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Hollow Nitrogen-Doped Carbon Spheres with Fe3O4 Nanoparticles Encapsulated as a Highly Active Oxygen-Reduction Catalyst Haitao Wang, Wei Wang, Yangyang Xu, Shuang Dong, Junwu Xiao, Feng Wang, Hongfang Liu, and Bao Yu Xia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15392 • Publication Date (Web): 07 Feb 2017 Downloaded from http://pubs.acs.org on February 8, 2017

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ACS Applied Materials & Interfaces

Hollow Nitrogen-Doped Carbon Spheres with Fe3O4 Nanoparticles Encapsulated as a Highly Active Oxygen-Reduction Catalyst Haitao Wang,† Wei Wang,† Yang Yang Xu,† Shuang Dong,† Junwu Xiao,† Feng Wang,† Hongfang Liu,*,† and Bao Yu Xia, *,†,‡ †

Key laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education,

Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology (HUST), Wuhan 430074, China ‡

Shenzhen Institute of Huazhong University of Science and Technology, Shenzhen 518000, P. R.

China.

KEYWORDS: core-shell, hollow, Fe3O4 nanoparticle, synergistic effect, oxygen reduction

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ABSTRACT: The development of nonprecious electrocatalyst with low cost and high efficiency for the oxygen reduction reaction (ORR) is a main challenge for electrochemical energy technology. In this work, a hierarchical hollow core-shell structured N-doped carbon spheres (N-HSCS), in which Fe3O4 nanoparticles are encapsulated (Fe3O4/N-HCSC) has been successfully prepared. The Fe3O4/N-HCSC electrocatalyst exhibits a remarkable catalytic performance towards ORR. The porous hollow core-shell structure and synergistic effect between Fe3O4 and protective nitrogen doped graphitic layers are mainly responsible for such an excellent ORR catalytic property and stability. This work demonstrates a promising strategy of nanostructure-engineering to the future design and preparation of highly efficient non-noble metal electrocatalysts.


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INTRODUCTION

The development of high-performance and low-cost electrocatalyst for the oxygen reduction reaction (ORR) is the key to promote the application of fuel cells and metal-air batteries.1-3 So far, platinum (Pt)-based noble metals are the best choice to accelerate the sluggish kinetic process of ORR.4,5 However, the high-cost and limited resourceful storage, as well as the poor stability are the practical problems for Pt-based noble metals.6,7 In this endeavor, various non-precious alternatives including metal oxides and carbides/nitrides are highly pursued for the ORR.8-10 However, their poor conductivity and limited active sites exposed prevent further enhancement of ORR. The combination of metal oxides with conductive nanocarbons represents a promising strategy to improve the overall catalytic performance for ORR. Especially, by doping of heteroatoms (N, S, etc) into carbon matrix, the heteroatom-doped carbon composites exhibit much improvement on the catalytic activity of ORR,11 which is considered by the modification of electronic and geometric effects.12 Unfortunately, there are much improvement of stability, but the catalytic activity is still inferior to Pt-based ORR electrocatalysts.13

Recently, transition metals (Mn, Fe, Co, Ni) contained N-doped nanocarbons have emerged as active ORR catalysts.14-16 Among of them, Fe-N/C materials demonstrate a better activity than those of containing other transition-metals (Mn, Co, Ni).17,18 This ORR activity originates from the synergistic effect between Fe-N species and the conductive carbon layers. Various nanocarbons including graphene,15 carbon nanorods,19 solid carbon sphere20 and carbon nanotubes3 are involved in the Fe/N-C composites. Typically, Fe3O4 decorated three-dimensional N-doped graphene aerogel and Fe-N decorated on carbon nanotube are reported as the ORR electrocatalysts.3, 3 ACS Paragon Plus Environment

21

Nevertheless,


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their ORR performance in half-wave potential and limited diffusion current density is much lower than expected, due to the poor porous structure together with the inefficient integration of components, which may still limit the intensity of ORR catalytic active sites and efficiency of mass transfer.22-25 Thus, it is still a challenge in controlling the structural uniformity of Fe/N-C, maximizing the number of active sites exposed and finding a new type carbon-based Fe/N catalyst with both desirable porosity and high conductivity for oxygen reduction.

Hollow core-shell structures with hierarchical architecture have attracted great interests in various systems involved drug delivery, cancer therapy and energy technologies.26-29 The distinctive microarchitecture with core, shell and hollow space brings the low density, high surface area and homogeneous reaction environment and so on. Especially, through the controllable assembly of Fe and N species supported on the core/shell nanocarbon to deliver the active sites, such a hierarchical structure may be therefore promising for the ORR occurring on the interfacial/surface of electrocatalyst. This structure design will not only improve the conductivity of the catalyst and facilitate the relevant mass transport,30 but also provide abundant active sites for ORR. However, the use of porous hollow core-shell spherical system to prepare Fe/N-C catalyst for ORR has never been reported so far.

In this work, we report a mesoporous hollow core-shell structured Fe/N-C type catalyst, in which active Fe-N species are dispersed uniformly in both carbon core and shell. The as-obtained Fe3O4/N-HCSC catalyst demonstrates an excellent electrocatalytic activity for ORR, compared to other transition-metal electrocatalysts and commercial Pt/C benchmarked. The large specific surface area and hierarchical carbon framework provides more positions to deposit active Fe and N species; 4 ACS Paragon Plus Environment

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the mesoporous carbon architecture facilitates not only mass exchange but also electron transfer in the course of catalytic process; the synergistic effect between Fe3O4 nanoparticle and protective N-doped graphitic layers enhances ORR catalytic activity and long-time stability. Therefore, this work would hold great potential to develop other Pt-alternatives and also provide some methodologies to achieve the controllable synthesis of other non-noble metal electrocatalysts with highly efficiencies.

EXPERIMENTAL SECTION

Synthesis of HCSC-700. Typically, resorcinol (0.4 g) and cetyltrimethyl ammonium bromide (CTAB 0.4 g) are added into the mixture solution including deionized water (45 mL), ethanol (18 mL) and ammonia solution (0.4 mL, 28 w t %) with constant stirring for 30 min.31 Formaldehyde solution (0.58 mL, 37 w t %) and tetraethyl orthosilicate (TEOS 1.8 mL) are then added and continuously stirred for 20 h at 30 °C. Subsequently, the red solution is poured into Teflon-lined autoclave maintained at 100 °C for 24 h. The products named CPS-SiO2 is collected, washed and dried at 60 oC. Then, CPS-SiO2 is annealed under flowing Ar at 700 °C for 2 h to form HCSC-SiO2. Finally, SiO2 is etched at 60 °C for 12 h in 2 M KOH solution to obtain HCSC-700.

Preparation of Fe3O4/N-HCSC, N-HCSC and Fe3O4/HCSC. 0.15 g of HCSC-700 powder is added into 15 mL of Fe(NO3)3 aqueous solution (0.24 M), and is continuously stirred for 12 h. The samples are collected and washed, then dried up in a vacuum oven at 40 °C. Subsequently, Fe(NO3)3/HCSC-700 is mixed with melamine (mass ratio 1:10), followed the thermal treatment at 900 °C (5 oC min-1) for 1 h under flowing Ar to obtain Fe3O4/N-HCSC. In a control, HCSC-700, the mixture of melamine and HCSC-700, and Fe(NO3)3/HCSC-700 are pyrolyzed under the same 5 ACS Paragon Plus Environment


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condition to form HCSC, N-HCSC, and Fe3O4/HCSC, respectively. N-HCSC is prepared with the different mass ratios of HCSC-700 and melamine (1:1, 1:5, 1:10, and 1:15), and Fe3O4/HCSC is also obtained with the different Fe(NO3)3 precursor introduced.

Materials characterization. Powder X-ray diffractometer (XRD, X'Pert PRO) is used to obtain the crystalline structures of catalysts. The microstructure and morphology of samples are examined by Nova Nano SEM 450 field-emission scanning electron microscopy (FE-SEM) and Tecnai G2 F20 S-TWIN

high-resolution

transmission

electron

microscopy

(TEM).

X-ray

photoelectron

spectroscopy (XPS) is carried out on an AXIS-ULTRA DLD-600W instrument. Raman spectroscopy is acquired using a confocal Raman microscope (LabRAM HR800). The pore structure of the samples are analyzed by N2 sorption on Micromeritics ASAP2020.

Electrochemical measurements. CHI760E electrochemical workstation (CH Instruments, China) with Pine electrochemical system are used to measure the electrochemical activity. Cyclic voltammograms (CVs) and linear sweeping voltammograms (LSVs) are recorded by rotating disc electrode (RDE, 5 mm diameter glassy carbon). For electrode preparation, 5 mg as-prepared catalysts mixing with Nafion solution (0.5 wt%, 20 µL) and isopropyl alcohol (980 µL) under ultra-sonication, then 12.5 µL of the homogeneous ink is dropped on the surface of pre-cleaned GC electrode, dried naturally, yielding a electrocatalyst loading of ~ 0.318 mg cm-2. Commercial Pt/C (20%, E-TEK) is also benchmarked with a loading of 0.102 mg cm-2. The modified GCE, platinum foil and saturated calomel electrode (SCE) together make up a typical three-electrode electrochemical test system. In our system, all potentials have been converted into ERHE (ERHE=ESCE + 0.242 + 0.059•pH). CVs plots are obtained in O2 or N2-staturated 0.1 M KOH electrolyte between 0 to 1.2 V (50 mV s-1). LSVs 6 ACS Paragon Plus Environment

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measurement are measured with a sweep rate of 5 mV s-1 (rotating speed: 1600 rpm) and corrected by i-R compensation in 0.1 M KOH solution. The ORR results are also studied in 0.5 M H2SO4 solution. The electron transfer number (n) can be analyzed by Koutecky-Levich (K-L) equation during the ORR with disk currents.32 J-1 = JL-1 + JK-1 = B-1ω-1/2 + JK-1

(1)

B = 0.2nFC 0 D02/3υ -1/6

(2)

Where J is the measured disk current density, J L and J k is the O2 diffusion limiting and kinetic current density; ω(rpm) is the rotation speed; F (96485 C mol-1) is the Faraday constant; C0 (1.21·10-6 mol cm-3) is the concentration of O2 dissolved; D0 (1.9·10-5 cm2 s-1) is the diffusion coefficient of dissolved O2;υ υ(0.01 cm2 s-1) is the kinematics viscosity of the solution.

RESULTS AND DISCUSSION

Figure 1. Illustration of the preparation of Fe3O4/N-HCSC. Figure 1 illustrates the preparation process of Fe3O4/N-HCSC. A typical sol-gel process is firstly 7 ACS Paragon Plus Environment


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taken place between negatively charged resorcinol formaldehyde (RF) emulsion droplets and silicate oligomers through a surfactant-directing co-assembly of cetyltrimethyl ammonium bromide (CTAB) in an alkaline solution.33 The negatively charged RF, cationic surfactant CTAB, and few silicate oligomers are firstly polymerized into a spherical polymer, as the hydrolysis of RF emulsion droplets is faster than tetraethyl orthosilicate.34 With prolonging the reaction time, the silicate oligomers together with the rest of RF droplets are co-assembled on the surface of spherical polymer preformed, then CPS-SiO2 is obtained in the subsequent hydrothermal process. After the carbonization at 700 °C, the core-shell structured HCSC-SiO2 is obtained. The spherical shell containing carbon and SiO2 is maintained because of a large number of rigid silicates,35 while a small amount of rigid silicate in the core shrink sharply due to the dehydrogenation of resin polymer. After the removal of silica and the impregnation of Fe3+ ions, HCSC samples are calcinated with melamine at 900 °C to form Fe3O4/N-HCSC.

Figure 2. (a-c) FESEM and (d-f) TEM images of CPS-SiO2, HCSC-SiO2, and HCSC-700, 8 ACS Paragon Plus Environment

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respectively. Figure 2 exhibits the morphology and structure of CPS-SiO2, HCSC-SiO2 and HCSC-700. The CPS-SiO2 exhibits a uniform sphere morphology with an average diameter of 600 nm (Figure 2a). CPS-SiO2 shows a core-shell structure with a shell thickness of ~ 75 nm (Figure 2d). This core-shell structure would be attributed to the different hydrolysis rates of RF and TEOS. After the subsequent carbonization process, HCSC-SiO2 still remains the sphere morphology and smooth surface (Figure 2b), while the thickness of shell decreases to ~45 nm and the inner core also shrink from 450 nm to 200 nm, leaving a large void in HCSC-SiO2 (Figure 2e). After etching, many pores appear on the shell of HCSC-700 due to the removal of SiO2 (Figure 2c), and thus a large cavity composed porous carbon can be observed in the inner of HCSC-700 (Figure 2f).



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Figure 3. (a) FESEM and (b) the corresponding HRTEM images of Fe3O4/N-HCSC: spherical shell (c) and core (d). SEM image in Figure 3a also reveals the similar surface of Fe3O4/N-HCSC, even after the second calcination process with the introduction of melamine for N-doping. The cracked sphere and the core particle indicate the large cavity in the sphere (inset of Figure 3a). TEM image in Figure 3b shows not only the well-remained sphere structure but also the uniform nanoparticles embedded in the carbon matrix (Figure S1a). A closer observation in Figure 3c shows the fine nanoparticles (~10 nm) in the shell are encapsulated by several graphite layers (Figure S1b). The nanoparticle in the inner core is also analyzed by HRTEM technique (Figure 3d and Figure S1d). The lattice distance of 0.302, 0.253 and 0.486 nm are corresponding to the lattice fringes (220), (311) and (111) of Fe3O4, respectively (Figure 3c and d, Figure S1c and d). It can be deduced that Fe3O4 nanoparticles are formed and embedded in porous shell and inner core simultaneously, which not only greatly enhances the density of accessible catalytic active sites, but also suppresses the possible agglomeration and dissolution of Fe3O4 nanoparticles during the future electrochemical operation,25, 36

thereby promising an outstanding catalytic activity and durability towards ORR.


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Figure 4. XRD patterns of different electrocatalysts (a). The high-resolution XPS spectra of N 1s (b) and Fe 2p (c), N2 sorption isotherm (d), and inset of (d) is the pore size distribution of Fe3O4/N-HCSC. XRD patterns of both HSCS and N-HCSC show two obvious diffraction peaks centered at 24.2 and 43.6o, which are corresponded to the (002) and (100) planes of the graphite carbon (JCPDS: 89-8487), respectively (Figure 4a). Compared with those of HCSC samples, both peaks of N-HCSC become broad and weak, implying graphitic structure becomes disordered with the introduction of N atoms. Moreover, the typical diffraction peaks of Fe3O4 are observed in Fe3O4/HCSC and Fe3O4/N-HCSC, which are assigned to the crystal planes (111), (220), (311), (222), (440), (422), (511) and (440) of Fe3O4 (JCPDS: 75-0033). XPS measurements are carried out to gain further insights into the composition and surface chemistry of Fe3O4/N-HCSC (Figure S2 and Table S1). The characteristic peak of N1s can only be clearly found at ~400 eV in the full XPS spectra of N-HCSC and Fe3O4/N-HCSC (Figure S2a), while no obvious N peaks in the Fe3O4/HCSC, which 11 ACS Paragon Plus Environment


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reveals that N atoms are successfully doped into HCSC when melamine is introduced. Moreover, the high-resolution C1s spectra of Fe3O4/N-HCSC can be deconvoluted into C-C, C-N, C=N and C=O at the binding energy of 284.6, 285.2, 285.9, and 290.3 eV, respectively, confirming the successful N-doping again (Figure S2b). Generally, N species are considered as a key role to promote the catalytic activity for carbon-based electrocatalysts.37, 38 This is because N-doping could modify the electronic properties of neighbor carbon or transition-metal atoms. The fitted N1s spectrum of Fe3O4/N-HCSC in Figure 4b demonstrates three N species at 398.4 eV (pyridinic N), 400.5 eV (pyrrolic N) and 401.3 eV (graphitic N). Obviously, there is a large binding energy shift between the N1s spectra of Fe3O4/N-HCSC and N-HCSC (Figure S3), indicating a strong electronic effect between Fe species and the neighbor N-doped carbons.39 It is worth mentioning that the fitted peak of graphitic N in Fe3O4/N-HCSC is much weaker as compared to N in N-HCSC (Figure 4b and Figure S2c). The contents of N in N-HCSC and Fe3O4/N-HCSC catalysts are determined as 2.58 wt% and 3.11 wt % (Table S2), which is close to XPS results of 2.27 wt% and 2.63 wt%. Moreover, the content of graphitic-N in Fe3O4/N-HCSC is much lower than that of N-HCSC (Tables S3), this means the presence of Fe3O4 in Fe3O4/N-HCSC facilitates the transformation of graphitic-N species and the formation of disordered carbons. Figure 4c exhibits the deconvoluted Fe 2p spectrum at ~ 710 eV (Fe 2p2/3) and 724 eV (Fe 2p1/2), while the other two peaks at ~709.8 eV and ~711.5 eV are the satellite peaks of Fe species.40 The content of Fe in Fe3O4/HCSC and Fe3O4/N-HCSC is measured by XPS technique as 0.83 wt% and 0.79 wt% (Tables S1), respectively, which is much lower than the results of TGA (7.83 wt% and 6.57 wt%, Figure S4) and ICP-AES (8.02 wt% and 7.05 wt%), due to the coverage by graphite layers. Raman spectra is carried out to obtain the carbon structure of HCSC, N-HCSC, Fe3O4/HCSC and Fe3O4/N-HCSC. All samples display two clearly 12 ACS Paragon Plus Environment

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separated peaks at ~1359 cm-1 for D band and ~1590 cm-1 for G band (Figure S5). It can be seen that the ratio of Fe3O4/HCSC (1.004), N-HCSC (1.027) and Fe3O4/N-HCSC (1.172) is much higher than that of HCSC. This increased disordered carbon structure would be resulted from the introduction of N dopant and the formation of Fe3O4/N-HCSC, as convinced by XRD and XPS results (Figure 4). Meanwhile, the high specific surface area and mesoporous structure are also confirmed by nitrogen sorption measurements (Figure S6 and Table S4). Even after the introduction of Fe and N, Fe3O4/N-HCSC also shows a high surface area (618.9 m2 g-1, Figure 4d). Especially, the distinctive mesoporous nanostructure with an average pore size of 6.54 nm and hollow carbon architecture can effectively facilitate O2 diffusion and further improve mass transfer efficiency for ORR (Table S4).41

The electrochemical activity of Fe3O4/N-HCSC is firstly evaluated by CVs in N2 or O2-saturated 0.1 M KOH electrolyte, while Pt/C electrocatalyst is also benchmarked. Before that, the N and Fe content is optimized (Figure S7). There is an obvious oxygen reduction peak at 0.836 V (EP, vs. RHE) for Fe3O4/N-HCSC in Figure 5a. The peak potential of Fe3O4/N-HCSC is more positive compared to HCSC (0.704 V) and N-HCSC (0.732 V), and also slightly surpassed than Pt/C benchmarked catalyst (0.828 V), which indicates a pronounced catalytic activity of Fe3O4/N-HCSC electrocatalyst for ORR (Figure S8). LSVs measurements are performed on the RDE for these HCSC-based materials and Pt/C electrocatalyst. The diffusion limiting current density Ji shows a typical enhancement with the increasing of rotation speed because of the shortened diffusion layer (Figure 5b).42 The K-L plots of Fe3O4/N-HCSC are analyzed (Figure 5b inset) and the corresponding transferred electron number (n) is also calculated. Unlike the K-L plots of HCSC (Figure S9), both Fe3O4/N-HCSC and Pt/C electrocatalysts exhibit a good linear relationship (Figure S10), indicating ORR on Fe3O4/N-HCSC and Pt/C is a first-order kinetics only with respect to the dissolved O2 13 ACS Paragon Plus Environment


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concentration.43 The average n of Fe3O4/N-HCSC (3.97) is closed to that of Pt/C catalyst (3.99) and also reveals a direct 4-electron process for ORR (Figure S11).

Figure 5. (a) CVs of Fe3O4/N-HCSC and commercial Pt/C electrocatalyst, (b) LSVs of Fe3O4/N-HCSC at rotation rate from 625 to 2500 rpm, inset of (b) is the corresponding K-L plots, (c) LSVs of different electrocatalysts at 1600 rpm. (d) The kinetic current densities (JK) of different electrocatalyst at 0.75 V. The ORR activity of catalysts are compared by LSVs at 1600 rpm (Figure 5c). The onset potential (E0) of Fe3O4/N-HCSC is 1.024 V, which is more positive than that of Pt/C catalyst (0.996 V), and much more positive than those of HCSC (0.865 V), N-HCSC (0.921 V), Fe3O4/HCSC (0.857 V) (Table S5). Moreover, Fe3O4/N-HCSC catalyst exhibits a higher half-wave potential (E1/2= 0.846 V), which is more positive than Pt/C (0.830 V) and other HCSC-based samples (

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