Pt Alloy Electrocatalysts for the Oxygen Reduction Reaction: From Model Surfaces to Nanostructured Systems

Polymer electrolyte membrane fuel cells are a promising alternative for future energy provision. However, their wider utilization is hindered by the slow rate of the oxygen reduction reaction (ORR) taking place at the cathode. In order to improve the ORR kinetics, alloys of Pt with late transition metals and lanthanides have been studied extensively, as they offer enhanced activity and in some cases acceptable stability. Nevertheless, many of these alloys are far from being "model objects" and their surface composition and structure are not stable under operating conditions in PEMFCs. The solute metal can dissolve from the surface and near-surface layers. This process often results in a structure in which several Pt-enriched layers cover the bulk alloy and protect it from further dissolution. In this work, we analyze the literature results on the properties of these alloys, from single crystals and polycrystalline materials to nanoparticles, gathered in the recent decades. As a result of this analysis, we additionally propose a relatively simple method to overview the activities of dealloyed Pt_nX-type alloys toward the ORR. Given that the Pt overlayer is several atomic layers thick, the so-called strain effects should primarily determine the behavior of these catalysts. The strain in the system is the result of the differences between the lattice parameters of the alloy and Pt-rich overlayers, causing dissimilar compressive strains in the lattice of the Pt-rich layer. This causes changes in the electronic structure and, consequently, in the binding properties of the surface. We propose that the atomic radius of the solute metal can be used in some particularly complex systems (e.g., polycrystalline and nanostructured alloys) as a simple semiempirical descriptor, statistically connected to the resulting lattice strain. The implications of this phenomenon can be used to qualitatively explain the behavior of e.g. some active Pt-alloy nanoparticles so far considered "anomalous".