FeNC Oxygen Reduction Electrocatalyst with High Utilization Penta-Coordinated Sites.

Atomic Fe in N-doped carbon (FeNC) electrocatalysts for oxygen (O 2 ) reduction at the cathode of proton exchange membrane fuel cells 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 remain challenging. A two-step synthesis approach has recently led to significant advances in terms of Fe-loading and mass activity; however, the Fe utilization remains low owing to the difficulty of building scaffolds with sufficient porosity that electrochemically exposes the active sites. Herein, this issue is addressed by coordinating Fe in a highly porous nitrogen-doped carbon support (≈3295 m 2  g -1 ), prepared by pyrolysis of inexpensive 2,4,6-triaminopyrimidine 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 utilization based on in situ nitrite stripping are achieved. The Fe single atoms are characterized 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 are further studied by density functional theory calculations.