Electronic and Defective Engineering of Electrospun CaMnO3 Nanotubes for Enhanced Oxygen Electrocatalysis in Rechargeable Zinc-Air Batteries

Rational design and massive production of bifunctional catalysts with superior oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities are essential for developing metal-air batteries and fuel cells. Herein, controllable large-scale synthesis of sulfur-doped CaMnO3 nanotubes is demonstrated via an electrospinning technique followed by calcination and sulfurization treatment. The sulfur doping can not only replace oxygen atoms to increase intrinsic electrical conductivity but also introduce abundant oxygen vacancies to provide enough catalytically active sites, which is further demonstrated by density functional theory calculation. The resulting sulfur-modified CaMnO3 (CMO/S) exhibits better electrocatalytic activity for ORR and OER in alkaline solution with higher stability performance than the pristine CMO. These results highlight the importance of sulfur treatment as a facile yet effective strategy to improve the ORR and OER catalytic activity of the pristine CaMnO3. As a proof-of-concept, a rechargeable Zn-air battery using the bifunctional catalyst exhibits a small charge-discharge voltage polarization, and long cycling life. Furthermore, a solid-state flexible and rechargeable Zn-air battery gives superior discharge-charge performance and remarkable stability. Therefore, the CMO/S nanotubes might be a promising replacement to the Pt-based electrocatalysts for metal-air batteries and fuel cells.