Macrocycles for Fuel Cell Cathodes

The direct four-electron reduction of O₂ to H₂O is an important issue in developing efficient cathode catalysts for fuel cells. Cobaltporphyrins have been demonstrated to serve as catalysts for the four-electron reduction of O₂, especially in the forms of dicobalt cofacial porphyrins and multinuclear cobaltporphyrins. Simple cobaltporphyrins with spontaneous face-to-face aggregation properties are also expected as efficient catalysts. For practical application as fuel cell cathode catalysts, it is necessary to use cobaltporphyrins dispersed on carbon particles with high surface area. Here, the recent developments on porphyrin-based catalysts and attempts to prepare face-to-face aggregates of cobaltporphyrins adsorbed dispersively on carbon black are summarized. The catalyst has been prepared by using a homogenizer in mixing cobaltporphyrin and carbon black, which gives rise to electroreduction of O₂ at a remarkably positive potential and shows a high selectivity for the direct four-electron reduction process.