Clarifying Sequential Electron-Transfer Steps in Single-Nanoparticle Electrochemical Process for Identifying the Intrinsic Activity of Electrocatalyst

Single-nanoparticle collision electrochemistry (SNCE) is an effective method for determining the intrinsic activity of electrocatalysts at the single-nanoparticle (NP) level. Despite fruitful advancements in the SNCE field, determining a quantitative relationship between the NP structure and its activity has remained difficult because of an unclear understanding of SNCE. In this study, we successfully uncovered the essential roles of the sequential electron-transfer steps in the SNCE system in regulating the apparent electrocatalytic activity of single NPs. By monitoring the oxygen reduction reaction of individual Pt NPs, significantly distinct apparent activities were observed at different electrodes owing to the rate-determining step-controlled electron transfer process. Furthermore, a new theoretical model is proposed for treating the electrochemical current, which involves NP-electrode electron transfer, heterogeneous electron transfer, and mass transfer in solution as sequential steps in the SNCE system. The combination of theoretical simulations and high-resolution electrochemical measurements allows for the corresponding parameters (contact resistance, heterogeneous kinetic constants, and adsorption possibility) of sequential electron-transfer steps to be quantified, resulting in the identification of a rate-determining step for improving the intrinsic activity of electrocatalysts. This work provides a clear picture for determining the intrinsic activity of single NPs in SNCE measurements and introduces a new conceptual route for the quantification of structure-activity relationships, which ultimately guide the rational design and optimization of electrocatalytic nanomaterials.