Assembly of Hollow Carbon Nanospheres on Graphene Nanosheets

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Assembly of Hollow Carbon Nanospheres on Graphene Nanosheets and Creation of Iron-Nitrogen-Doped Porous Carbon for Oxygen Reduction Haibo Tan, Jing Tang, Joel Henzie, Yunqi Li, Xingtao Xu, Tao Chen, Zhongli Wang, Jiayu Wang, Yusuke Ide, Yoshio Bando, and Yusuke Yamauchi ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b01502 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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Assembly of Hollow Carbon Nanospheres on Graphene Nanosheets and Creation of Iron-Nitrogen-Doped Porous Carbon for Oxygen Reduction

Haibo Tan,†1,2 Jing Tang,†*1 Joel Henzie,1 Yunqi Li,1,3 Xingtao Xu,1 Tao Chen,4 Zhongli Wang,1 Jiayu Wang,1 Yusuke Ide,1 Yoshio Bando,1 and Yusuke Yamauchi*5,6

[1] International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan [2] Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan [3] Department of Automotive Engineering, School of Transportation Science and Engineering, Beihang University, Beijing 100191, PR China [4] Beijing Synchrotron Radiation Facility (BSRF), Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China [5] School of Chemical Engineering & Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland, Brisbane, QLD 4072, Australia [6] Department of Plant & Environmental New Resources, Kyung Hee University, 1732 Deogyeong-daero, Giheunggu, Yongin-si, Gyeonggi-do 446-701, South Korea Email addresses of the corresponding authors: [email protected]; [email protected] † The authors contribute equally to this work

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ACS Paragon Plus Environment

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Abstract Triblock copolymer micelles coated with melamine-formaldehyde resin were self-assembled into closely-packed two-dimensional arrangements on the surface of graphene oxide sheets. Carbonizing these structures created a two-dimensional architecture composed of reduced graphene oxide (rGO) sandwiched between two monolayers composed of sub-40-nm diameter hollow nitrogen-doped carbon nanospheres (N-HCNS). Electrochemical tests showed that these hybrid structures had better performance for the oxygen reduction compared to physically mixed rGO and N-HCNS that were not chemically bonded together. Further impregnation of the sandwich structures with iron (Fe) species followed by carbonization yielded Fe1.6-N-HCNS/rGO-900 with a high specific surface area (968.3 m2 g-1), a high nitrogen doping (6.5 at%), and uniformly distributed Fe dopant (1.6 wt%). X-ray absorption near edge structure (XANES) analyses showed that most of the Fe in the nitrogen-doped carbon framework is composed of single Fe atoms each coordinated to four N-atoms. The best Fe1.6-N-HCNS/rGO-900 catalyst performed better in electrocatalytic oxygen reduction than 20 wt% Pt/C catalyst in alkaline medium, with a more positive half-wave potential of 0.872 V and the same diffusion-limited current density. Bottom-up soft-patterning of regular carbon arrays on free-standing two-dimensional surfaces should enable of conductive carbon supports that boost the performance of electrocatalytic active sites.

Keywords: monomicelle assembly, two-dimensional architecture, sandwich-like composite, iron and nitrogen doped carbon, oxygen reduction

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Carbon is a ubiquitous material in electrocatalysis because carbon is chemical/mechanical stable, can generate controllable porous architectures, and supports high conductivities all at a cost that is difficult to achieve with any other material. The microarchitecture and dimension of carbon supports would largely influence the kinetics of the catalysis process and the accessibility of the active sites, thus influence the electrocatalytic activity of the supported metal catalysts.1-4 Porous carbon nanomaterials with large effective surface areas and accessible channels expose the catalyst active sites to electrolytes (e.g., KOH or H2SO4 aqueous solution) and help accelerate mass transfer processes during electrochemical reactions.5-7 Among the various types of porous carbons, hollow carbon nanospheres (HCNS) are an interesting candidate support materials for metal catalysts due to their uniform size/shape, large interior void spaces, and the highly porous walls.8,9 Existing HCNS are amorphous, and have a low intrinsic electronic conductivity and high electrochemical polarization in electrochemical applications. HCNS also tend to aggregate in electrodes, which decrease the active surface area of the electrode. To overcome the shortcomings of HCNS, we sought to assemble them on two-dimensional (2D) graphene nanosheets. HCNS-graphene porous hybrid architectures offer complementary properties because: (i) graphene nanosheets are highly conductive and can stand in and improve the electron conductivity of HCNS, (ii) assembling thin layers of HCNS on graphene should limit their aggregation, and (iii) HCNS would in turn provide a steric barrier to prevent the restacking of the graphene nanosheets. All these properties taken together would generate conductive electrode materials with accessible diffusion pathways for reactants.10,11 Despite of being attractive, direct assembling of HCNS into regular arrays on ultrathin free-standing graphene nanosheets remains quite challenging.12-15 In order to maintain the thin planar structure, the diameters of HCNS assembled into graphene should be less than 50 nm, which has rarely been reported.16 HCNS-generating methods that use hard-templates such as silica or polystyrene (PS) spheres are not able to generate sub-50 nm HCNS due to the aggregation of the nanotemplates. Recently our group used amphiphilic triblock copolymers (PS-b-P2VP-b-PEO) sacrificial templates to synthesize uniform HCNS with external diameters of ~43 nm and hollow cores ~19 nm in diameter.17 An alternative soft-templating method based on one-step self-assembly of amphiphilic polymers might be a powerful approach to not only control the diameter of the HCNS at nanoscale but also simultaneously pattern HCNS on 2D substrates (e.g., graphene). 3


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In this manuscript we describe how to assemble HCNS into regular arrays on graphene nanosheets and examine the electrocatalytic properties of these 2D hierarchical porous hybrid architectures. HCNS were prepared using nitrogen-containing melamine-formaldehyde resin (M-FR) as the carbon and nitrogen precursor, which was assembled on the PS-b-P2VP-b-PEO micelles. Graphene oxide (GO) instead of graphene was used as the initial 2D free-standing substrate due to its abundant functional groups that are easy to be modified. The M-FR coated monomicelles assembled on the 2D GO surfaces, generating monomicelle@M-FR/GO sandwich-like structure. The material was impregnated with iron precursors and calcinated to generate 2D Fe-N-C catalyst on the 2D sandwich-like nanostructures with sub-40-nm diameter HCNS surrounding the thin graphene sheets. The high surface area and uniform network of pores should enable good performance in electrocatalysis. We examined their performance as an electrocatalyst for the oxygen reduction reaction (ORR).

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Results and discussion

Scheme 1. An illustration of the synthetic process used to prepare 2D Fey-N-HCNS/rGO-T nanosheets. The proposed synthetic process is illustrated in Scheme 1. Graphene oxide (GO) nanosheets were prepared via the chemical exfoliation of graphite (Figure S1).18 The triblock copolymer PS-b-P2VP-b-PEO was initially pre-assembled into spherical micelles (Figure S2). The PS-b-P2VP-b-PEO micelles were mixed with melamine-formaldehyde resin (M-FR) and the pH was adjusted to ca. 4.70 with HCl. After 10 min, an aqueous GO solution was added dropwise and held at 60 oC for 12 hours. The resulting gray precipitate was washed carefully with deionized water to remove the HCl and residual reactant to generate sandwich-like nanosheets denoted here as monomicelle@M-FR/GO. After impregnation of iron (Fe) species, the obtained iron-decorated monomicelle@M-FR/GO precursors were called Fex-monomicelle@M-FR/GO, where x represents the initial mass concentration of Fe (2.5, 5.0, and 7.5 wt%). Finally, the material was carbonized at high temperature under inert gas to generate 2D graphene sandwich-structured composites patterned with

sub-40nm

hollow

iron

and

nitrogen-doped

carbon

nanospheres,

assigned

as

Fey-N-HCNS/rGO-T, where y represents the concentration of Fe (in wt%) and T represents the calcination temperatures (700, 800, and 900 oC). Scanning electron microscope (SEM) (Figure 1a) and transmission electron microscope (TEM) (Figure 1b) images of the representative Fe5.0-monomicelle@M-FR/GO precursors show that monolayered solid nanospheres with an average diameter of 88 nm assemble on the GO nanosheets. After carbonization at 900 oC, those solid nanospheres were transformed into single-layered hollow nanospheres with an external diameter of ~ 28 nm and a hollow core of ~ 20 nm (Figures 1c-d). 5


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TEM observations show that the monomicelle cores in the monomicelle@M-FR composite are removed during calcination, and leaving voids in the carbonized M-FR nanospheres. In order to obtain isolated Fe-based active sites, the Fe-N-C catalysts are usually etched by the hot acid and then calcinated a second time.19-21 It is noteworthy that our Fe1.6-N-HCNS/rGO-900 was still composed of dispersed thin nanosheets without serious aggregation even after undergoing two separate calcination steps (Figure S3). As shown in Figures 1d-1e, crystalline nanoparticles composed of Fe were not observed in the TEM and high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) images. Elemental mapping images in Figure 1e reveal the uniform distributions of Fe and N species in the Fe1.6-N-HCNS/rGO-900 nanosheets. However, the high-magnified HAADF-STEM image shows not only the high dispersity of single Fe atom but also few ultra-small Fe-based crystal nanoparticles (Figure S4), implying the existence of Fe-based active sites towards oxygen reduction (e.g., Fe-Nx, metallic Fe clusters and/or Fe3C active sites).

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Figure 1. SEM and TEM images of (a, b) Fe5.0-monomicelle@M-FR/GO precursor and (c, d) Fe1.6-N-HCNS/rGO-900 product; (e) HAADF-STEM image and the corresponding elemental mappings of the yellow square region marked in (d). In these experiments, assembling micelles into closely-packed arrangements on GO nanosheets is the key synthetic step. To determine the formation mechanism of the monomicelle@M-FR/GO nanosheets, the intermediate morphologies of the precursors at different reaction stages were collected and analyzed. As shown in Figure 2a, ultra-small monomicelle@M-FR nanospheres (less than 60 nm) were first observed after adding PS-b-P2VP-b-PEO micelles into the M-FR solution. Adding the GO nanosheets caused the monomicelle@M-FR nanospheres to adsorb onto the GO, 7


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forming sub-monolayers that grew denser and denser over time (Figures 2b-e). After 12 hours of reaction, a monolayer of monomicelle@M-FR nanospheres covered nearly the entire GO nanosheet surface (Figure S5). During the reaction, the M-FR gradually cross-links to form a high-molecular weight polymer that strengthened the sandwich-like nanosheets.22,23 It is worthwhile mentioning that if the GO was not added until the completion of the polymerization of low-molecular weight of monomicelle@M-FR, an irregular mixture of bended GO and aggregated monomicelle@M-FR was obtained (denoted as monomicelle@M-FR&GO) (Figure 2f). To obtain a better understanding of the role of PS-b-P2VP-b-PEO micelles, M-FR nanospheres coated GO nanosheets composite (M-FR/GO) was prepared in the same synthetic condition only without adding PS-b-P2VP-b-PEO micelles. SEM image in Figure S6 shows that inhomogeneous M-FR nanospheres were anchored randomly on part of GO surface. This result highlights the importance of PS-b-P2VP-b-PEO micelles on promoting the uniform coating of M-FR nanospheres. The mass ratio between monomicelle@M-FR nanospheres and GO nanosheets is another critical factor in the construction of perfect sandwich-like monomicelle@M-FR/GO nanosheets. As demonstrated in Figure S7, the corresponding mass ratio between melamine and GO nanosheets was altered from 36 to 84. We found that a mass ratio of 60 is needed to form the sandwich-like 2D graphene-based composites (Figure 1a and Figure S5). On the basis of above observations, the mechanism for assembling closely-packed monomicelle@M-FR nanospheres on GO nanosheets was proposed (Scheme 1). Adding PS-b-P2VP-b-PEO micelles to the M-FR solution caused the materials to crosslink to generate uniform M-FR coated micelle nanospheres (monomicelle@M-FR).22,23 GO surfaces have abundant oxygen-containing functional groups (e.g., hydroxyls, carboxyls, lactones, and ketones) that enable the M-FR to interact via hydrogen bonding to form M-FR coated GO nanosheets (GO@M-FR) (Figure S8),24 although depletion forces caused by unadsorbed molecules might also play a significant role in assembly. As a result, with increasing reaction time, the further condensation polymerization of M-FR between monomicelle@M-FR and GO@M-FR led to the complete coverage of the GO nanosheets. Additionally, over time even the edges of the GO nanosheets are covered with strongly-anchored monomicelle@M-FR nanospheres due to the cross-linking of M-FR (Figure 1a).

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Figure 2. SEM images of (a) monomicelle@M-FR nanospheres before adding GO solution, (b-e) monomicelle@M-FR/GO nanosheets in continuous stages, and (f) monomicelle@M-FR&GO.

After post-treatment of the Fex-monomicelle@M-FR/GO precursors at different temperatures, the Fey-N-HCNS/rGO-T catalysts were obtained and characterized. Metallic iron or iron carbides crystal were not detectable in the XRD patterns, regardless of calcination temperature and initial Fe concentration (Figure 3a and Figure S9a), consistent with TEM measurements (Figure 1e). This is probably due to the special structure of the 2D single-layered hollow Fe-N/C nanospheres patterned on rGO nanosheets, which likely prevents diffusion and aggregation of the reduced iron species during calcination. The surface area is likely influenced by the calcination temperature and initial concentration of Fe. When the concentration of Fe was fixed at 5.0 wt%, the surface areas of the Fe1.6-N-HCNS/rGO-T samples increased from 666.4 to 968.3 m2 g-1 when the calcination temperature was increased from 700 to 900 °C. The increase in surface area is caused by an increase in micropores (Figure 3b and Table 1). When the temperature was fixed at 900 oC, Fe0.4-N-HCNS/rGO-900 with the lowest initial Fe concentration of 2.5 wt% had the highest specific surface area of 1042.5 m2 g-1 (Figure S9b and Table 1), probably due to the weak catalytic graphitization effect of the low concentration of Fe on carbon.

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Figure 3. (a) XRD patterns, (b) N2 adsorption-desorption isotherms, (c) high resolution N 1s and (d) Fe 2p XPS spectra of Fe1.6-N-HCNS/rGO-T (T = 700, 800, and 900); (e) Fe K-edge Fourier transformed extended X-ray absorption fine structure (FT-EXAFS) and (f) Fe K-edge wavelet transformed (WT) EXAFS (WT-EXAFS) for Fe1.6-N-HCNS/rGO-900 and Fe foil.

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Table 1. Structural properties of the 2D hollow Fey-N-HCNS/rGO-T nanosheets. Samples Fe1.6-N-HCNS/ rGO-700 Fe1.6-N-HCNS/ rGO-800 Fe1.6-N-HCNS/ rGO-900 Fe0.4-N-HCNS/ rGO-900 Fe0.8-N-HCNS/ rGO-900

SBET (m2 g-1)

SMicro (m2 g-1)

SMicro/ SBET

VPore (cm3 g-1)

VMicro (cm3 g-1)

VMicro/VPore

666.4

116.0

0.17

1.305

0.044

0.03

819.5

202.8

0.25

1.648

0.086

0.05

968.3

294.4

0.30

1.720

0.102

0.06

1042.5

368.1

0.35

2.147

0.152

0.07

926.1

254.9

0.28

1.906

0.107

0.06

SBET: Specific surface area; SMicro: Micropore surface area; VPore: Pore volume; VMicro: Micropore volume. Meanwhile, the surface elemental composition of Fey-N-HCNS/rGO-T nanosheets was investigated with X-ray photoelectron spectroscopy (XPS). By increasing the calcination temperature, the total nitrogen (N) concentration was gradually reduced from 18.3 at% (Fe1.6-N-HCNS/rGO-700) to 12.0 at% (Fe1.6-N-HCNS/rGO-800) and 6.5 at% (Fe1.6-N-HCNS/rGO-900) because of the loss of unstable N.25-27 The high-resolution N 1s spectra can be further deconvoluted into two main peaks located at 398.5 eV and 401.0 eV, which can be attributed to pyridinic-N and graphitic-N (Figure 3c and Figure S9c), respectively. These two types of N are critical in the formation of high-performance active sites for oxygen reduction reaction (ORR).28,29 The ratio of pyridinic-N to graphitic-N is expected to decrease at higher calcination temperatures, because pyridinic-N is less stable than graphitic-N at elevated temperatures (Figure S10).7, 30,31 As shown in Figure 3d and Figure S9d, the Fe 2p high-resolution XPS spectra of Fey-N-HCNS/rGO-T can be deconvoluted into two major peaks at 710.9 eV (Fe2+ 2p3/2) and 723.8 eV (Fe2+ 2p1/2), and a satellite peak at 714.7 eV (Fe3+ 2p3/2).32-34 Metallic iron or iron carbide crystals could not detected by XPS, probably because the concentration is below the detection limit of XPS. To elucidate the structure of the Fe species, the K-edge of the X-ray near edge structures (XANES) of the Fe1.6-N-HCNS/rGO-900 sample was measured, using Fe foil as a reference (Figure S11a). 11


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The XANES curve of Fe1.6-N-HCNS/rGO-900 has a higher near-edge absorption energy than Fe foil. This observation implies the presence of Fe atoms that are positively charged. Interestingly, Fe1.6-N-HCNS/rGO-900 displays a main peak at about 1.4 Å in Fe K-edge Fourier transformed extended X-ray absorption fine structure (FT-EXAFS) in Figure 3e without phase correction. This is the location of the first coordination shell of Fe−N(C), indicating that most of the Fe atoms are atomically dispersed.35,36 The small peak at ~2.5 Å likely belongs to Fe−Fe, implying there is some metallic Fe in the Fe1.6-N-HCNS/rGO-900 sample. Due to the powerful resolutions in both k and R spaces, Fe K-edge wavelet transformed (WT) EXAFS (WT-EXAFS) was conducted to more carefully examine the atomic dispersion of Fe. Figure 3f shows the WT-EXAFS maximum at 5 Å−1 for Fe1.6-N-HCNS/rGO-900 could be assigned to the Fe−N(C) bonding. A peak at 8 Å−1 in the WT-EXAFS plot corresponds to Fe-Fe bonds.35,36 Our sample had no peak at 8 Å−1, whereas the Fe foil standard had a strong peak as expected. EXAFS fitting was performed to confirm that Fe is coordinated with N, and to quantitatively determine the chemical configuration of the Fe atom. The fitting curves are shown in Figure S11b and the fitting parameters are listed in Table S1. The data indicates that the coordination number of the center atom of Fe in the Fe1.6-N-HCNS/rGO-900 is ~ 4 N atoms, and the average Fe-N bond length is 1.98 Å. Therefore, considering HAADF-STEM and XAFS results together, we are confident that: (1) most of the Fe atoms are atomically dispersed in the nitrogen-doped carbon framework, (2) the Fe atom is coordinated by ~ 4 N atoms, and (3) there is only trace ultrafine Fe clusters in the Fe1.6-N-HCNS/rGO-900 sample. Considering the small quantities of Fe-based active sites, sensitive cyclic voltammetry measurements were performed to identify the type of Fe species. As seen in Figure S12, cyclic voltammogram shows a very weak oxidation peak at -0.71 V (vs. Hg/HgO) and a reduction peak at -1.03 V (vs. Hg/HgO). These features can be assigned to the Fe(II)/Fe(0) redox couple,37 verifying the existence of trace metallic Fe in Fe1.6-N-HCNS/rGO-900. The high surface area and the 2D sandwich-like architecture of Fey-N-HCNS/rGO-T expose numerous active sites to electrolytes and enhance the mass transfer of the relevant species (e.g., H+/OH- and O2) in ORR. These features make Fey-N-HCNS/rGO-T a promising ORR catalyst. The electrocatalytic activity of 2D Fey-N-HCNS/rGO-T for ORR was evaluated initially by cyclic voltammetry (CV) in an O2-saturated 0.1 M KOH and 0.1 M HClO4 solutions (Figures 4a, S13a, 12


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S14a, and S15a). The typical oxygen reduction peaks of Fey-N-HCNS/rGO-T catalysts occurred in a potential range of 0.7-0.9 V and 0.5-0.7 V in alkaline and acidic solutions, respectively. As expected, calcination temperature and the Fe concentration play a crucial role in ORR performance because we already show that they determine the mesoscale and atomic scale structure of the catalyst. Compared with other catalysts, the Fe1.6-N-HCNS/rGO-900 with the highest Fe concentration has the most positive potential peak at 0.839 V and the highest current density in 0.1 M KOH. This can probably be attributed to the combined effects of abundant active sites including pyridinic-N, graphitic-N, and Fe-based active sites, a high effective surface area, a high electron conductivity and so on. To examine ORR performance of the Fe-based active sites, rotating ring-disk electrode (RRDE) measurements were conducted on the 2D Fey-N-HCNS/rGO-T and N-HCNS/rGO-900 samples. As expected, the N-HCNS/rGO-900 had worse ORR activity than Fe1.6-N-HCNS/rGO-700, regardless of electrolytes (Figures 4b, S13b, S14b, and S15b). This observation implies that the formation of Fe-based active sites in these samples is essential for catalyzing ORR. However, compared with N-HCNS&rGO-900 which was the mixture of aggregated N-HCNS and bended rGO nanosheets (Figure S16), N-HCNS/rGO-900 has a more positive half-wave potential (E1/2) and a higher diffusion-limited current density (jL), clearly demonstrating the structural advantage of the 2D sandwich-like architecture. Here rGO nanosheets maintain the dispersion of N-HCNS, resulting in more exposure of N-HCNS to the electrolyte and improving the utilization efficiency of active sites. N-HCNS in turn prevents re-stacking of the rGO nanosheets. On the other hand, rGO nanosheets can accelerate the electron transfer between N-HCNS and reduce the electrochemical polarization during ORR processes. Thus, such 2D sandwich-like N-HCNS/rGO-900 shows higher ORR activity than N-HCNS&rGO, and should serve as a better support for Fe-based active sites. Linear sweep voltammetry (LSV) measurements performed on the Fe1.6-N-HCNS/rGO-900 sample shows it has the best ORR performance, with more positive E1/2 and high jL among all the Fey-N-HCNS/rGO-T

catalysts

(Figures

4b,

S13b,

S14b,

and

S15b).

Remarkably,

Fe1.6-N-HCNS/rGO-900 exhibited a more positive E1/2 of 0.872 V than that of commercial Pt/C catalyst (E1/2=0.860 V), as well as the same jL of 5.58 mA cm-2 with Pt/C catalyst in alkaline condition. In acid conditions, Fe1.6-N-HCNS/rGO-900 has a jL of 5.68 mA cm-2 that approaching to Pt/C (jL=5.82 mA cm-2), and a high E1/2 of 0.683 V that is only 85 mV negative to Pt/C catalyst (Figure S14b). The Fe1.6-N-HCNS/rGO-T calcinated under much higher temperature (e.g., 950 and 13


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1000 °C) was not investigated in the present study because of low yield of carbonized product under the temperature higher than 950 oC. To further investigate ORR kinetics of Fe1.6-N-HCNS/rGO-900, LSV curves were recorded under different rotating speeds from 625 to 2500 rpm (Figure 4c and Figure S14c). The corresponding Koutecky-Levich (K-L) plots under different potentials are linear as shown in the inset of Figure 4c and Figure S14c. The number of transferred electrons (n) per oxygen molecule for Fe1.6-N-HCNS/rGO-900 was calculated based on K-L equation to be 3.98-4.02 between 0.40 and 0.70 V in 0.1 M KOH and 3.98-4.07 between 0.20 and 0.50 V in 0.1 M HClO4. Thus, an ideal four-electron ORR mechanism on Fe1.6-N-HCNS/rGO-900 was confirmed regardless of electrolytes. From RRDE experiments (Figure S17), the value of n for Fe1.6-N-HCNS/rGO-900 was estimated to be 3.91-3.99 between 0.2 and 0.8 V in 0.1 M KOH and 3.83-3.92 between 0.1 to 0.7 V in 0.1 M HClO4, further implying a highly efficient four-electron ORR process. Correspondingly, the yield of H2O2 was calculated to be < 5% and < 9% in 0.1 M KOH and 0.1 M HClO4 respectively. Such high n and low H2O2 yields demonstrate that the Fe1.6-N-HCNS/rGO-900 sample is a highly efficient and promising catalyst in ORR in both alkaline and acidic conditions. In order to determine the existence of Fe-Nx active sites for ORR, LSV curves were measured in the SCN- containing O2-saturated electrolytes. It is known that Fe-Nx sites can be poisoned by CN-,38,39 SCN-,40 and H2S,41 resulting in a loss of catalytic activity for ORR. As shown in Figure S18, Fe1.6-N-HCNS/rGO-900 shows a distinct degradation of catalytic activity for ORR especially in acidic electrolyte, which could be attributed to the deactivation of the essential Fe-Nx active sites by SCN-.32,40,42 However, Fe1.6-N-HCNS/rGO-900 still shows significantly better ORR activity than N-HCNS/rGO-900 after SCN- poisoning, implying the existence of other kinds of Fe-based active sites (e.g., metallic Fe clusters). Therefore, the superior ORR activity should be attributed to the co-existence of various kinds of Fe-based active sites. Moreover, the monodispersed 2D hollow structure of Fe1.6-N-HCNS/rGO-900 nanosheets also plays important role. On one hand, the single-layered hollow N/C nanospheres anchored on rGO nanosheets favor the homogeneous dispersion of Fe species, which increased the density of Fe-based active sites. On the other hand, the thin planar structure endows the Fe1.6-N-HCNS/rGO-900 catalyst with more exposed surface area, which facilitate the diffusion of both reactants and ORR-relevant products in the ORR process. Furthermore, the electrons can be rapidly transferred within the rGO plane that is linked to each 14


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hollow Fe-N/C nanospheres. This feature will reduce electrochemical polarization during ORR. The long term durability of Fe1.6-N-HCNS/rGO-900 for ORR was examined by cycling the potential between 0.6 and 1.0 V in O2-saturated 0.1 M KOH and between 0.5 and 0.9 V in O2-saturated 0.1 M HClO4 for 5000 times, respectively. As shown in Figure 4d, the E1/2 of Fe1.6-N-HCNS/rGO-900 slightly decreased by 15.0 mV after 5000 cycles, whereas Pt/C underwent a significant negative shift of 26.0 mV under alkaline condition. Similarly, as shown in Figure S14d, the E1/2 negative shift of 41.0 mV was observed on Fe1.6-N-HCNS/rGO-900 under acidic condition, which indicated a better stability than Pt/C (70 mV negative shift). The methanol crossover effect is a key obstacle for using electrocatalysts in direct methanol fuel cells. Thus, the chronoamperometric response measurement was carried out to estimate the tolerance of Fe1.6-N-HCNS/rGO-900 and Pt/C catalysts to methanol in O2-saturated 0.1 M KOH electrolyte by subsequent addition of methanol (Figure S19). The negative current of 20 wt% Pt/C catalyst changed to positive current after addition of 3 M methanol. Surprisingly, Fe1.6-N-HCNS/rGO-900 electrodes showed no obvious change in the ORR current upon addition of methanol, demonstrating that Fe1.6-N-HCNS/rGO-900 has a good tolerance to methanol poisoning. The results highlight the great potential of Fe1.6-N-HCNS/rGO-900 as noble-metal-free carbon-based catalyst in direct methanol fuel cells.

15


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Figure 4. (a) Cyclic voltammograms of Fey-N-HCNS/rGO-900 with a scan rate of 50 mV s-1; (b) LSV curves of Fey-N-HCNS/rGO-900, N-HCNS/rGO-900, N-HCNS&rGO-900, and Pt/C at 1600 rpm; (c) LSV curves of Fe1.6-N-HCNS/rGO-900 at different rotating speeds (insert is the corresponding Koutecky-Levich plots); and (d) LSV curves of Fe1.6-N-HCNS/rGO-900 and Pt/C catalysts at 1600 rpm before and after 5000 CV cycles. The electrolyte is 0.1 M KOH and the scan rate for (b-d) is 10 mV s-1.

Conclusion In summary, we described a simple method to generate 2D monomicelle@M-FR coated graphene oxide nanosheets by taking advantage of the self-assembly of asymmetric triblock copolymer micelles, melamine-formaldehyde resin, and graphene oxide nanosheets. Calcination and 16


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iron doping produces an ORR active catalyst which is composed of sub-40nm hollow iron-nitrogen-doped carbon nanospheres patterned on graphene nanosheets. In addition to pyridinic-N and graphitic-N active sites for ORR, the atomic Fe-N4 coordinate sites were well identified via X-ray absorption near edge structure analyses and electrochemical analyses. Electrochemical measurements show that the best Fe1.6-N-HCNS/rGO-900 catalyst has an impressive electrocatalytic activity for ORR in alkaline medium. Fe1.6-N-HCNS/rGO-900 featured a 12.0 mV of half-wave potential higher than 20 wt% Pt/C, a high diffusion-limited current density, an efficient four-electron ORR process, a substantially better durability, and a stronger resistance to methanol poisoning than 20 wt% Pt/C catalysts. Moreover, it is believed that this synthesis strategy can be further extended to prepare various 2D hollow non-precious metal-based carbon/rGO catalysts by using different metal- and nitrogen-doped carbon precursors.

Experimental Methods Materials.

Triblock

copolymer

of

poly(styrene-b-2-vinyl

pyridine-b-ethylene

oxide)

(PS(20,000)-b-P2VP(15,000)-b-PEO(27,000)) was bought from Polymer Source Inc. Melamine was purchased from Sigma-Aldrich. Formaldehyde solution (37 wt%), iron(III) chloride hexahydrate (FeCl3·6H2O), tetrahydrofuran (THF), and hydrochloric acid (2 M) were purchased from Nacalai Tesque. All chemicals were used directly without further purification. Preparation of graphene oxide (GO) nanosheets. The preparation of graphene oxide (GO) nanosheets followed procedures reported previously in the literature.18 In brief, 1.0 g of graphite powder was added to a mixture of concentrated nitric acid (15.8 M) and concentrated sulfuric acid (18.4 M) in a vial with a volume ratio of 1:2. The mixture was kept at 80 °C for 5 h before it was cooled down to room temperature. The mixture was then diluted with deionized (DI) water and left to stand overnight. The as-obtained pre-oxidized GO nanosheets in a vial was immersed an ice bath. 5.0 g of potassium permanganate was added slowly while stirring the solution for 2 h. Afterward, the solution is diluted with DI water followed by adding 30 wt% H2O2 until there was no bubbles generation. During this process, the color of the mixture changed from dark green to bright yellow. Finally, the products were centrifuged and washed with a dilute HCl solution (1:10 v/v), DI water and ethanol for multiple times. The final GO nanosheet powder can be obtained through freeze drying. Synthesis of PS-b-P2VP-b-PEO micelle aqueous solution. Typically, 0.125 g of triblock 17


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copolymer PS(20,000)-b-P2VP(15,000)-b-PEO(27,000) was completely dissolved in 24.5 mL of THF. Micellization of the triblock copolymer was stimulated by adding 500 µL of concentrated concentrated hydrochloric acid (HCl, 35 wt%) under stirring. Then the milk-white solution was transferred into a dialysis membrane tube (Mw cut-off: 14,000 Da), and was dialyzed against DI water for several cycles to remove HCl and THF. After dialysis process, a clear PS(20,000)-b-P2VP(15,000)-b-PEO(27,000) micelle aqueous solution was obtained and the concentration was ~ 3.6 mg mL-1. Synthesis of 2D hollow Fey-N-HCNS/rGO-T. A soluble melamine resol solution was initially prepared by the following process. 0.252 g of melamine was mixed with 640 µL formaldehyde (37 wt%) aqueous solution and 2 mL H2O and reacted at 70 oC under stirring for 5 min. Afterwards, 40 mL

H2O

was

added

to

above

transparent

clear

solution.

After

cooling,

4.7

mL

PS(20,000)-b-P2VP(15,000)-b-PEO(27,000) micelle aqueous solution was added to above solution and the initial pH was adjusted to ~ 4.70 by adding 2 M HCl aqueous solution. Then the mixed solution was reacted at 60 oC under stirring for 10 min. Afterwards, 8.0 mL GO aqueous solution (0.5 mg mL-1) was added into the above mixture drop by drop and was kept reaction for another 12 h. The generated gray precipitate was washed at least three times by using DI water to remove the residual reactant, and was collected by centrifugation at 14,000 rpm. The obtained precipitate was re-dispersed in DI water under sonication and mixed with a certain amount of 100 mM FeCl3 aqueous

solution.

The

products

with

various

initial

Fe

concentration

(denoted

as

Fex-monomicelle@M-FR/GO) were prepared for investigating the effect on electrocatalytic activity with various Fe loading concentration, where x represents the initial Fe concentration (x = 2.5, 5.0 and 7.5 wt%). After drying, the gray products were pyrolyzed at controlled temperatures in a flowing N2 atmosphere with a ramp rate of 2 °C min-1, and was kept at 400 oC and the target temperature (700, 800, and 900 °C) for 0.5 and 2 h, respectively. The carbonized product was leached in 0.5 M H2SO4 at 80 °C for 8 h under stirring and a second heat treatment was operated in flowing N2 atmosphere at the same target temperatures with ramping rate of 5 °C min-1. The finale carbonized product was assigned as Fey-N-HCNS/rGO-T, where y and T represent the concentration of Fe (wt%) and the target calcination temperature (700, 800, and 900 oC), respectively. Correspondingly, the finale carbonized product without Fe dopant was assigned as N-HCNS/rGO-T. As comparison, M-FR coated micelle nanospheres were physically mixed with GO nanosheets to produce monomicelle@M-FR&GO. The corresponding carbonized product was assigned as N-HCNS&rGO-T. Similarly, the M-FR coated micelle (monomicelle@M-FR) nanospheres was first prepared without adding GO. After washing with DI water to remove residual reactant, the generated white precipitate was dispersed in 42 mL DI water using sonication, and was mixed with 8.0 mL GO 18


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aqueous solution (0.5 mg mL-1). The pH of mixture was adjusted to ~ 4.70 by using 2 M HCl aqueous solution and stirred for 12 h. The monomicelle@M-FR&GO precipitate was collected by centrifugation and washed with DI water. Characterization. Fourier-transform infrared spectroscopy (FTIR, Thermoscientific Nicolet 4700) was performed to analyze the functional groups of GO nanosheets. The morphology of the as-prepared materials was initially observed on a field emission scanning electron microscope (FESEM, HITACHI SU-8230) with an accelerating voltage of 5 kV. The hollow structure of the as-prepared nanosheets were observed by transmission electron microscope (TEM, JEOL JEM-2100F) which was operated at 200 kV. The exact Fe concentration of carbonized Fey-N-HCNS/rGO-T material was estimated by inductively coupled plasma‒optical emission spectrometry (ICP-OES) (SPS3520UV-DD, Hitachi High-Technologies Co.). Wide-angle X-ray diffraction (XRD) patterns were measured on Rigaku Rint 2000 X-ray diffractometer with Cu Kα radiation at 40 kV and 40 mA, and the scanning rate is 1° min-1. The nitrogen adsorption-desorption isotherms were obtained by using a Belsorp 28 apparatus (Bel Japan, Inc.) at 77 K. The chemical state of nitrogen and iron was studied using X-ray photoelectron spectroscopy (PHI Quantera SXM) with Al Kα radiation at 20 kV and 5 mA. The C1s level at 284.5 eV was used to calibrate the shift of binding energy. Fe K-edge X-ray absorption spectra (XAFS) were obtained on 1W1B beamline of the Beijing Synchrotron Radiation Facility (BSRF) in fluorescence mode at a room temperature and utilizing a Si (111) double-crystal monochromator. The storage ring of BSRF was performed at 2.5 GeV with a maximum current of 250 mA in decay mode. After carrying out the standard procedures with the ATHENA program, the raw data of XAFS have been background-subtracted, normalized, and Fourier transformed. Least-squares curve fitting of the extended X-ray absorption fine structure χ(k) data was analyzed by using the ARTEMIS program. All fits were carried out in the R space with k-weight of 3. Electrochemical measurements. All as-prepared catalysts were ground before preparing inks. Commercial Pt/C (20 wt%, from Alfa Aesar, USA) served as a reference. In a typical experiment, 5 mg of catalyst was dispersed in 950 µL of isopropanol/water (1:2 v/v) mixture. Then 50 µL of 5.0 wt% Nafion solution was added and the suspension was further sonicated to obtain a homogenous ink. Then 5 µL of the black ink was dropped on a 4-mm diameter glassy carbon (GC) electrode. The GC electrode was pre-polished with 1 µm and 0.05µm alumina powder and was washed with water. The loading amounts of the commercial Pt/C and as-prepared catalysts were 0.2 mg cm-2. Electrochemical analyses. The details of electrochemical tests for estimating the oxygen reduction reaction (ORR) activity were reported in our previous work.43 All electrochemical analyses were 19


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performed in a conventional three-electrode cell using CHI 842B electrochemical analyzer (CH Instrument, USA). The catalyst film coated RRDE was the working electrode, while the Pt wire and the saturated calomel electrode (SCE) served as the counter electrode and reference electrode, respectively. The electrolytes were O2/N2 saturated 0.1 M KOH or 0.1 M HClO4 aqueous solutions and the flow of O2/N2 was maintained during tests. According to the Nernst equation (ERHE=E(SCE)+0.0591*pH+0.241), all potential values were normalized to the reversible hydrogen electrode (RHE) in this study. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements were conducted from 1.2 to 0.0 V. The scan rate for CV and LSV were 50 and 10 mV s-1, respectively. The rotating speeds for RRDE electrode ranged from 625 to 2500 rpm, while the potential for ring electrode was held at 1.2 V vs. RHE. Durability tests performed on the Fey-N-HCNS/rGO-T and Pt/C catalysts were conducted by cycling between 0.6 and 1.0 V in O2-saturated 0.1 M KOH and between 0.5 and 0.9 V in O2-saturated 0.1 M HClO4 at a scan rate of 50 mV s-1, respectively. The current density was calculated on the basis of the geometrical area (0.1256 cm2) of rotating disk electrode after correction of double layer capacitance. The Koutecky–Levich (K-L) equation was used to calculate the number of transferred electron (n):

1 1 1 1 1 = + = + (1)

/ = 0.62 / / (2) where j is the current density, jL and jK represent the diffusion-limited current density and the kinetic current density, respectively, ω means the angular velocity, n is the number of transferred electron, C0 represents the concentration of O2 (C0 = 1.2×10-6 mol cm-3), F is the Faraday constant (F = 96485 C mol-1), D0 is the diffusion coefficient of O2 (D0 = 1.9×10-5 cm2 s-1), and is the kinematic viscosity of the electrolyte ( = 0.01 cm2 s-1). Value of B can be calculated from the slope of the K-L equation. Based on RRDE measurements, n and the percentage of hydrogen peroxide yield (%H2O2) released during ORR can be calculated by following equations:

=

4 (3) + ( /!)

%H O = 200 ×

/! (4) + ( /!)

where, Id and Ir are the Faradaic current at the disk and the ring, respectively, N is the H2O2 collection coefficient of ring and the value is 0.4. Tolerance to methanol. The amperometric current-time chronoamperometric response measurement was carried out to estimate the tolerance of Fe1.6-N-HCNS/rGO-900 and Pt/C catalysts to methanol. 20


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The i-t chronoamperometric curve was recorded for 600 s at 0.65 V vs. RHE at 1600 rpm in O2-saturated 0.1 M KOH solution. Then the i-t chronoamperometric curve was recorded for another 600 s after the addition of 3 M methanol. SCN- poison measurement on Fe1.6-N-HCNS/rGO-900. The SCN- poison measurement on Fe1.6-N-HCNS/rGO-900 was carried out by adding 50 mM NaSCN into O2-saturated 0.1 M KOH and 0.1 M HClO4 at a scan rate of 10 mV s-1 and a rotation speed of 1600 rpm, respectively.

Supporting Information Available: Detailed characterizations of the structure of the graphene oxide nanosheets, micelles, GO nanosheets@M-FR spheres, N-HCNS&rGO-900 composite are available in the supporting information. This includes the structural details of Fey-N-HCNS/rGO-T with different Fe concentrations and processed at different calcination temperatures, and parameters for XANES and EXAFS fitting. Details of the electrochemical analyses are described as well, including analysis of Fey-N-HCNS/rGO-T in acidic conditions, and CV measurements used to identify active sites. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements This work was supported by an Australian Research Council (ARC) Future Fellow (Grant No. FT150100479), JSPS KAKENHI (Grant Numbers 17H05393 and17K19044), and the research funds by Qingdao University of Science and Technology and the Suzuken Memorial Foundation. The authors would like to thank New Innovative Technology (NIT) for helpful suggestions and discussions on materials fabrication. J.T was supported by Japan Society for the Promotion of Science International Research Fellows (JSPS project no. 17F17080).

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References (1) Zhu, Q.-L.; Xia, W.; Akita, T.; Zou, R.; Xu, Q. Metal-Organic Framework-Derived Honeycomb-Like Open Porous Nanostructures as Precious-Metal-Free Catalysts for Highly Efficient Oxygen Electroreduction. Adv. Mater. 2016, 28, 6391-6398. (2) Wu, Z.; Lv, Y.; Xia, Y.; Webley, P. A.; Zhao, D. Ordered Mesoporous Platinum@Graphitic Carbon Embedded Nanophase as a Highly Active, Stable, and Methanol-Tolerant Oxygen Reduction Electrocatalyst. J. Am. Chem. Soc. 2012, 134, 2236-2245. (3) Wan, Y.; Wang, H.; Zhao, Q.; Klingstedt, M.; Terasaki, O.; Zhao, D. Ordered Mesoporous Pd/Silica−Carbon as a Highly Active Heterogeneous Catalyst for Coupling Reaction of Chlorobenzene in Aqueous Media. J. Am. Chem. Soc. 2009, 131, 4541-4550. (4) Ma, Q.; Song, H.; Zhuang, Q.; Liu, J.; Zhang, Z.; Mao, C.; Peng, H.; Li, G.; Chen, K. Iron-Nitrogen-Carbon Species Boosting Fast Conversion Kinetics of Fe1-xS@C Nanorods as High Rate Anodes for Lithium Ion Batteries. Chem. Eng. J. 2018, 338, 726-733. (5) Aijaz, A.; Fujiwara, N.; Xu, Q. From Metal–Organic Framework to Nitrogen-Decorated Nanoporous Carbons: High CO2 Uptake and Efficient Catalytic Oxygen Reduction. J. Am. Chem. Soc. 2014, 136, 6790-6793. (6) Sun, Z.; Sun, B.; Qiao, M.; Wei, J.; Yue, Q.; Wang, C.; Deng, Y.; Kaliaguine, S.; Zhao, D. A General Chelate-Assisted Co-Assembly to Metallic Nanoparticles-Incorporated Ordered Mesoporous Carbon Catalysts for Fischer–Tropsch Synthesis. J. Am. Chem. Soc. 2012, 134, 17653-17660. (7) Tang, J.; Liu, J.; Li, C.; Li, Y.; Tade, M. O.; Dai, S.; Yamauchi, Y. Synthesis of Nitrogen-Doped Mesoporous Carbon Spheres with Extra-Large Pores through Assembly of Diblock Copolymer Micelles. Angew. Chem. Int. Ed. 2015, 54, 588-593. (8) Liu, J.; Wickramaratne, N. P.; Qiao, S. Z.; Jaroniec, M. Molecular-Based Design and Emerging Applications of Nanoporous Carbon Spheres. Nat. Mater. 2015, 14, 763. (9) Tang, J.; Liu, J.; Salunkhe, R. R.; Wang, T.; Yamauchi, Y. Nitrogen-Doped Hollow Carbon Spheres with Large Mesoporous Shells Engineered from Diblock Copolymer Micelles. Chem. Commun. 2016, 52, 505-508. (10) Lee, J. H.; Park, N.; Kim, B. G.; Jung, D. S.; Im, K.; Hur, J.; Choi, J. W. Restacking-Inhibited 3D Reduced Graphene Oxide for High Performance Supercapacitor Electrodes. ACS Nano 2013, 7, 22


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9366-9374. (11) Yang, X.; Zhu, J.; Qiu, L.; Li, D. Bioinspired Effective Prevention of Restacking in Multilayered Graphene Films: Towards the Next Generation of High-Performance Supercapacitors. Adv. Mater. 2011, 23, 2833-2838. (12) Li, M.; Zhang, Y.; Yang, L.; Liu, Y.; Yao, J. Hollow Melamine Resin-Based Carbon Spheres/Graphene Composite with Excellent Performance for Supercapacitors. Electrochim. Acta 2015, 166, 310-319. (13) Hu, H.; Wang, X.; Wang, J.; Wan, L.; Liu, F.; Zheng, H.; Chen, R.; Xu, C. Preparation and Properties

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Assembly of Hollow Carbon Nanospheres on Graphene Nanosheets

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