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.