A self-supported electrode as a high-performance binder- and carbon-free cathode for rechargeable hybrid zinc batteries

Developing highly efficient and flexible air-breathing electrodes is of significant importance for various energy conversion and storage technologies. In this work, we report the synthesis of the self-supported sandwich-structured electrode with Co3-xNixO4/Co3O4 nanowire arrays (NS@Co3-xNixO4/Co3O4). This novel nanostructure demonstrates very high reversible Faradaic redox reaction of CoNi-O -> CoNi-O-OH and excellent bifunctional activity toward both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Moreover, the air cathode, prepared by pressing the stainless steel (SS)@Co3O4 sandwiched between two Ni foam (NF)@Co3-xNixO4, possesses a dense and interconnected structure with a high loading of active catalyst. The Ni foam in this work not only acts as a substrate, but also serves as a nickel precursor for the formation of the Co3-xNixO4 catalyst. Meanwhile, combining the 3D porous structure of NF and intrinsic high OER activity of SS, the assembled porous air electrode can take full advantage of the sandwich structure to promote the electrocatalytic reaction kinetics of NS@Co3-xNixO4/Co3O4. Rechargeable battery system assembled from this NS@Co3-xNixO4/Co3O4 electrode based on both Zn-Co3-xNixO4 and Zn-air electrochemical reactions exhibits much higher energy efficiency and durability than those of commercial Pt/C and RuO2 electrocatalysts. This protocol opens a new avenue for the rational design of highly efficient and stable self-supported air electrode for metal-air batteries.