Enhanced Oxygen Reduction Activity on Silver-Palladium Alloyed Thin Film Electrocatalysts in Alkaline Media

The oxygen reduction reaction (ORR) requires a large overpotential, which is a factor limiting the large-scale deployment of clean energy technologies such as hydrogen fuel cells (H2 FCs), direct methanol FCs, and metal-air batteries. Designing strategies to enhance catalyst activity, stability, and selectivity for the ORR is one method towards enhancing H2 FCs technology. Many predicted active catalysts are not stable in acidic media, resulting in limited catalyst options for standard proton exchange membrane H2 FCs. Some theoretically active, yet acid-unstable, catalysts, such as Ag, can be stabilized in alkaline media making anion exchange membrane (AEM) fuel cells an attractive alternative. Ag-rich catalysts are specifically ideal because of their low cost and Ag’s high abundance relative to other catalysts materials such as Pt. We have previously shown that Cu atoms in Ag-Cu alloys can be modified by Ag to produce an enhanced activity for the ORR at high Cu atomic loadings. Better understanding the nature of intrinsic enhancements for the ORR on alloys will help in the development of new ORR alloy electrocatalysts. Besides, Ag-Cu alloys, several other Ag-bimetallics (including Ag-Pd, Ag-Mn, Ag-Fe, and Ag-Co) have been predicted to be good catalysts for the ORR. Alloying could lead to activity enhancements via changes in electronic structure, strain, and/or localized geometry. The activity of Ag-based catalysts for the ORR needs to be enhanced. One strategy to do this is to alloy Ag, which binds oxygen adsorbates too weakly, to a metal such as Pd, which binds oxygen adsorbates too strongly and is also very active. In this work, we use a thin film model system to show intrinsic activity enhancements for the ORR on Ag-Pd catalysts, resulting in activities higher than those of both parent materials even after reducing the Pd content by half. Ag-Pd thin films of different compositions are prepared using electron beam physical vapor co-deposition. X-ray diffraction indicate alloying across the full compositional range tested and atomic force microscopy shows that the films are highly flat (roughness factor < 1.02). Electrochemical testing in 0.1 M KOH shows significant activity improvements for the Ag-Pd films. The films with +40 at% Pd have activities, at 0.9 V vs RHE, greater than the linear combination of pure Ag and Pd activities. On a Pd content basis, the thin films with +50 at% Pd exhibit higher kinetic current densities, at 0.9 V vs RHE, than both pure Ag and Pd. Ag10Pd90 is the most enhanced thin film, both on a surface area and Pd loading bases, demonstrating a 5-fold activity enhancement relative pure Pd. By correlating density functional theory with experimental activity and physical characterization measurements, we hypothesize that Ag is modifying Pd’s surface electronic structure to adsorb OH more weakly via ligand and strain effects. These enhanced Ag-Pd ORR catalysts could facilitate the large-scale implementation of efficient clean energy electrochemical systems such as fuel cells and metal-air batteries.