Highly Efficient Fe-N-C electrocatalyst for Oxygen Reduction Derived

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Highly Efficient Fe-N-C electrocatalyst for Oxygen Reduction Derived from Core–shell-structured Fe(OH)3@Zeolitic Imidazolate Framework Jia-Wei Huang, Qing-Qing Cheng, Yi-Chen Huang, Hong-Chang Yao, Haibin Zhu, and Hui Yang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00023 • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019

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ACS Applied Energy Materials

Highly Efficient Fe-N-C electrocatalyst for Oxygen Reduction

Derived

from

Core–shell-structured

Fe(OH)3@Zeolitic Imidazolate Framework Jia-Wei Huang, a‡ Qing-Qing Cheng, b‡ Yi-Chen Huang, a Hong-Chang Yao, c Hai-Bin Zhu, a* and Hui Yang b* a

School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China.

b

Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210,

China. c

College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450002,

China.

KEYWORDS: Fe-N-C electrocatalysts, Iron oxide, Zeolitic imidazolate framework, Oxygen reduction reaction, H2-O2 proton exchange membrane fuel cell

ACS Paragon Plus Environment

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ABSTRACT: Fe-N-C electrocatalysts represent one of the most promising oxygen reduction catalysts to replace the expensive platinum (Pt)-based catalysts in fuel cells. Herein, we report a highly efficient zeolitic imidazolate framework (ZIF)-derived Fe-N-C electrocatalyst for the oxygen reduction reaction (ORR) in both alkaline and acidic solutions, which involves the formation of a core-shell-structured Fe(OH)3@ZIF-8. The encapsulated Fe(OH)3 in ZIF-8 gradually evolves into iron oxide with the increasing temperature during the carbonization, which plays several roles including creating Fe-Nx active sites, retaining morphology as a rigid template as well as tuning the carbon microstructure. The best-performing C-Fe(OH)3@ZIF-1000 catalyst features a hollow polyhedron (interior cavity: c.a 48 nm) with a thin carbon shell (c.a. 5 nm), exhibiting a high Brunner-Emmet-Teller (BET) surface area of 1021 m2 g-1. In alkaline solution, the ORR activity of C-Fe(OH)3@ZIF-1000 surpasses the benchmark Pt/C catalyst, with the onset potential (Eonset) of 0.99 V (vs. RHE) and the half-wave potential (E1/2) of 0.88 V (vs. RHE). In acidic solution, the difference in E1/2 between C-Fe(OH)3@ZIF-1000 and Pt/C is 60 mV (0.80 vs 0.86V), ranking it among the best Fe-N-C electrocatalysts in acidic media. H2-O2 proton exchange membrane fuel cell (PEMFC) with C-Fe(OH)3@ ZIF-1000 as the cathode catalyst delivers a maximum power density of 411 mW cm−2 at 0.35 V.


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1. INTRODUCTION Fuel cells and metal-air batteries are considered as promising clean energy conversion devices in numerous fields.1-5 However, the sluggish kinetics of the oxygen reduction reaction (ORR) on the cathode significantly limits the overall energy conversion efficiency of these devices,1, 6 which highly demands efficient electrocatalysts to lower the reaction barrier. Currently, precious platinum (Pt) (e.g. Pt/C) and its alloys still represent the state-of-the-art catalysts but high cost, scarce reserve and poor stability severely impede their large-scale implementation.7-9 Accordingly, recent years have witnessed soaring interest in developing cost-effective, earth-abundant and efficient nonprecious metal-based catalysts in this area. In this context, transition-metal (M) and nitrogen-co-doped carbon electrocatalysts (M-N-C), especially Fe-N-C materials with the Fe-Nx active sites, have been identified as one of the most promising candidates to replace the benchmark Pt/C catalysts, showing outstanding ORR performance with low cost, excellent methanol tolerance as well as environmental benignity.10-16 To date, a number of Fe-N-C electrocatalysts have been investigated, revealing that their morphologies and ORR activities are largely governed by the nature of the precursors used .17, 18 In this regard, metal-organic-frameworks (MOFs) with high surface area, intrinsic porosity and tunable structure, have been emerging as a unique class of precursors or self-sacrificed templates to fabricate Fe-N-C electrocatalysts.19,

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Amongst them, ZIF-8 (ZIF = Zeolitic Imidazolate

Framework), based on Zn2+ ion and 2-methylimidazole, has received the greatest attention owing to its excellent chemical/thermal stability, adjustable morphology/size, high N-content of imidazolate ligand, and easy evaporation of Zn (b.p. 907 °C).21-23 ZIF-8-derived Fe-N-C electrocatalysts generally involve combination of ZIF-8 with different Fe sources before annealing treatment, e.g. direct reactions of Fe2+ and Zn2+ in various ratios with 2-methylimidazole to form


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Fe/Zn-bimetallic ZIF), or anchoring Fe-containing species (e.g. ammonium ferric citrate) on the surface of ZIF-8.24, 25 As far as we know, the most frequently adopted Fe-doping strategy is the cage-encapsulation method which utilizes the cavity of ZIF-8 to accommodate small molecule iron-based coordination compounds (e.g. Fe(acac)3, Fe(Phen)32+, Fe(CO)5) by either ball milling or wet impregnation. 26-29 The cage-encapsulation method has proven advantageous in controlling the amount and distribution of the doped-Fe species, or inhibiting agglomeration of Fe atoms due to the confinement effect of ZIF-8 cavity.30, 31 Nevertheless, all above-mentioned Fe-containing components simply serve as Fe-sources alone to create the targeted Fe-Nx active sites, taking less specific consideration on the nanocarbon microstructure in the Fe-doping progress. Ideal Fe-N-C electrocatalysts for oxygen reduction require high density of exposed Fe-Nx active sites and optimal microstructure of carbon matrix, both equally indispensable to ORR catalysis.24, 29 Hence, it is highly desirable to achieve the two key factors simultaneously through a simple and facile Fedoping strategy. Li et al. reported on mesoporous Fe-N-C catalysts involving in-situ formed FeO(OH) nanorods, which were transformed into iron oxide (e.g, Fe2O3 and Fe3O4) during the pyrolysis, as both the Fe source and the thermally removal template, to help create the Fe-Nx active sites and abundant ORR-favorable mesopores.32 Similarly, He et.al prepared hierarchical porous carbons for supercapacitors, using nanosized Fe2O3 to tailor the porosity through reactions with carbon at higher temperatures.33 Inspired by these seminal findings, we conceive whether it is feasible to encapsulate iron oxide into the cavity of ZIF-8, simultaneously working as the Fe source and a microstructure-tuning template. Herein, we report a simple and effective approach to fabricating high-performance ZIF-8derived Fe-N-C catalysts for oxygen reduction, which can allow achieving dense Fe-Nx active sites and favorable carbon microstructure at the same time. First, FeCl3@ZIF-8 was prepared through


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a wet impregnation method. Second, treatment of FeCl3@ZIF-8 with KOH solution resulted in core-shell-structured Fe(OH)3@ZIF-8. Finally, Fe(OH)3@ZIF-8 was pyrolyzed to give the Fe-NC electrocatalysts, viz. C-Fe(OH)3@ZIF-T (T refers to the pyrolysis temperature). During the pyrolysis, the thermally labile Fe(OH)3 dehydrated to form iron oxide, serve to create Fe-Nx active sites and hollow structure. The optimal C-Fe(OH)3@ZIF-1000 shows abundant mesopores with a high Brunner-Emmet-Teller (BET) surface area of 1021 m2 g-1. In alkaline solution, the ORR activity of C-Fe(OH)3@ ZIF-1000 surpasses the benchmark Pt/C catalyst, displaying the onset potential (Eonset) of 0.99 V (vs RHE) and the half-wave potential (E1/2) of 0.88 V (vs RHE). In acidic solution, the difference in E1/2 between C-Fe(OH)3@ZIF-1000 and Pt/C is only 60 mV (0.80 vs 0.86V), ranking it among the best Fe-N-C electrocatalysts in acidic media. H2-O2 proton exchange membrane fuel cell (PEMFC) with C-Fe(OH)3@ ZIF-1000 as the cathode catalyst delivers a maximum power density of 411 mW cm−2 at 0.35 V. 2. EXPERIMENTAL SECTION Synthesis of ZIF-8 ZIF-8 nanoparticles in different sizes (20 nm, 50 nm and 100 nm) were prepared according to the literature procedure (Figure S1).34 Specifically, the 1:7 molar ratio between Zn(NO3)2·6H2O and 2-mehtylimidazole was kept constant, and the amount of methanol was adjusted with the molar ratio to Zn(NO3)2·6H2O at 420:1, 848:1 and 1800:1 to prepare 100 nm-, 50 nm-, and 20 nm-sized ZIF-8 nanocrystals, respectively. Typically, 8.4 g of Zn(NO3)2·6H2O was dissolved in 480 mL of methanol. A solution of 2-methylimidazole (16.1 g) in 480 mL methanol was added to the above solution, and vigorously stirred for 24 hrs. The precipitate collected by centrifugation was washed thoroughly with methanol twice, and then refluxed in methanol for 12 hrs. After being dried at 65


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°C overnight, the collected ZIF-8 powder was further activated at 200 °C under vacuum for 2 hrs prior to use. Synthesis of FeCl3@ZIF-8 ZIF-8 (1.832 g) was dispersed in 400 mL of methanol by sonication for 0.5 hr at room temperature. FeCl3 (129.6 mg) in 8 mL of methanol was subsequently injected into above solution under vigorously stirring for 6 hrs at room temperature. The resultant powder of FeCl3@ZIF-8 were collected by centrifugation, washed with methanol and dried at 60 °C overnight. Synthesis of Fe(OH)3@ZIF-8 FeCl3@ZIF-8 powder (1.86 g) was added into 200 mL freshly prepared aqueous KOH solution (14 mmol / L) with vigorous stirring under room temperature for 12 hrs. The resultant Fe(OH)3@ZIF-8 powder was collected by centrifugation, washed with methanol and dried in a vacuum. Synthesis of C-Fe(OH)3@ZIF-T Pyrolysis of C-Fe(OH)3@ZIF-T precursors was conducted in a tubular furnace at different temperatures (800-1100oC) under N2 atmosphere for 2hrs with a heating and cooling rate of 5 oC min-1. The pyrolyzed samples of C-Fe(OH)3@ZIF-T (T represents the pyrolysis temperature) were stirred in 100 mL of 0.5 M H2SO4 at 80 °C for 6 hrs to remove unstable species. After thorough washing with distilled water, the samples were dried in oven at 60 °C overnight. The optimal catalyst C-Fe(OH)3@ZIF-1000 was resulted at 1000oC. For comparison, C-ZIF-8-1000 and CFeCl3@ZIF-1000 were also prepared from ZIF-8 and FeCl3@ZIF-8, following the same procedure of C-Fe(OH)3@ZIF-1000 described above. The commercial 20% Pt/C (Vulcan carbon) catalyst was purchased from Johnson Matthey (UK).


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Physico-Chemical Characterization Methods Transmission Electron Microscopy (TEM), High-Resolution Transmission Electron Microscopy (HRTEM) and elemental mapping were performed on a JEOL JEM-2100F field emission electron microscope. Scanning electron microscopy (SEM) images were obtained by the Hitachi S-4800 system. Powder X-ray diffraction (PXRD) data were recorded on XRD diffractometer (Ultima IV, Rigaku Corporation) with Cu-Kα radiation (λ = 1.54056 Å). The Brunauer-Emmett-Teller (BET) surface area and pore size distribution were measured by nitrogen adsorption-desorption on a Micromeritics ASAP 2020 surface area and porosity analyzer. Raman spectra were recorded on a Thermo DXR 532 Raman spectrometer (USA) with a laser excitation wavelength of 532 nm. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Scientific Spectrometer with an Escalab 250 Xi X-ray as the excitation source, referenced to C 1s binding energy (BE) of 284.45 eV. XPS PEAK software was used to fit the spectra and calculate the quantification of C, N, O and Fe elements, and the spectra were fitted with mixed GaussianLorentzian component profiles after a Shirley background subtraction. Electrochemical Measurements Electrochemical measurements were performed on a CHI 604E workstation with a threeelectrode cell system. In 0.1 M HClO4 solution, graphite rod and Hg/HgCl2 were used as the counter and reference electrodes, respectively. The potential difference between the Hg/HgCl2 reference electrode and reversible hydrogen electrode (RHE) is 0.3 V in O2-saturated 0.1 M HClO4 solution. In 0.1 M KOH solution, graphite rod and Ag/AgCl were used as the counter and reference electrodes, respectively. The potential difference between the Ag/AgCl reference electrode and reversible hydrogen electrode (RHE) is 0.924 V in O2-saturated 0.1 M KOH electrolyte. Prior to use, the glass carbon rotating disk electrode (RDE, 3 mm in diameter) and rotating ring disk


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electrode (RRDE, SD: 0.2475 cm2) were polished with 0.3 and 0.05 μm Al2O3 slurry, and washed in ultrapure-water and ethanol. The Fe-N-C and Pt/C catalyst inks were prepared as follows: 6 mg of catalyst was dispersed ultrasonically in 1 mL of Nafion/ethanol (0.25% Nafion). The mass ratio between catalyst and Nafion is 1:0.25. In both basic and acidic electrolyte, the Fe-N-C catalyst loading was 0.6 mg cm–2, and the loading of commercial 20 wt % Pt/C was 0.2 mg cm–2. The RDE and RRDE tests were conducted in O2-saturated solution with a scan rate of 10 mV s–1 and at a rotating speed of 1600 rpm unless stated otherwise. The CV experiments were measured with stagnant electrolyte. The methanol tolerance was evaluated by comparing the CV before and after the addition of 2% CH3OH to the O2-saturated electrolyte. According to U.S. Department of Energy protocol,35 the accelerated durability tests (ADTs) were conducted by recording the LSV curves before and after the potential-cycling between 0.6 and 1.0 V (vs RHE) at a scan rate of 50 mV s–1 in O2saturated solution. The Koutecky–Levich (K–L) equations can be obtained at various rotating speeds. The electron transfer number (n) can be calculated from the K–L equation (Figure S9-11): 1 1 1 1 1 = + = + 𝐽 𝐽𝐿 𝐽𝐾 𝐵𝜔1/2 𝐽𝐾

(1)

B = 0.62n𝐹𝐶0𝐷2/3𝑉 ―1/6

(2)

𝐽𝑘 = n𝐹k𝐶0

(3)

where J is the measured current density, JK and JL are the kinetic and limiting current densities, JK is a current that is completely unaffected by mass transfer and is only controlled by kinetics, ω represents the angular velocity of the disk, n is the electron transfer number, F is the Faraday constant (96485 C mol–1), C0 is the bulk concentration of O2 (1.2 × 10–6 mol cm–3), D is the diffusion coefficient of O2 in electrolytes (1.9 × 10–5 cm2 s–1), V is the kinematic viscosity of the


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0.1 M KOH (0.011 cm2 s–1) and 0.1 M HClO4 (0.0089 cm2 s–1) and k is the rate constant for electron transfer. The H2O2 selectivity and the electron transfer number (n) can be calculated by the following equations: 𝐼𝑟 H2O2% = 200 ×

𝑁 𝐼𝑑 +

n=4×

𝐼𝑟 𝑁

𝐼𝑑 𝐼𝑑 +

(4)

𝐼𝑟

(5)

𝑁

Here, N is the current collection efficiency (0.37) for the Pt ring. Id and Ir are the disk and ring currents, respectively. PEMFC test Catalyst activity was evaluated in single fuel cell test. An ink for the catalyst was prepared by ultrasonically mixing 75 mg of C-Fe(OH)3@ZIF-1000 with 1.2 g of a 5 wt% Nafion solution and 3.5 mL of IPA (nominal amount of solid Nafion of 45 wt%) for 30 minutes. The anode ink was prepared by dispersing Pt/C with 20 wt.% Nafion in isopropanol/H2O solution (VIPA:Vwater=1:1) under ultrasonic for 3 h. The catalyst and Pt/C was deposited on the surface of 5 cm2 Ballard MB30 carbon paper by hand spray technique. The catalyst loading of CFe(OH)3@ZIF-1000 was 2 mg cm−2. The anode consisted of Pt/C with a metal loading of 0.1 mg cm-2. The MEA was made by sandwiching the Nafion membrane between the cathode and anode by hot-pressing at 135 °C and 2000lbs for 3 min. The test conditions were chosen to be Tcell = 80 °C and the back-pressure was fixed at 30 PSIG. The performance of PEMFCs were measured by polarization test on Arbin Fuel Cell Testing System (Arbin Instrument Inc., USA) and the polarization data was recorded per 2 min. The as-prepared MEAs were activated and tested in a


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PEMFC testing setup by purging H2 into the anode with the flow rate of 400 sccm and O2 or air into the cathode with flow rate of 200 and 1000 sccm. 3. RESULTS AND DISCUSSION

Figure 1. Schematic showing of evolution from ZIF-8 to C-Fe(OH)3@ZIF-T. The stepwise evolution from ZIF-8 to C-Fe(OH)3@ZIF-T is illustrated in Figure 1. Initially, FeCl3 was encapsulated into the inner cavity of ZIF-8 through a wet impregnation method, giving rise to FeCl3@ZIF-8. Treatment of FeCl3@ZIF-8 with KOH solution resulted in core-shellstructured Fe(OH)3@ZIF-8, wherein the transformation into water-insoluble Fe(OH)3 from the soluble FeCl3 enables the Fe3+ ions tightly sealed into the ZIF-8 cavity. Direct carbonization of Fe(OH)3@ZIF-8 at different temperatures followed by acid leaching afforded Fe-N-C electrocatalysts, namely. C-Fe(OH)3@ZIF-T (T represents the pyrolysis temperature). In the annealing process, the thermally labile Fe(OH)3 gradually dehydrated and formed iron oxide with the rising temperature (Figure S2). The in-situ formed iron oxide can serve multiple purposes. First, it acts as the source of Fe element to create the desired Fe-Nx active sites. Second, it works


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as a rigid template to prevent the rapid collapse of ZIF-8 framework at high temperatures (e.g. 1000 oC), maintaining the original ZIF-8 polyhedron morphology. Finally, its thermally removal nature can help adjust the carbon microstructure through complicated reactions with carbon at elevated temperatures.32, 33

Figure 2. TEM images of ZIF-8(a), FeCl3@ZIF-8(b), Fe(OH)3@ZIF-8(c), and C-Fe(OH)3@ZIF1000 (d); (e-g) HRTEM images of C-Fe(OH)3@ZIF-1000; (h) HADDF-STEM image and the corresponding element mappings for C, N, Fe atoms of C-Fe(OH)3@ZIF-1000. TEM images illustrate that both FeCl3@ZIF-8 and Fe(OH)3@ZIF-8 show the same polyhedron morphology as ZIF-8 with the particle size of about 50 nm (Figure 2a-c), indicating that the initial structure of ZIF-8 remains intact with its cage encapsulating FeCl3 and Fe(OH)3. Well-matched PXRD patterns of ZIF-8, FeCl3@ZIF-8 and Fe(OH)3@ZIF-8 further confirm that the structural integrity of ZIF-8 is well preserved in the cage-encapsulation progress (Figure S3). However, a striking difference in morphology was observed among C-ZIF-8-1000, C-FeCl3@ZIF1000 and C-Fe(OH)3@ZIF-1000, which were in turn obtained by pyrolysis of ZIF-8, FeCl3@ZIF-8 and Fe(OH)3@ZIF-8 at 1000oC. It is found that both C-ZIF-8-1000 and C-FeCl3@ZIF-1000 totally lose the polyhedron morphology and comprises nanoparticles with irregular shape and sizes


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(Figure S4). By sharp contrast, C-Fe(OH)3@ZIF-1000 roughly preserves the original polyhedron shape with a slight deformation, which might be ascribed to the supporting effect of the in-situ generated iron oxide template (Figure 2d). Particularly, TEM images of polyhedral nanoparticles of C-Fe(OH)3@ZIF-1000 feature light interiors and dark surrounding edges, indicative of a hollow structure which is supposed to be etched by thermally removal iron oxides at high temperatures (Eqs S1–4).32 High-resolution of TEM image of the surrounding edge further reveals that it consists of amorphous carbon without any metallic crystallites (Figure 2e-2g). HAADF-STEM and energy-dispersive X-ray spectroscopy (EDS) techniques manifest that C, N and Fe elements in C-Fe(OH)3@ZIF-1000 (Figure 2h) are homogeneously distributed within the entire carbon matrix, and the Fe and N signals according to each other infer that Fe atom might coordinate to N atom to generate highly active Fe-Nx catalytic sites.16 In the case of C-Fe(OH)3@ZIF-1000, the thin carbon shell (c.a 5 nm) with a short diffusion length coupled with hollow interior (c.a 48 nm) not only enables active sites more easily accessible to oxygen-relevant intermediates (e.g. OH-, OOH-) but also significantly reduce the mass transfer resistance.8


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Figure 3. (a) PXRD patterns of C-ZIF-8-1000, C-FeCl3@ZIF-1000 and C-Fe(OH)3@ZIF-1000; (b) Raman spectra of C-ZIF-8-1000, C-FeCl3@ZIF-1000 and C-Fe(OH)3@ZIF-1000; (c) N2 adsorption and desorption isotherms of C-ZIF-8-1000, C-FeCl3@ZIF-1000 and C-Fe(OH)3@ZIF1000; (d) Pore-size distributions of C-ZIF-8-1000, C-FeCl3@ZIF-1000 and C-Fe(OH)3@ZIF1000. PXRD patterns of C-ZIF-8-1000, C-FeCl3@ZIF-1000 and C-Fe(OH)3@ZIF-1000 are depicted in Figure 3a. Similar to C-ZIF-8-1000, C-Fe(OH)3@ZIF-1000 only exhibits two characteristic peaks at 2θ ≈ 26.0° and 44.0° corresponding to the (002) and (101) crystalline plane diffractions of graphite carbon, and no diffractions from metallic crystallites are observed, which


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agree well with its HRTEM images. Differently, three extra diffraction peaks at 2θ = 43.6°, 44.6° and 50.98°, assigned to metallic iron and Fe3C nanoparticles, 24, 36-39 are present in C-FeCl3@ZIF1000, suggesting that aggregation of Fe atoms occurs during the carbonization. Comparison between C-Fe(OH)3@ZIF-1000 and C-FeCl3@ZIF-1000 clearly points out that in-situ formed iron oxide can effectively inhibit the migration and agglomeration of Fe atoms especially under the circumstance of identical Fe content (vide infra: XPS data). The remarkable inhibition effect with iron oxide might lie in its strong Fe-O bonding interactions that prevents from the rapid breakage of Fe-O bonds to generate mobile active Fe species. Raman spectra of C-ZIF-8-1000, CFeCl3@ZIF-1000 and C-Fe(OH)3@ZIF-1000 display a typical D band at 1340 cm-1 (corresponding to the disordered carbon) and G band at 1575 cm-1 (corresponding to graphitic carbon) with a slight enhancement in the intensity ratio of D band to G band (ID/IG) (Figure 3b).The increasing ID/IG of C-Fe(OH)3@ZIF-1000 suggests more structure defects within the carbon matrix, which would contribute to more ORR active sites. The nitrogen adsorption– desorption isotherms of C-ZIF-8-1000, C-FeCl3@ZIF-1000 and C-Fe(OH)3@ZIF-1000 all show a pronounced hysteresis loop but with the obvious variations, suggesting the significant difference in the channel distribution within these materials.40 As shown in Figure 3c, C-Fe(OH)3@ZIF-1000 exhibits a H1-type hysteresis loop, indicative of the presence of dominant uniformly-distributed mesopores agreeing well with its morphology of hollow polyhedrons with the inner cavity of ca. 50 nm. By contrast, C-ZIF-8-1000 shows a H3 hysteresis loop matching well to its sheet granular morphology, and C-FeCl3@ZIF-1000 owns a H4 hysteresis loop characteristic of its layered structure. The pore size distributions (PSD) of three materials have been further evaluated (Figure 3d). Compared to C-ZIF-8-1000 and C-FeCl3@ZIF-1000, C-Fe(OH)3@ZIF-1000 possesses abundant mesopores centered at about 50 nm as well as large quantity of micropores (in the range


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of 1-2 nm and < 1 nm). Moreover, C-Fe(OH)3@ZIF-1000 possesses a Brunauer-Emmett-Teller (BET) surface area of 1021 m2 g−1, much higher than C-ZIF-8-1000 (767 m2 g−1) and CFeCl3@ZIF-1000 (571 m2 g−1). Dominant mesopores with C-Fe(OH)3@ZIF-1000 would help facilitate mass transport,41 whereas abundant micropores and large surface area are also considered to be beneficial for ORR, e.g. accommodating and exposing more active sites to boost the ORR performance.42, 43

Figure 4 (a) XPS survey spectra of C-FeCl3@ZIF-1000 and C-Fe(OH)3@ZIF-1000; (b) N 1s XPS spectra of C-FeCl3@ZIF-1000; (c) N 1s XPS spectra of C-Fe(OH)3@ZIF-1000; (d) N species percentage of C-FeCl3@ZIF-1000 and C-Fe(OH)3@ZIF-1000 determined by XPS measurements.


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XPS survey spectra indicate the presence of C, N, O and Fe elements on the surfaces of CFeCl3@ZIF-1000 and C-Fe(OH)3@ZIF-1000 (Figure 4a), further confirming that Fe atoms have been successfully doped into the carbon matrix. In comparison with C-FeCl3@ZIF-1000, CFe(OH)3@ZIF-1000 exhibits a higher fraction of N atom (5.78 at% vs. 4.5 at%) and the same Fe content (0.68 at%). As the same as C-FeCl3@ZIF-1000, the high-resolution N1s XPS spectra of C-Fe(OH)3@ZIF-1000 contains four distinct types of N atoms, viz. pyridinic-N (398.6 eV), Fe-Nx (399.6 eV), pyrrolic-N (400.6 eV), graphitic-N (401.5 eV) and oxidized-N (402.7 eV) (Figure 4b and 4c). 14, 25, 44 However, C-Fe(OH)3@ZIF-1000 contains a higher concentration of Fe-Nx species and graphitic N atom with a less amount of oxidized-N and pyrrolic-N atoms (Figure 4d and Table S2). The predominance of Fe-Nx and graphitic-N atoms in C-Fe(OH)3@ZIF-1000 would further boost its ORR performance, wherein Fe-Nx species are considered as highly active sites for ORR especially in acidic media, 45,46 and the catalytically active graphitic-N atoms favor a fourelectron ORR process.17,47,48


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Figure 5 (a) CV curves of C-Fe(OH)3@ZIF-1000 in N2- and O2-saturated HClO4; (b) CV curves of C-Fe(OH)3@ZIF-1000 in N2- and O2-saturated 0.1 M KOH; (c) ORR polarization curves of CZIF-8-1000, C-FeCl3@ZIF-1000, C-Fe(OH)3@ZIF-1000 and Pt/C at a sweep rate of 10 mV s−1 under rotating speeds of 1600 rpm in O2-saturated 0.1M HClO4 electrolyte; (d) Percentage of peroxide and electron transfer number of C-Fe(OH)3@ZIF-1000 in O2-saturated 0.1M HClO4


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electrolyte; (e) ORR polarization curves of C-ZIF-8-1000, C-FeCl3@ZIF-1000, C-Fe(OH)3@ZIF1000 and Pt/C at a sweep rate of 10 mV s−1 under rotating speeds of 1600 rpm in O2-saturated 0.1 M KOH electrolyte; (f) Effect of SCN- on ORR activity for C-Fe(OH)3@ZIF-1000 in O2-saturated 0.1M HClO4 electrolyte. The ORR activity of C-Fe(OH)3@ZIF-1000 catalyst was initially evaluated in acidic and alkaline electrolytes by the cyclic voltammetry (CV) technique (Figure 5a-b). In a N2-saturated electrolyte, featureless voltammetric curves with no obvious redox peaks were observed. In sharp contrast, a well-defined oxygen reduction peak appeared at 0.73 V (vs. RHE) in 0.1 M O2-saturated HClO4 solution, and 0.82 V (vs. RHE) in 0.1 M O2-saturated KOH solution, demonstrating that C-Fe(OH)3@ZIF-1000 has a prominent ORR catalytic activity. For comparison, the ORR activities of C-ZIF-8-1000, C-FeCl3@ZIF-1000, C-Fe(OH)3@ZIF1000 together with the reference 20% Pt/C were examined using LSV measurement under the same condition. In O2-saturated 0.1 M HClO4 solution, C-Fe(OH)3@ZIF-1000 displays the onset potential (Eonset) of 0.91V (vs. RHE ) and the half-wave potential (E1/2) of 0.80V (vs. RHE), which are superior to C-ZIF-8-1000 (Eonset = 0.74 V, E1/2 = 0.49 V) and C-FeCl3@ZIF-1000 (Eonset = 0.88 V, E1/2 =0.73 V). It is worth noting that the ORR activity of C-Fe(OH)3@ZIF-1000 is even comparable to the reference 20% Pt/C catalyst (Eonset = 0.99V, E1/2 = 0.86V), ranking it among the best Fe-N-C electrocatalysts in acidic media. (Figure 5c, Table S4). The ORR selectivity over CFe(OH)3@ZIF-1000 was confirmed by RRDE (rotating ring disk electrode) measurement, which follows a near-4 electron pathway (n = 3.7-3.95) in the potential window from 0.2 to 0.8 V (vs. RHE). (Figure 5d). In O2-saturated 0.1 M KOH solution, C-Fe(OH)3@ZIF-1000 (Eonset = 0.99 V, E1/2 = 0.88V) outperforms the commercial 20% Pt/C catalyst (Eonset = 1.02V, E1/2 = 0.85V) (Figure 5e). Moreover, RRDE tests with C-Fe(OH)3@ZIF-1000 in the alkaline electrolyte reveal a high


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electron transfer number (n > 3.9) together with a low H2O2 yield (< 5.0%), both of which are close to those of Pt/C (Figure S5). In contrast to Fe-free C-ZIF-8-1000, the Fe-doped C-Fe(OH)3@ZIF-1000 led to a remarkable enhancement in ORR performance. In order to uncover the significance of Fe species, KSCN was added as an ORR inhibitor to block Fe-Nx active sites during the ORR test.49 Compared to the initial C-Fe(OH)3@ZIF-1000, a striking negative shift of 50 mV in E1/2 potential was observed upon addition of KSCN (Figure 5f), strongly supporting the decisive catalytic role of Fe-Nx sites toward ORR. Besides, the size effect of ZIF-8 (20 nm, 50 nm and 100 nm) on the ORR performance of C-Fe(OH)3@ZIF-1000 was also investigated (Figure S6). Compared to 100 nmsized ZIF-8, a gradually positive shift in E1/2 is observed in 50 nm-sized ZIF-8, indicating an increase in the number of active sites. However, further reducing the particle size to 20 nm significantly deteriorates the ORR performance, which is likely caused by a sudden drop in the number of exposed active sites due to drastic agglomeration of particles with the reducing size. Taken all together, the outstanding ORR catalytic activity of C-Fe(OH)3@ZIF-1000 might be interpreted by the following factors. First, C-Fe(OH)3@ZIF-1000 has an ideal microstructure favorable for heterogeneous tri-phase ORR process including hollow structure of high surface area with thin carbon shell, and abundant mesopores and micropores. Second, C-Fe(OH)3@ZIF-1000 comprises a large number of exposed Fe-Nx active sites homogenously dispersed within the carbon matrix. In short, integration of highly active Fe-Nx sites into optimal microstructure endows CFe(OH)3@ZIF-1000 with impressive ORR activity in both acidic and alkaline electrolytes. Meanwhile, it also demonstrates that high density of active sites and favorable microstructure can be realized simultaneously by adopting appropriate Fe-doping strategy.


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Figure 6 (a) Methanol tolerance test with 2% methanol (in volume) in O2-saturated 0.1 M HClO4 solution for the C-Fe(OH)3@ZIF-1000 (the CV scan rate is 50 mV s −1); (b) The polarization curves of ORR on C-Fe(OH)3@ZIF-1000 before and after a 10000-cycle ADT in O2-saturated 0.1M HClO4 electrolyte at a rotating rate of 1600 rpm with the sweep rate of 10 mV s-1; (c) The polarization curves of ORR on C-Fe(OH)3@ZIF-1000 before and after a 10000-cycle ADT in O2-


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saturated 0.1M KOH electrolyte at a rotating rate of 1600 rpm with the sweep rate of 10 mV s-1; (d) H2/O2 fuel cell polarization plots. Cathode: 2 mg cm−2 of C-Fe(OH)3@ZIF-1000; Anode: 0.1 mg cm−2 Pt/C; H2/O2: 400 sccm / 200 sccm; Back pressure: 30 PSIG. (e) Steady-state polarization curves H2-air fuel cells prepared with C-Fe(OH)3@ZIF-1000 as cathode with a loading of 2 mg cm−2 and 0.1 mg cm−2 of Pt/C as anode; (f) 12 h stability test under H2-air condition at a constant voltage of 0.4 V. C-Fe(OH)3@ZIF-1000 also demonstrates a good methanol tolerance and long-term stability. As shown in Figure 6a, the CV curve of C-Fe(OH)3@ZIF-1000 in acidic electrolyte displays almost no change upon addition of methanol. On the contrary, the reference Pt/C catalyst shows prominent methanol oxidation peaks (Figure S7). The good durability of C-Fe(OH)3@ZIF-1000 was authenticated by accelerated duration test (ADT) (Figure 6b-c). In acidic medium, CFe(OH)3@ZIF-1000 shows a minor loss of 26 mV in E1/2 after 10 000 cycles, whereas a 50 mV negative shift in E1/2 was resulted with Pt/C even after 5000 cycles (Figure S8a). In basic electrolyte, after a 10 000 cycles a negligible negative shift (c.a. 5 mV) in E1/2 is resulted with CFe(OH)3@ZIF-1000, which is superior to Pt/C with a ∼20 mV drop even after a 3000 cycles (Figure S8b). Given that the excellent ORR performance of C-Fe(OH)3@ZIF-1000 in acidic media, the practical application performance of C-Fe(OH)3@ZIF-1000 was further evaluated as the cathode catalyst in PEMFC. In H2-O2 PEMFC test, the PEMFC polarization curve and power density plot at 80 °C show that the maximum power density reaches 411 mW cm−2 at the cell voltage of 0.35 V and the current density of 1.19 A cm−2 (Figure 6d). By contrast, H2-air fuel cell delivers a similar OCV (open circuit voltage, 0.88 vs 0.91V), but a declined peak power density (193 mW cm−2) was observed possibly due to the restriction of mass-transport under high current regions (Figure 6e). Furthermore, the stability measurement of the H2-air cell performed at a


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constant voltage of 0.4 V indicates that the current density decays by approximately 12.5% in 12 hours (Figure 6f). The degradation in the durability test might be attributed to multiple factors. First, our catalyst produces more hydrogen peroxide than Pt/C during oxygen reduction in an acidic medium. Exposure to H2O2 leaves iron-based catalytic sites untouched but would oxidize the carbon surface, weakening the O2-binding on iron-based sites with a decrease in their turnover frequency (TOF).50-52 Second, demetalation of Fe-Nx active sites situated in the micropores, according to the Le Chatelier principle, would occur in the flux of water running into the micropores.53 Third, demetalation and carbon oxidation can also strengthen each other. On the one hand, it is known that iron ions (Fe2+/Fe3+) from demetalation catalyze the formation of radicals from H2O2 via the Fenton mechanism, which act as potential carbon oxidizers. On the other hand, chemical and/or electrochemical oxidation of carbon may accelerate demetalation, either by affecting the Fe-Nx sites or by destroying the local carbon structure.54,55 4. CONCLUSION In summary, we have successfully synthesized high-performance and robust Fe-N-C carbon electrocatalysts for oxygen reduction from a core-shell-structured Fe(OH)3@ ZIF-8, wherein the in-situ formed iron oxide from Fe(OH)3 can achieve the Fe-doping and microstructure-tuning simultaneously. In the process of catalyst preparation, in-situ formed iron oxide not only serves as the Fe-source to create of Fe-Nx active sites but also as a rigid and thermally removal template to generate hollow polyhedral morphology characteristic of hollow interior (c.a. 48 nm) and thin carbon shell (c.a. 5 nm), which both help boost the ORR performance. Our current work provides a new strategy to explore highly efficient ZIF-derived Fe-N-C electrocatalysts for oxygen reduction.


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ASSOCIATED CONTENT Supporting Information Complementary SEM images, PXRD, TEM images, K-L plots, Methanol tolerance test, H2/air fuel cell polarization plots and other related data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. (match statement to author names with a symbol) Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT This work was financially supported by the National Key Research Program (2017YFA0206500), the National Natural Science Foundation of China (21533005, 216673275, 21573107, and 21401099), and the Fundamental Research Funds for the Central Universities (No. 3207047406). REFERENCES 1. Debe, M. K. Electrocatalyst Approaches and Challenges for Automotive Fuel Cells. Nature 2012, 486, 43-51.


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Graphic for manuscript

Highly Efficient Fe-N-C electrocatalyst for Oxygen Reduction Derived from Core-shell-structured Fe(OH)3@Zeolitic Imidazolate Framework Jia-Wei Huang a‡, Qing-Qing Cheng b‡, Yi-Chen Huang a, Hong-Chang Yao c, Hai-Bin Zhu a*, Hui Yang b*

A highly efficient Fe-N-C electrocatalyst derived from core-shell-structured Fe(OH)3@ZIF-8 shows a superior oxygen reduction performance in both acidic and alkaline media.


Highly Efficient Fe-N-C electrocatalyst for Oxygen Reduction Derived

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