Defect-Driven Electrochemical Domain Modulation in Prussian Blue Revealed by Single-Entity Analysis

Electrochemical domains, defined as the spatial region of the electrode material where charge transfer occurs, are central to charge storage and electrocatalysis. In Prussian Blue (PB), a prototypical mixed-valence framework, these domains extend throughout the three-dimensional lattice and are shaped by structural accessibility and defect distribution. However, ensemble measurements obscure this spatial heterogeneity, averaging local variations in the electrochemical behavior. Here, we apply single-entity electrochemistry to individual PB nanocubes to resolve electrochemical domain behavior during K⁺ insertion and H₂O₂ reduction. Correlative electron microscopy and electrochemical analysis reveal a defect-driven reversal in function: smaller nanocubes exhibit a greater volumetric capacity for K⁺ storage, whereas larger nanocubes show greater catalytic activity for H₂O₂ reduction. This contrast originates from the dual role of structural defects, which limit ion-accessible volume by disrupting lattice connectivity and simultaneously expose coordinatively unsaturated Fe sites that promote catalytic activity. Our findings establish a mechanistic framework that connects structure, electrochemical domain accessibility, and function, demonstrating the power of integrating single-particle electrochemistry with high-resolution structural imaging in resolving spatially heterogeneous interfacial processes in redox-active materials.