Tuning the Coordination\nEnvironment in Single-Atom\nCatalysts to Achieve Highly Efficient Oxygen Reduction Reactions

Abstract

Designing atomically dispersed metal catalysts for oxygen\nreduction\nreaction (ORR) is a promising approach to achieve efficient energy\nconversion. Herein, we develop a template-assisted method to synthesize\na series of single metal atoms anchored on porous N,S-codoped carbon\n(NSC) matrix as highly efficient ORR catalysts to investigate the\ncorrelation between the structure and their catalytic performance.\nThe structure analysis indicates that an identical synthesis method\nresults in distinguished structural differences between Fe-centered\nsingle-atom catalyst (Fe-SAs/NSC) and Co-centered/Ni-centered single-atom\ncatalysts (Co-SAs/NSC and Ni-SAs/NSC) because of the different trends\nof each metal ion in forming a complex with the N,S-containing precursor\nduring the initial synthesis process. The Fe-SAs/NSC mainly consists\nof a well-dispersed FeN₄S₂ center site where\nS atoms form bonds with the N atoms. The S atoms in Co-SAs/NSC and\nNi-SAs/NSC, on the other hand, form metal–S bonds, resulting\nin CoN₃S₁ and NiN₃S₁ center\nsites. Density functional theory (DFT) reveals that the FeN₄S₂ center site is more active than the CoN₃S₁ and NiN₃S₁ sites, due to the\nhigher charge density, lower energy barriers of the intermediates,\nand products involved. The experimental results indicate that all\nthree single-atom catalysts could contribute high ORR electrochemical\nperformances, while Fe-SAs/NSC exhibits the highest of all, which\nis even better than commercial Pt/C. Furthermore, Fe-SAs/NSC also\ndisplays high methanol tolerance as compared to commercial Pt/C and\nhigh stability up to 5000 cycles. This work provides insights into\nthe rational design of the definitive structure of single-atom catalysts\nwith tunable electrocatalytic activities for efficient energy conversion.