Dynamic stability of Pt-based alloys for fuel-cell catalysts calculated from atomistics†

Abstract

The oxygen reduction reaction (ORR) is the fundamental electron-accepting reaction in aqueous electrochemistry, and is crucial to technologies such as fuel cells and batteries. Alloys of Pt that produce a surface Pt layer under mildly compressive strain are generally the most reactive catalysts for this reaction; however, their long-term durability can vary widely with preparation. In this work, we develop atomistic models based on electronic structure calculations to compare and rationalize the stability of such electrocatalysts, focusing on contrasting face-centered tetragonal (FCT or L1₀) alloys of Fe, Ni and Co with that of their face-centered cubic (FCC or L1₂) counterparts. We first describe how the non-noble elements Ni, Fe, Co and Sc of the near-surface alloy have the driving force to undergo rapid dissolution at fuel-cell operating conditions, whereas Pt is quite stable against dissolution, leading to the well-known surface Pt enrichment. Post dissolution, we discuss the kinetics associated with diffusion of sub-surface elements through a vacancy mediated diffusion model. Through the diffusion models, we compare and discuss the effect of geometry and surface structure on an electrocatalyst's stability. We show that alloying Pt with non-noble elements results in significantly higher kinetic stability of the core as compared to pure Pt. Our calculations suggest that the diffusion energetics at the bulk can be quite different from the near-surface region; we conclude the surface rates are more essential. We find that L1₀ structures of Fe and Co could provide better stability than the L1₂ systems, especially in the presence of Pt overlayers. In contrast, for Sc-containing species, we argue that presence of Pt overlayers destabilizes the catalytic surface, as also reported by an experimental study.