A series of second generation core-shell alloy structures have been investigated for their catalytic performance towards the oxygen reduction reaction1-3 (ORR) using periodic, self-consistent density functional theory (DFT) calculations. These alloys are layered structures of pseudomorphically-deposited, homogenous materials on top of a substrate. All the alloys studied include at least one monolayer of Pt as a skin, and often include one or two monolayers of Pd separating the Pt skin from the bulk substrate material. Using a simple electrochemical model4, three distinct ORR mechanisms were studied on the close-packed facets of these catalysts, categorized by the method in which the O-O bond is cleaved. The mechanisms include (1) a direct route involving dissociation of O2 (2) hydrogen-assisted dissociation of peroxyl (OOH) and (3) hydrogen-assisted dissociation of hydrogen peroxide (HOOH). A Sabatier analysis of catalytic activity leads to a volcano5 relation between the catalyst's activity and the binding energy of OH. A series of alloy compositions are studied to determine the most-dominant mechanism as well as the optimal binding energy of OH to maximize ORR activity. The results of the study show that some of these alloys have the possibility of achieving higher energy efficiency. Also, the catalysts show higher (or comparable) activity to a pure Pt catalyst, at the operating potential of 0.80 V. This study provides useful insights for designing more active, more efficient and cheaper catalysts for cathodes of H2 proton-electrolyte-membrane (PEM) fuel cells.
1. Nilekar, A. U.; Mavrikakis, M., Improved oxygen reduction reactivity of platinum monolayers on transition metal surfaces. Surface Science 2008, 602, L89.
2. J. Zhang, M.B. Vukmirovic, Y. Xu, M. Mavrikakis, R. R. Adzic, Controlling the Catalytic Activity of Platinum Monolayer Electrocatalysts for Oxygen Reduction with Different Substrates, Angewandte Chemie International Edition 2005, 44, 2132.
3. J. Zhang, M.B. Vukmirovic, K. Sasaki, A.U. Nilekar, M. Mavrikakis, R.R. Adzic, Mixed-Metal Pt Monolayer Electrocatalysts for Enhanced Oxygen Reduction Kinetics, Journal of the American Chemical Society (Communication) 2005, 127, 12480.
4. NÝrskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H., Origin of the overpotential for oxygen reduction at a fuel-cell cathode. Journal of Physical Chemistry B 2004, 108, 17886.
5. Bligaard, T.; NÝrskov, J. K.; Dahl, S.; Matthiesen, J.; Christensen, C. H.; Sehested, J., The Bronsted-Evans-Polanyi relation and the volcano curve in heterogeneous catalysis. Journal of Catalysis 2004, 224, 206.