Non-noble metals as activity sites for ORR catalysts in proton exchange membrane fuel cells (PEMFCs)

Proton exchange membrane fuel cells (PEMFCs) have great potential to become the next generation green energy technique, but their application is limited by the slow kinetics of the cathode oxygen reduction reaction (ORR) in acidic medium. Meanwhile, the high price of Pt-based catalysts, which are now widely used commercially, has raised the cost of PEMFCs. Therefore, non-noble metal ORR catalysts as alternatives to Pt-based group metals (PGM) have attracted much attention. However, there is still a big gap between the performance of non-noble metal catalysts and commercial Pt/C catalysts in acidic environment. Recently, it has been realized that the performance of catalysts is closely related to the structure of catalytically active sites. Inspired by this, in this review, we firstly introduced the development and breakthrough of non-noble metals as activity sites. We then briefly summarized their catalytic mechanisms, and put forward some suggestions on how to improve the activity and stability of non-noble metal ORR catalysts.