Cobalt (II) oxide nanosheets with rich oxygen vacancies as highly efficient bifunctional catalysts for ultra-stable rechargeable Zn-air flow battery

The development of highly efficient bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is of paramount importance for rechargeable Zn-air batteries. Herein, cobalt (II) oxide (L-CoO) nanosheets with hierarchical nanostructures were grown on the stainless steel (SS) substrate via the facile heat treatment of 2D layered Co(OH)2; and this binder-free 3D air electrode exhibits superior ORR/OER activity and stability (over 1000 h) in the rechargeable Zn-air flow battery. Importantly, X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) were employed to reveal the origins of the excellent bifunctional catalytic performance of this catalyst. Moreover, the evolution of the atomic-scale structures revealed by operando XAS strongly supports the excellent stability of L-CoO during electrocatalytic operation. Our work suggests a facile strategy to construct oxygen vacancy active sites; meanwhile, it offers a deep understanding at the atomic level on the excellent cycling stability in the Zn-air flow battery through X-ray spectroscopy.