Sodium chloride-assisted green synthesis of a 3D Fe-N-C hybrid as a highly active electrocatalyst for the oxygen reduction reaction

To promote the oxygen reduction reaction (ORR) on a non-precious-metal catalyst, integrating two-dimensional (2D) nanosheets and one-dimensional (1D) nanotubes in one catalyst is considered as one of the desirable approaches since this hybrid architecture can host more useful active sites and enhance mass/electron transfer. Herein, we demonstrated a sodium chloride-assisted strategy for the in situ synthesis of a three-dimensional (3D) hybrid of carbon nanosheets and nanotubes. The micrometer-scale sodium chloride (NaCl) crystal acted as a recyclable skeleton to adsorb the precursors on its surfaces, which assisted the formation of micrometer-sized graphitic carbon nanosheets with nanometer thickness by the template effect during the pyrolysis, and iron-based nanocrystals with a size of tens of nanometers by helping the distribution of iron sources and preventing their aggregation. The small iron-based nanocrystals favored the growth of long CNTs connected to carbon nanosheets and the outmigration of carbon atoms during the cooling process, which led to the formation of carbon-layer encapsulated metallic iron nanoparticles between the carbon nanosheets or inside the carbon nanotubes. Benefiting from these features, the developed hybrid exhibited a significantly enhanced electrocatalytic activity and durability for the ORR. The results may open up opportunities for exploring cost-effective high-performance electrocatalysts for energy applications.