In situ coating of a N, S co-doped porous carbon thin film on carbon nanotubes as an advanced metal-free bifunctional oxygen electrocatalyst for Zn–air batteries

Abstract

Reversible oxygen catalysts with good catalytic performance and economic efficiency are urgently needed for the rapid development of metal–air batteries and reversible fuel cells. Herein, a metal-free N, S co-doped porous carbon thin film (NSCF) was coated in situ on carboxylic carbon nanotubes (CNT@NSCF) via a simple and scalable low temperature polymerization–high temperature pyrolysis strategy. By analyzing the surface structure of comparable carbon supports including ketjenblack, Vulcan XC-72 carbon, and activated carbon, it is found that the surface oxygen-containing functional groups of the support are the key factor to obtaining a uniform coating of NSCF with the formation of a large amount of efficient N, S active species. As a result, the optimal CNT@NSCF shows an excellent bifunctional oxygen catalytic activity with a narrow potential difference (ΔE = E_{j = 10,OER} − E_1/2,ORR) value of 0.74 V, which is superior to that of Pt/C + RuO₂ (0.75 V) and most metal-free carbon-based oxygen catalysts. Notably, the CNT@NSCF-based rechargeable Zn–air battery delivers a remarkable performance with a high open-circuit voltage and outstanding charge–discharge stability after 213 h, which fully demonstrates the promising practical application potential of the CNT@NSCF catalyst.