Understanding Polymer-Ion Interactions for Improved OMIECs

Organic mixed ionic-electronic conductors (OMIECs) are soft electronic materials that can solvate and transport ions as well as electronic charges, and are of interest for applications such as sensing, electrochromics, catalysis and energy storage, often using aqueous electrolytes.[1] OMIECs offer a high degree of flexibility in their structure and function which allows them to be customized for specific applications. However, it is difficult to disentangle how different factors, such as the polymer chemical structure, ion type and electrolyte influence the properties of the OMIECs such as their transport properties, capacity and stability under electrochemical cycling. A common strategy to investigate polymer-electrolyte interactions is systematic variation of the polymer side chains.[2, 3] In this work, we go further and investigate the effect of varying side chain structure, electrolyte concentration and ion type in order to probe the mechanism of electrochemical doping in a series of oxybithiophene based p-type polymers. We show that introducing hydrophobic groups on to the polar side chains influences redox performance, specific capacity, rate capability, swelling behaviour of the polymers. Interestingly, the choice and concentration of electrolyte strongly modulates the behaviour of the polymers. With the help of in situ characterization methods, we observed an anomalous swelling trend of one polymer that is controlled through choice of electrolyte. We interpret this unusual swelling phenomenon in terms of the interactions between polymer and both anions and cations and will discuss how electrolytes choice may impact the electrochemical performance of OMIECs.<br/><br/><b>References:</b><br/><br/>[1] B. D. Paulsen, K. Tybrandt, E. Stavrinidou, and J. Rivnay, "Organic mixed ionic–electronic conductors," <i>Nature Materials, </i>Review article vol. 19, no. 1, pp. 13-26, 1/1 2020.<br/>[2] N. Siemons<i> et al.</i>, "Controlling swelling in mixed transport polymers through alkyl side-chain physical cross-linking," vol. 120, no. 35, p. e2306272120, 2023.<br/>[3] D. Moia<i> et al.</i>, "Design and evaluation of conjugated polymers with polar side chains as electrode materials for electrochemical energy storage in aqueous electrolytes," <i>Energy & Environmental Science, </i>10.1039/C8EE03518K vol. 12, no. 4, pp. 1349-1357, 2019.