PEM fuel cell electrocatalysts based on transition metal macrocyclic compounds

•The paper provides an overview of transition metal macrocyclic (TMM) complex-based electrocatalysts used in ORRs in PEMFCs.•Performances of TMM catalysts, including both ORR activity rates and stabilities, are discussed.•The effects of different types and natures of metals, macrocyclic ligands, and so on are analyzed and reviewed.•Required improvements of TMM catalysts in ORR activity, stability and commercial viability are identified.•Potential research directions to overcome current challenges are suggested.

This paper provides a comprehensive overview of the current state of development of transition metal macrocyclic (TMM) complex-based electrocatalysts used in oxygen reduction reactions (ORRs) in polymer electrolyte membrane fuel cells (PEMFCs).Up to date performances of carbon material-supported TMM catalysts, including both ORR activity rates and stabilities are, discussed. The effects of different types and natures of metals, macrocyclic ligands, ligand substitutions, catalyst supporting materials, electrode preparation, and various acid/alkaline solutions are thoroughly analyzed and reviewed based on the published literature. The ORR mechanisms facilitated by these TMM catalysts are discussed from both a theoretical and experimental observation point of view. Required improvements in ORR activity, stability and commercial viability in order for TMM complex-based electrocatalysts to be utilized in PEMFCs are identified. Potential research directions to overcome current challenges are also suggested to facilitate future efforts in this area.