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.

Producing thick films of conducting polymers by a low‐cost manufacturing technique would enable new applications. However, removing huge solvent volume from diluted suspension or dispersion (1–3 wt%) in which conducting polymers are typically obtained is a true manufacturing challenge. In this work, a procedure is proposed to quickly remove water from the conducting polymer poly(3,4‐ethylenedioxythiophene:poly(4‐styrene sulfonate) (PEDOT:PSS) suspension. The PEDOT:PSS suspension is first flocculated with 1 m H2SO4 transforming PEDOT nanoparticles (≈50–500 nm) into soft microparticles. A filtration process inspired by pulp dewatering in a paper machine on a wire mesh with apertures dimension between 60 µm and 0.5 mm leads to thick free‐standing films (≈0.5 mm). Wire mesh clogging that hinders dewatering (known as dead‐end filtration) is overcome by adding to the flocculated PEDOT:PSS dispersion carbon fibers that aggregate and form efficient water channels. Moreover, this enables fast formation of thick layers under simple atmospheric pressure filtration, thus making the process truly scalable. Thick freestanding PEDOT films thus obtained are used as electrocatalysts for efficient reduction of oxygen to hydrogen peroxide, a promising green chemical and fuel. The inhomogeneity of the films does not affect their electrochemical function.