Jesús Barrio, Angus Pedersen, Saurav Ch. Sarma, Alexander Bagger, Mengjun Gong, Silvia Favero, Chang‐Xin Zhao, Ricardo Garcia‐Serres, Alain Y. Li, Qiang Zhang, Frédéric Jaouen, Frédéric Maillard, Anthony Kucernak, Ifan E. L. Stephens, Maria‐Magdalena Titirici, Imperial College London, Department of Chemistry [Imperial College London], Department of Chemical Engineering, Tsinghua University, Physiochimie des Métaux (PMB), Laboratoire de Chimie et Biologie des Métaux (LCBM - UMR 5249), Institut de Chimie du CNRS (INC)-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche Interdisciplinaire de Grenoble (IRIG), Direction de Recherche Fondamentale (CEA) (DRF (CEA)), Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Direction de Recherche Fondamentale (CEA) (DRF (CEA)), Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Université Grenoble Alpes (UGA)-Institut de Chimie du CNRS (INC)-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche Interdisciplinaire de Grenoble (IRIG), Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Université Grenoble Alpes (UGA), Bolin Centre for Climate Research, Stockholm University, Institut Charles Gerhardt Montpellier - Institut de Chimie Moléculaire et des Matériaux de Montpellier (ICGM), Institut de Chimie du CNRS (INC)-Centre National de la Recherche Scientifique (CNRS)-Université de Montpellier (UM)-Ecole Nationale Supérieure de Chimie de Montpellier (ENSCM), Université de Montpellier (UM), Electrochimie Interfaciale et Procédés (EIP), Laboratoire d'Electrochimie et de Physico-chimie des Matériaux et des Interfaces (LEPMI), Institut de Chimie du CNRS (INC)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes (UGA)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP ), Université Grenoble Alpes (UGA)-Institut de Chimie du CNRS (INC)-Université Savoie Mont Blanc (USMB [Université de Savoie] [Université de Chambéry])-Centre National de la Recherche Scientifique (CNRS)-Université Grenoble Alpes (UGA)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP ), Université Grenoble Alpes (UGA), WPI Advanced Institute for Materials Research (WPI-AIMR), Tohoku University [Sendai], ANR-17-EURE-0003,CBH-EUR-GS,CBH-EUR-GS(2017), ANR-11-LABX-0003,ARCANE,Grenoble, une chimie bio-motivée(2011), European Project: 866402,NitroScission, European Project: 892614,HAEMOGLOBIN, and European Project: 896637 ,DimerCat
International audience; Atomic Fe in N-doped carbon (FeNC) electrocatalysts for oxygen (O$_2$) reduction at the cathode of proton exchange membrane fuel cells (PEMFCs) are the most promising alternative to platinum-group-metal catalysts. Despite recent progress on atomic FeNC O$_2$ reduction, their controlled synthesis and stability for practical applications remains challenging. A two-step synthesis approach has recently led to significant advances in terms of Fe-loading and mass activity; however, the Fe utilisation remains low owing to the difficulty of building scaffolds with sufficient porosity that electrochemically exposes the active sites. Herein, we addressed this issue by coordinating Fe in a highly porous nitrogen doped carbon support (~3295 m$^2$ g$^{-1}$), prepared by pyrolysis of inexpensive 2,4,6triaminopyrimidine and a Mg$^{2+}$ salt active site template and porogen. Upon Fe coordination, a high electrochemical active site density of 2.54×10$^{19}$ sites g$_{FeNC}$$^{-1}$ and a record 52% FeN$_x$ electrochemical utilisation based on in situ nitrite stripping was achieved. The Fe single atoms are characterised pre-and post-electrochemical accelerated stress testing by aberration-corrected high-angle annular dark field scanning transmission electron microscopy, showing no Fe clustering. Moreover, ex situ X-ray absorption spectroscopy and low-temperature Mössbauer spectroscopy suggest the presence of penta-coordinated Fe sites, which were further studied by density functional theory calculations.