Novel electrocatalysts for oxygen reduction using cobaltporphyrins that undergo facile electropolymerization

[Meso-tetra (thiophen-3-yl)porphinato]cobalt (II) (CoT3ThP) complex, which undergoes facile electropolymerization, was synthesized by the dehydro-condensation reaction of pyrrole and 3-thienylaldehyde. The electroconductive thin film of CoT3ThP interconnected by 2,5-thienylene chains, was produced at the glassy carbon electrode by electrolysis. The catalytic activity for oxygen reduction at the modified electrode was evaluated by cyclic voltammetry. The catalytic current for the O₂ reduction appeared near E_p = 0.2 V vs. SCE. It was found that the electrode modified with CoT3ThP film has a higher stability than that modified with porphyrin by absorption. Next, we chose carbon black with a high surface-area as a catalyst support, and CoT3ThP was electropolymerized under the condition of suspension of carbon black in an electrolyte solution. We found that the polymerized CoT3ThP was produced on the surface of the carbon nanoparticle. The carbon nanoparticles modified with polymerized CoT3ThP (polyCoT3ThP/C) were suspended in a Nafion solution, and a pseudo MEA (Membrane Electrode Assembly) was made by casting polyCoT3ThP/C on an edge-plane pyrolytic graphite electrode. The polyCoT3ThP/C catalyst was found to reduce O₂ mainly with four electrons (the number of electrons transferred n=3.8) at a positive potential of E_p = 0.44 V vs. SCE. The catalytic activity of polyCoT3ThP/C was improved by heat treatment under an inert gas atmosphere (E_p = 0.47 V vs. SCE, n = 3.8). The results show the possibility of a novel cathode catalyst for use in fuel cells.