Thin-Film Electrodes Based on Conjugated Polyelectrolytes for High-Performance Pseudocapacitance in Aqueous Media

Abstract

Conjugated polyelectrolytes (CPEs) are an emerging class of materials that exhibits competitive pseudocapacitive performances in their hydrogel form due to their mixed electronic/ionic conductive nature. Interest in developing solid-state electrodes based on CPEs for practical applications is needed. However, the water-soluble nature of the CPEs hinders their thin-film stability in aqueous media. Herein, we report that the slow-dried drop-casted CPE-K thin films formed a solid-state electrode insoluble in aqueous electrolyte solutions when the salt concentration is higher than 0.1 M. XRD measurements revealed that slow-dried, drop-casted CPE-K film possesses a more ordered macromolecular structure when compared to its pre-deposited powder form. DLS measurements revealed that the extent of CPE-K aggregation increases as a function of salt concentration, which enhances the stability of the films in the electrolyte solutions. In addition, AFM imaging suggests well-ordered and uniform nanostructures. As a result, it is possible to use CPE-K thin-film as a pseudocapacitive electrodes capable to maintain 85% (71 F g-1 ) of its original capacitance (84 F g-1) at a specific current of 500 A g-1. In addition, CPE-K film electrodes exhibit good cycling stability where a capacitance retention of 93% after 100,000 cycles at a current density of 35 A g-1 was obtained. To our knowledge, such high charge/discharge rates and stabilities are greater than those reported for state-of-the-art CP in any form. Further electrochemical characterization revealed that surface-controlled pseudocapacitive behavior dominates the charge-storing mechanism of the CPE-K electrodes. In addition, our electrochemical analysis suggests that facile ionic diffusion attributed to the presence of an ionic lattice in the ordered structure of the CPE-K films is responsible for the unprecedented charging and cycling performances. These findings provide a new strategy to obtain solid-state electrodes based on CPEs without the need for binders or additives. Additional advantages such as water processability, low cost, and bio-compatibility may allow CPE-K to be a sustainable solution for the future of electrochemical energy storage.