Correlating Structure and Oxygen Reduction Activity on Y/Pt(111) and Gd/Pt(111) Single Crystals
Authors: Ulrikkeholm Elisabeth Therese , Pedersen Anders Filsøe , Johansson Tobias Peter , Malacrida Paolo , Escudero-Escribano María , Hernandez-Fernandez Patricia , Vej-Hansen Ulrik Grønbjerg , Pedersen Christoffer Mølleskov , Friebel Daniel , Rossmeisl Jan , Stephens Ifan E. L. , Nilsson Anders , Chorkendorff Ib 
Published: February 2020

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 at the cathode to a level comparable to Pt-group metal loading in internal combustion engines.

PtxY and PtxGd exhibit exceptionally high activity for oxygen reduction, both in the polycrystalline form and the nanoparticulate form. [1,2,3,4]. Moreover, their negative alloying energy may make them inherently less prone to degradation via dealloying than the more commonly investigated alloys of Pt and late transition metals such as Ni, Co, Fe and Cu. In order to understand the origin of the enhanced activity of these alloys, we have investigated Y/Pt(111) [5] and Gd/Pt(111) single crystals, formed by depositing large amounts of Y and Gd on Pt(111) single crystals under Ultra-High Vacuum (UHV) conditions and annealing to high temperatures.  We subsequently characterised the surface using low energy electron diffraction, ion scattering spectroscopy and temperature programmed desorption of CO.  After the characterization in UHV, the ORR activity was measured. Angle resolved X-ray photoelectron spectroscopy measurements were carried out after the electrochemical measurements. These experiments revealed, that thick platinum overlayers had been formed, and that the structure formed under reaction conditions was significantly different from our initial expectations. The structures of the overlayers were investigated using surface sensitive X-ray diffraction using synchrotron radiation, and correlated to the oxygen reduction activity.

[1] M. Escudero-Escribano, A. Verdaguer-Casadevall, P. Malacrida, U. Grønbjerg, B. P. Knudsen, A. K. Jepsen, J. Rossmeisl, I. E. L. Stephens, and I. Chorkendorff,  Journal of the American Chemical Society, 134(40):16476–16479, Oct 10 2012.

[2] 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.

[3] A. Velázquez-Palenzuela, F. Masini, A. F. Pedersen, M. Escudero-Escribano, D. Deian, P. Malacrida, T. W. Hansen, D. Friebel, A. Nilsson, I. E. L. Stephens, I. Chorkendorff, J. Catal. 2015, in press.

[4] Hernandez-Fernandez, P., Masini F., McCarthy D. N., Strebel C. E., Friebel D., Deiana D., Malacrida P., Nierhoff A., Bodin A., Wise A. M., Nielsen J. H., Hansen T. W., Nilsson A., Stephens I. E. L., and Chorkendorff I. Nat Chem, 6(8): 732-738, Aug 2014

[5] T. P. Johansson, E. T. Ulrikkeholm, P. Hernandez-Fernandez, M. Escudero-Escribano,P. Malacrida, I. E. L. Stephens, and I. Chorkendorff. Physical Chemistry Chemical Physics, 16(27):13718–13725, 2014.