Abstract
Precious metal-free Fe-N-C catalysts efficiently electrocatalyze the oxygen reduction reaction both in acid and alkaline electrolyte. Their stability is however limited in acidic medium, but generally accepted to be much higher in alkaline electrolyte. Herein, by combining advanced electron and X-ray based techniques, we provide comprehensive evidence of a Fe dissolution/reprecipitation mechanism, which partially transforms single Fe atoms into Fe oxide nanoparticles and Fe carbide nanoparticles into Fe carbide core@Fe oxide shell nanoparticles, and is independent on the gas atmosphere used during the accelerated stress tests. Our work shows that Fe-N-C materials based on zero-valent Fe nanoparticles should be designed so that all Fe nanoparticles are protected by a defect-free graphite shell, for improved durability. For single atom Fe-N-C catalysts, the present study raises the question of a possible synergy between minute amount of Fe oxide nanoparticles and Fe-NxCy single-atom sites, leading to the higher apparent durability of this catalyst.