Oxygen reduction reaction by metal complexes containing non-macrocyclic ligands

Dioxygen reduction reaction (ORR) has paramount importance in fuel cell application and air batteries. In nature, 4e−/4H+ reduction of dioxygen to water occurs in cytochrome c oxidase and multi-copper oxidase. Such natural reactions inspired chemists to develop artificial analogs to mimic ORR. The development of small-molecule model complexes for ORR is a burgeoning interest considering the clear structure, easy tunability, direct observation of the reaction intermediates. Several molecular ORR electrocatalysts have been developed inspired by natural systems based on 3d transition metal complexes supported by small-molecule cyclic and acyclic ligands in the last decades. Such studies pointed out the effect of ligand donor atoms, primary and secondary sphere effects, pH of the reaction medium, etc., on the ORR activity, and such mechanistic information is very crucial to develop cathode material for ORR, which can fulfill the ultimate goal of developing ORR catalyst based on earth-abundant transition metals. Different molecular catalysts based on Cu, Fe, Co, and Mn metal ions have been developed for chemical and electrochemical ORR activity in the last decade. In addition, different polyoxometalate anions, redox-active transition metal-oxide molecular clusters, have also been employed as cathode materials in air batteries and in the due course, mechanistic detail of electron transfer during oxygen reduction reactions are also explored. This chapter discussed the ORR catalyzed by 3d transition metal complexes supported by acyclic ligands and polyoxometalates.