High-density iron nanoparticles encapsulated within nitrogen-doped carbon nanoshell as efficient oxygen electrocatalyst for zinc–air battery

doi:10.1016/j.nanoen.2015.02.025

Nano Energy

Volume 13, April 2015, Pages 387-396

https://doi.org/10.1016/j.nanoen.2015.02.025 Get rights and content

Highlights

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High-density carbon-encapsulated iron nanoparticles are successfully obtained.
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Dicyandiamide and ammonium ferric citrate are used as pyrolysis precursors.
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Iron surface area and nitrogen content in the material can be tuned conveniently.
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The material demonstrates excellent bifunctionality for oxygen electrolysis.
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The material shows high performance and cycling durability in zinc-air battery.

Abstract

Exploring highly efficient electrocatalysts toward oxygen reduction and evolution reactions are critical for the development of rechargeable zinc–air batteries. As a novel class of electrocatalyst, transition metal nanoparticles encapsulated within nitrogen-doped carbon have been regarded as competitive alternative to replace noble metal electrocatalysts. Herein, we report successful synthesis of high-density iron nanoparticles encapsulated within nitrogen-doped carbon nanoshell (Fe@N–C) by solid-phase precursor׳s pyrolysis of dicyandiamide and ammonium ferric citrate. The resulting Fe@N–C material shows excellent bifunctionality for ORR and OER in alkaline medium compared to state-of-the-art commercial Pt/C and IrO₂, which demonstrates high performance and cycling durability in zinc–air battery as efficient oxygen electrocatalyst.

Graphical abstract

High-density iron nanoparticles encapsulated within nitrogen-doped carbon nanoshell demonstrates excellent bifunctionality for oxygen reduction and evolution reactions in alkaline medium, showing high performance and cycling durability in zinc–air battery as efficient oxygen electrocatalyst.

Introduction

Renewable energy sources, such as solar and wind energy, have been developed rapidly due to the world׳s growing energy demands and serious environmental threat caused by the depletion of natural fossil resources [1], [2]. The intermittent and local nature of renewable energy sources make them difficult to merge with the electricity grid. Electrochemical energy storage and conversion technologies provide a promising solution for efficient use of renewable electricity [3], [4], [5]. In recent years, rechargeable zinc–air batteries, consisting of a zinc electrode, electrolyte and an air electrode, have attracted increasing attention due to their high energy density, low cost and compact structure [6]. The renewable electricity is stored into rechargeable zinc–air battery by the charge process, and then the zinc–air battery delivers electricity by the discharge process as portable and transportation power sources, accomplished with oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in the air electrode. There are several challenges in oxygen electrocatalysis for rechargeable zinc–air batteries, such as intrinsically sluggish kinetics of ORR and OER, high overpotential and poor cycling durability, etc [7], [8], [9]. Noble metal electrocatalysts containing Pt, Ru or Ir have demonstrated high activity and durability in ORR or OER, however, these electrocatalysts exhibit poor bifunctionality. For example, Pt shows a high ORR activity but a poor OER activity, while RuO₂ and IrO₂ exhibit high OER activities but poor ORR activities [10], [11], [12]. Furthermore, the high cost and terrestrial scarcity of noble metals hinder an extensive use in zinc–air battery. Therefore, development of competent and inexpensive electrocatalysts using earth-abundant elements is highly demanded to address the major issues currently plaguing oxygen electrocatalysis.

Up to now, some transition metal- and nitrogen-functionalized carbon materials have shown remarkable activities in oxygen electrocatalysis [6], [13], [14], [15], [16], [17], [18], as well as high performances in zinc–air and lithium–oxygen batteries [13], [14], [16], [17], [18]. Among different active sites, transition metal-N_x, such as Fe–N_x, is considered as the most active species for oxygen electrocatalysis [13], [18], however, the acidic and oxidant attacks lead to leaching of iron out of Fe–N_x/C catalysts during the electrocatlytic process. Furthermore, the optimum iron content in Fe–N_x/C catalysts is usually less than 0.2 wt% [15], therefore, the leaching problem and low active site density cause a poor durability, especially in acidic medium [19]. Very recently, transition metal nanoparticles encapsulated in carbon are being spotlighted as a competitive candidate to replace noble metal electrocatalysts [20], [21], [22]. The surface carbon layer in the unique encapsulation structure prevents acid-leaching, oxidation and aggregation of transition metal nanoparticles in the electrocatalytic process, showing excellent durability in a wide pH range [23], [24]. Transition metal nanoparticles not only increase the graphitization degree of surface carbon layer during carbonization [25], but also transfer electron to the surface carbon layer [26]. The doping nitrogen in carbon lattice further induces uneven charge distribution of the adjacent carbon atoms [27], [28]. A synergetic role of the doped nitrogen in carbon lattice and the encapsulated transition metal nanoparticles stimulates an enhanced intrinsic electrocatlytic activity on the carbon-based materials [21], [26]. Consequently there are urgent demands to synthesize high-density transition metal nanoparticles encapsulated within nitrogen-doped carbon for increased active site density and enhanced electrocatalytic performance.

Various routes have been well developed for preparing high-purity carbon nanomaterials [29], [30], [31], [32]. In sharp contrast, synthesis of high-density transition metal nanoparticles encapsulated in carbon is still in their early stages of development. Some encapsulation structures have been successfully synthesized recently [20], [21], [22], [26], [33], [34], however, the content of transition metal is low [20], [34] and the particle size of encapsulated transition metal is rather large [33], resulting in a low surface area of transition metal and active site density. Therefore, there is still a great challenge to synthesize carbon encapsulated transition metal nanoparticles with high loading and dispersion. Recently, solid phase precursors have been widely used to synthesize carbon nanomaterials due to several advantages such as high yield, convenience of nitrogen doping and low cost using cheap equipment and a simple process [24], [35], [36], [37]. In this work, we report using dicyandiamide (DCDA) together with ammonium ferric citrate (AFC) as solid-phase precursors to prepare high-density iron nanoparticles encapsulated within nitrogen-doped carbon nanoshell (labeled as Fe@N–C) by pyrolysis and acid-leaching. The carbon structure and morphology show strong dependence on the composition of iron precursor and pyrolysis temperature. With an increased pyrolysis temperature in a range of 600–900 °C, the form of carbon evolves from graphitic C₃N₄ into carbon nanoshell and carbon nanotube, while the crystalline size of encapsulated iron nanoparticles increases and content of doped nitrogen decreases. The optimized high-density Fe@N–C material shows excellent bifunctionality for ORR and OER in alkaline medium compared to state-of-the-art commercial Pt/C and IrO₂. Furthermore, the high-density Fe@N–C material demonstrates high performance and cycling durability in zinc–air battery as efficient oxygen electrocatalyst.

Section snippets

Material synthesis

In a typical experiment, 8 g of dicyandiamide (C₂H₄N₄, Alfa Aesar, denoted as DCDA) and 1 g of ammonium ferric citrate (C₆H₁₁FeNO₇, J&K Chemical Ltd., denoted as AFC) were dissolved in 100 mL of de-ionized water. The solution was continuously stirred and dried at 80 °C. The obtained mixture was placed in a quartz tube of a horizontal furnace. The pyrolysis of the mixture was performed in Ar atmosphere at a flow rate of 50 mL min⁻¹. The furnace was heated to the target temperature at a rate of 10 ^°C min

Results and discussion

SEM image of Fe@N–C-700 (Figure 1a) shows that iron nanoparticles are uniformly dispersed on the carbon materials, and TEM image in Figure 1b reveals that the iron nanoparticles are encapsulated within carbon nanoshell. High resolution TEM (HRTEM) image in Figure 1c exhibits a completely encapsulated metallic iron nanoparticle showing (110) fringes. The formation of graphitic carbon around the surface of iron nanoparticles occurs through the dissolution of amorphous carbon into iron

Conclusions

High-density iron nanoparticles encapsulated within nitrogen-doped carbon nanoshell were successfully synthesized by solid-phase precursor׳s pyrolysis of DCDA and AFC followed by acid-leaching. Physicochemical characterization revealed that the surface area of iron nanoparticles and the content of nitrogen could be simply tuned by the iron precursor and pyrolysis temperature. The high-density Fe@N–C material demonstrated high ORR and OER activities as bifunctional catalyst in alkaline medium,

Acknowledgments

We gratefully acknowledge the financial support from the Ministry of Science and Technology of China (Grants 2012CB215500 and 2013CB933100), the National Natural Science Foundation of China (Grant 21103178).

Jing Wang received her B. S. in Materials Chemistry from Ocean University of China in 2010. Now, she is a Ph.D. candidate in Prof. Xinhe Bao׳s group at Dalian Institute of Chemical Physics (DICP), Chinese Academy of Sciences (CAS). Her research is focused on non-precious metal catalysts for electrochemical energy conversion and storage.

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Haihua Wu received his B. S. in Chemistry and Materials Science from Hebei Normal University in 2012. She is currently a Ph.D. candidate in Prof. Xinhe Bao׳s group at DICP, CAS. Her research is focused on synthesis and characterization of non-precious metal catalysts for water electrolysis and electrochemical reduction of carbon dioxide.

Dunfeng Gao received his BSc in materials chemistry from China University of Petroleum (East China) in 2009. He is currently a PhD candidate in physical chemistry in Prof. Xinhe Bao׳s group at DICP, CAS. His research interest is focused on water and carbon dioxide electrolysis.

Shu Miao received his B. S. and Master degree in Materials Science from Tsinghua University in 1999 and 2002, and got his Ph. D. in Materials Science from California Institute of Technology in 2007. He worked as a postdoctoral fellow at CEMES, France from 2007 to 2008, and then took a second postdoctoral fellow at the University of Sheffield, UK from 2008 to 2011. He joined DICP in 2011 as an Associate Professor. His research interests include investigation on microstructure of catalysts and energy materials using electron microscopy, and development of new electron microscopy techniques and instruments.

Guoxiong Wang received his B. S. from Wuhan University in 2000 and Ph. D in Physical Chemistry from DICP, CAS in 2006. After working at Catalysis Research Center, Hokkaido University, Japan from 2007 to 2010 as postdoctoral researcher, he joined State Key Laboratory of Catalysis, DICP as an Associate Professor. His research interests include highly efficient electrocatalytic materials and processes for electrochemical energy conversion and storage.

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Rapid communicationHigh-density iron nanoparticles encapsulated within nitrogen-doped carbon nanoshell as efficient oxygen electrocatalyst for zinc–air battery

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Material synthesis

Results and discussion

Conclusions

Acknowledgments

Nature

Science

Adv. Energy Mater.

Nanoscale

Angew. Chem. Int. Ed.

Chem. Soc. Rev.

Nature

J. Am. Chem. Soc.

J. Am. Chem. Soc.

Angew. Chem. Int. Ed.

Acs Catal.

J. Power Sources

J. Am. Chem. Soc.

Angew. Chem. Int. Ed.

Adv. Mater.

J. Phys. Chem. C

Chem. A

Adv. Energy Mater.

Energy Environ. Sci.

Carbon

Angew. Chem. Int. Ed.

Angew. Chem. Int. Ed.

J. Mater. Chem. A

J. Mater. Chem. A

Rapid communication
High-density iron nanoparticles encapsulated within nitrogen-doped carbon nanoshell as efficient oxygen electrocatalyst for zinc–air battery