CoFeNi/N-codoped carbon nanotubes with small diameters derived from spherical Prussian blue analog as bifunctional oxygen electrocatalysts for rechargeable Zn-air batteries

Owing to sluggish kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), it is still challenging to rationally design and construct bifunctional electrocatalysts with high performance, low-cost and good stability to substitute precious-metal catalysts. Herein, we introduce Ni ions to obtain spherical Prussian blue analog (PBA) doped with Co/Fe/Ni and further to synthesize nitrogen and Co/Fe/Ni co-doped carbon nanotubes (denoted as CoFeNi-CNTs) with small diameters (20–30 nm) by pyrolyzing the mixture of melamine and PBA. The optimized CoFeNi-CNTs have abundant metal-nitrogen-carbon (M-N-C) structures and CoFeNi nanoparticles respectively as effective ORR and OER active sites, which lead to outstanding electrocatalytic activities and durability with a half-wave potential of 0.85 V for ORR and an overpotential of 440 mV for OER. Especially, the rechargeable Zn-air battery based on CoFeNi-CNTs catalyst delivers a high peak power density of 138.7 mW cm-2, specific capacity of 793 mAh gZn-1 and stable charge-discharge cycling over 500 h, outperforming the Pt/C-RuO2 based devices. And all-solid-state Zn-air battery also displays excellent discharge-charge performance and stability. This work explores the optimization of oxygen catalytic activity for carbon nanotubes by reducing diameters and constructing dual-active-sites strategy.