Tannic acid-mediated surface engineering of CNTs for enhanced bifunctional oxygen electrocatalysis

Developing stable and efficient bifunctional electrocatalysts for oxygen reduction and evolution reactions is essential for rechargeable metal-air batteries. Here, we reported a facile surface modification strategy of carbon nanotubes (CNTs) as air electrocatalysts for zinc-air batteries (ZABs). By leveraging the versatile binding affinity of tannic acid with metal ions and carbon surfaces, the CNTs surface was coated with a uniform, thin layer of nitrogen-doped carbon featuring atomically dispersed cobalt. The atomic cobalt and rich nitrogen endow the material with high intrinsic activity in oxygen electrocatalysis. Moreover, the carbon layer optimizes the hydrophilicity of CNTs, and the interwoven CNTs network enables fast electron transfer and accelerated reactant diffusion. The assembled aqueous and solid-state ZABs deliver good rate performance (discharging current density in the liquid-state ZAB: 0-100 mA cm^-2, solid-state ZAB: 0.5-10 mA cm^-2) and nice cycling stability.