In-situ Electrochemical Spectroscopic Ellipsometry Characterization of Polymer and MXene Films

Abstract In-situ electrochemical spectroscopic ellipsometry (EC-SE) can enable real-time monitoring of optical properties and thickness changes of surfaces and thin films. This method can provide a broad range of information on the evolution of the microenvironment of an electrode during electrochemical reactions. However, to avoid modeling artifacts, accurate data analysis is critical. Here, we outline key steps in data analysis that can help prevent overfitting and misinterpretation of results. For example, determining the refractive index of the electrolyte at the air/electrolyte interface, along with measurements of the electrode’s dielectric data in dry conditions, can yield more accurate results than simultaneous fitting of all parameters for the electrolyte and electrode. We also present detailed modeling strategies tailored to fitting in-situ cyclic voltammetry (CV) data on various types of thin films, by providing two distinct examples: the thickness change of catalyst polymer films due to swelling by the electrolyte, and the simultaneous change in thickness and optical properties of Ti3C2Tx MXene films due to both reversible (electrochromic effect) and irreversible (degradation) processes. These details can help guide the informed selection of models in similar conditions by providing practical guidelines. The step-by-step approach to modeling can be more broadly adopted to enhance the accuracy and reproducibility of EC-SE experiments for interfacial studies relevant to energy conversion and storage, as well as other electrochemical reactions.