Cobalt Porphyrin Functionalized Carbon Nanotubes for Oxygen Reduction

Carbon nanotube (CNT) compositions were prepared by covalently grafting a Co(II) porphyrin to functionalized multiwalled carbon nanotubes (MWCNTs) via zwitterionic functionalization of the CNT sidewalls followed by a SN2 substitution reaction. The MWCNT−Co−porphyrin compositions, mixed with Nafion, displayed excellent catalytic performance for oxygen reduction in acidic media (pH range, 0.0−5.0) at room temperature. With low catalyst loading, the oxygen reduction rates achieved are more than 1 order of magnitude higher than previously reported values for similar Co−porphyrin catalysts. These results demonstrate the advantages of systems of MWCNTs covalently linked to electrocatalytic molecules. The electrodes are easily fabricated by a drop-casting vacuum drying procedure. Rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) measurements revealed the mechanism to be a direct four-proton and four-electron reduction of oxygen to water. These results demonstrate that new MWCNT electrocatalytic systems are potential substitutes for platinum or other metal-based cathode materials in proton conducting membrane fuel cells.