Real-Time and Spatially Resolved Analysis of Ion (De)Intercalation in Nickel Hexacyanoferrate Nanofilms Using Plasmonic Electrochemical Microscopy

Nickel hexacyanoferrate (NiHCF), a Prussian blue analog, is a promising electrode material for energy storage and ion separation technologies. Studying the ion (de)intercalation process in NiHCF films is critical because the reversible insertion and removal of ions into its crystal lattice governs the material's performance in applications such as sodium-ion batteries, electrochromic devices, and selective ion sensing. However, conventional analytical techniques are limited to bulk measurements, which average the electrochemical response and obscure the critical role of local heterogeneities, such as defects and grain boundaries. To overcome this, we employ plasmonic electrochemical microscopy (PEM), a technique that spatially resolves refractive index changes, to study NiHCF films with high resolution. We demonstrate the unique capabilities of PEM to monitor the electrodeposition process in real time, visualize heterogeneous ion (de)intercalation dynamics and their correlation with film thickness and local ion concentrations, and generate a spatially resolved electrochemical impedance map of NiHCF. This provides unprecedented insight into local charge transfer kinetics and establishes PEM as a powerful methodology for deconvoluting complex, localized phenomena in energy storage materials, paving the way for the rational design of more efficient electrodes.