Recent advancements on designing transition metal-based carbon-supported single atom catalysts for oxygen electrocatalysis: Miles to go for sustainable Zn-air batteries

Metal-air batteries, especially, rechargeable zinc-air batteries (ZABs) have recently rejuvenated extensive research attention as a promising sustainable energy technology, owing to its environment friendliness, low manufacturing cost, and high theoretical specific energy density. However, the real-time application of ZABs is yet to be achieved and it is mainly hindered due to the sluggish kinetics of oxygen-involved reactions. The two fundamental electrode reactions, specifically, oxygen reduction reaction and oxygen evolution reaction reinforce discharging and charging processes of ZABs. Therefore, uninterrupted research endeavours in developing novel design strategies is the crucial step to realize effective bifunctional electrocatalysts for oxygen electrocatalysis, which will rapid-up further progress of ZABs for commercialization. Recently, single atom catalysts (SACs) supported on various carbon scaffolds with maximized atom-utilization efficiency, unique metal coordination environments, abundant anchoring sites with exceptional tunability, ordered porosity, and selectivity have emerged as potential candidates for motivating oxygen electrocatalysis. Significant advances have been accomplished in designing SACs with commendable oxygen electrocatalytic activity, still great challenges need to be surmounted in order to make them viable for electrochemical energy conversion and storage devices. In this review, we have provided an overview of synthesis strategies, oxygen electrocatalytic performances, and identification of carbon-supported SACs. We have critically examined the role of coordination environment, engineering of isolated reactive centers, tailoring of the metal active centers, and modulation of the configuration of carbon substrates on the oxygen electrocatalytic activity. Finally, we concluded by highlighting the existing challenges and future research directions for further innovation of carbon-supported SACs for ZAB applications.