Beneficial Effects on the Cobalt-Catalyzed Hydrogen Evolution Reaction Induced by Corrole Chelation

Current technologies for the mass production of hydrogen gas, needed for fuel cells, the synthesis of ammonia, and many other purposes, have an enormous carbon footprint. Less than 5% of H-2 is manufactured by the much cleaner water electrolysis process, mainly because electrodes are based on precious platinum. In line with the worldwide effort of replacing platinum by earth-abundant metals, this study focused on electrocatalytic proton reduction by cobalt. A series of cobalt(III) corroles that greatly varies in the electronic and steric effects of the meso-C substituents was fully characterized for investigating how the complexes differ in terms of their reduction potentials and as electrocatalysts for proton reduction to hydrogen gas. The smallest and most electron-rich derivative, with H atoms rather than larger and more electron-withdrawing substituents on the macrocycle, displayed the most interesting catalytic activity. Despite of its most negative Co-II/Co-I reduction potential in the absence of acid, catalysis by this complex was characterized by the lowest overpotential and the largest faradaic efficiency, as well as an activity that was almost as good as that of platinum under heterogeneous conditions. Mechanism-of-action investigations via experimental and computational analyses exposed that the superior performance of the most electron-rich complex is attributable to its capability of reducing protons by singly (rather than doubly) reduced cobalt.