Activity Descriptor Identification for Oxygen Reduction on Platinum-Based Bimetallic Nanoparticles: In Situ Observation of the Linear Composition–Strain–Activity Relationship

[Image: see text] Despite recent progress in developing active and durable oxygen reduction catalysts with reduced Pt content, lack of elegant bottom-up synthesis procedures with knowledge over the control of atomic arrangement and morphology of the Pt–alloy catalysts still hinders fuel cell commercialization. To follow a less empirical synthesis path for improved Pt-based catalysts, it is essential to correlate catalytic performance to properties that can be easily controlled and measured experimentally. Herein, using Pt–Co alloy nanoparticles (NPs) with varying atomic composition as an example, we show that the atomic distribution of Pt-based bimetallic NPs under operating conditions is strongly dependent on the initial atomic ratio by employing microscopic and in situ spectroscopic techniques. The Pt(x)Co/C NPs with high Co content possess a Co concentration gradient such that Co is concentrated in the core and gradually depletes in the near-surface region, whereas the Pt(x)Co/C NPs with low Co content possess a relatively uniform distribution of Co with low Co population in the near-surface region. Despite their different atomic structure, the oxygen reduction reaction (ORR) activity of Pt(x)Co/C and Pt/C NPs is linearly related to the bulk average Pt–Pt bond length (R(Pt–Pt)). The R(Pt–Pt) is further shown to contract linearly with the increase in Co/Pt composition. These linear correlations together demonstrate that (i) the improved ORR activity of Pt(x)Co/C NPs over pure Pt NPs originates predominantly from the compressive strain and (ii) the R(Pt–Pt) is a valid strain descriptor that bridges the activity and atomic composition of Pt-based bimetallic NPs.