Aqueous asymmetric pseudocapacitor featuring high areal energy and power using conjugated polyelectrolytes and Ti₃C₂T_x MXene.

Abstract

Despite the development of various pseudocapacitive materials, full-cell pseudocapacitors have yet to surpass the power density of conventional electric double layer capacitors, primarily due to the lack of high-rate positive pseudocapacitive materials. This work reports a solid-state conjugated polyelectrolyte that achieves high-rate charge storage as a positive electrode, facilitated by a co-ion desorption mechanism. The conjugated polyelectrolyte retains 70% of its capacitance at 100 A g^-1 with a mass loading of 2.8 mg cm^-2 and exhibits a long cycling life of 100,000 cycles in a Swagelok cell configuration. Increasing the electrode thickness fourfold has minimal impact on ion diffusivity and accessibility, yielding a high areal capacitance of 915 mF cm^-2. When paired with a high-rate negative pseudocapacitive electrode Ti₃C₂T_x, the device leverages the redox-active potentials of both materials, achieving a device voltage of 1.5 V and supports operation rates up to 10 V s^-1 or 50 A g^-1. This configuration enables the pseudocapacitor to deliver an areal power of 160 mW cm^-2, while significantly increasing the areal energy (up to 71 μWh cm^-2). The high areal performance, combined with the additive-free and water-based fabrication process, makes pseudocapacitors promising for on-chip and wearable energy storage applications.