Lewis-Basic Lanthanide Metal-Organic Framework-Derived Versatile

Jun 11, 2018 - ... Metal-Organic Framework-Derived Versatile Multi-Active-Site Synergistic Catalysts ... Oxygen reduction reaction underpins the devel...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 22023−22030

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Lewis-Basic Lanthanide Metal-Organic Framework-Derived Versatile Multi-Active-Site Synergistic Catalysts for Oxygen Reduction Reaction Zhong Zhang, Shumei Liu, Xiaohui Li, Tao Qin, Ling Wang, Xiangjie Bo,* Yiwei Liu, Li Xu, Shuang Wang, Xiuwei Sun, Ying Lu, Fang Luo, and Shuxia Liu*

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Key Laboratory of Polyoxometalate Science of the Ministry of Education, College of Chemistry, Northeast Normal University, Jilin 130024, P. R. China S Supporting Information *

ABSTRACT: Oxygen reduction reaction underpins the development of the whole fuel-cell field, where there is a strong impetus to develop efficient and stable catalysts that can replace the precious metal Pt/C. Herein, a series of excellent catalysts for ORR derived from Ce/La dual lanthanide metal-organic framework with functional Lewisbasic sites were synthesized for the first time. The synergistic effect of high concentration of oxygen vacancies from Laembedded CeO2 and Fe-Nx sites as well as porous structure endows the catalyst superior performance to Pt/C, with a halfwave potential (E1/2) of 0.870 V and a current density (j) of 5.43 mA/cm2. Furthermore, the catalysts are also effective for other nonelectrocatalytic reactions. It is expected that this research will contribute to synthesis of an excellent nonplatinum electrocatalyst for fuel-cell applications, and the oxygen vacancies stabilized in carbon matrix offer a method for versatile catalyst design for other reactions. KEYWORDS: lanthanide metal-organic framework, synergistic catalysts, oxygen reduction reaction, La-doped ceria, N-doped porous carbon

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molecules,22−25 and other carbon sources,26−30 such as graphene oxide through adsorption or host−guest interaction, can greatly expand the types of active sites and enhance the performance of the final material. The last one is through the construction of isomorphic MOFs to optimize the composition and structure of the final catalyst after pyrolysis, like the most typical ZIF-8 and ZIF-67 pairs.31−33 It is noted that M−N/C (such as Fe, Co) catalysts obtained have shown much higher activity and stability than that of the commercial Pt/C catalyst among all of the nonprecious metal catalysts.34 In addition, ceria (CeO2), as an important oxygen storage material used in the three-way catalysts, possesses unique oxygen storage capacity and exhibits reversible transformation between Ce3+/Ce4+, properties beneficial for CO oxidation, NOx reduction,35 ORR,36−39 etc. Furthermore, the doping of aliovalent ions (La3+, Gd3+, Cu2+, etc.) results in oxygen vacancies40,41 and thus induces more attractive properties, including enhanced sintering resistance,42 electron transfer, and adsorption of reactant species,37 which can greatly improve performance of CeO2.

INTRODUCTION As a new form of energy conversion, fuel cells are considered as highly promising technology to solve energy and environmental problems.1,2 To this end, optimization of the core of fuel cells, cathodic oxygen reduction reaction (ORR), has attracted much attention.3−5 Although the commercial Pt/C catalyst demonstrates excellent performance for ORR, its low earth abundance, high cost, ease of poisoning, and poor stability limit further development.6−9 Therefore, the demand for nonprecious metal catalysts with efficient and stable ORR is imminent. In recent years, numerous studies have demonstrated that functionalized porous carbon-based material derived from metal-organic frameworks (MOFs) exhibits exceptional activity and stability for ORR catalysis,10−13 owing to the distinctive features of MOF with crystalline porous structure and large surface area built from metal ions or clusters and suitable organic linkers. These features include rich composition, atomic-level dispersion, tunable premodification/postsynthesis performance, and unique template as well as precursor effect.14−16 At present, MOF-derived, functionalized carbonbased catalysts are mainly manufactured by three strategies. In the first strategy, MOFs are pyrolyzed directly, leading to limited active sites of the resulting catalysts.17,18 In the second, functionalization of MOF with metal ions, 19−21 small © 2018 American Chemical Society

Received: March 6, 2018 Accepted: June 11, 2018 Published: June 11, 2018 22023

DOI: 10.1021/acsami.8b03742 ACS Appl. Mater. Interfaces 2018, 10, 22023−22030

Research Article

ACS Applied Materials & Interfaces Scheme 1. Illustrated Preparation Process of xLa-CeNC-Fe from MOF-Ce/La-x

Figure 1. (a) XRD patterns of xLa-CeNC-Fe. (The vertical purple line represents bulk CeO2 PDF# 34-0394.) (b, c) N2 adsorption/desorption isotherms and pore size distribution of xLa-CeNC-Fe. Ce/La-x@Fe3+ was heated to 900 °C for 2 h under high-purity N2 (99.999%) atmosphere with a heating rate of 5 °C/min. The prepared carbon material (denoted as xLa-CeNC-Fe NAE, NAE represents the sample without acid etching treatment) was immersed in 1 M HCl solution for 10 h, then centrifuged, washed, and dried. Preparation of xLa-CeNC. MOF-Ce/La-x (400 mg) was directly pyrolyzed as above without introduction of Fe3+ as well as acid etching treatment.

Our group has reported a series of isomorphic fluorescent lanthanide MOFs (Ln-BTPCA) with multiple Lewis-basic (LB) sites based on rare-earth metal ions and nitrogencontaining carboxylic acid ligands, which exhibit superior performance in metal ion sensing (especially Fe3+ ions) and tunable white light emission.43,44 Inspired by previous research, herein, we synthesized a series of Fe/N-doped porous carbons in collaboration with La-embedded CeO2 catalysts (xLaCeNC-Fe, here x stands for the atomic ratio of La in original MOF-Ce/La, the same in MOF-Ce/La-x, see below) derived from Ce/La dual rare-earth metal MOF (MOF-Ce/La-x). Because of the synergistic effect of Fe/N-doped porous carbon with massive oxygen vacancies in La-embedded CeO 2 nanoparticles, the catalyst 0.5La-CeNC-Fe shows the best performance for ORR catalysis among the catalysts evaluated in this study.

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RESULTS AND DISCUSSION

The Ce/La bimetal MOFs were synthesized by a simple coprecipitation method between the corresponding acetate hydrate with different Ce/La ratios and ligand (Scheme 1). Several lines of evidence suggest that acetate probably forms some precursor fragment structures45,46 with metal ions in solution, and acetate ions can accelerate the deprotonation of the ligand47,48 at the same time. The rapid nucleation rate resulted in nanometer dimension MOFs with uniform size about 200 nm (Figure S1). To determine the phase purity of the obtained powder, X-ray diffraction (XRD) patterns of MOF-Ce/La-x were obtained (Figure S2). XRD patterns are in good agreement with the simulated ones. However, sharp peaks broaden with increasing ratios of La3+, which can probably be attributed to the incorporation of La3+ with a larger radius, leading to poor crystallinity. The element content of Ce and La in all MOF-Ce/La-x was determined by energydispersive X-ray (EDAX) data (Table S1). Then, Fe3+-modified MOF-Ce/La-x (designated as MOFCe/La-x@Fe3+) was collected by immersing MOF-Ce/La-x in DMF solutions containing FeCl3. This was further confirmed by the fluorescence-quenching experiment (Figure S3). After pyrolysis in an inert atmosphere followed by a simple one-step acid etching, the corresponding catalysts were obtained (designated as xLa-CeNC-Fe). The phase constitution was confirmed by powder X-ray diffraction (XRD). The characteristic diffraction peaks were assigned to the bulk CeO2 (PDF# 34-0394). Both before and after acid etching, the diffraction peaks of the correlative Fe phase were not found (Figures 1a

EXPERIMENTAL SECTION

Chemical Reagents and Materials. Except the ligand of 1,1′,1″(benzene-1,3,5-triyl)tripiperidine-4-carboxylic acid (H3BTPCA) was synthesized referring to the previous literatures,43,44 all other reagents were commercially available and used without further purification. Cerium(III) acetate hydrate [(CH3CO2)3Ce·xH2O, Mw: 317.25 g/ mol] and lanthanum(III) acetate hydrate [(CH3CO2)3La·xH2O, Mw: 316.04 g/mol] were purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). Deionized water was used for all experiments. Synthesis of MOF-Ce/La-x. Typically, cerium(III) acetate hydrate and lanthanum(III) acetate hydrate with the different ratios of Ce3+/La3+ were dissolved in 10 mL deionized water (the total molar amounts of La3+ and Ce3+ were 0.4 mmol). H3BTPCA (0.4 mmol, 0.93 g) was dissolved in 10 mL N,N-dimethylformamide (DMF) and added dropwise to the earth metal acetate solution with continuous stirring at room temperature. The precipitate was washed with alcohol and distilled water several times and followed by drying in an oven at 60 °C for 10 h. Preparation of xLa-CeNC-Fe Electrocatalysts. Typically, 400 mg of MOF-Ce/La-x was immersed in 8 mL of 0.05 M FeCl3 DMF solution for 20 h (Fe3+/MOF-Ln = 0.6, mole ratio), then washed with ethanol three times and drying process as above. The prepared MOF22024

DOI: 10.1021/acsami.8b03742 ACS Appl. Mater. Interfaces 2018, 10, 22023−22030

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Figure 2. (a−d) SEM images and (e−j) TEM and high-resolution TEM (HRTEM) images of 0.5La-CeNC-Fe. (k−r) Corresponding element mapping of 0.5La-CeNC-Fe.

and S4a,b). The diffraction peaks of Ce-NC-Fe without La3+ incorporation did not shift, indicating that Fe3+ was not doped into the ceria lattice (Figure 1a). As expected, with increasing ratios of La3+ in the MOF-Ce/La-x, the diffraction peaks of CeO2 gradually shifted toward lower 2θ values, suggesting that the doping amount of La3+ gradually increased due to the larger ionic radius of La3+ (0.116 nm) than that of Ce4+ (0.097 nm).49,50 However, when the content of La3+ increased to x = 0.6, no CeO2 diffraction and scattering peaks existed in the corresponding sample of 0.6La-CeNC-Fe, indicating excessive doping and unstable crystal structure (Figure S4c,d), which was also corroborated by EDAX data (Figure S5) and X-ray photoelectron spectroscopy (XPS) survey spectra (Figure S6). Unexpectedly, when MOF-Ce/La-x without Fe 3+ was pyrolyzed, almost no diffraction peak for the corresponding La oxide was observed (Figure S4a). However, the characteristic diffraction peaks of La2O3 (PDF# 05-0602) were clearly observed when Fe3+ was immersed (Figure S4b). We speculate that trace iron could apparently catalyze crystallization of La2O3 during pyrolysis. The unexpected formation of La2O3 could be advantageous, in that the catalyst’s surface area and porosity can be improved after these nanoparticles were etched, as verified by N2 sorption/desorption experiments. The obvious characteristics of micropore were shown for CeNC-Fe, whereas other xLa-CeNC-Fe samples showed typical typed-IV isotherms, suggesting the coexistence of micropores and mesopores 51,52 (Figure 1b). Brunauer−Emmett−Teller (BET) surface areas of xLa-CeNC-Fe (corresponding x = 0, 0.2, 0.5, and 1) were determined to be 218.23, 280.49, 418.55, and 561.72 m2/g, which increased rapidly in line with the

hypothesis above (Figure S7). Such a high BET surface area is critical to adequate exposure of the catalytic active sites. At the same time, the size distribution of 0.5La-CeNC-Fe is in the mesoporous range and primarily within 2−8 nm range, which is highly beneficial for mass transfer during the reaction (Figure 1c). On the other hand, after the La2O3 nanoparticles being removed, the carbon content was also increased, which also contributed to large surface area and high conductivity of catalysts (Figure S7 and Table S2). To further investigate the composition and morphology of catalysts, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images (Figures 2a−j, S8, and S9) were acquired. Because of the catalytic graphitization effect of Fe,53−55 a large amount of carbon nanotubes (CNTs) with about 20 nm wide and several micrometers long appeared in the 0.5La-CeNC-Fe (Figures 2a−h and S8f). This may be because volatilized organic fragments assemble to form carbon nanotubes under the catalysis of Fe. Besides CNTs, graphenewrapped three-dimensional porous carbon (GWPC) was also presented (Figures 2i,j and S8e). TEM images showed that small La-doped CeO2 nanoparticles with sizes of 5−10 nm were homogeneously embedded in the CNTs/GWPC structure (Figure 2i,j). On the contrary, as for other samples of La3+ incorporation, significant agglomeration of material (Figure S8a−d) occurred with the appearance of large-size ceria nanoparticles (Figure S9a−d). The La3+ in MOF facilitated the dispersion of Ce3+ and effectively hindered the agglomeration of CeO2 particles. On the other hand, removal of the produced La2O3 nanoparticles after acid treating greatly improved the surface 22025

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Figure 3. (a) Raman spectrum of xLa-CeNC-Fe. High-resolution XPS spectra of (b) C 1s, (c) N 1s, (d) O 1s, and (e) Ce 3d in the 0.5La-CeNCFe. (f) The content of Ce3+ based on XPS data of xLa-CeNC-Fe and xLa-CeNC-Fe-Ad.

oxygen molecules during the reactions.36,58 With increasing ratios of La3+, the concentration of oxygen vacancies increased as shown in Figure 3a. High-resolution XPS spectra of C 1s for 0.5La-CeNC-Fe showed a predominant peak at 284.7 eV, which was assigned to sp2 hybrid carbon59,60 (Figure 3b). The result is in accordance with the strong carbon (200) diffraction peak in XRD (Figure 1a) and graphitic carbon layers with welldefined lattice spacing (Figure 2g). Highly sp2 hybridized carbon is extremely crucial for rapid electron transfer for fast ORR kinetics.21 The large amounts of graphitic nitrogen (GrN) and pyridinic nitrogen (Py-N) appearing at 401.2 and 398.2 eV in the high-resolution N 1s XPS spectrum (Figure 3c) are important active sites to adsorb and activate O2 because of the N doping-induced charge redistribution stemmed from the different electronegativities between C and N.61 In addition, the high content of graphitic nitrogen benefits from the large amount of the aromatic N present in the ligand of MOF-Ce/La-x.16 Other types of N also appeared, such as pyrrolic N (Py-N) and oxidized N (Ox-N) (Figures 3c and S10a). The corresponding possible C−N bond also appeared in Fourier transform infrared spectrum62 (Figure S11). The Fe signal was present in the prepared catalyst by the identification of possible satellite peak (about 718 eV) in the high-resolution Fe 2p XPS spectrum59,63 (Figure S10b) and possible Fe-N peak (399.4 eV)19,21 in the N 1s spectrum (Figure 3c). Fe content is found to be ca. 1.42 wt % (Table S3). The O 1s high-resolution spectrum was deconvoluted into three typical peaks at 529.1 (Oα), 531.7 (Oβ), and 533.16 eV (Oγ) (Figure 3d). Oα is ascribed to lattice oxygen, whereas Oβ is attributable to chemisorbed oxygen, which serves as reactive oxygen species to promote ORR.36 The Oγ located on 533.2 eV is identical with C−O, suggesting that CeO2 particles with high vacancy were stabilized by the surrounding carbon substrate.64 Red peaks in the high-resolution Ce 3d spectrum of catalysts were assigned to be Ce3+ 37−39 (Figures 3e and S12a,c). On the basis of the peak area quantification, the Ce3+ content is shown in Figure 3f with various La3+ ratios. On the

area and permeability of the catalyst, consistent with the N2 adsorption/desorption experiment results. Furthermore, the fact that pure porous CNTs were collected after pyrolysis of MOF-La also confirmed the effect of La3+ (Figures S8g,h and S9e,f). High-resolution TEM (HRTEM) images further showed the homogeneous distribution of nanoparticles (Figure 2j). Corresponding lattice space (Figure 2g,j) is identical with the (200) planes of graphitized carbon and doped CeO2, in consistent with the calculation results by Bragg’s law based on XRD data (Figure 1a). The crystal facet spacing value of 0.271 nm of the (200) plane for bulk CeO2 (PDF# 34-0394) is smaller than that of the experimental data (0.279 nm, Figure 2j), which are in accordance with the left shift of diffraction peaks of CeO2 (Figure 1a), indicating the successful incorporation of La3+.49,50 The mapping images of 0.5LaCeNC-Fe revealed that as-prepared catalyst was with homogeneous N/Fe doping, and La-doped CeO2 nanoparticles were uniformly distributed (Figure 2k−r). The exact content of La-doped CeO2 was 21.02 wt %, according to elemental analysis experiment (Tables S3 and S4). To gain more information about the surface of catalyst on atomic level, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were performed. As can be concluded from the G band (about 1580 cm−1) in the Raman spectrum,56,57 all of the catalysts held an extraordinary degree of graphitization due to the catalysis effect of Fe53−55 (Figure 3a). In addition, all catalysts except La-NC-Fe showed a peak at about 465 cm−1, which was attributed to the F2g vibration of CeO2 with fluorite structure58 (Figure 3a). Nonetheless, the peak intensity of 465 cm−1 weakened with increasing the amount of La3+, which is ascribed to the reduction of CeO2 content and decrease of symmetry. F2g bands of the 0.2 and 0.5La-CeNC-Fe shifted slightly toward lower wavenumbers probably because the presence of La3+ affected the vibrational frequency of the Ce−O bonds49 (Figure 3a). The band at about 570 cm−1 indicates the formation of oxygen vacancies (Vo••), which acted as the active site for binding and activating 22026

DOI: 10.1021/acsami.8b03742 ACS Appl. Mater. Interfaces 2018, 10, 22023−22030

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ACS Applied Materials & Interfaces basis of previous reports, the content of Ce3+ is another manifestation of the concentration of oxygen vacancies because of charge balance in the final material.37,39 And then, ceria nanoparticles with high concentrations of oxygen vacancies hold excellent oxygen storage capacity, which is extremely beneficial for enhancing the ORR rate by increasing the concentration of local O2.35,38,65 O2 adsorption/desorption measurement was performed to further probe the oxygen vacancies of the catalyst (Figure 4).

As expected, 0.5La-CeNC-Fe reached the maximum O2 adsorption capacity at 1 bar due to the synergistic effect between more oxygen vacancies and higher surface area than that of other samples. Under the same conditions, O2 adsorption capacity of CeNC-Fe is higher than that of 0.2LaCeNC-Fe, suggesting that ceria is crucial for O2 adsorption. The O2 desorption isotherms of La-doped CeNC-Fe catalysts showed significant hysteresis during the low-pressure section, indicating the strong interactions between the adsorbed oxygen and the catalysts.66,67 Although the desorption isotherm of LaNC-Fe is almost completely closed, the slight hysteresis may be attributed to Fe-Nx and the other corresponding nitrogen sites.61 These results suggest that the oxygen vacancies of catalysts could bind O2 effectively. 0.5La-CeNC-Fe was characterized with XPS after oxygen adsorption/desorption experiment (designated as xLa-CeNC-Fe-Ad) (Figure S13). The percentage of chemisorbed oxygen improved significantly, whereas the Ce3+ content decreased (Figures 3f and S13), which indicated that oxygen vacancies could bind and activate O238,65 to promote ORR. All ORR measurements were investigated with a threeelectrode system (detailed in Supporting Information). Cyclic voltammetry (CV) measurements of xLa-CeNC-Fe were performed, which implied that these catalysts are ORR active because of the appearance of well-defined cathodic reduction peaks (Figures 5a and S14). The sample of 0.5La-CeNC-Fe is superior to other catalysts, even Pt/C (0.875 V and 2.36 mA/ cm2), with the most positive peak potential (0.886 V) and the largest current density (2.72 mA/cm2). The linear sweep voltammetry (LSV) polarization curves obtained by the rotating disk electrodes of the catalysts and Pt/C were displayed in Figures 5b and S15. The catalyst of 0.5LaCeNC-Fe with a half-wave potential (E1/2) of 0.870 V exhibits

Figure 4. O2 adsorption/desorption isotherms of (a) CeNC-Fe, (b) 0.2La-CeNC-Fe, (c) 0.5La-CeNC-Fe, and (d) La-NC-Fe (P0 = 1 bar). Filled symbols, adsorption; open symbols, desorption.

Figure 5. (a) CV curves for 0.5La-CeNC-Fe and Pt/C (20 wt %) at a scan rate of 20 mV/s. (b) LSV curves for xLa-CeNC-Fe and Pt/C with the rotation rate of 1600 rpm. (c) LSV curves for 0.5La-CeNC-Fe with different rotation rates at a scan rate of 5 mV/s. (d) K−L plots for 0.5LaCeNC-Fe at different electrode potentials from 0.20 to 0.70 V. (e) The calculated electron-transfer number from K−L plots data under various potentials and (f) Tafel plots of 0.5La-CeNC-Fe and Pt/C catalysts. All of the tests were conducted in O2-saturated 0.1 M KOH solution. 22027

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with La-embedded CeO2 nanoparticles. On the one hand, the introduction of La3+ improves the surface area and porosity of the catalyst, which is conducive to the mass transfer during reaction. On the other hand, La3+ embedded into the lattice of CeO2 results in high concentration of oxygen vacancies and effectively activated the oxygen molecules. In addition, this versatile catalyst is demonstrated to be suitable for the efficient activation of other oxygen species, showcasing their potential in other nonelectrochemical reactions (Figure S21). We are also trying out the use of this catalyst for other reactions using oxygen as the oxidant, and we think oxygen vacancies stabilized by the carbon substrate played an extremely important role during the catalytic reaction, which paved the way for constructing the catalyst with a high concentration of oxygen vacancies for other practical applications.

a significantly better ORR activity than that of other catalysts and is slightly better than that of Pt/C with E1/2 of 0.862 V. Meanwhile, the ORR activity of 0.5La-CeNC-Fe is also better than that of most CeO2-based and Fe-N-C-based catalysts reported recently (Tables S5 and S6). Compared with CeNCFe and La-NC-Fe, the oxygen vacancies played an extremely important role during ORR, in consistency with the analysis above. As for the sample without and with low-content Fe, they also showed poor ORR activity, which may be attributed to the poor conductivity and absent Fe-Nx sites (Figures S15 and S16). When a high amount of Fe was used for experiments, unexpected unknown phases were produced (Figure S17). To gain further understanding of the ORR mechanism, LSV polarization curves were collected at different rotation speeds (Figure 5c) and used for Koutecký−Levich (K−L) plots, upon which the electron-transfer numbers were calculated (Figures 5d,e and S18). At all potentials, the calculated electron-transfer number of 0.5La-CeNC-Fe is about 3.94, which indicated a four-electron pathway during ORR, like Pt/C (3.96). However, there are obvious twoelectron and four-electron mixing processes for other catalysts, which indicated poor selectivity during electrochemical catalysis59 (Figure S18). As shown in Figure 5f, the Tafel slope of 0.5La-CeNC-Fe (86.8 mV/dec) is much smaller than that of Pt/C (125.7 mV/dec), indicating a faster electrontransfer rate,68 which is attributed to the high-crystalline carbon layer and oxygen vacancies in ceria.37,49 Besides the role of oxygen vacancies played in ORR, probable Fe-Nx is also the important catalysis sites for ORR. LSV curves were obtained with and without 0.01 M SCN− in O2-saturated 0.1 M KOH solution (Figure S19a). Because of SCN− poisoning,22 E1/2 of 0.5La-CeNC-Fe decreased by 53 mV, which showed the important role of Fe-Nx for promoting ORR. Compared with Pt/C (20 mV), catalyst 0.5La-CeNC-Fe exhibited excellent cycle stability with only a 7 mV loss after 1000 cycles (Figure S19b,c). Durability tests lasting 20 h (72 000 s) for 0.5La-CeNC-Fe, and commercial Pt/C catalysts further demonstrated the excellent stability of the catalyst with only 10.0% attenuation of current (Figure S19d) by using current−time (i−t) chronoamperometry technology. The crossover effect caused by methanol (MeOH) was interrogated by LSV in O2-saturated 0.1 M KOH solution with 3 M MeOH. The introduction of MeOH leads to a decrease of current density with the Pt/C system due to the accompanied MeOH oxidation, a process not observed with 0.5La-CeNC-Fe, indicating a strong tolerance to MeOH crossover of this new catalyst (Figure S19e). The rotating ring-disk electrode experiment was performed to confirm the ORR kinetics and monitor the formation of intermediate peroxide species (Figure S20). The HO2− yield (HO2− %) of 0.5La-CeNC-Fe is 2.26−5.55% over a wide potential range from −0.2 to −0.8 V (vs Ag/AgCl), which is comparable to that of the Pt/C catalyst (Figure S20b). A range of the electron-transfer number of 3.90−3.96 suggests of a four-electron pathway dominated ORR process16,18 (Figure S20c).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b03742. Electrochemistry measurements details and additional characterizations of all materials (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.B.). *E-mail: [email protected] (S.L.). ORCID

Xiangjie Bo: 0000-0002-0407-0976 Shuxia Liu: 0000-0003-0235-8594 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant nos. 21231002, 21371029, 21671033, 21571030, and 91622108) and the Open Research Fund of the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry (Jilin University, Grant no. 2015-01).

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REFERENCES

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CONCLUSIONS A series of Fe/N-doped porous carbon and La-embedded CeO2 composite catalysts derived from Ce/La dual rare-earth metal MOF were synthesized for the first time. Impressively, 0.5La-CeNC-Fe stands out with the best ORR catalytic activity arising from a synergistic effect of Fe/N-doped porous carbon 22028

DOI: 10.1021/acsami.8b03742 ACS Appl. Mater. Interfaces 2018, 10, 22023−22030

Research Article

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