First principles investigation of the activity of thin film Pt, Pd and Au surface alloys for oxygen reduction

Abstract

Further advances in fuel cell technologies are hampered by kinetic limitations associated with the sluggish cathodic oxygen reduction reaction. We have investigated a range of different formulations of binary and ternary Pt, Pd and Au thin films as electrocatalysts for oxygen reduction. The most active binary thin films are near-surface alloys of Pt with subsurface Pd and certain PdAu and PtAu thin films with surface and/or subsurface Au. The most active ternary thin films are with pure metal Pt or Pd skins with some degree of Au in the surface and/or subsurface layer and the near-surface alloys of Au with mixed Pt-Pd skins. The activity of the binary and ternary catalysts is explained through weakening of the OH binding energy caused by solute elements. However, given the low alloy formation energies it may be difficult to tune and retain the composition under operating conditions. This is particularly challenging for alloys containing Au due to a high propensity of Au to segregate to the surface. We also show that once Au is on the surface it will diffuse to defect sites, explaining why small amounts of Au retard dissolution of Pt nanoparticles. For the PtPd thin films there is no pronounced driving force for surface segregation, diffusion to defects or surface self-assembling. On the basis of stability and activity analysis we conclude that the near surface alloy of Pd in Pt and some PdAu binary and PtPdAu ternary thin films with a controlled amount of Au are the best catalysts for oxygen reduction.

Oxygen Reduction Reaction on Pt Overlayers Deposited onto a Gold Film: Ligand, Strain, and Ensemble Effect

“…5). Some of the present authors argued in a recent article that Au containing alloys most likely will not be active for ORR because of the large Au lattice constant; 14 however, here we see that there are some active formulations after all. It should be further noted that Au(100) is known to undergo reconstruction in an electrochemical environment 15 that results in growth of an epitaxially strained hexagonal surface layer.…”

“…It is expected that the surface will become pure Au or highly Au enriched over time due to Au segregation. However, the effect can be reversed by oxygen induced Pt segregation at higher potentials (O favors Pt sites) 14 . Hence, in order to maintain active compositions in fuel cells the potential needs to be cycled between potentials at which ORR operates and potentials at which lost Pt surface can be recovered.…”

Molecular dynamics simulations of ternary PtxPdyAuz fuel cell nanocatalyst growth

“…and Pt 70 Pd 15 Au 15 lead to improved catalytic activity towards ORR and that gold could be responsible for improved catalyst stability. The choice for gold content below 25% was dictated by the necessity to obtain a good compromise between catalytic activity and stability, due to gold segregation to nanocatalyst surface [27,32]. Similar trends in chemical ordering were revealed when more comprehensive searches were performed, utilizing genetic algorithms to elucidate stable structures of PtAu and PtPdAu nanoparticles, though these searches were performed at 0K in vacuum [46,47].…”

“…binary catalysts have been proposed as possible candidates for both PEMFC electrodes, the anode for the hydrogen oxidation reaction and the cathode for improved oxygen reduction reaction (ORR) compared with pure platinum [18][19][20][21][22]. At the same time, Au addition was proposed to improve durability of the nanocatalysts [23][24][25][26][27]. On the other hand, the atomic structure and morphology [28] also play very important roles in the electrocatalytic efficiency for ORR.…”

“…This is of interest for comparing structure predictions and consistency with ORR efficiency and cyclic voltammetry results. The present work is devoted to MD simulations applied to the synthesis of PtPdAu ternary catalysts by both a wet chemical method at room temperature and a plasma sputtering co-deposition, based on the availability of recent detailed experimental [32] and DFT results [27].…”