PGM-free carbon-based catalysts for the electrocatalytic oxygen reduction reaction: active sites and activity enhancement

Exploring high-activity, low-cost platinum group metal-free (PGM-free) oxygen reduction reaction (ORR) electrocatalysts to replace precious metal Pt is critical for large-scale fuel cell applications. Owing to their wide source, controllable composition, low price, and excellent performance, the PGM-free carbon-based electrocatalysts have attracted great interest in academia and are expected to be an ideal replacement for precious metal electrocatalysts. In this review, we mainly focus on PGM-free carbon-based electrocatalysts and first introduce the ORR mechanisms and the active site classification of PGM-free carbon-based electrocatalysts. Then, we propose four strategies to enhance the ORR activity of electrocatalysts from the active site perspective based on the relationship between the structure and function of active sites. Finally, we present the current challenges and prospects for developing ORR electrocatalysts exhibiting high performance and stability.