Unraveling the high-activity nature of Fe-N-C electrocatalysts for the oxygen reduction reaction: The extraordinary synergy between Fe-N₄ and Fe₄N

The scarcity and weak durability of precious metal catalysts are among the biggest obstacles to achieving cost-effective electrocatalysts in fuel cells and metal-air batteries. Hence, it is imperative to develop low-cost non-precious metal catalysts with comparable oxygen reduction reaction (ORR) activity to precious metal catalysts. Herein, we report a highly effective strategy for the facile synthesis of Fe/N-functionalized 3D porous carbon networks. A major advantage of the newly designed catalyst is that ultrafine Fe₄N nanoparticles are grown and uniformly mounted on the carbon framework upon pyrolysis treatment at 800 °C, and co-exist with numerous in situ formed Fe-N₄ moieties in the carbon matrix, being evidenced by using X-ray absorption and photoelectron spectroscopy. The new electrocatalysts exhibit high ORR activity, comparable/superior to that of the state-of-the-art Fe/N-carbon based catalysts reported to date. Specifically, the catalysts show a half-wave potential of 0.890 V (vs. RHE) and a limited current density of 6.18 mA cm^-2. By resorting to experimental measurements and density-functional theory (DFT) calculations, the synergistic effects between Fe-N₄ moieties and the Fe₄N support are identified for the first time, which play a key role in boosting the catalytic performance of the Fe/N-functionalized porous carbon networks.