Study on the formation mechanism of self-assembly bamboo-shaped carbon nanotubes enriched with highly stable Fe-N₄ sites for Zn-air batteries

Carbon nanotubes' (CNTs) preparation and formation mechanisms are of great interest. To improve their performance in the oxygen reduction reaction (ORR), a post-treatment to add active centers is required. This study presents a simple method using a molten-salt template to self-assemble bamboo-shaped CNTs (B-Fe-N-C) with highly stable Fe-N₄ (Fe-pyridinic-N₄) sites. By analyzing the evolution of high-temperature materials and the intermolecular bonding process, the formation mechanism of B-Fe-N-C follows a two-dimensional material synthesis--bonding--self-assembly--growth mechanism. Impressively, B-Fe-N-C exhibits superior ORR performance (a half-wave potential of 0.868 V) and stability (only 3 mV attenuation after 10000 cycles of accelerated durability test) in alkaline medium. Density functional theory calculations demonstrate that Fe-Pyridinic-N₄ sites can lower the energy barrier for ORR. Furthermore, Comsol multiphysics simulations confirm the significant enhancement of oxygen mass transfer by bamboo-shaped B-Fe-N-C. Therefore, B-Fe-N-C is employed in zinc-air batteries, resulting in nice discharge performance and high peak power density.