FeNC Oxygen Reduction Electrocatalyst with High Utilisation Penta-coordinated sites

Atomic Fe in N-doped carbon (FeNC) electrocatalysts for oxygen (O2) 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 O2 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 m2 g-1), prepared by pyrolysis of inexpensive 2,4,6-triaminopyrimidine and a Mg2+ salt active site template and porogen. Upon Fe coordination, a high electrochemical active site density of 2.54×10^19 sites gFeNC-1 and a record 52% FeNx 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.