Differential Surface Elemental Distribution Leads to Significantly Enhanced Stability of PtNi-Based ORR Catalysts

PtNi-based nanomaterials represent an emerging class of highly active catalysts for the oxygen reduction reaction (ORR) in fuel cells. However, they suffer from poor stability in operating conditions, which is the key challenge in maintaining their activity advantage over Pt in practice. We report significantly enhanced stability and activity of octahedral PtNi nanoparticles by tuning their surface elemental distribution through the introduction of a third element (Cu) during synthesis. To uncover the mechanism behind this observation, we performed kinetic Monte Carlo (KMC) simulations initialized using growth-tracking experiments and demonstrated that PtNiCu has improved Ni and Cu retention compared with PtNi, in agreement with experiments. The tracked movement of individual atoms in KMC reveals that the enhanced stability can be attributed to increased surface Pt composition in as-synthesized catalysts, which reduces the generation of surface vacancies and suppresses the surface migration and subsequent dissolution of subsurface Cu and Ni atoms. One of the main challenges in rationally designing high-performance nanoscale catalysts is to understand the atomic-scale mechanisms that contribute to the properties and structural evolution of the catalyst. This is primarily due to the limitations in atomic-scale experimental characterizations and the lack of computational models to describe the realistic in situ evolution of atomic arrangements. We have used a novel combination of experimental characterization and atomic-scale simulations to explain the improvement in the stability and activity of PtNi nanoparticles for the oxygen reduction reaction when Cu is added. Our simulations reveal that the improved catalytic activity and stability are primarily due to enhanced initial surface Pt composition in the nanoparticles that contain Cu, which we attribute to the higher reduction potential of Cu relative to Ni. This study opens the door to new design strategies for high-performance nanoscale catalysts. The introduction of Cu into PtNi nanoparticles has improved the catalytic activity and stability of the particles for the oxygen reduction reaction. To explain this improvement, we ran kinetic Monte Carlo (KMC) simulations initialized using layer-by-layer compositions deduced from the experimental growth trajectory of the particles. The KMC runs revealed that enhanced initial surface Pt composition in the particles containing Cu suppressed the dissolution of subsurface Cu and Ni atoms, leading to improved stability and activity.