Single-metal-atom catalysts: An emerging platform for electrocatalytic oxygen reduction

Electrochemical oxygen reduction reaction (ORR) is at the heart in many sustainable energy conversion technologies such as rechargeable fuel cells and metal-air batteries. Currently, various noble/transition metal-based materials have been developed as catalysts for boosting their catalytic performances. Among them, single-atom catalysts (SACs) have received increasing interest as promising electrocatalysts owing to their maximum utilization of active species, low-coordination environment, quantum size effect and tunable metal-support interaction. Over the past few years, tremendous SACs have been fabricated by using various approaches and are further used for the advanced energy conversion process. In this review, we offer a critical overview on the state-of-the-art design of SACs under the framework of bottom-up and top-down strategies and in-situ/operando characterizations. We also comprehensively present recent advances in the development of SACs for ORR electrocatalysis, fuel cells and zinc-air batteries, and describe key challenges and future opportunities in this emerging field.