Silica-Protective-Layer-Assisted Formation of Active Fe−Nx Sites in Fe−N/C Electrocatalysts for Efficient Oxygen Reduction Reaction

Abstract

Iron- and nitrogen-doped 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. In this presentation, we present 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 less-active 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 that the coordination of silica layer can 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 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 performances 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.