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

Nanoscale Horizon, 2025, 10, 1988-1996

Xiangmin Tang, Luyao Zou, Xiaopeng Li, Zhipeng Xu, Huilin Fan, Chao Lin and Jung-Ho Lee

doi.org/10.1039/D5NH00179J

Abstract

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.

New concepts

The widespread application of ZABs faces significant challenges due to high voltage polarization caused by slow oxygen reduction/evolution reactions. Transition metal/N-doped carbon bifunctional oxygen electrocatalysts, typically derived from pyrolyzed metal–organic compounds, have shown promise due to their high activity. However, their high cost and scalability limitations remain critical barriers to ZABs commercialization. Here, we introduce a coordination-driven approach using naturally abundant tannic acid to create ultrathin cobalt-chelated polyphenol coatings on CNTs. This polyphenol layer transforms into a carbon layer rich in Co–N–C and nitrogen species (Co–N/CNTs), which exhibit excellent bifunctional oxygen electrocatalytic activity. Additionally, the carbon layer improves the hydrophobicity of CNTs, while the interconnected CNTs network facilitates rapid electron transfer and efficient diffusion of reactants such as oxygen and ions. When the optimized electrocatalyst is integrated into the air electrode, both aqueous and solid-state ZABs demonstrate high power densities and remarkable cycling stability. This CNTs modification strategy provides a cost-effective method for developing efficient electrocatalysts, with potential applications extending to other sustainable energy conversion and storage technologies.