Understanding the High Activity of Fe–N–C Electrocatalysts in Oxygen Reduction: Fe/Fe₃C Nanoparticles Boost the Activity of Fe–N_x

Wen-Jie Jiang, Lin Gu, Li Li, Yun Zhang, Xing Zhang, Lin-Juan Zhang, Jian-Qiang Wang, Jin-Song Hu,Zidong Wei, and Li-Jun Wan

J. Am. Chem. Soc., 2016, 138 (10), pp 3570–3578

DOI: 10.1021/jacs.6b00757

Publication Date (Web): February 23, 2016

Abstract (click  for pdf file)

Understanding the origin of high activity of Fe–N–C electrocatalysts in oxygen reduction reaction (ORR) is critical but still challenging for developing efficient sustainable nonprecious metal catalysts in fuel cells and metal–air batteries. Herein, we developed a new highly active Fe–N–C ORR catalyst containing Fe–N_x coordination sites and Fe/Fe₃C nanocrystals (Fe@C-FeNC), and revealed the origin of its activity by intensively investigating the composition and the structure of the catalyst and their correlations with the electrochemical performance. The detailed analyses unambiguously confirmed the coexistence of Fe/Fe₃C nanocrystals and Fe–N_x in the best catalyst. A series of designed experiments disclosed that (1) N-doped carbon substrate, Fe/Fe₃C nanocrystals or Fe–N_x themselves did not deliver the high activity; (2) the catalysts with both Fe/Fe₃C nanocrystals and Fe–N_x exhibited the high activity; (3) the higher content of Fe–N_x gave the higher activity; (4) the removal of Fe/Fe₃C nanocrystals severely degraded the activity; (5) the blocking of Fe–N_x downgraded the activity and the recovery of the blocked Fe–N_x recovered the activity. These facts supported that the high ORR activity of the Fe@C-FeNC electrocatalysts should be ascribed to that Fe/Fe₃C nanocrystals boost the activity of Fe–N_x. The coexistence of high content of Fe–N_x and sufficient metallic iron nanoparticles is essential for the high ORR activity. DFT calculation corroborated this conclusion by indicating that the interaction between metallic iron and Fe–N₄ coordination structure favored the adsorption of oxygen molecule. These new findings open an avenue for the rational design and bottom-up synthesis of low-cost highly active ORR electrocatalysts.