Effect of Zn atom in Fe-N-C catalysts for electro-catalytic reactions: theoretical considerations

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-N_(x) 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-N_(6)(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-N_(5) shows the lowest η^(HER) of−0.26 V.A few structures with a better activity than that of FeN_(4) or ZnN_(4) are attributed to the synergistic effects between Fe and Zn atoms.The calculated ORR reaction pathways on Fe-Zn-N6(III)show that H_(2)O 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.