Urchin-shaped hollow iron-nitrogen-doped carbon microspheres as high-performance electrocatalysts for oxygen reduction

Oxygen reduction reaction (ORR) kinetics are enhanced in alkaline media. Hence, alternative non-platinum (Pt)-group metal electrocatalysts have been investigated extensively in this medium to compete with Pt in terms of performance and durability. Among various non-Pt catalysts, one of the most popular class of electrocatalysts is iron- and nitrogen-doped carbon-based (Fe-N-C) by the high electrocatalytic activity and selectivity in ORR. However, the inherent catalytic reactivity of such non-Pt electrocatalysts remains inferior to that of state-of-the-art Pt electrocatalysts. Here, we explore the ORR of hollow and urchin-like, three-dimensional (3D) nanostructured Fe-N-Cs prepared via polymerization-induced self-assembly of aniline followed by carbonization. The resulting Fe-N-Cs consist of a hollow microsphere framework coupled with nanorod bundles, and exhibit large surface areas (874 m²g^-1), hierarchical cavities, and excellent electrical conductivities (0.63 Scm^-1) as electrodes. They are of particular interest as oxygen reduction electrocatalyst for proton exchange membrane fuel cells (PEMFCs). These unique features, which enhance electrocatalytic efficiency, are attributed to efficient mass- and electro-transport ORR kinetics. Electrochemical experiments reveal improved onset (ca. 1.04 V) and half-wave potentials (ca. 0.9 V), which is comparable to those of commercial Pt electrocatalysts. The 3D hierarchical porous network with high interdigitation of well-dispersed nanorod building blocks is thought to be key to facilitating the ORR reaction.