Deconvoluting Surface\nand Bulk Charge Storage Processes\nin Redox-Active Oxides by Integrating Electrochemical and Optical\nInsights

Redox-active transition metal oxides (TMOs) play crucial\nroles\nin diverse energy storage and conversion technologies, such as batteries\nand pseudocapacitors. These materials show intricate electrochemical\ncharge storage processes, encompassing both bulk ion-intercalation,\ntypical of battery electrodes, and pseudocapacitive-like behavior\nlocalized near the surfaces. However, understanding the underlying\nmechanisms of charge storage in redox-active TMOs is challenging due\nto the coexistence of these behaviors. In this study, we propose an\nintegrated approach that combines <i>operando</i> electrochemical\nand optical techniques to disentangle the contributions of bulk and\nsurface phenomena. Using birnessite δ-MnO<sub>2–<i>x</i></sub> as a model system, we account for surface pseudocapacitive-like\nlayers and employ a refined model that incorporates both surface reactions\nand bulk chemical diffusion. This methodology allows us to extract\nessential kinetic parameters, establishing a fundamental framework\nfor unraveling surface and bulk electrochemical processes. This advancement\nprovides a valuable tool for the rational design of energy storage\ndevices, enhancing our ability to tailor these materials for specific\napplications.