Design of Highly Active Fe-N/C Catalysts with Abundant Fe-Nx Sites for Oxygen Reduction Reaction

Abstract

The development of highly active, durable, and low-cost oxygen reduction reaction (ORR) catalysts is central to making polymer electrolyte fuel cells (PEFCs) commercially viable. In this direction, tremendous recent efforts have been devoted to replacing expensive, scarce Pt-based electrocatalysts with non-precious metal catalysts (NPMCs) for the ORR. Among a wide range of NPMCs, the Fe-N/C catalysts have emerged as the most promising ORR catalysts due to their high ORR activities. A growing body of literature based on spectroscopic studies suggests that the active sites of these catalysts involve Fe-Nx coordination. We have endeavored to develop general synthetic strategies towards high-performance Fe-N/C catalysts that can host a high density of catalytically active Fe-Nx sites. We have prepared transition metal-doped ordered mesoporous porphyrinic carbons (M-OMPCs) by nanocasting mesoporous silica templates. The M-OMPC catalysts contain predominantly molecularly dispersed Fe-Nx sites and have high surface areas and tunable pore structures. Among the M-OMPC catalysts, the FeCo-OMPC catalyst exhibited an excellent ORR activity in an acidic medium, higher than other non-precious metal catalysts. It showed higher kinetic current at 0.9 V and superior long-term durability and MeOH-tolerance than Pt/C catalyst. Second strategy is based on a generalized “silica-protective-layer-assisted” method that can preferentially produce catalytically active Fe-Nx sites towards highly efficient Fe-N/C electrocatalysts. This method is applicable to any type of carbon supports including carbon nanotubes, reduced graphene oxide, and carbon black. One of resulting catalysts, consisting of CNT wrapped with thin porphyrinic carbon layer (CNT/PC), contained relatively high density of Fe-Nx sites, and showed very high ORR activity and remarkable stability in alkaline media. Importantly, the CNT/PC-based cathode demonstrated excellent performances in both alkaline anion exchange membrane fuel cell as well as acidic proton exchange membrane fuel cell.