The on-demand engineering of metal-doped porous carbon nanofibers as efficient bifunctional oxygen catalysts for high-performance flexible Zn-air batteries

Developing efficient bifunctional oxygen electrocatalysts is an essential step in the realization of flexible metal-air batteries to power emerging flexible electronics. Herein, we use a dual-functional metal template to achieve the on-demand control of dispersed active M-N-C sites, porous structures, and surface wettability in a carbon nanofiber catalyst. The resulting engineered carbon nanofibers possess a high surface area (612.2 m(2) g(-1)), greatly improved accessibility to active catalytic sites, excellent surface hydrophilicity, and enhanced Fe(Co)-N-x/C interactions, demonstrating excellent bifunctional catalytic activities for both oxygen reduction and evolution reactions with long-term stability. When employed in air electrodes for aqueous rechargeable Zn-air batteries (ZABs), the ZABs show a high specific capacity (740 mA h g(Zn)(-1)), excellent rate capabilities, and, in particular, exceptional cycling stability over 2000 cycles. Furthermore, flexible ZABs fabricated using air electrodes containing this catalyst and a hydrogel electrolyte demonstrate outstanding performance, with a high open circuit potential (1.42 V), large peak power density (188.6 mW cm(-2)), high specific capacity (647 mA h g(Zn)(-1)), excellent round-trip efficiency of >64% over 500 cycles, and performance retention under various mechanical deformation processes. This unique and tunable carbon nanofiber engineering approach can create noble-metal-free high-performance bifunctional oxygen catalysts, outperforming Pt/C-IrO2 and bringing us one step closer to realizing a reliable energy storage solution for future flexible electronics.