Melt-salt-assisted direct transformation of solid oxide into atomically dispersed FeN4 sites on nitrogen-doped porous carbon

doi:10.1016/j.nanoen.2020.104670

Nano Energy

Volume 72, June 2020, 104670

https://doi.org/10.1016/j.nanoen.2020.104670 Get rights and content

Highlights

•
Melt-Salt-Assisted Direct Transformation of Solid Oxide into isolated MN₄ Sites on Nitrogen-Doped Porous Carbon.
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The strong polar force of MS breaks the ionic bonds in oxides and prevents the aggregation of free metal ions.
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Fe-SAC/NC shows excellent catalytic activity and superior stability for ORR in both alkaline and acidic electrolyte.

Abstract

Emerging heterogeneous catalysts with metal atomically dispersed on supports (tentatively termed single-atom catalysts, SACs) exhibit many appealing features in a wide variety of catalytic reactions, such as high activity and nearly 100% atom utilization. However, the synthesis of SACs currently requires not only multiple procedures but also appropriate precursors with special structures. Herein, the molten-salt assistance is presented as an effective means that enables the use of common metal oxides (such as Fe₂O₃ and Co₂O₃) and small organic molecules as precursors for the preparation of carbon-supported SACs through one-pot pyrolysis. Molten salts as templates that can be easily removed after synthesis also contribute to the formation of porous and nitrogen-doped carbon with a high specific area. More importantly, benefiting from the strong polarizing force of molten salts, the ionic bonds in oxides can be destabilized at high temperature. Subsequently, metal ions released from solid oxides are transformed into atomically dispersed active sites after being trapped by nitrogen (N) atoms on the carbon support. The as-prepared SAC with atomically dispersed Fe–N₄ sites demonstrates high activity, outstanding stability and good methanol tolerance in the oxygen reduction reaction (ORR) in both alkaline and acidic media.

Graphical abstract

A novel strategy is developed to directly transform common metal oxides into isolated single-atom sites through molten-salt-assisted pyrolysis, by which SACs of Fe-SAC/NC and Co-SAC/NC can be prepared successfully and Fe-SAC/NC demonstrates superior electrocatalytic performance for the ORR.

Introduction

Single-atom catalysts (SACs) are emerging heterogeneous catalysts on which isolated metal atoms are chemically anchored to form effective active sites. SACs have aroused widespread interest due to their maximal atom utilization (~100%) and ideal catalytic performance [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10]]. In addition, SACs have a similar high efficiency and selectivity as homogeneous catalysts and simultaneously have similar stability and recyclability as heterogeneous catalysts. Extensive experimental and theoretical studies have demonstrated that SACs have outstanding catalytic potential for the oxygen reduction reaction (ORR), the carbon dioxide reduction reaction (CO₂RR), the hydrogen evolution reaction (HER) and other interesting conversions [[11], [12], [13]]. Simultaneously, SACs also show excellent performance in the battery systems [14,15]. Nevertheless, the fabrication of SACs still faces great challenges due to the sharply increasing surface free energy and the migration and aggregation trends of single-metal atoms during the synthesis or the subsequent catalytic reaction. To prevent the aggregation of metal atoms during synthesis, researchers have proposed a variety of strategies.

Piernavieja-Hermida et al. prepared a palladium SAC using atomic layer deposition (ALD), in which the individual Pd atoms confined in the TiO₂ nanocavity enabled the formation of isolated active sites [16]. Through a photochemical method, Zheng et al. developed a palladium-titanium oxide (Pd₁/TiO₂) SAC with a Pd loading of up to 1.5 wt% [17]. However, physical routes such as ALD or other mass-selected soft-landing techniques usually suffer from low yields and require advanced equipment. One facile approach routinely used to prepare carbon-supported SACs is high-temperature carbonization of functional polymers or metal organic frameworks (MOFs) whose skeletons hold Co or Zn (such as ZIF-67 or ZIF-8) [[18], [19], [20], [21], [22], [23]]. Although the SACs derived from these precursors can preserve the original microstructures to a certain degree, this strategy also results in low yields and requires complicated steps (especially the synthesis of precursors and post treatment). Hence, for large-scale production and popularization of SACs, it is an urgent and challenging task to develop some facile and low-cost synthesis strategies.

Compared to the restricted dynamics of solid-state reactions, material synthesis in the liquid phase is usually the most favored due to the reversible dynamics, fast convection and diffusion of reactants. Thus far, apart from water, various types of high-temperature solvents, including different kinds of melting metals and salts, have been extensively explored for liquid phase synthesis [24,25]. Salt melts (MSs) have a long history in research involving the electrolysis of refractory metals and the synthesis of metallic and nonmetallic materials [[24], [25], [26], [27], [28]]. MS is a class of unique reaction media because most salts are soluble in water and can be easily separated from the product after reaction. More importantly, ionized cations and anions enable MS to be a solvent with a very strong polarity capable of breaking metallic, covalent or ionic bonds, which allows the direct use of common bulk materials as precursors for SACs synthesis. In turn, in MS, the aggregation of metal ions is suppressed robustly, which is highly desired for the formation of atomically dispersed metal sites at high temperature. Therefore, the potential of MS in the synthesis of SACs should be exploited further.

Herein, we demonstrate that MS is a powerful medium that enables bulk oxides (Fe₂O₃ and Co₂O₃) to be directly used as precursors for one-pot facile synthesis of nitrogen-doped carbon-supported SACs (termed M-SAC/NC) at high temperature. The strong polarizing force of MS is critical because it is capable of dissolving oxides, after which the released metal ions can move freely and are finally trapped by nitrogen (N) atoms stemming from small organic molecules. Using this approach, Fe-SAC/NC and Co-SAC/NC are synthesized successfully, and the former exhibits outstanding ORR activity in both alkaline and acid media. In particular, in KOH solution, Fe-SAC/NC exhibits better ORR performance than the most commonly used commercial catalyst (20 wt% Pt/C) due to the numerous atomically dispersed Fe–N₄ moieties. In addition, Fe-SAC/NC also demonstrates good stability and methanol tolerance for the ORR.

Section snippets

Results and discussion

In our synthesis, nitrogen-rich adenine was selected as the carbon and nitrogen precursor, while bulk ferric oxide (Fe₂O₃) served as the iron source, which has never been reported before. Both adenine and Fe₂O₃ are naturally abundant and environmentally benign. A schematic illustration of our synthesis strategy is depicted in Fig. 1. Briefly, Fe₂O₃ and adenine were mixed well with inorganic salts (NaCl/ZnCl₂) by ball milling, followed by pyrolysis at 900 °C in an inert atmosphere (see details

Conclusion

In conclusion, based on a solvent with ultrahigh polarity at high temperature, we demonstrate a molten-salt-assisted pyrolysis strategy enabling direct transformation of bulk metal oxides into isolated metal atoms supported on nitrogen-doped porous carbon (Fe-SAC/NC and Co-SAC/NC). Advantages of this approach have been well illustrated, such as low cost, simplicity and high applicability. The thought of utilizing molten salts to destabilize chemical bonds and suppress metal aggregation is

Chemicals

Zinc chloride (ZnCl₂, 98%) was purchased from Shanghai Macklin Biochemical Technology Co. Ltd. (Shanghai, China). Sodium chloride (NaCl, 99.5%) was purchased Tianjin Kermel Chemical Reagent Co. Ltd. (Tianjin, China). The adenine (98%), ferric sesquioxide (Fe₂O₃, 99.5%) and cobalt trioxide (Co₂O₃, AR) were purchased from Shanghai Aladdin Biochemical Technology Co. Ltd. (Shanghai, China).

Sample preparation

In a typical synthesis, ZnCl₂ (5.0 g, 36.68 mmol), NaCl (5.0 g, 85.56 mmol), adenine (1.0 g, 7.4 mmol) and Fe₂O

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 51773025) and the Natural Science Foundation of Liaoning Province (Materials Joint Foundation, Grant No. 20180510027), Dalian science and technology innovation fund (Grant No. 019J12GX032).

Jinwen Hu received the M. S. degree from Dalian University of Technology (DUT), China, in 2017. Currently, he is a Ph.D. candidate in DUT. His current research is focused on the preparation of single-atom catalysts and their application in the field of electrocatalysis.

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Danyang Wu graduated from Beijing University of Chemical Technology with a Bachelor's degree in 2009. Then she received her M. S. degree and Ph.D from Dalian University of Technology (DUT). Currently, she worked at Dalian Maritime University. Her research focuses on the controllable synthesis of single-atom catalysts and their application in electrocatalysis.

Chao Zhu is currently a research fellow of School of Materials Science and Engineering, Nanyang Technological University. He received his B.S. from Department of Physics, Nanjing University in 2009 and his Ph.D. from Department of Physics, Hong Kong University of Science and Technology in 2013. His researches focus on the atomic-scale (scanning) transmission electron microscopy study of 2D, catalysis and energy materials.

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Jiangwei Zhang is currently a associate research fellow of State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics. He received his B.S. from Beijing University of Chemical Technology in 2011 and his Ph.D. from Department of Chemistry, Tsinghua University in 2016. Currently, His researches focus atomically precise material structure determination and their corresponding catalytic applications in cutting edge of energy-related filed.

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Guifeng Zhang graduated from Wuhan University of Technology in 1984 with a Bachelor's degree, and received his Master’ and Doctor's degree from Northwestern Polytechnical University respectively in 1989 and 1992. He worked as an associate Professor in the Department of Materials and Technology, Northwestern Polytechnical University, and as an AvH scholar at the Department of Physics, Duisburg-Essen of University in 1997–1998 and 2001–2003. In 2004 he joined Dalian University of Technology with focus on the superhardness thin film materials, energy materials and devices, preparation of new materials.

Yantao Shi graduated from Lanzhou University in 2005 with a Bachelor's degree, then studied at Tsinghua University for his Master's degree and Ph.D from 2005 to 2010. As a visiting scholar, he worked at the Department of Physics, Hong Kong University of Science and Technology (HKUST) in the following two years. In 2012, he joined Dalian University of Technology with focus on the third generation thin film solar cells, e.g. dye-sensitized solar cells (DSCs) and perovskite solar cells (PSCs). His contributions include synthesizing novel photoelectric materials, revealing the effects of nanostructures on cell performance, improving long-term durability, and so on.

View full text

Melt-salt-assisted direct transformation of solid oxide into atomically dispersed FeN4 sites on nitrogen-doped porous carbon

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Results and discussion

Conclusion

Chemicals

Sample preparation

Declaration of competing interest

Acknowledgements

Joule

Single-atom catalysis of CO oxidation using Pt1/FeOx

Nat. Chem.

Single-atom catalysts: a new frontier in heterogeneous catalysis

Acc. Chem. Res.

Single-atom electrocatalysts

Angew. Chem. Int. Ed.

Atomically dispersed nickel-nitrogen-sulfur species anchored on porous carbon nanosheets for efficient water oxidation

Nat. Commun.

A copper single-atom catalyst towards efficient and durable oxygen reduction for fuel cells

J. Mater. Chem. A.

Fe-N-C oxygen reduction fuel cell catalyst derived from carbendazim: synthesis, structure, and reactivity

Adv. Energy Mater.

Identification of catalytic sites for oxygen reduction in iron- and nitrogen-doped graphene materials

Nat. Mater.

Iron-based cathode catalyst with enhanced power density in polymer electrolyte membrane fuel cells

Nat. Commun.

Identification of catalytic sites in cobalt-nitrogen-carbon materials for the oxygen reduction reaction

Nat. Commun.

A polymer encapsulation strategy to synthesize porous nitrogen-doped carbon-nanosphere-supported metal isolated-single-atomic-site catalysts

Adv. Mater.

Efficient visible-light-driven carbon dioxide reduction by a single-atom implanted metal-organic framework

Angew. Chem.

Heteroatom-mediated interactions between ruthenium single atoms and an MXene support for efficient hydrogen evolution

Adv. Mater.

Atomic cobalt as an efficient electrocatalyst in sulfur cathodes for superior room-temperature sodium-sulfur batteries

Nat. Commun.

Fabrication of superior single-atom catalysts toward diverse electrochemical reactions

Small Methods

Towards ALD thin film stabilized single-atom Pd1 catalysts

Nanoscale

Photochemical route for synthesizing atomically dispersed palladium catalysts

Science

Melt-salt-assisted direct transformation of solid oxide into atomically dispersed FeN₄ sites on nitrogen-doped porous carbon

Single-atom catalysis of CO oxidation using Pt₁/FeO_x