Oxygen electroreduction on multi-walled carbon nanotube supported metal phthalocyanines and porphyrins in alkaline media.

Metal phthalocyanine and porphyrin modified electrodes were prepared using multi-walled carbon nanotubes (MWCNTs) as a support material. The catalyst materials were heat-treated before electrochemical testing. X-ray photoelectron spectroscopic study was carried out in order to examine the surface composition. The electroreduction of oxygen has been investigated on Fe phthalocyanine/MWCNT, Co phthalocyanine/MWCNT, Fe porphyrin/MWCNT and Co porphyrin/MWCNT catalysts. Electrochemical experiments were carried out in 0.1 M KOH employing the rotating disk electrode (RDE) method. The glassy carbon (GC) disk electrodes were modified with MN4 macrocycle/MWCNT catalysts using Tokuyama AS-4 ionomer. Electrochemical characterization of the materials showed that all the MN4 macrocycle/MWCNT modified GC electrodes are highly active for the reduction of oxygen in alkaline solutions. Particularly high electrocatalytic activity was observed for porphyrin-based electrodes heat-treated at 800 degrees C. The RDE results obtained are significant for the development of alkaline membrane fuel cells.

Electrochemical modification of gold electrodes with azobenzene derivatives by diazonium reduction.

An electrochemical study of Au electrodes electrografted with azobenzene (AB), Fast Garnet GBC (GBC) and Fast Black K (FBK) diazonium compounds is presented. Electrochemical quartz crystal microbalance, ellipsometry and atomic force microscopy investigations reveal the formation of multilayer films. The elemental composition of the aryl layers is examined by X-ray photoelectron spectroscopy. The electrochemical measurements reveal a quasi-reversible voltammogram of the Fe(CN)6 (3-/4-) redox couple on bare Au and a sigmoidal shape for the GBC- and FBK-modified Au electrodes, thus demonstrating that electron transfer is blocked due to the surface modification. The electrografted AB layer results in strongest inhibition of the Fe(CN)6 (3-/4-) response compared with other aryl layers. The same tendencies are observed for oxygen reduction; however, the blocking effect is not as strong as in the Fe(CN)6 (3-/4-) redox system. The electrochemical impedance spectroscopy measurements allowed the calculation of low charge-transfer rates to the Fe(CN)6 (3-) probe for the GBC- and FBK-modified Au electrodes in relation to bare Au. From these measurements it can be concluded that the FBK film is less compact or presents more pinholes than the electrografted GBC layer.