New nitrogen-doped graphitic carbon nanosheets with rich structural defects and hierarchical nanopores as efficient metal-free electrocatalysts for oxygen reduction reaction in Zn-Air batteries

Design Principles and Mechanistic Understandings of Non-Noble-Metal Bifunctional Electrocatalysts for Zinc-Air Batteries

Zinc-air batteries (ZABs) are promising energy storage systems because of high theoretical energy density, safety, low cost, and abundance of zinc. However, the slow multi-step reaction of oxygen and heavy reliance on noble-metal catalysts hinder the practical applications of ZABs. Therefore, feasible and advanced non-noble-metal electrocatalysts for air cathodes need to be identified to promote the oxygen catalytic reaction. In this review, we initially introduced the advancement of ZABs in the past two decades and provided an overview of key developments in this field. Then, we discussed the working mechanism and the design of bifunctional electrocatalysts from the perspective of morphology design, crystal structure tuning, interface strategy, and atomic engineering. We also included theoretical studies, machine learning, and advanced characterization technologies to provide a comprehensive understanding of the structure-performance relationship of electrocatalysts and the reaction pathways of the oxygen redox reactions. Finally, we discussed the challenges and prospects related to designing advanced non-noble-metal bifunctional electrocatalysts for ZABs.

Recent Progress of Non-Noble Metal Catalysts for Oxygen Electrode in Zn-Air Batteries: A Mini Review

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) play crucial roles in energy conversion and storage devices. Particularly, the bifunctional ORR/OER catalysts are core components in rechargeable metal–air batteries, which have shown great promise in achieving "carbon emissions peak and carbon neutrality" goals. However, the sluggish ORR and OER kinetics at the oxygen cathode significantly hinder the performance of metal–air batteries. Although noble metal-based catalysts have been widely employed in accelerating the kinetics and improving the bifunctionality, their scarcity and high cost have limited their deployment in the market. In this review, we will discuss the ORR and OER mechanisms, propose the principles for bifunctional electrocatalysts design, and present the recent progress of the state-of-the-art bifunctional catalysts, with the focus on non-noble metal-based materials to replace the noble metal catalysts in Zn–air batteries. The perspectives for the future R&D of bifunctional electrocatalysts will be provided toward high-performance Zn–air batteries at the end of this paper.