Templating Synthesis of Mesoporous Fe3C-Encapsulated Fe-N

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Templating Synthesis of Mesoporous Fe3C-Encapsulated FeN-Doped-Carbon Hollow Nanospindles for Electrocatalysis Xin Xin, Haili Qin, Huai-Ping Cong, and Shu-Hong Yu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00548 • Publication Date (Web): 06 Apr 2018 Downloaded from http://pubs.acs.org on April 7, 2018

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Templating Synthesis of Mesoporous Fe3C-Encapsulated Fe-N-Doped-Carbon Hollow Nanospindles for Electrocatalysis Xin Xin,† Haili Qin,† Huai-Ping Cong,†,* and Shu-Hong Yu‡,*

†

Anhui Province Key Laboratory of Advanced Catalytic Materials and Reaction Engineering,

School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, P. R. China ‡

Division of Nanomaterials & Chemistry, Hefei National Research Center for Physical

Sciences at Microscale, CAS Center for Excellence in Nanoscience, Collaborative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. China

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ABSTRACT Developing cost-efficient alternatives to the noble metal catalysts towards oxygen reduction reaction (ORR) has attracted much attention. Herein, a kind of mesoporous hollow spindle-like Fe-N-C electrocatalysts with iron carbide nanoparticles encased in the N-doped graphitic layers has been synthesized by a novel “reactive hard template” strategy through the Fe3+-assisted polymerization of dopamine on the Fe2O3 cores and the following calcinations. The Fe2O3 nanospindles not only as the hard template guide the formation of well-defined shape and structure of the catalyst, but also as the reactive template provide Fe reservoir to generate Fe3C nanoparticles in the catalyst during the thermochemical process. The superiorities in accessible active sites of Fe-Nx species, Fe3C nanoparticles in graphene-like layers and highly mesoporous hollow structure enable the catalysts to exhibit excellent ORR performances including high catalytic activity, outstanding long-term cycling stability and good tolerance to methanol.

KEYWORDS: reactive template, mesoporous, Fe-N-C electrocatalyst, Fe3C nanoparticle, ORR elctrocatalysis

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INTRODUCTION Fuel cells are new energy devices which convert chemical energy directly into electricity in an efficient and environmentally friendly way.1-2 Although Pt and other noble metals are considered as most efficient electrocatalysts,3 a serious problem of these catalysts is the sluggish oxygen reduction reaction (ORR) kinetics at the cathode.4 Moreover, other drawbacks including methanol crossover effect, high cost, poor durability and low supply seriously restrict their commercialization to a large extent.5-6 A large number of studies have therefore been performed to develop cost-efficient alternatives to the noble metal catalysts, such as non-precious metal containing carbon materials,7 chalcogenide-containing compounds,8 heteroatom doped carbon materials9 and pyrolyzed metal-organic framework (MOF).10 Recently, a kind of transition metal-coordinating nitrogen-doped carbon (M-N-C, M = Fe, Co, Ni) catalysts has been regard as promising candidates to replace Pt-based ORR catalysts.11-17 It is believed that the incorporation of N and Fe into the graphene layer improves the ORR performance through tuning the structural and electronic environments of carbon framework.18-19 Pyrolyzed Fe-porphyrin and Fe-phthalocyanine encapsulated in ordered mesoporous carbon have been frequently employed in preparation of M-N-C systems.20-21 Furthermore, a new type of Fe-N-C materials with Fe3C nanoparticles encased in graphitic layers was developed to effectively enhance the ORR activity.22 Although carbide did not contact with electrolyte directly, it played a crucial role in electrocatalysis. It was assumed that the encapasulated Fe3C nanoparticles could activate the surrounding N-doped graphitic layers and enhance the oxygen adsorption and reduction.23 In addition to Fe3C 3


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nanoparticles, the formation of Fe-N species and N-C sites related with the doped nitrogen in the catalysts was also proved to be a decisive factor in attractive performance.24 Till now, transition metal and heteroatom co-doped carbon materials in many cases have exhibited outstanding electrocatalytic activity and endurance in alkaline and acidic media.25 Nevertheless, it is still under discussion that whether transition metals serving as the active centers or transition metals with respect to heteroatoms plays a synergistic effect on creating more active sites of Fe-N-C.26 Although the nature of high catalytic activity for Fe3C-encapsulated Fe-N-C catalysts has not been fully understood, it was confirmed that large specific surface area and highly mesoporous structure (2-50 nm) made remarkable contributions to the accessibility of active sites and high-flux transport of oxygen reduction-related species, and therefore, leading to the optimized electrocatalytic capability.27-29 Hard templates such as mesoporous silica combined with the post-treatment of the impregnated Fe, N and C-containing precursors were commonly used to create porous Fe-N-C catalysts.30 However, complicated and tedious procedures were inevitably involved in these cases including synthesis of high-quality porous templates, preparation of precursors mixed with monodispersed silica particles and removal of templates with corrosive hydrofluoric acid or alkaline solution. Actually, these drawbacks not only prevented the scale-up production of catalysts, but also damaged the homogeneity of the resultant product and inevitably depressed the catalytic activity and stability. Therefore, developing a simple and efficient strategy to synthesize mesoporous Fe3C-encased and N-doped carbon catalysts with homogeneous composition and microstructure is highly demanded and appealing. 4


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Herein, we demonstrated a novel “reactive hard template” method for the fabrication of mesoporous hollow spindle-like Fe-N-C catalyst with Fe3C nanoparticles encapsulated in N-doped graphitic layers by Fe3+-assisted polymerization of dopamine on the Fe2O3 nanospindle templates and the following calcinations of the composites at an elevated temperature. The Fe2O3 nanospindles not only acted as hard templates to enable carbon shell well-shaped during carbonization process but also as reactive templates to provide iron sources for the formation of Fe3C nanoparticles with a thermochemical reaction. Additionally, metal ions (Fe3+) involved in polydopamine (PDA) shell as catalyst promoted the decomposition of polymer into carbon species, facilitating the deposition of Fe3C during pyrolysis. Benefited from the superiority in accessible active sites of Fe-Nx species and Fe3C nanoparticles encased in graphene-like layers and highly mesoporous hollow structure, the catalysts exhibited excellent ORR performances in terms of high catalytic activity, outstanding long-term cycling stability and good tolerance to methanol.

EXPERIMENTAL SECTION Synthesis of α-Fe2O3 spindles. In a typical synthesis of the α-Fe2O3 spindles, 0.02 M of FeCl3·6H2O and 0.45 mM of NaH2PO4·2H2O were dissolved in 50 mL of deionized water under stirring, followed by a ultrasonic treat for 30 min to form a clear solution. The obtained yellow solution was transferred to a Teflon-lined stainless steel autoclave and heated at 105 oC for 48 h. After the autoclave was cooled to room temperature, the product was collected by centrifuging and washing three times with deionized water and then dried at 60 oC for 12 h. Synthesis of hollow Fe-N-C-1 catalyst. In a typical synthesis, 1 mg/mL of FeCl3·6H2O and 1 5


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mg/mL of dopamine were dissolved into deionized water with constant stirring, after which the pH value was adjusted to 8.5 with tris(hydroxymethyl)aminomethane (Tris). Then, α-Fe2O3 spindles were dispersed in dark green solution and stirred at room temperature for 24 h. The resulting solution was collected by centrifuging and washing three times with deionized water and dried at 60 oC for 12 h. Subsequently, the sample was transferred to tube furnace and annealed at 800 oC for 1 h in Ar atmosphere with a rate of 5 oC min-1. The inactive Fe impurities were removed in 0.5 M H2SO4 solution at 80 oC for 8 h. Finally, the obtained products were heated in Ar atmosphere at the same temperature for 2 h. In this way, the hollow Fe-N-C catalyst with Fe3C nanoparticles encapsulated in graphitic layer was fabricated and denoted as Fe-N-C-1. Synthesis of comparative electrocatalysts (Fe-N-C-2 and Fe-N-C-3). For the synthesis of Fe-N-C-2, SiO2@Fe2O3 was firstly prepared and used as the template. α-Fe2O3 nanospindles were dispersed in a mixture of 40 mL deionized water and 160 mL ethanol glycol. Then, 2 mL ammonium hydroxide (28 wt%) was dissolved in the mixture under stirring, followed by dropwise adding 150 µL tetraethyl orthosilicate (TEOS) and stirring at room temperature for 4 h. The resultant solution was collected by centrifuging and washing three times with ethanol and deionized water, and then dried at 60 oC for 12 h. The following step of polymerizing dopamine on spindle template kept the same as that of Fe-N-C-1. After annealing at 800 oC in Ar atmosphere, the sample was treated with 2 M NaOH solution and 0.5 M H2SO4 solution at 80 oC for 8 h to remove SiO2 and inactive Fe impurities, respectively. Finally, the obtained products were heated again in Ar atmosphere at 800 oC for 2 h, and denoted as Fe-N-C-2. The synthesis process of Fe-N-C-3 was similar to that of hollow Fe-N-C-1 spindles, except 6


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that FeCl3·6H2O was not added into the dopamine solution. Characterization. Scanning electron microscopy (SEM) images were obtained on Carl Zeiss Microscopy GmbH field-emission scanning electron microscope at an acceleration voltage of 5 kV (Merlin Compact). Transmission electron microscopy (TEM) images were carried out on a Hitachi H7700 transmission electron microscope at 120 kV. High-resolution TEM (HRTEM), elemental mappings and energy dispersive X-ray spectroscopy (EDX) were taken on JEM-2100F transmission electron microscope operating at 200 kV equipped with Oxford Inca energy instrument. X-ray diffraction (XRD) was conducted on PANalytical X’Pert PRO MPD. X-ray photoelectron spectra (XPS) were acquired on ESCALAB 250Xi (Thermo Scientific) X-ray photoelectron with monochromatic Al Kα source. Barrett-Emmett-Teller (BET) measurements were carried out on a Quantachrome Autosorb-6B nitrogen adsorption apparatus. Electrochemical measurements. The electrochemical measurements were carried out by an electrochemical workstation (PGATAT302N, Autolab, Netherlands). The Fe-N-C sample ink was prepared by mixing the catalyst powder (10 mg) with 100 µL Nafion solution (0.5 wt%) and 920 µL ethanol under sonication. The Pt/C catalyst (20 wt%, Johnson Matthey) ink was prepared by 50 µL Nation (0.5 wt%) and 970 µL ethanol under sonication. Then, 12 µL of Fe-N-C catalyst or 4 µL of Pt/C catalyst was loaded on glassy carbon rotating disk electrode (RDE geometric area 0.196 cm2) or rotating ring-disk electrode (RRDE) at room temperature, respectively. In this way, the amount of Fe-N-C catalyst and Pt/C catalyst dropped onto GC surface was 0.6 mg cm-2 and 0.1 mg cm-2, respectively. Before each test, electrolyte must be saturated with N2/O2 by bubbling gas for 30 min. The cyclic voltammetry (CV) curves were 7


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tested in 0.1 M KOH solution with saturated O2/N2, accompanied with a scan rate of 50 mV s-1. RDE measurement was performed with liner sweep voltammetry (LSV) in KOH solution at a scan rate of 10 mV s-1 with varying rotating speeds from 225 to 2025 rpm. RRDE was the same to RDE and the ring electrode potential was set to be 0.2 V. The hydrogen peroxide yield (%H2O2) and electron transfer number (n) were calculated by the following equations: n=

4 ID ID + ( IR / N )

%H2O2=100

2 IR / N ID + ( IR / N )

(1)

(2)

where ID and IR are the Faradaic current at the disk and ring, respectively. N is the H2O2 collection efficiency at the ring. The number of electrons was calculated by Koutecky-Levich equation: 1 1 1 1 1 = + = + 1/ 2 J JL JK Bω JK B=0.62nFC0D0 2/3ν -1/6 where J is the measured current density, JL and JK are the diffusion-limiting and kinetic current densities, respectively, n is the number of electron, F (96485 C mol-1) is the Faraday constant, C0 (1.2×10-6 mol cm-3) is the bulk concentration of O2, D0 (1.9×10-5cm2 s-1) is diffusion coefficient of O2 in KOH. ν (0.01 cm s-1) is the kinetic viscosity of the electrolyte. ω is the angular velocity of the disk. All outcomes were obtained at room temperature. Chronoamperometric approach for studying methanol crossover and evaluating durability was performed in 0.1 M KOH solution with saturated O2 at 1600 rpm and a fixed voltage of -0.4 V. The arrow indicated that 1 M methanol at 300 s was injected. The ORR stability of catalysts before and after 10000 cycles was cycled in N2/O2 saturated 0.1 M KOH solution at 8


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1600 rpm between -0.8 V and 0.05 V with a scan rate of 100 mV s-1.

RESULTS AND DISCUSSION Synthesis of Fe3C nanoparticles encapsulated Fe-N-C catalyst. Figure 1 shows the typical synthesis process of hollow Fe-N-C catalysts. Uniform spindle-like Fe2O3 crystals with the length of 500 nm and width of 100 nm were firstly synthesized by hydrothermal method (Figure S1).31 Then, Fe3+-incorporated polydopamine (PDA) shell was coated on Fe2O3 spindle through the polymerization of dopamine at pH 8.5 with the existence of FeCl3 (Fe2O3@Fe3+-PDA) (Step 1).32 The thickness of PDA shell from 15 to 80 nm was easily controlled by changing the dopamine concentration in the range of 0.5-3.0 mg/mL (Figure S2). With the pyrolysis of Fe2O3@Fe3+-PDA composites at 800 oC under Ar atmosphere, the hollow structure with thin carbon shell formed due to the synergistic reaction of C species in polymer assisted with both Fe2O3 core and Fe3+ species (Step 2). With such annealed process, the iron carbide nanoparticles with and without graphitic layer enwrapped were generated, forming pod-like architecture, as revealed from XRD pattern and SEM and TEM images (Figure S3 and S4). It was worth to note that the integrated spindle morphology was still preserved. Subsequently, the pyrolyzed product was etched in H2SO4 solution to remove unstable and non-electroactive iron carbide species (Step 3). The pre-leached powder was annealed again and the iron carbide nanoparticle encapsulated Fe-N-C catalyst was finally obtained, denoted as Fe-N-C-1.

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Figure 1. Schematic illustration for the fabrication process of hollow Fe-N-C-1 catalyst. In the Step 1, the Fe2O3 nanospindles were coated with a shell of Fe3+-incorporated PDA. In the Step 2, the obtained core-shell structures were pyrolyzed, forming hollow Fe-N-C product encased with the graphitic layer-enwrapped Fe3C nanoparticles and non-electroactive Fe3C nanoparticles. In the Step 3, with the acid leach and pyrolysis process, the non-electroactive Fe3C species were removed and hollow Fe-N-C-1 catalyst was obtained.

Figure 2. (a) SEM image and (b) TEM image of Fe-N-C-1. (c) HRTEM image of graphitic layer-enwrapped Fe3C nanoparticle in Fe-N-C-1. The inserts in (c) show enlarged HRTEM images of the squared areas. (d, e) Highly magnified TEM image and the corresponding elemental mappings of Fe-N-C-1. (f) EDX image of Fe-N-C-1 in (d). SEM image in Figure 2a showed that Fe-N-C-1 catalysts delivered well-defined spindle morphology with 500 nm in length and 150 nm in width after the pyrolysis and subsequent etching process. TEM image in Figure 2b revealed the hollow feature of the obtained sample 10


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with the shell thickness of about 20 nm. Moreover, the unique porous and nanoparticle-encapsulated structure was clearly presented in carbon layer of Fe-N-C-1. The magnified TEM image of Fe-N-C-1 in Figure 2d and corresponding elemental mapping images in Figure 2e revealed the homogeneous distributions of C, N, O elements and intermittent distribution of Fe element in the nanospindles. EDX analysis in Figure 2f further indicated the existence of above four elements in Fe-N-C-1 catalyst. HRTEM image in Figure 2c gave the detailed structure of the nanoparticles encapsulated in the spindle. It was clearly observed that graphite-like layers with an interlayer spacing of 0.348 nm, corresponding to (002) plane of graphitic carbon, tightly wrapped the nanoparticle. The crossed 2D crystal lattices with the same spacing distance of 0.21 nm and a characteristic angle of 68.4o were well matched with (-211) and (211) planes of Fe3C phase. Previous report indicated that although Fe3C was not in direct contact with the electrolyte, the synergy between N-doped graphitic layers and Fe3C nanoparticles was prone to improve ORR activity.33

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Figure 3. (a) XRD pattern, (b) Survey XPS spectrum, (c) High-resolution N1s XPS spectra and (d) High-resolution Fe2p XPS spectra of Fe-N-C-1. XRD pattern of Fe-N-C-1 catalyst in Figure 3a presented a broad diffraction peak at 26.5o, indexed as the (002) plane of graphitized carbon. All the other diffraction peaks were the characteristics of Fe3C phase (JCPDS 35-0772), indicating that the Fe3C nanoparticles generated in hollow Fe-N-C-1 nanospindles. XPS measurement was employed to analyze the element composition and chemical bonding configuration in Fe-N-C-1. Survey XPS spectrum in Figure 3b showed the presence of C, N, O, Fe in catalyst with the contents of 86.5 at%, 4.7 at%, 7.8 at% and 1.0 at%, respectively. The core-leveled N1s XPS spectrum of Fe-N-C-1 was deconvoluted into several subpeaks with different kinds of N species, including pyridinic N at 398.6 eV, pyrrolic N at 399.9 eV, graphitic N at 400.9 eV and oxidized N at 403.8 eV (Figure 3c). It was well accepted that graphitic N played a key role in enhancing ORR activity in 12


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N-doped carbons. Herein, pyridinic and pyrrolic N provided long-pair electrons and served as the coordination sites to combine with metal, which was another beneficial factor in improving the electrocatalytic performance.13 Therefore, in consideration of the overlapped pyridinic N and N-Fe bonding, the peak at 398.6 eV was partly resulted from the N-Fe configuration by N atoms coordinating with Fe.16 The high-resolution Fe 2p XPS spectra of Fe-N-C-1 suggested that there existed two kinds of iron species, that was Fe2+ and Fe3+, as confirmed by two sets of the Fe 2p3/2 and Fe 2p1/2 bands with signals at 711.8 eV, 723.4 eV and 714.2 eV, 727.4 eV, respectively (Figure 3d).34 The appearance of Fe 2p3/2 peak at 711.8 eV was a proof of the formation of the coordinated Fe-N moieties by ferrous ions bonded with nitrogen.35 Moreover, a signal at 719.0 eV related to the satellite peak indicated the absence of iron oxide phase in the obtained catalyst.36 To reveal the porous structure of Fe-N-C-1, the N2 adsorption/desorption isotherms were collected in Figure 4. The catalyst demonstrated a typical Type IV isotherm with an obvious type H2 hysteresis loop, indicating the existence of abundant mesopores. Its Brunauer-Emmett-Teller (BET) specific surface area was calculated as 200.7 m2 g-1 and pore volume was 0.56 cm3 g-1 with the pore size centered at 5 nm. The large pore volume and high specific surface area of Fe-N-C-1 favored rapid transport of electrolytes and electrons and more exposure of active sites, playing important roles in enhancing the electrochemical performances.37

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Figure 4. (a) N2 adsorption-desorption isotherms and (b) Corresponding pore size distribution of Fe-N-C-1. The dopamine concentration was investigated to reveal the influence of the thickness of PDA shell on the morphology and structure of the resulted Fe-N-C-1 spindles. With the low concentration of 0.5 mg/mL, the carbon shell arising from the thermal decompose of PDA was too thin to support the integrated hollow spindle morphology (Figure S5a and S5d). Moreover, the carbon shell would be partly consumed due to the formation of Fe3C by the reaction of carbon with iron sources. With improving the dopamine concentration higher than 1.0 mg/mL, the well-defined hollow N-doped carbon spindles with varied shell thickness encased with Fe3C nanoparticles were fabricated (Figure 2b, Figure S5b-5c and S5e-5f). As revealed in the XPS spectra, the content of N in the obtained catalysts was well controlled from 3.58 to 5.19 % when the dopamine concentration used in the range of 0.5 to 3.0 mg/mL (Figure S6a). It was speculated that the integrated structure with the thinner carbon shell would be beneficial to fast transport of electrons and electrolytes, and therefore, resulting in efficient electrocatalysis. Based on above analyses, 1.0 mg/mL of dopamine was selected to prepare the target Fe-N-C-1 catalyst, which was testified in the following electrocatalytic measurements. 14


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Additionally, it was found that the concentration of FeCl3 used was another important control parameter to achieve the optimal structure and composition (Figure S6b). With the FeCl3 contents lower than 1.0 mg/mL, the products showed non-uniform hollow structure with uneven shell thickness and a small quantity of encapsulated nanoparticles (Figure S7a-7b and S7d-7e). When improving the corresponding content to 1.2 mg/mL, the obtained spindle delivered a collapsed morphology due to the remained ultrathin carbon shell by excessive reaction of carbon with iron species (Figure S7c and S7f). Additionally, the architecture of Fe-N-C-1 catalyst was greatly influenced by pyrolysis temperature. At low temperature of 700 o

C, the product remained spindle morphology with coarse surface. The typical porous

structure and encapsulated Fe3C nanoparticles were not shown in these samples (Figure S8a and S8d). With raising the pyrolysis temperature to 900 and 1000 oC, the obtained products were not well-shaped, presenting the broken structure (Figure S8b-8c and S8e-8f).

Synthesis of two comparative catalysts. To reveal the synergistic effects of Fe2O3 core and Fe3+ in the PDA shell on the formation of Fe3C nanoparticle-encapsulated Fe-N-C spindles, we prepared two control samples, named as Fe-N-C-2 and Fe-N-C-3. For the fabrication of Fe-N-C-2, Fe2O3 spindles were coated with a first shell of silica, and then, a second shell of Fe3+ incorporated PDA, enabling Fe2O3 core to isolate from PDA shell in the pyrolysis procedure. As shown from TEM image (Figure S9), Fe2O3@SiO2@Fe3+-PDA precursor presented well-dispersed spindle morphology with SiO2 and PDA shell thickness of 25 nm and 40 nm, respectively. With the removal of SiO2 and inactive species in the annealed product, Fe-N-C-2 with the carbon layer thickness of 20 nm showed well-defined hollow spindle structure with abundant pores (Figure S10). However, different from Fe-N-C-1, no 15


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obvious nanoparticles were detected in the carbon shell. Only a broad band at 26o, corresponding to (002) plane of graphite was investigated in XRD pattern of Fe-N-C-2, confirming the absence of Fe3C nanoparticles incorporated in Fe-N-C-2 (Figure S11). Survey XPS spectrum of Fe-N-C-2 provided the elemental information with 92.2 at% of C, 2.8 at% of N, 4.6 at% of O and little amount of Fe (0.4 at%) (Figure 5a, Table S1). The typical band of Fe 2p3/2 at 711.8 eV as deconvoluted from high-resolution Fe 2p XPS spectra indicated that Fe ions were coordinated with N, forming Fe-N bondings (Figure 5b).35

Figure 5. (a) Survey XPS spectra and (b) high-resolution Fe2p XPS spectra of Fe-N-C-2 and Fe-N-C-3. As for the preparation of Fe-N-C-3, we adopted similar procedures to that of Fe-N-C-1, except for no addition of Fe3+ in the dopamine solution. The obtained sample showed hollow spindle structure with faint surface (Figure S12). Also, no nanoparticles were detected in the spindles, which were testified by the absence of characteristic diffraction peaks of Fe3C in the corresponding XRD pattern (Figure S11). Furthermore, the dominant C of 90.5 at% and negligible Fe of 0.3 at% were revealed in the corresponding survey XPS spectrum (Figure 5a, Table S1). The two control samples also delivered the mesoporous structures, as revealed from the Type-IV N2 sorption isotherms with obvious hysteresis loops (Figure S13). However, 16


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they gave smaller pore volumes than Fe-N-C-1 (0.31 cm3 g-1 of Fe-N-C-2 and 0.42 cm3 g-1 of Fe-N-C-3, respectively). All these analyses indicated that both Fe species including the core of Fe2O3 and Fe3+ in the polymer shell synergistically participated in the generation of Fe3C nanoparticles during the pyrolysis procedure. The Fe2O3 nanospindles served as the reactive hard templates not only to guide the shape and structure of the production but also provide the Fe sources for the formation of Fe3C in the carbonization process. The Fe3+ incorporated into the PDA shell could promote the decomposition of polymer into small-molecular hydrocarbons and catalyze the graphitization of non-crystalline carbon,38 which reacted with the above Fe sources and in-situ formed Fe3C nanoparticles through a thermochemical reaction. On the other hand, during pyrolysis, the graphitization of non-crystalline hybrids impelled by Fe species was gradually migrated and grew into iron compound nanoparticles, and then generated porous structure in the etching of above unstable intermediate products by acid solution.11

Figure 6. (a) CV curves of Fe-N-C-1 and Pt/C. (b) LSV curves of Fe-N-C-1, Fe-N-C-2, Fe-N-C-3 and Pt/C. (c) Histogram of half-wave potentials of Fe-N-C-1, Fe-N-C-2, Fe-N-C-3 and Pt/C. The loading amount of Fe-N-C catalysts and Pt/C catalyst was 0.60 mg/cm2 and 0.20 mg/cm2, respectively. ORR performance of Fe-N-C catalysts. Microstructure, morphology and composition of the catalyst were well accepted to be tightly related to ORR catalytic activity including specific surface area, porous property, heteroatom doping and transition metal nanoparticle 17


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incorporation.25 Pyrolysis process was crucial to prepare high-performance carbonized Fe-N-C catalyst and carbonization temperature certainly influenced the corresponding ORR activity. The catalytic performance of Fe-N-C-1 catalysts obtained at pyrolysis temperatures ranging from 700 to 1000 oC was investigated by rotating disk electrode (RDE) in O2 saturated 0.1 M KOH solution (Figure S14). The catalyst prepared at 800 oC exhibited the best ORR activity, as demonstrated by the most positive onset and half-wave potentials than those of samples prepared at other temperatures. At this carbonization temperature, the sample could hold the optimal N species, facilitating the formation of high-density Fe-Nx active sites for enhanced ORR.39 Remarkably, catalysts with different pyrolysis temperatures possessed different structures and graphitization degrees. The catalyst with a pyrolysis temperature of 700 oC, although held a similar hollow structure to that of Fe-N-C-1 at 800 oC, showed no Fe3C nanoparticles encapsulated in the graphitic carbon layers (Figure S8). When elevating the pyrolysis temperature above 900 oC, the integrated structure could not maintain. Furthermore, it was found that the dopamine concentration related with the thickness of pyrolyzed-carbon shell and the involved FeCl3 amount associated with the composition and structure of the catalyst significantly influenced the ORR electrochemical performance (Figure S15 and S16). Through the linear sweep voltammogram (LSV) curves, the optimal concentrations of dopamine at 1.0 mg/mL and FeCl3 at 1.0 mg/mL were screened, probably due to that a balance was achieved between ORR-related factors including active site density, surface area and electron conductivity at these optimized conditions.11, 25 Additionally, the ORR electrochemical performance of the target Fe-N-C-1 catalyst and two comparative samples was manifested by cyclic voltammetry (CV) in N2/O2 saturated 0.1 18


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M KOH solution. In N2 saturated CV curve of Fe-N-C-1 catalyst, no obvious featured peaks were observed in Figure 6a. However, there existed an evident cathodic oxygen reduction peak at -0.225 V of the Fe-N-C-1 vs. -0.182 V of 20 wt% Pt/C in O2 saturated solution. Compared with Fe-N-C-1, the Fe-N-C-2 and Fe-N-C-3 had lower peak potentials at -0.281 V and -0.313 V, respectively (Figure S17). Then, the ORR activities of three catalysts were further demonstrated by RDE curves at a rotation rate of 1600 rpm in O2 saturated 0.1 M KOH solution. A commercial 20 wt% Pt/C catalyst was also measured under the same conditions for comparison. In Figure 6b, the LSV curve of Fe-N-C-1 reflected high onset potential of -0.061 V and half-wave potential of -0.171 V, which were close to that of Pt/C with onset potential of -0.052 V and half-wave potential of -0.167 V. In stark contrast to Fe-N-C-1, Fe-N-C-2 exhibited very poor ORR activity with onset potential of -0.125 V and half-wave potential of -0.237 V. Fe-N-C-3 also presented negative onset potential of -0.146 V and half-wave potential of -0.255 V, which was plotted in Figure 6c for clearness. Furthermore, Fe-N-C-1 displayed a higher diffusion-limiting current density (5.52 mA cm-2) at -0.4 V than Fe-N-C-2 (4.34 mA cm-2) and Fe-N-C-3 (4.15 mA cm-2). Based on above analyses, the Fe-N-C-1 catalyst exhibited enhanced ORR performance compared with the comparative materials and the reported literatures (Table S2) benefiting from the superiorities in structure and composition. Firstly, the porous structure of Fe-N-C-1 with high pore volume facilitated exposing more active sites and transporting the ORR-related species ( OH-, O2, H+, H2O ).30 Secondly, high content of active sites including Fe-Nx and N-C gave high activity of the Fe-N-C-1 catalyst. Thirdly, although the encapsulated Fe3C nanoparticles were not accessed to the electrolyte solution, they not only enabled the carbons more active towards ORR by 19


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activating the surrounding graphitic layers,23 but also boosted the activity of Fe-Nx sites and brought about excellent ORR performance.18

Figure 7. LSV curves of (a) Fe-N-C-1, (c) Fe-N-C-2 and (e) Fe-N-C-3 at different rotation rates. Koutecky-Levich (K-L) plots (j-1 vs. ω-1/2) of (b) Fe-N-C-1, (d) Fe-N-C-2 and (f) Fe-N-C-3 at different potentials. To better investigate the ORR kinetics of Fe-N-C-1, electron transfer number (n) was 20


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calculated based on Koutecky-Levich (K-L) plots (j-1 vs. ω-1/2) at different potentials and various rotation rates from 400 to 2025 rpm (Figure 7a). The good linearity and coincidence in Figure 7b suggested that first-order reaction kinetics of catalyst toward the O2 concentration with a similar electron transfer number at different potentials.11 The n value of Fe-N-C-1 was calculated to be 3.92-3.94 in the potential range from -0.4 to -0.6 V. The result indicated that the ORR kinetics of Fe-N-C-1 was 4e- transfer pathway, which was very close to that of the commercial Pt/C (n = 3.98-4.0) (Figure S18). In contrast, Fe-N-C-2 and Fe-N-C-3 exhibited poor linearity with inconsistent slope, leading to low n values (n = 3.42 for Fe-N-C-2 and 3.83 for Fe-N-C-3 at -0.4 V, respectively) (Figure 7c-7f), which further proved that core of Fe2O3 and Fe incorporated carbon shell synergistically influenced the structure, composition and the resulted performance of the catalysts. Furthermore, the rotating ring-disk electrode (RRDE) measurements were further employed to reveal the electrocatalysis selectivity of these catalysts. Remarkably, the H2O2 yield of Fe-N-C-1 kept up less than 6 % with potential changing from -0.8 V to -0.1 V and the corresponding electron transfer number was 3.94-3.98, much better than that of Fe-N-C-2 (n = 3.21-3.71) and Fe-N-C-3 (n = 3.67-3.85), and commercial Pt/C (n = 3.80-3.92). (Figure 8a, Figure S19). A comparison of RDE and RRDE test results at -0.4 V was clearly shown in Figure 8b. Evidently, the Fe-N-C-1 delivered outstanding electrocatalytic selectivity based on coincident RDE and RRDE results.

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Figure 8. (a) H2O2 yield and electron transfer number (n) in RRDE measurement of Fe-N-C-1 and Pt/C. (b) Histogram of electron transfer number (n) in RRDE and RDE measurements of Fe-N-C-1, Fe-N-C-2, Fe-N-C-3 and Pt/C at the potential of -0.4 V. The solid and dashed column represents the n value from RRDE and RDE measurement, respectively.

Figure 9. (a) Chronoamperometric responses to the addition of 1 M methanol into Fe-N-C-1 and commercial Pt/C, respectively. (b) Durability evaluation of Fe-N-C-1 and Pt/C catalysts by chronoamperometric curves. LSV curves of (c) commercial Pt/C and (d) Fe-N-C-1 before and after 10000 cycles in O2-saturated 0.1 M KOH solution. 22


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The resistance to methanol crossover effect and the electrocatalytic durability are two important indicators of catalyst in commercialization of direct methanol fuel cells. The influence of methanol crossover on the catalysts was shown in Figure 9a. The same amount of catalysts was loaded on the electrode as that of aforementioned electrochemical measurements in O2-saturated 0.1 M KOH solution. Upon the injection of 1 M methanol into KOH solution, the Pt/C showed an obvious current jump arising from the oxidation of methanol on the electrode. Then, its current was gradually stable and reduced to half of the original value. In contrast, no noticeable change was detected with the methanol injection from the electrochemical curve of Fe-N-C-1 electrode, indicating its strong methanol-tolerant property. The stability of our catalyst was also systematically studied compared with commercial Pt/C as shown in Figure 9b-9d. The ORR current-time chronoamperometric curve in Figure 9b revealed excellent electrochemical durability of Fe-N-C-1 with a decrement of 8.3 % over 20000 s. However, commercial Pt/C exhibited big reduction with current density drop of 30 % due to the aggregation, deactivation and even loss of Pt nanoparticles.40 LSV curves of Fe-N-C-1 and Pt/C electrodes were carried out before and after long-term cycles to further reveal their electrocatalytic durability (Figure 9c and 9d). After 10000 cycles, the Fe-N-C-1 showed negative shift of the half-wave potential by 5 mV. However, a large potential change with a 72 mV negative shift was generated on Pt/C electrode. These results suggested that the Fe-N-C-1 exhibited much better resistance to methanol crossover effect and ORR stability when compared with Pt/C.

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CONCLUSION In conclusion, we have developed a new kind of catalysts with hollow porous structure, in which Fe3C nanoparticles were encased in N-doped graphitic layers. The α-Fe2O3 nanospindles as hard reactive template played dual roles in guiding the formation of well-defined hollow architecture and providing Fe sources for the generation of Fe3C nanoparticles through the thermochemical reaction with carbon species by Fe3+-catalyzed graphitization of polymer. Owing to the synergistic effects of unique structure, effective composition and abundant active sites, the catalyst exhibited superior ORR electrochemical performance, methanol-tolerance and long-term durability when compared with commercial Pt/C in alkaline media. The method presented in this work opens new avenue for the access to high-performance electrocatalysts as promising candidates in replacement of Pt/C for practical fuel cells.

ASSOCIATED CONTENT Supporting Information SEM images, TEM images, XRD patterns, XPS spectra, N2 adsorption-desorption isotherms, LSV curves, CV curves, H2O2 yield in RRDE measurement and tables for element contents in the catalysts and comparison of ORR performance of different catalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author 24


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* E-mail: [email protected] (H.P. Cong); [email protected] (S.H. Yu)

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grants. 21571046, 21761132008, 21431006, 51732011, 21503063), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant 21521001), the National Basic Research Program of China (Grants 2014CB931800), the Fundamental Research Funds for the Central Universities (Grants JZ2016HGPA0735, JZ2017HGTB0197), the Users with Excellence and Scientific Research Grant of Hefei Science Center of CAS (2015HSC-UE007) , and Anhui Provincial Natural Science Foundation (1708085MB30).

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Templating Synthesis of Mesoporous Fe3C-Encapsulated Fe-N

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