Carbon nanotube-supported Pd catalysts (Pd/CNT) as the cathode for polymer electrolyte fuel cells (PEFCs) were modified with various transition metals (Fe, Co, Ni and Cu) to improve their catalytic activity for the oxygen reduction reaction (ORR). Modification with these transition metals enhanced the activity of Pd/CNT for the ORR, especially the activity of Pd-Co/CNT, which was 2 times higher than that of Pd/CNT. X-ray diffraction and X-ray absorption spectroscopy indicated that the metal species in Pd-Co/CNT was mainly present as a Pd-Co alloy, which acts as catalytically active sites for the ORR. However, the Pd-Co/CNT catalysts were rapidly deactivated for the ORR in 0.1 M HClO 4 electrolyte due to the dissolution and diffusion of metal species out of the catalysts. Pd-Co/CNT was covered with silica layers with a thickness of a few nanometers, which prevented the diffusion of Co and Pd species into the HClO4 electrolyte. Thus, silica-coated Pd-Co/CNT catalysts exhibited high activity for the ORR and excellent durability under severe cathode conditions.
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