Transport parameters are key to optimizing the performance of electrochromic devices, yet a fundamental understanding of ionic transport in intercalation materials is still incomplete. Conventional techniques such as galvanostatic and potentiostatic intermittent titration (GITT/PITT) and electrochemical impedance spectroscopy (EIS) probe transport properties averaged over the sample volume and may therefore fail to spatially resolve transport phenomena, particularly in materials that undergo structural transformations and exhibit concentration-dependent diffusion.
Here, we demonstrate that electrochromic transition-metal oxides provide excellent model systems for investigating such transport processes. By covering parts of the thin film with a protective PMMA layer and exposing only a narrow stripe-like gap to the electrolyte, hydrogen insertion is spatially confined and induces lateral diffusion underneath the PMMA layer perpendicular to the stripe. The electrochromic response, combined with the Beer-Lambert law, enables in situ mapping of hydrogen concentration profiles during electrochemical intercalation and deintercalation.
This lateral geometry as shown in Fig. 1 extends the accessible diffusion length scale from the mere film thickness of a few hundred nanometers to several hundred micrometers in the plane of the film, allowing long diffusion times and highly resolved concentration profiles. Complementary Raman spectroscopy at different distances from the PMMA gap provides local information on structural phase transformations as the hydrogen concentration evolves.
Using this approach, we identify pronounced concentration-dependent diffusion coefficients on both polycrystalline WO₃ and β-MoO₃ and directly correlate changes in transport behavior with structural phase transitions. Furthermore, we extend the method to WO₃/NiO heterostructures to investigate how a p-n semiconductor junction and its associated depletion regions influence diffusive transport.
Our results demonstrate that lateral diffusion experiments provide a powerful platform for resolving local ionic transport phenomena in electrochromic materials and offer new insights into the interplay between diffusion, structure, and interfaces in intercalation systems.
 

hydrogen intercalation,lateral diffusion,in-situ Raman spectroscopy,transition-metal oxides