Electrical and Capacitive Response of Hydrogel Solid-Like Electrolytes for Supercapacitors.

Flexible hydrogels are attracting significant interest as solid-like electrolytes for energy storage devices, especially for supercapacitors, because of their lightweight and anti-deformation features. Here, we present a comparative study of four ionic conductive hydrogels derived from biopolymers and doped with 0.1 M NaCl. More specifically, such hydrogels are constituted by κ-carrageenan (κC), carboxymethyl cellulose (CMC), poly-γ-glutamic acid (PGGA) or a phenylalanine-containing polyesteramide (PEA). After examining the morphology and the swelling ratio of the four hydrogels, which varies between 483% and 2356%, their electrical and capacitive behaviors were examined using electrochemical impedance spectroscopy. Measurements were conducted on devices where a hydrogel film was sandwiched between two identical poly(3,4-ethylenedioxythiophene) electrodes. The bulk conductivity of the prepared doped hydrogels is 76, 48, 36 and 34 mS/cm for PEA, PGGA, κC and CMC, respectively. Overall, the polyesteramide hydrogel exhibits the most adequate properties (i.e., low electrical resistance and high capacitance) to be used as semi-solid electrolyte for supercapacitors, which has been attributed to its distinctive structure based on the homogeneous and abundant distribution of both micro- and nanopores. Indeed, the morphology of the polyestermide hydrogel reduces the hydrogel resistance, enhances the transport of ions, and results in a better interfacial contact between the electrodes and solid electrolyte. The correlation between the supercapacitor performance and the hydrogel porous morphology is presented as an important design feature for the next generation of light and flexible energy storage devices for wearable electronics.