Atomic-layer-deposited ultrathin Co9S8 on carbon nanotubes: an efficient bifunctional electrocatalyst for oxygen evolution/reduction reactions and rechargeable Zn–air batteries

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are the key processes in many renewable energy conversion systems, and the development of high-performance non-precious bifunctional OER/ORR electrocatalysts is of crucial importance for various energy conversion devices, such as rechargeable metal–air batteries and regenerative fuel cells. Herein, we report a novel efficient bifunctional OER/ORR catalyst of Co9S8/CNT, which is synthesized using an advanced technique of atomic layer deposition (ALD) for conformally coating a uniform thin layer of Co9S8 on a high-surface-area carbon-nanotube (CNT) network scaffold. The ALD-synthesized Co9S8/CNT catalyst displays remarkable electrocatalytic performance with excellent catalytic activity and stability toward both the OER and ORR, and is further demonstrated to be a superior bifunctional oxygen catalyst for high-performance rechargeable Zn–air batteries. The fabricated aqueous rechargeable Zn–air batteries are able to deliver a remarkably high power density with superior long-term cycling stability, and the fabricated solid-state rechargeable Zn–air batteries are able to display very good flexibility and stability upon bending. Therefore, we believe that the ALD-synthesized bifunctional Co9S8/CNT electrocatalyst will have broad and promising applications for renewable energy conversion devices.