The development of suitable catalyst materials for oxygen reduction in the cathode half-cell reaction of PEM fuel cells has received tremendous attention in recent years. Metalloporphyrins supported on carbon blacks have proven to be potential candidates as suitable modification materials for the cathode with two and four electron reduction of oxygen to hydrogen peroxide and water respectively, as reported in literature. Heat treatment of these catalysts at temperatures between 400°C and 900°C has lead to enhanced stability. Basic investigations on those materials in principle use dip- or drop- coating techniques and sometimes combined with a heat treatment [1, 2]. Such modified surfaces have been applied extensively in the catalytic reduction of oxygen. An immediately observable limitation however is the possibility of quantifying effectively, the porphyrin-loading in the case of dip or drop coated materials and a drastic reduction in catalytic activity in the case of a heat-coated surface due to a destruction of the heat polymerized metalloporphyrin under high temperature conditions. In order to obtain quantifiable porphyrin loadings as modification materials, electro-polymerized metalloporphyrins of Fe and Mn using pulse potentials or cyclic votammetric deposition procedures have been established. Fundamental parameters such as ring substituents, porphyrin loading and their effects on activity and stability for oxygen reduction have also been investigated, with Fe-porphyrins showing better initial activity but Mn-porphyrins have shown better stability. A combination of two transition metalloporphyrins of Fe and Mn was also investigated. A further step was then taken to visualize the local electro-catalytic activity using scanning electrochemical microscopy (SECM). An estimation of the number of electrons involved in the oxygen electro reduction, to water or hydrogen peroxide was attempted, using the redox competition mode of scanning electrochemical microscopy (RC-SECM) [3]. Details of experimental procedures and results shall be discussed.

References

1. Jianying Qu, Yan Shen, Xiaohu Qu & Shaojun Dong. Chem. Commun., (2004), 34 - 35

2. Deryn Chu, Rongzhong Jiang. Solid State Ionics (2002), 148, 591 - 599

3. Kathrin Eckhard, Xinxing Chen, Florin Turcu & Wolfgang Schuhmann. Phys. Chem. Chem. Phys, (2006), 8, 5359 - 5365

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