Direct growth of ordered N-doped carbon nanotube arrays on carbon fiber cloth as a free-standing and binder-free air electrode for flexible quasi-solid-state rechargeable Zn-Air batteries

The development of an air electrode that is flexible in physical property and highly active and durable at different geometric status for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is of crucial importance for the rational design of flexible rechargeable Zn-air batteries (ZABs). Considering their good elasticity, high conductivity, and superior thermal and chemical stability, carbon nanotubes have been widely used as a catalyst support in various electrocatalysts, while oxide or metal nanoparticles have been frequently deposited on the carbon nanotube substrate to perform as the active materials. Considering the poor contact between active materials and carbon nanotubes may introduce a challenge for long-term operating stability, in particular in flexible devices, pure carbon electrocatalyst is highly appreciated. Herein, a free-standing air electrode with cobalt nanoparticles encapsulated N-codoped carbon nanotube arrays uniformly grown on the surface of carbon fiber cloth is developed by a two-step in situ growth method. Such a carbon-based electrode shows outstanding activity for both ORR and OER. The flexible ZAB with such air electrode shows superior flexibility and stability working under extreme bending conditions. Moreover, the polarization and round-trip efficiency for the flexible battery is 0.67V and 64.4% at 2mA/cm(2), respectively, even after being operated for 30hours. This study provides a feasible way to design all carbon-based free-standing and flexible electrode and enlightens the electrode design for flexible energy conversion/storage devices.