Ionic Solvent Shell Drives Electroactuation in Organic Mixed Ionic‐Electronic Conductors

Abstract

The conversion of electrochemical processes into mechanical deformation in organic mixed ionic‐electronic conductors (OMIECs) enables artificial muscle‐like actuators but is also critical for degradation processes affecting OMIEC‐based devices. To provide a microscopic understanding of electroactuation, the modulated electrochemical atomic force microscopy (mEC‐AFM) is introduced here as a novel in‐operando characterization method for electroactive materials. The technique enables multidimensional spectroscopic investigations of local electroactuation and charge uptake giving access to the electroactuation transfer function. For poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) based microelectrodes, the spectroscopic measurements are combined with multichannel mEC‐AFM imaging, providing maps of local electroactuation amplitude and phase as well as surface morphology. The results demonstrate that the amplitude and timescales of electroactuation are governed by the drift motion of hydrated ions. Accordingly, slower water diffusion processes are not limiting, and the results illustrate how OMIEC microactuators can operate at sub‐millisecond timescales.

Subsurface Profiling of Ion Migration and Swelling in Conducting Polymer Actuators with Modulated Electrochemical Atomic Force Microscopy

“…The resulting electroactuation mechanism for Ppy:DBS differs from findings obtained for PEDOT:PSS (see Supp. Inf.…”

“…We perform a first quantitative analysis on the actuation data by computing the average actuation transfer function A = S̅ / Q̅ . 34 The transfer function A links the input charge Q̅ = ∫ I AC dt (see Supp. Inf.…”

“…12). 34 To explain the more complex electroswelling behavior of Ppy:DBS, we hypothesize that ionic transport and electronic transport are limiting and cause a nonuniform distribution of charge and swelling in the polymeric layer. To test this hypothesis, we introduce an advanced analysis scheme of the multidimensional spectroscopy data set {I AC (ω,E) and S̅ (ω,E)} provided by mEC-AFM experiments.…”

“…However, these methods all required macroscopic electroactive thin films and could only reveal slow actuation processes occurring on the time scale of seconds and the monitoring of fast local ionic exchange processes remains elusive. For this reason, we recently introduced modulated electrochemical atomic force microscopy (mEC-AFM) as an operando characterization technique to study local volume changes and interface forces resulting from dynamical electroactuation effects in OMIECs . Through the introduction of the actuation transfer function, charge accumulation in PEDOT doped with polystyrenesulfonate (PEDOT:PSS) was correlated to morphological changes in terms of drift transport of hydrated ions, achieving high-frequency operation in OMIEC-based microactuators.…”