Current Status on the Development of Homogenous Molecular Electrocatalysts for Oxygen Reduction Reaction (ORR) Relevant for Proton Exchange Membrane Fuel Cell Applications

Oxygen reduction reaction (ORR) is an essential component in aerobic biological energy transduction, where the oxidation prowess of O<inf>2</inf> is employed to harvest the energy stored in reduced carbon sources. This experimental blueprint is mimicked in renewable energy technology, such as fuel cell. However, the harsh chemical conditions encountered in fuel cells have restricted the direct use of fragile biological ORR catalysts: the copper-based oxidase enzymes. Thus, a number of homogeneous synthetic ORR catalysts were developed in the past few years that can be used directly as an alternative cathodic substance in fuel cell. In this review, we have depicted the rationale behind the evolution of various ORR catalysts along with their developmental history.