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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*,⊥,# †
International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan § Department of Automotive Engineering, School of Transportation Science and Engineering, Beihang University, Beijing 100191, P.R. China ∥ Beijing Synchrotron Radiation Facility (BSRF), Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China ∧ Australian Institute for Innovative Materials (AIIM), University of Wollongong (UOW), North Wollongong, NSW 2500, Australia ⊥ School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland, Brisbane, Queensland 4072, Australia # Department of Plant and Environmental New Resources, Kyung Hee University, 1732 Deogyeong-daero, Giheunggu, Yongin-si, Gyeonggi-do 446-701, South Korea S Supporting Information *
ABSTRACT: Triblock copolymer micelles coated with melamine-formaldehyde resin were self-assembled into closely packed two-dimensional (2D) arrangements on the surface of graphene oxide sheets. Carbonizing these structures created a 2D architecture composed of reduced graphene oxide (rGO) sandwiched between two monolayers of sub-40 nm diameter hollow nitrogen-doped carbon nanospheres (N-HCNS). Electrochemical tests showed that these hybrid structures had better performance for 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 fine structure analyses showed that most of the Fe in the nitrogendoped 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 limiting current density. Bottom-up soft-patterning of regular carbon arrays on free-standing 2D surfaces should enable 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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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
arbon is a ubiquitous material in electrocatalysis because carbon is chemical/mechanical stable, generates controllable porous architectures, and supports high conductivities all at a cost that is difficult to achieve with any other material. The microarchitecture and dimensions of carbon supports influence the kinetics of catalytic processes in part by increasing the accessibility of metal active sites and enhancing charge transfer in electrocatalysis.1−4 Porous carbon nanomaterials with © XXXX American Chemical Society
Received: February 26, 2018 Accepted: May 3, 2018 Published: May 3, 2018 A
DOI: 10.1021/acsnano.8b01502 ACS Nano XXXX, XXX, XXX−XXX
Article
Cite This: ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano Scheme 1. An Illustration of the Synthetic Process Used To Prepare 2D Fey-N-HCNS/rGO-T Nanosheets
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).
carbons, hollow carbon nanospheres (HCNS) are an interesting candidate support material 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 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 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
