Understanding the Oxygen Reduction Reaction on a Y/Pt(111) Single Crystal

Polymer electrolyte membrane fuel cells (PEMFC) hold promise as a zero-emission source of power, particularly suitable for automotive vehicles. However, the high loading of Pt required to catalyse the oxygen reduction reaction (ORR) at the PEMFC cathode, prevents the commercialisation of this technology. Improving the activity of Pt by alloying it with other metals could decrease the loading of Pt. An earlier theoretical study conducted at our laboratory identified Pt_xY as an active and stable catalyst for oxygen reduction. Experiments conducted on sputter-cleaned polycrystalline Pt₃Y confirmed that it showed exceptional activity for the ORR [1]. Later studies also revealed that Pt₅Y, prepared in the same manner also showed similar high activity, as shown in Figure (a) below. However, our X-ray photoelectron spectroscopy measurements revealed that the structure formed under reaction conditions was significantly different from our initial expectations.

In order to understand this phenomenon, we investigated a Y/Pt(111) single crystal, formed by depositing large amounts of Y om Pt(111) under ultra-high vacuum (UHV) conditions and annealing to high temperatures. We subsequently characterised the surface using electrochemical measurements, low energy electron diffraction, ion scattering spectroscopy, angle resolved X-ray photoelectron spectroscopy, temperature programmed desorption of CO, and synchrotron based X-ray absorption spectroscopy and surface sensitive X-ray diffraction. These measurements were supported by DFT calculations. From the experimental investigations of the crystal structure, it could be concluded that we have obtained a Pt₅Y structure with a 5.5 % compressive strain compared to Pt. Electrochemical measurements show that the activity approximates that of sputter-cleaned Pt₃Y, as shown in Figure A. Since there is no Y in the first few atomic layers, we attribute the high activity to the compressive strain exerted onto the surface by the alloy bulk.

[1] J. Greeley, I.E.L. Stephens, A.S. Bondarenko, T.P. Johansson, H.A. Hansen, T.F. Jaramillo, J. Rossmeisl, I. Chorkendorff, J.K. Nørskov, Nature Chemistry, 1 (2009) 552-556.