High-strength, anti-freezing and recyclable soy protein isolate/poly(vinyl alcohol) gels empowered by deep eutectic solvent-augmented Hofmeister effect

Abstract

Highly tough hydrogels formed via the Hofmeister effect exhibit broad applications in flexible electronics. However, high concentrations of salting-out salts inevitably induce phase separation of the solvent and polymer chains, thereby affecting the mechanical properties and conductivity of the hydrogels. Herein, we introduced a deep eutectic solvent (DES) to suppress the strong salting-out effect of trisodium citrate (Na₃Cit), thereby preventing phase separation in the soy protein isolate (SPI)/poly(vinyl alcohol) (PVA) mixture. This strategy enabled the formation of a homogeneous SPI/PVA/DES/Na₃Cit composite solution, which was subsequently processed into a gel electrolyte (denoted as SPDNH) through a freeze-thaw cycle. The optimized organohydrogel exhibited the tensile strength of 1.65 ± 0.03 MPa and the elongation at break of 518.8 ± 7.49%, along with an ionic conductivity of 2.01 ± 0.04 S/m. Moreover, the incorporation of DES imparted remarkable freeze resistance to the SPDNH, with a freezing point as low as –33 °C. A flexible all-solid-state supercapacitor was fabricated using the SPDNH electrolyte and activated carbon electrodes. The device delivered a high areal capacitance of 125.61 mF/cm² and demonstrated stable cycling performance at room temperature, retaining 80.20% of its initial capacitance after 1000 charge–discharge cycles. Furthermore, the supercapacitor maintained reliable operation under various bending conditions and at low temperatures (-20 °C), highlighting its potential for use in flexible and low-temperature energy storage applications.