Quantitative Imaging of Electroswelling in Organic Mixed Ionic Electronic Conductors

Ionic transport and accumulation into active layers of electrodes is of crucial relevance for energy storage and conversion processes. Organic mixed ionic electronic conductors (OMIECs) are a well characterized model system that exhibits a 3D electrified interface. As a result, unconventional properties emerge such as volumetric capacitance or strong electroswelling. Electroswelling allows to achieve volume changes and mechanical actuation controlled by small electrical potentials. The electroswelling effect is exploited in electrochemical actuators with low-voltage drive, nanoscale precision and miniaturization, with applications in soft robotics, soft micromechanical devices or actuated biomedical devices. However, electroswelling can also trigger delamination in OMIECs thin film devices used for sensors or electrochemical energy storage and puts their long-term stability at risk.<br/><br/>To investigate ionic transport and accumulation at the microscale we introduce a novel type of modulated electrochemically controlled AFM experiment (mEC-AFM) that combines local surface strain measurements with electrochemical impedance spectroscopy. We test the experiment on OMIEC covered microelectrodes and obtain multidimensional spectroscopic data that explains the relevant processes and their timescales in electroswelling. Combining the technique with the PinPoint imaging mode, we record high resolution maps that report the local amplitude and the phase of electroswelling on soft polymer thin films. The data demonstrates that electroswelling in PEDOT:PSS used as OMIEC material is driven by the volume of hydrated ions entering the thin film and does not depend on slower diffusive processes.[1] In more complex materials such as PPY-DBS ionic transport is more hindered and the multidimensional spectroscopic data allows for in-depth profiling of ionic transport and swelling properties.<br/><br/>[1] F. Bonafe et al. Adv. Sci. 2024, https://doi.org/10.1002/advs.202308746