Oxygen reduction reaction in nature and its importance in life

Oxygen reduction reaction (ORR) has emerged as a key reaction in renewable energy applications as they can seamlessly partner with a range of energy conversion reactions. Hence, the presence of ORR catalyst in the cathodic compartment of a fuel cell has become an essential feature. Currently, the expensive platinum, rhodium, iridium metals, or combinations of their composites are reckoned as the best ORR catalysts, which have severely impacted their possible worldwide applications. The biology has provided iron and copper-based cytochrome c oxidase and multicopper oxidase enzymes, respectively, that can perform the O2 reduction to water following a four proton/four-electron pathway with unparalleled efficiency. However, the existence of a sensitive protein scaffold surrounding the active site has limited the practical applications of the enzymes under hazardous fuel cell operational conditions. The recombinant and mutated versions of the native enzymes were developed to resolve this problem. These synthetic metalloenzymes allowed their strategic immobilization on electroactive surfaces for probing their thorough electrocatalytic activity and practical applications. Additionally, the presence of directed covalent linkage provided the must-needed long-term stability for these heterogeneous metalloenzyme assemblies. Both the native and synthetic metalloenzymes inspire the development of first-row transition metal-based artificial catalysts for a complete 4e−/4H+ reduction of O2 to water. The journey of the development of both synthetic metalloenzymes and bio-inspired catalysts is portrayed in this chapter detailing the underlying unique structure-function relationship.