Altering the spin state of Fe-N-C through ligand field modulation of single-atom sites boosts the oxygen reduction reaction. (January 2023)

• Record Type: Journal Article
• Authors: Xue, Dongping; Yuan, Pengfei; Jiang, Su; Wei, Yifan; Zhou, Ying; Dong, Chung-Li; Yan, Wenfu; Mu, Shichun; Zhang, Jia-Nan
• Is Part Of: Nano energy. Volume 105(2023)
• Publication Date: 2023-01
• Subjects: Electron-spin state -- Fe-N-C catalysts -- Ligand field modulation -- d-band center -- Oxygen reduction reaction; Nanoscience -- Periodicals; Nanotechnology -- Periodicals; Nanostructured materials -- Periodicals; Power resources -- Technological innovations -- Periodicals; Nanoscience; Nanostructured materials; Nanotechnology; Power resources -- Technological innovations; Periodicals; 621.042
• DOI: 10.1016/j.nanoen.2022.108020
• Languages: English
• ISSNs: 2211-2855

Abstract

Abstract: Atomically dispersed Fe-N-C catalysts are the most promising candidates alternatively to Pt-based catalysts for oxygen reduction reaction (ORR). However, the ORR activity of Fe-Nx electrocatalysts in acid are still far from satisfactory; thus far, although some discussions demonstrate the important role of ligand fields of single atom metal-N-C sites on improving catalytic properties, the behind mechanism is still ambiguous. Herein, based on the ligand field theory, the electron spin-state modulation of Fe active centers in SA Fe-N-C achieved from a low-spin state (LS) for FeN5 (Fe-N5 -LS) to a high-spin state (HS) for FeN4 (Fe-N4 -HS) and FeN3 (Fe-N3 -HS) was realized by converting defect-rich pyrrolic N-coordinated FeNx sites, which tune the electron readily penetrating the antibonding π-orbital of oxygen. The Fe-N4 -HS exhibits a 3d-electronic structure of t2g 3 eg 2 and significantly accelerate the ORR reaction kinetics. Taking advantage of activity-boosting high spin state (S5/2 ) of Fe (III), the designed Fe-N4 -HS (with two longitudinal parallel coordinated pyrr-N and pyri-N, respectively) catalyst displays excellent ORR activity, which is comparable to commercial Pt/C catalyst. In addition, Fe-N4 -HS presents higher proton exchange membrane fuel cell (PEMFC) and Zn-air battery performances than most non-precious-metal electrocatalysts. Our findings provide fundamental and technological insights into the correlation between the electronic spin states/geometric structure and high-efficiency SA Fe-N-C catalysts for ORR process. Graphical Abstract: The experimental and theoretical evidences have been provided that spin altering of the FeNx configuration was manipulated by transferring the N-coordination number of Fe-Nx (x = 3–5) in Fe-N-C catalysts, which lays the foundation for the understanding of ORR mechanism of spin cross-involvement at the electronic level. As a result, the designed Fe-N4 -HS catalyst with high spin state (S5/2 ) display excellent acidic/alkaline ORR activity, proton exchange membrane fuel cell and Zn-air battery performance. ga1 Highlights: Based on the ligand field theory, the electron spin modulation is realized by converting pyrrolic N-coordinated FeNx sites. Fe-N4 -HS catalyst with high-spin Fe(III) (t2g 3 eg 2 ) is more likely to penetrate the antibonding π-orbital of oxygen. The designed high spin state (S5/2 ) of Fe-N4 -HS catalyst displays excellent ORR activity. Fe-N4 -HS presents higher proton exchange membrane fuel cell and Zn-air battery performances. This work provides guidance for investigating the ORR mechanism of spin cross-involvement at the electronic level.