A porphyrin covalent organic framework cathode for flexible Zn-air batteries

Flexible and aqueous Zn-air batteries constitute promising next-generation energy storage devices to meet the growing demand for a safe and stable energy supply for flexible displays, wearable electronics, and implantable medical devices. However, their practical applications are severely hindered by the sluggish kinetics of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) at the air cathode. Herein, a porphyrin covalent organic framework (POF) was innovatively designed and applied as a cathode electrocatalyst for flexible Zn-air batteries. Porphyrin active sites were periodically constructed into well-defined two-dimensional (2D) frameworks precisely controlled at the atomic level, and carbon nanotubes (CNTs) served as scaffolds for further morphology regulation. Interwoven into a free-standing, robust, and flexible film, the as-fabricated CNT@POF hybrid demonstrates impressive performance in rechargeable liquid and flexible all-solid-state Zn-air batteries. Specifically, the liquid Zn-air battery with the CNT@POF cathode exhibits a small voltage gap of 0.71 V and outstanding stability for 200 cycles, even better than the noble-metal-based cathode. Flexible all-solid-state Zn-air batteries demonstrate a high energy efficiency of 61.6% at 1.0 mA cm^-2 and the flexibility to stably light a red light-emitting diode (LED, 2.0 V) when bent to different degrees. This contribution demonstrates versatile and ingenious strategies for the multiscale regulation of advanced materials for energy electrocatalysis and thereby facilitates their applications in flexible and safe energy storage and conversion.