Article pubs.acs.org/cm
Atomic Fe Embedded in Carbon Nanoshells−Graphene Nanomeshes with Enhanced Oxygen Reduction Reaction Performance Congwei Wang,† Huinian Zhang,†,‡ Junying Wang,† Zheng Zhao,†,‡ Jie Wang,†,‡ Yan Zhang,†,‡ Miao Cheng,†,‡ Huifang Zhao,†,‡ and Junzhong Wang*,† †
CAS Key Laboratory of Carbon Materials, State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *
ABSTRACT: Low-cost preparation of durable electrocatalysts is vital for energy storage and conversion. Here, we integrated two methods of synthesizing isolated iron atoms into a special carbon matrix as an advanced electrocatalyst. Atomic Fe isolation and graphene nanomeshes or curved carbon nanoshells were almost synthesized simultaneously. The hierarchical atomic Fe/ carbon material with 0.53 atom % Fe exhibited superior oxygen reduction reaction (ORR) performance to Pt−C (20 wt % Pt) with 40 mV more positive onset potential, larger current density, and stronger methanol-tolerant capability. We demonstrated that the catalytic active sites were Fe isolation and coordinated with nitrogen in the porous curved carbon-graphene matrix. This strategy could be developed into a general approach to prepare atomic metal/carbon electrocatalysts.
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INTRODUCTION Boosting the sluggish kinetics of the cathodic oxygen reduction reaction (ORR) is crucial in optimizing electrochemical energy conversion efficiency of fuel cells and metal−air batteries.1 Precious metal Pt and its alloys loaded on carbon are excellent electrocatalysts in ORR, but suffer from high cost and methanol poisoning. Cost-effective non-precious-metal electrocatalysts have attracted intense attention. The transition metals, featured as the unsaturated d subshell, such as Fe, Co, Ni, etc., have shown to be suitable candidates as efficient and durable ORR catalysts.2−5 Specifically, nanostructured Fe-based alloys and derivatives with modified electronic structures have exhibited excellent ORR performances.5−7 However, the heart of improving Fe-based catalysts lies in the mystery of the atomic-scale structures of active sites. The catalytic mechanism, regarding the role of transition-metal precursors as the central component of the active site structure or as a template or catalyst to assist formation of C−N active sites, is still debated.6,8 Strategies for increasing the number of active sites, either by reducing the size of catalyst9,10 or by introducing nanoparticles with a high index,11 have been demonstrated as effective. Remarkably, single-atom catalysts (SACs), which represent the lowest size limit to obtain full atom utility in a catalyst, have © 2017 American Chemical Society
recently emerged as a research gold mine. Indeed, a growing number of SAC systems has been reported, such as Pt, Au, Pd, Fe, etc.,12−18 among which the transition-metal SAC systems have triggered more interest due to their inexpensive, abundant, and variability nature.15−18 However, the practical wide employment of SAC is hampered mainly because of the sideeffects of downsizing the metal to the subnanometer or the fact that single atoms are structurally unstable. Even under inert environments, their high reactivity could bring serious aggregation and coarsening, interfering with the dispersion of active sites and limiting the catalytic durability and efficiency.19,20 Among the current SAC substrate, graphene (G) stands out in its unique place owing to its intriguing physicochemical features.21−23 More interestingly, proper chemical manipulation and altering of the electronic properties of G through heteroatom doping could cause electron modulation to provide a desirable surface electronic structure for both the catalytic process and SAC deposition of practical significance;14,16,17 meanwhile, introducing surface engineering to make porous instead of intact G could further enhance the Received: July 23, 2017 Revised: October 30, 2017 Published: November 10, 2017 9915
DOI: 10.1021/acs.chemmater.7b03100 Chem. Mater. 2017, 29, 9915−9922
Article
Chemistry of Materials
Figure 1. Schematic illustration of the synthesis of atomic Fe into N-doped graphene nanomeshes coated by hollow carbon nanoshell (Fe−NGM/ C−Fe).
Figure 2. In situ TEM images of thermally etching and doping a graphene flake into graphene nanomeshes through annealing FeSO4 and melaminedecorated graphene at the temperature range 350−1300 °C.
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DOI: 10.1021/acs.chemmater.7b03100 Chem. Mater. 2017, 29, 9915−9922
Article
Chemistry of Materials
Figure 3. (a) HAADF image of Fe−NGM; the arrow shows the nanopores on G sheets. (b) High-resolution HAADF image of Fe−NGM; the inset shows the FFT pattern of the G sheet in part b. (c) Fe−NGM/C−Fe; the arrows highlight the nanoholes of the carbon matrix with folded graphene. (d) High-resolution HAADF image of Fe−NGM/C−Fe; the densely dispersed atomic iron in both carbon shell and G nanomeshes. (e) EELS atomic spectrum of Fe−NGM/C−Fe.
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RESULTS AND DISCUSSION The synthesis of targeted material Fe−NGM/C−Fe is illustrated in Figure 1. It included two-stage annealing and an in situ N-doping process for forming graphene nanomeshes followed by coating with carbon nanoshells embedded with atomic Fe. Electrochemically exfoliated graphene sheets with an average of ∼20 μm literal size (Figure S1) were synthesized by a patented process. Graphene was mixed with etching agent (ferrous sulfate, FeSO4) and melamine and dried into a solid form. Figure S2 presents the thermogravimetric analysis (TGA) of two and three components in argon. The jumping weight losses of the mixture of G and FeSO4 at 450 and 800 °C could be assigned to the decomposition of FeSO4 and the etching of carbon into gases, respectively. At the presence of melamine, we observed a broadened, much larger weight loss, which probably resulted from more complicated reactions involving melamine, including the anchoring and formation of atomic Fe in G nanomeshes and in situ N doping to form Fe−NGM (Figure S4). The annealing temperature affected the pores and
electrocatalytic properties of G itself and facilitate mass transport.17,24 Rational synthesis to achieve a high density of very active and corrosion-resistant sites could help in the understanding of how atomic-scale structure couples to both activity and durability. However, it is quite challenging to tailor graphene loading rich SAC at the atomic scale. Here, we demonstrated a scalable synthesis of a single-atom catalyst system of hierarchical atomic Fe/carbon materials with Fe atom isolation into N-doped graphene nanomeshes coated by carbon nanoshells (Fe−NGM/C−Fe). The electrochemically exfoliated graphene25 and melamine were used as raw materials to obtain atomic Fe in N-doped graphene nanomeshes (Fe−NGM) by a thermal annealing and in situ doping process. Next, iron-oleate complex was solventlessly pyrolysized onto Fe−NGM to obtain Fe−NGM/C−Fe. This atomically tailored material exhibited superior ORR performance compared to commercial Pt−C (20 wt %) with more positive onset/half-wave potential, larger current density, and better methanol poisoning resistance. 9917
DOI: 10.1021/acs.chemmater.7b03100 Chem. Mater. 2017, 29, 9915−9922
Article
Chemistry of Materials
Figure 4. (a) XPS full spectra of Fe−NGM/C−Fe and Fe−NGM. (b) Chart showing the percentages of Fe, N, C, and O in both Fe−NGM and Fe−NGM/C−Fe measured by XPS and ICP-OES. (c) XPS spectra of N 1s peaks with the deconvolution. (d) Bonding configurations of doped N element.
nanoparticles seemingly started melting; the G matrix moved nonuniformly, and some curved lines appeared around the ironbased particles, indicating that the sp2 carbon of G was partially destroyed and rearranged with the formation of iron compound (such as iron carbide). Liquid-like iron-based compound particles could be seen at 1000 °C. This phenomenon is similar to carbon nanotube formation via a vapor−liquid−solid mechanism.26 At above 1150 °C, the iron-based nanoparticle evaporated, and the voids in G were left. As shown in Figure 2h, a number of 4−6 nm voids around a big 30 nm hole were clearly presented, as highlighted by the white dashed lines. Lots of
