In alignment with widespread interest in carbon neutralization and sustainable practices, we disclose that conjugated polyelectrolyte (CPE) hydrogels are a type of recyclable, electrochemically stable, and environmentally friendly pseudocapacitive material for energy storage applications. By leveraging ionic-electronic coupling in a relatively fluid medium, one finds that hydrogels prepared using a fresh batch of an anionic CPE, namely, Pris-CPE-K, exhibit a specific capacitance of 32.6 ± 6.6 F g^-1 in 2 M NaCl and are capable of 80% (26.1 ± 6.5 F g^-1) capacitance retention after 100,000 galvanostatic charge-discharge (GCD) cycles at a current density (J) of 10 A g^-1. We note that equilibration under a constant potential prior to GCD analysis leads to the K⁺ counterions in the CPE exchanging with Na⁺ and, thus, the relevant active material Pris-CPE-Na. It is possible to remove the CPE material from the electrochemical cell via extraction with water and to carry out a simple purification through dialysis to produce a recycled material, namely Re-CPE-Na. The recycling workup has no significant detrimental impact on the electrochemical performance. Specifically, Re-CPE-Na hydrogels display an initial specific capacitance of 26.3 ± 1.2 F g^-1 (at 10 A g^-1) and retain 77% of the capacitance after a subsequent 100,000 GCD cycles. Characterization by NMR, FTIR, and Raman spectroscopies, together with XPS and GPC measurements, revealed no change in the structure of the backbone or side chains. However, rheological measurements gave evidence of a slight loss in G′ and G′′. Overall, that CPE hydrogels display recyclability argues in favor of considering them as a novel materials platform for energy storage applications within an economically viable circular recycling strategy.

A subclass of organic semiconductors known as conjugated polyelectrolytes (CPEs) is characterized by a conjugated backbone with ionic pendant groups. The water solubility of CPEs typically hinders applications of thin films in aqueous media. Herein, it is reported that films of an anionic CPE, namely CPE-K, drop cast from water produces single-component solid-state pseudocapacitive electrodes that are insoluble in aqueous electrolyte. That X-ray diffraction experiments reveal a more structurally ordered film, relative to the as-obtained powder from chemical synthesis, and dynamic light scattering measurements show an increase in aggregate particle size with increasing [KCl] indicate that CPE-K films are insoluble because of tight interchain contacts and electrostatic screening by the electrolyte. CPE-K film electrodes can maintain 85% of their original capacitance (84 F g⁻¹) at 500 A g⁻¹ and exhibit excellent cycling stability, where a capacitance retention of 93% after 100 000 cycles at a current density of 35 A g⁻¹. These findings demonstrate that it is possible to use initially water soluble ionic-organic materials in aqueous electrolytes, by increasing the electrolyte concentration. This strategy can be applied to the application of conjugated polyelectrolytes in batteries, organic electrochemical transistors, and electrochemical sensors, where fast electron and ion transport are required.

Conjugated polymer hydrogels (CPHs) are emerging pseudocapacitive materials capable of forming redox-active hydrogels. Current efforts focus on increasing their areal capacitance (C_Areal) and cycling stabilities by using binders tolerant to H₂SO₄-based electrolytes, while alternatives in more environmentally friendly electrolytes underperform due to low-capacity values. Herein, we demonstrate that it is possible to use conjugated polyelectrolyte (CPE), namely CPE-K, to create a single-component binder-free pseudocapacitive gel in environmentally friendly electrolytes (2 M: NaCl, MgCl₂, and MgSO₄), with C_Areal 1.9 times larger than those reported for single-component binder-free CPHs. The resulting pseudocapacitive gel exhibited C_Areal (523 mF cm⁻² at 0.25 mA cm⁻²) scalable with its thickness in NaCl electrolytes, providing an attractive solution to improve the capacitance of devices while maintaining a minimal charge-collecting electrode surface footprint. In addition, the CPE-K gel demonstrates 86% capacitance retention after 100 000 cycles at 10 mA cm⁻², which is higher than those reported for conventional state-of-the-art conjugated polymers. Electrochemical characterization revealed that C_Areal at all cycling rates tested is proportional to d_Thk up to 750 μm, primarily due to facile ionic diffusion within the 3D conductive network of the gel. Thicker electrodes (d_Thk = 1250 μm) can be operated at a rate of 15 mA cm⁻² with minimal capacity loss. These results demonstrate the potential applications of self-doped CPE gels in designing the next generation of multi-functional electrochemical energy storage and conversion technologies for targeting high energy and power density applications.

Conjugated polyelectrolyte (CPE) hydrogels synergize the electrical properties of redox-active polymers with the physical properties of hydrogels. Of particular relevance is their implementation as pseudocapacitors due to their high ionic conductivity, strong ionic-electronic coupling, and large electroactive surface area. To date, efforts to improve the cycling stability of such hydrogels are predominated by the use of additives - optimization of the CPE's intrinsic properties remains underexplored. Herein, the systematic increase in the molecular weight (MW) of a self-doped CPE, namely CPE-K, has been demonstrated as an effective strategy to enhance the cycling stability of the resulting hydrogel. At high MW, mechanically stronger hydrogels were obtained with a specific capacitance as high as 88 ± 4 F g⁻¹ at 0.25 A g⁻¹ and a cycling stability of 76% capacitance retention after 100 000 cycles at 2.5 A g⁻¹. Furthermore, this strategy yields a wider working pseudocapacitive window, less internal resistance, and higher ionic conductivity within the 3D conductive network. We attribute the enhanced electrochemical performance to stronger inter-chain contacts for optimal morphological organization, as revealed by rheological measurements, resulting in stress-tolerant hydrogels with a higher degree of percolation within a 3D conductive network. These results position CPE-K hydrogels as a state-of-the-art organic material for long-term pseudocapacitive technologies and potentially for the next generation of multi-functional pseudocapacitive devices that go beyond high energy density and power density.