Oxygen reduction reaction by metallocorroles and metallophthalocyanines

Globally escalating energy demands and environmental restrictions concerning the use of fossil fuels have stimulated extensive research in the area of sustainable, clean, low-cost, and above all environmentally benign energy sources. Aerobic respiration is the most important life process that relies on oxygen reduction reaction (ORR), where molecular oxygen is selectively reduced to water by the perfectly arranged tris-copper and bis-heme centers present in cytochrome c oxidase. This process leads to the release of energy, which is stored as ATP. Getting inspiration from nature, many scientists have designed different model systems that can be useful as alternative energy resources. This ORR also plays a pivotal role in the energy transformation process in alternative energy generation devices, such as metal-air batteries and fuel cells (FCs). ORR as a FC cathodic reaction is a key step that affects the efficiency of overall energy conversion. Catalysts play important roles in this process by improving the kinetics of these reactions. The kinetics of ORR at the cathode is a major factor for the performance of electrocatalysts. Low kinetic rates, high over-potential, and consequential energy loss are the bottlenecks, which must be prevented to make such devices commercially viable. Pt-based electrocatalysts are most efficient, although they offers limited scope of commercialization of FCs as it is scarce and costly. It is also reported that Pt still shows over-potentials of over 200 mV from the equilibrium reversible potentials. This has forced the scientific community to extensively research on developing alternative catalysts. Tetrapyrrolic ligands like porphyrin, phthalocyanine, and corrole-based systems are versatile and can very efficiently catalyze ORR when bound to metals like Fe, Co, and Cu. First row transition metallophthalocyanine (MPc) and metallocorroles are potential electrocatalysts for many reactions, and they may eventually be used as true alternatives to the precious metals currently in use. Regarding ORR, halogenated cobalt-corroles and iron-phthalocyanines can match the potentiality of an industry standard, platinum-based catalyst. In this chapter, we would like to focus on the progress that has been made in this field of metallocorrole and metallophthalocyanine for ORR activity.