Deconvoluting Surface and Bulk Charge Storage Processes in Redox-Active Oxides by Integrating Electrochemical and Optical Insights

Redox-active transition metal oxides (TMOs) are pivotal in enhancing electrochemical energy storage and conversion technologies, encompassing applications in supercapacitors, batteries, and electrolyzers. As electrode materials, redox-active TMOs facilitate charge storage through redox processes at different length scales, from surface pseudo-capacitive-like behavior (<i>i.e.</i>, surface process) to bulk battery-type ion intercalation (<i>i.e.</i>, bulk process). Understanding these processes is crucial for optimizing energy storage and conversion devices due to their coexistence in real applications. However, the significant kinetic differences between surface and bulk processes pose challenges for explicit deconvolution and quantitative analysis. Although recent studies combining <i>operando</i> physical characterizations with electrochemical testing have shown promise in visualizing and validating the kinetic behaviors of surface and bulk processes, there remains a lack of methodologies that can simultaneously provide a quantitative interpretation of these processes based on physico-electrochemical responses within a unified physical framework. This framework requires a refined electrochemical model capable of accurately reconstructing the overall redox-active behavior by properly describing the individual contributions of surface and bulk processes. Achieving this level of precision remains a significant challenge for existing models.<br/><br/>In this study, we propose an integrated approach that combines <i>operando</i> electrochemical and optical techniques to disentangle the contributions of surface and bulk processes. Using birnessite δ-MnO_2-x as a model system, we account for surface pseudo-capacitive-like layers (denoted as <i>L_Surf</i>) and employ a refined model that incorporates both the kinetic parameters of surface reaction rate (denoted as <i>k</i>) and bulk chemical diffusion coefficient (denoted as <i>D</i>). This methodology enables the deconvolution of electrochemical and optical responses corresponding to surface and bulk processes based on the same set of kinetic parameters, thereby precisely elucidating the variations in electrochemical kinetics and <i>operando</i> optical properties for each process within a consistent physical framework. Our method provides a robust framework for investigating the complex interplay of kinetic factors in redox-active TMOs, offering valuable insights for designing and optimizing a wide range of electrochemical technologies.