X-Ray Absorption Spectroscopy—A Probe for Electroactive Polymer-Charge Interactions

Semiconducting polymers have sparked interest in electrochemical processes such as catalysis and energy storage due to their strong charge transport properties and ionic conductivity. Despite the importance of polymer-electrolyte interactions in device performance, there is a lack of understanding of the fundamental interphasial interactions that occur between the polymer and electrolyte. In this study, we use near-edge X-ray absorption fine structure spectroscopy (NEXAFS) to study the local atomic electronic and chemical structure of a pi-conjugated redox active polymer. We use N2200, a well-studied polymer¹ due to its high crystallinity, conductivity, and charge carrier mobility which has monomeric units consisting of an naphthalene diimide (NDI) with alkyl sidechains coupled with a bithiophene unit.<br/>N2200 has two redox events correlating to the formation of polarons (-1 charge) and bipolarons (-2 charge) during charging that are accompanied by the absorption of counter-cations from the surrounding electrolyte for charge neutralization. We will introduce and discuss an <i>ex-situ</i> study in which carbon and oxygen NEXAFS is measured at different points in the polymer charging process and compare to an <i>operando</i> study in which the polymer charge is held potentiostatically while measuring NEXAFS. Using simulations, we assign electron excitation events to experimental NEXAFS spectra, identify polaron localization, and discuss how polymer-electrolyte interfacial interactions dictate charge transfer. Our results show how the charge localizes on the polymer and how elemental environments of the polymer change as a function of charge. This work addresses the impact of polymer choice as electrode materials on device performance and operation.<br/>References:<br/>(1) <i>Adv. Mater.</i> <b>2010</b>, <i>22</i> (39), 4359–4363. https://doi.org/10.1002/adma.201001202.<br/>(2) <i>Chem. Mater.</i> <b>2018</b>, <i>30</i> (9), 2945–2953. https://doi.org/10.1021/acs.chemmater.8b00321.