A General Approach to Preferential Formation of Active Fe Nx Sites in Fe N/C Electrocatalysts for Efficient Oxygen Reduction Reaction.
Iron nitrogen on carbon (Fe N/C) catalysts have emerged as promising nonprecious metal catalysts (NPMCs) for oxygen reduction reaction (ORR) in energy conversion and storage devices. It has been widely suggested that an active site structure for Fe N/C catalysts contains Fe Nx coordination. However, the preparation of high performance Fe N/C catalysts mostly involves a high temperature pyrolysis step, which generates not only catalytically active Fe Nx sites, but also less active large iron based particles. Herein, we report a general "silica protective layer assisted" approach that can preferentially generate the catalytically active Fe Nx sites in Fe N/C catalysts while suppressing the formation of large Fe based particles. The catalyst preparation consisted of an adsorption of iron porphyrin precursor on carbon nanotube (CNT) silica layer overcoating, high temperature pyrolysis, and silica layer etching, which yielded CNTs coated with thin layer of porphyrinic carbon (CNT/PC) catalysts. Temperature controlled in situ X ray absorption spectroscopy during the preparation of CNT/PC catalyst revealed the coordination of silica layer to stabilize the Fe N4 sites. The CNT/PC catalyst contained higher density of active Fe Nx sites compared to the CNT/PC prepared without silica coating. The CNT/PC showed very high ORR activity and excellent stability in alkaline media. Importantly, an alkaline anion exchange membrane fuel cell (AEMFC) with a CNT/PC based cathode exhibited record high current and power densities among NPMC based AEMFCs. In addition, a CNT/PC based cathode exhibited a high volumetric current density of 320 A cm 3 in acidic proton exchange membrane fuel cell. We further demonstrated the generality of this synthetic strategy to other carbon supports.
Journal of the American Chemical Society