2418 | Energy Environ. Sci., 2016, 9, 2418--2432 This journal is © The Royal Society of Chemistry 2016 Cite this: Energy Environ. Sci., 2016, 9, 2418 Structural and mechanistic basis for the high activity of Fe–N–C catalysts toward oxygen reduction† Jingkun Li, a Shraboni Ghoshal, a Wentao Liang, b Moulay-Tahar Sougrati, c Fre ´ de ´ ric Jaouen, c Barr Halevi, d Samuel McKinney, d Geoff McCool, d Chunrong Ma, e Xianxia Yuan, e Zi-Feng Ma, e Sanjeev Mukerjee* a and Qingying Jia* a The development of efficient non-platinum group metal (non-PGM) catalysts for oxygen reduction reaction (ORR) is of paramount importance for clean and sustainable energy storage and conversion devices. The major bottleneck in developing Fe–N–C materials as the leading non-PGM catalysts lies in the poor understanding of the nature of active sites and reaction mechanisms. Herein, we report a scalable metal organic framework-derived Fe–N–C catalyst with high ORR activity demonstrated in practical H 2 /air fuel cells, and an unprecedented turnover frequency (TOF) in acid in rotating disk electrode. By characterizing the catalyst under both ex situ and operando conditions using combined microscopic and spectroscopic techniques, we show that the structures of active sites under ex situ and working conditions are drastically different. Resultantly, the active site proposed here, a non-planar ferrous Fe–N 4 moiety embedded in distorted carbon matrix characterized by a high Fe 2+/3+ redox potential, is in contrast with those proposed hitherto derived from ex situ characterizations. This site reversibly switches to an in-plane ferric Fe–N 4 moiety poisoned by oxygen adsorbates during the redox transition, with the population of active sites controlled by the Fe 2+/3+ redox potential. The unprecedented TOF of the active site is correlated to its near-optimal Fe 2+/3+ redox potential, and essentially originated from its favorable biomimetic dynamic nature that balances the site-blocking effect and O 2 dissociation. The porous and disordered carbon matrix of the catalyst plays pivotal roles for its measured high ORR activity by hosting high population of reactant-accessible active sites. Broader context Oxygen reduction reaction (ORR) constitutes a critical element in the commercialization of electrochemical energy conversion and storage devices. Consequently, the replacement of unsustainable noble metal catalysts with earth-abundant materials constitutes a vital technological strategy towards fixing the twin challenge of energy security and climate change. The success in this regard requires the development of scalable non-precious metal catalysts with high performance in practical devices, and proper understanding of the nature of active sites and reaction kinetics. In this work we developed a scalable Fe–N–C catalyst with high ORR activity demonstrated in practical H 2 /air fuel cells, as well as an unprecedented turnover frequency in acid, measured with a rotating disk electrode. Combined ex situ and in situ characterizations identify a non-planar ferrous Fe–N 4 moiety embedded in distorted carbon matrix characterized by a high Fe 2+/3+ redox potential as the active site, and verify the redox mechanism. Such insights form a critical step in our ability to design and scaleup future ORR catalysts. 1. Introduction Eliminating noble metals such as those of the platinum group (PGM) for cathodic oxygen reduction especially at the interface with polymer membranes provides for market transformation of several key technologies ranging from energy conversion in fuel cells to energy storage devices. 1,2 The rapid escalation of catalytic activity of non-platinum group metal (non-PGM) catalysts reported recently has shown great promise for M–N–C materials (M = Fe and/or Co). 3,4 M–N–C materials synthesized through pyrolysis of precursors comprising of transition metals, nitrogen, and carbon at high temperatures (700–1100 1C) a Department of Chemistry and Chemical Biology, Northeastern University, Boston, Massachusetts, 02115, USA. E-mail: s.mukerjee@neu.edu, Q.jia@neu.edu b Department of Biology, Northeastern University, Boston, Massachusetts, 02115, USA c Institut Charles Gerhardt Montpellier, UMR CNRS 5253, Universite ´ Montpellier, Agre ´gats, Interfaces et Mate ´riaux pour l’Energie, Montpellier, 34095, France d Pajarito Powder, LLC (PPC), Albuquerque, New Mexico 87102, USA e Shanghai Electrochemical Energy Devices Research Center, Department of Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China † Electronic supplementary information (ESI) available: Additional information including the EXAFS and XANES analysis, effective electrochemical surface area calculations, turnover frequency calculations, BET surface area summary, H 2 /air PEMFC performance with the scale-up catalyst, and the RDE testing results. See DOI: 10.1039/c6ee01160h Received 19th April 2016, Accepted 10th June 2016 DOI: 10.1039/c6ee01160h www.rsc.org/ees Energy & Environmental Science PAPER Published on 10 June 2016. Downloaded by Northeastern University on 27/01/2017 03:26:05. View Article Online View Journal | View Issue