FeNC Oxygen Reduction Electrocatalyst with High Utilization Penta‐Coordinated Sites

Abstract

AbstractAtomic Fe in N‐doped carbon (FeNC) electrocatalysts for oxygen (O2) 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 O2 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 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 × 1019 sites gFeNC−1 and a record 52% FeNx 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.