Elucidating Carbon Structures in Highly Active Fe-N-C Catalysts for Oxygen Reduction
Authors: Wu Gang 
Published: February 2020

In the past decade, a variety of NPMC formulations have been studied including organometallic components, nonprecious-metal chalcogenides, and nitrogen-doped carbon catalysts. Recent progresses in the development of high-performance NPMCs for the ORR suggest that the M-N-C (M: Fe or Co) catalysts synthesized from iron, cobalt, nitrogen, and carbon via a high-temperature approach offer the highest activity to efficiently catalyze the ORR among studied NPMCs [1-11]. Although decent ORR activity in alkaline media has been achieved on the metal-free nitrogen-doped carbon catalysts, addition of transition metals is indispensable for enhancing activity in more harsh acidic media. Under these conditions, the best performing M-N-C catalysts are also far away from the Pt catalysts in terms of activity and long-term durability, thereby not being able to provide sufficient performance yet for practical applications. Therefore, the significant gaps between the most active Fe-N-C (PANI-Fe-C) and the state of the art Pt/C catalysts must be bridged for viable fuel cell applications [1].

During the catalyst development through the high-temperature approach, the catalysts are dominated by in-situ formed graphitized carbon nanostructures derived from carbon/nitrogen precursors[4]. These carbon nanostructures likely link to the oxygen reduction activity and may be critical to active sites. However, the role of carbon structures in the M-N-C catalyst seems to be still controversial. Initially, we speculated that the in situ formation of highly graphitized carbon nanostructures in the M-N-C catalysts seems to be a critical factor dictating active site generation and is directly linked to the observed ORR activity [4,12]. The carbon nanostructures (e.g., tubes, onion-like carbon, and multiple-layered graphene) once were observed in highly active M-N-C catalysts (Fig. 1). However, in our recent effort to develop the M-N-C catalyst derived from MOFs, dominant amorphous carbon morphology with uniform distribution of N and Fe was found in a new type of Fe-N-C catalysts showing even higher ORR activity (E
1/2 up to 0.82 V vs RHE) and stability relative to state-of-the-art graphitized carbon-rich Fe-N-C catalysts (0.80 V). Thus, it seems that the graphitized carbon structures in the M-N-C catalysts are not necessary for generating high ORR activity. The discrepancy continuously puzzles us in terms of the optimal carbon structures in the M-N-C catalysts for the ORR. In this presentation, we are focusing on elucidating the role of carbon structures and local bonding environment, which are fundamentally important to predict and design the optimal M-N-C catalysts.

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Figure 1