Controlling Ion Uptake in Carboxylated Mixed Conductors

Due to their ability to simultaneously transport ions and electrons, organic mixed ionic-electronic conductors (OMIECs) have emerged as promising materials, positioned at the forefront of applications such as energy storage, neuromorphic computing, and bioelectronics. Despite their potential, the intricacies of charge compensation during polymer redox processes remain underexplored, often reduced to simplified models of single-ion injection with minimal focus on the role of counterions. To deepen insights and inform design strategies for next-generation OMIECs, we examine a set of p-channel carboxylated mixed conductors, focusing on the impact of side-chain functionality. Our investigation reveals distinct swelling behaviors during electrochemical doping and dedoping, driven by interactions with chaotropic and kosmotropic electrolytes. Polymers functionalized with carboxylic acid (COOH) exhibit notable deswelling and mass reduction during doping, attributed to substantial cation expulsion, while those with ethoxycarbonyl (COOEt) groups show significant mass increases, pointing to an anion-dominated mechanism. Utilizing operando X-ray fluorescence (XRF) as a real-time in situ technique, we provide the first evidence that COOH-functionalized polymers strongly interact with cations, whereas COOEt-functionalized counterparts engage in minimal cation interactions. Importantly, we demonstrate that cations help mitigate swelling by counterbalancing anions, facilitating efficient anion uptake during p-channel doping without compromising material performance. By further operating the COOH-functionalized polymer in acidic pH electrolytes, we observed significant swelling differences, particularly in the inhibition of cation interactions. This pH-responsive behavior likely arises from proton exchange, which modulates the electrostatic interactions between the polymer and electrolyte. This ability to selectively control ion uptake and viscoelastic properties through pH modulation represents a distinct discovery in the field, highlighting a new mechanism for tailoring the performance of OMIECs. These results highlight the critical role of side-chain chemistry in modulating ion uptake and conductivity, offering a fresh framework for the design of OMIECs with enhanced performance and reduced swelling.