Cathode catalysts for low-temperature fuel cells : analysis of surface phenomena
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The electrochemical oxygen reduction reaction (ORR) steps on a noble metal catalyst in an acidic aqueous electrolyte depend on the nature of the catalytic surface with which the O₂ molecule interacts. It has been assumed that the O₂ molecules interact directly with a bare noble-metal surface. By studying the nature of chemisorbed species on the surface of a metal catalyst as a function of the voltage on the anodic and cathodic sweeps, it is shown here that the O₂ reacts with a surface covered with oxide species extracted from the aqueous electrolyte and not from the O₂ molecules; the ORR is more active when the surface species are OH rather than O. Moreover, the strength of the chemical bond of the adsorbed species was shown to depend on the relative strengths of the metal-metal versus metal-oxide bonds. The Pt-Pt bonds are stronger than the Pd-Pd bonds, and the relative Pd-O bonds are stronger than the relative Pt-O bonds. As a result, the chemisorbed O species is stable to lower anodic potentials on Pd. CO oxidation to CO₂ occurs at a higher potential on Pd than on Pt, which is why Pd (not Pt) is tolerant to methanol. Experiments with alloys show the following: (1) methanol tolerance decreases with the increase of Pt in the Pd-Pt alloys with Pd₃Pt/C showing an initial tolerance that decreases with cycling; (2) OH is formed on Pt₃Co/C and core-shell Pt-Cu/C, which results in a higher activity and durability for the ORR on these catalysts; (3) a 300°C anneal is needed to stabilize the Pd₃Au/C catalyst that forms an O adsorbate; and (4) OH is formed on Pd₃Co/C and Pd₃CoNi/C. These studies provide a perspective on possible pathways of the ORR on oxide-coated noble-metal alloy catalysts.