Enhancing ORR Performance of Bimetallic PdAg Electrocatalysts by Designing Interactions between Pd and Ag

Abstract

Precise tuning of the electronic properties of Ag/C using under potentially deposited (UPD) Cu and subsequent galvanic displacement to deposit atomically dispersed loading of Pd resulted in a robust bimetallic alloy with significant activity for the oxygen reduction reaction in alkaline media. The specific design of the catalyst and atomic arrangement of Pd−Ag outperforms conventional Pd/C and Ag/C commercial catalysts. The ORR activity of Pd deposited onto Ag/C was determined on the basis of rotating disk electrode voltammetry studies, showing a 2-fold increase in Pd mass activities compared to results obtained using Pd/C. While scanning transmission electron microcopy (STEM) coupled with electron energy loss spectroscopy (EELS) probed the uniformity of the nanoparticles, the origin of the outstanding activity was traced to the structural properties of the Pd−Ag interface as shown by X-ray absorption spectroscopy (XAS), along with X-ray photoelectron spectroscopy (XPS). Segregation of metals with a suitable geometric arrangement of the Ag to Pd ratio at the interface resulted in an increased performance where the active sites were key steps of oxygen bond breaking.

Tuning the electronic structure of Ag-Pd alloys to enhance performance for alkaline oxygen reduction

“…1f ) of select Ag 1– x Pd x compositions ( x = 1, 0.9, 0.8, 0.5, 0.2). Together, the Ag 3 d XPS and Pd L 3 -edge XAS support a model, similar to that seen in previous Ag-Pd alloys 23 , 32 , 33 , in which the Ag and Pd d -bands become more filled as each metal dilutes the other. According to the d -band theory description of metal-adsorbate interactions, this change in d -band electron density has important catalytic implications that will be described further below.…”

“…Ag-Pd alloys have been previously synthesized and are thermodynamically stable and miscible at all compositions 20 – 23 , 26 – 30 . Here, we synthesize a series of Ag 1- x Pd x (0 ≤ x ≤ 1) thin films (~70 nm thick) via electron-beam physical vapor deposition (PVD) and probe the crystal structure, extent of alloying, electronic structure, and topography of the as-prepared thin films with x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), x-ray absorption spectroscopy (XAS), and atomic force microscopy (AFM).…”

“…Although Pd XPS is complicated to interpret, the L 3 -edge XAS of Pd has been used effectively in the literature to elucidate the charge density within Ag-Pd alloys 23 , 32 , 33 . The small deviations from the single-particle-model for 4 d metals makes the use of the L 3 -edge quantitatively meaningful 34 .…”

“…Ag shows promise as a secondary species because of its relatively low cost, high abundance, base stability, and high 4 e − selectivity 14 – 16 . Furthermore, previous experimental studies have demonstrated enhancement both in Ag-based bimetallics (e.g., Cu-Ag 17 , Co-Ag 18 , and Sn-Ag 19 ) and Ag-Pd nano-configurations 20 – 23 , and physics-based modeling 24 has predicted enhancement for the ORR based on d -band changes. Understanding the mechanism behind alloy-based enhancement could guide the discovery of structure-activity relationships for more complicated ternary or quaternary systems.…”

Recent Advances in Electrocatalysts for Proton Exchange Membrane Fuel Cells and Alkaline Membrane Fuel Cells

“…[245][246][247][248][249][250][251][252][253][254] and PGM-free (Ni, Co, Ag, etc.) [255][256][257][258] cores have also been explored and illustrated in Table 3.…”