Attaching Cobalt Corroles onto Carbon Nanotubes: Verification of Four-Electron Oxygen Reduction by Mononuclear Cobalt Complexes with Significantly Improved Efficiency

Cobalt complexes have been extensively explored in catalyzing the oxygen reduction reaction (ORR), which is the cathode reaction in fuel cells. Although they show high activities, mononuclear Co complexes typically mediate the 2e reduction of O-2 to H2O2, and two Co sites are generally required to catalyze the 4e reduction of O-2 to H2O. Herein we report the significantly improved efficiency of covalently grafted Co corroles on carbon nanotubes (CNTs) for the 4e ORR Azide-containing Co corroles can be attached to alkyne-modified CNTs via azide-alkyne cycloaddition. This attachment can avoid the formation of dimeric face-to-face Co corroles. The resulted hybrid catalyzes the 4e ORR in 0.5 M H2SO4 aqueous solutions with a half-wave potential at 0.78 V versus reversible hydrogen electrode (RHE). This performance makes this hybrid one of the most efficient Co-based molecular ORR electrocatalysts. Control studies using Co corrole analogues loaded on CNTs via noncovalent interactions give ORR. half-wave potentials at 0.68-0.61 V versus RHE. This work is significant in demonstrating that with proper covalent bond interactions to CNTs mononuclear Co corroles can become intrinsically active for the 4e ORR with significantly improved efficiency.