Iridium Application in Low-Temperature Acidic Fuel Cells: Pt-Free Ir-Based Catalysts or Second/Third Promoting Metal in Pt-Based Catalysts?

Pt and Ir have similar properties, that is, same period and block (6d) of the periodic table, same fcc crystal structure, similar mass and atomic size. The cost of iridium, however, is ca. 60% lower than that of platinum, so it could be a good substitute for Pt as a catalyst in fuel cells, also if it has to be taken into account that a large use of iridium in fuel cells could noticeably increase its price. Moreover, iridium is stable towards dissolution in acid medium. For these reasons, Pt-free Ir-based catalysts and Ir-containing Pt-based catalysts have been tested both as anode and cathode materials in low-temperature acid fuel cells. In this work an overview of the application of Ir and Ir-containing catalysts as anode materials for hydrogen, methanol and ethanol oxidation, and as cathode materials for oxygen reduction in low-temperature polymer electrolyte fuel cells fuelled with hydrogen or low molecular weight alcohols, is presented. The more suitable application of iridium in fuel cell electrodes, either as non-Pt Ir-based catalysts, maintaining an acceptable catalytic activity, or as co-catalysts in Pt-based materials, to increase fuel cell performance simultaneously to a slight decrease of the cost, is discussed.