Probing the Active Site Structures of Iron-Based ORR Catalysts

Abstract

Highly active and durable electrocatalysts for the oxygen reduction reaction (ORR) are undoubtedly essential for the commercialization of polymer exchange membrane fuel cells. Recently, two types of iron-based catalysts have been extensively studied and reported to be active and stable towards ORR in acidic electrolyte. One is the well-recognized FeNx/C catalyst, and the other is the catalyst with structures of graphitic layer encapsulated metal particles. Generally, catalysts synthesized by pyrolysis of different iron- and nitrogen-containing precursors always contain both structures, making it impossible to distinguish the role of each structure during the ORR. Using a high temperature autoclave approach, we synthesized two model catalysts with almost exclusively one desired structure in each catalyst. Based on detailed characterizations in terms of catalyst structure and composition as well as electrochemical tests, the active site structures of these two catalysts and their correlation with the ORR performance are explored. Figure 1. Schematic illustration of the two possible active sites. Figure 1