Effect of Zn atom in Fe-N-C catalysts for electro-catalytic reactions: theoretical considerations
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hool of Materials Science and Engineering, Beihang University, Beijing, 100191, China Department of Physics, Capital Normal University, Beijing 100048, China
© Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020 Received: 22 July 2020 / Revised: 21 August 2020 / Accepted: 23 August 2020
ABSTRACT Due to the high specific surface area, abundant nitrogen and micropores, ZIF-8 is a commonly used precursor for preparing high performance Fe-N-C catalysts. However, the Zn element is inevitably remained in the prepared Fe-N-C catalyst. Whether the residual Zn element affects the catalytic activity and active site center of the Fe-N-C catalyst caused widespread curiosity, but has not been studied yet. Herein, we built several Fe, Zn, and N co-doped graphene models to investigate the effect of Zn atoms on the electrocatalytic performance of Fe-N-C catalysts by using density functional theory method. The calculation results show that all the calculated Fe-Zn-Nx structures are thermodynamically stable due to the negative formation energies and relative stabilities. The active sites around Fe and Zn atoms in the structure of Fe-Zn-N6(III) show the lowest oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) overpotentials of 0.38 and 0.43 V, respectively. The bridge site of Fe-Zn in Fe-Zn-N5 shows the lowest ηHER of −0.26 V. A few structures with a better activity than that of FeN4 or ZnN4 are attributed to the synergistic effects between Fe and Zn atoms. The calculated ORR reaction pathways on Fe-Zn-N6(III) show that H2O is the final product and the ORR mechanism on the catalyst would be a four-electron process, and the existence of Zn element in the Fe-N-C catalysts plays a key role in reducing the ORR activation energy barrier. The results are helpful for the deep understand of high-performance Fe-N-C catalysts.
KEYWORDS Fe-N-C, Zn-N-C, oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, density functional theory.
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Introduction
With the ever-increasing consumption of fossil fuels and worldwide environmental concerns, the search for more sustainable, clean and renewable energy sources has become one of the most important challenges today [1, 2]. For the advantages of high energy conversion efficiency, high power density and zero emission of pollutants, fuel cells and rechargeable metal–air batteries have attracted much attention in recent years [3–6]. However, the large-scale application of the new energy conversion devices has been hindered due to the scarcity and high price of precious metals catalysts (PMCs) [7–10]. Therefore, it is of great significance to develop efficient nonprecious metal catalysts (NPMCs) as alternatives [4, 11–15]. Single atom catalysts (SACs) have attracted intensive attention recently due to the maximum atom-utilization efficiency, excellent activity, large surface coverage and the property of controllable coordination environment with homogeneous catalysts [16–20]. Among various NPMCs, M-N-C (M: Fe, Co, Zn, Cu,
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