Electrochemistry of and Electrocatalysis by Low-Symmetry N4 Macrocycles
Authors: Raslin Arik , Gross Zeev 
Published: February 2024

The introduction of efficient and affordable catalysts for three seemingly simple electrochemical reactions could improve very much the sustainability of our energy supply: reduction of protons to hydrogen gas (the hydrogen evolution reaction, HER), water oxidation, and selective reduction of oxygen to water (the oxygen reduction reaction, ORR).1–6 Platinum group metals (PGM) are outstanding catalysts for all these reactions, but being rare and expensive prevents their utility for large scale operations.7 Within the search for catalysts based on earth abundant 1st row transition metals, we now introduce two different molecular catalysts: Co(III) complexes of corroles, with very different in size meso-C substituents (C6F5 >> CF3 >> H), as well by newly synthesized and fully characterized Co(II), Cu(II) and Ni(II) monoazaporphyrins (MAzP) with meso-CF3 substituents. All the catalyst were adsorbed on carbon supports with very different porosity (Vulcan << BP2000). The corrole-modified electrodes were studied as ORR catalysts, revealing that the onset potential of the best performing cobalt corrole is approaching that of Pt and with low percentage formation of undesired hydrogen peroxide. Testing that complex as a cathode for an anion exchange membrane fuel cell (AEMFC) revealed high power density relative to other molecular catalysts and excellent stability over 12 hours.

Metal complexes of a novel MAzP with CF3 on the three meso-C positions were studied as catalysts for HER in PBS buffer (pH=7.4), as well as for water oxidation and ORR at pH 13. The cobalt complex performed best in all processes. Compared to cobalt porphyrins with either three or four CF3 groups on the meso-C positions, Co-MAzP performed better for water oxidation. For HER catalysis, it has the same onset potential and current as the tetra-CF3 cobalt porphyrin and more current than the tris-CF3 analogue. In ORR catalysis, Co-MAzP performs with the lowest onset potential, but less H2O2 is formed via catalysis by the other porphyrins.

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