Microporous Framework Induced Synthesis of Single-Atom Dispersed Fe-N‑C Acidic ORR Catalyst and Its in Situ Reduced Fe‑N 4 Active Site Identification Revealed by X‑ray Absorption Spectroscopy Meiling Xiao, † Jianbing Zhu, † Liang Ma, ‡ Zhao Jin, § Junjie Ge,* ,§ Xin Deng, ∥ Yang Hou, ∥ Qinggang He,* ,∥ Jingkun Li, ⊥ Qingying Jia, ⊥ Sanjeev Mukerjee, ⊥ Ruoou Yang, # Zheng Jiang, # Dangsheng Su, & Changpeng Liu, § and Wei Xing* ,† † State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ‡ Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, People’s Republic of China § Laboratory of Advanced Power Sources, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ∥ College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China ⊥ Department of Chemistry and Chemical Biology, Northeastern University, 360 Huntington Avenue, Boston, Massachusetts 02115, United States # Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, People’s Republic of China & Shenyang National Laboratory for Material Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, People’s Republic of China * S Supporting Information ABSTRACT: Developing highly efficient, low-cost oxygen reduction catalysts, especially in acidic medium, is of significance toward fuel cell commercialization. Although pyrolyzed Fe-N-C catalysts have been regarded as alternatives to platinum- based catalytic materials, further improvement requires precise control of the Fe-N x structure at the molecular level and a comprehensive understanding of catalytic site structure and the ORR mechanism on these materials. In this report, we present a microporous metal−organic-framework-confined strategy toward the preferable formation of single-atom dispersed catalysts. The onset potential for Fe-N-C is 0.92 V, comparable to that of Pt/C and outperforming most noble-metal-free catalysts ever reported. A high-spin Fe 3+ -N 4 configuration is revealed by the 57 Fe Mö ssbauer spectrum and X-ray absorption spectroscopy for Fe L-edge, which will convert to Fe 2+ - N 4 at low potential. The in situ reduced Fe 2+ -N 4 moiety from high-spin O x -Fe 3+ -N 4 contributes to most of the ORR activity due to its high turnover frequency (TOF) of ca. 1.71 e s −1 sites −1 . KEYWORDS: oxygen reduction reaction, single atom, electrocatalyst, active site, X-ray absorption spectroscopy ■ INTRODUCTION Efficient synthesis and implementation of non-platinum-group- metal (non-PGM) catalysts toward the oxygen reduction reaction (ORR) is highly desirable but challenging for chemical energy conversion and storage. 1−4 Pyrolyzed metal−nitrogen− carbon catalysts (M-N x -C, M = Co, Fe) are regarded as the most promising candidates, initiated by using cobalt phthalo- cyanine as an ORR catalyst in 1964. 5 Later, researchers recognized that the activity and durability in acidic solution can be largely enhanced by heat treatment. 6−8 In addition to metal−nitrogen-coordinated macrocycles, inorganic metal salts, nitrogen-containing polymers (polyaniline, polypyrrole) or small molecules (NH 3 , melamine), and carbon black were selected as pyrolysis precursors for the synthesis of the M-N x -C catalysts. 4,9−20 Significant breakthroughs in ORR performance using nonmacrocycle precursors to prepare the Fe-N x -C catalysts were reported by Dodelet et al. 4 and Zelenay et al. 9 However, current progress primarily involves pyrolysis conditions and precursor optimization to maximize the performance. 21−26 The lack of a precise control strategy for the synthesis of catalyst and ongoing debate on the real active site structure have seriously limited further progress. The debate is associated with whether the transition metal participates in the ORR process. 27−33 Although most of the Received: January 11, 2018 Revised: February 21, 2018 Published: February 21, 2018 Research Article pubs.acs.org/acscatalysis Cite This: ACS Catal. 2018, 8, 2824-2832 © XXXX American Chemical Society 2824 DOI: 10.1021/acscatal.8b00138 ACS Catal. 2018, 8, 2824−2832