Beyond Extended Surfaces: Understanding the Oxygen Reduction Reaction on Nanocatalysts.

Increasing the platinum utilization efficiency is the key to the advancement and broad dissemination of proton-exchange-membrane fuel cells (PEMFCs). Central to the task is the creation of highly active and durable Pt-based catalysts for the cathodic oxygen reduction reaction (ORR), which demands a comprehensive understanding of the ORR processes on these catalysts under reaction conditions. Past efforts have accumulated a vast wealth of knowledge of the ORR on extended Pt and Pt-alloy model surfaces. While the knowledge has been applied to understanding and designing ORR catalysts, it has also been recognized that these understandings cannot always translate into nanoscale systems. In this Perspective, we will review the progress that the theoretical descriptor has evolved to reconcile the observed differences between extended and nanoscale Pt surfaces, and we highlight the needs in advancing both characterizations and theories in order to understand ORR in the more complex Pt-alloy nanocatalysts. Particularly, understanding the dynamic structure-composition-function relation of Pt-alloy nanocatalysts during ORR demands concerted efforts in precision synthesis, advanced atomistic-scale in situ characterization, and comprehensive computational models.