Metal Centers Drive Selectivity of Electrocatalytic Oxygen Reduction Reaction Promoted by Tetrapyrazinoporphyrazine Complexes

Abstract

Fuel cells represent a promising technology for the future decarbonization of the mobility sector. However, the efficient use of H₂ and O₂ to produce electricity still requires noble metal catalysts such as platinum and ruthenium. In particular, the Oxygen Reduction Reaction (ORR) is complex and limiting due to its mechanism, which involves the transfer of four electrons and four protons to produce water. The search for alternative catalysts exhibiting high selectivity is progressing at a rapid pace. In this context, this group previously unveiled a homogeneous catalyst based on titanium-centered tetrapyrazinoporphyrazines (TPyzPz) for the ORR, noting a certain modularity in the selectivity toward either a two- or four-electron reduction reaction. In this study, the influence of different metal centers (magnesium (Mg), cobalt (Co), copper (Cu), and zinc (Zn)) and various substituents is investigated on the tetrapyrazinoporphyrazine ring. The findings indicate a strong dependence of activity and selectivity on these modifications. Notably, cobalt and copper catalysts exhibit a selectivity greater than 90% toward H₂O production in the ORR. However, alterations to the macrocycle structure significantly affected the reactivity of these catalysts. These new insights highlighted the importance of careful structural design in the development of the next generation of ORR catalysts.