a).

shown in Figure 6c, located at 284.6, 285.5, 285.8, 286.9, and 289.4 eV, respectively. This can be attributed to CC bonding (sp2 carbon), C−C bonding (sp3 carbon), C−N bonding, C−O of epoxy group, and metal−C groups.43,44 By contrast, the signal of sp2 hybridized C is obviously higher than those of other carbon bonds. Thus, the graphitic sp2 is dominant on the surface, mainly due to sp3 hybridized C had shifted into graphitic carbon by the synergistic catalysis of iron and nickel during the solvothermal process. This change of carbon species is very beneficial for enhancing the ORR performance. In the Ni 2p spectrum (Figure 6d), the two stronger signals at 854.83 and 872.27 eV are the binding energies of Ni 2p3/2 and Ni 2p1/2, considered as Ni2+.45 Simultaneously, the weaker shake-up satellite peak is also observed at 861.89 eV, indexed as Ni3+.42 As shown in Figure 6e, the N 1s peak is resolved into three deconvoluted peaks at 398.64, 401.075, and 403.55 eV, which are attributed to pyridinic N, graphitic N, and oxidized N, respectively.46−50 This further indicates that the amino-type nitrogen (399.4 eV) of urea has reacted with some functional groups on graphene and other functional groups and then transformed into other electronic structures of nitrogen. Notably, the long-range π−π conjugate structures have been formed among the sixmembered rings of pyridinic N, graphitic N, and six-membered

shown in Figure 5. The results of the above measurements together indicate that bimetallic Fe/Ni-MOFs as a new type of bimetallic MOFs have been successfully prepared in our work. The chemical states of the elements are the key to the ORR catalytic performances. Therefore, XPS measurements were carried out to identify the chemical composition and state of the as-prepared samples. As shown in Figure 6a, the survey spectra of as-prepared samples show the coexistence of Fe, C, Ni, N, and O in all samples, and the XPS survey results of the three samples are similar. However, as the growth time of solvothermal reaction is different, the content of each element in the samples is different, judging from the peak intensity. The contents of C, N, Fe, and Ni in all samples are all increasing with the increase of time for solvothermal reaction, while the content of O is decreasing. This is one of the reasons why the ORR performances of Fe/Ni-MOFs/NG-20 exhibit the best among all samples. As shown in Figure 6b, the XPS Fe 2p shows four peaks. The peaks at 710.38 and 724.36 eV correspond to the lower energy spin−orbital (2p3/2) of the Fe2+ species and to the higher energy spin−orbital (2p1/2) region of Fe3+ species, respectively. The peak at 715.17 eV (Fe 2p) is assigned to the coexistence region of Fe2+ and Fe.3,40−42 The peak at 732.24 eV belongs to a satellite peak. Five characteristic deconvolution peaks of the C 1s spectrum are 10228


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Figure 5. (a) HAADF-STEM image of Fe/Ni-MOFs-20 and its corresponding elemental mapping images: (b) Fe, (c) Ni, (d) O, (e) C, and (f) N, respectively.

20, the intensity of the CC peak is obviously weakened, the C−N peak disappears, and the intensities of the C−O and metal−C peaks are enhanced. In the O 1s spectrum of Fe/NiMOFs-20, position and intensity of each bond has some change. These changes indicate that chemical adsorption and chemical reactions occur by carbon and oxygen functional groups between Fe/Ni-MOFs-20 and NG. The significant differences of the as-prepared samples are observed in the surface pore characteristics, measured by N2 adsorption/desorption measurements at 77 K. As shown in Figure 7a,b and Table 1, the typical IV isotherms with H1 hysteresis loops are obtained for all the samples with the specific surface areas of 25.793, 86.939, and 189.79 m2 g−1, respectively. The mesoporous and microporous structures all

rings of graphene in the course of transformation, which not only increase the conductivity of the Fe/Ni-MOFs/NG-20 nanocomposite, but also considerably improve the ORR activity. The binding energy at 530 eV for O 1s can originate from O2− ions.51 Four deconvolution peaks in in Figure 6f belong to carbon oxide (OI at 529.70 eV and oxygen−oxygen bond OII at 529.93 eV), Fe−oxygen bond (OIII at 530.87 eV), and Ni−oxygen bond (OIV at 531.95 eV), respectively. In order to compare the valence changes, the Fe/Ni-MOFs-20 were also tested by XPS; the results are shown in Figure S1 (the Supporting Information). A comparison of Figure 6 and Figure S1 shows that there are slight changes in the positions of the Fe 2p and Ni 2p peaks, while the C 1s and O 1s spectra have changed greatly. In the C 1s spectrum of Fe/Ni-MOFs10229


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Figure 6. (a) XPS survey spectra of as-prepared samples. (b−f) XPS high resolution spectra of Fe/Ni-MOFs/NG-20: (b) Fe 2p, (c) C 1s, (d) Ni 2p, (e) N 1s, and (f) O 1s, respectively.

the activity of the kinetic reaction for ORR. Therefore, it is anticipated that Fe/Ni-MOFs/NG-20 will demonstrate better catalytic activity and kinetics, which is also confirmed by ORR, linear sweep voltammetry (LSV), and Tafel results (seen in Figure 8). As shown in Figure 7c,d, the change of the carbon structure and defect sites induced by N-doping were further supported by Raman spectroscopic results. Similar to the spectra of all sp2 carbons, two obvious peaks (D- and G-bands) emerge near 1340 and 1580 cm−1 in all of the prepared products, respectively.53,54 However, the ratios of ID/IG increase from 0.93 for GO, to 1.03 for NG, and to 1.032 for Fe/Ni-MOFs/ NG-2; to 1.041 for Fe/Ni-MOFs/NG-6; and to 1.062 for Fe/ Ni-MOFs/NG-20, indicating the decrease of the degree of graphitization with the reduction of sp2 carbon. This reveals the incorporation of heterogeneous atoms (N, Fe, Ni). The D peak is the disordered carbon, while the G peak is graphitic carbon or ordered carbon. This reveals the defect sites and structural distortion are caused by doped heteroatoms. Also, this important fact suggests that some hydrocarbons and nitrogen atoms are introduced into graphitic lattices. Addi-

exist in all samples. Just like the changes of surface areas for samples, the values of total pore volumes for samples increase from 0.029 to 0.279 cm3 g−1. It can also be seen that the distribution of mesoporous structures decreases gradually with the increase of Fe/Ni-MOFs growth time, while the microporous structures increase gradually. Specifically, mesopores are predominant for Fe/Ni-MOFs/NG-2 with a mean pore diameter of 7.3896 nm, but the value of the mesoporous mean diameter is decreased to 3.6571 nm in Fe/Ni-MOFs/NG-20. This change in the mean diameter is explained by the following mechanisms. The microporous structure may be inherited from frameworks of the Fe/Ni-MOFs, while the mesoporous structures are mainly originated from the external surface and edges of the thin flexible graphene sheets, or from the interspaces between nanoparticles, which agree well with TEM observation (Figure 2i). The hierarchical pores are important for the ORR, based on the fact that the hierarchically porous structure is beneficial to the mass transfer in the ORR process.52 Moreover, micropores and mesopores with numerous exposed ORR active sites can provide many channels and pores for transportation of electrons and molecules, improving 10230


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Figure 7. (a) N2 adsorption−desorption isotherms of Fe/Ni-MOFs/NG-2, Fe/Ni-MOFs/NG-6, and Fe/Ni-MOFs/NG-20 at 77 K and (b) their BJH pore size distribution curves. (c, d) Raman spectra of GO, NG, Fe/Ni-MOFs/NG-2, Fe/Ni-MOFs/NG-6, and Fe/Ni-MOFs/NG-20.

Table 1. Summary of Surface Properties of As-Prepared Samples sample

SBET (m2 g−1)

Vtotal (cm3 g−1)

diametermeso (nm)

diametermicro (nm)

Fe/Ni-MOFs/NG-2 Fe/Ni-MOFs/NG-6 Fe/Ni-MOFs/NG-20

25.793 86.939 189.79

0.029 0.104 0.279

7.3896 5.5218 3.6571

0.9103 0.886 0.765

tionally, the 2D peaks of samples can be seen at 2750 cm−1 in all samples, which illustrates that the multilayers exist. The Raman results have also been consistent with the results of TEM and SEM (Figure 2). Comparative cyclic voltammogram (CV) measurements of as-prepared samples and commercial 20% Pt/C catalyst were performed to evaluate the ORR catalytic activities. For comparison, all CV tests were also carried out under identical conditions. The commercial 20% Pt/C and other samples, except Fe/Ni-MOFs/NG-2 and NG, exhibit good ORR catalytic activities, and obviously observed cathodic ORR peaks, as shown in Figure 8a. The ORR activity of Fe/NiMOFs/NG-2 is too poor to observe a cathodic peak. Impressively, Fe/Ni-MOFs/NG-20 stands out from these samples, due to its positive potential of the ORR peak at 0.72 V (vs RHE) and the largest value of cathodic current density for the ORR peak (5.49 mA cm−2), while those of Pt/C are 173 mV and 3.42 mA cm−2. The increase of peak current density and the positive shift of peak potential have revealed the strongly electrocatalytic activity of Fe/Ni-MOFs/NG-20 for ORR. LSV curves were obtained to further evaluate the ORR activities of different samples at 1600 rpm in O2saturated 0.1 M KOH at a potential scanning rate of 10 mV s−1, as shown in Figure 8b. The catalysts of Fe/Ni-MOFs/NG6 and Fe/Ni-MOFs/NG-20 also possessed excellent activities with onset potentials of 1.09 and 1.01 V, respectively. These

are positive 180 and 100 mV to the 20% Pt/C electrode (the onset potential of 0.91 V for 20% Pt/C, close to the published literature27,50). The values of the limiting current densities (JL) for Fe/Ni-MOFs/NG-6 and Fe/Ni-MOFs/NG-20 are 6.92 and 8.56 mA cm−2, which are both larger than that of 20% Pt/ C (JL = 5.46 mA cm−2). Obviously, The ORR activity of Fe/ Ni-MOFs/NG-2 is the worst among them, due to that the intact structure of the Fe/Ni-MOFs has not yet formed during the solvothermal process of 2 h. The bulk morphology of Fe/ Ni-MOFs-2 is seen in Figure 2 SEM images. Fe/Ni-MOFs/ NG-20 can possess the best ORR activity among the samples, because the perfect MOF porous skeleton has formed by 20 h of solvothermal reaction. Thus, for Fe/Ni-MOFs-20, more exposed channels and pores are pathways for electrons, ions, and molecules to access the ORR active sites, which can greatly improve the rate of the ORR kinetic reaction. To better illustrate the ORR properties, ORR active performances are exhibited in Table 2. By comparison, Fe/ Ni-MOFs/NG-20 shows excellent ORR activity, superior to those of commercial 20% Pt/C catalyst and other previously reported MOFs. To further obtain the deep ORR mechanism of Fe/NiMOFs/NG-20, LSV plots were recorded at different rotation speeds, under saturated N2 from that of saturated O2 in 0.1 M KOH at scan rate of 10 mV s−1. The cathodic current densities in Figure 8c are orderly increased from 800 to 2000 rpm, 10231


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Figure 8. (a) Comparative CV curves of as-prepared samples and commercial 20% Pt/C catalyst in O2 saturated 0.1 M KOH at scan rate of 50 mV s−1. (b) Comparison of polarization plots of as-prepared samples and commercial 20% Pt/C catalyst at 1600 rpm in O2-saturated 0.1 M KOH at scan rate of 10 mV s−1. (c) LSV polarization plots of Fe/Ni-MOFs/NG-20 with different rotation speeds under saturated N2 from that of saturated O2 in 0.1 M KOH at scan rate of 10 mV s−1, (d) its K−L plots, and (e) and its electron transfer number. (f) Tafel plots for Fe/Ni-MOFs/NG-20 and commercial 20% Pt/C catalyst in 0.1 M KOH electrolyte. (g) RRDE results for Fe/Ni-MOFs/NG-20 and 20% Pt/C in O2-saturated 0.1 M KOH electrolytes under a rotation rate of 1600 rpm and at scan rate of 10 mV s−1 and (h) their corresponding peroxide yields and electron-transfer numbers (n) at potential ranges from 0.2 to 0.8 V.

four-electron (4e−) ORR pathway, and also indicates that Fe/

which indicates that a typical mass transfer is dominated by diffusion control. This result can be illustrated as that diffusion distances are shortened with the increased rotaion at the electrode surface. As seen in Figure 8d,e, all the Koutecky− Levich (K−L) plots are parallel in the potential range from 0.3 to 0.7 V, indicating first-order reaction kinetics for ORR. The average number (n) per oxygen molecule was approximately 3.925 in the potential range from 0.3 to 0.7 V (close to the theoretical value of 4.0). This demonstrates that the diffusioncontrolled process belongs to the efficient one step followed

Ni-MOFs/NG-20 has high catalytic selectivity for the reaction of O2 + 2H2O + 4e− = 4OH−. The transferred electron numbers per oxygen involved in ORR are based on the K−L equation as in the following relationship (from eqs 2 and 3): 1 1 1 1 1 = + = + 1/2 J JL JK nFKC0 Bω 10232

(2)


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implies faster kinetic velocity of electron transport;62 that is, it permits a faster attainment of higher catalytic current density under lower overpotential. This shows that Fe/Ni-MOFs/NG20 has good performance in dynamics. To further study the ORR kinetics of Fe/Ni-MOFs/NG-20, rotating ring disk electrode (RRDE) tests were carried out in O2-saturated 0.1 M KOH electrolyte, at the rotation rate of 1600 rpm and with a scan rate of 10 mV s−1; commercial 20% Pt/C was used as a comparison. As shown in Figure 8g, whether Fe/Ni-MOFs/NG-20 or 20% Pt/C, the values of the current densities for the disks are obviously much larger than those of the rings, which illustrates that the ORR catalytic processes mainly undergo four-electron pathways.63 Moreover, the calculated values of the average electron transfer numbers (n) for Fe/Ni-MOFs/NG-20 and 20% Pt/C are 3.89 and 3.96 at potentials from 0.2 to 0.8 V, both close to the theoretical value of 4.0 for the 4e− ORR pathway, and also in good consistency with the result from the K−L plots. The values of hydrogen peroxide yields (%) are both below 5%, as seen in Figure 8h, which implies high catalytic efficiency toward ORR.64 The 4e− ORR pathway is superior to the 2e− ORR pathway, because there is no H2O2 formation in the 4e− process. H2O2 is poisonous to proton exchange membranes in fuel cells. The measured and calculated values of the commercial 20% Pt/C are near those in recently reported literature.65−67 The above values of n and peroxide yields were evaluated by RRDE data, based on eqs 4 and 5.

Table 2. Comparison of ORR Performances of Fe/NiMOFs/NG-20 and Other MOF Catalysts in 0.1 M OH− E0 (V)

JL (mA·cm−2)

loading (mg·cm−1)

Fe/Co-MOF Te@ZIF-8/Fe Fe/ZIF-7 Cu/ZIF-8 UiO-66-NH2 Co−Ni-ZIF(900) 20% Pt/C

1.05 0.946 1.04 0.914 0.92 0.923 0.91

3.7 6.1 5.6 5.5 4.86 5.1 5.46

0.10 0.15 0.978 0.250 0.592 − 0.141

Fe/Ni-MOFs/NG-20

1.09

8.56

0.141

material

B = 0.62nFC0D0 2/3v−1/6

ref 55 56 57 58 59 60 our work our work

(3)

where J represents the current density; JL and JK express the diffusion-limiting and kinetic-limiting current densities; ω is the angular frequency of the disk (rad s−1); n is defined as the number of electrons per transferred oxygen molecule; F, C0, and D0 are 96 485 C·moL−1, 1.2 × 10−6 moL cm−3, and 1.9 × 10−5 cm2 s−1 in 0.1 M KOH; and v is 0.01 cm2 s−1, respectively. The kinetic mechanism of catalytic ORR of Fe/Ni-MOFs/ NG-20 is further investigated by Tafel plots (in Figure 8f). The value of the Tafel slope is 58.17 mV dec−1, which is better than that of commercial 20 wt % Pt/C catalyst (68.49 mV dec−1, closely related literature50,61). A lower value of the Tafel slope

Figure 9. EIS curves in 0.1 M KOH O2-saturated electrolyte with frequency range from 105 to 100 Hz: (a) 20% Pt/C (a) and (b) Fe/Ni-MOFs/ NG-20. (c) Durability tests at 0.6 V under 0.1 M KOH O2-saturated solution. (d) Chronoamperometric responses in 0.1 M KOH O2-saturated before and after addition of absolute methanol. 10233


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Industrial & Engineering Chemistry Research H 2O2 % = 200 n=4

IR /N ID + IR /N

ID/N ID + IR /N

To further evaluate the crossover effect for fuel molecules in the ORR process, current−time chronoamperometry was applied at 0.6 V in O2-saturated 0.1 M KOH electrolyte. As shown in Figure 9d, on addition of methanol, an immediate and sharp increase in relative current is observed on the 20% Pt/C electrode, as reported in previous literature.73−75 This sharp increase of relative current is due to the rapid catalytic oxidation of methanol on the 20% Pt/C surface, which makes the ORR activity of 20% Pt/C decrease sharply. This phenomenon indicates that the catalytic selectivity of 20% Pt/C for methanol fuel is better than that for oxygen, and this result is detrimental to the ORR process of the fuel cell cathode. The current density of Fe/Ni-MOFs/NG-20 is almost unaffected under the same testing conditions. These observations indicate that Fe/Ni-MOFs/NG-20 possesses better fuel selectivity toward ORR and higher tolerance to methanol crossover, compared with 20% Pt/C.

(4)

(5)

where IR and ID are ring and disk current densities. N is the collection efficiency of the Pt ring electrode (N = 0.37). The electrochemical impedance spectroscopic (EIS) measurements were measured in 0.1 M KOH O2-saturated electrolyte with a frequency range from 105 to 100 Hz to further investigate the electrochemical mechanism, as shown in Figure 9a,b. The EIS patterns can be fitted into an equivalent circuit. Rs represents the electrolyte resistance,68 while Rp corresponds to the catalyst surface resistance, related to the migration of electrons and ions, W is the Warburg impedance, and CPE1 and C1 represent the interfacial resistance and charge-transfer resistance, respectively. As we have seen, the Nyquist plots both show half of the semicircles, followed by the lines. Rp of Fe/Ni-MOFs/NG-20 (4.82 Ω) obviously decreases, compared with that of 20 wt % Pt/C (6.01Ω). Values of the Warburg impedance (W) also have a similar decrease. This shows that Fe/Ni-MOFs/NG-20 has better penetrability of electrons and ions than 20 wt % Pt/C. It can also be concluded that Fe/Ni-MOFs/NG-20 has better electrical conductivity. This is mainly attributed to the intact MOFs architecture, large π−π conjugated bonds, hierarchically porous structures, and nitrogen-doped graphitic carbon. The excellent ORR activity of the Fe/Ni-MOFs/NG-20 nanocomposite can be attributed to its unique structure, which is mainly manifested in the following three aspects. First, porous Fe/Ni-MOFs are not only the ORR catalyst itself, but also the cocatalyst carriers with NG in our work. The bimetallic Fe/Ni-MOFs, as mixed-metal−organic frameworks, can produce more potential metal−carbon−nitrogen ORR active sites, compared with monometallic MOFs. Also, Fe/Ni− C−N sites have been proven to be highly efficient ORR active sites.69−72 Moreover, porous 3D architectures of bimetallic Fe/ Ni-MOFs are intact, without high temperature treatments of annealing technologies. Second, the hierarchically porous structure of the Fe/Ni-MOFs/NG-20 nanocomposite with a large specific surface area can provide numerous exposed channels and pores as pathways for electrons, ions, and molecules to access the ORR active sites, which can greatly improve the rate of ORR kinetic reaction. Third, graphene C, graphitic N, and pyridinic N in Fe/Ni−C−N active sites can form a large π−π conjugated structure, which can also enhance the rate of ORR kinetic reaction. Just as reported in the literature, graphene C, graphitic N, and pyridinic N are the ORR active sites, which can improve the ORR activity.38,40,47,52 The durability is also important parameter for evaluating an ORR to directly impact fuel cell application.62 The durability of the Fe/Ni-MOFs/NG-20 and Pt/C electrodes were assessed for ORR by chronoamperometric methods at 0.6 V in 0.1 M KOH O2-saturated electrolyte at a rotation rate of 1600 rpm. The current density of the Fe/Ni-MOFs/NG-20 electrode exhibits a lower loss after 12 000 s, up to 6.31% decrease from its original activity, while Pt/C loses 24.32% of its original activity under the same condition, as shown in Figure 9c. The excellent durability of Fe/Ni-MOFs/NG-20 is attributed to a robust MOFs skeleton embedded and wrapped by the graphene and hierarchically porous distribution.

4. CONCLUSION In summary, this research work provides a promising strategy to design and fabricate hierarchically porous M-MOFs nanocomposites. As-prepared M-MOFs skeleton embedded and wrapped by nitrogen-doped graphene can form robust architecture, as a highly efficient ORR catalyst for fuel cells. Fe/Ni-MOFs/NG-20 exhibited a highly efficient ORR performance at the fuel cell cathode under alkaline media. These excellent ORR properties are mainly due to possession of intact M-MOFs architecture, hierarchically porous structures, much highly graphitic carbon, nitrogen doping to form pyridinic N and graphitic N, long-range π−π conjugated bonds in molecular chain, and an interconnected 3D conductive network. Moreover, the addition of urea to form hierarchically porous structures is not only favorable for promoting the migration and penetration of molecules, electrons, and ions in the fuel cell electrode, but also is beneficial to improve activity, stability, durability, and tolerance to the fuel molecule crossover effect. Our work shows that the Fe/Ni-MOFs/NG20 nanocomposite is a new candidate to improve the ORR performance of catalysts in alkaline fuel cells in the future.

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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/acs.iecr.9b01412. Details on XPS survey spectrum of Fe/Ni-MOFs-20 and XPS high resolution spectra of Fe 2p, Ni 2p, C 1s, and O 1s (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.H.). ORCID

Ying Huang: 0000-0001-5677-8426 Tingting Xu: 0000-0003-0896-3701 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Scientific Research Program Funded by Shaanxi Provincial Education Department (Grant No. 14JK2145) and the Innovation 10234


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Foundation of Shanghai Aero-space Science and Technology (Grant No. SAST2016114). We would like to thank the Analytical & Testing Center of Northwestern Polytechnical University for chemical characterization tests.

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Highly Efficient Oxygen Reduction Reaction Catalyst Derived from Fe

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