A General Approach to Preferential Formation of Active Fe-Nx Sites in Fe-N/C Electrocatalysts for High-Performance Polymer Electrolyte Fuel Cells

Abstract

Non-precious-metal catalysts (NPMCs) for oxygen reduction reaction (ORR) have been of tremendous interests in energy conversion and storage devices. Among several classes of NPMCs, iron and nitrogen supported on carbon (Fe-N/C) have shown the most promising ORR activity. It has been widely suggested that an active site structure for Fe-N/C catalysts contains Fe-Nx coordination. However, the preparation of Fe-N/C catalysts mostly involves a high-temperature pyrolysis step, which generates not only Fe-Nx sites, but also a significant portion of less active large iron-based particles. This poses a great challenge to rational design of Fe-N/C catalysts with abundant Fe-Nx species. We developed “silica-protective-layer-assisted” synthetic 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 Fe porphyrinic precursor on carbon nanotubes (CNTs), silica layer overcoating, high-temperature pyrolysis, and silica layer etching, which yielded CNTs coated with thin layer of porphyrinic carbon (CNT/PC) catalysts. In situ X-ray absorption spectroscopy during the preparation of CNT/PC catalyst revealed that the interaction between the silica layer and Fe-N4 in a porphyrin precursor appears to protect the Fe-N4 site and to prevent the formation of large Fe-based particles. The CNT/PC catalyst showed very high ORR activity and remarkable 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, comparable with 2020 DOE target (300 A cm-3). We further demonstrated the generality of this synthetic strategy to other carbon supports including reduced graphene oxides and carbon blacks.