(Invited) Efficient Electrocatalysis of Oxygen Reduction with Metallo-Corroles

Abstract

The most active non-precious metal catalysts (NPMCs) for oxygen reduction (ORR) to date are the pyrolyzed catalysts, inspired from Heme-like complexes. These are usually composed of iron and/or cobalt coordinated by nitrogens, claimed to resemble the structure of porphyrins and phthalocynines, on the surface of a carbon support. Unfortunately, the exact structure of the catalytic sites remains a mystery and the solution for this conundrum seems be almost impossible. The advantages of using such catalysts are: (1) their high activity and (2) low price, which is derived from the cost of their precursors. The disadvantages are: (1) low durability compared to precious metal catalysts, and (2) their unknown structure, which limits their further improvement in order to obtain both the activity and durability benchmarks needed to become good alternatives for precious metals. One of the most promising options to resolve these issues is to find non-pyrolyzed molecular non-precious metal catalysts for ORR that could be tuned to match the necessary requirements. Recently, Metallo-corroles, a relatively new family of molecular catalysts was reported to have very good potential as non-precious metal catalysts for ORR. The electro-reduction of oxygen was investigated, using a series of first row transition metal β-pyrrole-brominated 5,10,15-tris(pentafluorophenyl)-corroles [M(tpfcBr8), M=Mn, Fe, Co, Ni and Cu] as catalysts in alkaline solutions. The kinetics of the oxygen reduction reaction (ORR) on these catalysts was evaluated using the rotating disk electrode method in a solution of 0.1M KOH. These corroles were adsorbed on a high surface area carbon powder (BP2000) prior to electrochemical measurements, to create a unique composite material. The comparison between the corroles with different metal centers showed a favorable catalytic performance of the ORR in the case of Fe and Co Corroles. The mechanism by which these corroles catalyze the ORR was studied by the rotating ring disk electrode (RRDE) technique, showing that in the case of alkaline solutions, there is a favorable 4 electron transfer mechanism. When comparing the performance of these catalysts in alkaline solutions, to their performance in acidic media, it was clear that the former was substantially higher, both in the onset potentials of the ORR and in the overall kinetic parameters.