Highly active and durable carbon nitride fibers as metal-free bifunctional oxygen electrodes for flexible Zn–air batteries

Abstract

The design of flexible, highly energetic, and durable bifunctional oxygen electrocatalysts is indispensable for rechargeable metal–air batteries. Herein we present a simple approach for the development of carbon nitride fibers co-doped with phosphorus and sulfur, grown in situ on carbon cloth (PS-CNFs) as a flexible electrode material, and demonstrate its outstanding bifunctional catalytic activities toward ORR and OER compared to those of precious metal-based Pt/C and IrO₂ on account of the dual action of P and S, numerous active sites, high surface area, and enhanced charge transfer. Furthermore, we demonstrate the flexibility, suitability, and durability of PS-CNFs as air electrodes for primary and rechargeable Zn–air batteries. Primary Zn–air batteries using this electrode showed high peak power density (231 mW cm⁻²), specific capacity (698 mA h g⁻¹; analogous energy density of 785 W h kg⁻¹), open circuit potential (1.49 V), and outstanding durability of more than 240 h of operation followed by mechanical recharging. Significantly, three-electrode rechargeable Zn–air batteries revealed a superior charge–discharge voltage polarization of ∼0.82 V at 20 mA cm⁻², exceptional reversibility, and continuous charge–discharge cycling stability during 600 cycles. This work provides a pioneering strategy for designing flexible and stretchable metal-free bifunctional catalysts as gas diffusion layers for future portable and wearable renewable energy conversion and storage devices.