Size Effect of Single Nanoparticle Electrodes on the Electrochemical Interface and Charge Transfer Kinetics and New Insights into Their Size-Activity Relations

Abstract

Understanding the particle size effects on the electrochemical reaction kinetics is of particular importance for rational design of nanoparticle materials for energy conversion and storages such as fuel cell electrocatalysts and battery materials. Our present knowledge on the size effects of nanoparticle catalysts is generally related to the impact of particle size on the atomic or electronic structures of nanoparticle surfaces. For electrocatalytic reactions, it is well known that the charge transfer kinetics not only depends on the electrode surface structure but also, perhaps more importantly, rely on the electrochemical solid-liquid interfaces, namely, the electric double layers (EDL). Effect of particle size on the latter aspect and thus the charge-transfer kinetics, however, has remained largely unexplored. Exemplified by two redox reactions with different electron transfer kinetics (such as oxygen reduction reaction in fuel cells), we performed finite element analysis of the EDL potential distributions, concentration distributions of redox and intermediate species, and the overall voltammetry behaviors of single platinum (Pt) nanoparticle electrodes as functions of the nanoparticle size for the first time. Our results reveal significant influence of particle size on the potential drop distributions within the EDL of nanoparticle electrodes; decreasing particle size led to an increase of the potential at the compact layer, translating into apparent size effects on the charge transfer kinetics particularly for slow electrochemical reactions. These results provide important new insights into the size effect of nanoparticle materials for fuel cell electrocatalysis and battery electrodes.