The Trans Axial Ligand Effect on Oxygen Reduction. Immobilization Method May Weaken Catalyst Design for Electrocatalytic Performance

To investigate the effect of trans axial ligand on the oxygen reduction reaction (ORR), Co corroles with axial pyridine (py) and triphenylphosphine (PPh3) ligands were examined. Two methods were used to immobilize these Co corroles on carbon nanotubes (CNTs): Co 5,15-bis(pentafluorophenyl)-10-(4)-(1-azido)phenylcorrole (1) was grafted on CNTs via covalent bonds, whereas Co tris(pentafluorophenyl)corrole (2) was directly loaded through simple physical adsorption. When examined in alkaline media, covalently grafted Co corroles displayed much higher electrocatalytic ORR efficiency than simply adsorbed analogues. ORR half-wave potential at 0.85 V versus reversible hydrogen electrode was achieved by using grafted Co corroles with PPh3 axial ligand, which represents the state-of-the-art performance for molecular catalysis under alkaline conditions. Remarkably, 1 with PPh3 ligand outperforms 1 with py ligand in terms of anodically shifted half-wave potential by 130 mV, but 2 with either PPh3 or py axial ligand displays almost identical efficiency. These results show that (1) improved ORR performance can be achieved by using axial ligand of stronger electron-donating ability but (2) immobilization method may weaken catalyst design for molecular electrocatalysis. Therefore, caution should be paid when analyzing structural effects of molecular catalysts that are loaded on supporting materials.