Aerosol-assisted synthesis of bimetallic nanoparticle-loaded bamboo-like N-doped carbon nanotubes as an efficient bifunctional oxygen catalyst for Zn-air batteries

Considering fabrication of highly efficient and low-cost bifunctional oxygen electrocatalysts, which can substitute noble metal-based catalysts, is imperative for advanced rechargeable Zn-air batteries (ZABs). Among the various candidates, N-doped carbon and transition metal composite catalyst is considered as promising bifunctional oxygen electrocatalysts. In this study, one-dimensional structured bamboo-like N-doped carbon nanotubes (bNCNTs) were synthesized by spray pyrolysis followed by a post-treatment. CoFe nanoalloys embedded within SiO₂ microspheres were employed as a nanocatalyst to grow homogeneous bNCNTs, which have known as excellent oxygen reduction reaction (ORR) activity. To compensate for the low oxygen evolution reaction (OER) activity of bNCNTs, NiFe nanoparticles were loaded on bNCNTs (NiFe/bNCNT) by a “drop and dry” process and post-treatment. Synergistic effect arising from bNCNTs showing high surface area and controlled infiltration method, NiFe nanoparticles exhibited high dispersity on bNCNTs. Owing to the synergistic effect of NiFe metal nanoparticles and bNCNTs, NiFe/bNCNT exhibited superior ORR properties with a high limiting current density (5.1 mA cm⁻²) and a low Tafel slope (94 mV dec⁻¹), as well as outstanding OER activity with a low overpotential (350 mV) in an alkaline media compared to those of single metal-loaded bNCNTs and Pt/C-RuO₂. Furthermore, when applied as ZAB cathode catalyst, NiFe/bNCNT demonstrated a high power density (224 mW cm⁻²) and 330 hours long cycling stability.