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

Abstract

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 n X-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”.

Why conclusions from platinum model surfaces do not necessarily lead to enhanced nanoparticle catalysts for the oxygen reduction reaction

“…1 summarizes original and literature experimental data reported by different groups on the ORR activities and *OH adsorption potentials of stepped single-crystal Pt surfaces (a similar collection of data on single-crystal Pt alloys can be found in ref. 40).…”

Lattice mismatch as the descriptor of segregation, stability and reactivity of supported thin catalyst films

“…[29][30][31][32][33][34] In order to link the activity and composition of nanoparticles and polycrystalline alloys based on Pt, significant efforts have been made in organizing and analyzing the available experimental results. 35,36 In TFC (or CSC) the surface composition and the electronic structure of the coating (shell) can be affected by underlying support (core). The effects can be due to the chemical environment (ligand effect) or due to the lattice mismatch (strain effect).…”

Formation of Pt Skin Layer on Ordered and Disordered Pt-Co Alloys and Corrosion Resistance in Sulfuric Acid

“…The enhancement of the ORR activity by alloying is attributed to the formation of the Pt skin layer; the change in the electronic structure of Pt atoms at the surface due to the existence of alloying elements in the sub-surface layer leads to a shift in the Pt d-band center, which affects the binding energy between adsorbed oxygenated species (OH, O) and the Pt surface [18][19][20][21]. The adsorption of oxygenated species which interfere with ORR on the Pt surface is thought to be suppressed at the Pt skin layer formed on Pt alloy catalysts in the potential region near the open-circuit voltage of PEFCs by the shift in the Pt dband center [18,22]. Among various Pt alloys, Pt-Co is a leading candidate for use as an alternative to Pt because it has the highest ORR activity; this is attributed to the appropriate arrangement of the Pt d-band center, as shown in experimental results and density function theory (DFT) simulations [23,24].…”