Mechanism of Oxygen Electro-Reduction on Au-Modified Pt: Minimizing O Coverage and Pt Site Exposure toward Highly Stable and Active Cathode

作者：Fang Ya Hui; Liu Zhi Pan^*

来源：Journal of Physical Chemistry C, 2011, 115(35): 17508-17515.

DOI：10.1021/jp205376b

To catalyze oxygen reduction reaction (ORR) under electrochemical conditions approaching its thermodynamic limit, i.e., 1.23 V vs NHE, has been consistently pursued by chemists. It is known that metal electrodes, even noble metals, undergo severe corrosion at the high potential, accompanied with the rapid decrease in activity. A comprehensive understanding of both the stability and the catalytic activity of the Pt surface is urgently called for toward the rational design of a new ORR catalyst. This work, by utilizing chemically inert Au as a modifier to the Pt surface, investigated the stability and the activity of a set of Au/Pt composites using first-principles-based theoretical methods designed for the modeling of solid/liquid electrocatalysis. By computing the surface phase diagram and the corrosion thermodynamics, we demonstrate that the presence of Au can remarkably reduce the in situ 0 atom coverage to be below a critical local 0.5 monolayer (ML) and thus protect the neighboring Pt sites from corrosion. The AuPt surface alloy with a very low amount of Au dispersing in the Pt surface layer is sufficient to stabilize the whole catalyst surface. With the calculated ORR profiles, we show that ORR activity in AuPt surface alloys is not sensitive to the Au concentration, and a good activity is maintained with the Au concentration up to 7/8 ML. Fundamentally, this is because the minimum active site of ORR requires only two neighboring Pt atoms (with an OOH pathway), and the majority of surface Pt sites on pure Pt are in fact nonactive spectators that are terminated by 0 atoms at the working potentials. The key factors controlling the ORR activity and surface stability are therefore unified as the minimum 0 coverage and the minimum exposure of Pt (active) sites. The theory presented here suggests that the structural engineering to separate active sites (e.g., two Pt atoms) by inert elements (e.g., Au) is an effective approach for yielding a stable, active, and economic catalyst. Experimental observations on Au/Pt composite catalysts are discussed in the context of current findings, focusing on the thermodynamic tendency for AuPt surface alloy formation.

• 出版日期2011-9-8
• 单位复旦大学