Fe, N-decorated three dimension porous carbonaceous matrix for highly efficient oxygen reduction reaction

Journal Pre-proofs Full Length Article Fe, N-Decorated Three Dimension Porous Carbonaceous Matrix for Highly Efficient Oxygen Reduction Reaction Xin Chen, Kaicai Fan, Lingbo Zong, Yaowen Zhang, Di Feng, Mengyun Hou, Qi Zhang, Dehua Zheng, Yanan Chen, Lei Wang PII: DOI: Reference:

S0169-4332(19)33450-6 https://doi.org/10.1016/j.apsusc.2019.144634 APSUSC 144634

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

21 August 2019 31 October 2019 7 November 2019

Please cite this article as: X. Chen, K. Fan, L. Zong, Y. Zhang, D. Feng, M. Hou, Q. Zhang, D. Zheng, Y. Chen, L. Wang, Fe, N-Decorated Three Dimension Porous Carbonaceous Matrix for Highly Efficient Oxygen Reduction Reaction, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144634

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Fe, N-Decorated Three Dimension Porous Carbonaceous Matrix for Highly Efficient Oxygen Reduction Reaction Xin Chena, Kaicai Fanb, Lingbo Zong*a, Yaowen Zhanga, Di Fenga, Mengyun Houa, Qi Zhanga, Dehua Zhenga, Yanan Chenc, Lei Wang*a a Key Laboratory of Eco-chemical Engineering, Taishan Scholar Advantage and Characteristic Discipline Team of Eco Chemical Process and Technology, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China. b Centre for Clean Environment and Energy, School of Environment and Science, Griffith University, Queensland 4222, Australia c School of Materials Science and Engineering, Key Laboratory of Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin Key Laboratory of Composite and Functional Materials, Tianjin University, Tianjin 300072, China

E-mail:[email protected] E-mail: [email protected] Xin Chen and Kaicai Fan contributed equally to this work. Keyword: Porous carbon matrix, Nonpreciousmetal electrocatalyst, Oxygen reduction reaction, High activity

Abstract: Rational engineering of simple, economical and durable nonprecious metal electrocatalyst with excellent oxygen reduction reaction (ORR) activity is crucial for the electrochemical energy conversion devices, such as metal-air batteries and fuel cells. Here, we developed a highly efficient ORR electrocatalyst with rich and well-dispersed Fe-Nx active sites and iron nanocrystals decorated three dimension (3D) porous carbonaceous matrix by a rational designed carbonaceous spheres templated 1

strategy. The fabricated electrocatalyst exhibits outstanding ORR activity with onset potential of 1.0 V and half-wave potential of 0.90 V versus reversible hydrogen electrode in alkaline media, which is much better than benchmark Pt/C. In addition, it presents particularly better performance in terms of superior stability and excellent methanol tolerance capacity. Experimental studies display that the remarkable ORR activity primarily originates from the synergetic effect of rich Fe-Nx active sites, sufficient

carbon

encapsulated

metallic

iron

nanocrystals

and

desired

meso/microporous structure. What worth to point out is that the facile synthetic technique can be extended to fabricate other low cost highly active transition metal-N-carbon scaffolds ORR electrocatalysts.

1 Introduction： Environmental pollutions and foreseeable energy shortage have become the urgent issues. Developing clean and renewable energy technologies are allimportant to alleviate the confronting problems. The electrochemical oxygen reduction reaction (ORR) is a critical and limiting process step for the next-generation energy conversion and storage devices, such as fuel cells and metal-air batteries[1, 2]. However, the intrinsically sluggish kinetics process of ORR induced high cathodic overpotentials and required superior electrocatalysts to accelerate the reaction rate[3]. Currently, the most efficient ORR electrocatalysts are Pt-based materials. Unfortunately, the scarce abundance and high-cost have impeded the large-scale commercialization applications[4]. Meanwhile, Pt-based materials also suffer from insufficient durability and poor poisoning tolerance[5]. Thus, it is highly desirable to develop cost-effective and stable alternative ORR electrocatalysts with outstanding performance. Up to now, considerable research efforts have been devoted to discovering low-cost and high activity ORR electrocatalysts[6, 7], including earth-abundant metal alloys[8], metal oxides[9], transition metal compounds[10, 11], metal-nitrogen-doped carbons materials (TM-N-C)[12, 13] and metal-free heteroatom-doped carbon materials[14, 15]. In particular, Fe-N-C has been deemed to be appealing alternatives 2

for precious metal electrocatalysts because of earth abundance and high activities toward ORR[16]. What worth to point out is that the high activities are derived from the stubborn interactions between N and Fe atoms on carbon substrates and modified electronic structure, which can boost ORR performance in terms of the theoretical predictions[17]. Lately, Fe nanocrystals have also been demonstrated to promote ORR activity of electrocatalysts by dramatically increasing the activity of Fe-Nx active sites[18, 19]. In addition to the intrinsic feature of active sites, which is determined by the chemical compositions and interactions, ORR activity is intensively manifested to be depended on active sites exposure and mass diffusion of ORR interrelated species[20]. Therefore, valid engineering with optimized microarchitectures not only improves the interaction between active sites and supports, but also benefits the increase of defects, exposure of accessible reactive sites and migration of ORR-relevant species in the reaction process. In these regards, the coexistence of moderate metallic iron nanocrystals, high content of active sites and well-defined microarchitectures are essential for the superior ORR activity. Among many kinds of carbonaceous nanostructure’ scaffolds, carbonaceous frameworks generally possess economic viability, high conductivity and unique structural features. Specifically, porous structures play an important role in providing abundant exposure of active sites and facilitating mass diffusion during electrocatalytic process[21, 22]. More importantly, these mesoporous carbonaceous frameworks could also prominently enhance the preservation of the exposed active sites and improve resistance to corrosion during electrochemical reaction, remarkably increasing the durability of electrocatalysts[23, 24]. It is therefore sensible to postulate that the monodispersed Fe-containing moieties and Fe-Nx active sites anchored on N-decorated mesoporous carbonaceous matrix with uniform spherical morphology could be a promising alternative as a robust ORR electrocatalysts. Herein, we develope an efficient carbonaceous spheres templates and vapor deposition-assistant approach to prepare Fe, N-decorated mesoporous carbonaceous 3

spheres with well-dispersed carbon encapsulated Fe nanocrystals, abundant Fe-Nx active sites, high content of active pyridinic N and improved porous structures (named Fe@Fe-N-CSs). In addition, within the carbonaceous spheres templates, the captured target ions, such as Fe3+ will promote the oxidative polymerization of pyrrole leading to N-doped mesoporous carbonaceous spheres with uniform Fe nanocrystals[25]. The as-fabricated samples with large surface area, well-defined meso/macroporous characteristic, monodispersed Fe nanocrystals and high content Fe-Nx active sites show remarkable ORR activity in alkaline medium with outstanding stability and excellent methanol tolerance, and surpass commercial Pt/C. We reveal that pyrolysis treatment not only creates Fe-Nx active sites and graphitic layer structure encapsulated Fe nanocrystals, but also promotes the formation of porosity structures. Electrocatalytic studies demonstrate that Fe nanocrystals play important role in enhancing electrocatalytic activity of Fe-Nx active sites for ORR. The superb ORR performance may attribute to synergistic effects between high content of ORR active Fe-Nx sites, abundant Fe nanocrystals, high active pyridine N and porous structure with sufficient contact of electrolyte. This research may provide novel approach in efficient design and fabrication of highly active and stable carbon based nonprecious metal electrocatalysts.

2. Experimental Section 2.1 Reagents The FeCl3·6H2O (purity ≥ 99.0%), C6H12O6·H2O and Pyrrole were purchased from the Sinopharm Chemical Reagent Co., Ltd and Energy Chemical, Sigma-Aldrich. All chemicals were analytical-grade reagents and used without further purification. 2.2 Synthesis of uniform Fe, N- decorated carbonaceous spheres (Fe@Fe-N-CSs) Synthesis of carbonaceous spheres templates Carbonaceous spheres templates were fabricated using hydrothermal technique. Typically, sucrose solution (89 g sucrose, 250 Ml deionized water) was hydrothermally treated at 200 ºC for 2 h. Then, the products were washed with 4

deionized water and ethanol. The SEM image of the obtained brown carbonaceous spheres templates were shown in Fig. S1. Adsorption of Fe3+ and vapor deposition 4.0545 g FeCl3·6H2O was dissolved in 75 Ml solution (56.25 Ml ethanol, 18.75 Ml deionized water). Then, the as-prepared carbonaceous spheres templates (1.5 g) were added into the above solution with the assistance of ultrasonication for 10 min. Subsequently, the mixture was adequately stirred for 6 h at room temperature, and then the product named Fe/C was collected and dried at 80 °C for 12 h. The obtained Fe/C sample was exposed to the pyrrole vapors for 24 h in a 100 Ml Teflon-lined stainless-steel autoclave. The dark solid obtained by the vapor deposition method was heated under N2 to 900 °C (5 °C min−1) for 3 h. The mesoporous carbonaceous spheres with N and Fe have been designated as Fe@Fe-N-CSs. 2.3 Synthesis of uniform N-decorated carbonaceous spheres (C-N) The C-N electrocatalyst was prepared according to the procedure for Fe@Fe-N-CSs but without FeCl3·6H2O. 2.4 Synthesis of Fe-decorated carbonaceous spheres (Fe-C) The Fe-C electrocatalyst was prepared according to the procedure for Fe@Fe-N-CSs but without carbonaceous spheres templates. 2.5 Physical characterization Powder X-ray diffraction of samples were examined using Rigaku-Dmax 2500 diffractometer with Cu-

=0.15405 nm). The morphologies and

structure information of the as-prepared samples were inspected on the field emission scanning electron microscope (FE-SEM, S4800, Hitachi), transmission electron microscopy (TEM, JEM-2100F). The chemical composition was analyzed by X-ray photoelectron spectroscopy (Thermo Scientific K-Alpha) and the C1s peak at 284.8 eV as an internal standard. Nitrogen sorption isotherms were measured at 77 K with a

quantachrome autosorb iQ-c gas sorption analyzer (Quantachrome Instruments). Before measurements, samples were degassed under the vacuum conditions at 300 °C 5

for 3 h. The Brunauer-Emmett−Teller (BET) approach was adopted to calculate the specific surface area. Raman spectra were recorded with the confocal Raman microscope (LabRAM HR800, Laser Wavelength 532 nm). 2.6 Electrochemical characterizations The electrochemical measurements were carried out in a standard three-electrode system at room temperature using Gamry reference 600 electrochemical workstation (Gamry, USA). A rotating glassy carbon electrode with an area of 0.196 cm2, Ag/AgCl (Saturated KCl) and platinum sheet were used as working electrode, reference electrode, and counter electrode, respectively. All potentials in the ORR were calibrated to the reversible hydrogen electrode (RHE), using the E(RHE) = E(Ag/AgCl) + 0.059·pH+0.1976. The catalyst ink was prepared by ultrasonically dispersing 5 mg catalyst in 1 Ml solution (0.98 Ml of isopropyl alcohol and 0.02 Ml of 5 wt % Nafion solution). Next, 20 μl of the ink was coated onto the surface of glassy carbon electrode (dried in air). For comparison, the catalyst loading of Pt/C (20%) is 0.153 mg cm−2. All electrochemical measurements were conducted at room temperature in 0.1 M KOH or 0.5 M H2SO4 electrolyte, which was bubbled with O2 or N2 for 30 min before measurement and maintained at the same atmosphere during the measurements. Before the measurement, the glassy carbon working electrode was cycled for several circles until the curve was stable to get rid of the possible contaminant on the surface of catalyst. The LSV curves were conducted from 1.2 to 0.2 V vs RHE at different rotating speed with the scan rate of 5 mV s-1. The obtained LSV curves were corrected with 95% iR-correction. The transfer number of the ORR was calculated by the Koutecky-Levich equations:

1 1 1 1 1 = + = 1 + j jL jK B 2 jK B = 0.62nFC0 ( D0 ) 2 3 1 6 6

jK = nFkC0 In this equation, jK, jL and j represented the kinetic current density, diffusion limited cathodic current density and measured current density, respectively. ω was the angular velocity of the disk, n was the overall electron transfer number during the ORR, F was the Faraday constant (F=96485 C mol-1), C0 was the bulk concentration of oxygen in 0.1 M KOH (1.2*10-3 mol L-1), ν was the kinematic viscosity of the 0.1 mol L-1 KOH electrolyte (0.01 m2 s−1), D0 was the diffusion coefficient of O2 in the electrolyte (1.9*10-5 cm2 s-1) and k was the electron transfer rate constant.

3 Results and discussion:

Scheme 1. Schematic diagram of the synthesis procedure

The Fe@Fe-N-CSs electrocatalyst was prepared by a facile two-step approach, followed by pyrolysis technique (Scheme 1). Specifically, Fe3+ target ions were firstly captured by the negative charged carbonaceous spheres hard templates. Then, these Fe3+ occupied carbonaceous spheres were impregnated with pyrrole vapors, which can be rapidly converted into the polypyrrole[26]. The Fe3+ and pyrrole infiltrated composite spheres were further carbonized into the porous Fe, N decorated 3D porous carbon framework and the residual Fe3+ were reduced into the metal Fe nanocrystals, simultaneously. Finally, Fe@Fe-N-CSs electrocatalyst was afforded.

7

Fig. 1 (a, b) SEM and (c, d) TEM images of Fe@Fe-N-CSs; (e) HRTEM image of Fe nanocrystals in the Fe@Fe-N-CSs; (f) HAADF-STEM image and (g, h, i, j) elemental mapping of C, Fe, O, N; (k) XRD pattern of Fe@Fe-N-CSs; (l) N2 adsorption/desorption isotherm curve of Fe@Fe-N-CSs and the corresponding pore size distribution (inset).

The metallographic characteristics of the fabricated Fe@Fe-C-NSs samples were characterized by scanning electron microscopy (SEM). The SEM images (Fig. 1.a,b) revealed well-defined spherical structure, which perfectly preserved the primal morphology of the hard templates. Moreover, these samples were highly coarse, with rich cavities after pyrolysis in N2 atmosphere. The microstructures of electrocatalyst were further investigated by the transmission electron microscope (TEM) images (Fig. 1c,d). The corresponding TEM images indicated that the small Fe nanocrystals were uniformly decorated in spherical carbonaceous matrix. The enlargement of Fig. 1d and HRTEM image of Fig. 1e further testified that the embedded nanocrystals with 8

the d-spacing of (110) crystallographic planes were entirely encased by amorphous graphitic layer. These particular confinement structures not only promoted the graphitic layer to boost the electrocatalytic activity, but also suppressed the dissolution or erosion in acidic medium and random aggregation of nanocrystals during electrochemical reaction, thereby exhibiting enhanced electrocatalytic performance and strong stability for ORR[27, 28]. To elucidate the formation of Fe nanocrystals,

high-angle

annular

dark-field

scanning

transmission

electron

microscope (HAADF-STEM) was also executed. Uniformly distributed Fe nanocrystals existed in the 3D carbonaceous matrix (Fig. 1f), and the elemental mapping imaging (Fig. 1g-j) declared the presence of C, Fe, O and N elements throughout Fe@Fe-N-CSs. To distinguish the chemical composition of electrocatalysts, X-ray diffraction (XRD) patterns was demonstrated in Fig. 1k. It disclosed that the XRD pattern of the Fe@Fe-N-CSs electrocatalyst exhibited two characteristic peaks at around 26° and 43°, which were assigned to graphitic carbon. The other characteristic peak at about 45° and 65° were well indexed in accord with the standard Fe reference (JCPDS No. 06-0696), which agreed well with HRTEM images. According to the knowledge of literatures, it can be speculated that the Fe nanocrystals should originate from the reduction reaction of the target Fe3+ by carbon during pyrolysis[29]. The porous characteristic of Fe@Fe-N-CSs was assessed by N2 adsorption-desorption isotherms analysis. The remarkable type-IV hysteresis loop of Fig. 1l presented the dominant mesopores and macropores structure. Meanwhile, the pore diameter distribution pattern (inset) acquired from the desorption curves indicated that the mesopore diameter distribution was focus on 20 nm. More importantly, the holey structures with high surface area (360 m2 g−1) will promote the exposure of active sites. In particular, the meso/macroporous structure has been manifested to efficiently improve accessibility to active sites, enlarge catalyst/medium contact area and benefited to the mass transport during ORR process[30]. In this regard, the characteristic of 9

Fe@Fe@Fe-N-C with high surface area and meso/macroporous structure should remarkably enhance the ORR performance.

Fig 2. (a) XPS survey spectra of Fe@Fe-N-CSs; (b) The high-resolution XPS spectra of N 1s; (c) N content in Fe@Fe-N-CSs; (d) High-resolution XPS spectrum of Fe in Fe@Fe-N-CSs.

The X-ray photoelectron spectroscopy (XPS) was conducted to investigate the chemical composition and bonding state of nitrogen and iron for the samples. The typical survey spectra (Fig. 2a) of the electrocatalysts testified the coexistence of C, N, O and Fe elements. The high-resolution N1s signals of Fig. 2b showed several distinct peaks at 405.7, 401.1, and 398.2 eV, which were in accordance with the oxidized N, graphitic N, and pyridinic N, respectively[31]. The pyridinic N may also include the iron-bound nitrogen (Fe-N) from Fe-Nx coordination on account of the small binding energies difference[32]. Notably, the pyridinic N and graphitic N were the dominant N species with the percentage of 25.41% and 51.27%, respectively (Fig. 2c). It is well known that the pyridinic N has significant effect on the formation of Fe-Nx species and the graphitic N is good for the 4e transfer pathway of ORR process[28]. In other words, these two forms of N, account for 76.68 % of the total N content in Fe@Fe-N-CSs were beneficial to boost the ORR performance[33]. Such phenomenon clearly highlighted a desired nitrogen doping efficiency for the catalysts. High resolution XPS of Fe2p spectra was recorded to further investigate the surface state of Fe species. As depicted in Fig. 2d, signals at approximately 710.4 eV and 713.4 eV in 10

Fe2p spectrum of Fe@Fe-N-CSs were clearly observed, which were ascribed to the 2p3/2 of Fe2+ and Fe3+[34]. Apart from that, generation of carbon encapsulated metallic zero valence Fe nanocrystals were also confirmed by the peak appeared at approximately 706.8 eV[35, 36]. The presence of metal Fe is also in good agreement with the XRD result. These results intensively demonstrated the coexistence of multiple active sites, including carbon encapsulated metal Fe nanocrystals and Fe-Nx moieties, which were manifested to promote the ORR activity.

Fig 3. (a) ORR polarization of Fe@Fe-N-CSs synthesized at various temperatures; (b) LSVs of different electrocatalysts at 1600 rpm, 5 mV s−1; (c) Tafel slope curves of different electrocatalysts; (d) LSVs of Fe@Fe-N-CSs-900 with various rotation rates. The inset in (d) is the corresponding −

K-L plots with a sweep rate of 5 mV s 1; (e) Chronoamperometric responses of Fe@Fe-N-CSs-900 and Pt/C in O2-saturated 0.1 M KOH, 1600 rpm; (f) Methanol tolerance of Fe@Fe-N-CSs-900 and Pt/C in O2-saturated 0.1 M KOH, 1600 rpm. 11

To better understand the ORR activity, the electrochemical performance for ORR was evaluated by using a rotating disk electrode (RDE) in 0.1 M KOH electrolyte solution saturated with O2. It is widely known that annealing temperature has great significance to the ORR activities, and the ORR features of Fe@Fe-N-CSs obtained at different annealing temperatures (800, 900 and 1000 oC) was recorded in Fig. 3a. It is interesting to survey that Fe@Fe-N-CSs synthesized at 900 oC displayed the optimized catalytic activity with the highest onset potential (1.0 V), largest half-wave potential (0.90 V) and larger limiting current density (5.06 mA cm−2) among all electrocatalysts. The inferior ORR performance of Fe@Fe-N-CSs-800 may be ascribed to the lower graphitization testified by Raman test. For the Fe@Fe-N-CSs-1000, despite the high graphitic degree, it may undergo the destruction of graphic structure and leaching of active N, as revealed by Raman and XPS spectra (Fig. S2, Fig. S3 and Table S1). Thus, the balanced graphitic degree and reserved active species may account for the best activity of Fe@Fe-N-CSs-900. Taking into account the above-mentioned analysis, 900 °C was considered to be the optimal pyrolysis temperature to gain Fe@Fe-N-CSs with the highest ORR activity. Due to the abundant Fe-N-coordination, sufficient carbon encapsulated metallic Fe nanocrystals, rich pore structure and high surface area, the resultant electrocatalysts were expected to have superb ORR performance[37]. To elucidate the ORR activities, linear sweep voltammetry (LSV) were conducted. Fig. 3b revealed the representative LSV of these four samples under the rotating rate of 1600 rpm with the scan rate of 5 mV s−1. The LSV curves stated that C-N and Fe-C showed the worst ORR activity in terms of onset potential (Eonset), half-wave potential (E1/2) and diffusion-limiting current (j). After the introduction of iron precursor, the sample named Fe@Fe-N-CSs-900 showed high Eonset (1.0 V), large E1/2 (0.90 V) and j (5.06 mA cm2). Notably, the Eonset and E1/2 of Fe@Fe-N-CSs-900 (Eonset 1.0 V, E1/2 0.90 V respectively) are significantly more positive than commercial Pt/C (Eonset 0.97 V, E1/2 12

0.84 V respectively) and other electrocatalysts (Fig. S4, Fig. S5)[38, 39]. Thus, the Fe@Fe-N-CSs-900 possesses outstanding ORR activity than benchmark Pt/C in alkaline medium. Furthermore, as shown in the Tafel plots (Fig. 3c), Fe@Fe-N-CSs-900 exhibited the smallest slope of 52 mV dec−1 which was also much smaller than Pt/C (81.6 mV dec−1). This phenomenon confirmed that the Fe@Fe-N-CSs-900 had favorable ORR kinetics, good electrochemical reaction activity and the first electron transfer path may be the rate-determining procedure in the ORR process[40, 41]. In attempt to insight into the ORR kinetics of Fe@Fe-N-CSs-900, LSV plots at different rotating speeds were carried out. The LSV profiles in O2-saturated 0.1 M KOH electrolyte media showed that the diffusion limiting current density was enhanced significantly with the increasing of rotation rate from 250 to 2500 rpm (Fig. 3d). In order to further understand the ORR kinetics, the Koutechy-Levich (K-L) analysis was conducted (Fig. 3d inset). The corresponding K-L plot originated from the LSV curves demonstrated good linearity over a wide potential range (Fig. 3d), meaning first-order reaction kinetics[42]. The transferred electron number (n) dating from the slopes of K-L plots, were 3.96, which was consistent with the four-electron transfer mechanism for ORR. The outstanding ORR activity of Fe@Fe-N-CSs-900 can be ascribed to the advantageous chemical composition and characteristic microarchitecture. High proportion of pyridinic-N in the electrocatalysts can boost the spin density and the π states’ density of carbon atom near the Fermi level, and therefor facilitate the intrinsic ORR activity[43, 44]. Moreover, the Fe nanocrystals embedded into the Fe, N-decorated carbonaceous matrix can reduce HO2, thus facilitating the 4e process of ORR[45]. Especially, metallic Fe nanocrystals favoured the adsorption of oxygen molecule on the adjacent Fe-Nx sites and promote ORR activity of Fe-Nx active sites[19]. Moreover, the carbonaceous matrix with desired specific surface area and meso/macroporous peculiarity can effectively afford sufficient mass transfer during the ORR process and adequate contact area of electrolyte and electrocatalysts. The 13

high content of active nitrogen doping, rich Fe-Nx sites, moderate Fe nanocrystals and benign meso/macroporous structure promise the higher ORR activity. Besides the catalytic activity, the stability and methanol tolerance are crucial index for practical application of ORR electrocatalyst. The stability and methanol tolerance of Fe@Fe-N-CSs-900 was assessed by using the current-time chronoamperometric method. The current feedback was measured at the potential of 0.8 V and 1600 rpm. The Fe@Fe-N-CSs-900 showed a superior stability which preceded benchmark Pt/C, with a small relative current loss of 4.8%, and demonstrated strong robustness (Fig. 3e). The intense affinity between Fe, N and carbon matrix not only safeguarded the homogeneous distribution of Fe-Nx active sites, but also guaranteed the reinforced stability. After injection of methanol, a dramatic decay of cathodic current density of Pt/C was registered, displaying the rapid degradation of ORR performance. Remarkably, the Fe@Fe-N-CSs-900 did not show obvious change of current density, implying excellent tolerance to methanol (Fig. 3f). The strong durability and methanol tolerance may be attributed to the successful pinning of metal Fe nanocrystals into the Fe, N-decorated porous carbonaceous framework as well as the strong interaction of those components[46]. To explore the universality of the proposed synthetic strategy, it was used to prepare other TM@TM-N-CSs. For instance, Co@Co-N-CSs were also successfully fabricated by employing the similar approach (Fig. S6). The ORR performance of the obtained Co@Co-N-CSs electrocatalyst in alkaline electrolyte was also certified by LSV. The as-obtained catalyst demonstrated comparable ORR activity to benchmark Pt/C. Researches showed that this approach was universal which hold potential to fabricate other high efficiency electrocatalysts.

Conclusion: In conclusion, high-performance Fe, N decorated 3D porous carbonaceous matrix electrocatalyst for ORR has been suggested through a facile and scalable two-step synthetic strategy. The obtained electrocatalysts successfully integrates the 14

excellent merits such as rich and accessible Fe-Nx active sites, moderate metallic Fe nanocrystals and three-dimensional porosity, which guarantee significantly high activity for ORR. The optimal catalyst with long-term stability and excellent resistance to methanol crossover displays large onset potential at 1.0 V, high half-wave potential at 0.90 V, which are 30 mV and 60 mV higher than benchmark Pt/C. Moreover, the present research not only demonstrates a simple and high-performance ORR electrocatalyst for alternative of platinum group metal electrocatalysts, but also provides a reliable technique to synthesize cost-effective electrocatalysts for large scale applications.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements This work was supported financially by the National Natural Science Foundation of China (Grant No. 51702180, 51772162), The Scientific and Technical Development Project of Qingdao, China (Grant No.18-2-2-52-jch), The Taishan Scholar Advantage and Characteristic Discipline Team of Eco Chemical Process and Technology.

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High performance Fe, N-decorated porous carbon for oxygen reduction reaction was successfully prepared

by templates assisted strategy. The obtained electrocatalyst with

moderate Fe nanocrystals, high content of Fe-Nx coordination sites and desired meso/microporous structure exhibited large onset potential at 1.0 V, high half-wave potential at 0.90 V, superior stability and excellent methanol tolerance capacity.

Xin Chena, Kaicai Fanb, Lingbo Zong*a, Yaowen Zhanga, Di Fenga, Mengyun Houa, Qi Zhanga, Dehua Zhenga, Yanan Chenc, Lei Wang*a

Keyword: Porous carbon matrix, Nonpreciousmetal electrocatalyst, Oxygen reduction reaction, High activity a Key Laboratory of Eco-chemical Engineering, Taishan Scholar Advantage and Characteristic Discipline Team of Eco Chemical Process and Technology, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China b Centre for Clean Environment and Energy, School of Environment and Science, Griffith University, Queensland 4222, Australia c School of Materials Science and Engineering, Key Laboratory of Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin Key Laboratory of Composite and Functional Materials, Tianjin University, Tianjin 300072, China

E-mail: [email protected] E-mail: [email protected]

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Title: Fe, N-Decorated Three Dimension Porous Carbonaceous Matrix for Highly Efficient Oxygen Reduction Reaction

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1. Highly Efficient electrocatalyst was constructed by using template-assisted approach. 2. Fe nanocrystals, high content of active sites and porous structure contributed to the remarkable ORR performance. 3. It opens up a general way to design of TM, N-decorated 3D porous electrocatalysts.

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The authors declare no conflict of interest.

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