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

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 & mu;m, primarily due to facile ionic diffusion within the 3D conductive network of the gel. Thicker electrodes (d(Thk) = 1250 & mu;m) 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.