Economic and sustainable electrocatalysts for the oxygen reduction reaction (ORR) are needed to make hydrogen fuel cells and metal-air batteries viable for wide-spread use. Nature uses iron-containing cytochrome c oxidase to catalyze ORR by chemical reductants, but small molecule ORR electrocatalysts containing first-row transition metals typically either operate at high effective overpotentials or preferentially produce hydrogen peroxide, which damages other components of the devices.
By incorporating a secondary redox-active component into first-row transition metal complexes, we have been able to promote low overpotential ORR that is selective for the four-electron reduction of O2 to water. Derivatives of the additional redox-active component, 1,4-hydroquinone (quinol), are used extensively for electron transfer processes in biology.
We report electrocatalytic ORR by a series of Fe(II), Fe(III), and Co(II) complexes with quinol-containing ligands and analyze how the ligand framework and identity of the metal ion influence the activity, water selectivity, and effective overpotential of the electrocatalysis.
Comparisons to catalytically active phenolic derivatives demonstrate that the quinol improves the water selectivity and/or activity without increasing the overpotential. With iron, the catalytic cycle can be entered through either Fe(II) or Fe(III); entering the cycle via Fe(III) lowers the effective overpotential without altering either the rate or water selectivity of the electrocatalyzed ORR.