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Carbon supported platinum is commonly used as anode and cathode electrocatalyst in lowtemperature fuel cells fuelled with hydrogen or low molecular weight alcohols. The cost of Pt and the limited world supply are significant barriers to the widespread use of these types of fuel cells. Moreover, platinum used as anode material is readily poisoned by carbon monoxide, present in the reformate gas used as H 2 carrier in the case of polymer electrolyte fuel cells, and a byproduct of alcohol oxidation in the case of direct alcohol fuel cells. In addition, Pt alone does not present satisfactory activity for the oxygen reduction reaction when used as cathode material. For all these reasons, binary and ternary platinum-based catalysts and non-platinum-based catalysts have been tested as electrode materials for low temperature fuel cells. Palladium and platinum have very similar properties because they belong to the same group in the periodic table. The activity for the oxygen reduction reaction (ORR) of Pd is only slightly lower than that of Pt, and by addition of a suitable metal, such as Co or Fe, the ORR activity of Pd can overcome that of Pt. Conversely, the activity for the hydrogen oxidation reaction (HOR) of Pd is considerably lower than that of Pt, but by adding of a very small amount (5 at%) of Pt, the HOR activity of Pd attains that of pure Pt. This paper presents an overview of Pd and Pdcontaining catalysts, tested both as anode and cathode materials for low-temperature fuel cells.

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Among noble metal electrocatalysts, only iridium presents high activity for both the oxygen reduction reaction (ORR) in acid medium, in the oxide form, and the oxygen evolution reaction (OER) in acid medium, alloyed with first row transition metals. Indeed, platinum, the best catalyst for the ORR, has poor activity for the OER in any form, and ruthenium, the best catalyst for the OER, in the oxide form, possess poor activity for the ORR in any form. In this work, an overview of the application of Ir and Ir-containing catalysts for the OER in protonexchange membrane water electrolyzer anodes, for the ORR in proton exchange membrane fuel cell cathodes, and for both OER and ORR in unit regenerative fuel cell oxygen electrodes is presented.

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