Iron and nitrogen co-doped carbon (Fe-N-C) catalysts are the most promising platinum group metal (PGM) free catalysts for oxygen reduction reaction (ORR) in acid media for proton exchange membrane fuel cell (PEMFC) application. Despite remarkable progresses have been made in the design of new materials and the study of active sites structures for PGM-free catalysts [1-5], the synthetic methods are primarily dependent on the pyrolysis of metal, nitrogen and carbon sources simultaneously. The formation mechanism of Fe-N4 active sites has remained elusive due to the accompanying process involved in decomposition process of nitrogen and carbon precursors and carbon graphitization, therefore hindering to understand the interaction between Fe and N during the high temperature. In this presentation, we report a non-pyrolysis method that enable high-performance by employing a well-defined model system consist of nitrogen doping, iron and carbon. The as prepared Fe-N-C catalysts showed superior activity in both acid solution (E1/2 =0.84 V vs. RHE) and fuel cell test. Correlation between ORR performance (activity, stability) and thermal activation was also established. A low-temperature activation is enough to generate high activity (E1/2 =0.79 V vs. RHE). Importantly, the formation of Fe-N x active sites can separate from nitrogen-doping and carbonization process. Comprehensive studies including X-ray absorption spectroscopy and atomic-resolution electron microscopy proved that high-active Fe-N x sites can be formed at low temperature. Molecular dynamics simulation and first-principle calculations further proved that the structure evolution and charge redistribution during the formation of FeNx active sites. These findings provide knowledge of Fe-N-C catalysts as well as other electrocatalysts with metal-N x active site. References Zhang, S. Ding, S. Hwang, X. Zhao, D. Su, H. Xu, H. Yang, G. Wu, Journal of The Electrochemical Society 2019, 166, F3116.Zhang, S. Hwang, M. Wang, Z. Feng, S. Karakalos, L. Luo, Z. Qiao, X. Xie, C. Wang, D. Su, Y. Shao, G. Wu, Journal of the American Chemical Society 2017, 139, 14143.Li, M. Chen, D. A. Cullen, S. Hwang, M. Wang, B. Li, K. Liu, S. Karakalos, M. Lucero, H. Zhang, C. Lei, H. Xu, G. E. Sterbinsky, Z. Feng, D. Su, K. L. More, G. Wang, Z. Wang, G. Wu, Nature Catalysis 2018, 1, 935.He, S. Hwang, D. A. Cullen, M. A. Uddin, L. Langhorst, B. Li, S. Karakalos, A. J. Kropf, E. C. Wegener, J. Sokolowski, M. Chen, D. Myers, D. Su, K. L. More, G. Wang, S. Litster, G. Wu, Energy & Environmental Science 2019, 12, 250.X. Wang, D. A. Cullen, Y.-T. Pan, S. Hwang, M. Wang, Z. Feng, J. Wang, M. H. Engelhard, H. Zhang, Y. He, Y. Shao, D. Su, K. L. More, J. S. Spendelow, G. Wu, Advanced Materials 2018, 30, 1706758.