Atomic-level dispersed catalysts for PEMFCs: Progress and future prospects

Atomic-level dispersed metal catalysts (including single atoms and clusters), a unique system for electrocatalysis, have attracted enormous attention in electrochemical energy conversion and storage because of their maximal atom utilization and high catalytic efficiency. This review is intended to be a comprehensive, unequivocal, and readable account of general interest in the chemical community. We summarize the latest advances in the catalytic reactions of atom-dispersed metal catalysts in proton exchange membrane fuel cells, mainly including the oxygen reduction reaction (ORR), methanol oxidation reaction (MOR) and hydrogen oxidation reaction (HOR), where the interaction between catalytically active sites and supports is discussed as well. The synthetic routes and characterization methods for the catalysts are also presented in detail. On the structure-activity relationship side, both single atom and metal cluster tend to form new active sites by bonding to adjacent non-metal atoms, which could introduce a unique variable for designing and synthesizing efficient catalysts. The outcome of the catalysts is discussed based on data collected from RDE experiments and fuel cell tests. In the end, the application prospects and challenges of atomic-level dispersed metal catalysts in proton exchange membrane fuel cells are generalized.