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

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 (Delta E = E-j = 10,E-OER - E-1/2,E-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.