3D hierarchical porous hydrogel polymer electrolytes for flexible quasi-solid-state supercapacitors

Abstract

Hydrogel polymer electrolytes (GPEs) represent a promising solution for the development of safe and stable energy storage devices. However, GPEs often demonstrate limited ionic conductivity, insufficient mechanical properties, and inadequate performance under the extreme temperatures. Herein, GPEs with a unique 3D interconnected hierarchical porous structure have been exploited through a two-step process involving both chemical and physical crosslinking, followed by an activation of KOH electrolytes. As a result, GPEs exhibit an exceptionally high ionic conductivity of 48.8 mS/cm, a tensile strain of 894 %, and a tensile strength of 2.9 MPa. Additionally, GPEs maintain stable charge/discharge across a wide temperature range of − 18 °C to 50 °C. Symmetric coin and pouch supercapacitors (SCs) were assembled using activated carbon as electrodes. The effective contact between electrodes and GPEs endows the SCs low resistance and enhances electrochemical reversibility. The coin SC demonstrates a specific capacitance (C_s) of 157.4F/g at a current density of 1.5 A/g, 100 % capacitance retention after 40,000 cycles at room temperature (RT). The flexible pouch SC endures a minimum of 100 cycles of 180° bending and is capable of withstanding the weight of a car rolling over it. Furthermore, the pouch SC, despite experiencing multiple instances of bending and crushing, maintains a capacitance of 99 % after 100,000 cycles at a current density of 2 A/g. The research offers novel insights into the design and optimization of GPEs, and proves the feasibility for the substitution of GPEs for both commercial separators and liquid electrolytes in SCs, while also facilitating lightweight and flexible quasi-solid-state design for energy storage devices.