Solid phase microwave-assisted fabrication of Fe-doped ZIF-8 for single-atom Fe-N-C electrocatalysts on oxygen reduction

doi:10.1016/j.jechem.2020.06.046

Journal of Energy Chemistry

Volume 54, March 2021, Pages 579-586

https://doi.org/10.1016/j.jechem.2020.06.046 Get rights and content

Highlights

•
A microwave-assisted strategy to rapidly prepare Fe-doped ZIF-8 is proposed.
•
The derived Fe-N-C has abundant atomic FeN₄ sites and hierarchical pores.
•
Outstanding ORR activity, stability and methanol tolerance in acids are obtained.
•
P_max of 61 mW cm⁻² and excellent stability are achieved in DMFCs.

Abstract

Fe–N–C endowed with inexpensiveness, high activity, and excellent anti-poisoning power have emerged as promising candidate catalysts for oxygen reduction reaction (ORR). Single-atom Fe–N–C electrocatalysts derived from Fe-doped ZIF-8 represent the top-level ORR performance. However, the current fabrication of Fe-doped ZIF-8 relies on heavy consumption of time, energy, cost and organic solvents. Herein, we develop a rapid and solvent-free method to produce Fe-doped ZIF-8 under microwave irradiation, which can be easily amplified in combination with ball-milling. After rational pyrolysis, Fe–N–C catalysts with atomic FeN₄ sites well dispersed on the hierarchically porous carbon matrix are obtained, which exhibit exceptional ORR performance with a half-wave potential of 0.782 V (vs. reversible hydrogen electrode (RHE)) and brilliant methanol tolerance. The assembled direct methanol fuel cells (DMFCs) endow a peak power density of 61 mW cm⁻² and extraordinary stability, highlighting the application perspective of this strategy.

Graphical abstracts

Fe-doped ZIF-8 is prepared by a microwave-assisted method, featured with fast, energy-saving and solvent-free. The derived Fe-N-C has abundant atomic FeN₄ sites dispersed on porous carbon, served as efficient ORR electrocatalysts for fuel cells.

Introduction

The blossoms of the fuel cell technology are heavily dependent on electrocatalysts for the oxygen reduction reaction (ORR) [1], [2], [3]. Benefitted from the excellent performance of Pt-based ORR electrocatalysts, hydrogen proton exchange membrane fuel cells (H₂-PEMFCs) and direct methanol fuel cells (DMFCs) show broad application prospects in vehicles, portable devices and backup power due to their high efficiency and environmental friendliness features [4], [5]. However, their widespread commercialization is severely constrained by the high-cost and scarcity of Pt [6], [7].

In last decades, the dramatic evolution in non-precious metal catalysts (NPMCs) for ORR cherishes hopes on the replacement of Pt-based catalysts [8], [9], [10], [11]. Among that, Fe–N–C have cropped up as one of the most auspicious candidates due to excellent ORR activity, stability and instinct anti-poisoning power [12], [13], [14]. With the deepening unveiling of reaction mechanism, atomically dispersed FeN_x sites have been considered extremely efficient on ORR [15], [16], [17], [18]. To date, Fe–N–C single-atom electrocatalysts derived from Fe-doped zeolitic imidazolate framework-8 (ZIF-8) precursors represent the second to none performances of Fe–N–C in electrochemical tests [19], H₂-PEMFCs [20] and DMFCs [21]. However, the conventional solvent synthesis of the Fe-doped ZIF-8 is not only time consuming but also low in yield [22], [23]. The residual solvent with numerous unreacted metal ions and ligands cannot be recycled, thus increasing the environmental burden and fabrication cost [24], [25], [26]. The solid phase transformation from Fe salts, ZnO and imidazole ligands into Fe-doped ZIF-8 can be achieved by a heating method [27], [28], but the elevated temperatures and prolonged times increase extra energy and time consumption. In this regard, a green and energy saving method for rapid and scalable production of Fe-doped ZIF-8 precursors is highly fascinating but remains predicament.

Herein, we design a novel route to synthesize Fe-doped ZIF-8 precursors by microwave-assistant under low-power (900 W) and short-time (15 min) consumption without using any solvent, which can be facilely scaled up for mass production. After rational heating treatment, the obtained Fe–N–C materials (M₁₅-FeNC-NH₃) featured with abundant atomic FeN₄ active sites and hierarchically micro/meso-porous structure, simultaneously boosting the electrochemical activation and mass-transport properties. M₁₅-FeNC-NH₃ shows comparable activity, better durability and methanol resistance in acid electrolyte in comparison with the commercial Pt/C. When used in the cathode of DMFCs, it can operate at different temperatures under O₂ or air conditions and exhibits a peak power density of 61 mW cm⁻² at 90 °C when fed with O₂. Importantly, the fabricated DMFCs possess milestone stability under different current desities among the ever-reported DMFCs based Fe–N–C catalysts.

Section snippets

Experimental

All reagents in this paper were used without further purification. Specially, the size of ZnO purchased from Shanghai Maclin Biochemical Co., Ltd. is 90 ± 10 nm.

Investigation of the microwave-assistant synthesis

Scheme 1 illustrates the microwave-assisted synthesis of Fe-doped ZIF-8 and subsequent pyrolysis to procure Fe–N–C catalysts. The ball-milled mixture of 2-MIM, ZnO and Fe(Ac)₂ is light yellow powder. After 15 min microwave irradiation, dark red bulks denoted as ZnO@Zn/Fe-ZIF₁₅ are obtained (Fig. S1). ZnO@Zn/Fe-ZIF₁₅ exhibits irregular granular morphology under SEM and TEM observation and the uniform dopant of Fe is confirmed by elemental mapping and EDS spectrum (Figs. S2–S4).

The formation

Conclusions

In summary, a rapid and environmentally friendly synthesis of Fe-doped ZIF-8 precursors by microwave-assistance is proposed. After rational heat treatment, the resultant M₁₅-FeNC-NH₃ catalyst with atomic FeN₄ active sites and abundant pore structure endows superior ORR performance, excellent stability and methanol tolerance in acid medium. DMFCs based on M₁₅-FeNC-NH₃ demonstrate a highest peak power density of 61 mW cm⁻² as well as excellent durability. This work provides new opportunities for

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.

Acknowledgments

This work was financially supported by the Key Program of the Chinese Academy of Sciences (KFZD-SW-419), China, and the Major Research Plan of the National Natural Science Foundation of China (91834301), China. We thank Alexandre I. Rykov for his help in Mössbauer spectrum analysis. The DFT calculations utilized resources at the High Performance Computing Center, Jilin University.

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View full text

Solid phase microwave-assisted fabrication of Fe-doped ZIF-8 for single-atom Fe-N-C electrocatalysts on oxygen reduction

Highlights

Abstract

Graphical abstracts

Introduction

Section snippets

Experimental

Investigation of the microwave-assistant synthesis

Conclusions

Declaration of Competing Interest

Acknowledgments

Appl. Catal. B-Environ.

Curr. Opin. Electrochem.

J. Energy Chem.

Electrochem. Commun.

Chem. Eng. J.

Micropor. Mesopor. Mat.

Appl. Catal. B-Environ.

Appl. Catal. B-Environ.

J. Power Sour.

J. Power Sour.

J. Power Sour.

Nature

Chem. Soc. Rev.

Chem. Rev.

Science

Science

Nat. Chem.

Nature

Science

Science

Science

J. Am. Chem. Soc.

J. Phys. Chem. Lett.

Nat. Mater.

Science

Energy Environ. Sci.

Nat. Cat.

J. Mater. Chem. A

Angew. Chem. Int. Ed.