Role of Iridium in Pt-based Alloy Catalysts for the ORR: Surface Adsorption and Stabilization Studies

Pt2IrCo and Pt2IrCr ternary alloys are studied using density functional theory and compared with the bimetallic Pt3Co and Pt3Cr alloys to determine the effect of the substitution of platinum by iridium on surface adsorption properties and catalytic activity toward the oxygen reduction reaction (ORR) as well as electrochemical stability against dissolution of the Pt atoms from the ternary alloy surfaces. Ternary alloys exhibit a strong Pt surface segregation tendency leading to the formation of a Pt monolayer (ML) on their surfaces. The presence of such top Pt ML induces relatively weaker binding strengths of O and OH in the ternary alloys, suggesting further activity enhancement for the ORR compared to the corresponding bimetallic alloy surfaces. Moreover, the Pt-skin surfaces of Pt2IrCo and Pt2IrCr show an enhanced electrochemical stability with respect to the dissolution of Pt atoms from the alloy surface. The origin of the catalytic activity and stability enhancement for the alloys is analyzed. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3409559] All rights reserved.