Pseudocapacitive gels based on conjugated polyelectrolytes: thickness and ion diffusion limitations

Abstract

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.