Volumetric Double-Layer Charge Storage in Composites Based on Conducting Polymer PEDOT and Cellulose

Energy storage technology incorporating conducting polymers as the active component in electrode structures, in part based on natural materials, is a promising strategy toward a sustainable future. Electronic and ionic charge transport in poly­(3,4-ethylenedioxythiophene) (PEDOT) provides fundamentals for energy storage, governed by volumetric PEDOT:counterion double layers. Despite extensive experimental investigations, a solid understanding of the capacitance in PEDOT-based nanocomposites remains unsatisfactory. Here, we report on the charge storage mechanism in PEDOT composited with cellulose nanofibrils (termed as “power paper”) from three different perspectives: experimental measurements, density functional theory atomistic simulations, and device-scale simulations based on the Nernst–Planck–Poisson equations. The capacitance of the power paper was investigated by varying the film thickness, charging currents, and electrolyte ion concentrations. We show that the volumetric capacitance of the power paper originates from electrostatic molecular double layers defined at atomistic scales, formed between holes, localized in the PEDOT backbone, and their counterions. Experimental galvanostatic cycling characteristics of the power paper is well reproduced within the electrostatic Nernst–Planck–Poisson model. The difference between the specific capacitance and the intrinsic volumetric capacitance is also outlined. Substantial oxygen reduction reactions were identified and recorded in situ in the vicinity of the power paper surface at negative potentials. Purging of dissolved oxygen from the electrolyte leads to the elimination of currents originating from the oxygen reduction reactions and allows us to obtain well-defined electrostatic-capacitive behavior (box-shaped cyclic voltammetry and triangular galvanostatic charge–discharge characteristics) at a large operational potential window from −0.6 V to +0.6 V. The obtained results reveal that the fundamental charge storage is a result of electrostatic Stern double layers in both oxidized and reduced electrodes, and the developed theoretical approaches provide a predictive tool to optimize performance and device design for energy storage devices based on high-performance conducting polymer electrodes